2011 Field Season at Petrified Forest National Park Officially Starts on Tuesday
This year I will have two student interns, newcomer Susan Drymala from the University of Maryland who some of you may know from her blog Crurotarsi:The Forgotten Archosaurs, and 2009 alumnus Rachel Guest. I'm hoping we will hit the ground running this year so wish us luck. Our goal this year: try to find a bit more of Chindesaurus bryansmalli. I'm planning on blogging quite a bit on what we are doing so stay tuned. It may be at this site or I may start a new site specifically for PEFO fieldwork. I don't know yet. I hope that Susan and Rachel have a blast and enjoy their time doing some intense paleontology at the park for the next 10 weeks or so.
Fossil Evidence on Origin of the Mammalian Brain.
Rowe, T., Macrini, T. E., and Z.-X. Luo. 2011. Fossil Evidence on Origin of the Mammalian Brain. Science 332:955. DOI: 10.1126/science.1203117 [supp. info]
Abstract - Many hypotheses have been postulated regarding the early evolution of the mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium sheds light on this history. We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses. The initial pulse was probably driven by increased resolution in olfaction and improvements in tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.
Abstract - Many hypotheses have been postulated regarding the early evolution of the mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium sheds light on this history. We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses. The initial pulse was probably driven by increased resolution in olfaction and improvements in tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.
"Dinosaur Rock" at Petrified Forest National Park
The Earth Science Picture of the Day is what the photographer is calling "Dinosaur Rock" in Petrified Forest National Park (aka Battleship Rock). I will agree that to make this a dinosaur you need to right angle and lighting. I'll admit I'll never look at this outcrop the same again.
BTW... the name Battleship Rock stems from the petrified logs sticking out of the side of the butte that are reminiscent of cannon on a ship.
I'm glad Mr. Lachman enjoyed the park and featured it in his picture of the day.
BTW... the name Battleship Rock stems from the petrified logs sticking out of the side of the butte that are reminiscent of cannon on a ship.
I'm glad Mr. Lachman enjoyed the park and featured it in his picture of the day.
Miniaturisation and the Origin of Mammals
Probably the most awkward abstract to read that I have ever seen because of the included references.
Bonaparte, J. F. 2011. Miniaturisation and the origin of mammals. Historical Biology (first published online, May 24, 2011). DOI: 10.1080/08912963.2011.559079.
Abstract - In this paper, the hypothesis of miniaturisation to explain the origin of mammals (Rowe 1993, Mammals phylogeny: mesozoic differentiation, multituberculates, monotremes, early therians, and marsupials. New York: Springer-Verlag, p. 129-145) is discussed, based on three lines of evidence resulting from new discoveries of eucynodonts in the Late Triassic of Southern Brazil (Bonaparte et al. 2003, Rev Bras Paleont 5:5-27; 2005, Rev Bras Paleont 8:25-46; 2006, New Mexico Museum Nat Hist Sci Bull 37:1-8; 2010, Rev Bras Paleont) that are: (1) the incomplete fossil record of eucynodonts known until 2003; (2) the structure of the primary palate rejects the ancestral condition of thrinaxodontids, probainognathids, chiniquodontids and cynognathids to the earliest mammals; and (3) the relatively large postdentary bones of the Middle Triassic brasilodontids that are otherwise very small in size (skull 44 mm long) suggest that small size per se did not help to improve the middle ear or other sophisticated organs present in the earliest mammals (Rowe 1993; Kemp 2005, The origin and evolution of mammals. Oxford University Press, p. 1-391). Small size possibly was not a secondary character, but a persistent primitive one. This new interpretation has resulted from comparative study of non-mammalian eucynodonts discovered in the Middle and Late Triassic of Brazil and those known previously. The general acceptance of the hypothesis of miniaturisation is thus a consequence of the poor fossil record of Middle and Late Triassic eucynodonts before 2003.
Bonaparte, J. F. 2011. Miniaturisation and the origin of mammals. Historical Biology (first published online, May 24, 2011). DOI: 10.1080/08912963.2011.559079.
Abstract - In this paper, the hypothesis of miniaturisation to explain the origin of mammals (Rowe 1993, Mammals phylogeny: mesozoic differentiation, multituberculates, monotremes, early therians, and marsupials. New York: Springer-Verlag, p. 129-145) is discussed, based on three lines of evidence resulting from new discoveries of eucynodonts in the Late Triassic of Southern Brazil (Bonaparte et al. 2003, Rev Bras Paleont 5:5-27; 2005, Rev Bras Paleont 8:25-46; 2006, New Mexico Museum Nat Hist Sci Bull 37:1-8; 2010, Rev Bras Paleont) that are: (1) the incomplete fossil record of eucynodonts known until 2003; (2) the structure of the primary palate rejects the ancestral condition of thrinaxodontids, probainognathids, chiniquodontids and cynognathids to the earliest mammals; and (3) the relatively large postdentary bones of the Middle Triassic brasilodontids that are otherwise very small in size (skull 44 mm long) suggest that small size per se did not help to improve the middle ear or other sophisticated organs present in the earliest mammals (Rowe 1993; Kemp 2005, The origin and evolution of mammals. Oxford University Press, p. 1-391). Small size possibly was not a secondary character, but a persistent primitive one. This new interpretation has resulted from comparative study of non-mammalian eucynodonts discovered in the Middle and Late Triassic of Brazil and those known previously. The general acceptance of the hypothesis of miniaturisation is thus a consequence of the poor fossil record of Middle and Late Triassic eucynodonts before 2003.
Blogroll Updates
Added a few new links and removed a few stagnant ones (no new posts for 5 months or more). There are new blogs popping up almost daily, anything key that I'm missing?
"Belodon" from Creatures of Other Days by Hutchinson and Flower (1894)
The small animals in the reconstruction are the aetosaur Aetosaurus. The one of the left seems to be wondering why the phytosaur is wearing aetosaur armor.
New Skull Description of the Early Jurassic Ornithischian Dinosaur Heterodontosaurus tucki
Norman, D. B., Crompton, A. W., Butler, R. J., Porro, L. B., and A. J. Charig. 2011. The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: cranial anatomy, functional morphology, taxonomy, and relationships. Zoological Journal of the Linnaean Society published first online 20 May 2011. DOI: 10.1111/j.1096-3642.2011.00697.x
Abstract - The cranial anatomy of the Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962 is described in detail for the first time on the basis of two principal specimens: the holotype (SAM-PK-K337) and referred skull (SAM-PK-K1332). In addition several other specimens that have a bearing on the interpretation of the anatomy and biology of Heterodontosaurus are described. The skull and lower jaw of Heterodontosaurus are compact and robust but perhaps most notable for the heterodont dentition that merited the generic name. Details of the cranial anatomy are revealed and show that the skull is unexpectedly specialized in such an early representative of the Ornithischia, including: the closely packed, hypsodont crowns and ‘warping’ of the occlusal surfaces (created by progressive variation in the angulation of wear on successive crowns) seen in the cheek dentition; the unusual sutural relationships between the bones along the dorsal edge of the lower jaw; the very narrow, deeply vaulted palate and associated structures on the side wall of the braincase; and the indications of cranial pneumatism (more commonly seen in basal archosaurs and saurischian dinosaurs). Evidence for tooth replacement (which has long been recognized, despite frequent statements to the contrary) is suggestive of an episodic, rather than continuous, style of tooth replacement that is, yet again, unusual in diapsids generally and particularly so amongst ornithischian dinosaurs. Cranial musculature has been reconstructed and seems to conform to that typically seen in diapsids, with the exception of the encroachment of M. adductor mandibulae externus superficialis across the lateral surface of the temporal region and external surface of the lower jaw. Indications, taken from the unusual shape of the occlusal surfaces of the cheek dentition and jaw musculature, are suggestive of a novel form of jaw action in this dinosaur. The taxonomy of currently known late Karoo-aged heterodontosaurids from southern Africa is reviewed. Although complicated by the inadequate nature of much of the known material, it is concluded that two taxa may be readily recognized: H. tucki and Abrictosaurus consors. At least one additional taxon is recognized within the taxa presently named Lanasaurus and Lycorhinus; however, both remain taxonomically problematic and their status needs to be further tested and may only be resolved by future discoveries. The only other named taxon, Geranosaurus atavus, represents an invalid name. The recognition of at least four distinct taxa indicates that the heterodontosaurids were speciose within the late Karoo ecosystem. The systematics of Heterodontosaurus and its congeners has been analysed, using a restricted sample of taxa. A basal (nongenasaurian) position within Ornithischia is re-affirmed. There are at least four competing hypotheses concerning the phylogenetic placement of the Heterodontosauridae, so the evidence in support of the various hypotheses is reviewed in some detail. At present the best-supported hypothesis is the one which places Heterodontosauridae in a basal (non-genasaurian) position; however, the evidence is not fully conclusive and further information is still needed in respect of the anatomy of proximate outgroups, as well as more complete anatomical details for other heterodontosaurids. Heterodontosaurids were not such rare components of the late Karoo ecosystem as previously thought; evidence also suggests that from a phylogenetic perspective they occupied a potentially crucial position during the earliest phases of ornithischian dinosaur evolution.
Abstract - The cranial anatomy of the Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962 is described in detail for the first time on the basis of two principal specimens: the holotype (SAM-PK-K337) and referred skull (SAM-PK-K1332). In addition several other specimens that have a bearing on the interpretation of the anatomy and biology of Heterodontosaurus are described. The skull and lower jaw of Heterodontosaurus are compact and robust but perhaps most notable for the heterodont dentition that merited the generic name. Details of the cranial anatomy are revealed and show that the skull is unexpectedly specialized in such an early representative of the Ornithischia, including: the closely packed, hypsodont crowns and ‘warping’ of the occlusal surfaces (created by progressive variation in the angulation of wear on successive crowns) seen in the cheek dentition; the unusual sutural relationships between the bones along the dorsal edge of the lower jaw; the very narrow, deeply vaulted palate and associated structures on the side wall of the braincase; and the indications of cranial pneumatism (more commonly seen in basal archosaurs and saurischian dinosaurs). Evidence for tooth replacement (which has long been recognized, despite frequent statements to the contrary) is suggestive of an episodic, rather than continuous, style of tooth replacement that is, yet again, unusual in diapsids generally and particularly so amongst ornithischian dinosaurs. Cranial musculature has been reconstructed and seems to conform to that typically seen in diapsids, with the exception of the encroachment of M. adductor mandibulae externus superficialis across the lateral surface of the temporal region and external surface of the lower jaw. Indications, taken from the unusual shape of the occlusal surfaces of the cheek dentition and jaw musculature, are suggestive of a novel form of jaw action in this dinosaur. The taxonomy of currently known late Karoo-aged heterodontosaurids from southern Africa is reviewed. Although complicated by the inadequate nature of much of the known material, it is concluded that two taxa may be readily recognized: H. tucki and Abrictosaurus consors. At least one additional taxon is recognized within the taxa presently named Lanasaurus and Lycorhinus; however, both remain taxonomically problematic and their status needs to be further tested and may only be resolved by future discoveries. The only other named taxon, Geranosaurus atavus, represents an invalid name. The recognition of at least four distinct taxa indicates that the heterodontosaurids were speciose within the late Karoo ecosystem. The systematics of Heterodontosaurus and its congeners has been analysed, using a restricted sample of taxa. A basal (nongenasaurian) position within Ornithischia is re-affirmed. There are at least four competing hypotheses concerning the phylogenetic placement of the Heterodontosauridae, so the evidence in support of the various hypotheses is reviewed in some detail. At present the best-supported hypothesis is the one which places Heterodontosauridae in a basal (non-genasaurian) position; however, the evidence is not fully conclusive and further information is still needed in respect of the anatomy of proximate outgroups, as well as more complete anatomical details for other heterodontosaurids. Heterodontosaurids were not such rare components of the late Karoo ecosystem as previously thought; evidence also suggests that from a phylogenetic perspective they occupied a potentially crucial position during the earliest phases of ornithischian dinosaur evolution.
China Fossil Shows Bird, Crocodile Family Trees Split Earlier than Thought
From the University of Washington:
A fossil unearthed in China in the 1970s of a creature that died about 247 million years ago, originally thought to be a distant relative of both birds and crocodiles, turns out to have come from the crocodile family tree after it had already split from the bird family tree, according to research led by a University of Washington paleontologist.
The only known specimen of Xilousuchus sapingensis has been reexamined and is now classified as an archosaur. Archosaurs, characterized by skulls with long, narrow snouts and teeth set in sockets, include dinosaurs as well as crocodiles and birds.
The new examination dates the X. sapingensis specimen to the early Triassic period, 247 million to 252 million years ago, said Sterling Nesbitt, a UW postdoctoral researcher in biology. That means the creature lived just a short geological time after the largest mass extinction in Earth's history, 252 million years ago at the end of the Permian period, when as much as 95 percent of marine life and 70 percent of land creatures perished. The evidence, he said, places X. sapingensis on the crocodile side of the archosaur family tree.
"We're marching closer and closer to the Permian-Triassic boundary with the origin of archosaurs," Nesbitt said. "And today the archosaurs are still the dominant land vertebrate, when you look at the diversity of birds."
The work could sharpen debate among paleontologists about whether archosaurs existed before the Permian period and survived the extinction event, or if only archosaur precursors were on the scene before the end of the Permian.
"Archosaurs might have survived the extinction or they might have been a product of the recovery from the extinction," Nesbitt said.
The research is published May 17 online in Earth and Environmental Science Transactions of the Royal Society of Edinburgh, a journal of Cambridge University in the United Kingdom.
Co-authors are Jun Liu of the American Museum of Natural History in New York and Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. Nesbitt did most of his work on the project while a postdoctoral researcher at the University of Texas at Austin.
The X. sapingensis specimen – a skull and 10 vertebrae – was found in the Heshanggou Formation in northern China, an area with deposits that date from the early and mid-Triassic period, from 252 million to 230 million years ago, and further back, before the mass extinction.
The fossil was originally classified as an archosauriform, a "cousin" of archosaurs, rather than a true archosaur, but that was before the discovery of more complete early archosaur specimens from other parts of the Triassic period. The researchers examined bones from the specimen in detail, comparing them to those from the closest relatives of archosaurs, and discovered that X. sapingensis differed from virtually every archosauriform.
Among their findings was that bones at the tip of the jaw that bear the teeth likely were not downturned as much as originally thought when the specimen was first described in the 1980s. They also found that neural spines of the neck formed the forward part of a sail similar to that found on another ancient archosaur called Arizonasaurus, a very close relative of Xilousuchus found in Arizona.
The family trees of birds and crocodiles meet somewhere in the early Triassic and archosauriforms are the closest cousin to those archosaurs, Nesbitt said. But the new research places X. sapingensis firmly within the archosaur family tree, providing evidence that the early members of the crocodile and bird family trees evolved earlier than previously thought.
"This animal is closer to a crocodile, but it's not a crocodile. If you saw it today you wouldn't think it was a crocodile, especially not with a sail on its back," he said.
REFERENCE
Nesbitt, S. J., Liu, J., and C. Li. 2011. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (for 2010):271-284.
A fossil unearthed in China in the 1970s of a creature that died about 247 million years ago, originally thought to be a distant relative of both birds and crocodiles, turns out to have come from the crocodile family tree after it had already split from the bird family tree, according to research led by a University of Washington paleontologist.
The only known specimen of Xilousuchus sapingensis has been reexamined and is now classified as an archosaur. Archosaurs, characterized by skulls with long, narrow snouts and teeth set in sockets, include dinosaurs as well as crocodiles and birds.
The new examination dates the X. sapingensis specimen to the early Triassic period, 247 million to 252 million years ago, said Sterling Nesbitt, a UW postdoctoral researcher in biology. That means the creature lived just a short geological time after the largest mass extinction in Earth's history, 252 million years ago at the end of the Permian period, when as much as 95 percent of marine life and 70 percent of land creatures perished. The evidence, he said, places X. sapingensis on the crocodile side of the archosaur family tree.
"We're marching closer and closer to the Permian-Triassic boundary with the origin of archosaurs," Nesbitt said. "And today the archosaurs are still the dominant land vertebrate, when you look at the diversity of birds."
The work could sharpen debate among paleontologists about whether archosaurs existed before the Permian period and survived the extinction event, or if only archosaur precursors were on the scene before the end of the Permian.
"Archosaurs might have survived the extinction or they might have been a product of the recovery from the extinction," Nesbitt said.
The research is published May 17 online in Earth and Environmental Science Transactions of the Royal Society of Edinburgh, a journal of Cambridge University in the United Kingdom.
Co-authors are Jun Liu of the American Museum of Natural History in New York and Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China. Nesbitt did most of his work on the project while a postdoctoral researcher at the University of Texas at Austin.
The X. sapingensis specimen – a skull and 10 vertebrae – was found in the Heshanggou Formation in northern China, an area with deposits that date from the early and mid-Triassic period, from 252 million to 230 million years ago, and further back, before the mass extinction.
The fossil was originally classified as an archosauriform, a "cousin" of archosaurs, rather than a true archosaur, but that was before the discovery of more complete early archosaur specimens from other parts of the Triassic period. The researchers examined bones from the specimen in detail, comparing them to those from the closest relatives of archosaurs, and discovered that X. sapingensis differed from virtually every archosauriform.
Among their findings was that bones at the tip of the jaw that bear the teeth likely were not downturned as much as originally thought when the specimen was first described in the 1980s. They also found that neural spines of the neck formed the forward part of a sail similar to that found on another ancient archosaur called Arizonasaurus, a very close relative of Xilousuchus found in Arizona.
The family trees of birds and crocodiles meet somewhere in the early Triassic and archosauriforms are the closest cousin to those archosaurs, Nesbitt said. But the new research places X. sapingensis firmly within the archosaur family tree, providing evidence that the early members of the crocodile and bird family trees evolved earlier than previously thought.
"This animal is closer to a crocodile, but it's not a crocodile. If you saw it today you wouldn't think it was a crocodile, especially not with a sail on its back," he said.
REFERENCE
Nesbitt, S. J., Liu, J., and C. Li. 2011. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101 (for 2010):271-284.
A Late Triassic Vertebrate Paleontology Extravaganza - ‘Late Triassic Terrestrial Biotas and the Rise of Dinosaurs’ Special Issue
It's out. The special volume in Earth and Environmental Science Transactions of the Royal Society of Edinburgh on Late Triassic Terrestrial Biotas and the Rise of Dinosaurs. These papers stem from the Society of Vertebrate Paleontology Symposium at the 2009 annual meeting in Bristol, U. K. I've included a list of the papers and their abstracts below. There is an amazing amount of information here on Late Triassic vertebrates and early dinosaurs. It will take me awhile to get through the papers, but for now I'd like to say it is an honor to be included in this volume. I think Randy, Richard, Max, and the staff of EESTRSE did an excellent job in the production of this volume and I hope everyone who has an interest in Late Triassic terrestrial biotas enjoys the volume and finds the papers interesting and useful.
Abstract - Herbivorous and omnivorous dinosaurs were rare during the Carnian stage of the Late Triassic. By contrast, the succeeding Norian stage witnessed the rapid diversification of sauropodomorphs and the rise of the clade to ecological dominance. Ornithischians, by contrast, remained relatively rare components of dinosaur assemblages until much later in the Mesozoic. The causes underlying the differential success of ornithischians and sauropodomorphs remain unclear, but might be related to trophic specialisation. Sauropodomorphs replaced an established herbivore guild consisting of rhynchosaurs, aetosaurs and basal synapsids, but this faunal turnover appears to have been opportunistic and cannot be easily attributed to either competitive interactions or responses to floral change. Consideration of diversity patterns and relative abundance suggests that the ability to eat plants might have been a major factor promoting sauropodomorph success, but that it was less important in the early evolution of Ornithischia. On the basis of current evidence it is difficult to determine the diet of the ancestral dinosaur and scenarios in which omnivory or carnivory represent the basal condition appear equally likely.
13) Irmis, R. B. Evaluating hypotheses for the early diversification of dinosaurs.
Abstract - Many hypotheses have been proposed for the rise of dinosaurs, but their early diversification remains poorly understood. This paper examines the occurrences, species diversity and abundance of early dinosaurs at both regional and global scales to determine patterns of their early evolutionary history. Four main patterns are clear: (1) sauropodomorph dinosaurs became abundant during the late Norian–Rhaetian of Gondwana and Europe; (2) Triassic dinosaurs of North America have low species diversity and abundance until the beginning of the Jurassic; (3) sauropodomorphs and ornithischians are absent in the Triassic of North America; and (4) ornithischian dinosaurs maintain low species diversity, relative abundance and small body size until the Early Jurassic. No one hypothesis fully explains these data. There is no evidence for a Carnian–Norian extinction event, but sauropodomorphs did become abundant during the Norian in some assemblages. No clear connection exists between palaeoenvironment and early dinosaur diversity, but environmental stress at the Triassic–Jurassic boundary is consistent with changes in North American dinosaur assemblages. Elevated growth rates in dinosaurs are consistent with the gradual phyletic increase in body size. This study demonstrates that early dinosaur diversification was a complex process that was geographically diachronous and probably had several causes.
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| Issue cover featuring artwork by Jeff Martz. |
Table of Contents - Volume 101 - Special Issue 3-4 (Late Triassic Terrestrial Biotas and the Rise of Dinosaurs)
1) Butler, R. J., Irmis, R. B., and M. C. Langer. Preface to ‘Late Triassic Terrestrial Biotas and the Rise of Dinosaurs’ Special Issue.
Abstract - The early Mesozoic records an important transition in the history of the Earth’s terrestrial ecosystems. As they recovered from the largest known mass extinction (the end-Permian event), organisms in these ecosystems transitioned to new forms that eventually evolved into the classic Mesozoic biotas, and laid the foundations for many groups still flourishing today (Fraser 2006; Irmis & Whiteside 2010; Sues & Fraser 2010). All of this was set against a backdrop of dynamic climatic and physical events that shaped these biotas. This early Mesozoic terrestrial transition reached its culmination in many ways during the Late Triassic, when ecosystems had largely recovered from the end-Permian extinction, but had not yet been affected by the end-Triassic mass extinction (Fraser & Sues this volume). Thus, we see a combination of taxa, with some groups that would not survive the end of the Triassic living alongside early representatives of lineages that flourished later in the Mesozoic (e.g., Fraser 2006; Irmis et al. 2007; Brusatte et al. 2008; Sues & Fraser 2010, this volume) and in some cases are still diverse today. Just one example of this transition, recorded during the Late Triassic, is the origin and diversification of non-avian dinosaurs, the iconic representatives of Mesozoic terrestrial ecosystems (Brusatte et al. 2010; Langer et al. 2010). Although small and rare components of their respective biotas when they first evolved ∼231 Ma, dinosaurs were abundant and had a near-worldwide distribution by the beginning of the Jurassic Period (∼201·3 Ma).
2) Fraser, N. C., and H. D. Sues. The beginning of the ‘Age of Dinosaurs’: a brief overview of terrestrial biotic changes during the Triassic.
Abstract - The first appearance of dinosaurs during the early Late Triassic coincided with marked faunal changes in terrestrial ecosystems. Most of the major groups of extant tetrapods (or their proximate sister-taxa), including mammaliaforms, crocodyliform archosaurs, lepidosaurs and turtles, also first appeared in the fossil record during the Late Triassic. On the other hand, a number of Palaeozoic ‘holdovers’, such as procolophonid parareptiles, dicynodont therapsids and many groups of temnospondyls, vanished near or at the end of the Triassic. The tempo and mode of this faunal turnover have long been debated, but there has been growing acceptance of a rather sudden event, although the precise dating of such an event remains controversial. However, new discoveries have cast doubt on this assumption. The persistence of non-dinosaurian dinosauromorphs alongside dinosaurs well into Norian times hints at a more protracted turnover. New data on Triassic insect assemblages indicate that turnover among insects may also have been more protracted and possibly not co-incident with the faunal changes among tetrapods. Future work directed toward improved absolute age assessments for major faunal assemblages will be critical for a better understanding of the transition from therapsid-dominated to dinosaur-dominated communities during the early Mesozoic.
3) Olsen, P. E., Kent, D. V., and J. H. Whiteside. Implications of the Newark Supergroup-based astrochronology and geomagnetic polarity time scale (Newark-APTS) for the tempo and mode of the early diversification of the Dinosauria.
Abstract - The Newark-APTS established a high-resolution framework for the Late Triassic and Early Jurassic. Palaeomagnetic polarity correlations to marine sections show that stage-level correlations of continental sequences were off by as much as 10 million years. New U–Pb ages show the new correlations and the Newark basin astrochronology to be accurate. Correlation of Newark-APTS to the Chinle Formation/Dockum Group, Glen Canyon Group, Fleming Fjord Formation and Ischigualasto Formation led to the following conclusions: (1) there are no unequivocal Carnian-age dinosaurs; (2) the Norian Age was characterised by a slowly increasing saurischian diversity but no unequivocal ornithischians; (3) there was profound Norian and Rhaetian continental provinciality; (4) the classic Chinle-, Germanic- and Los Colorados-type assemblages may have persisted to the close of the Rhaetian; (5) the distinct genus-level biotic transition traditionally correlated with the marine Carnian–Norian is in fact mid-Norian in age and within published error of the Manicouagan impact; (6) the end-Triassic marine and continental extinctions as seen in eastern North America were contemporaneous; and (7) compared to Triassic communities, Hettangian and Sinemurian age terrestrial communities were nearly globally homogenous and of low diversity. Consequently, the complex emerging picture of dinosaur diversification demands biostratigraphically-independent geochronologies in each of the faunally-important regions.
4) Parker, W. G., and J. W. Martz. The Late Triassic (Norian) Adamanian–Revueltian tetrapod faunal transition in the Chinle Formation of Petrified Forest National Park, Arizona.
Abstract - Recent stratigraphic revisions of the Upper Triassic Chinle Formation of Petrified Forest National Park, in conjunction with precise and accurate documentation of fossil tetrapod occurrences, clarified the local biostratigraphy, with regional and global implications. A significant overlap between Adamanian and Revueltian faunas is rejected, as is the validity of the Lamyan sub-land vertebrate faunachron. The Adamanian–Revueltian boundary can be precisely placed within the lower Jim Camp Wash beds of the Sonsela Member and thus does not occur at the hypothesised Tr-4 unconformity. This mid-Norian faunal turnover, may coincide with a floral turnover, based on palynology studies and also on sedimentological evidence of increasing aridity. Available age constraints bracketing the turnover horizon are consistent with the age of the Manicouagan impact event. The rise of dinosaurs in western North America did not correspond to the Adamanian–Revueltian transition, and overall dinosauromorph diversity seems to have remained at a constant level across it. The paucity of detailed Late Triassic vertebrate biostratigraphic data and radioisotopic dates makes it currently impossible to either support or reject the existence of globally synchronous Late Triassic extinctions for tetrapods.
5) Sulej, T., Bronowicz, R., Tałanda, M., and G. Niedźwiedzki. A new dicynodont–archosaur assemblage from the Late Triassic (Carnian) of Poland.
Abstract - This paper reports a new assemblage from the Late Triassic (mid–late Carnian) at Woźniki near Częstochowa (Poland). The Woźniki vertebrate assemblage is similar to that of Lisowice–Lipie Śląskie, a new locality bearing vertebrates from latest Triassic (latest Norian–early Rhaetian) strata of southern Poland, in the presence of dicynodonts, shark spines, plagiosaurs and a cyclotosaur, but conchostracans and bivalves are similar to those from the Krasiejów site (late Carnian). The most complete specimen from Woźniki belongs to a dicynodont, and consists of cranial and postcranial bones of a single individual. It demonstrates that large dicynodonts were part of the Late Triassic vertebrate assemblage in Central Europe. Numerous tetrapod tracks and traces are associated with skeletal fossils at Woźniki.
6) Nesbitt, S. J., Liu, J., and C. Li. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China.
Abstract - The two major lineages of extant archosaurs, crocodylians and avians, diverged in the Triassic, but the details and timing of this event are incompletely understood. Fragmentary and phylogenetically uninformative specimens, in addition to poor temporal constraints on rock units from the Early and Middle Triassic, typify obstacles in identifying early archosaurs. This paper re-describes the partial skeleton of the only known specimen of Xilousuchus sapingensis Wu, 1981 from the Early Triassic Heshanggou Formation in north-central China. Originally assigned to the non-archosaurian archosauriform clade Proterosuchidae, an extensive phylogenetic analysis posits X. sapingensis as a crown-group archosaur within Suchia, thus making this taxon the unequivocally oldest known member of Archosauria. The age and phylogenetic position of X. sapingensis indicate that many archosaurs, including all major clades of non-archosaurian archosauriforms, the avianline, ornithosuchids, aetosaurs and paracrocodylomorph lineages, must have diverged by the end of the Early Triassic. X. sapingensis is part of a possible clade of sail-backed poposauroids that were common components of archosaur assemblages during the Early to Middle Triassic.
7) Benton, M. J., and A. D. Walker. Saltopus, a dinosauriform from the Upper Triassic of Scotland.
Abstract - Saltopus elginensis, reported in 1910 from the yellow sandstones of the Lossiemouth Sandstone Formation (Late Triassic) of Morayshire, NE Scotland, has long been controversial. It was described first as a theropod dinosaur, but others disagreed. Reanalysis of the type, and only, specimen using casts from the natural rock moulds, as well as X-rays and CT scans, has revealed new anatomical data not available to previous researchers. Saltopus was a small, 800–1000 mm-long biped, whose tail made up more than half its length. It is an avemetatarsalian because it has elongated and tightly bunched metatarsals, the tibia is longer than the femur, the calcaneal tuber is rudimentary or absent, and metatarsal II is equal to or longer than metatarsal IV; a unique assemblage of characters diagnosing this clade. Saltopus is a dinosauromorph on the basis of the reduced fingers IV and V, the saddle-shaped dorsal margin of the iliac blade, and the articulation of sacral rib 1 close to the front of the iliac blade. Saltopus is a dinosauriform on the basis of the trochanteric shelf and lesser trochanter on the proximal end of the femur, the waisted sacral ribs, and perhaps the rod-like and straight pubis. However, it lacks all apomorphies of Dinosauria, retaining for example the primitive condition of two sacral vertebrae. Cladistic analyses place Saltopus within Dinosauromorpha and Dinosauriformes, and between the basal dinosauriform Pseudolagosuchus and the derived clade consisting of Silesauridae and Dinosauria, so making it one of a radiation of small pre-dinosaurian bipedal archosaurs in the Triassic found so far in North and South America and in Europe.
8) Langer, M. C., Bittencourt, J. S., and C. L. Schultz. A reassessment of the basal dinosaur Guaibasaurus candelariensis, from the Late Triassic Caturrita Formation of south Brazil.
Abstract - The dinosaur record of the Santa Maria beds of Rio Grande do Sul (Mid–Late Triassic; south Brazil) includes the herrerasaur Staurikosaurus pricei, and two basal members of the sauropodomorph lineage: Saturnalia tupiniquim and Unaysaurus tolentinoi. The most enigmatic of the saurischian taxa is, however, Guaibasaurus candelariensis, previously regarded as either a basal theropod or a basal sauropodomorph. This study provides a detailed anatomical revision of all specimens originally referred to G. candelariensis by Bonaparte and co-authors, including its type-series and a more recently excavated partial postcranium. Although coming from different sites, these specimens share a unique combination of traits, and at least one possible autapomorphic feature of the pelvis, which support the inclusivity and uniqueness of the species. G. candelariensis was a medium-sized (nearly 2 m long) biped with an intriguing mix of plesiomorphic and derived (eusaurischian/theropod) features. Phylogenetic studies reveal weak support for the nesting of G. candelariensis within Theropoda, but this affinity is bolstered by various traits it shares with neotheropods. The Norian age of G. candelariensis corroborates previous studies that suggest the continuous radiation of more basal dinosauromorphs until the end of the Triassic, after the appearance of the three main dinosaur clades.
9) Novas, F. E., Ezcurra, M. D., Chatterjee, S., and T. S. Kutty. New dinosaur species from the Upper Triassic Upper Maleri and Lower Dharmaram formations of Central India.
Abstract - The beginning of dinosaur evolution is currently known based on a handful of highly informative Gondwanan outcrops of Ischigualastian age (late Carnian–early Norian). The richest Triassic dinosaur records of the southern continents are those of South America and South Africa, with taxonomically diverse faunas, whereas faunas from India and central Africa are more poorly known. Here, the known diversity of Gondwanan Triassic dinosaurs is increased with new specimens from central India, which allow a more comprehensive characterisation of these dinosaur assemblages. Five dinosauriform specimens are reported from the probable late Norian–earliest Rhaetian Upper Maleri Formation, including two new sauropodomorph species, the non-plateosaurian Nambalia roychowdhurii and the plateosaurian Jaklapallisaurus asymmetrica, a guaibasaurid and two basal dinosauriforms. The Lower Dharmaram Formation, probably latest Norian–Rhaetian in age, includes basal sauropodomorph and neotheropod remains, providing the second record of a Triassic Gondwanan neotheropod. The currently available evidence suggests that the oldest known Gondwanan dinosaur assemblages (Ischigualastian) were not homogeneous, but more diverse in South America than in India. In addition, the Upper Maleri and Lower Dharmaram dinosaur assemblages resemble purported coeval South American and European beds in the presence of basal sauropodomorphs. Accordingly, the current available evidence of the Triassic beds of the Pranhita–Godavari Basin suggests that dinosaurs increased in diversity and abundance during the late Norian to Rhaetian in this region of Gondwana.
10) Porro, L. B., Butler, R. J., Barrett, P. M., Moore-Fay, S., and R. L. Abel. New heterodontosaurid specimens from the Lower Jurassic of southern Africa and the early ornithischian dinosaur radiation.
Abstract - Heterodontosaurids are poorly understood early ornithischian dinosaurs with extensive geographic and stratigraphic ranges. The group is best known from the Lower Jurassic upper ‘Stormberg Group’ (upper Elliot and Clarens formations) of southern Africa, previously represented by at least three distinct species and ten described specimens. This paper describes four additional heterodontosaurid specimens from southern Africa. A partial skull of a large individual of Heterodontosaurus tucki (NM QR 1788) is approximately 70 longer than that of the type specimen of Heterodontosaurus, and provides new information on allometric changes in mandibular morphology during growth in this taxon. It is the largest known heterodontosaurid cranial specimen, representing an individual approximately 1·75 metres in length, and perhaps 10 kg in body mass. NHMUK R14161 is a partial skull that appears to differ from all other heterodontosaurids on the basis of the proportions of the dentaries, and may represent an unnamed new taxon. Two additional partial skulls (NHMUK RU C68, NHMUK RU69) are referred to cf. Lycorhinus. At least four, and possibly five or more, heterodontosaurid species are present in the upper ‘Stormberg’. This high diversity may have been achieved by dietary niche partitioning, and suggests an adaptive radiation of small-bodied ornithischians following the end Triassic extinctions.
11) Brusatte, S. L., Benton, M. J., Lloyd, G. T., Ruta, M., and S. C. Wang. Macroevolutionary patterns in the evolutionary radiation of archosaurs (Tetrapoda: Diapsida).
Abstract - The rise of archosaurs during the Triassic and Early Jurassic has been treated as a classic example of an evolutionary radiation in the fossil record. This paper reviews published studies and provides new data on archosaur lineage origination, diversity and lineage evolution, morphological disparity, rates of morphological character change, and faunal abundance during the Triassic–Early Jurassic. The fundamental archosaur lineages originated early in the Triassic, in concert with the highest rates of character change. Disparity and diversity peaked later, during the Norian, but the most significant increase in disparity occurred before maximum diversity. Archosaurs were rare components of Early–Middle Triassic faunas, but were more abundant in the Late Triassic and pre-eminent globally by the Early Jurassic. The archosaur radiation was a drawn-out event and major components such as diversity and abundance were discordant from each other. Crurotarsans (crocodile-line archosaurs) were more disparate, diverse, and abundant than avemetatarsalians (bird-line archosaurs, including dinosaurs) during the Late Triassic, but these roles were reversed in the Early Jurassic. There is no strong evidence that dinosaurs outcompeted or gradually eclipsed crurotarsans during the Late Triassic. Instead, crurotarsan diversity decreased precipitously by the end-Triassic extinction, which helped usher in the age of dinosaurian dominance.
12) Barrett, P. M., Butler, R. J., and S. J. Nesbitt. The roles of herbivory and omnivory in early dinosaur evolution.Abstract - Herbivorous and omnivorous dinosaurs were rare during the Carnian stage of the Late Triassic. By contrast, the succeeding Norian stage witnessed the rapid diversification of sauropodomorphs and the rise of the clade to ecological dominance. Ornithischians, by contrast, remained relatively rare components of dinosaur assemblages until much later in the Mesozoic. The causes underlying the differential success of ornithischians and sauropodomorphs remain unclear, but might be related to trophic specialisation. Sauropodomorphs replaced an established herbivore guild consisting of rhynchosaurs, aetosaurs and basal synapsids, but this faunal turnover appears to have been opportunistic and cannot be easily attributed to either competitive interactions or responses to floral change. Consideration of diversity patterns and relative abundance suggests that the ability to eat plants might have been a major factor promoting sauropodomorph success, but that it was less important in the early evolution of Ornithischia. On the basis of current evidence it is difficult to determine the diet of the ancestral dinosaur and scenarios in which omnivory or carnivory represent the basal condition appear equally likely.
13) Irmis, R. B. Evaluating hypotheses for the early diversification of dinosaurs.
Abstract - Many hypotheses have been proposed for the rise of dinosaurs, but their early diversification remains poorly understood. This paper examines the occurrences, species diversity and abundance of early dinosaurs at both regional and global scales to determine patterns of their early evolutionary history. Four main patterns are clear: (1) sauropodomorph dinosaurs became abundant during the late Norian–Rhaetian of Gondwana and Europe; (2) Triassic dinosaurs of North America have low species diversity and abundance until the beginning of the Jurassic; (3) sauropodomorphs and ornithischians are absent in the Triassic of North America; and (4) ornithischian dinosaurs maintain low species diversity, relative abundance and small body size until the Early Jurassic. No one hypothesis fully explains these data. There is no evidence for a Carnian–Norian extinction event, but sauropodomorphs did become abundant during the Norian in some assemblages. No clear connection exists between palaeoenvironment and early dinosaur diversity, but environmental stress at the Triassic–Jurassic boundary is consistent with changes in North American dinosaur assemblages. Elevated growth rates in dinosaurs are consistent with the gradual phyletic increase in body size. This study demonstrates that early dinosaur diversification was a complex process that was geographically diachronous and probably had several causes.
Underwater View Between the North American and Eurasian Plates
Check out these amazing photos between two tectonic plates that started diverging in the Late Triassic. It's probably a good idea to ignore the comments.
http://www.dailymail.co.uk/sciencetech/article-1385589/The-growing-gap-Eurasia-North-American-tectonic-plates.html
http://www.dailymail.co.uk/sciencetech/article-1385589/The-growing-gap-Eurasia-North-American-tectonic-plates.html
1980s Ghost Ranch Coelophysis Quarry Excavations
Palaeoblog has posted photos from the early 1980s Carnegie Museum excavations of the Coelophysis Quarry at Ghost Ranch, New Mexico. These are courtesy of Greg McDonald who served as the quarry chief.
Why We Don't Find Cynodonts in the Chinle Formation
Whiteside, J. H., Grogan, D. S., Olsen, P. E., and D. V. Kent. 2011. Climatically driven biogeographic provinces of Late Triassic tropical Pangea. PNAS Published online before print May 13, 2011,
doi: 10.1073/pnas.1102473108 [supplemental info]
Abstract - Although continents were coalesced into the single landmass Pangea, Late Triassic terrestrial tetrapod assemblages are surprisingly provincial. In eastern North America, we show that assemblages dominated by traversodont cynodonts are restricted to a humid 6° equatorial swath that persisted for over 20 million years characterized by “semiprecessional” (approximately 10,000-y) climatic fluctuations reflected in stable carbon isotopes and sedimentary facies in lacustrine strata. More arid regions from 5–20°N preserve procolophonid-dominated faunal assemblages associated with a much stronger expression of approximately 20,000-y climatic cycles. In the absence of geographic barriers, we hypothesize that these variations in the climatic expression of astronomical forcing produced latitudinal climatic zones that sorted terrestrial vertebrate taxa, perhaps by excretory physiology, into distinct biogeographic provinces tracking latitude, not geographic position, as the proto-North American plate translated northward. Although the early Mesozoic is usually assumed to be characterized by globally distributed land animal communities due to of a lack of geographic barriers, strong provinciality was actually the norm, and nearly global communities were present only after times of massive ecological disruptions.
doi: 10.1073/pnas.1102473108 [supplemental info]
Abstract - Although continents were coalesced into the single landmass Pangea, Late Triassic terrestrial tetrapod assemblages are surprisingly provincial. In eastern North America, we show that assemblages dominated by traversodont cynodonts are restricted to a humid 6° equatorial swath that persisted for over 20 million years characterized by “semiprecessional” (approximately 10,000-y) climatic fluctuations reflected in stable carbon isotopes and sedimentary facies in lacustrine strata. More arid regions from 5–20°N preserve procolophonid-dominated faunal assemblages associated with a much stronger expression of approximately 20,000-y climatic cycles. In the absence of geographic barriers, we hypothesize that these variations in the climatic expression of astronomical forcing produced latitudinal climatic zones that sorted terrestrial vertebrate taxa, perhaps by excretory physiology, into distinct biogeographic provinces tracking latitude, not geographic position, as the proto-North American plate translated northward. Although the early Mesozoic is usually assumed to be characterized by globally distributed land animal communities due to of a lack of geographic barriers, strong provinciality was actually the norm, and nearly global communities were present only after times of massive ecological disruptions.
Placerias Quarry Panorama
Here is a panorama of the area where the Late Triassic Placerias Quarry is located in the Chinle Formation near St. Johns, Arizona. Sharp eyes can pick out a red car at the quarry just left of center of the photo above the juniper trees. Historically this quarry is one of the most productive sites in the Chinle Formation, and has produced numerous skeletons of aetosaurs, phytosaurs, "rauisuchians", crocodylomorphs, dinosauriforms, and of course more than 40 individuals of the dicynodont Placerias hesternus. The limited exposures have made stratigraphic correlation of this site to other Chinle localities problematic; however, it is a beautiful field area.
The South American Aetosaur Aetosauroides is Not Referable to Stagonolepis
In a series of papers Heckert and Lucas (1999, 2000, 2002) and Lucas and Heckert (2001) proposed that the American aetosaur taxon Aetosauroides was a junior synonym of Stagonolepis, and that as a result the Ischigualasto (Argentina) and the Santa Maria (Brazil) formations were Adamanian (latest Carnian) in age based upon vertebrate biostratigraphy and correlable with other Adamanian strata such as the lower part of the Chinle Formation in western North America. However, these synonomies were based on very superficial resemblances and not on apomorphy based comparisons (Parker, 2008).
The new issue of the Journal of Vertebrate Paleontology has a paper by Julia Desojo and Martin Ezcurra redescribing some of the Aetosauroides material and refuting the synonomy with Stagonolepis using a detailed comparison of characters. Especially important are characters of the skull, which clearly demonstrates strong differences between the two taxa.
As a result Stagonolepis is not known from South America and therefore the proposed biostratigraphic correlations between South America, North America, and Europe using aetosaurs is not supported by this study. This is also supported by radiometric dates which show a 10 million year difference between the main fossil horizon in ht eIschigualasto Formation and the base of the main fossil bearing beds in the Chinle Formation (Irmis and Mundil, 2008).
I have suggested elsewhere (Parker, 2008) that Stagonolepis does not occur in North America either and thus aetosaur taxa appear much more endemic than argued by Heckert and Lucas and thus of limited (only regional) biostratigraphic utility.
Biostratigraphy and taxonomy aside, this redescription highlights the plesiomorphic nature of Aetosauroides, especially regarding features of the skull including a gracile, fully-toothed dentary, different from the "slipper-shaped", anteriorly edentulous mandibles of more derived aetosaurs, and the meeting of the premaxilla and nasal to exclude the maxilla from the margin of the external naris. Thus, Aetosauroides is an interesting and clearly plesiomorphic form with an armor ornamentation convergent with more derived forms such as Stagonolepis and Calyptosuchus.
Desojo, J. B., and M. D. Ezcurra. 2011. A reappraisal of the taxonomic status of Aetosauroides (Archosauria, Aetosauria) specimens from the Late Triassic of South America and their proposed synonymy with Stagonolepis. Journal of Vertebrate Paleontology 31:596-609. DOI: 10.1080/02724634.2011.572936
Abstract - The South American record of early Late Triassic aetosaurs is composed of two species: Aetosauroides scagliai and “Aetosauroides subsulcatus.” Previously undescribed materials belonging to “Aetosauroides subsulcatus” allow us to reassess its taxonomy, leading us to consider it a junior synonym of Aetosauroides scagliai. Based on the emended diagnosis of the species provided here, we recognize that specimens assignable to Aetosauroides scagliai are less common than thought previously and several of them are not diagnostic beyond indeterminate non-typothorasicine aetosaurins. Previous assignments of Aetosauroides as a junior synonym of Stagonolepis are not followed because the South American taxon is distinct due to the presence of a maxilla excluded from the external narial margin, tooth crowns with a straight distal margin and without a constriction between the root and crown, a gradually convex ventral margin of the dentary, oval fossae ventral to the neurocentral suture on the lateral sides of the centra, and a ratio between the length and the width between the distal-most tips of the postzygapophyses equal to or lower than 0.75. The evidence provided here bolsters the validity of Aetosauroides and extends the distribution of Aetosauroides scagliai into southern Brazil. Although Stagonolepis was employed as an index taxon for the Adamanian LVF, this genus is currently restricted to Europe and North America. Thus, no overlapping genera or species of aetosaur are shared between South America and other landmasses. Accordingly, the record of aetosaurs is not useful for providing biostratigraphical correlations between Late Triassic South American beds and those in other regions.
REFERENCES
Heckert, A. B., and S. G. Lucas. 1999. A new aetosaur from the Upper Triassic of Texas and the phylogeny of aetosaurs. Journal of Vertebrate Paleontology 19:50–68.
Heckert, A. B., and S. G. Lucas. 2000. Taxonomy, phylogeny, biostratigraphy, biochronology, paleobiogeography, and evolution of the Late Triassic Aetosauria (Archosauria: Crurotarsi). Zentralblatt fur Geologie und Palaontologie, 1998, Teil I, Heft 11–12:1539–1587.
Heckert, A. B., and S. G. Lucas. 2002. South American occurrences of the Adamanian (Late Triassic, Latest Carnian) index taxon Stagonolepis (Archosauria: Aetosauria) and their biochronological significance. Journal of Paleontology 76:852–863.
Irmis, R. B., and R. Mundil. 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. Journal of Vertebrate Paleontology 28:95A.
Lucas, S. G., and A. B. Heckert. 2001. The aetosaur Stagonolepis from the Upper Triassic of Brazil and its biochronological significance. Neues Jahrebuch fur Geologie und Palaontologie, Monatshefte 2001:719–732.
Parker, W. G. 2008. How many valid aetosaur taxa are there? Reviewing the alpha-taxonomy of the Aetosauria (Archosauria: Pseudosuchia) and its implications for Late Triassic global biostratigraphy. Journal of Vertebrate Paleontology 28:125A.
The new issue of the Journal of Vertebrate Paleontology has a paper by Julia Desojo and Martin Ezcurra redescribing some of the Aetosauroides material and refuting the synonomy with Stagonolepis using a detailed comparison of characters. Especially important are characters of the skull, which clearly demonstrates strong differences between the two taxa.
As a result Stagonolepis is not known from South America and therefore the proposed biostratigraphic correlations between South America, North America, and Europe using aetosaurs is not supported by this study. This is also supported by radiometric dates which show a 10 million year difference between the main fossil horizon in ht eIschigualasto Formation and the base of the main fossil bearing beds in the Chinle Formation (Irmis and Mundil, 2008).
I have suggested elsewhere (Parker, 2008) that Stagonolepis does not occur in North America either and thus aetosaur taxa appear much more endemic than argued by Heckert and Lucas and thus of limited (only regional) biostratigraphic utility.
Biostratigraphy and taxonomy aside, this redescription highlights the plesiomorphic nature of Aetosauroides, especially regarding features of the skull including a gracile, fully-toothed dentary, different from the "slipper-shaped", anteriorly edentulous mandibles of more derived aetosaurs, and the meeting of the premaxilla and nasal to exclude the maxilla from the margin of the external naris. Thus, Aetosauroides is an interesting and clearly plesiomorphic form with an armor ornamentation convergent with more derived forms such as Stagonolepis and Calyptosuchus.
Desojo, J. B., and M. D. Ezcurra. 2011. A reappraisal of the taxonomic status of Aetosauroides (Archosauria, Aetosauria) specimens from the Late Triassic of South America and their proposed synonymy with Stagonolepis. Journal of Vertebrate Paleontology 31:596-609. DOI: 10.1080/02724634.2011.572936
Abstract - The South American record of early Late Triassic aetosaurs is composed of two species: Aetosauroides scagliai and “Aetosauroides subsulcatus.” Previously undescribed materials belonging to “Aetosauroides subsulcatus” allow us to reassess its taxonomy, leading us to consider it a junior synonym of Aetosauroides scagliai. Based on the emended diagnosis of the species provided here, we recognize that specimens assignable to Aetosauroides scagliai are less common than thought previously and several of them are not diagnostic beyond indeterminate non-typothorasicine aetosaurins. Previous assignments of Aetosauroides as a junior synonym of Stagonolepis are not followed because the South American taxon is distinct due to the presence of a maxilla excluded from the external narial margin, tooth crowns with a straight distal margin and without a constriction between the root and crown, a gradually convex ventral margin of the dentary, oval fossae ventral to the neurocentral suture on the lateral sides of the centra, and a ratio between the length and the width between the distal-most tips of the postzygapophyses equal to or lower than 0.75. The evidence provided here bolsters the validity of Aetosauroides and extends the distribution of Aetosauroides scagliai into southern Brazil. Although Stagonolepis was employed as an index taxon for the Adamanian LVF, this genus is currently restricted to Europe and North America. Thus, no overlapping genera or species of aetosaur are shared between South America and other landmasses. Accordingly, the record of aetosaurs is not useful for providing biostratigraphical correlations between Late Triassic South American beds and those in other regions.
REFERENCES
Heckert, A. B., and S. G. Lucas. 1999. A new aetosaur from the Upper Triassic of Texas and the phylogeny of aetosaurs. Journal of Vertebrate Paleontology 19:50–68.
Heckert, A. B., and S. G. Lucas. 2000. Taxonomy, phylogeny, biostratigraphy, biochronology, paleobiogeography, and evolution of the Late Triassic Aetosauria (Archosauria: Crurotarsi). Zentralblatt fur Geologie und Palaontologie, 1998, Teil I, Heft 11–12:1539–1587.
Heckert, A. B., and S. G. Lucas. 2002. South American occurrences of the Adamanian (Late Triassic, Latest Carnian) index taxon Stagonolepis (Archosauria: Aetosauria) and their biochronological significance. Journal of Paleontology 76:852–863.
Irmis, R. B., and R. Mundil. 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. Journal of Vertebrate Paleontology 28:95A.
Lucas, S. G., and A. B. Heckert. 2001. The aetosaur Stagonolepis from the Upper Triassic of Brazil and its biochronological significance. Neues Jahrebuch fur Geologie und Palaontologie, Monatshefte 2001:719–732.
Parker, W. G. 2008. How many valid aetosaur taxa are there? Reviewing the alpha-taxonomy of the Aetosauria (Archosauria: Pseudosuchia) and its implications for Late Triassic global biostratigraphy. Journal of Vertebrate Paleontology 28:125A.
Arcusaurus pereirabdalorum, A New Basal Sauropodomorph From The Early Jurassic Of South Africa
Yates, A. M., Bonnan, M. F., and J. Neveling. 2011. A new basal sauropodomorph dinosaur from the Early Jurassic of South Africa. Journal of Vertebrate Paleontology 31:610-625. DOI: 10.1080/02724634.2011.560626
Abstract - A new basal sauropodomorph dinosaur, Arcusaurus pereirabdalorum, sp. nov., is named and described on the basis of a partial, disarticulated but associated skull and dispersed cranial and postcranial elements from at least two individuals. Arcusaurus is part of a distinctive local fauna from the upper Elliot Formation (Lower Jurassic) in the Senekal District, Free State, South Africa. It can be diagnosed by various details of the premaxilla, nasal, and dentary in the skull and the shape of the distal caudal vertebrae. The taxon displays an unusual mix of characteristics. It lacks several synapomorphies of Plateosauria (Plateosaurus + Massospondylus and all descendants of their most recent common ancestor) but does display other derived characteristics that are otherwise known only from less inclusive clades within Plateosauria. In a cladistic analysis a position outside Plateosauria as the sister group of the clade of Efraasia + more-derived sauropodomorphs is supported; however, this position was not found to be a significantly better explanation of the data as a relatively derived position within Plateosauria. If the basal position for Arcusaurus is accepted, then a divergence from other sauropodomorphs in the middle Norian and a ghost lineage up to 35 Ma is implied. No other non-plateosaurian sauropodomorphs are known from the Jurassic, making Arcusaurus a potentially relictual taxon in the Early Jurassic.
Abstract - A new basal sauropodomorph dinosaur, Arcusaurus pereirabdalorum, sp. nov., is named and described on the basis of a partial, disarticulated but associated skull and dispersed cranial and postcranial elements from at least two individuals. Arcusaurus is part of a distinctive local fauna from the upper Elliot Formation (Lower Jurassic) in the Senekal District, Free State, South Africa. It can be diagnosed by various details of the premaxilla, nasal, and dentary in the skull and the shape of the distal caudal vertebrae. The taxon displays an unusual mix of characteristics. It lacks several synapomorphies of Plateosauria (Plateosaurus + Massospondylus and all descendants of their most recent common ancestor) but does display other derived characteristics that are otherwise known only from less inclusive clades within Plateosauria. In a cladistic analysis a position outside Plateosauria as the sister group of the clade of Efraasia + more-derived sauropodomorphs is supported; however, this position was not found to be a significantly better explanation of the data as a relatively derived position within Plateosauria. If the basal position for Arcusaurus is accepted, then a divergence from other sauropodomorphs in the middle Norian and a ghost lineage up to 35 Ma is implied. No other non-plateosaurian sauropodomorphs are known from the Jurassic, making Arcusaurus a potentially relictual taxon in the Early Jurassic.
April Fieldwork - 2011 - Petrified Forest National Park Area
In April, despite the high winds, I was fortunate enough to spend a couple of days with the local landowners exploring parcels in the expansion area of Petrified Forest National Park. These sites are a continuation of the area in the park locally known as the "Devils Playground", named by paleontologist Charles L. Camp in the early 1920s.
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| Incredible clast supported conglomerate in the upper Brown Sandstone (Sonsela Member, Chinle Formation), a rare facies for this unit but one that clearly shows deposition in good sized channels. |
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| Isolated mesa and one of Charles Camp's documented fossil localities. These strata represent belong to the upper part of the Sonsela Member. |
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| In-situ phytosaur snout fragment found by one of the local landowners. |
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| The 'persistent red silcrete' horizon and upper Sonsela strata near the Devil's Playground. |
Dawn of the Dinosaurs Article in Science News
There is a really good popular article on the origins and rise of dinosaurs in the Triassic in the new issue of Science News.
Although uncredited the incredible "Triassic Neighborhood" mural was done by Victor Leshyk and commissioned by the National Park Service for the Rainbow Forest Museum at Petrified Forest National Park (Leshyk does retain copyright for uses such as this). This is a scene of animals from the time when the Blue Mesa Member was being deposited.
Although uncredited the incredible "Triassic Neighborhood" mural was done by Victor Leshyk and commissioned by the National Park Service for the Rainbow Forest Museum at Petrified Forest National Park (Leshyk does retain copyright for uses such as this). This is a scene of animals from the time when the Blue Mesa Member was being deposited.
End-Triassic Extinction Reset Evolution of Marine Apex Predators
Thorne, P. M., Ruta, M., and M. J. Benton. 2011. Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary. PNAS Early Online, DOI: 10.1073/pnas.1018959108. [suppl. info]
Abstract - Ichthyosaurs were important marine predators in the Early Jurassic, and an abundant and diverse component of Mesozoic marine ecosystems. Despite their ecological importance, however, the Early Jurassic species represent a reduced remnant of their former significance in the Triassic. Ichthyosaurs passed through an evolutionary bottleneck at, or close to, the Triassic-Jurassic boundary, which reduced their diversity to as few as three or four lineages. Diversity bounced back to some extent in the aftermath of the end-Triassic mass extinction, but disparity remained at less than one-tenth of pre-extinction levels, and never recovered. The group remained at low diversity and disparity for its final 100 Myr. The end-Triassic mass extinction had a previously unsuspected profound effect in resetting the evolution of apex marine predators of the Mesozoic.
Abstract - Ichthyosaurs were important marine predators in the Early Jurassic, and an abundant and diverse component of Mesozoic marine ecosystems. Despite their ecological importance, however, the Early Jurassic species represent a reduced remnant of their former significance in the Triassic. Ichthyosaurs passed through an evolutionary bottleneck at, or close to, the Triassic-Jurassic boundary, which reduced their diversity to as few as three or four lineages. Diversity bounced back to some extent in the aftermath of the end-Triassic mass extinction, but disparity remained at less than one-tenth of pre-extinction levels, and never recovered. The group remained at low diversity and disparity for its final 100 Myr. The end-Triassic mass extinction had a previously unsuspected profound effect in resetting the evolution of apex marine predators of the Mesozoic.
A New Exhaustive Phylogenetic Analysis of Archosauria
I've been hinting at this for a bit but it is finally out. Congratulations to Sterling Nesbitt on an amazingly detailed and robust study of archosaurian relationships and their early evolution and distributions. This will be the new standard for archosaurian phylogeny and biogeography.
Nesbitt, S. J. 2011. The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades. Bulletin of the American Museum of Natural History 352:1-292. [Free Download]
Abstract - Archosaurs have a nearly 250 million year record that originated shortly after the Permian-Triassic extinction event and is continued today by two extant clades, the crocodylians and the avians. The two extant lineages exemplify two bauplan extremes among a diverse and complex evolutionary history, but little is known about the common ancestor of these lineages. Renewed interest in early archosaurs has led to nearly a doubling of the known taxa in the last 20 years.
This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset of 412 characters. Each terminal taxon is explicitly described and all specimens used in the analysis are clearly stated. Additionally, each character is discussed in detail and nearly all of the character states are illustrated in either a drawing or highlighted on a specimen photograph. A combination of novel characters and comprehensive character sampling has bridged previously published analyses that focus on particular archosauriform subclades.
A well-resolved, robustly supported consensus tree (MPTs = 360) found a monophyletic Archosauria consisting of two major branches, the crocodylian-line and avian-line lineages. The monophyly of clades such as Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria is supported in this analysis. However, phytosaurs are recovered as the closest sister taxon to Archosauria, rather than basal crocodylian-line archosaurs, for the first time. Among taxa classically termed as “rauisuchians,” a monophyletic poposauroid clade was found as the sister taxon to a group of paraphyletic “rauisuchians” and monophyletic crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of “rauisuchians,” and are not more closely related to aetosaurs than to taxa such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and similar forms (“Sphenosuchia”) were found as a paraphyletic grade leading to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is well supported. A monophyletic clade containing Silesaurus and similar forms is well supported as the sister taxon to Dinosauria. Pterosaurs are robustly supported at the base of the avian line.
A time-calibrated phylogeny of Archosauriformes indicates that the origin and initial diversification of Archosauria occurred during the Early Triassic following the Permian-Triassic extinction. Furthermore, all major basal archosaur lineages except Crocodylomorpha were established by the end of the Anisian. Early archosaur evolution is characterized by high rates of homoplasy, long ghost lineages, and high rates of character evolution. These data imply that much of the early history of Archosauria has not been recovered from the fossil record. Not only were archosaurs diverse by the Middle Triassic, but they had nearly a cosmopolitan biogeographic distribution by the end of the Anisian.
Nesbitt, S. J. 2011. The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades. Bulletin of the American Museum of Natural History 352:1-292. [Free Download]
Abstract - Archosaurs have a nearly 250 million year record that originated shortly after the Permian-Triassic extinction event and is continued today by two extant clades, the crocodylians and the avians. The two extant lineages exemplify two bauplan extremes among a diverse and complex evolutionary history, but little is known about the common ancestor of these lineages. Renewed interest in early archosaurs has led to nearly a doubling of the known taxa in the last 20 years.
This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset of 412 characters. Each terminal taxon is explicitly described and all specimens used in the analysis are clearly stated. Additionally, each character is discussed in detail and nearly all of the character states are illustrated in either a drawing or highlighted on a specimen photograph. A combination of novel characters and comprehensive character sampling has bridged previously published analyses that focus on particular archosauriform subclades.
A well-resolved, robustly supported consensus tree (MPTs = 360) found a monophyletic Archosauria consisting of two major branches, the crocodylian-line and avian-line lineages. The monophyly of clades such as Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria is supported in this analysis. However, phytosaurs are recovered as the closest sister taxon to Archosauria, rather than basal crocodylian-line archosaurs, for the first time. Among taxa classically termed as “rauisuchians,” a monophyletic poposauroid clade was found as the sister taxon to a group of paraphyletic “rauisuchians” and monophyletic crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of “rauisuchians,” and are not more closely related to aetosaurs than to taxa such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and similar forms (“Sphenosuchia”) were found as a paraphyletic grade leading to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is well supported. A monophyletic clade containing Silesaurus and similar forms is well supported as the sister taxon to Dinosauria. Pterosaurs are robustly supported at the base of the avian line.
A time-calibrated phylogeny of Archosauriformes indicates that the origin and initial diversification of Archosauria occurred during the Early Triassic following the Permian-Triassic extinction. Furthermore, all major basal archosaur lineages except Crocodylomorpha were established by the end of the Anisian. Early archosaur evolution is characterized by high rates of homoplasy, long ghost lineages, and high rates of character evolution. These data imply that much of the early history of Archosauria has not been recovered from the fossil record. Not only were archosaurs diverse by the Middle Triassic, but they had nearly a cosmopolitan biogeographic distribution by the end of the Anisian.
Postosuchus v.2.0 by Jeff Martz
I think this is one of the most amazing reconstructions he has done and is completely revamped from his previous version. Look at the details! I won't comment about the stance for once.
Jeff, when are the posters and T-shirts coming out?
Jeff, when are the posters and T-shirts coming out?
April Recap
Back on April 1st I noted that this month had the potential to be the best month ever for Triassic vertebrate paleontology. Unfortunately several items did not materialize during the month like I had supposed such as Sterling Nesbitt's new massive archosaur phylogeny and the special volume on early dinosaurs based on the SVP symposium in Bristol awhile back. Hopefully these will be out next month, but I still think it was a pretty good month for Triassic vertebrate paleontology. Here is a recap of what we saw in April:
April 1 - Nine articulated and associated rauisuchian skeletons from Brazil (Decuriasuchus quartacolonia).
April 12 - A new basal theropod from the Chinle Formation (Daemonosaurus chauliodus).
April 18 - Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus.
April 25 - A new nearly complete articulated skeleton of the bipedal pseudosuchian Poposaurus gracilis from the Upper Triassic of Utah.
April 25 - Redescription of the Lower Triassic Therocephalian synapsid Olivierosuchus parringtoni.
April 29 - New postcranial material of the rauisuchian Saurosuchus galilei from the Late Triassic of Argentina.
I also had a record number of hits ever for this month mainly based on the Daemonosaurus post and the following three posts. Thanks to everyone for their interest and there is much more to come.
April 27 - Dinosaurs Are Crurotarsans.
April 8 - The Laws of Fieldwork.
April 1 - Triassic officially loses status as the first period of the Age of Dinosaurs because it just doesn't have many.
April 1 - Nine articulated and associated rauisuchian skeletons from Brazil (Decuriasuchus quartacolonia).
April 12 - A new basal theropod from the Chinle Formation (Daemonosaurus chauliodus).
April 18 - Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus.
April 25 - A new nearly complete articulated skeleton of the bipedal pseudosuchian Poposaurus gracilis from the Upper Triassic of Utah.
April 25 - Redescription of the Lower Triassic Therocephalian synapsid Olivierosuchus parringtoni.
April 29 - New postcranial material of the rauisuchian Saurosuchus galilei from the Late Triassic of Argentina.
I also had a record number of hits ever for this month mainly based on the Daemonosaurus post and the following three posts. Thanks to everyone for their interest and there is much more to come.
April 27 - Dinosaurs Are Crurotarsans.
April 8 - The Laws of Fieldwork.
April 1 - Triassic officially loses status as the first period of the Age of Dinosaurs because it just doesn't have many.
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