Stegops spikes?

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.

Figure 1. Stegops skull retraced and reconstructed in dorsal and lateral views, plus the version provided by Moodie 1916. Here the squamosal 'spikes' are reconstructed as a deeply excavated squamosal like the one in Tersomius.

Figure 1. Stegops skull retraced and reconstructed in dorsal and lateral views, plus the version provided by Moodie 1916. Here the squamosal ‘spikes’ are reconstructed as a deeply excavated squamosal like the one in Tersomius.

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.”

Figure 3. Tersomius texensis, an amphibamid lepospondyl close to Dendrerpeton.

Figure 2. Tersomius texensis, an amphibamid lepospondyl close to Dendrerpeton.

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.

References
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.

wiki/Stegops

2 thoughts on “Stegops spikes?

  1. Dude, the paper that resulted from Milner & Schoch’s talk has long been published. Google Scholar is your friend!

    The new reconstruction bears little resemblance to the Moodie illustration (Fig. 1).

    Perhaps that’s because it’s based on such a crappy photo.

    • Oops! Sorry. The paper is not out; what is out is the short description in the appropriate Handbuch volume (Schoch & Milner 2014). There are new reconstruction drawings of the skull in dorsal and ventral view (fig. 38A, D); the authors trace the nomenclature and find out the species name is actually Stegops newberryi; there’s a detailed new diagnosis; and there’s the following on p. 82:

      Systematic position: Romer (1930, 1947) suggested that Stegops was zatracheid, highlighting the shared possession of spines and flanges on the backs of the skull and mandible. Other authors such as Boy (1989, citing Hook) and Milner (1993) suggested that Stegops is a dissorophid and only minimally convergent with the zatracheids. In fact, Stegops shares various dissorophoid apomorphies (abbreviated jugal, LEP), has a typically dissorophoid palate, and is therefore assigned to that clade here, albeit with unresolved affinities within Dissorophoidea.”

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