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

Lethiscus: oldest of the tetrapod crown group?

Figure 1. Lethiscus stock skull, drawing from Pardo et al. 2017 and colorized here.

Figure 1. Lethiscus stocki skull, drawing from Pardo et al. 2017 and colorized here. Note the loss of the postfrontal and the large orbit. Pardo et al. nest this taxon between Acanthostega and Pederpes in figure 3. There is very little that is plesiomorphic about this long-bodied legless or virtually legless taxon. Thus it should nest as a derived taxon, not a basal plesiomorphic one.

Pardo et al. 2017
bring us new CT scan data on Lethiscus stocki (Wellstead 1982; Viséan, Early Carboniferous, 340 mya) a snake-like basal tetrapod related to Ophiderpeton (Fig. 2) in the large reptile tree (LRT, 1018 taxa), but with larger orbits.

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

Figure 2. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Lethiscus is indeed very old (Middle Viséan)
but several reptiles are almost as old and Tulerpeton, a basal amniote, comes from the even older Late Devonian. So the radiation of small burrowing and walking tetrapods from shallow water waders must have occurred even earlier and Tulerpeton is actually the oldest crown tetrapod.

Figure 2. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles). None of this is supported by the LRT.

Figure 3. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles), resurrecting an old idea not supported in the LRT. Actually not much of this topology is supported by the LRT.

Pardo et al. nested Lethicus
between Acanthostega (Fig. 4) and Pederpes (Fig. 3) using a matrix that was heavily weighted toward brain case traits. Ophiderpeton and Oestocephalus (Fig. 2) were not included in their taxon list, though the clade is mentioned in the text: “Overall, the skull morphology demonstrates underlying similarities with the morphologies of both phlegethontiid and oestocephalid aïstopods of the Carboniferous and Permian periods.” So I’m concerned here about taxon exclusion. No other basal tetrapods share a lateral temporal fenestra or share more cranial traits than do Lethiscus, OphiderpetonOestocephalus and RileymillerusAll bones are identified here as they are in Pardo et al. so bone ID is not at issue. I can’t comment on the Pardo team’s braincase traits because so few are examined in the LRT. Dr. Pardo said they chose taxa in which the brain case traits were well known and excluded others.

Figure 4. Acanthostega does not have much of a neck.

Figure 4. Acanthostega is basal to Lethiscus in the Partdo et al. tree.

Pardo et al. considered
the barely perceptible notch between the tabular and squamosal in Lethiscus (Fig. 1) to be a “spiracular notch” despite its tiny size. I think they were reaching beyond reason in that regard. They also note: “The supratemporal bone is an elongate structure that forms most of the dorsal margin of the temporal fenestra, and is prevented from contacting the posterior process of the postorbital bone by a lateral flange of the parietal bone.” The only other taxon in the LRT that shares this morphology is Oestocephalus, Together they nest within the Lepospondyli (Fig. 3) in the LRT. I think it is inexcusable that Pardo et al. excluded  Ophiderpeton and Oestocephalus. 

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Summarizing,
Pardo et al. report, “The braincase and its dermal investing bones [of Lethiscus] are strongly indicative of a very basal position among stem tetrapods.”  and “The aïstopod braincase was organized in a manner distinct from those of other lepospondyls but consistent with that seen in Devonian stem tetrapods.” It should also be noted that the skull, body and limbs were likewise distinct from those of other lepospondyls, yet they still nest with them in the LRT because no other included taxa (1018) share more traits. ‘Distinct’ doesn’t really cut it, in scientific terms. As I mentioned in an email to Dr. Pardo, it would have been valuable to show whatever bone in Lethiscus compared to its counterpart in Acanthostega and Oestocephalus if they really wanted to drive home a point. As it is, we casual to semi-professional readers are left guessing.

Pardo et al. references the clade Recumbirostra.
Wikipedia lists a number of microsaurs in this clade with Microbrachis at its base, all within the order Microsauria within the subclass Leposondyli. Pardo et al. report, “Recumbirostrans and lysorophians are found to be amniotes, sister taxa to captorhinids and diapsids.” The LRT does not support this nesting. Pardo et al. also report, “This result is consistent with early understandings of microsaur relationships and also reflects historical difficulties in differentiating between recumbirostrans and early eureptiles.” Yes, but the later studies do not support that relationship. Those early understandings were shown to be misunderstandings that have been invalidated in the LRT and elsewhere, but now resurrected by Pardo et al.

Ophiderpeton granulosum (Wright and Huxley 1871; Early Carboniferous–Early Permian, 345-295mya; 70cm+ length; Fig. 2, dorsal view)

Oestocephalus amphiuminus (Cope 1868; Fig. 2,  lateral views) is known from tiny immature and larger mature specimens.

Figure 7. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

Figure 5. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

A side note:
The recent addition of several basal tetrapod taxa has shifted the two Phlegethontia taxa (Fig.5) away from Colosteus to nest with Lethiscus and Oestocephalus, their traditional aistopod relatives. That also removes an odd-bedfellow, tiny, slender taxon from a list of large robust stem tetrapods.

References
Pardo JD,Szostakiwskyj M, Ahlberg PE and Anderson JS 2017. Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966
Wellstead CF 1982. A Lower Carboniferous aïstopod amphibian from Scotland. Palaeontology. 25: 193–208.
Wright EPand Huxley TH 1871. On a Collection of Fossil Vertebrata, from the Jarrow Colliery, County of Kilkenny, Ireland. Transactions of the Royal Irish Academy 24:351-370

wiki/Acherontiscus
wiki/Adelospondylus
wiki/Adelogyrinus
wiki/Dolichopareias
wiki/Ophiderpeton
wiki/Oestocephalus
wiki/Rileymillerus
wiki/Acherontiscus