SVP 2021 abstracts – 02: Origin of amniotes

Marjanović 2021 adds more taxa and characters
to better understand the origin of Amniota. Unfortunately, too few taxa.

Marjanović reports,
“Ever since phylogenetic analysis was introduced to the study of early limbed vertebrates in the mid-1990s, there have been exactly two types of such analyses: those that extend from the origin of limbs to the origin of Amniota (but barely reach into Amniota or Diadectomorpha) and
those that try to investigate amniote phylogeny and barely reach beyond Amniota.”

Make that exactly three such analyses. The large reptile tree (LRT, 1988+ taxa; subset Fig. 2) extends far beyond both limits.

“Yet, some questions have recently surfaced that cannot be researched if this dichotomy is
upheld; whether Seymouriamorpha or a grouping of mostly “microsaurs” is closer to the amniote–diadectomorph clade, whether at least some “microsaurs” are stem-sauropsid amniotes, and the monophyly and position of Diadectomorpha (in the stem group of Pan-
Amniota or that of Pan-Mammalia).”

By adding taxa the LRT nests diadectomorphs deep within the new Lepidosauromorpha. Neither microsaurs nor seymouriamorphs nests closer to the last common ancestor of all reptiles, Silvanerpeton (Fig. 1). The LRT recognizes no clade ‘Sauropsida‘.

In the LRT microsaurs include limbless caececilians. Salamanders and frogs nest together with other lepospondyls like Dissorophus and Dendrerpeton.

“Seymouriamorpha is placed next to the amniote–diadectomorph clade with very weak support.”

In the LRT Seymouriamorpha nests alongside Reptilomorpha with strong support.

“Successively less close lie Amphibia, Solenodonsaurus, Chroniosuchia, Temnospondyli, a “gephyrostegid” grade, Anthracosauria, Caerorhachis and the “Parrsboro jaw”, and a baphetid-colosteid clade.

In the LRT Solenodonsaurus and Chroniosuchia nest with Reptilia within Archosauromorpha as amphibian-like reptiles. The others nest in a broad swath at various points between basalmost tetrapods and Reptilia.

“Westlothiana is found as an amphibian, Casineria as a temnospondyl or a “gephyrostegid”.

In the LRT (Fig. 3) Westothiana nests within the Archosauromorpha. So does Casineria. More taxa will resolve this issue in Marjanovic’s study.

“Further redescriptions of taxa and of character complexes will be necessary, though not sufficient, to improve our understanding of amniote origins. My results also underscore the still
underestimated importance of “microsaurs” as close amniote relatives, and the closeness of the origins of Amniota and Lissamphibia – a narrow gap largely bridged by Albanerpetidae, Lysorophia and the “microsaurs”.

Marjanovic does not mention the reptilomorphs recovered by the LRT (Fig. 2), including the oldest one, Late Devonian, Tulerpeton (Fig. 1).

“Depending on the exact size of this gap, extant amphibians may be better model organisms for amniotes like ourselves than used to be thought: phylogenetics is important even for biomedical research.”

Doubtful. The LRT does not support this conclusion.

References
Marjanović D 2021. The origin of Amniota in phylogenetic context. Journal of Vertebrate Paleontology abstracts: pp. 177–178.