THEODORE H. EATON, JR.
INTRODUCTION
In trying to determine the ancestral relationships of modern orders of Amphibia it is not possible to select satisfactory structural ancestors among a wealth of fossils, since very few of the known fossils could even be considered possible, and scarcely any are satisfactory, for such a selection. The nearest approach thus far to a solution of the problem in this manner has been made with reference to the Anura. Watson's paper (1940), with certain modifications made necessary by Gregory (1950), provides the paleontological evidence so far available on the origin of frogs. It shows that several features of the skeleton of frogs, such as the enlargement of the interpterygoid spaces and orbits, reduction of the more posterior dermal bones of the skull, and downward spread of the neural arches lateral to the notochord, were already apparent in the Pennsylvanian Amphibamus (Fig. 1), with which Gregory synonymized Miobatrachus and Mazonerpeton. But between the Pennsylvanian and the Triassic (the age of the earliest known frog, Protobatrachus) there was a great lapse of time, and that which passed between any conceivable Paleozoic ancestor of Urodela and the earliest satisfactory representative of this order (in the Cretaceous) was much longer still. The Apoda, so far as known, have no fossil record.
Nevertheless it should be possible, first, to survey those characters of modern Amphibia that might afford some comparison with the early fossils, and second, to discover among the known Paleozoic kinds those which are sufficiently unspecialized to permit derivation of the modern patterns. Further circumstantial evidence may be obtained by examining some features of Recent Amphibia which could not readily be compared with anything in the fossils; such are the embryonic development of the soft structures, including cartilaginous stages of the skeleton, the development and various specializations of the ear mechanism, adaptive characters associated with aquatic and terrestrial life, and so on.
COMPARISON OF MODERN ORDERS WITH THE LABYRINTHODONTS AND LEPOSPONDYLS
Fig. 1. Saurerpeton (× 1/2, after Romer, 1930, fig. 6); Amphibamus, the palatal view × 2-1/4, from Watson, 1940, fig. 4 (as Miobatrachus), the dorsal view × 2-1/2, from Gregory's revised figure of Amphibamus (1950, Fig. 1); Protobatrachus, × 1, from Watson, 1940, fig. 18, 19.
In both Anura and Urodela the skull is short, broad, relatively flat, with reduced pterygoids that diverge laterally from the parasphenoids leaving large interpterygoid vacuities, and with large orbits. (These statements do not apply to certain larval or perennibranchiate forms.) The skull in both orders has lost a number of primitive dermal bones in the posterior part; these are: basioccipital, supraoccipital, postparietal, intertemporal, supratemporal, and tabular. The exoccipitals form the two condyles but there are no foramina for the 11th and 12th nerves, since these are not separate in modern Amphibia. The opisthotic is missing in all except Proteidae (but see discussion of the ear). Although the skull is normally autostylic, a movable basipterygoid articulation is present among Hynobiid salamanders and in at least the metamorphic stages of primitive frogs, and therefore should be expected in their ancestors. The vertebrae are, of course, complete; see discussion in later section. The quadratojugal, lost in salamanders, is retained in frogs, and conversely the lacrimal, absent in frogs, occurs in a few primitive salamanders. The situation in Apoda is different, but postfrontal and jugal should be noted as bones retained in this order while lost in the others.