Anatomy of Eocaecilia Micropodia, A Limbed Caecilian of the Early Jurassic

Abstract

Eocaecilia micropodia, an Early Jurassic caecilian from the Kayenta Formation of northeastern Arizona, is structurally comparable to Recent gymnophionans in numerous aspects but also possesses characters that are primitive or appear to be uniquely derived. The skull of Eocaecilia exhibits such distinctively caecilian features as (1) a sulcus along the orbital rim indicating the presence of a ten-taculum; (2) an os basale representing consolidation of the supraoccipital, exoccipital, basisphenoid, basioccipital, pleurosphenoid, and parasphenoid elements; (3) an internal naris posterior to the premaxillary–maxillary suture and medial to the tooth rows on the vomer and palatine; (4) enlarged nasal capsules; and (5) an olfactory eminence on the vomer. As in Recent caecilians, the lower jaw comprises a pseudodentary and pseudoangular that are joined along an elongate, oblique suture. The pseudoangular bears a robust internal process and an elongate re-troarticular process. The teeth are bicuspid and pedicellate, but are minute in size and are more numerous than in most living caecilians.Several features of the skull and lower jaw of Eocaecilia are unexpectedly divergent from the pattern known in Recent gymnophionans. The apparent fusion of the stapes and quadrate is unique. The obliquely oriented, more or less planar jaw joint would appear to provide little stability, and is thus structurally and functionally unlike that known in any other caecilian. The internal process of the lower jaw is very robust, and projects into the adductor chamber.Eocaecilia also presents primitive and/or transitional features that might be expected in forms representing an intermediate stage in the development of a specialized life style. The skull retains separate jugal, quadratojugal, postparietal, and ?tabular (or ?supratemporal) bones, elements that in living forms are either coossified with adjacent bones or lost. The limb girdles and limbs of Eocaecilia are a primitive retention, but the reduction in their relative size would appear to be transitional toward the limbless, gymnophionan condition.Elongation of the body in Eocaecilia is estimated to be comparable to that in primitive extant gymnophionans, but a precise comparison cannot be made because of uncertainty over the number of vertebrae. In general, the postcranial axial skeleton is relatively primitive. Intercentra are present. The parapophyses are not protracted as elongate processes, as in living forms, nor is there a pronounced longitudinal keel on the ventral aspect of centra. In contrast to the atlases of Recent caecilians, an interglenoid tubercle is present. Processes projecting from the internal walls of the neural canal of the atlas and at least the next four postatlantal vertebrae represent attachment points for a suspensory ligament of the spinal cord. Such processes have not been previously reported in living caecilians but are now known to be present in representatives of various families (ichthyophiids, typhlonectids, and caeciliids). Neural spines are absent in the postatlantal and dorsal regions, as in living caecilians. Haemal arches are present in the tail, and distal caudal vertebrae bear posterodorsally recurved neural processes, as in rhinatrematids.Although our knowledge of caecilian evolution and diversity now extends into the Early Jurassic, Eocaecilia micropodia does not provide sufficient evidence to securely recognize the origin of gymnophionans among known Paleozoic amphibians. The discovery of an operculum in Eocaecilia micropodia is novel confirmatory evidence that the opercular apparatus is a character shared with other lissamphibians. The absence of a separate operculum in extant gymnophionans may be hypothesized to relate to the loss of the shoulder girdle and the muscular link between the girdle and operculum. Nonetheless, a substantial morphological and temporal gap still intervenes in the identification of caecilian origins. Lissamphibia still remains at