Late Triassic Dispersal And Endemism

I last left you with a Rube Goldberg machine and a walrus sucking its own cock. That was a high note for this blog, but I think it is time we got back to the Triassic. Focus, people.

A new paper by Martin Ezcurra attempts to quantify geographic variation in Middle and Late Triasic tetrapod faunas, and I want to elaborate on the problems that Bill pointed out due to absent data. Ezcurra took big composite phylogenetic trees meshing phylogenies of different groups of Triassic tetrapods, and ran them through something called “tree reconciliation analysis.” I don’t know exactly what that means other than it involves numbers, and my eyes tend to slide quickly and nervously past sections of papers containing lots of numbers they might slide quickly and nervously past an alley where two homeless people are butchering a third for the meat. Occasionally, as a matter of principle, I might force myself to read through the procedure to get a basic grasp on the calculations, the same way I might, as a matter of principle shout “hey, you guys shouldn’t be doing that!” into the alley before running. But not this time.

That seemed funnier when I thought it up at 3:00 this morning.

In a nutshell, Ezcurra compared phylogenetic trees with the geographic distribution of taxa to establish which areas of Pangea tended to have the most similar faunas, and what that tells us about patterns of dispersal. For the Middle Triassic, Ezcurra found evidence that vertebrate faunas tended to be more cosmopolitan with a “polyphyletic Gondwana”, meaning that different regions of Gondwana (South America, India, and Africa) had more similarities to various parts of Laurasia than they did to each other. Ezcurra interpreted this to mean that there were few barriers (climatic, geographic, or otherwise) to tetrapod dispersal across the supercontinent.

However, for the Late Triassic, Ezcurra found more evidence of faunal endemism, both between Gondwana and Laurasia, and between equatorial Laurasia (North America), and higher latitudes (Europe). It has been recognized for a long time that Late Triassic Laurasian and Gondwanan faunas showed some key differences, with Benton’s (1983) paper being the real landmark. However, Ezcurra’s analysis, largely as a result of leaving out some key taxa, missed some of the more important patterns noted by Benton (1983) which are still valid, and probably exaggerated the differences between Europe and North America. In particular, I have some issues with the decidedly selective treatment of phytosaurs and aetosaurs.

Part of the problem is the phylogenetic tree used for phytosaurs. Phytosaurs were a hugely important group in the northern hemisphere during the Late Triassic (e.g. Long and Murry, 1995; Hungerbeuhler, 2002), and virtually unknown from South America and Africa except for some scrappy specimens (Kischlatt and Lucas, 2003). However, Ezcurra’s phylogeny used Parker and Irmis’ (2006) phytosaur phylogeny, which (like Hungerbeuhler's 1998 phylogeny) was almost entirely concerned with pseudopalatines, the most derived phytosaur clade. As a result, other phytosaurs were pretty much ignored; Ballew’s (1989) analysis was the last published phylogeny to include the forms now assigned to Parasuchus, Angistorhinus, Leptosuchus, and/or Rutiodon, which show a lovely correspondence between stratigraphy and phylogeny in the western United States. Subsequent (but as of yet unpublished) phylogenies by Axel Hungerbeuhler and Michelle Stocker fit the broad patter Ballew recognized.

Phytosaurs show some really important geographic patterns. Basal phytosaurs (sometimes lumped into the genera Paleorhinus or Parasuchus; e.g. Hunt and Lucas, 1991a; Long and Murry, 1995) had a broad distribution and are known from Europe, North America, North Africa, and India (e.g. Chatterjee, 1978; Long and Murry, 1995). A Leptosuchus-like form from India links it to North America (Hungerbeuhler and Chatterjee, 2002). Moreover, Leptosuchus and the more derived Pseudopalatus are known from the from the Adamanian and Revueltian (which Ezcurra lumps into the South American “Ischigualastian and “Coloradian”) of North America respectively, but the European pseudopalatines Nicrosaurus and Mystriosuchus fall out in between them phylogenetically (Ballew, 1989; Hungerbeuhler, 2002; Parker and Irmis, 2006; Stocker, 2008). Phytosaurs were really getting around throughout Laurasia, and to at least some parts of Gondwana, and dispersal seems to have been occurring continuously as phytosaurs evolved during the Late Triassic.

The same is true of some aetosaurs. Ezcurra (2010) used Bill’s analysis from his Heliocanthus paper (Parker, 2007), which established the three broad groupings of aetosaurs (basal aetosaurines, typothoracisines, and desmatosuchines) that agree with my own observations of aetosaur morphology. As suggested by Ezcurra, the spiky desmatosuchine aetosaurs do seem to be restricted to North America (at least, so far), but the same isn’t true of aetosaurines. Although Ezcurra claims that the broad-bodied typothoracisine aetosaurs were also endemic to North America, Paratypothorax is also known from Europe. Moreover, I have seen photos of Indian aetosaur material that suggest to me that Typothorax and Paratypothorax might both have been present there, so like phytosaurs, typothoracisines were moving widely across Laurasia and probably at least down the east coast of Gondwana.

Most importantly, Ezcurra claims that “aetosaurines” (by which he probably means BASAL aetosaurines, as typothoracisines are also aetosaurines) were restricted to high latitudes and absent in North America. However, both Aetosaurus and Stagonolepis-like forms are known from western North America, so basal aetosaurines actually had a VERY cosmopolitan distribution, all the way from the deserts of Scotland to the more tropical conditions in western North America, back into higher latitudes of Brazil and Argentina.

From Paleo Errata

In summary, I don’t think all of Ezcurra’s latitudinal differences really hold up. The abundance of rhynchosaurs at high latitudes, while they are virtually absent in North America (Elder, 1978; Hunt and Lucas, 1991b), may be valid. The absence of prosauropods in North America (e.g. Nesbitt et al, 2005) until things really dried out there during the Early-Middle Jurassic may also be an important latitudinal pattern. Perhaps prosauropods and rhynchosaurs really did like dry conditions. It had also never occurred to me for some reason that an entire aetosaur clade (the desmatosuchines) was restricted to North America, while aetosaurines were more cosmopolitan. Perhaps the desmatosuchines liked more wetter conditions, which falls in pretty nicely with the timing of their extinction in North America (Parker and Martz, in prep). However, North America was not nearly as isolated as Ezcurra indicates, and many of the groups that he lists were a lot more cosmopolitan than he realized.

As noted by Benton (1983), India is a very interesting anomaly, forming an important faunal link between Gondwana and Laurasia, and between high and low latitudes. Rhynchosaurs and therapsids link it to the rest of Gondwana, while it also seems to contain aetosaurs and phytosaurs linking it to both North America and Europe. Its placement along the western margin of Tethys makes me wonder if oceans currents might have given the east coast of Gondwana a different climate from the rest of the southern supercontinent. Ezcurra’s analysis, though it contains a lot of problems, has certainly given me a lot to think about.

Oh yeah, here is something funny and unrelated. It just goes on and on and should stop being funny, but somehow it doesn’t. Well, it kind of does. It comes and goes.

“Female asses are mysterious creatures. They come and go as they please, and much of their behavior seems unfathomable to an outsider.”
-David Attenborough, “Planet Earth”

REFERENCES

Ballew, K.L. 1989. A phylogenetic analysis of Phytosauria from the late Triassic of the western United States. In Lucas, S.G., and Hunt, A.P. (eds.) Dawn of the Age of Dinosaurs in the American Southwest, pp. 309-339. New Mexico Museum of Natural History, Albuquerque, NM.

Benton, M.J. 1983a. Dinosaur success in the Triassic: a noncompetitive ecological model. Quarterly Review of Biology, vol. 58, no. 1 (March), pp. 29-55.

Chatterjee, S. 1978. A primitive parasuchid (phytosaur) reptile from the Upper Triassic Maleri Formation of India. Paleontology, vol. 21, pt. 1, pp. 83-127.

Ezcurra, M. D. 2010. Biogeography of Triassic tetrapods: evidence for provincialism and driven sympatric cladogenesis in the early evolution of modern tetrapod lineages. Proceedings of the Royal Society B.

Hungerbühler, A. 2002b. The late Triassic phytosaur Mystriosuchus westphali, with a revision of the genus. Palaeontology, vol. 45, pt. 2, pp. 377-418.

Hungerbühler, A., and Chatterjee, S. 2002. New phytosaurs from the Upper Triassic of India. Journal of Vertebrate Paleontology, vol. 22 (suppl. to no. 3, September 19th), p. 68A.

Hunt, A.P., and Lucas, S.G. 1991a. The Paleorhinus biochron and the correlation of the non-marine Upper Triassic of Pangaea. Palaeontology, vol. 34, pt. 2, pp. 487-501.

Hunt, A.P., and Lucas, S.G. 1991b. A new rhynchosaur from the Upper Triassic of West Texas, and the biochronology of Late Triassic rhynchosaurs. Paleontology, vol. 34, pt. 4, pp. 927-938.

Kischlat, E.-E., and S.G. Lucas. 2003. A phytosaur from the Upper Triassic of Brazil. Journal of Vertebrate Paleontology, vol. 23, no. 2, pp. 464-467.

Parker, W.G. 2007. Reassessment of the aetosaur "Desmatosuchus chamaensis" with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology, vol. 5, no. 1, pp. 41-68.
Long and Murry, 1995

Nesbitt, S.J., Irmis, R.B., and Parker, W.G. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology, vol. 5, no. 2 (May 25th), pp. 209-243.

Parker, W.G., and Irmis, R.B. 2006a. A new species of the Late Triassic phytosaur Pseudopalatus (Archosauria: Pseudosuchia) from Petrified Forest National Park, Arizona. In Parker, W.G., Ash, S.R., and Irmis, R.B. (eds.) A Century of Research at Petrified Forest National Park 1906-2006. Museum of Northern Bulletin no. 62, pp. 126-143. Flagstaff, AZ.

Stocker, M.R. 2008. The relationships of the phytosaur Leptosuchus Cape 1922 with decriptions of new material from Petrified Forest National Park, Arizona. Unpublished master’s thesis, University of Iowa, IA, 220 pp.