Junggarsuchus: not a crocodyliform sister, a dino sister

Updated Dec. 22, 2014 with a new tree (Fig. 3), moving the SMNS 12352 specimen between Terrestrisuchus and Gracilisuchus — and that makes the last common ancestor of crocs and dinos Lewisuchus and pulls the Poposauridae into the Archosauria.

Junggarsuchus (Fig. 1, Clark et al. 2004) is known from a Jurassic fossil from China that was purported to be the sphenosuchian closest to the Crocodyliformes (all higher crocs, Fig. 2).

Figure 1. Junggarsuchus colorized. Once thought to be the crocodylomorph closest to crocodylformes, it now nests as a pre-dinosaur.

Figure 1. Junggarsuchus colorized. Once thought to be the crocodylomorph closest to crocodylformes, it now nests as a pre-dinosaur.

Clark et al. (2004, Fig. 2) nested Junggarsuchus between Dibothrosuchus and Protosuchus (Fig. 5) and both basal to the living Alligator.

Figure 2. Crocodylomorph tree according to Clark et al. 2004. Green taxa are crocodylomorphs in the large reptile tree.

Figure 2. Crocodylomorph tree according to Clark et al. 2004. Green taxa are crocodylomorphs in the large reptile tree.

Adding Junggarsuchus to the large reptile tree
When I added Junggarsuchus to the large reptile tree I was not surprised to see it nesting with Pseudhesperosuchus (Figs. 3, 4). The two are more similar to each other than either is to any other tested taxon. Among many other traits, they both share a double mandibular fenestrae, high arching postorbitals, and anterior nares.

Figure 3. Pseudhesperosuchus colorized to show how the maxilla has slipped ventrally. Moving it back in place removes that extra antorbital fenestra that others have noticed.

Figure 3. Pseudhesperosuchus colorized to show how the maxilla has slipped ventrally. Moving it back in place removes that extra antorbital fenestra that others have noticed.

Parrish (1997) and Clark et al. (2000) both illustrated a second antorbital fenestra in Pseudhesperosuchus (Fig. 4). No other sisters have this trait and it disappears when the maxilla is rotated back into place (Fig. 3).

Figure 4. Pseudhesperosuchus by Clark et al. 2000 showing the second antorbital fenestra caused by taphonomic slippage of the maxilla.

Figure 4. Pseudhesperosuchus by Clark et al. 2000 showing the second antorbital fenestra caused by taphonomic slippage of the maxilla.

Adding Caiman (extant croc) to the large reptile tree
When I added Caiman (a living relative of crocodiles and alligators) to the large reptile tree, I expected it to nest with Protosuchus, the conventional sister, as shown above (Fig. 2). Instead Caiman nested with the SMNS 12352 specimen, an anterior skull with anterodorsal nares and a few other traits (like prefrontals that contact each other at the midline) and both of these were sisters to Sphenosuchus (Fig. 5). Shifting Caiman to Protosuchus adds 15 steps.

Figure 1. Ten basal bipedal crocodylomorphs descending from a sister to Decuriasuchus.

Figure 5. Click to enlarge. Ten basal bipedal crocodylomorphs descending from a sister to Decuriasuchus. Here sphenosuchus is basal to living crocs, not Protosuchus.

But wait, there’s more!
Let’s get back to Junggarsuchus and Pseudhesperosuchus. With new data they now nest between basal bipedal crocs and basal bipedal dinosaurs. That new nesting makes more and more sense as sisters begin to look more and more like one another.

Figure 7. Pseudhesperosuchus nests near Trialestes at the base of the Dinosauria.

Figure 6. Pseudhesperosuchus nests near Trialestes at the base of the Dinosauria. Note how closely all sister taxa resemble each other.

Lewisuchus, a tiny basal poposaur
We’ve seen this before: miniaturized taxa at the base of major clades. This time it’s Lewisuchus (Fig. 5) at the base of the Poposauria, basal to Poposaurus and Turfanosuchus. All known poposaurs are larger than Lewisuchus.

Elongate coracoids and proximal carpals. Problem?
Both Junggarsuchus and Pseudhesperosuchus have a narrow (elongate) coracoid, a shape typical of all but the most basal crocs. By contrast, basal dinosaurs have a disc-like coracoid. Unfortunately we don’t know of an in-between coracoid in transitional taxa, Trialestes (Fig. 5) and the PVL specimen. Apparently in dinosaurs the coracoid evolved from a strut to a disc.

In like fashion, Trialestes has the elongate proximal carpals (ulnare and radiale) typical of crocs. Herrerasaurus, a basal dinosaur, does not have elongate proximal carpals. Apparently, as the forelimbs were no longer used for quadrupedal locomotion, the proximal carpals returned to their plesiomorphic state as short discs.

Figure 7. Portion of the large reptile tree showing the ancestry of archosaurs (crocs + dinos) updated with new taxa and new data. Small black squares are miniaturized taxa at the base of several clades.

Figure 7. Portion of the large reptile tree showing the ancestry of archosaurs (crocs + dinos) updated with new taxa and new data. Small black squares are miniaturized taxa at the base of several clades.

It is important
to let the facts recover your conclusions. But if something is untenable, it probably means the tree data is in need of repair, or at least a second look.

References
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Clark JM et al. 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20 (4): 683–704.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull.
Irmis RB, Nesbitt SJ and Sues H-D 2013. Early Crocodylomorpha. Geological Society Special Publications 379:275-302.
Parrish JM 1993. Phylogeny of the crocodylotarsi, with reference to archosaurian and crurotarsian monophyly. Journal of Vertebrate Paleontology 13:287-308.

wiki/Pseudhesperosuchus

wiki/Junggarsuchus

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