The Coal Measures Amphibia of North America

by Roy Lee Moodie

Horizon and locality: Mazon Creek shales, near Morris, Illinois.

The form of the skull of Amphibamus grandiceps Cope is not unlike that of Tuditanus minimus Moodie ([462]) from the Linton, Ohio, beds, but it is less acuminate than in that form. The large size of the orbits is especially striking. The shape of the skull is triangular, with concavities in the posterior table which correspond to the ear-slits so characteristic of Metoposaurus ([242]) from the Keuper of Germany. The narrowed posterior table of the skull is truncate, as in several other genera of Microsauria, notably Tuditanus and Saurerpeton. In structure the skull differs but little from many of the other Carboniferous forms, but the arrangement of the elements of the skull is more regular than in other genera.

The premaxillaries are very small elements in the anterior tip of the skull. They border the nares. The skull is rather peculiar among the Microsauria in the possession of a distinct lacrimal. I have detected this element in the cranium of Stegops divaricata Cope. As here defined the lacrimal is triangular, with its posterior border formed exclusively by the prefrontal. Its other relations are the normal ones. The nasal is elongate, with the usual relations of that element. The frontal is slightly longer and broader than the nasal. It apparently forms a portion of the inner border of the orbit. The parietal foramen lies in the anterior fourth of the parietal, a rather unusual position for this structure. The parietals, as in so many of the Microsauria, together form the largest element of the skull and are roughly a triangular area in the postero-median portion of the skull. The postparietal and the tabulare are clearly distinguishable and they have the usual relations for those elements. The maxillary, jugal, and quadratojugal together form the greater part of the maxillary border. The postero-lateral angle of the skull is, as visual, formed by the squamosal. The orbit is bounded posteriorly by the postorbital and the postfrontal, which include in the angle between them the quadrangular squamosal. The orbit is especially remarkable for its size as compared with the dimensions of the skull, being without a parallel among other known Microsauria. Around the border of the orbit in the specimen Cope studied ([105]) there were found 14 quadrangular plates which he called "superciliary plates." Hay ([316]) was inclined to regard them as sclerotic plates. In the Yale Museum specimen ([plate 4, figs. 5, 6]) there are 20 of these plates, and there seems to be no doubt that they are sclerotic elements. In the restoration ([fig. 26]) 29 sclerotic plates are given, but there is no assurance that this number is the exact one. They may also have been slightly larger, but not as large as in Branchiosaurus.

Fig. 26. Restoration of body outline and skeleton of Amphibamus grandiceps Cope, from Mazon Creek, Illinois, shales. Restoration is based on complete specimens of the species and on Cope's drawing. Form of body is indicated in one specimen, that in possession of Mr. Daniels. × 1.5.

Skull: pmx, premaxilla; n, nasal; fr, frontal; par, parietal; la, lacrimal; pf, prefrontal; pof, postfrontal; po, postorbital; pp, postparietal; spt, supratemporal; mx, maxilla; j, jugal; qj, quadratojugal; sq, squamosal; tab, tabulare.

Skeleton: ic, interclavicle; cl, clavicle; sc, scapula; h, humerus; r-u, radius, ulna; r, carpus; pu, pubis; il, ilium; f, femur; t, tibia; fb, fibula; ts, tarsus; x, ischium.

The vertebral column is preserved nearly entire in the Daniels specimen and quite entire ([478]) in the Yale specimen. Cope, in his study of the type ([105], [107]), thought there could be no more than 13 presacrals, but the specimen was poorly preserved and indecisive on this point. Dr. Hay ([316]) was inclined to the opinion that there were less than 20. The Yale specimen shows 22 centra, which are elongate, hour-glass-shaped bodies, with the neural spine a long, low crest running the entire length of the centrum, with a median elevation, so that in lateral view the spine would be triangular in form. The body of the centrum is expanded laterally into a diapophysis which extends anteriorly. The posterior vertebræ, at least, had the notochord largely persistent. The osseous part of the vertebra seems to have been but a thin shell, and the structure of the zygapophyses can not be determined. That they were dorsal in position is, however, evident from several vertebræ. The points of these structures project laterally. The tail is short and the caudal vertebræ weakly developed.

There are distinct impressions of at least 12 pairs of ribs in the Daniels specimen. They are long, slender, and curved, and there is no definite assurance that there were as many ribs as are indicated ([fig. 26]) in the restoration ([462]). The ribs are intercentral ([469]) and probably occupied the full length of the vertebral column. There may have been as many as indicated in the restoration.