Horizon and locality: Mazon Creek, near Morris, Illinois.
The species is very imperfectly known. Sufficient is present, however, to show its wide generic differences from other forms of the Cricotidæ. These characters are of a phylogenetic nature and indicate the more primitive nature of the present form, as we would expect from its geological position. The sutures separating the four vertebral elements are clearly apparent. The pleurocentral-neurocentral suture is apparent in 4 vertebræ.
Fig. 39. The vertebræ of Spondylerpeton spinatum Moodie, the only known temnospondyle from the the Mazon Creek shales. × 1. hy, hypocentrum; inc, intercentrum; pc, pleurocentrum; nc, neurocentrum.
There is but a single pleurocentrum preserved complete. This shows the form of the attached neurocentrum and chevron, which corresponds to the hypocentrum pleurale according to Fritsch. The pleurocentrum is flattened laterally, with a rather large canal for the notochord. Its sides are marked with 4 longitudinal grooves. Surfaces for the attachment of the ribs are not present, and for this reason, as well as the presence of chevrons, the vertebræ are supposed to be caudals. As such they represent an animal of some 3 or 4 feet in length. It was the giant of the Mazon Creek Amphibia. ([Plate 4, figs. 1, 2.])
Attached to the upper side of pleurocentrum by a sutural union occurs the neurocentrum. The neural arch is quite large and is oval in outline, although somewhat constricted at the tip. The spine of the neurocentrum is rather long and broad at the base, measuring 12 mm. across the anterior zygapophysis. The neurocentrum is laterally flattened and ends in a rather acute and somewhat rugose point. It was probably tipped with cartilage. The anterior zygapophysis occurs well down on the neurocentrum, its lower edge being 5 mm. from the suture separating the pleurocentrum and the neurocentrum. The posterior zygapophysis occurs quite high up on the neurocentrum and lies at a distance of 15 mm. from the pleuro-neurocentral suture, thus indicating an extreme posterior inclination of the neural spine. The posterior zygapophysis of the best preserved vertebra is separated from its mate, the anterior zygapophysis, in the next succeeding vertebra by a space of 5 mm.
The ventral surface of the pleurocentrum bears a structure, which is, without doubt, a chevron, although the character of its opening can not be determined. It is elongated and is united by a broad base to the pleurocentrum. Its union is by a clearly defined suture, which is apparent in 3 vertebræ. The condition represented by the specimen duplicates almost exactly the condition figured by Cope for the caudal region of Cricotus Cope.[D]
[D] COPE, E. D., Trans. Am. Phil. Soc., XVI, p. 246, 1890.