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The skull of Litorosuchus in detail

Monday, November 14, 2016 11:27
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(Before It's News)

Earlier we looked at Litorosuchus (Li et al. 2016; Figs. 1, 2), a new macrocnemid 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, but the nesting of Litorosuchus did not change from within its Macrocnemus nesting. Many traits were added from the palate.

Litorosuchus still nests between two Macrocnemus species, which currently makes the genus, Litorosuchus, a junior synonym. However, I would suggest that each of the Macrocnemus specimens now known are distinct enough to represent distinct genera, so Litorosuchus will be the first new genus named for this clade and hopefully others will follow.

Figure 1. Litorosuchus in situ and as originally traced. See text for new interpretations of certain bones.

Figure 2. Litorosuchus in situ and as originally traced. See text for new interpretations of certain bones. Note how color helps to visually segregate bones from one another and helps identify displaced smaller bones.

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 their ancestors, the macrocnemids. 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, a trait often seen in pterosaurs, which also arise from Macrocnemus.
  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 displaced quadratojugal is smaller than a single sclerotic ossicle, as in Macrocnemus and pterosaurs.

Hopefully this specimen
will draw attention to the origin of the lepidosaur tritosaur fenestrasaur clade that also has a diapsid temporal architecture and an antorbital fenestra, but is unrelated to archosauriforms according to the LRT, now at 866 taxa.

It remains important
that sister taxa look alike when nested in cladograms. If they don’t look similar, then please expand your inclusion set! Remember, we’re dealing with microevolution here.

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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