The fact that the arrangement of the sense-organs of Micrerpeton corresponds so exactly to the condition found in Necturus is of considerable interest. Necturus alone among the modern tailed Amphibia has the arrangement described for the lateral-line system of Micrerpeton. All other forms of the Caudata, as also the larval forms of the Salientia, have an arrangement of the lateral-line system which is perfectly distinct from that found in Necturus, although the basis of the same arrangement is found in all. In Amblystoma, for instance, the median lateral-line is not present on the tail, and the dorsal line is incompletely developed. The close similarity of the arrangement of the systems of sense-organs in the two forms, Micrerpeton and Necturus, may be of genetic significance with regard to the latter form. The lateral-line sense-organs are of a very fundamental significance, and it is not at all improbable that the same arrangement of the lines has existed from the Carboniferous period or earlier. We know that such has been the case in a great many of the fishes. The ancestors of the modern Caudata must have originated somewhere in the basal Carboniferous or earlier periods, and, in the writer's opinion, the Branchiosauria represent the ancestral group of the Caudata. This suggestion is by no means new, since Baur and others have held the same view. This topic has been discussed at length elsewhere ([459]) by the writer.
The relations of the form Micrerpeton caudatum are readily determined. The number of the presacral vertebræ, the form and position of the ribs, the shape of the skull, the arrangement of the cranial elements, the structure of the pectoral girdle and the character of the ventral armature all clearly bespeak a close relationship with Branchiosaurus, Melanerpeton, Pelosaurus, and other European branchiosaurian forms from the Upper Carboniferous and Lower Permian.
The above-described species, with others given below, is the earliest geological evidence of the Branchiosauria, since the oldest European forms are from the Stephanian (Upper Carboniferous) , which probably lies somewhat above the horizon of the Allegheny series of North America. The presence of the Branchiosauria in America is of considerable interest in the bearing it has on the distribution and migration of the Paleozoic animals. Knowledge of how the group came to occur in such widely separated localities in approximately contemporary geological strata is an unsolved problem of paleontology. It is possible that the piscian ancestors of the Amphibia migrated across or along the borders of the seas and began the amphibian phase of development independently in the two continents. That evolution should, in this case, have followed almost exactly parallel lines seems incredible.
| Measurements of the Type. | |
| mm. | |
| Length of entire animal | 49 |
| Length of head in median line | 6.5 |
| Width of head at posterior border | 8 |
| Length of orbit | 2.5 |
| Width of orbit | 2 |
| Interorbital space | 2 |
| Length of the vertebral column | 33 |
| Length of the vertebral centrum in dorsal series | 0.5 |
| Length of trunk from base of skull to sacrum | 22 |
| Length of rib | 1.5 |
| Length of scapula | 3 |
| Maximum width of clavicle | 2 |
| Length of humerus | 2.5 |
| Length of ilium | 1.5 |
| Length of femur | 2 |
| Length of tibia | 1.5 |
| Length of tail impression | 21.5 |
| Width of tail impression at base | 4 |
Moodie, Kans. Univ. Sci. Bull., VI, No. 2, p. 330, 1912.
Type: Eumicrerpeton parvum Moodie.
The genus is established on three well-preserved specimens representing nearly the entire anatomy. The generic characters are found in the very broad posterior table of the skull, with its short length, reduction of tympanic notch, and shortness of body. The body-length of Eumicrerpeton ([plate 5, fig. 1]) is less proportionately than that of other closely allied genera. Other generic characters are found in the sharp postero-lateral angle of the skull, and it is to be distinguished from Micrerpeton, especially, by the short, stumpy limb bones. The narrow, elongate eye, placed close to the edge of the skull, is a character not observed hitherto in the Branchiosauria. The genus is closely allied to Branchiosaurus of Europe.
Eumicrerpeton parvum Moodie.
Moodie, Proc. U. S. Nat. Mus., 40, p. 430, fig. 1, 1911.