The early formation of the skull in extant and Paleozoic amphibians

Abstract

Understanding of evolutionary changes in the vertebrate skull is greatly influenced by the knowledge of ontogeny. Extant amphibians are an outstanding example in this field, because their life cycles are complex and have been intensively studied. At the same time, fossil material of Paleozoic amphibians has become available that sheds light on the ontogeny of a long-extinct clade, prompting comparison with recent forms. In this paper, the formation of the skull of a Paleozoic amphibian (the branchiosaurid temnospondyl Apateon) is analyzed in comparison with that of an extant salamander (the hynobiid Ranodon). Both temporal patterns (sequence of ossification) and spatial patterns (morphological changes) are described. The general results are that (1) the sequence of ossification is similar in many aspects, and (2) most dermal bones share fundamental similarities in morphogenesis, although sometimes the morphological result in adults may differ conspicuously.The comparison reveals that the parasphenoid, premaxillae, maxillae, frontals, parietals, squamosals, and prefrontals are strikingly similar in their mode of growth. In particular, the appearance of the earliest primordia and the subsequent stages of morphological transformation are almost identical. The development of the pterygoids and nasals is different in the earliest stages, but the ontogenetic trajectories converge in later stages. In Ranodon and other transforming urodeles, the pterygoids and vomers experience extensive resorption during metamorphic climax, whereas in branchiosaurids the morphology of this region remains stable throughout ontogeny.In the sequence of cranial ossifications, the early appearance of the premaxilla and tooth-bearing elements of the palate characterize both genera, but the maxilla forms much later in Ranodon. The ectopterygoid, absent in all salamanders, is the last palatal element to appear in branchiosaurids. In the skull roof, the parietals, frontals, and squamosals are the first bones to form in both groups. Conversely, the circumorbital elements and tabular are among the last ossifications in branchiosaurids, and the prefrontal and lacrimal (the only circumorbital bones present in salamanders) form within the same interval in urodele ontogeny. The septomaxilla is the last dermal element to ossify in both groups. Comparison with caecilians and anurans indicates that salamanders are much more similar to Paleozoic branchiosaurids than to other extant lissamphibians. A major difference between salamanders and branchiosaurids is that the morphology of the latter is much less affected by developmental perturbations, such as larval specializations and metamorphosis.