Abstract
The scant fossil record of caecilians has obscured the origin and evolution of this lissamphibian group. Eocaecilia micropodia from the Lower Jurassic of North America remains the only stem-group caecilian with an almost complete skull preserved. However, this taxon has been controversial, engendering re-evaluation of traits considered to be plesiomorphic for extant caecilians. Both the validity of the placement of E. micropodia as a stem caecilian and estimates of the plesiomorphic condition of extant caecilians have been questioned. In order to address these issues, the braincase of E. micropodia was examined via micro-computed tomography. The braincase is considered to be a more reliable phylogenetic indicator than peripheral regions of the skull. These data reveal significant new information, including the possession of an ossified nasal septum, ossified anterior wall of the sphenethmoid, long anterolateral processes on the sphenethmoid, and paired olfactory nerve foramina, which are known only to occur in extant caecilians; the latter are possibly related to the evolution of the tentacle, a caecilian autapomorphy. A phylogenetic analysis that included 64 non-amniote taxa and 308 characters represents the first extensive test of the phylogenetic affinities of E. micropodia. The results place E. micropodia securely on the stem of extant caecilians, representing a clade within Temnospondyli that is the sister taxon to batrachians plus Gerobatrachus. Ancestral character state reconstruction confirms the braincase of E. micropodia to be largely representative of the plesiomorphic condition of extant caecilians. Additionally, the results refine the context within which the evolution of the caecilian form can be evaluated. The robust construction and pattern of the dermal skull of E. micropodia is interpreted as symplesiomorphic with advanced dissorophoid temnospondyls, rather than being autapomorphic in its robust construction. Together these data increase confidence in incorporating E. micropodia into discussions of caecilian evolution.
Highlights
Of the three orders of living amphibians, caecilians remain the most poorly represented in the fossil record
Micro-computed tomography confirms that the braincase of Eocaecilia micropodia consists of two bones (Figure 2A–C), similar in general morphology to the two composite bones comprising the braincase in extant caecilians, i.e., the sphenethmoid and the os basale [32]
New Morphological Features The application of micro-computed tomography to the skull of Eocaecilia micropodia revealed the presence of several new features of the braincase previously inaccessible through traditional methods
Summary
Of the three orders of living amphibians, caecilians remain the most poorly represented in the fossil record. Only six accounts of crown-group fossil caecilians (Apoda) have been published to date (four Cenozoic and two Mesozoic forms [6,7,8,9,10,11]), five of which are based on specimens consisting only of vertebrae. As recognized members of the crown-group, these fossils do little to shed light on the origin of the caecilian form. Two stem-group fossil caecilians (Gymnophiona) have been found (both Mesozoic [12,13]), and only one of these, Eocaecilia micropodia from the Lower Jurassic of North America [12,14], is represented by substantial parts of the cranial and postcranial skeleton. As the only well preserved stem-group caecilian, E. micropodia represents our best opportunity to understand the major morphological transformations that took place during the evolution of the highly specialized caecilian form, and of lissamphibians in general
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