Litorosuchus: an armored protorosaur, not an archosauriform

Revised November 17, 2018
with a new nesting of Litorosuchus with the aquatic, armored protorosaur, Jaxtasuchus.

Li et al. 2016 describe
a Middle Triassic (240mya) semi-aquatic reptile,  Litorosuchus somnii based on a complete and articulated skeleton (Fig. 1) together with hard and soft dermal tissue (see below). Note the antorbital fenestra without a fossa. That’s a key trait that led the Li team astray.

Figure 1. Litorosuchus somnii was wrongly considered a sister to Vancleavea and wrongly considered an archosauriform. In the LRT it nests with Macrocnemus, a tritosaur lepidosaur that also has members with an antorbital fenestra. Click to see an enlarged rollover image.

Figure 1. Litorosuchus somnii was wrongly considered a sister to Vancleavea and wrongly considered an archosauriform. In the LRT it nests with Macrocnemus, a tritosaur lepidosaur that also has members with an antorbital fenestra. Click to see an enlarged rollover image.

Unfortunately 
Li et al. nested Litorosuchus with the armored and aquatic Vancleavea (Nesbitt et al 2009, Fig. 2), which they still insist is an aberrant stem archosaur (Nesbitt et al. 2009, Nesbitt 2011) despite lacking an upper temporal or antorbital fenestera, among a long list of other autapomorphies. Five years ago here Vancleavea was nested with Helveticosaurus in the Thalattosauria in the large reptile tree (LRT) where it remains today. Taxon exclusion burned the Nesbitt et al. 2009 study on Vancleavea and the Nesbitt 2011 study on archosaurs. Two of those team members (Nesbitt and Stocker) are also on the Li et al. 2016 team. Sadly, Li et al. 2016 got burned again for the same reason.

Vancleavea campi

Figure 2. Vancleavea skeleton, sans osteoderms.

Taxon exclusion
So certain that Litorosuchus was an archosauriform, Jaxtasuchus was not included in the Li et al. study matrix (based on Nesbitt et al. 2009 and additions thereafter) despite their overall resemblance to the new taxon. In the LRT Litorosuchus nests strongly within Jaxtasuchus, (Fig. 3) another clade that has members with an antorbital fenestra, a trait that appears at least 4 times within the Reptilia.

Figure 2. The armored aquatic protorosaur, Jaxtasuchus, is smaller and less complete than Litorosuchus.

Figure 3. The armored aquatic protorosaur, Jaxtasuchus, is smaller and less complete than Litorosuchus.

Let’s take a look at the Li et al. abstract,
then discuss the protorosaur affinities of the new taxon.

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

Figure 4. Litorosuchus in situ with a new tracing of the inverted and displaced posterior premaxilla with several teeth.

From the Li et al. abstract:
“Reptiles have a long history of transitioning from terrestrial to semi-aquatic or aquatic environments that stretches back at least 250 million years. Within Archosauria, both living crocodylians and birds have semi-aquatic members. Closer to the root of Archosauria and within the closest relatives of the clade, there is a growing body of evidence that early members of those clades had a semi-aquatic lifestyle [1]. However, the morphological adaptations to a semi-aquatic environment remain equivocal in most cases. Here, we introduce a new Middle Triassic (245–235 Ma) archosauriform, Litorosuchus somnii, gen. et sp. nov., based on a nearly complete skeleton from the Zhuganpo Member (Ladinian [241–235 Ma]) of the Falang Formation, Yunnan, China. Our phylogenetic analyses suggest [2] that Litorosuchus is a stem archosaur closely related to the aberrant Vancleavea just outside of Archosauria. The well-preserved skeleton of L. somnii bears a number of morphological characters consistent with other aquatic-adapted tetrapods including: a dorsally directed external naris, tall neural spines and elongate chevrons in an elongated tail, a short and broad scapula, webbed feet, long cervical vertebrae with long slender ribs, and an elongated rostrum with long and pointed teeth [3]. Together these features represent one of the best-supported cases of a semi-aquatic mode of life for a stem archosaur [4]. Together with Vancleavea campi, the discovery of L. somnii demonstrates a growing body of evidence that there was much more diversity in mode of life outside Archosauria. Furthermore, L. somnii helps interpret other possible character states consistent with a semi-aquatic mode of life for archosauriforms, including archosaurs.” [5]

Notes

  1. Why restrict the taxon search to the clade Archosauria (crocs + dinos) a priori when Litorosuchus looks nothing like any of them? Better to add the new taxon to a large gamut analysis and let the scores determine the clade nesting, especially knowing that an antorbital fenestra, if present, is convergent over several unrelated clades.
  2. The results only ‘suggest’ [we call this a weasel-word because you can weasel your way out of it] because Litorosuchus bears no resemblance to its ‘by default‘ sister taxa. In the LRT the nesting is sure and secure, supported by a long list of traits used as evidence and readily apparent just by looking (Fig. 3).
  3. These are the basic traits of Jaxtasuchus (Fig. 3). Someone evidently had their blinders on.
  4. Stem archosaurs (sensu LRT, crocs + dinos only) were bipedal and terrestrial.  Basal archosauriformes (sensu LRT, like Proterosuchus) were indeed semi-aquatic. Jaxtasuchus also had a semi-aquatic lifestyle. Perhaps they competed.
  5. Jimi Hendrix said it best, “Castles made of sand slip into the sea, eventually.” The only question is, how long with the Nesbitt, Stocker excluded taxon cladogram take to find more solid footing with a larger gamut matrix?

I only had to take a look at the foot of Litorosuchus
to know that the Li team had missed a golden opportunity to discuss protorosaur affinities. The long pointed skull with the long premaxillary ascending process together with the long neck and short limbs all link Litorosuchus to Jaxtasuchus, not Vancleavea. The length and depth of the tail occur within this clade, along with the presence of dermal  armor. The antorbital fenestra was present in Jaxtasuchus and Pamelaria.

Diandongosuchus nests as a basal phytosaur when choristoderes and basal younginoids are included, far from Qianosuchus, which also does not nest with poposaurs, which are all bipedal (or formerly bipedal) herbivores, a far cry from Diandongosuchus.

Figure 4. Diandongosuchus nests as a basal or stem phytosaur and was coeval with Litorosuchus.

Diandongosuchus is discussed within the Li et al. text
because it is semi-aquatic, coeval and an archosauriform. However it is unrelated to archosaurs except through Youngina UC1528 and basalmost Proterosuchus specimens. Diandongosuchus is therefore unrelated to Litorosuchus in the LRT except through archosauriform reptiles. And yes, Nesbitt and Stocker claimed credit for discovering that Diandongosuchus (Fig. 4) was a basal phytosaur when you heard that here first, four years ago. Either this is proof that they don’t Google or proof that they like to stick to outmoded paradigms while ignoring the greater evidence.

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.

 

1 thought on “Litorosuchus: an armored protorosaur, not an archosauriform

  1. I’m not comfortable with Litorosuchus being by Vancleavea either (I say add it to Ezcurra’s archosauromorph matrix), but your “plumes” on the tail are obviously the neural arches with tall neural spines, while that area on the back are overlapping osteoderms. You got a ton of things wrong otherwise too, probably since you don’t have the actual paper with its excellent high res images. I’ll send it to you, and look forward to your revised opinion.

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