Restoring the little crocodylomorph, Coloradisuchus

In 2017 Martinez, Alcober and Pol introduced a new
small (6cm skull length) crocodylomorph, Coloradisuchus abelini (Figs. 1, 2). The specimen is only known from the bottom half of its small skull + mandibles (Fig. 1). Unique for such a small Late Triassic croc, the nares are confluent at the snout tip, facing anteriorly. The premaxilla/maxilla suture is marked by a large oval fenestra exposing the lower canine in lateral view. This trait is typically found in protosuchids (Fig. 5), but also to a lesser extent in Gracilisuchus (Fig. 3) and Dibothrosuchus (Fig. 2).

Figure 1. Coloradisuchus skull from Martinez, Alcover and Pol 2017. Colors added.

Figure 1. Coloradisuchus skull from Martinez, Alcover and Pol 2017. Colors added. Skull length 6 cm. Restoration according to Dibothrosuchus.

The question is:
where to nest Coloradisuchus?

Figure 2. Dibothrosuchus compared to scale with the much smaller Coloradisuchus.

Figure 2. Early Jurassic Dibothrosuchus compared to scale with the much smaller Triassic Coloradisuchus.

From the abstract:
“Protosuchids are known from the Late Triassic to the Early Cretaceous and form a basal clade of Crocodyliformes. We report here a new protosuchid crocodyliform, Coloradisuchus abelini, gen. et sp. nov., from the middle Norian Los Colorados Formation, La Rioja, northwestern Argentina. Our phylogenetic analysis recovers Coloradisuchus abelini within Protosuchidae, as the sister group of the clade formed by Hemiprotosuchus and two species of Protosuchus (P. richardsoni and P. haughtoni). The new protosuchid C. abelini increases the diversity of crocodyliforms in the Late Triassic and, together with H. leali from the same stratigraphic levels of the Los Colorados Formation, shows that the diversification of basal crocodyliforms was probably faster and/or older than thought previously.”

It is easy to see why the authors assumed Coloradisuchus was a protosuchid, but adding convergent taxa moves it away. On the other hand, distinctly different Hemiprotosuchus (Fig. 4) clearly nests elsewhere.

Figure 5. Gracilisuchus skull updated with new colors.

Figure 3. Gracilisuchus skull updated with new colors. Skull length = 8cm.

Here in the LRT
Hemiprotosuchus nested far from protosuchids, at the base of the Aetosauria (Fig. 4). Protosuchids are terminal taxa also arising form small bipedal ancestors.

Figure 3. Hemiprotosuhus image from Desojo and Ezccura 2016. Colors added. This taxon is derived from Ticinosuchus, basal to aetosaurs.

Figure 4. Hemiprotosuhus image from Desojo and Ezccura 2016. Colors added. This taxon is derived from Ticinosuchus, basal to aetosaurs.

Adding Coloradisuchus
to the large reptile tree (LRT, 1737+ taxa) nests it between the much larger Early Jurassic Dibothrosuchus (Fig. 2) and the similarly-sized Middle Triassic Gracilisuchus (Fig. 3). These taxa also share a fenestra between the naris and antorbital fenestra, though much narrower than in protosuchids and Coloradisuchus. Martinez, Alcober and Pol did not test Dibothrosuchus and Gracilisuchus in their abbreviated cladogram consisting only of protosuchids and putative protosuchids.

Figure 2. Protosuchus skull. The high cranium and low triangular rostrum evidently made Bonaparte 1969 consider Hemiprotosuchus similar enough to Protosuchus.

Figure 5. Protosuchus skull. The high cranium and low triangular rostrum evidently made Bonaparte 1969 consider Hemiprotosuchus similar enough to Protosuchus.

Martinez, Alcobar and Pol note:
“The only known Triassic record of Protosuchidae is Hemiprotosuchus leali, from the upper levels of the middle Norian Los Colorados Formation (Bonaparte, 1971; Kent et al., 2014), and a putative, unnamed protosuchid from thelate Norian–Rhaetian Quebrada del Barro Formation (Martınez et al., 2015), both from northwestern Argentina.” 

Figure x. Subset of the LRT focusing on Euarchosauriformes and Crocodylomorpha.

Figure x. Subset of the LRT focusing on Euarchosauriformes and Crocodylomorpha.

With Hemiprotosuchus now nesting with coeval aetosaurs,
Coloradisuchus in the Triassic nests temporally and phylogenetically apart from other protosuchids. Unfortunately, due to preservation issues (Fig.1), relatively few traits can be scored for Coloradisuchus. Even so, moving Coloradisuchus to the protosuchid lineage adds five steps. That may change with further study or better data. Let’s keep working on this one.


References
Martinez RN, Alcober OA and Pol D 2017. A new protosuchid crocodyliform (Pseudosuchia, Crocodylomorpha) from the Norian Los Colorados Formation, northwestern Argentina. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1491047.

 

4 thoughts on “Restoring the little crocodylomorph, Coloradisuchus

  1. Your reconstruction of the Ticinosuchus skull bears little resemblance to the actual skull of Ticinosuchus. Just saying.

      • Well….Sterling actually saw the specimen. I would regard his interpretation to be more robust. You agree, I assume?

      • Chris: you are arguing on authority, not evidence. Take a look at Sterling’s paper. He identified one premaxilla as a “?right lacrimal” (his question mark, not mine). He did not identify a premaxilla. Two are clearly present. His mind was not ready for the possibility of a triangular, aetosaur-like premaxilla, so he was stymied. That’s the value of coloring bone elements. You find out you have four lacrimals and no premaxillae. Then you realize if you move two of those purported ‘lacrimals’ (the ones with little teeth on one rim) to the front of the skull… they fit! And they fit phylogenetically. So, I do not consider his interpretation more robust, but by his own indication, more tentative.

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