Paleontologists have long recognized
that Spathicephalus (Fig. 1) was a close relative of baphetids, like Baphetes, ever since Watson (1929). Two main features link Spathicephalus with baphetids: antorbital fenestrae that have fused with the orbits, and a closed palate formed mostly from a pair of broad pterygoid bones.
Spathicephalus mirus (Watson 1926; Late Carboniferous, 320 mya) was described, “unlike that of any other early tetrapod, with a flattened, square-shaped skull and jaws lined with hundreds of very small chisel-like teeth.” The extended orbit shape traditionally allied Spathicephalus with Baphetes, but here in the large reptile tree (LRT, 978 taxa) it nests with two other flat-headed temnospondyls, Gerrerpeton and Koilops (Fig. 2), apart from Baphetes. The fossil does not show tooth replacement as every tooth is present without gaps. Distinct from derived temnospondyls, but like basal forms, the palate is closed on this bottom-feeder. Long before the publication of Koilops Milner et al. 2009 nested Spathicephalus close to Eucritta and Baphetes, but that relationship was not recovered in the LRT, despite sharing a closed palate. The elongate orbit without intrusions is convergent with that of Baphetes.
Koilops herma (Clack et al. 2016; NMS G. 2013.39/14) Tournasian, early Carboniferous ~375 mya) is a temnospondyl with a flat skull and large orbits nesting between Greererpeton and Spathicephalus. The nares were close to the rim of the short rorstrum. The pineal foramen was enormous. The teeth were small and sharp. The nasals were broad.
Clack et al. (14 other authors) 2016. Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature ecology & evolution 1(0002):1-11.
Milner AC, Milner AR, Walsh SA 2009. A new specimen of Baphetes from Nýřany, Czech Republic and the intrinsic relationships of the Baphetidae. Acta Zoologica 90: 318.
Watson DMS 1929. Croonian Lecture. The evolution of the Amphibia. Philosophical Transactions of the Royal Society, London B 214:189-257.