SVP 2021 abstracts – 19: A new (unnamed) basal sea croc

Wilberg et al. 2021 describe
a new basal thalattosuchian, the clade that ultimately produced crocodilians with flippers instead of toes (Fig. 1).

Add Middle Triassic Dyoplax (Fig. 2). Look for the last common ancestor. Added taxa might not have the traits you are looking for, but this is how the LRT recovered the origin of pterosaurs, turtles, snakes, cephalopods, reptiles, tetrapods, ichthyosaurs, etc.

Willberg et al. continue:
“All phylogenetic hypotheses imply a ghost lineage extending at least to the Sinemurian (c. 199 Ma), and a lack of older or more plesiomorphic forms may contribute to the uncertain phylogenetic placement of the group.

Dyoplax needs to be added.

“Fragmentary postcranial material from the Sinemurian and Pliensbachian of South America, Europe, and India has been attributed to Thalattosuchia, but lacks thalattosuchian apomorphies.”

Perhaps only the plesiomorphic parts are preserved.

“Here we describe new material from the early Pliensbachian (c. 190 Ma) Belemnite Marl Member of the Charmouth Mudstone Formation (Dorset, UK). The specimen includes partially
articulated cranial, mandibular, axial, and appendicular elements. It can be attributed to Thalattosuchia based on the presence of a distinct fossa on the posterolateral corner of the squamosal, a broad ventrolateral process of the otoccipital broadly covering the dorsal surface of the quadrate body, large supratemporal fenestrae that lack a flattened skull table, a broadly exposed prootic, and an orbital process of the quadrate lacking bony attachment with the braincase. This specimen therefore represents the earliest thalattosuchian currently known from diagnostic material.”

So this specimen has thalattosuchian apomorphies. Good. I still wish workers would add this taxon to their cladogram and then announce that this taxon is the last common ancestor of thalattosuchians. Traits can converge. Their is only one last common ancestor (until a better one comes along, of course).

“To determine the phylogenetic position of the new taxon, we performed two separate analyses based on different published datasets.”

So they are borrowing and trusting. That’s not good science.

“Both analyses recover the new taxon as the earliest diverging thalattosuchian, sister to Teleosauroidea + Metriorhynchoidea.”

Does ‘earliest diverging thalattosuchian’ = last common ancestor? If so, good!.
No name was applied. For that we’ll wait for the paper.

“We also performed Bayesian time-scaling analyses using a fossilized birth-death model to investigate the impact of the inclusion of the new specimen on estimates of divergence times of Thalattosuchia within Crocodylomorpha. The results show a likely Late Triassic origin for Thalattosuchia (median age=211 Ma), which is constrained between 224.33 and 196.70 Ma (95% highest posterior density). The new specimen extends the fossil record of Thalattosuchia, but the time-scaling analyses demonstrate that a significant ghost lineage remains.”

First author Wilberg 2015 wrote:
“Outgroup sampling is a central issue in phylogenetic analysis. However, good justification is rarely given for outgroup selection in published analyses.”

This was insightful. The ideal is to have so many outgroup taxa that the software tells the scientist the short list of proximal outgroup taxa, as a large number of taxa do in the LRT.

Wilberg 2015 continues:
“A new analysis of crocodylomorphs with increased outgroup sampling recovers Thalattosuchia as the sister group to Crocodyliformes, distantly related to long-snouted crocodyliforms.”

Various borrowed analyses from that paper omitted Dyoplax (Fig. 1) and considered Pelagosaurus an ingroup thalattosuchian, rather than an outgroup relative of Gavialis. which nests with Crocodylus in the Wilberg 2015 borrowed studies. This is distinct from the LRT (Fig. 2) where Gavialis nests apart from other living crocodilians.

References
Wilberg E, Godoy PL, Griffiths E, Turner AH and Benson RB 2021. A new basal thalattosuchian crocodylomorph from the Early Jurassic (Pliensbachian) of Dorset, UK, and implications for the origin of the group. Journal of Vertebrate Paleontology abstracts.
Wilberg E 2015. What’s in an Outgroup? The Impact of Outgroup Choice on the Phylogenetic Position of Thalattosuchia (Crocodylomorpha) and the Origin of Crocodyliformes. Syst. Biol. 64(4):621–637.

wiki/Dyoplax