Extinction and the Rise of Dinosaurs - What Will the Microvertebrates Tell Us?

Brian Switek has a great year end article (based mainly on the recent Allen et al., study in Palaeontology) regarding the loss of most pseudosuchian groups in the end-Triassic extinction and discussion on why the dinosaurs were mostly unaffected.

According to their findings body size was not a factor; however, there are not too many data regarding Middle Triassic - Early Jurassic microfaunas. My intern, colleague and Virginia Tech grad student Ben Kligman is adding to this information. Ben started a couple of years ago at Petrified Forest National Park looking at a new microsite in the Blue Mesa Member of the park. This unit and roughly the same horizon had been the subject of several previous microvertebrate studies that despite being nearly two decades apart had generated roughly the same results, a lot of teeth and scales that could only be assigned to broad taxonomic levels. Thus, I was not enthusiastic about this at first. However, Ben tackled this new site with gusto and developing a new sampling technique with the help of PEFOs lead preparator and curator Matt Smith, very quickly built a sample of over 100 different morphotypes. Even more important this new technique allowed preservation of relatively complete jaw elements.

<i>Palacrodon browni</i> from the Chinle Formation of Arizona.
From Kligman et al 2018. Acta Palaeotologica Polonica 63(1).

Ben already has several publications about new these finds (Kligman et al., 2017; Kligman et al., 2018) and several more in the works. Furthermore, this research won the Student Poster Prize at the 2018 Society of Vertebrate Paleontology meeting. Ben is now sampling a wider stratigraphic range of sites and his work will help us further understand the effects of the end-Triassic extinction.

Surprise! Scientists now say that Crocodiles are not "Living Fossils".

Here are some excerpts from a BBC article that was posted online today:

"Crocodiles can no longer be referred to as "living fossils", according to scientists".


"Members of the crocodilian [sic] family have previously been thought to have changed little since prehistoric times. However, new fossil analyses suggests that modern crocodilians actually evolved from a very diverse group".

"Recently discovered ancient ancestors include small cat-like specimens, giant "supercrocs" and a pug-nosed vegetarian species".

"Modern crocodilians are adapted to aquatic environments with long snouts, strong tails and powerful jaws. Yet contrary to popular belief, scientists now suggest that the basic body structure of crocodiles, alligators and ghariels [sic] evolved from a diverse group of prehistoric reptiles with different body shapes".

You can read the rest of this story here.  I am always flabbergasted how these "popular beliefs" still persist and how this "revelation" can be considered new.

Can't wait to get my copy of the new JVP memoir though.  It sounds incredible.

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).