The skull of Litorosuchus in detail

Revised November 17, 2018
with a new nesting for Litorosuchus next to the protorosaur, Jaxtasuchus.

Earlier we looked at Litorosuchus (Li et al. 2016; Figs. 1, 2), a new protorosaur with an antorbital fenestra that was originally considered to be an aquatic basal archosauriform, nesting with the thalattosaur, Vancleavea, which was also considered to be an aquatic basal archosauriform. Unfortunately, neither resembles each other and neither resembles any other archosauriform because they both nest elsewhere in the family tree of reptiles when given the opportunity. Both suffered from academic taxon exclusion.

Figure 1. Litorosuchus skull reconstructed from tracings in figure 2. That antorbital fenestra does not make it an archosaurifom. At least two other clades also produce an antorbital fenestra.

Figure 1. Litorosuchus skull reconstructed from tracings in figure 2. That antorbital fenestra does not make it an archosaurifom. At least two other clades also produce an antorbital fenestra. The gracile temporal bones are in contrast to the robust maxilla and long teeth.

Today,
thanks to M. Mortimer, I have the paper which includes closeups of the skull (Fig. 2). I’ll start off by saying I was able to add or change 36 scores for Litorosuchus with the new data. Many traits were added from the palate.

Litorosuchus nests
the armored aquatic protorosaur, Jaxtasuchus.

Figure 2. Litorosuchus in situ with a new tracing of the inverted and displaced posterior premaxilla with several teeth.

Figure 2. Litorosuchus in situ with a new tracing of the inverted and displaced posterior premaxilla with several teeth.

First of all,
this is an excellent specimen. And virtually all the bones are well exposed enabling the creation of an accurate reconstruction (Fig. 1). Unfortunately, when Li et al. saw an antorbital fenestra they assumed Litorosuchus was related to archosauriformes and so excluded unrelated taxa that also have an antorbital fenestra, like fenestrasaurs and derived protorosaurs. As a result, some bones not found in archosauriformes were ignored originally in Litorosuchus.

  1. The Li et al. lacrimal is actually the medial descending process of the nasal
  2. The Li et al. surangular is actually several posterior mandible bones, including the coronoid, surangular and articular.
  3. The actual lacrimal is a small bone displaced to the middle portion of the right nasal. It is small and somewhat tear-shaped, as in Macrocnemus.
  4. The Li et al. jugal is too deep because part of it includes a slender pterygoid.
  5. The small postfrontal, supratemporal and squamosal were not identified by Li et al. but are indeed present.
  6. A tiny pineal opening is present.
  7. The straight and robust quadratojugal is still firmly attached to the posterior jugal, doubling its length.

This turned out to be
an overlooked opportunity for the Li team, who, unfortunately, restricted their inclusion set to just archosauriforms and their outgroups (plus one by-default nested thalattosaur and two by-default nested pterosaurs).

References
Li C, Wu X-C, Zhao L-J, Nesbitt SJ, Stocker MR, Wang L-T 2016. A new armored archosauriform (Diapsida: Archosauromorpha) from the marine Middle Triassic of China, with implications for the diverse life styles of archosauriforms prior to the diversification of Archosauria. The Science of Nature 103: 95. doi:10.1007/s00114-016-1418-4
Nesbitt SJ 2011. The early evolution of archosaurians: relationships and the origin of major clades. Bull Amer Mus Nat Hist 352:1–292.
Nesbitt SJ, Stocker MR, Small BJ and Downs A 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnean Society 157 (4): 814–864. doi:10.1111/j.1096-3642.2009.00530.x.

 

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