SVP abstracts 16: A 3D aïstopod points to yet another transition to land

Marjanović and Jansen 2020 suggest
a transition to terrestrial life independent from any crown-group tetrapods in the snake-like microsaur aîstopod clade. In the LRT that clade includes extant aquatic snake-like caecilians. In the LRT terrestrial and fossorial snakes likewise had aquatic ancestors by convergence.

From the Marjanović and Jansen 2020 abstract:
“A complete, articulated, three-dimensional and stunningly well-prepared skeleton from the Saar-Nahe basin (western Germany) phenetically resembles Oestocephalus, but achieves a lower head-to-body length ratio by possessing more elongate and more numerous vertebrae.”

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Continuing from the Marjanović and Jansen 2020 abstract:
“Despite the rather young ontogenetic age indicated by size and skull proportions, the shape range of the dorsal scales is that of Colosteus, including rhombic scales around the dorsal midline.”

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Figure 2. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Continuing from the Marjanović and Jansen 2020 abstract:
“As in the “nectridean” Keraterpeton, the dorsal scales bear microscopic honeycombed sculpture; we also report this in Oestocephalus.”

Figure 3. Keraterpeton, basal to the Diplocaulus clade in the LRT.

Figure 3. Keraterpeton, basal to the Diplocaulus clade in the LRT.

Continuing from the Marjanović and Jansen 2020 abstract:
“Such sculpture is also seen on the ventral scales of the new specimen, which are nonetheless as narrow as in other aïstopods.”

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The 'gill bones' are removed. They are gastralia.

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The ‘gill bones’ are removed. They are gastralia.

Continuing from the Marjanović and Jansen 2020 abstract:
“The presence of the braincase and the first complete, undistorted aïstopod palate is confirmed by μCT; hyobranchial bones, endochondral girdles or a tail-fin skeleton are absent. The tail tapers to a point, is not laterally flattened, and the scales do not leave room for a soft-tissue tail fin; no gill slit is apparent in the scale cover behind the head.”

These indicators of terrestrial life contrast with the mandibular lateral-line canal previously identified in Coloraderpeton and suggest that the new specimen, together with the phlegethontiids from the contemporaneous fossil forest floor of Chemnitz (eastern Germany), represents a transition to terrestrial life independent from any crown-group tetrapods.”

The basalmost taxon in this legless clade is nearly legless Acherontiscus, (Fig. 5) considered an aquatic animal due to a few lateral lines on the skull. Living legless microsaurs, the caecilians, are also secondarily aquatic. The authors consider their new taxon and Phlegethontia (Fig. 4) secondarily terrestrial.

In a similar fashion 
extant snake ancestors in the LRT were aquatic, making most living snakes secondarily terrestrial, by convergence. Derived sea snakes and others, like the water moccasin, went back to an aquatic existence making the snake-like morphology rather flexible with regard to niche.

Figure 6. Acherontiscus is a basal taxon in the aïstopod clade.

Figure 5. Acherontiscus is a basal taxon in the legless aïstopod clade.

Continuing from the Marjanović and Jansen 2020 abstract:
“Yet, despite the stem-tetrapodomorph plesiomorphies in the braincase, lower jaw and scales of Aïstopoda, a preliminary phylogenetic analysis of an improved and greatly enlarged dataset finds no support for a whatcheeriid-grade position, and less support for a more crownward colosteid-grade position (as recently proposed) than for an amphibian one.”

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 6. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Continuing from the Marjanović and Jansen 2020 abstract:
“Only Andersonerpeton, an isolated lower jaw described as an aïstopod, joins Densignathus in the whatcheeriid grade. Redescriptions of additional “nectrideans” and other supposed “lepospondyls” will be needed to resolve this conundrum.”

Figure 6. Living caecilian photo.

Figure 7. Living caecilian photo.

According to Wikipedia,
Aïstopoda include: Lethiscus, Ophiderpeton, Oestocephalus, Coloraderpeton and Phlegethontia among taxa tested by the large reptile tree (LRT, subset Fig. 6) nesting in the clade Microsauria. Aïstopods have been variously grouped with other lepospondyls, or placed at or prior to the batrachomorph-reptiliomorph divide. However, a cladistic analysis by Pardo et al. (2017) recovered Aistopoda at the base of Tetrapoda.

The aîstopod, Lethiscus, is from Viséan strata 340 mya,
coeval with Silvanerpeton, the last common ancestor of all reptiles in the LRT. There are no legless taxa proximal to reptiles in the LRT (subset Fig. 6).


References
Marjanović D and Jansen M 2020. A complete, three-dimensional early Permian aïstopod (Tetrapodomorpha) illuminates the phylogeny, ontogeny and terrestrialization of early limbed and limbless vertebrates. SVP abstracts 2020.

wiki/Aistopoda

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