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.
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.
Laosuchus naga traits include:
- an extremely reduced pineal foramen
- absence of palatal dentition
- well-developed transverse flange of the pterygoid that contacts the maxilla
- internal crest on and above the dorsal side the palate
- otic notch closed by the tabular horn and the posterior part of the squamosal, forming a continuous wall
- thin and high ventromedial ridge on parasphenoid.
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.
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