Westlothiana 2026

Thursday, October 1, 2026 0:17

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From the Igielman et al 2026 abstract:
“Tetrapods (limbed vertebrates) include lissamphibians and amniotes, and now comprise half of all vertebrate species, with particular importance on land. Limbs first appeared in stem tetrapods by the Late Devonian, 365 million years ago^1,2,3,4,5.

Note: from this time period a derived tetrapod close to the origin of the clade Reptilia is Tulerpeton (Fig 1). This phylogenetic evidence indicates a rather long run of tetrapods from earlier in the Devonian. The first tetrapod tracks are known from the Middle Devonian.

Figure 1. Tulerpeton restored based on the bauplan of Silvanerpeton and to the same scale.

“However, many taxa either have limbs with fin-like functional anatomy or have secondarily lost them, raising questions about the ecological context and early evolution of the tetrapod limb⁶.

“Many taxa” is EXTREMELY vague. Be more specific. Plesiosaurs have a fin-like functional anatomy.

Figure 2. Colosteus relatives according to the LRT scaled to a common skull length. Their sizes actually vary quite a bit, as noted by the different scale bars. Note the tiny limbs with fingers and toes. Many studies overlook and omit these late-surviving Carboniferous taxa.

Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Four fingers turns out to be the primitive number. Five is a recent mutation, convergent in Proterogyrinus and Reptilomorpha, like Tulerpeton in figure 1.

“Here we provide considerable new data on the enigmatic Early Carboniferous Westlothiana lizziae, widely cited as the earliest terrestrially adapted stem amniote^7,8.

Considerable new data is always welcome.

In the large reptile tree (LRT, 2340 taxa) Westlothiana is one of several members of the Archosauromorpha – Reptilia from the Viséan, Early Carboniferous. Coeval Silvanerpeton (Fig 1), is the last common ancestor of all reptiles (Amniota is a junior synonym) in the LRT.

“We find unexpected plesiomorphies of the skull roof, palate, braincase and mandible, alongside aquatic adaptations, including an ossified internal gill skeleton and extensive denticles on the palate and mandible. These traits return Westlothiana as a stem tetrapod in our phylogenetic analyses, demonstrating that terrestrial adaptations such as an amniote-like pedal formula, loss of fin-like forelimb function and superficially claw-like terminal phalanges evolved in a mosaic fashion, and that these traits evolved in a transitional, amphibious context before the origin of the tetrapod crown group.

This Igielman et al study suffered from too few taxa compared to the LRT. Several LRT reptiles, including Diadectes and Limnoscelis nested as pre-reptiles in the Igielman et al study. Among the basalmost tetrapods and their ancestors, Panderichthys, Trypanognanthus, Colosteus and Pholidogaster (Fig 2) are missing from the authors’ cladogram.

re: an ossified internal gill skeleton

The supplementary data featuring palatal elements do not label any bones as ‘gill skeleton’.
I do note robust cervical ribs. I hope those are not misidentified.

re: extensive denticles on the palate and mandible.

The huge number of short palatal teeth is impressive. Not as impressive as the palate of the leatherback sea turtle (Dermochelys), but still impressive and in smaller size such palatal teeth are common to closely related taxa, like Gephyrostegus, in the LRT.

“Our findings reinforce recent suggestions of a taxonomically inclusive tetrapod-stem lineage, but also illuminate substantial gaps in anatomical knowledge, especially for small-bodied taxa, that obstruct our understanding of early tetrapod evolution.”

Adding more taxa would illuminate those substantial gaps. Cherry-picking too few taxa almost always leads one down the wrong path.

A reconstruction of the in situ elements (Fig 4) would also be helpful.

Figure 4. Westlothiana reconstructed. Gray area indicates a gravid (egg-bearing) female indicated by the short area with long dorsal ribs and the long area with short dorsal ribs enabling abdominal expansion.

Bottom line:
Westlothiana (Fig 4) is well known from head to tail. Casineria (Fig 5) and Brouffia (Fig 6) are related archosauromorph reptiles with long lumbar regions in the LRT, but not included in the authors’ cladogram.

Figure 5. Casineria reconstructed. Note the large lumbar area, a trait common to early reptiles.

Figure 6. Brouffia reconstruction. This is another basal archosauromorph reptile in the LRT.

References
Igielman B et al 2026. Evolution of terrestrial anatomy revealed by a derived stem tetrapod. Nature. https://doi.org/10.1038/s41586-026-11090-6

Source: https://pterosaurheresies.wordpress.com/2026/10/01/westlothiana-2026/

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