The Coal Measures Amphibia of North America

by Roy Lee Moodie

Moodie, Proc. U. S. Nat. Mus., 40, pp. 431-433, fig. 2, 1911.

Moodie, Kans. Univ. Sci. Bull., VI, No. 2, pp. 347-349, pl. 5, fig. 2, 1912.

Type: Specimen No. 4306, U. S. National Museum.

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

The type is a part of the collection of Mr. R. D. Lacoe, in the U. S. National Museum. The fossil is very poorly preserved, but the remains are to be seen on both halves of the nodule, so that considerable can be made out as to its structure.

The chief diagnostic characters which will at once distinguish the species are the elongate arm, large interclavicle, shape of the vertebra, and triangular skull.

The portions of the animal which are preserved are the impression of the skull with one orbit, the right humerus and radius with portions of others, and traces of ventral scutellæ. These remains are so intermingled with the remains of plants that it has been quite difficult to distinguish bone impression from plants. This, however, has been done by whitening the fossils with ammonium chloride, when the texture of the fossils serves to distinguish the one from the other. Parts of the plants have been converted into galena and kaolin, as have also parts of the bones, so the task has been rendered doubly difficult. There can be no doubt, however, that the observations recorded below are correct. The position of the arm in relation to the pectoral girdle and the position of the girdle in relation to the skull impression first called attention to the possible presence of a fossil amphibian.

There is little to be said of the skull. It is merely an impression in the nodule. It is triangular in form, with the snout an acute angle. The angle is, however, exaggerated by the compression to which the fossil has been subjected. The right side of the skull lies over a portion of some plant. The animal is preserved on its back, so that this gives a good opportunity for the study of the pectoral girdle, which is partially preserved. The interclavicle is very large and from it the species has been given its specific name (thoracatus—armed with a breast plate). It is an exaggerated T, with the stem very short with its anterior margin curved, and ending in a rather sharp, elongate point. The interclavicle recalls, in a measure, the same element of the Branchiosauria, although it is much more expanded anteriorly and has a shorter spine. In these respects it resembles more nearly a reptilian interclavicle ([fig. 14 B]) .

The clavicle is of the simple triangular shape so characteristic of the Microsauria. It is somewhat displaced backward and its inner margin is slightly obscured. The humerus is elongate, apparently cylindrical, and with expanded ends, resembling very closely the humerus of Amphibamus grandiceps, although its proportions are much greater than in that species. Its length is almost equal to the length of the skull, while in A. grandiceps the length of the humerus is only half that of the skull. The radius (ulna?) resembles in its general proportions those of the humerus. It is a more elongate, slender, lighter bone. The impression of the other bone of the forearm is obscured.

A portion of a single vertebral centrum from the posterior part of the dorsal series is preserved. It is apparently amphicœlous; its width is nearly half greater than its length.