Untangling the Sclerothorax chimaera

Tough one today
with many puzzle pieces.

The genus Sclerothorax
was first named by Huene 1932 based on two giant salamander-sized Early Triassic specimens. One was a torso and anterior tail lacking a skull and ventral pectoral girdle (HLD-V 608; Fig. 1). The other was a skull and ventral pectoral girdle (HLD-V 607; Fig. 1). Apparently there were no bones in common. Both are shown here at about one-quarter natural size.

The 608 specimen torso and tail are notable
for their exceedingly tall neural spines topped by spine tables, like those of Eryops, and overlapping ribs, like those of Eryops, Sclerocephalus, Mastodontsaurus (Fig. 2) and Petobatrachus.

The 607 skull is notable
for being shorter than the interclavicle like no other basal tetrapods.. Schoch et al. 2007 report, “At first sight, this (second) specimen seemed so different from the first find that Huene himself was struck. Yet his efforts in further preparing the second specimen revealed the morphology of the dorsal spines which he found similar to the first specimen, albeit affected by compaction and consequently distorted.”

Contra earlier studies,
in the large reptile tree (LRT, 1443 taxa) the NMK S-118 posterior skull referred specimen nested with the similarly-sized Early Permian Trimerorhachis (Fig. 6), a flat-head, flat-torso taxon without overlapping dorsal ribs or high dorsal spines. Distinct from most basal tetrapods, the jugal does not contribute to the orbit rim. The HLD-V 608 torso and tail holotype specimen nest at the base of Peltobatrachus + Sclerocephlaus, and between the Ossinodus clade and the Eryops clade. So the two specimens are not congeneric in the LRT and that skull does not belong with that torso and tail (Fig. 4). (As always, I am willing to be convinced otherwise with better data.)

When it used skull traits from Schoch et al. (2007)
(Fig. 5) the LRT lost resolution. I also discovered the lateral view of the reconstructed skull  did not match the dorsal view with regard to the placement of nares, orbits and certain sutures. A repair of the lateral view is presented here (Fig. 5).

The change in the skull sutures
presented here (Fig. 3) and the subsequent nesting of the 607 skull specimen with Trimerorhachis (Fig. 6) is supported by the preservation of long, only slightly curved and laterally oriented ribs in the NMK S-117 specimen (Fig. 3), like those of Trimerorhachis (Fig. 6).

Fossils typically come to rest
with their major axis parallel to the bedding plane. In this way taxa with a wide, flat, skull and torso will usually be preserved in dorsal aspect. By contrast, taxa preserved in lateral view are more likely to have a deeper than wide torso, as in the Sclerothorax holotype (Fig. 1).

Eryops also had tall neural spines,
overlapping ribs and a deep pelvis in common with Sclerothorax. Perhaps Sclerothorax had a large skull and strong limbs, like Eryops, suitable for terrestrial locomotion.

By contrast
the 607 skull retains lateral line canals, like those of Trimerorhachis, so we might expect shorter limbs and an aquatic environment for the 607 skull specimen.

Phylogenetically separating
the 607 skull from the holotype torso resolves the lateral line canal issue when the skull was joined to the torso as a chimaera.

References
Huene F v 1932. Ein neuartiger Stegocephalen−Fund aus dem oberessischen Buntsandstein. Palaönontologische Zeitschrift 14: 200–229.
Schoch RR, Fastnacht M, Fichter J and Keller T 2007. Anatomy and relationships of the Triassic temnospondyl Sclerothorax. Acta Palaeontologica Polonica 52 (1): 117–136.

wiki/Sclerothorax