Ignored by Baron et al. 2017, and everybody else
the Junggarsuchus clade (including Pseudhesperosuchus, Carnufex and Trialestes in order of increasing quadrupedality, Figs. 1–4) nests as the proximal ancestors to Herrerasaurus (Fig. 1) and the rest of the Dinosauria (Fig. 5) in the large reptile tree (LRT). That cladogram tests a wider gamut of taxa in greater detail than any other reptile cladogram ever published, attempting to not overlook anything. The Junggarsuchia is a sister clade to the Crocodylomorpha with both arising from a taxon near Lewisuchus (Fig. 1). Traditional paleontology (see Wikipedia) nests this largely ignored clade with the sphenosuchian crocodylomorphs (Fig. 4)… and for two good reasons!
Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.
One: Paleontologists never seem to include Dinosauria
in their smaller gamut croc analyses because they’re looking at crocs!~. So once again, taxon exclusion is holding some workers back from seeing ‘the big picture’. ReptileEvolution.com and the blog you are currently reading is all about examining ‘the big picture.’
Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur, closely related to Junggarsuchus.
Two: Junggarsuchians ALSO have elongate proximal wrist bones
Elongate proximal carpals are found in both sphenosuchian crocs and derived members of the Junggarsuchus clade. Paleontolgists wrongly assumed such odd wrist bones were homologous. It’s an easy mistake to make. However, the LRT makes clear that intervening taxa, including Junggarsuchus, do not have elongate wrist bones.
Among taxa that preserve the manus,
(Fig. 3) it is Junggarsuchus that nests closest to Herrerasaurus and the Dinosauria.
Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.
Like the basal members of the Crocodylomorpha
the Junggarsuchus clade (the Prodinosauria here) transition from bipedal basal members to quadrupedal derived members, with the longest forelimbs belonging to the most derived member, Trialestes (Fig. 3). Distinct from the others and contra the original interpretation, I think Trialestes may have had a larger ulnare than radiale, to match its larger ulna.
Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.
Let’s not forget
PVL 4597 (Fig. 6) which was mistakenly considered a specimen of Gracilisuchus by (Lecuona and Desojo 2011), but under phylogenetic analysis in the LRT, still nests as the proximal outgroup to Herrerasaurus. It is tiny specimen, supporting the hypothesis of phylogenetic miniaturization at clade origin. And it retains a small proximally oriented calcaneal tuber, as found in other Junggarsuchians.
Figure 5. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).
We looked at
phylogenetic miniaturization at the origin of several pterosaur clades. Well, it happens here too, at the base of the Dinosauria (Fig. 1) with PVL 4597 (Fig. 6), easily overlooked, easily mistaken for something else.
One should not ‘choose’ outgroup taxa
based on paradigm, tradition, guessing, convenience or opinion. Rather outgroup taxa should ‘choose themselves’ based on rigorous testing of a large gamut of outgroup candidates in phylogenetic analysis. To minimize selection bias, the LRT provides 858 outgroup taxa the opportunity to nest close to dinosaurs.
Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.
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