Expanding the Late Triassic Record of the Dinosaur Precursor Dromomeron romeri: A New Record from the Chinle Formation of Arizona.

A new open access paper from my co-worker, friend, and colleague Adam Marsh documenting a new record of the dinosaur precursor Dromomeron romeri from the Chinle Formation Arizona. This further demonstrates the importance of museum collections and apomorphy based identification work to identify stratigraphic, chronologic, and bibliographical extensions, improving our understanding of early dinosaur distributions.  

Marsh, A. D. 2018. A new record of Dromomeron romeri Irmis et al., 2007 (Lagerpetidae) from the Chinle Formation of Arizona, U.S.A. PaleoBios, 35. Retrieved from https://escholarship.org/uc/item/8w5755sg

Abstract - The relatively recent discovery and contextualization of silesaurid and lagerpetid dinosauromorphs has led to a revolution in understanding the early evolutionary history of the dinosaurian lineage. Lagerpetids are known from North America and South America in Middle and Upper Triassic rocks, especially the Chinle Formation of New Mexico and the Dockum Group of Texas. Until now, only a single specimen of Dromomeron gregorii was known from the Upper Triassic Chinle Formation of Arizona. However, a new lagerpetid astragalus specimen (MNA V7237) from the Owl Rock Member of the Chinle Formation found on Ward Terrace in the Navajo Nation of Arizona is referred to Dromomeron romeri. MNA V7237 represents the youngest radioisotopically-dated record of Lagerpetidae, indicating that D. romeri persisted throughout the entire Norian (Otischalkian into the Apachean) in North America.

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.

A New Lagerpetid Dinosauromorph from the Late Triassic of Argentina

Martínez, R. N., Apaldetti, C., Correa, G. A., and D. Abelín. 2015. A Norian lagerpetid dinosauromorph from the Quebrada del Barro Formation, northwestern Argentina. Ameghiniana (future issue) doi:10.5710/AMGH.21.06.2015.2894

Abstract: The early evolution of Ornithodira, the clade that includes pterosaurs and dinosaurs, is poorly known. Until a decade ago, the basal radiation of Dinosauromorpha, the clade including dinosaurs and birds, was poorly understood because of a scarce of fossil record, which was restricted to specimens known of the Ladinian Chañares Formation from Argentina. In the last years the discovery of several non-dinosaurian dinosauromorphs dramatically expanded this record and also demonstrated that this group—previously restricted to the Middle Triassic—persisted at least well into the Norian. Although Norian non-dinosaurian dinosauromorphs have been reported from several places around the world, the only known Norian non-dinosauriform dinosauromorphs—Dromomeron romeri and Dromomeron gregorii—come from North America. We report here the first record from the Southern Hemisphere of a non-dinosauriform dinosauromorph, Dromomeron gigas sp. nov., from the Norian Quebrada del Barro Formation, northwestern Argentina. A phylogenetic analysis recovers Dromomeron gigas nested into the monophyletic group Lagerpetidae, and as the sister taxon to Dromomeron romeri. The inclusion of D. gigas within Lagerpetidae suggests that body size increased in this lineage over time, as was previously demonstrated for Dinosauriformes as a whole, and that lagerpetids reached a larger size than previously thought. Finally, the new finding provides novel information on the basal radiation of Dinosauromorpha constituting the first record of a Norian association of dinosaurs with non-dinosauriform dinosauromorphs outside North America.

A New Silesaurid from the Middle Triassic of Zambia and Evidence for Rapid Diversification of Silesauridae

Peecook, B. R., Sidor, C. A., Nesbitt, S. J., Smith, R. M. H., Steyer, J. S., and K. D. Angielczyk. 2013. A new silesaurid from the upper Ntawere Formation of Zambia (Middle Triassic) demonstrates the rapid diversification of Silesauridae (Avemetatarsalia, Dinosauriformes) Journal of Vertebrate Paleontology 33:1127-1137 DOI:10.1080/02724634.2013.755991 


Abstract - Recent discoveries have shown that non-dinosaurian dinosauromorphs were morphologically diverse, globally distributed, and have a stratigraphic range extending into the Upper Triassic. Silesauridae, the sister group to Dinosauria, contains at least seven species. Here we describe Lutungutali sitwensis, gen. et sp. nov., the first silesaurid from the upper portion of the Ntawere Formation of the Luangwa Basin, Zambia. The upper Ntawere Formation has been correlated with subzone C of the Cynognathus Assemblage Zone of the Karoo Basin in South Africa and the Lifua Member of the Manda beds in the Ruhuhu Basin in Tanzania, both of which are considered Anisian in age and the latter has yielded the silesaurid Asilisaurus kongwe. The results of our phylogenetic analysis, including a new pelvic character, allies Lutungutali with Upper Triassic silesaurids such as Silesaurus, Sacisaurus, and Eucoelophysis rather than with the possibly coeval Asilisaurus. The Zambian silesaurid shares a laterally oriented brevis fossa on the ilium and a transversely thin ischium in cross-section with Upper Triassic forms. Silesaurids were more diverse during their early evolution in the Anisian than previously suspected. Lutungutali and Asilisaurus are the two oldest known members of the bird-line archosaurs represented by body fossils. Together they show that a subclade of bird-line archosaurs was diversifying soon after its origin, building further support for the rapid diversification of Archosauria in the wake of the Permo-Triassic extinction.

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.

Biomechanical Comments about Triassic Dinosaurs from Brazil

Delcourt, R., de Azevedo, S. A. K., Grillo, O. N., and F. O. Deantoni. 2012. Biomechanical comments about Triassic dinosaurs from Brazil. Papáis Avulsos de Zoologia 52:341-347.
 Abstract - Triassic dinosaurs of Brazil are found in Santa Maria and Caturrita formations, Rio Grande do Sul state, Brazil. There are three species known from the Santa Maria Formation (Staurikosaurus pricei, Saturnalia tupiniquim and Pampadromaeus barberenai), and two from Caturrita Formation (Guaibasaurus candelariensis and Unaysaurus tolentinoi). These dinosaur materials are, for the most part, well preserved and allow for descriptions of musculature and biomechanical studies. The lateral rotation of the Saturnalia femur is corroborated through calculations of muscle moment arms. The enhanced supracetabular crest of Saturnalia, Guaibasaurus, Staurikosaurus, Herrerasaurus ischigualastensis, Efraasia minor and Chormogisaurus [sic] novasi suggests that basal dinosaurs may have maintained an inclination of the trunk at least 20° on the horizontal axis. The pectoral girdle articulation of basal sauropodomorphs (Saturnalia and Unaysaurus) was established using a new method, the Clavicular Ring, and the scapular blade remains near 60° on the horizontal axis. This is a plesiomorphic condition among sauropodomorphs and is also seen in the articulated plateosauridae Seitaad ruessi. The Brazilian basal dinosaurs were lightweight with a body mass estimated around 18.5 kg for Staurikosaurus, 6.5 kg for Saturnalia, and 17 kg for Guaibasaurus. Pampadromaeus probably weighed 2.5 kg, but measures of its femur are necessary to confirm this hypothesis. The Triassic dinosaurs from Brazil were diversified but shared some functional aspects that were important in an evolutionary context.

Possible New Plateosaurid Sauropodomorph From Brazil

Bittencourt, J.S., Leal, L.A., Langer, M.C. & Azevedo, S.A.K., iFirst article. An additional basal sauropodomorph specimen from the Upper Triassic Caturrita Formation, southern Brazil, with comments on the biogeography of plateosaurids. Alcheringa, 1–10.

Abstract - We describe an additional saurischian specimen from the Caturrita Formation (Norian) of the Parana Basin, southern Brazil. This material was collected in the 1950s and remained unstudied due to its fragmentary condition. Detailed comparisons with other saurischians worldwide reveal that some characters of the ilium, including the low ventral projection of the medial wall of the acetabulum and its concave ventral margin, together with the short triangular shape of the pre-acetabular process and its mound-like dorsocaudal edge, resemble those of sauropodomorphs such as Plateosaurus and Riojasaurus. This set of traits suggests that MN 1326-V has affinities with basal Sauropodomorpha, probably closer to plateosaurians than to Saturnalia-like taxa. Previous records of this clade in the Caturrita Formation include Unaysaurus, which has been related to Plateosaurus within Plateosauridae. Alternative schemes suggest that plateosaurids include Plateosaurus plus the Argentinean ‘prosauropods’ Coloradisaurus and Riojasaurus. Both hypotheses raise biogeographic questions, as a close relationship between faunas from South America and Europe excluding Africa and North America is not supported by geological and biostratigraphical evidence. Additionally, the absence of plateosaurids in other continents suggests that the geographical distribution of this taxon is inconsistent with the geological history of western Pangaea, and this demands further investigations of the phylogeny of sauropodomorphs or improved sampling.

Evidence of Avian-like Behavior in a Basal Saurischian Dinosaur

There has been a rash of Triassic vertebrate papers in the last two weeks.  Here is one that looks at the taphonomy of a specimen of the basal saurischian Guaibasaurus and suggests that it had a avian style of resting.

Agnolin, F., and Martinelli, A.G. 2012. Guaibasaurus candelariensis (Dinosauria, Saurischia) and the early origin of avian-like resting posture. Alcheringa (1–5) [advance online] DOI:10.1080/03115518.2012.634203

Abstract - A specimen of the basal saurischian Guaibasaurus candelariensis Bonaparte, Ferigolo and Ribeiro (UFRGS-PV-0725-T) from the Faxinal do Soturno locality, Rio Grande do Sul State, Brazil (Caturrita Formation; Late Triassic) lacks any sign of post-mortem transport and burial deformation, and exhibits features (flexed forelimbs, folded hindlimbs under the body and curved neck) that indicate a typical avian-like resting position. The presence in Guaibasaurus of an avian-like resting posture and related physiological implications would extend this unique trait, previously considered restricted to derived maniraptoran theropods, to the base of the Theropoda (or even
Saurischia) clade.