Updated January 28, 2019
with a new reconstruction of Stegops and a nesting with Tersomius.
Stegops divaricata (Cope 1885; AMNH 2559; 5.6 cm skull length; Westphalian, Late Carboniferous, 310 mya) is a basal tetrapod that has bounced around the family tree without settling down.
Moodie 1916 reported
the skull of Stegops was small, oval and “the quadrate angles project into sharp horns.” One can presume Moodie meant the squamosal had horns, because that’s how he drew them (Fig. 1). The quadrates in this and related taxa are hidden beneath the cheek bones. He considered Stegops a microsaur.
According to Wikipedia:
“Stegops is an extinct genus of euskelian temnospondyl from the Late Carboniferous of the eastern United States. Fossils are known from the Pennsylvanian coal deposits of Linton, Ohio. It was once classified in the eryopoid family Zatrachydidae because it and other zatrachydids have spikes extending from the margins of its skull, but it is now classified as a dissorophoid that independently evolved spikes.”
After Moodie 1916,
this taxon was largely ignored for decades.
Then Milner and Schoch 2005 reported:
“The spiky-headed temnospondyl amphibian Stegops divaricata from the Middle Pennsylvanian coal of Linton, Ohio has remained neglected and enigmatic for several decades. It has been argued to be the ancestor of the Permian Zatrachydidae, also spiky-headed temnospondyls, although there are few resemblances other than the spikes. An examination of previously undescribed material of Stegops, along with a re-evaluation of the original specimens, permits a redescription and partial systematic assignment of it. All specimens have bony spikes on the tabular, quadratojugal and angular, but in apparent dimorphism, only some have squamosal and supratemporal spikes. A phylogenetic analysis of 52 characters in 15 temnospondyl taxa places Stegops within the dissorophoid clade but leaves its position uncertain within that clade. The Zatrachydidae, represented by Acanthostomatops, fall outside the Dissorophoidea, and the zatrachydid affinities of Stegops asserted by previous workers are based on homoplasious similarities in ornamentation. Internal relationships of the Dissorophoidea remain unresolved and Stegops shares conflicting similarities with Amphibamidae in some resolutions and with an Ecolsonia + Dissorophidae + Trematopidae clade in others.”
After phylogenetic analysis
this specimen of Stegops nested with Tersomius (Fig. 2) contra Milner and Schoch. The open palate with palatine exposure on the cheek, together with a deeply emarginated squamosal roofed over by tabular spikes are traits uniting thiese taxa. In the large reptile tree (LRT, 1390 taxa) dissorphids nest with basal lepospondyls.
Cope ED 1885. Second continuation of researches among the Batrachia of the Coal Measures of Ohio. Proceedings of the American Philosophical Society 22:405–408.
Milner AR and Schoch RR 2005. Stegops. A problematic spiky-headed temnospondyl
SVPCA Platform Presentation, (London)
Milner A and Schoch R 2006. Stegops, a problematic spiky-headed temnospondyl. Journal of Vertebrate Paleontology. 26 (3, Suppl.): 101A.
Moodie RL 1909. Journal of Geology 17(1):79
Moodie RL 1916. The microsaurian family stegpidae. The coal measures amphibia of North America. Carnegie Institution of Washintion 238: 222pp.