Laosuchus naga enters the LRT

Updated January 30, 2019
with a new nesting of Laosuchus between Eryops and the Cochleosaurus clade, not as a chroniosuchid, as originally nested.

The question today is:
what are chroniosuchians? Are they reptiles or not? Arbez, Sidor and Steyer 2018 say: ‘not’ (Fig. 1). Here that mistake is due to tradition and taxon exclusion, based on their cherry-picked outgroups. Heretically. chroniosuchians are amphibian-like reptiles.

Figure 1. Cladogram from xx 2018 with Laosuchus nesting with chroniosuchians in the absence of Solenodonsaurus.

Figure 1. Cladogram from Arbez, Sidor and Steyer 2018 with Laosuchus nesting with chroniosuchians in the absence of Solenodonsaurus.

Arbez, Sidor and Steyer report from their abstract:
“Chroniosuchians were a clade of non-amniotic tetrapods known from the Guadalupian (middle Permian) to Late Triassic, mainly from Russia and China.” Asaphestera is the proximal outgroup followed by Limnoscelis, Seymouria, Gephyrostegus and other taxa.

By contrast and using more outgroup taxa
the large reptile tree (LRT 1391 taxa, ) nests chroniosuchians within the base of the archosauromorph branch of reptiles. When more taxa are included in the LRT, Limnoscelis and Gephyrostegus nest as reptiles (= amniotes) while Asaphestra and Seymouria nest as unrelated traditional microsaur lepospondyls and seymouriamorphs respectively.

Arbez, Sidor and Steyer 2018 introduce a new taxon,
Laosuchus naga (Fig 3), as a long-snouted chroniosuchian, but here nests with long-snouted eryopid temnospondyls. 

Figure 1. Laosuchus in dorsal and lateral views. Colors added with some difficulty here as all the bones are fused and their surfaces are ornamented.

Figure 1. Laosuchus in dorsal and lateral views. Colors added with some difficulty here as all the bones are fused and their surfaces are ornamented.

Laosuchus naga traits include:

  1. an extremely reduced pineal foramen
  2. absence of palatal dentition
  3. well-developed transverse flange of the pterygoid that contacts the maxilla
  4. internal crest on and above the dorsal side the palate
  5. otic notch closed by the tabular horn and the posterior part of the squamosal, forming a continuous wall
  6. thin and high ventromedial ridge on parasphenoid.
Figure 4. Solenodonsaurus skull in situ and reconstructed. That brown bone on top of the frontal/parietal suture is a displaced lacrimal that nicely fills the gap in the reconstruction.

Figure 4. Solenodonsaurus skull in situ and reconstructed. That brown bone on top of the frontal/parietal suture is a displaced lacrimal that nicely fills the gap in the reconstruction.

Something I learned while reexamining Solenodonsaurus
The displaced bone atop the skull is actually part of the broken lacrimal. The quadratojugal is displaced on the posterior mandible. The prefrontal is broken but not very displaced. The posterior jugal is broken into several pieces. Using DGS allows one to cut and paste and fit these puzzle pieces back into the missing parts of the skeleton where they belong. If they don’t fit, they don’t belong, but they never fit perfectly. It’s like putting Humpty Dumpty together again. There are always a few pieces left over.

References
Arbez T, Sidor CA and Steyer J-S 2018. Laosuchus naga gen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology. Journal of Systematic Palaeontology DOI: 10.1080/14772019.2018.1504827

http://zoobank.org/urn:lsid:zoobank.org:pub:11D07FA3-0F4C-4EF9-A416-E8E6BE76C970

 

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