Further Investigating the Biogeographic Origins of Dinosauria

New paper out in Palaeontology.

Marsola, J. C., Ferreira, G. S., Langer, M. C., Button, D. J., and R. J. Butler. 2018. Increases in sampling support the southern Gondwanan hypothesis for the origins of dinosaurs. Palaeontology. Early Online. https://doi.org/10.1111/pala.12411

Abstract - Dinosaurs were ubiquitous in terrestrial ecosystems through most of the Mesozoic and are still diversely represented in the modern fauna in the form of birds. Recent efforts to better understand the origins of the group have resulted in the discovery of many new species of early dinosaur and their closest relatives (dinosauromorphs). In addition, recent re‐examinations of early dinosaur phylogeny have highlighted uncertainties regarding the interrelationships of the main dinosaur lineages (Sauropodomorpha, Theropoda and Ornithischia), and questioned the traditional hypothesis that the group originated in South Gondwana and gradually dispersed over Pangaea. Here, we use an historical approach to examine the impact of new fossil discoveries and changing phylogenetic hypotheses on biogeographical scenarios for dinosaur origins over 20 years of research time, and analyse the results in the light of different fossil record sampling regimes. Our results consistently optimize South Gondwana as the ancestral area for Dinosauria, as well as for more inclusive clades including Dinosauromorpha, and show that this hypothesis is robust to increased taxonomic and geographic sampling and divergent phylogenetic results. Our results do not find any support for the recently proposed Laurasian origin of dinosaurs and suggest that a southern Gondwanan origin is by far the most plausible given our current knowledge of the diversity of early dinosaurs and non‐dinosaurian dinosauromorphs.

An interesting new study out today that uses phylogeny based biogeographical analyses to test the hypothesis of Baron et al. (2017a,b) that stem-dinosaurs originated in Laurasia. Their results reject that hypothesis in favor of the long standing hypothesis of a Gondwanan origin. I couldn't access the supplemental data in Dryad because the article isn't officially out. I'm not suggesting that the conclusions are wrong, but do have a few questions/comments about the data and how specimen sampling issues from Western North America could affect a rerun of the analysis. 

1) Some of the separation of taxa into biogeographical bins is confusing. For example most of the Chinle Formation taxa Chindesaurus, Tawa,  and Eucoelophysis are shown in green depicting the 'Equatorial Belt' as the ancestral zone, yet taxa from the same formation and localities therein such as Dromomeron gregorii and D. romeri are shown in yellow from the 'Euroamerica' zone. Why are they separated?

2) I am not aware of any Rhaetian occurrences of Eucoelophysis baldwini. This taxon occurs in a couple of quarries from around Ghost Ranch, New Mexico and one of them, the Hayden Quarry, is solidly dated in the middle-late Norian at about 212 Ma (Irmis et al., 2011). There is a purported occurrence (Rinehart et al., 2009) of Eucoelophysis from the Coelophysis Quarry at Ghost Ranch that is most likely Rhaetian in age; however, this occurrence is based on the rejected hypothesis that Eucoelophysis remains a neotheropod dinosaur with the holotype representing a highly weathered individual (Rinehart et al., 2009). This referred specimen is simply another specimen of Coelophysis. Thus all presently known occurrences of Eucoelophysis are Norian in age.

3) Recent fieldwork in the Chinle Formation, especially at Petrified Forest National Park, has recovered a significant amount of early dinosaur material. This includes the earliest known dated neotheropod specimen and early occurrences of dinosauriforms. This material is presently under study. Combined with already published accounts of silesaurids and coelophysids (e.g., Padian, 1986; Parker et al., 2006), these show a robust record of dinosauromorphs in the early-middle Norian of Arizona. Thus a specimen based study using autapomorphy-based identifications would pull Equatorial Laurasian silesaurids and neotheropods down into the early-middle Norian. Also important is a recently mentioned occurrence of Dromomeron gregorii from the Otischalkian Boren Quarry in the Dockum Group of Texas which is older than any of the Chinle Formation occurrences and pulls these occurrences down even further (Lessner et al., 2018).

4) Many of the specimens mentioned above are of interest because they originate from some of the lowest fossil bearing beds in the Chinle Formation, the Blue Mesa Member. To date no diagnostic vertebrate fossils from the Chinle Formation are known from below the middle of the Blue Mesa Member, thus the vertebrate faunas of the lower Chinle (lower Blue Mesa, Mesa Redondo, Shinarump) are unknown. This is significant because these units represent the earliest Norian based on the 'long-Norian' hypothesis (227-208 Ma). Coupled with the Moenkopi/Chinle unconformity much of the well-sampled Triassic of the western U.S. is apparently lacking the Carnian and earliest Norian. This is a bias that should not be ignored. Possible Carnian rocks elsewhere such as the base of the Dockum Group in Texas and units in Wyoming need to be better sampled.  

It's difficult to say how these details would affect these early dinosaur biogeographical studies, but there are data out there that should be included in future analyses.

REFERENCES

Baron, M. G., Norman, D. B. and Barrett, P. M. 2017a. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543, 501–506.

Baron, M. G., Norman, D. B. and Barrett, P. M. 2017b. Baron et al. reply. Nature, 551, E4–E5.

Irmis, R. B., Mundil, R., Martz, J. W., and W. G. Parker. 2011. High resolution U-Pb ages from the Upper Triassic Chinle Formation (New Mexico, U.S.A.) support a diachronous rise of dinosaurs. Earth and Planetary Science Letters 309:258-267.

Lessner, E. J., Parker, W. G., Marsh, A. D., Nesbitt, S. J., Irmis, R. B. and B. Mueller. 2018. New insights into Late Triassic dinosauromorph-bearing assemblages from Texas using apomorphy-based identifications. PaleoBios, 35.ucmp_paleobios_39960.

Padian, K.. 1986. On the type material of Coelophysis Cope (Saurischia: Theropoda), and a new specimen from the Petrified Forest of Arizona (Late Triassic: Chinle Formation), p. 45-60. In K. Padian (ed.), The beginning of the Age of Dinosaurs: Faunal change across the Triassic-Jurassic boundary. Cambridge University Press, Cambridge.

Parker, W. G., Irmis, R. B., and S. J. Nesbitt. 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:160-161.

Rinehart, L. F., Lucas, S. G., Heckert, A. B., Spielmann, J. A., and M. D. Celeskey. 2009. The paleobiology of Coelophysis bauri (Cope) from the Upper Triassic (Apachean) Whitaker Quarry, New Mexico, with detailed analysis of a single quarry block. New Mexico Museum of Natural History and Science Bulletin 45.

New Open Access Paper Discussing the Rise of Dinosaurs

Benton, M.J., Forth, J., and M.C. Langer. 2014. Models for the rise of dinosaurs. Modern Biology 24:R87-R95. doi:10.1016/j.cub.2013.11.063

Abstract - Dinosaurs arose in the early Triassic in the aftermath of the greatest mass extinction ever and became hugely successful in the Mesozoic. Their initial diversification is a classic example of a large-scale macroevolutionary change. Diversifications at such deep-time scales can now be dissected, modelled and tested. New fossils suggest that dinosaurs originated early in the Middle Triassic, during the recovery of life from the devastating Permo-Triassic mass extinction. Improvements in stratigraphic dating and a new suite of morphometric and comparative evolutionary numerical methods now allow a forensic dissection of one of the greatest turnovers in the history of life. Such studies mark a move from the narrative to the analytical in macroevolutionary research, and they allow us to begin to answer the proposal of George Gaylord Simpson, to explore adaptive radiations using numerical methods.

The Problem of Dinosaur Origins: Integrating Three Approaches to the Rise of Dinosauria

An excellent essay looking on depth at the question of dinosaur origins. I think the ideas put forth are worthy of dicussion and will be heavily debated, especially since it is written from a North American perspective.

Padian, K., 2013. The problem of dinosaur origins: integrating three approaches to the
rise of Dinosauria. Earth and Environmental Science Transactions of the Royal Society of
Edinburgh First View Article. DOI: http://dx.doi.org/10.1017/S1755691013000431

Abstract - The problem of the origin of dinosaurs has historically had three dimensions. The first is the question of whether Dinosauria is monophyletic, and of its relationships to other archosaurs. This
question was plagued from the beginning by a lack of relevant fossils, an historical burden of confusing taxonomic terms and a rudimentary approach to devising phylogenies. The second dimension concerns the functional and ecological adaptations that differentiated dinosaurs from other archosaurs, a question also marred by lack of phylogenetic clarity and testable biomechanical hypotheses. The third dimension comprises the stratigraphic timing of the origin of dinosaurian groups with respect to each other and to related groups, the question of its synchronicity among various geographic regions, and some of the associated paleoenvironmental circumstances. None of these dimensions alone answers the question of dinosaur origins, and they sometimes provide conflicting implications. Since Dinosauria was named, one or another set of questions has historically dominated academic discussion and research. Paradigms have shifted substantially in recent decades, and current evidence suggests that we are due for more such shifts. I suggest two changes in thinking about the beginning of the “Age of Dinosaurs”: first, the event that we call the (phylogenetic) origin of dinosaurs was trivial compared to the origin of Ornithodira; and second, the “Age of Dinosaurs” proper did not begin until the Jurassic. Re-framing our thinking on these issues will improve our understanding of clade dynamics, timing of macroevolutionary events, and the effects of Triassic climate change on terrestrial vertebrates.

Finally a Dinosaur Textbook with a Section on Dinosaur Origins! Dinosaurs: A Concise Natural History

In 2010 when I reviewed the first edition of Fastovsky and Weishampel "Dinosaurs: a Concise Natural History" for the Journal of Vertebrate Paleontology I gave it a good review, recommending it for classroom use.  However, one complaint I had was that as a book on dinosaurs it had almost no coverage of dinosaur origins.  David Fastovsky took note and assured me that the next edition would remedy this. 

Well I'm happy to say that the second edition is out and does indeed have a much expanded section on dinosaur origins. Thanks to David and David for not taking my constructive criticism too hard and adding this section. In my opinion this makes what I think is the best dinosaur textbook available even better. 

Rediscovered Specimen Draws Dinosaur Origins Down Into the Middle Triassic

I've been peripherally involved in the recent renaissance regarding dinosaur origins since my discovery of the skeleton of Revueltosaurus callenderi in 2004 and the subsequent recognition that it was not a dinosaur. With my good friends and colleagues Sterling Nesbitt and Randall Irmis, I proceeded to reexamine much of the Late Triassic dinosaur record using an apomorphy-based approach emphasized by UT Austin professor Chris Bell and his colleagues for Neogene vertebrates. Our findings were that Triassic dinosaurs were actually rarer than previously believed, especially in North America. In addition, extensive fieldwork by Sterling and Randy led to the discoveries of Dromomeron romeri, Tawa hallae, and Asilisaurus kongwe, filling in some key gaps in our understanding of the early diversification of the Ornithodira.  Nonetheless it has become readily apparent that many important specimens crucial to this issue were not weathering out of outcrops in the field, but rather were sitting un- or misidentified in museum collections around the globe. Our new understanding of character states and polarities for early diverging dinosauriforms provided us with specific search criteria leading to the discoveries and/or reinterpretations of taxa such as Eucoelophysis baldwini, Dromomeron gregorii, Technosaurus smalli, Daemonosaurus chauliodus, and of course Effigia okeeffae, all from previously collected material including fossils collected for Edward Cope in the 1800s. Furthermore, Randy's work found that the rise of dinosaurs was diachronous, although the timing is still poorly understood.

The discovery of the silesaurid Asilisaurus kongwe (published in 2010) pulled the split between Silesauridae and Dinosauria into the Middle Triassic creating a significant ghost lineage for Dinosauria as the earliest known bona fide dinosaurs do not appear until the end of the Carnian stage of the Late Triassic. Amazingly it appears that we did not need to wait very long for this ghost lineage to be filled.

Today's issue of Biology Letters has a paper by Sterling Nesbitt, Paul Barrett, Sarah Werning, Christian Sidor, and the late Alan Charig on a probable new dinosaur from the Middle Triassic of Tanzania. Even more amazing is that this is not a new specimen, but was actually collected in the 1930s, and never described, except in Charig's 1950s dissertation, until today. Charig named the new specimen Nyasasaurus parringtoni and until now this name has been a nomen nudum.

Known from a partial humerus and several vertebrate (three cervical, five presacral and three sacral), reanalysis places Nyasasaurus as either a dinosaur or as the sister taxon to Dinosauria. The humerus bears a ventrally elongated deltopectoral crest with a deflected apex, both synapomorphies of Dinosauria. The cervical vertebrae are elongate and possess deep lateral fossae, consistent with character states found in dinosaurs. The presence of three sacral vertebrae, although not restricted to Dinosauria, also supports this placement. This interpretation is supported not only by a phylogenetic analysis but also by bone histology, which shows high, continuous growth rates similar to that of early diverging dinosaurs. 

This material suggests that dinosaurs probably appeared in the fossil record 10 to 15 million years earlier than we expected. Furthermore, Nesbitt et al. argue that this strongly supports the hypothesis that dinosaurs were not a dominant group during their early history. Finally, Nyasasaurus also provides more support for a Gondwanan origin of dinosaurs.

One thing that is now definitely clear is that workers interested in dinosaur origins will need to spend more time in Middle Triassic terrestrial units. Back in 2004 I don't think any of us fathomed what discoveries and interpretations the next decade would bring. As I often state, it is not necessarily what we already know that dirves our work, but what is still out there for us to learn.

Nesbitt, S. J., Barrett, P. M., Werning, S., Sidor, C. A., and A. J. Charig. 2012. The oldest dinosaur? A Middle Triassic dinosauriform from Tanzania. Biology Letters.
 

Abstract - The rise of dinosaurs was a major event in vertebrate history, but the timing of the origin and early diversification of the group remain poorly constrained. Here, we describe Nyasasaurus parringtoni gen. et sp. nov., which is identified as either the earliest known member of, or the sister-taxon to, Dinosauria. Nyasasaurus possesses a unique combination of dinosaur character states and an elevated growth rate similar to that of definitive early dinosaurs. It demonstrates that the initial dinosaur radiation occurred over a longer timescale than previously thought (possibly 15 My earlier), and that dinosaurs and their immediate relatives are better understood as part of a larger Middle Triassic archosauriform radiation. The African provenance of Nyasasaurus supports a southern Pangaean origin for Dinosauria.

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.

Dispersal as a Key Factor in Early Dinosaur Diversification of North America and the Myth of a 'Cosmopolitan' Dinosaur Fauna in the Early Mesozoic

New and available free from the Royal Society website. Popular news story here.

Rowe, T. B., Sues, H.-D., and R. Reisz. 2010. Dispersal and diversity in the earliest North American sauropodomorph dinosaurs, with a description of a new taxon. Proceedings of the Royal Society B, First Cite. doi:10.1098/rspb.2010.1867 

Abstract - Sauropodomorph dinosaurs originated in the Southern Hemisphere in the Middle or Late Triassic and are commonly portrayed as spreading rapidly to all corners of Pangaea as part of a uniform Late Triassic to Early Jurassic cosmopolitan dinosaur fauna. Under this model, dispersal allegedly inhibited dinosaurian diversification, while vicariance and local extinction enhanced it. However, apomorphy-based analyses of the known fossil record indicate that sauropodomorphs were absent in North America until the Early Jurassic, reframing the temporal context of their arrival. We describe a new taxon from the Kayenta Formation of Arizona that comprises the third diagnosable sauropodomorph from the Early Jurassic of North America. We analysed its relationships to test whether sauropodomorphs reached North America in a single sweepstakes event or in separate dispersals. Our finding of separate arrivals by all three taxa suggests dispersal as a chief factor in dinosaurian diversification during at least the early Mesozoic. It questions whether a ‘cosmopolitan’ dinosaur fauna ever existed, and corroborates that vicariance, extinction and dispersal did not operate uniformly in time or under uniform conditions during the Mesozoic. Their relative importance is best measured in narrow time slices and circumscribed geographical regions.

Evidence for an Early Triassic Origin for the Dinosaur Stem Lineage

Brusatte, S. L., Niedźwiedzki, G., and R. J. Butler. 2010. Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic. Proceedings of the Royal Society B. doi: 10.1098/rspb.2010.1746

Abstract - The ascent of dinosaurs in the Triassic is an exemplary evolutionary radiation, but the earliest phase of dinosaur history remains poorly understood. Body fossils of close dinosaur relatives are rare, but indicate that the dinosaur stem lineage (Dinosauromorpha) originated by the latest Anisian (ca 242–244 Ma). Here, we report footprints from the Early–Middle Triassic of Poland, stratigraphically well constrained and identified using a conservative synapomorphy-based approach, which shifts the origin of the dinosaur stem lineage back to the Early Olenekian (ca 249–251 Ma), approximately 5–9 Myr earlier than indicated by body fossils, earlier than demonstrated by previous footprint records, and just a few million years after the Permian/Triassic mass extinction (252.3 Ma). Dinosauromorph tracks are rare in all Polish assemblages, suggesting that these animals were minor faunal components. The oldest tracks are quadrupedal, a morphology uncommon among the earliest dinosauromorph body fossils, but bipedality and moderately large body size had arisen by the Early Anisian (ca 246 Ma). Integrating trace fossils and body fossils demonstrates that the rise of dinosaurs was a drawn-out affair, perhaps initiated during recovery from the Permo-Triassic extinction.

Popular news story here with a horrible title as they are not true dinosaur footprints.

Asilisaurus kongwe, the Oldest Avian-line Archosaur and the Early Diversification of Ornithodira

Nesbitt, S.J., Sidor, C.A., Irmis, R.B., Angielczyk, K.D., Smith, R.M.H., and L. A. Tsuji. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature. doi:10.1038/nature08718

Abstract - The early evolutionary history of Ornithodira (avian-line archosaurs) has hitherto been documented by incomplete (Lagerpeton) or unusually specialized forms (pterosaurs and Silesaurus). Recently, a variety of Silesaurus-like taxa have been reported from the Triassic Period of both Gondwana and Laurasia, but their relationships to each other and to dinosaurs remain a subject of debate. Here we report on a new avian-line archosaur from the early Middle Triassic (Anisian) of Tanzania. Phylogenetic analysis places Asilisaurus kongwe gen. et sp. nov. as an avian-line archosaur and a member of the Silesauridae, which is here considered the sister taxon to Dinosauria. Silesaurids were diverse and had a wide distribution by the Late Triassic, with a novel ornithodiran bauplan including leaf-shaped teeth, a beak-like lower jaw, long, gracile limbs, and a quadrupedal stance. Our analysis suggests that the dentition and diet of silesaurids, ornithischians and sauropodomorphs evolved independently from a plesiomorphic carnivorous form. As the oldest avian-line archosaur, Asilisaurus demonstrates the antiquity of both Ornithodira and the dinosaurian lineage. The initial diversification of Archosauria, previously documented by crocodilian-line archosaurs in the Anisian, can now be shown to include a contemporaneous avian-line radiation. The unparalleled taxonomic diversity of the Manda archosaur assemblage indicates that archosaur diversification was well underway by the Middle Triassic or earlier.

Life restoration of Asilisaurus with sail-backed poposauroid in the background. Image by M.H. Donnelly (Field Museum).

Prior to 2003 the non-dinosaurian dinosauriforms known as silesaurids were unrecognized in the fossil record. Specimens existed in collections, collected as early as the 1930s, while others were given tentative identifications (e.g., the ornithosuchid of Long and Murry, 1995 and the ornithischian Technosaurus). Dzik (2003) described the first, Silesaurus opolensis from the Carnian of Poland, with its very distinctive femoral and morphologies. Suddenly similar forms were recognized from all over the globe (e.g., Eucoelophysis, Sacisaurus; Ezcurra 2006; Nesbitt et al. 2007; Irmis et al. 2007a), whereas new specimens were being discovered from the Chinle Formation of New Mexico and Arizona (Parker et al. 2006; Irmis et al. 2007b).

Still, because the earliest pseudosuchian archosaurs were known from the Anisian (e.g., the Moenkopi Formation of Arizona), whereas the earliest ornithodirans were from the Ladinian of Argentina, there was a proposed ghost lineage for Ornithodira existing back into the Anisian.

Sterling Nesbitt looks over the Ruhuhu Valley in 2007. Photo by L. Tsuji.

One of the strengths of phylogentic analysis is the ability to make predictions as to where in time and space certain groups should and could be found. Sterling Nesbitt clearly recognized the strong possibility that the earliest representatives of ornithodira could be found in the Manda beds of Tanzania as this fauna was known for its pseudosuchian constituents. A few years back this prediction paid off, as a team lead by Christian Sidor in 2007 to explore the Permian and Triassic rocks of Tanzanian, uncovered an amazing deposit of early archosaurs including numerous specimens of a silesaurid. Named Asilisaurus kongwe (Ancient ancestor lizard) these specimens represent the earliest known member of the lineage leading to dinosaurs and strongly supports the diversification of the Archosauria by the early Middle Triassic (~243 million years ago).

The tibia of Asilisaurus, following excavation in 2007. Photo by R. Smith.

This find also strongly suggests that adaptations for an omnivorous or herbivorous diet evolved independently in silesaurids, ornithischians, and sauropodomorphs, from carnivorous ancestors. The paper also phylogenetically defines the Silesauridae and proposes that the South American archosaurs Lewisuchus and Pseudlagosuchus are members of this clade and probably also synonymous. Another early appearance of a non-dinosaurian dinosauriform in
Gondwana also provides further support for a southern origin for this group.

Skeletal reconstruction of Asilisaurus, with missing bones in gray. Image by S. Nesbitt.

One final point, silesaurs have been proposed to be basal ornithischians (e.g., Ferigolo & Langer, 2007) and represent the Late Triassic radiation of that clade. However, if this is the case, despite the lack of support in phylogentic analyses (e.g., Irmis et al., 2007b), the discovery of early Middle Triassic ornithichians would pull the split with saurischians back as early as the Early Triassic and create a sizeable ghost lineage for the base of Saurischia.

More photos and information on Ruhuhu Basin Research can be found here:
http://protist.biology.washington.edu/sidor/Ruhuhu/asilisaurus.html

REFERENCES

Dzik, J. 2003. A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23:556-574.

Ezcurra, M.D. 2006. A review of the systematic position of the dinosauriform archosaur Eucoelophysis baldwini from the Upper Triassic of New Mexico, U.S.A. Geodiversitas 28:649-684.

Ferigolo, J. and M. Langer. 2007. A late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology 19:23-33.

Irmis, R.B., Parker, W.G., Nesbitt, S.J., and J. Liu. 2007a. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19:3-22.

Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D., and A. Downs. 2007a. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317:358-361.

Nesbitt, S.J., Irmis, R.B., and W.G. Parker. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5:209-243.

Parker, W.G., Irmis, R.B., and S.J. Nesbitt. 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:160-161.

Triassic Ornithischian? Krzyzanowskisaurus hunti revisited.

http://www.azstarnet.com/news/local/article_d4138dc4-60a2-544c-8ba8-45ed550df2c2.html

The Arizona Daily Star (Tucson, AZ), published this story earlier on local fossil collector Stan Krzyzanowski. Those of you familiar with dinosaur taxonomy may recognize the name in regards to the Triassic tooth taxon Krzyzanowskisaurus hunti, to which the story alludes.

This taxon, known from only a handful of isolated teeth, was originally described as a new species of the putative ornithichian dinosaur Reveultosaurus by Heckert (2002). When it was subsequently discovered that the type species of Revueltosaurus, R. callenderi, actually represented a non-dinosaurian pseudosuchian archosaur (Parker et al. 2005), Heckert (2005) referred R. hunti to this new genus, Krzyzanowskisaurus. Heckert's (2005:77) support for this new referral is that the teeth of K. hunti "appear more derived than R. callenderi, and are in fact more 'typically' ornithischian than those of R. callenderi". By more 'typically' ornithischian Heckert (2005:78) is referring to the presence of a "pronounced bulge on the lingual surface", which he hypothesizes as being homologous with the 'cingulum' found in basal ornithischian teeth. However, there is no strong evidence for this homology and thus there are no apomorphies whatsoever to support the referral of K. hunti to Ornithischia (Irmis et al. 2007). Likewise, no phylogentic analysis exists to support the claim that this tooth morphology would be more derived. In fact co-occurring fossils from the St. Johns area actually support the original referral of these teeth to Revueltosaurus, thus I maintain that no material from North America currently exists that can be unambiguously referred to Ornithischia (Parker et al. 2005; Nesbitt et al. 2007; Irmis et al. 2007), and the reconstruction below is most likely erroneous (and apparently plagiarized from Greg Paul, see the copyright and read the comments below).



The Krzyzanowski bonebed is a potentially interesting vertebrate microsite from the Chinle Formation near St. Johns (similar to sites in the same area collected by Charles Camp in the late 1920s), but to date has only received limited coverage in publications (e.g., Heckert et al. 2004; Heckert et al. 2005), including a claim by these authors as to the presence of what would be the only sauropodomorph material from the Upper Triassic of North America (besides Greenland, which is physiogeographically considered part of that continent).

REFERENCES

Heckert, A.B. 2002. A revision of the upper Triassic ornithischian dinosaur Revueltosaurus, with a description of a new species. New Mexico Musuem of Natural History and Science Bulletin 21:253–267.

Heckert, A.B. 2005. Krzyzanowskisaurus, a new name for a probable ornithischian dinosaur from the upper Triassic Chinle Group, Arizona and New Mexico, USA. New Mexico Museum of Natural History and Science Bulletin 29: 77–83.
Heckert, A.B., Lucas, S.G., and A.P. Hunt. 2005. Triassic vertebrate fossils in Arizona. New Mexico Museum of Natural History and Science Bulletin 29:16-44.

Heckert, A. B., Rinehart, L. F., Krzyzanowski, S. E., Lucas, S. G., and S.K. Harris. 2004. The Krzyzanowski bonebed: an Upper Triassic (Adamanian: latest Carnian) vertebrate fauna and its implications for microvertebrate studies. New Mexico Geology 26:64.

Irmis, R.B., Parker, W.G., Nesbitt, S.J., and J. Liu. 2006. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19:3-22.
Nesbitt, S.J., Irmis, R.B., and W.G. Parker. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5:209-243.

Parker, W.G., Irmis, R.B., Nesbitt, S.J., Martz, J.W., and L.S. Browne. 2005. The
Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings of the Royal Society of London, B 272:963–969.
The image above is from the linked article.

New Paper on the Origin and Early Evolution of Dinosaurs

Langer, M.C., Ezcurra, M.D., Bittencourt, J.S., and F.E. Novas. 2009. The origin and early evolution of dinosaurs. Biological Reviews 84:1-56. doi:10.1111/j.1469-185X.2009.00094.x

ABSTRACT-The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks(approximately 230 Ma)accumulated over extensional rift basins in southwestern Pangea. The better known of these are Herrerasaurus ischigualastensis, Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos from the Ischigualasto Formation, Argentina, and Staurikosaurus pricei and Saturnalia tupiniquim from the Santa Maria Formation, Brazil. No uncontroversial dinosaur body fossils are known from older strata, but the Middle Triassic origin of the lineage may be inferred from both the footprint record and its sister-group relation to Ladinian basal dinosauromorphs. These include the typical Marasuchus lilloensis, more basal forms such as Lagerpeton and Dromomeron, as well as silesaurids: a possibly monophyletic group composed of Mid-Late Triassic forms that may represent immediate sister taxa to dinosaurs. The first phylogenetic definition to fit the current understanding of Dinosauria as a node-based taxon solely composed of mutually exclusive Saurischia and Ornithischia was given as ‘‘all descendants of the most recent common ancestor of birds and Triceratops’’. Recent cladistic analyses of early dinosaurs agree that Pisanosaurus mertii is a basal ornithischian; that Herrerasaurus ischigualastensis and Staurikosaurus pricei belong in a monophyletic Herrerasauridae; that herrerasaurids, Eoraptor lunensis, and Guaibasaurus candelariensis are saurischians; that Saurischia includes two main groups, Sauropodomorpha and Theropoda; and that Saturnalia tupiniquim is a basal member of the sauropodomorph lineage. On the contrary, several aspects of basal dinosaur phylogeny remain controversial, including the position of herrerasaurids, E. lunensis, and G. candelariensis as basal theropods or basal saurischians, and the affinity and/or validity of more fragmentary taxa such as Agnosphitys cromhallensis, Alwalkeria maleriensis, Chindesaurus bryansmalli, Saltopus elginensis, and Spondylosoma absconditum. The identification of dinosaur apomorphies is jeopardized by the incompleteness of skeletal remains attributed to most basal dinosauromorphs, the skulls and forelimbs of which are particularly poorly known. Nonetheless, Dinosauria can be diagnosed by a suite of derived traits, most of which are related to the anatomy of the pelvic girdle and limb. Some of these are connected to the acquisition of a fully erect bipedal gait, which has been traditionally suggested to represent a key adaptation that allowed, or even promoted, dinosaur radiation during Late Triassic times. Yet, contrary to the classical ‘‘competitive’’ models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. In fact, the radiation of the group comprises at least three landmark moments, separated by controversial (Carnian Norian, Triassic-Jurassic) extinction events. These are mainly characterized by early diversification in Carnian times, a Norian increase in diversity and (especially) abundance, and the occupation of new niches from the Early Jurassic onwards. Dinosaurs arose from fully bipedal ancestors, the diet of which may have been carnivorous or omnivorous. Whereas the oldest dinosaurs were geographically restricted to south Pangea, including rare ornithischians and more abundant basal members of the saurischian lineage, the group achieved a nearly global distribution by the latest Triassic, especially with the radiation of saurischian groups such as ‘‘prosauropods’’ and coelophysoids.