You heard it here first: Recumbirostran ‘microsaurs’ are not amniotes

Reisz, Maho and Modesto 2024 reported,
“For over three decades, Amniota has been treated as a crown group, but the content of stem amniotes has varied among large-scale analyses of Palaeozoic tetrapods.”

That content has not wavered in the large reptile tree (LRT, Fig 1) for the past 12 years.

“Nine recent phylogenetic studies on Palaeozoic tetrapods, all derived from each other, have potentially changed our perspectives on amniote origins and early evolution. The first of these analyses posited that two particular clades of Palaeozoic tetrapods, Recumbirostra and Lysorophia, both previously considered to be lepospondyl stem-amniotes, are actually amniotes.”

That hypothesis was never confirmed or recovered by the LRT.

“This raises the possibility that early amniotes included both small fossorial and terrestrial animals, some of which show strong evidence of limb reduction.”

These authors are not naming a last common ancestor genus for Reptilia = Amniota. That simple process resolves ALL such problems.

Try not to be nebulous and cagey like this. No basal amniotes in the LRT are fossorial (burrowing). All (likely) maintained a swampy niche. Silvanerpeton (Viséan, Early Carboniferous) is the last common ancestor of all reptiles (Amniota is a junior synonym) in the LRT.

“Here we show that the strange position of these small, often fossorial and limb-reduced tetrapods in Amniota is the result of two fundamental flaws: (1) the phylogenetic analysis combines two separate data matrices that were each designed to test only patterns of relationships among early anamniote tetrapods; and (2) the absence of key amniote characters.”

The key characters the authors seek, or think they know beforehand, will actually make themselves known only _after_ the last common ancestor of reptiles is established. Do this first. Everything else will fall into place.

“Our reanalysis of Palaeozoic tetrapod interrelationships reinstates recumbirostrans and lysorophians outside Amniota.”

In the LRT these taxa have always nested outside the Reptilia/Amniota,
within the Microsauria (Fig 1).

“The results of our study impact other recent studies that have simply accepted the codings in the data matrix that resulted in the hypothesis that recumbirostrans are amniotes and raises important questions about the initial diversification of amniotes and the evolution of fossoriality and limb reduction among tetrapods.”

More nebulous chatter. Any taxon nesting outside of Silvanerpaton nests outside the Reptilia/Amniota. It’s that simple.

According to Wikipedia (Recumbirostra),
“Recumbirostra includes the families Pantylidae, Gymnarthridae, Ostodolepidae, Rhynchonkidae and Brachystelechidae, with additional families such as Microbrachidae and Molgophidae being included by some authors.”

These are all members of the Microsauria
and outgroup taxa to the Reptilomorpha in the LRT (Fig 1).

“Recumbirostra was erected as a clade in 2007 to include many of the taxa traditionally grouped in “Microsauria“, which has since been shown to be a paraphyletic or polyphyletic grouping.”

The LRT does not confirm that result. Rather Microsauria is a clade.

Figure 1. Subset of the LRT focusing on the origin of the Reptilia and Microsauria. ” data-image-caption=”

Figure 1. Subset of the LRT focusing on the origin of the Reptilia and Microsauria.

” data-medium-file=”https://pterosaurheresies.files.wordpress.com/2024/02/microsauria2024-588.jpg?w=100″ data-large-file=”https://pterosaurheresies.files.wordpress.com/2024/02/microsauria2024-588.jpg?w=341″ src=”https://pterosaurheresies.files.wordpress.com/2024/02/microsauria2024-588.jpg” alt=”Figure 1. Subset of the LRT focusing on the origin of the Reptilia and Microsauria. ” class=”wp-image-84193″ />

Coincidentally,
Marjanovic 2024 also reported on the origin of the Amniota in a preprint also presented as an abstracts at SVP 2021 and 2023. The title indicates this author does not yet realize Amniota is a junior synonym. Even so, like Reisz, Maho and Modesto, Marjanovic reports, “Still, it appears that the recently proposed membership of many traditional “microsaurs” in Amniota can be excluded with reasonable confidence.”

Marjanovic’s cladogrram was hobbled by taxon exclusion. Perhaps that is why his results nested a dozen reptiles outside the “Amniota”.

References
Marjanovic D (preprint) 2024. The origin of Amniota in phylogenetic context. DOI:
10.31233/osf.io/tja8b
Reisz RR, Maho T and Modesto SP 2024. Recumbirostran ‘microsaurs’ are not amniotes. Journal of Systematic Palaentology 22(1): Article: 2296078

wiki/Recumbirostra