SVP abstracts – 12: on larval stem tetrapods

Pardo and Mann 2022 reported,
“Tetrapoda consists of two major lineages, Amniota and Lissamphibia.”

The large reptile tree (LRT, 2171 taxa) does not recover this basal dichotomy. Amniota is a junior synonym for Reptilia. In addition, Stegocephalia, Temnospondylii, Reptilomorpha and Microsauria are missing from the Pardo and Mann list of Tetrapod lineages — IF Tetrapoda includes all taxa with fingers rather than fins (but see below for various definitions).

“Each of these is characterized by novel life histories (the amniotic egg and biphasic metamorphosis, respectively) that serve as adaptations to a life on land. Although it is clear that amniote embryogenesis is novel within vertebrata, it is unclear whether amphibian metamorphosis is similarly novel or whether amphibian-like biphasic metamorphosis was present in the earliest tetrapods.”

Evidently lacking are small/ juvenile versions of adult stem tetrapods (but how do the authors define this clade? See below). Is this true? I know we have juvenile basal tetrapods (e.g. Greererpeton), which do not have adult proportions in the skull (Fig 2).

Mazon Creek strata represents “approximately 309 million years ago in the mid-Pennsylvanian epoch of the Carboniferous period”, or 51 million years after the Late Devonian appearances of the earliest tetrapods and reptiliomorphs (e.g. Tulerpeton).

Just to be clear, stem tetrapods should include Panderichthys, Tiktaalik and other pre-finger air-breathing taxa, not including lungfish. However, when you look up the definition Acanthostega, Ichthyostega and Tulerpeton are included. These all have four pods (= legs) with fingers and toes.

Wikipedia also reports,“The stem tetrapods may also include one or both of Temnospondyli and Lepospondyli, depending on author.”

And in case you missed this phrase without it sinking in, the authors reported, “delayed morphogenesis of the pectoral and pelvic appendages…” That sounds like no arms and legs, nor any pectoral or pelvic fins. The authors do not tell us which genus this embryo most closely resembles based on skull and axial traits, leaving this abstract in the vague, but hopeful nebula of suprageneric taxa.

“Absence of external gills, which are widely preserved in definitive stem-lissamphibians from the same locality, strongly suggests that stem-tetrapods lacked a lissamphibian-like metamorphosis (tadpole stage), and that metamorphosis did not play an important role in the initial tetrapod terrestrialization process. This also bolsters an emerging recognition that thyroid-mediated metamorphoses in diverse vertebrate lineages evolved convergently.”

Looking forward to seeing this paper: 1) to see the late-surviving, embryo stem tetrapods lacking fins or fingers), and 2) to seeing how the authors define their terms.

No matter how you view or judge the results presented here,
I hope all readers gain insight into the way paleontologists report results. Look for what isn’t in the text, but should be in the text. Try to be specific rather than vague. Try to run an analysis before presenting results. And try to include as many competing candidate taxa as necessary when you are dealing with an enigma or an embryo.

References
Pardo JD and Mann A 2022. Exceptionally preserved larval stem tetrapods from Mazon Creek. SVP abstracts 2022: 275.

wiki/Stem_tetrapoda
wiki/Tetrapod