Recently Li et al. (2012) announced the discovery of Diandongosuchus (Fig. 1), described as a “basal poposaurid” close to Qianosuchus. There seemed to be something wrong with this nesting (because poposaurids are all dinosaur herbivores derived from bipeds and Qianosuchus is a sort of piscivorous rauischid close to Ticinosuchus), so I tested those results in phylogenetic analysis. The new test recovered Diandongosuchus between Proterochampsa and the phytosaurs. It’s the basalmost phytosaur! Moving Diandongosuchus to the poposaurids added 51 steps, a substantial number.
The Problems were Many and Basic
The problem with the Li et al. (2012) study is that they relied on the Nesbitt (2011) study on archosaurs, which included several taxa that do not belong (Vancleavea, pterosaurs and Mesosuchus) and several taxa that are needed but were not included (Youngina, choristoderes). More importantly, the Nesbitt (2012) study did not include taxa and character traits that could recover lineages with gradual accumulations of characters. Rather the Nesbitt (2012) study produced “strange bedfellows,” as discussed earlier over seven posts ending here. Finally the Nesbitt (2012) study was largely unresolved outside of the Diandongosuchus clade, which gave this new taxon many possible sisters and ancestors. Not good. The large reptile tree solves all these problems.
Here is what the Nesbitt (2012) study produces:
A real mixed bag (Fig. 2). In the large reptile tree none of these taxa nest together. While Qianosuchus has a skull that appears piscivorous (a sister, Ticinosuchus includes fish in the belly), like other euarchosauriforms, the skull was taller than wide, unlike the wider, flatter skull of Diandongosuchus and its true kin (Fig. 3),
If you’re going to look for the relatives of Diandongosuchus…
Look for the relatives that look like it. Diandongosuchus has a very crocodilian appearance down to the bony scutes on its back. The BPI 2871 specimen referred to Youngina (Fig. 3) has that look, but lacks an antorbital fenestra and mandibular fenestra. It shares with Diandongosuchus a rostrum longer than the dentary, dorsal nares and a wide, flat triangular (in dorsal view) skull. Like Proterochampsa, in Diandongosuchus the premaxilla was just starting to lengthen, which goes to extremes in parasuchians. Neither of these relatives to Diandongosuchus include post-crania. For those bones we’ll take a closer look at parasuchians.
For the four separate origins of the antorbital fenestra look here.
Parasuchians have long been considered analogs to modern crocodilians with the major difference in the location of the external nares, either back toward the eyes in the parasuchians or at the snout tip in crocs. Like all pararchosauriforms (except Champsosaurus and perhaps Doswellia) the nares in Diandongosuchus are displaced posteriorly, but not quite dorsal in location. The orbit is midway in relative size between BPI 2871 and Parasuchus.
The cervicals are robust in Diandongosuchus, taller than long, as in parasuchians. The leaf-like cervical ribs are closer to those seen in choristoderes, like Champsosaurus, than the narrow ribs of parasuchians. The dorsal ribs are more robust than those of parasuchians, again closer to those in choristoderes. The tail was broad proximally and narrow distally as in choristoderes and parasuchians.
The coracoid of Diandongosuchus is a close match to that of Parasuchus, with its deep anterior notch, a trait rarely–if ever–found elsewhere within the Reptilia The clavicles were quite long in Diandongosuchus, as in choristorderes. They were smaller in parasuchians. The ilium of Diandongosuchus was a simple posterior process, as in choristoderes, bearing no trace of an anterior process as in parasuchians. The shapes of the pubis and ischium are close matches to those found in Parasuchus. Both were oriented largely medially, creating a not-so-deep pelvic area. The sigmoidal femur of Diandongosuchus is a close match to that of Parasuchus. The fibula includes a long, low ridge-like trochanter in both taxa. The tarsus was similar in both taxa. Manus and pes proportions were similar in both taxa to the exclusion of all other candidates.
While we’re on the subject…
Keep in mind that as relatives to parasuchians, proterochampsids like Gualosuchus and Chanaresuchus are not far from Diandongosuchus, as you’ll see for yourself (Fig. 4). The skulls are strikingly similar with similar limb to torso proportions.
The Li et al. (2012) study nested parasuchians (phytosaurs) basal to their “Archosauria” with so many unresolved derived branches (including pterosaurs) that really anything goes here, including Diandongosuchus and Qianosuchus as two possibilities among many (at least nine taxa).
The images above (Figs. 2, 3) should prove to be good guides. Diandongosuchus, in all regards, belongs with the taxa in figure 3 more parsimoniously than those in figure 2. The details await anyone caring to see the data matrix of the large reptile tree or to duplicate the taxon list in their own study.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Li C, Wu X-C, Zhao L-J, Sato T and Wang LT 2012. A new archosaur (Diapsida, Archosauriformes) from the marine Triassic of China, Journal of Vertebrate Paleontology, 32:5, 1064-1081.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.