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.
Showing posts with label phylogentic analysis. Show all posts
Showing posts with label phylogentic analysis. Show all posts
Eoraptor is a Sauropodomorph, and a New Basal Dinosaur, Eodromaeus murphi, from the Late Triassic of Argentina
The last decade has seen a renaissance in the study of basal dinosaurs and as a result of the resurgence in study we have seen a bunch of new dinosaur taxa coming from the Upper Triassic of North and South America (see excellent syntheses by Langer et al. 2010, and Brusatte et al., 2010). These include the stem-sauropodomorphs Saturnalia, Panphagia, and most recently Chromigosaurus (Langer, 2003; Martinez and Alcober, 2009; Ezcurra, 2010). Other new early dinosaurs include the herrerasaurid Sanjuansaurus, and the basal theropod Tawa (Nesbitt et al. 2010; Alcober and Martinez, 2010). We have also seen updated description and discussion of older taxa such as Chindesaurus and Pisanosaurus as well as the discovery and reinvestigation of numerous dinosauromorphs (e.g., Dzik, 2003; Ferigolo and Langer, 2007; Irmis et al., 2007a, b; Nesbitt et al., 2007, 2009, 2010). Furthermore, a plethora of phylogentic studies are attempting to work out the relationships of all of these taxa (e.g., Langer, 2004; Langer and Benton, 2006; Ezcurra, 2006, 2010; Irmis et al., 2007; Upchurch et al., 2007; Yates, 2007; Martinez and Alcobar, 2009; Nesbitt et al., 2009).
One point of contention in all of these analyses is the phylogenetic position of Eoraptor lunensis from the Upper Triassic Ischigualasto Formation. Originally considered the basal most theropod (e.g., Sereno et al. 1993), recent analysis have supported this hypothesis (Ezcurra, 2006, 2010; Nesbitt et al., 2009) or cast it as a basal saurischian outside of Eusaurischia (e.g., Langer, 2004; Langer and Benton, 2004; Upchurch et al., 2007; Yates, 2007; Martinez and Alcober, 2009).
A new paper out today in Science by Martinez et al. describes a new basal theropod taxon, Eodromaeus murphi, from the Ischigualasto Formation. This new taxon is known by much of the skeleton and represents the new theropod previously mentioned by Martinez et al. (2008). A phylogenetic analysis places Eodromaeus within Theropoda as the sister taxon to Neotheropoda (a position previously held by Tawa, which in this new analysis is a neotheropod) and the sister taxon to the herrerasaurid theropods.
Very striking in this analysis is the recovery of Eoraptor lunensis not only as a Eusaurischian but as a stem-sauropodomorph in a polytomy with Panphagia and Saturnalia (thus it would be a saturnaliine according to Ezcurra 2010; Guaibasaurus and Chromogisaurus were not included in the analysis). This positioning for Eoraptor had previously been alluded to by Martinez and Alcobar (2009), but this is the first time it has been supported by a phylogenetic analysis.
I'm sure this new analysis will cause quite a debate among basal dinosaur workers, especially regarding the ideas of taxon sampling and character inclusion/codings in phylogenetic analyses. The analysis by Martinez et al. excludes lots of incomplete/poorly known taxa as well as taxa that these authors percieve as being too far out phylogentically to have consequence to the question of basal dinosaur relationships. The character matrix is an updated version of Sereno (1999) and does not utilize many characters found in other studies such as Langer and Benton (2006), Ezcurra (2006), and Nesbitt et al. (2009). I don't really have a horse in this race given that I don't have a basal dinosaur analysis that I am personally working on, but as someone outside looking in, it is really difficult to compare across these various analyses given the strong differences in datasets being utilized. Hopefully we will see a consensus at some point.
Nonetheless, if Eoraptor is indeed found to be a basal sauropodomorph, this is of great interest because it gets us a step closer to seeing what the common ancestor of theropods and sauropodomorphs may have looked like.
Some other neat tidbits and conclusions from the paper:
-Like Eoraptor, Eodromaeous possesses a small row of teeth on the palatal ramus of the pterygoid.
-A new radioisotopic date for the top of the Ischigualasto Formation constrains the majority of the formation between 231.4 Ma and 225.9Ma, thus the Ischigualasto Formation spans the Carnian/Norian boundary and does not overlap with dinosaur bearing strata in North America supporting the hypothesis of Irmis and Mundil, 2008, 2010. The previously reported date of ~217 Ma for the middle of the formation by Shipman (2004) and Currie et al (2009) has been considered unreliable because of a lack of stratigraphic control and the inability to reproduce the methodology.
-The Ischigualasto is divided into three biozones based on vertebrate fossil occurrences. One of the boundaries is supposed to represent the Carnian/Norian boundary, although it seems to me that these boundaries are somewhat ambiguous as they all depend on negative evidence.
-Placing the vertebrate occurrences in stratigraphic order suggests to the authors that rhynchosaurs went extinct at the Carnian/Norian boundary. Interestingly they also propose that dinosaurs went extinct locally and did not reappear in the area until deposition of the Los Colorados Formation later in the Norian.
Martinez, R. N., Sereno, P. C., Alcober, O. A., Colombi, C. E., Renne, P. R., Montañez, I. P., and B. S. Currie. 2011. A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea. Science 331:206-210 DOI: 10.1126/science.1198467.
Abstract - Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.
New Scientist article by Jeff Hecht here. Other links at Live Science and the BBC.
One point of contention in all of these analyses is the phylogenetic position of Eoraptor lunensis from the Upper Triassic Ischigualasto Formation. Originally considered the basal most theropod (e.g., Sereno et al. 1993), recent analysis have supported this hypothesis (Ezcurra, 2006, 2010; Nesbitt et al., 2009) or cast it as a basal saurischian outside of Eusaurischia (e.g., Langer, 2004; Langer and Benton, 2004; Upchurch et al., 2007; Yates, 2007; Martinez and Alcober, 2009).
A new paper out today in Science by Martinez et al. describes a new basal theropod taxon, Eodromaeus murphi, from the Ischigualasto Formation. This new taxon is known by much of the skeleton and represents the new theropod previously mentioned by Martinez et al. (2008). A phylogenetic analysis places Eodromaeus within Theropoda as the sister taxon to Neotheropoda (a position previously held by Tawa, which in this new analysis is a neotheropod) and the sister taxon to the herrerasaurid theropods.
Very striking in this analysis is the recovery of Eoraptor lunensis not only as a Eusaurischian but as a stem-sauropodomorph in a polytomy with Panphagia and Saturnalia (thus it would be a saturnaliine according to Ezcurra 2010; Guaibasaurus and Chromogisaurus were not included in the analysis). This positioning for Eoraptor had previously been alluded to by Martinez and Alcobar (2009), but this is the first time it has been supported by a phylogenetic analysis.
I'm sure this new analysis will cause quite a debate among basal dinosaur workers, especially regarding the ideas of taxon sampling and character inclusion/codings in phylogenetic analyses. The analysis by Martinez et al. excludes lots of incomplete/poorly known taxa as well as taxa that these authors percieve as being too far out phylogentically to have consequence to the question of basal dinosaur relationships. The character matrix is an updated version of Sereno (1999) and does not utilize many characters found in other studies such as Langer and Benton (2006), Ezcurra (2006), and Nesbitt et al. (2009). I don't really have a horse in this race given that I don't have a basal dinosaur analysis that I am personally working on, but as someone outside looking in, it is really difficult to compare across these various analyses given the strong differences in datasets being utilized. Hopefully we will see a consensus at some point.
Nonetheless, if Eoraptor is indeed found to be a basal sauropodomorph, this is of great interest because it gets us a step closer to seeing what the common ancestor of theropods and sauropodomorphs may have looked like.
Some other neat tidbits and conclusions from the paper:
-Like Eoraptor, Eodromaeous possesses a small row of teeth on the palatal ramus of the pterygoid.
-A new radioisotopic date for the top of the Ischigualasto Formation constrains the majority of the formation between 231.4 Ma and 225.9Ma, thus the Ischigualasto Formation spans the Carnian/Norian boundary and does not overlap with dinosaur bearing strata in North America supporting the hypothesis of Irmis and Mundil, 2008, 2010. The previously reported date of ~217 Ma for the middle of the formation by Shipman (2004) and Currie et al (2009) has been considered unreliable because of a lack of stratigraphic control and the inability to reproduce the methodology.
-The Ischigualasto is divided into three biozones based on vertebrate fossil occurrences. One of the boundaries is supposed to represent the Carnian/Norian boundary, although it seems to me that these boundaries are somewhat ambiguous as they all depend on negative evidence.
-Placing the vertebrate occurrences in stratigraphic order suggests to the authors that rhynchosaurs went extinct at the Carnian/Norian boundary. Interestingly they also propose that dinosaurs went extinct locally and did not reappear in the area until deposition of the Los Colorados Formation later in the Norian.
Martinez, R. N., Sereno, P. C., Alcober, O. A., Colombi, C. E., Renne, P. R., Montañez, I. P., and B. S. Currie. 2011. A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea. Science 331:206-210 DOI: 10.1126/science.1198467.
Abstract - Upper Triassic rocks in northwestern Argentina preserve the most complete record of dinosaurs before their rise to dominance in the Early Jurassic. Here, we describe a previously unidentified basal theropod, reassess its contemporary Eoraptor as a basal sauropodomorph, divide the faunal record of the Ischigualasto Formation with biozones, and bracket the formation with 40Ar/39Ar ages. Some 230 million years ago in the Late Triassic (mid Carnian), the earliest dinosaurs were the dominant terrestrial carnivores and small herbivores in southwestern Pangaea. The extinction of nondinosaurian herbivores is sequential and is not linked to an increase in dinosaurian diversity, which weakens the predominant scenario for dinosaurian ascendancy as opportunistic replacement.
New Scientist article by Jeff Hecht here. Other links at Live Science and the BBC.
The First Detailed 3D visualizations of the Braincase and Vestibular System in a Permian Diapsid Reptile
Not Triassic but sill extremely significant for work on Triassic archosauromorphs as Youngina is often used as an outgroup for phylogenetic studies of this clade. Great new information, extremely cool, and of course open access!
Gardner, N. M., Holliday, C. M., anf F. R. O'Keefe. 2010. The braincase of Youngina capensis (Reptilia: Diapsida): new insights from high-resolution CT scanning of the holotype. Palaeontologia Electronica 13.3.19A.
Abstract - Detailed descriptions of braincase anatomy in early diapsid reptiles have been historically rare given the difficulty of accessing this deep portion of the skull, because of poor preservation of the fossils or the inability to remove the surrounding skull roof. Previous descriptions of the braincase of Youngina capensis, a derived stem-diapsid reptile from the Late Permian (250 MYA) of South Africa, have relied on only partially preserved fossils. High resolution X-ray computed tomography (HRXCT) scanning, a new advance in biomedical sciences, has allowed us to examine the reasonably complete braincase of the holotype specimen of Youngina capensis for the first time by digitally peering through the sandstone matrix that filled the skull postmortem. We present the first detailed 3D visualizations of the braincase and the vestibular system in a Permian diapsid reptile. This new anatomical description is of great comparative and phylogenetic relevance to the study of the structure, function and evolution of the reptilian head.
Gardner, N. M., Holliday, C. M., anf F. R. O'Keefe. 2010. The braincase of Youngina capensis (Reptilia: Diapsida): new insights from high-resolution CT scanning of the holotype. Palaeontologia Electronica 13.3.19A.
Abstract - Detailed descriptions of braincase anatomy in early diapsid reptiles have been historically rare given the difficulty of accessing this deep portion of the skull, because of poor preservation of the fossils or the inability to remove the surrounding skull roof. Previous descriptions of the braincase of Youngina capensis, a derived stem-diapsid reptile from the Late Permian (250 MYA) of South Africa, have relied on only partially preserved fossils. High resolution X-ray computed tomography (HRXCT) scanning, a new advance in biomedical sciences, has allowed us to examine the reasonably complete braincase of the holotype specimen of Youngina capensis for the first time by digitally peering through the sandstone matrix that filled the skull postmortem. We present the first detailed 3D visualizations of the braincase and the vestibular system in a Permian diapsid reptile. This new anatomical description is of great comparative and phylogenetic relevance to the study of the structure, function and evolution of the reptilian head.
Morphological Diversity and Biogeography of Procolophonids
Cisneros, J. C., and M. Ruta. 2010. Morphological diversity and biogeography of procolophonids (Amniota: Parareptilia). Journal of Systematic Paleontology 8:607-625. DOI: 10.1080/14772019.2010.491986.
Abstract - A recent phylogenetic analysis of procolophonid parareptiles is used as the basis for a study of morphological diversity (disparity) in these amniotes. Disparity values are compared in three groups of procolophonids (a paraphyletic series of basal taxa and two monophyletic sister groups: procolophonines and leptopleuronines), two ecophenotypic assemblages (one based upon inferred diet - non high-fibre versus high-fibre species; the other based upon cranial sculpture - non horned versus horned species), and two temporal assemblages (Lower Triassic versus Middle and Upper Triassic). The mean disparity values are comparable in the case of temporal and ecophenotypic assemblages. High-fibre species are marginally less disparate than non high-fibre species. The combined Middle and Upper Triassic species are slightly less disparate than Lower Triassic species. Finally, horned species are only slightly more disparate than non-horned species. The paraphyletic series of basal taxa and the leptopleuronines show similar disparity values, marginally higher than those for procolophonines. Phylogenetic analysis is also used to reconstruct the biogeographical history of procolophonids. Both ancestral area analysis and dispersal-vicariance analysis show that South Africa was the most likely ancestral area for procolophonids as a whole. North China - either as a single area or in combination with Russia or South Africa - was the most likely ancestral area for the leptopleuronine-procolophonine clade.
Abstract - A recent phylogenetic analysis of procolophonid parareptiles is used as the basis for a study of morphological diversity (disparity) in these amniotes. Disparity values are compared in three groups of procolophonids (a paraphyletic series of basal taxa and two monophyletic sister groups: procolophonines and leptopleuronines), two ecophenotypic assemblages (one based upon inferred diet - non high-fibre versus high-fibre species; the other based upon cranial sculpture - non horned versus horned species), and two temporal assemblages (Lower Triassic versus Middle and Upper Triassic). The mean disparity values are comparable in the case of temporal and ecophenotypic assemblages. High-fibre species are marginally less disparate than non high-fibre species. The combined Middle and Upper Triassic species are slightly less disparate than Lower Triassic species. Finally, horned species are only slightly more disparate than non-horned species. The paraphyletic series of basal taxa and the leptopleuronines show similar disparity values, marginally higher than those for procolophonines. Phylogenetic analysis is also used to reconstruct the biogeographical history of procolophonids. Both ancestral area analysis and dispersal-vicariance analysis show that South Africa was the most likely ancestral area for procolophonids as a whole. North China - either as a single area or in combination with Russia or South Africa - was the most likely ancestral area for the leptopleuronine-procolophonine clade.
The Phylogenetic Relationships of Eucynodontia
Liu, J., and P. Olsen. 2010. The Phylogenetic Relationships of Eucynodontia (Amniota: Synapsida). Journal of Mammalian Evolution. Published online April 13 2010. doi: 10.1007/s10914-010-9136-8
Abstract - The phylogeny of Eucynodontia is an important topic in vertebrate paleontology and is the foundation for understanding the origin of mammals. However, consensus on the phylogeny of Eucynodontia remains elusive. To clarify their interrelationships, a cladistic analysis, based on 145 characters and 31 species, and intergrating most prior works, was performed. The monophyly of Eucynodontia is confirmed, although the results slightly differ from those of previous analyses with respect to the composition of both Cynognathia and Probainognathia. This is also the first numerical cladistic analysis to recover a monophyletic Traversodontidae. Brasilodon is the plesiomorphic sister taxon of Mammalia, although it is younger than the oldest mammals and is specialized in some characters. A monophyletic Prozostrodontia, including tritheledontids, tritylodontids, and mammals, is well supported by many characters. Pruning highly incomplete taxa generally has little effect on the inferred pattern of relationships among the more complete taxa, although exceptions sometimes occur when basal fragmentary taxa are removed. Taxon sampling of the current data matrix shows that taxon sampling was poor in some previous studies, implying that their results are not reliable. Two major unresolved questions in cynodont phylogenetics are whether tritylodontids are more closely related to mammals or to traversodontids, and whether tritylodontids or tritheledontids are closer to mammals. Analyses of possible synapomorphies support a relatively close relationship between mammals and tritylodontids, to the exclusion of traversodontids, but do not clearly indicate whether or not tritheledontids are closer to mammals than are tritylodontids.
Abstract - The phylogeny of Eucynodontia is an important topic in vertebrate paleontology and is the foundation for understanding the origin of mammals. However, consensus on the phylogeny of Eucynodontia remains elusive. To clarify their interrelationships, a cladistic analysis, based on 145 characters and 31 species, and intergrating most prior works, was performed. The monophyly of Eucynodontia is confirmed, although the results slightly differ from those of previous analyses with respect to the composition of both Cynognathia and Probainognathia. This is also the first numerical cladistic analysis to recover a monophyletic Traversodontidae. Brasilodon is the plesiomorphic sister taxon of Mammalia, although it is younger than the oldest mammals and is specialized in some characters. A monophyletic Prozostrodontia, including tritheledontids, tritylodontids, and mammals, is well supported by many characters. Pruning highly incomplete taxa generally has little effect on the inferred pattern of relationships among the more complete taxa, although exceptions sometimes occur when basal fragmentary taxa are removed. Taxon sampling of the current data matrix shows that taxon sampling was poor in some previous studies, implying that their results are not reliable. Two major unresolved questions in cynodont phylogenetics are whether tritylodontids are more closely related to mammals or to traversodontids, and whether tritylodontids or tritheledontids are closer to mammals. Analyses of possible synapomorphies support a relatively close relationship between mammals and tritylodontids, to the exclusion of traversodontids, but do not clearly indicate whether or not tritheledontids are closer to mammals than are tritylodontids.
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