Stereospondyls were the most diverse clade of early tetrapods, spanning 190 million
years, with over 250 species belonging to eight taxonomic groups. They had a range of
morphotypes and have been found on every continent. Stereospondyl phylogeny is widely
contested and repeatedly examined but despite these studies, we are still left with the
question, why were they so successful and why did they die out? A group-wide analysis
of functional morphology, informing us about their palaeobiology, was lacking for this
group and was carried out in order to address the questions of their success and demise.
Based on an original photograph collection, size independent skull morphometrics were
used, in conjunction with analyses of the fossil record and comparative anatomy, to
provide a synthesis of the functional morphology of stereospondyl amphibians.
Stereospondyls originated in the Carboniferous and most taxonomic groups were extinct
at the end of the Triassic. The early Triassic had exceptionally high numbers of shortlived genera, in habitats that were mostly arid but apparently experienced occasional
monsoon rains. Genera turnover slowed and diversity was stable in the Middle Triassic,
then declined with a series of extinctions of the Late Triassic. Stereospondyls showed the
pattern of ‘disaster’ taxa: rapidly diversifying following a mass extinction, spreading to a
global distribution, although this high diversity was relatively short-lived.
Geometric morphometrics on characteristics of the skull and palate was carried out to
assess general skull morphology and identified the orbital position and skull outline to be
the largest sources of skull variation. Comparing anatomy of stereospondyls with extant
species revealed that the differences in head shape and orbit positions between
stereospondyls allows inference of a range of feeding behaviours, ranging from rapid head
swipes, to crushing invertebrates with wide palatal bones, and crocodilian-type
ambushing. The range of feeding modalities meant stereospondyl species were able to
coexist with minimal competition. The success was, however, short lived, as highly
specialised shapes in the middle and Late Triassic probably meant that stereospondyls
were not able to adapt to a more generalised lifestyle when the end Triassic extinction
eliminated ecosystems
