How informative is vertebral development for the origin of lissamphibians

The origin of lissamphibians remains unresolved and different origins within Paleozoic early tetrapods have been proposed. Both the vertebral morphology as well as the ossification sequence of the vertebrae are potentially important character traits to test these different hypotheses. Here, we aim to determine if vertebral formation and ossification sequence in lissamphibians and early tetrapods are valuable indicatives of phylogenetic relationships. To answer this question, we examined the vertebral development in growth series of different salamander taxa and compared the results with literature data on vertebral development in frogs, caecilians and early tetrapods. In salamanders, caecilians and early tetrapods, the vertebral centrum develops by chordacentral and perichordal centrum formation, whereas in frogs, no chordacentral centrum formation has been observed so far. In the salamander taxa studied here, the following chondrification and ossification sequences are observed: chondrification first of the neural arches and then of the centra followed by ossification first of the centra and then of the neural arches. In frogs, in contrast, the neural arches ossify prior to the centra and it can be assumed that this developmental sequence represents the ancestral condition for tetrapods, as it characterizes some stem-tetrapods and all temnospondyls from which growth series are known. However, the ossification sequence of the vertebral column is only incompletely known in stem-amniotes which makes a comparison with extant lissamphibians difficult. This indicates that the mode of centrum formation and the ossification sequence are highly variable and cannot be used to determine the origin of lissamphibians within early tetrapods.