Earliest Mammalian Evolutionary Adaptations

Zhou, C.-F., Wu, S., Martin T., and Z.-X. Luo. 2013. A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations. Nature 500 (7461): 163--167 doi:10.1038/nature12429

Abstract - The earliest evolution of mammals and origins of mammalian features can be traced to the mammaliaforms of the Triassic and Jurassic periods that are extinct relatives to living mammals. Here we describe a new fossil from the Middle Jurassic that has a mandibular middle ear, a gradational transition of thoracolumbar vertebrae and primitive ankle features, but highly derived molars with a high crown and multiple roots that are partially fused. The upper molars have longitudinal cusp rows that occlude alternately with those of the lower molars. This specialization for masticating plants indicates that herbivory evolved among mammaliaforms, before the rise of crown mammals. The new species shares the distinctive dental features of the eleutherodontid clade, previously represented only by isolated teeth despite its extensive geographic distribution during the Jurassic. This eleutherodontid was terrestrial and had ambulatory gaits, analogous to extant terrestrial mammals such as armadillos or rock hyrax. Its fur corroborates that mammalian integument had originated well before the common ancestor of living mammals.

Timing of the Earliest Known Feathered Dinosaurs

Edging a bit closer......


Liu, Y.-Q., Kuang, H.-W., Jiang, X.-J., Peng, N., Xu, H.,  and H.-Y. Sun. 2012. Timing of the earliest known feathered dinosaurs and transitional pterosaurs older than the Jehol Biota. Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication) http://dx.doi.org/10.1016/j.palaeo.2012.01.017


Abstract - The Early Cretaceous Jehol Biota in China has produced numerous well preserved fossils of feathered theropods and early birds. Recent discoveries of feathered dinosaurs, as well as transitional pterosaurs and a sexually mature individual of Darwinopterus preserved together with an egg from the Daohugou Biota of an earlier age than the Jehol Biota, in northeastern China, have greatly enriched our knowledge of the transition from dinosaurs to birds and primitive to derived pterosaurs. The age estimate of fossils or host strata, however, has proven to be contentious and varies widely from the Middle Jurassic to the Early Cretaceous. Here, we report a SHRIMP U-Pb zircon date unambiguously associated with the fossil horizons, and thus, for the first time, provide an age calibration for the earliest appearance of feathered dinosaurs and transitional pterosaurs. Date results indicate that the feathered dinosaurs of China were present more than 161 Ma ago, unquestionably older than Archaeopteryx in Germany, and are the earliest known feathered dinosaurs in the world. Furthermore, feathers appeared in ornithischians before 159 Ma rather than late in the Early Cretaceous. The known transitional pterosaurs first emerged before 161 Ma. The Daohugou Biota, containing mammals, primitive pterosaurs, insects and plants, in addition to the feathered dinosaurs, was living in Inner Mongolia, western Liaoning and northern Hebei in northeastern China during the Middle Jurassic.

A New Paper That is Sure to Stir Up Some Debate

I'm not an expert on feathers so I can't offer an opinion but I'm sure lots of others will.

Dzik, J., Sulej, T., and G. Niedwiedzki. 2010. Possible link connecting reptilian scales with avian feathers from the early Late Jurassic of Kazakstan. Historical Biology 22: 394-402.


Abstract - Organic tissue of a recently found second specimen of feather-like Praeornis from the Karabastau Formation of the Great Karatau Range in southern Kazakstan, has a stable carbon isotope composition indicative of its animal affinity. Three-dimensional preservation of its robust carbonised shaft indicates original high contents of sclerotic organic matter, which makes the originally proposed interpretation of Praeornis as a keratinous integumental structure likely. The new specimen is similar to the holotype of Praeornis in the presence of three 'vanes' on a massive shaft not decreasing in width up to near its tip. Unlike it, the vanes are not subdivided into barbs and the pennate structure is expressed only in the distribution of organic-matter-rich rays. Similar continuous blades border the 'barbs' in the holotype, but the organic matter was removed from them by weathering. It is proposed that the three-vaned structure is a remnant of the ancestral location of scales along the dorsum and their original function in sexual display, similar to that proposed for the Late Triassic probable megalancosaurid Longisquama. Perhaps subsequent rotation around the shaft, in the course of evolution from an ancestral status similar to Praeornis towards the present aerodynamic and protective function of feathers, resulted in the tubular appearance of their buds.

The Oldest Fossil Feather from Europe

No it's not Triassic, but I'll bet I got a few hearts racing with that title on this blog!  Anyhow, I couldn't resist, plus I'm not really sure what I think of this.  I'm just seeing a smear, but then again I'm not a fossil feather expert.  What do you think?

Schweigert, G., Tischlinger, H. & Dietl, G. 2009. The oldest fossil feather from Europe. Neues Jahrbuch für Geologie und Paläontologie, Abh., DOI: 10.1127/0077-7749/2009/0061.

Abstract: We describe a fossil feather from Nusplingen, an Upper Jurassic Solnhofen-type Fossil - lagerstätte in SW Germany. It is Late Kimmeridgian in age and thus stratigraphically older than the isolated Archaeopteryx feather from the Lower Tithonian of Solnhofen, Bavaria, described in 1861. The features of the new find are unique, therefore it is impossible to identify the animal from which this feather came, but despite this uncertainty, the specimen may play an important role in our understanding of the evolution of feathers.

from Schweigert et al. 2010

New Juvenile Specimen of Lufengosaurus huenei from the Lower Jurassic of China

Sekiya, T., and Dong, Z. 2010. A New Juvenile Specimen of Lufengosaurus huenei Young, 1941(Dinosauria: Prosauropoda) from the Lower Jurassic Lower Lufeng Formation of Yunnan, Southwest China. Acta Geologica Sinica (English Version) 84:11-21.

Abstract - A skull and a series of associated cervical vertebrae (ZLJ0112)discovered from the Lower Lufeng Formation (Lower Jurassic) are determined as a juvenile specimen of Lufengosaurus huenei Young 1941 based on amended autapomorphies. Differences between ZLJ0112 and the holotype (subadult specimen) are considered as ontogenetic characteristic changes of L. huenei. Since some of these differences are present in other prosauropod dinosaurs (i.e., forms of the maxillary vascular foramen are irregular; the frontal contribution to the dorsal margin of the orbit is substantial; the frontal contribution to the supratemporal fossa is absent; the supratemporal fenestra is visible in lateral view; the supraoccipital inclined at 75 degrees; the parasphenoid rostrum lies level with the occipital condyle; the retroarticular process is short; the axial postzygapophysis project caudally beyond the end of the centrum) they may be common ontogenetic changes in prosauropod dinosaurs.

Cruxicheiros newmanorum, a new Middle Jurassic Theropod from England

Benson, R. B. J., and J. D. Radley. in press. A new large-bodied theropod dinosaur from the Middle Jurassic of Warwickshire, United Kingdom. Acta Palaeontologica Polonica, available online 20 Nov 2009.

Abstract - Previously undocumented postcranial material from the Chipping Norton Limestone Formation (Middle Jurassic: Lower Bathonian) of Cross Hands Quarry, near Little Compton, Warwickshire represents a new large-bodied theropod dinosaur, distinct from the contemporaneous Megalosaurus bucklandii. Cruxicheiros newmanorum gen. et sp. nov. is diagnosed by a single autapomorphy, the presence of a proximomedially inclined ridge within the groove that marks the lateral extent of the posterior flange of the femoral caput (trochanteric fossa). C. newmanorum shows three tetanuran features: widely separated cervical zygapophyses, a swollen ridge on the lateral surface of the iliac blade and an anterior spur of the caudal neural spines. However, due to fragmentary preservation its affinities within Tetanurae remain uncertain: phylogenetic analysis places it as the most basal tetanuran, the most basal megalosauroid (= spinosauroid) or the most basal neotetanuran.

Aardonyx celestae, a New Transistional Sauropodomorph from the Early Jurassic of South Africa

Yates, A. M., Bonnan, M. F., Neveling, J., Chinsamy, A., and M. G. Blackbeard. 2009. A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proceedings of the Royal Society B. doi:10.1098/rspb.2009.1440. Published online

Abstract - Aardonyx celestae gen. et sp. nov. is described from the upper Elliot Formation (Early Jurassic)of South Africa. It can be diagnosed by autapomorphies of the skull, particularly the jaws, cervical column, forearm and pes. It is found to be the sister group of a clade of obligatory quadrupedal sauropodomorphs (Melanorosaurus + Sauropoda) and thus lies at the heart of the basal sauropodomorph–sauropod transition. The narrow jaws of A. celestae retain a pointed symphysis but appear to have lacked fleshy cheeks. Broad, U-shaped jaws were previously thought to have evolved prior to the loss of gape-restricting cheeks. However, the narrow jaws of A. celestae retain a pointed symphysis but appear to have lacked fleshy cheeks, demonstrating unappreciated homoplasy in the evolution of the sauropod bulk-browsing apparatus. The limbs of A. celestae indicate that it retained a habitual bipedal gait although incipient characters associated with the pronation of the manus and the adoption of a quadrupedal gait are evident through geometric morphometric analysis (using thin-plate splines) of the ulna and femur. Cursorial ability appears to have been reduced and the weight bearing axis of the pes shifted to a medial, entaxonic position, falsifying the hypothesis that entaxony evolved in sauropods only after an obligate quadrupedal gait had been adopted.

Tyrannosauroid from the Bathonian (Middle Jurassic) of England

Important new Jurassic paper for those of you not subscribed to the VRTPALEO listserve...

RAUHUT, O.W.M., A.C. MILNER, and S. MOORE-FAY. 2009. Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi(Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society Early View doi: 10.1111/j.1096-3642.2009.00591.x

ABSTRACT
The cranial osteology of the small theropod dinosaur Proceratosaurus from the Bathonian of Minchinhampton, England, is described in detail, based on new preparation and computed tomography (CT) scan images of the type, and only known, specimen. Proceratosaurus is an unusual theropod with markedly enlarged external nares and a cranial crest starting at the premaxillary-nasal junction. The skull is highly pneumatic, with pneumatized nasals, jugals, and maxillae, as well as a highly pneumatic braincase, featuring basisphenoid, anterior tympanic, basipterygoid, and carotid recesses. The dentition is unusual, with small premaxillary teeth and much larger lateral teeth, with a pronounced size difference of the serrations between the mesial and distal carina. The first dentary tooth is somewhat procumbent and flexed anteriorly. Phylogenetic analysis places Proceratosaurus in the Tyrannosauroidea, in a monophyletic clade Proceratosauridae, together with the Oxfordian Chinese taxon Guanlong. The Bathonian age of Proceratosaurus extends the origin of all clades of basal coelurosaurs back into the Middle Jurassic, and provides evidence for an early, Laurasia-wide, dispersal of the Tyrannosauroidea during the late Middle to Late Jurassic.

Just In Case You Missed This One

This came out about three weeks ago in PLoS ONE. Pretty cool study showing how different types of dinosaurs would negotiate a particularily difficult terrain.

Wilson, J. A., Marsicano, C. A., and R. M. H. Smith. 2009. Dynamic Locomotor Capabilities Revealed by Early Dinosaur Trackmakers from Southern Africa. PLoS ONE 4(10): e7331. doi:10.1371/journal.pone.0007331.

ABSTRACT

Background

A new investigation of the sedimentology and ichnology of the Early Jurassic Moyeni tracksite in Lesotho, southern Africa has yielded new insights into the behavior and locomotor dynamics of early dinosaurs.

Methodology/Principal Findings

The tracksite is an ancient point bar preserving a heterogeneous substrate of varied consistency and inclination that includes a ripple-marked riverbed, a bar slope, and a stable algal-matted bar top surface. Several basal ornithischian dinosaurs and a single theropod dinosaur crossed its surface within days or perhaps weeks of one another, but responded to substrate heterogeneity differently. Whereas the theropod trackmaker accommodated sloping and slippery surfaces by gripping the substrate with its pedal claws, the basal ornithischian trackmakers adjusted to the terrain by changing between quadrupedal and bipedal stance, wide and narrow gauge limb support (abduction range = 31°), and plantigrade and digitigrade foot posture.

Conclusions/Significance

The locomotor adjustments coincide with changes in substrate consistency along the trackway and appear to reflect ‘real time’ responses to a complex terrain. It is proposed that these responses foreshadow important locomotor transformations characterizing the later evolution of the two main dinosaur lineages. Ornithischians, which shifted from bipedal to quadrupedal posture at least three times in their evolutionary history, are shown to have been capable of adopting both postures early in their evolutionary history. The substrate-gripping behavior demonstrated by the early theropod, in turn, is consistent with the hypothesized function of pedal claws in bird ancestors.

Limusaurus and Bird Digit Identity

For those of you who may have missed this or are not subscribed to the Dinosaur Mailing List, this is a comment left for my recent post on Limusaurus, the strange new beaked ceratosaur from China. One of the purported significances of this specimen is that it shed light on the debate on dinosaur digit homologies. As you can see below, this new article is a response to that claim.

A. Vargas has left a new comment on your post "Limusaurus inextricabilis, a Bizarre Beaked Cerato...":

Vargas, AO, Wagner GP, and Gauthier, JA. Limusaurus and bird digit identity.

hdl.handle.net/10101/npre.2009.3828.1

Here is our response to the Limusaurus paper. It was recently rejected by nature, not for any technical reason but because it was considered not to be of sufficient interest/importance.
We have uploaded it at the nature precedings citable archive, because we think it is important there is a quick and citable reply that unlike Xu’s proposal, is consistent with the view of the larger community of theropod paleontologists, namely, that tetanuran digits still are I, II, III. We are preparing a longer paper on this topic.

Limusaurus inextricabilis, a Bizarre Beaked Ceratosaur from the Late Jurassic of China

OK...OK....This post is regarding the Jurassic, and the Late Jurassic specifically, but it is almost Triassic right? Sort of? Still, this is just to cool of a discovery not to mention.

Today's issue of Nature contains an article by Dr. James Clark (George Washington University) and Xu Xing (Chinese Academy of Science's Institute of Vertebrate Paleontology and Paleoanthropology in Beijing) and colleagues titled "A Jurassic ceratosaur from China and its significance fortheropod digit reduction and avian digital homologies". The paper describes a new beaked ceratosaur (yes, beaked ceratosaur) from the Jurassic of China. This new specimen also offers key information regarding the interpretation of digit homology between non- avian and avian dinosaurs.

As Tom Holtz noted to me in an earlier e-mail message....this is a "ceratosaur convergent on Effigia (a Triassic pseudosuchian): Truly weird!". I could not have summed it up any better!

Kudos also to my friend and colleague Sterling Nesbitt and to fellow blogger David Hone who are co-authors on this paper.

The following text is from the Reuters News Release . Photos are from here.

Limusaurus inextricabilis (meaning "mire lizard who could not escape") was found in 159 million-year-old deposits located in the Junggar Basin of Xinjiang, northwestern China. The dinosaur earned its name from the way its skeletons were preserved, stacked on top of each other in fossilized mire pits that were the subject of a 2008 National Geographic film, "Dino Death Trap."

A close examination of the fossil shows that its upper and lower jaws were toothless, demonstrating that the dinosaur possessed a fully developed beak. Its lack of teeth, short arms without sharp claws and possession of gizzard stones suggest that it was a plant-eater, though it is related to carnivorous dinosaurs.

The newly discovered dinosaur's hand is unusual and provides surprising new insights into a long-standing controversy over which fingers are present in living birds, which are theropod dinosaur descendants. The hands of theropod dinosaurs suggest that the outer two fingers were lost during the course of evolution and the inner three remained. Conversely, embryos of living birds suggest that birds have lost one finger from the outside and one from the inside of the hand.

Unlike all other theropods, the hand of Limusaurus strongly reduced the first finger and increased the size of the second. Drs. Clark and Xu and their co-authors argue that Limusaurus' hand represents a transitional condition in which the inner finger was lost and the other fingers took on the shape of the fingers next to them. The three fingers of most advanced theropods are the second, third and fourth fingers -- the same ones indicated by bird embryos -- contrary to the traditional interpretation that they were the first, second and third.

Limusaurus is the first ceratosaur known from East Asia and one of the most primitive members of the group. Ceratosaurs are a diverse group of theropods that often bear crests or horns on their heads, and many have unusual, knobbyfingers lacking sharp claws.The fossil beds in China that produced Limusaurus have previously yielded skeletons of a variety of dinosaurs and contemporary animals described by Drs.Clark and Xu and their colleagues. These include the oldest tyrannosaur, Guanlong wucaii; the oldest horned dinosaur, Yinlong downsi; a new stegosaur, Jiangjunosaurus junggarensis; and the running crocodile relative, Junggarsuchus sloani.

REFERENCE

Xing Xu et al, 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature 459:940. doi:10.1038/nature08124

Using Forensic Science to Determine the Taphonomic History of a Sauropod Dinosaur Quarry

Congratulations to Kenneth Bader. Kenneth has worked in the fossil preparation lab at Petrified Forest National Park since last summer. Prior to this he completed his Master of Science Degree at Kansas University. Kenneth's first paper from his thesis was just published in the journal Palaios. It is an interesting paper which looks at the taphonomy of sauropod skeletons from a quarry in the Jurassic Morrison Formation of Wyoming.

Kenneth closely examined the skeletons and documented hundreds of trace fossils of varying morphologies. Some were interpreted as tooth marks from predators and/or scavengers, but more interestingly many of the trace fossils can be attributed to various arthropods. Comparisons were made with arthropod traces and modern bones and from this a taphonomic hypothesis using forensic entomology can be proposed.

Bader et al. hypothesize that based on this evidence these sauropods died during the dry season, were scavenged, and then went through several detailed stages of decomposition, including a "dry stage" where tracemakers tunneled through dried flesh and created pupation chambers adjacent to bones. Larvae hatched and then contributed to the final removal of soft tissue from the bones. Total time from death until complete burial was estimate to be about 1-3 years.

Congrats again to Kenneth on a very interesting study.

BADER, K., HASIOTIS, S., & MARTIN, L. (2009). APPLICATION OF FORENSIC SCIENCE TECHNIQUES TO TRACE FOSSILS ON DINOSAUR BONES FROM A QUARRY IN THE UPPER JURASSIC MORRISON FORMATION, NORTHEASTERN WYOMING PALAIOS, 24 (3), 140-158 DOI: 10.2110/palo.2008.p08-058r

Abstract- Trace fossils on sauropod skeletons from a quarry in fluvial deposits of the Morrison Formation, Wyoming, are used to reconstruct the taphonomic history of the dinosaur bone accumulation. Shallow pits; rosettes; hemispherical pits; thin, curvilinear, branching grooves; and U- to V-shaped linear grooves make up trace fossils found on sauropod skeletons. The traces were interpreted by comparisons to traces on modern bone. Rosettes are circular rings of modified bone and are likely an early stage in the production of shallow pits. They are interpreted as pupation chambers constructed in dried flesh in contact with sauropod bone. Hemispherical pits are circular with a U-shaped cross section and interpreted as dermestid pupation chambers completed in sauropod bone. Thin, curvilinear, branching grooves are semicircular in cross section, form irregular dendritic or looping patterns, and are interpreted as root etchings. U- to V-shaped linear grooves are interpreted as theropod or crocodilian bite marks. Skeletal articulation and condition and distribution of bone modification traces suggest the skeletons accumulated at this site over no more than 3.5 years, with the bulk of the skeletons contributed during the dry season in the final 3–6 months. Carcasses went through all stages of decomposition—including the dry stage, represented by shallow pits, rosettes, and hemispherical pits. Vertebrate scavengers and necrophagous arthropods fed on the carcasses during all decomposition stages prior to burial of the assemblage.

New Early Middle Jurassic Tetanuran Theropod from China

Well this appears to be Jurassic week on Chinleana. Again not a Triassic specimen, but early Middle Jurassic dinosaurs tend to be rare and this one appears to be a fairly plesiomorphic tetanuran. The authors claim that it is more plesiomorphic than Sinraptor, Yangchuanosaurus, and "Szechuanosaurus", although they were not able to support this with a phylogenetic analysis at this time.

Wu, X., Currie, P.J., Dong, X., Pan, S., and T. Wang. 2009. A new theropod dinosaur from the Middle Jurassic of Lufeng, Yunnan, China. Acta Geologica Sinica 83:9-24.

Abstract – A new theropod dinosaur, Shidaisaurus jinae gen. et sp. nov., has been described on the basis of an incomplete skeleton. The specimen was found near the base of the Upper Lufeng Formation (early Middle Jurassic) in Yunnan, China. It is the first theropod dinosaur from the Middle Jurassic of Yunnan. Shidaisaurus jinae is distinguishable from other Jurassic theropods by certain features from the braincase, axis, and pelvic girdle. The absence of any pleurocoels in the axis or in any anterior dorsal vertebrae suggests that the new Lufeng theropod is relatively primitive and more plesiomorphic than most of the Middle to Late Jurassic theropods from China. Most Chinese taxa of Jurassic theropod dinosaurs have not been well described; a further detailed study will be necessary for us to determine their phylogenetic relationships with Shidaisaurus jinae.

New Long Necked Stegosaur Miragaia longicollum

Not Triassic but still really cool. This is a new advance paper in Proceedings of the Royal Society B on a new long necked stegosaur from the Late Jurassic of Portugal. Appropriately named Miragaia longicollum it has at least 17 cerival vertebrate due to "cervicalization" of the dorsal column. Very cool. Even cooler is that it is available for free from the Royal Society website. The photo below is from here.

REFERENCE

Mateus, O., Maidment, S.C.R., and N.A. Christiansen. 2009. A new long-necked ‘sauropod-mimic’ stegosaur and the evolution of the plated dinosaurs. Proceedings of the Royal Society B, Published online on February 25, 2009. DOI 10.1098/rspb.2008.1909.

What exactly is going on with the dinosaurs in the Early Jurassic?

It is considered by some to be, and should have been, a classic example of adaptive radiation. At the end of the Triassic the majority of pseudosuchians go extinct, removing the biggest competitors of the dinosaurs and leaving the door open for an evolutionary explosion of the dinosaurs. Interestingly, however, this is not what happened according to a new study by Steve Brusatte and colleagues (Brusatte et al., 2008b) who found, in the continuation of their research comparing morphospace disparity between ornithidirans and pseudosuchians (Brusatte et al., 2008a), that dinosaur disparity remained relatively unchanged through the Triassic/Jurassic boundary. It would be expected that once the extinction of the pseudosuchians freed up a large amount of morphospace, the dinosaur record (with whom the pseudosuchians occupied a lot of the same niches and had similar body plans) would show a strong response, yet the dinosaurs show only a "slight non-significant increase" (Brusatte et al., 2008b). Thus, these authors argue, "different aspects of dinosaur radiation (diversity, disparity, and abundance) were decoupled, and the overall macroevolutionary pattern of the first 50 Myr of dinosaur evolution is more complex than often considered (Brusatte et al., 2008b).

Adam Yates had discussed this (and his hypothesis) a few weeks ago at Dracovenator and I had provided some follow-up discussion here. Nonetheless, despite the timing of the extinction it is apparent that not to much is going on for the dinosaurs immediately after the TR/J extinction (which by the way took out the non-dinosaurian dinosauromorphs). Sure to the record of coelophysoids and sauropodomorphs you add a few large theropods such as Dilophosaurus and you see the first good records of heterodontosaurids and the earliest thyreophorans (including the first ornithischians and sauropodomorphs in N. America), but you do not see a true explosion of dinosaur diversity until you get to the Late Jurassic. How much of this is a sampling and/or preservation problem is unclear, but simply look at the Weishampel et al. (2004) chapter on dinosaur distribution in the 2nd edition of The Dinosauria and compare the faunal lists for these epochs. You really have to clean up the Late Triassic portion removing many of the Ornithischia references, indeterminate theropods (could be shuvosaurids), and all of the footprint evidence (no ornithischian or sauropodomorph tracks in N. America; the "theropod" tracks worldwide could be made by convergent dinosauriforms, and pseudosuchians), not to mention the really messed up stratigraphy for the Chinle and Dockum which caused some duplicate entries. Now compare the Late Triassic, Early Jurassic, and Middle Jurassic lists to the rest of the chapter. Surprised? I commend Brusatte et al. (2008a, 2008b) for setting the stage and providing a baseline framework for some much needed future research to address this enigma.

REFERENCES

Brusatte, S.L., Benton, M.J., Ruta, M., and G.T. Lloyd. 2008a. Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs. Science 321:1485-1488.

Brusatte, S.L., Benton, M.J., Ruta, M., and G.T. Lloyd. 2008b. The first 50 Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity. Biology Letters, doi:10.1098/rsbl.2008.0441, published online.

Weishampel, D. B., Barrett, P. M., Coria, R. E., Le Loeuff, J., Gomani, E. S., Zhao Z., Xu X., Sahni, A., and C. Noto. 2004. Dinosaur distribution. In: Weishampel, D. B., Dodson, P., and Osmólska, H. eds. The Dinosauria. 2nd edition. Univ. California Press, Berkeley. pp. 517-606.

The TR-J Terrestrial Extinction Actually Early Jurassic?

Adam Yates most recent post over at Dracovenator and a new abstract by Zeigler and Geissman has got me thinking more about faunal transitions between the Late Triassic and Middle Jurassic. As I stated in an earlier post, Chinle Formation faunal composition remains relatively consistent from the oldest to youngest localities and it is not until you get into the uppermost units of the formation and higher that you start to see some changes. Lucas and Tanner (2007) provides a good documentation of faunal change in the western U.S.A. through this interval and demonstrates that the lowermost Dinosaur Canyon Member (Moenave Formation) and the basal portion of the Wingate Sandstone (both units previously argued to be Jurassic in age and in the Glen Canyon Group) are most likely latest Triassic in age. This is based several lines of evidence including magnetostratigraphy, lithostratigraphic correlation, and biostratigraphy (the presence of phytosaur body fossils and pseudosuchian trace fossils). The upper Moenave, upper Wingate, and the Kayenta Formation lack these fossils. In addition, Lucas and Tanner (2007) place the youngest known Chinle Formation fossil assemblage (the Ghost Ranch Coelophysis Quarry) in the Rock Point Member, which they consider to be laterally equivalent to the base of the Wingate and the lower Dinosaur Canyon Member. They also consider this assemblage to be latest Norian in age based on palynology and the presence of the aetosaur Aetosaurus.

Zeigler and Geissman (2008) argue that based on magnetostratigraphy that the Ghost Ranch Coelophysis Quarry is not in the Rock Point and that it may be even younger than previously supposed. As I have noted previously, Zeigler (2008) correlates the site (using magnetostratigraphy) with the lower Moenave and now Zeigler and Geissman (2008) suggest that the uppermost Chinle Formation is at least Rhaetian and may even be Hettangian in age! This would extend the range of phytosaurs and other non-crocodylomorph pseudosuchians into the Early Jurassic. Thus there would be no terrestrial Triassic/Jurassic extinction, at least not in western North America.

Furthermore, Adam Yates recent post suggests that there may have been an end Early Jurassic extinction that spelled the end of coelophysoids and basal sauropodomorphs, followed by the rise of tetanurans and eusauropods in the Middle Jurassic. If Zeigler and Geissman and Yates are correct there would have been two major faunal turnovers in the very short period of time (approx. 30 million years) encompassing the Early Jurassic. In the earliest Jurassic we would see the disappearance of non-crocodylomorph pseudosuchians and the rise of a dinosaur dominated fauna, including the first basal sauropodomorphs in North America (which are not found in the Late Triassic of that continent*). Approximately 24 million years later we get the Early-Middle Jurassic turnover discussed by Yates and an explosion in dinosaurian diversity. Very interesting and the reason why research on the vertebrate fossil record of the lower Glen Canyon Group in becoming very important and needs to be expanded.

*Note: the only purported evidence of Late Triassic sauropodomorphs in North America are the ichnotaxa Tetrasauropus and Pseudotetrasauropus (e.g., Lucas and Tanner, 2007); however, Rainforth (2003) has determined that these taxa probably represent tracks made by pseudosuchians.

REFERENCES

Lucas, S.G., and L.H. Tanner. 2007. Tetrapod biostratigraphy and biochronology of the Triassic–Jurassic transition on the southern Colorado Plateau, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 244:242–256.

Rainforth, E.C. 2003. Revision and re-evaluation of the Early Jurassic dinosaurian ichnogenus Otozoum. Palaeontology 46, 803–838.

Zeigler, K.E., and J.W. Geissman. 2008. Magnetostratigraphy of the Upper Triassic Chinle Group and Implications for the Age and Correlation of Upper Triassic Strata in North America. Geological Society of America Abstracts with programs (online).