Abstract

The origin of limbed vertebrates (tetrapods) from their fish-like ancestors is an iconic
example of a major evolutionary radiation. The group of vertebrates that covered this
transition, the early tetrapods, encompassed 12-15 groups, ranging from the Late Devonian to
the Middle Cretaceous. The tempo and mode of the evolutionary diversification undergone by
these organisms, however, remains largely unknown. To fill this gap in our understanding of
the early proliferation of vertebrates, the core objective of the project is to quantify models of
skeletal transformation across the transition from fish-like ancestors to early tetrapods in light
of a revised, amended, and expanded database of cranial traits. Studies so far have relied on a
limited database of osteological characters, often with few characters for certain complexes or
a reduced taxon set for tetrapods over the transition. Here, I create a new database that will be
used to retrieve a tree for the group which will be subjected to macroevolutionary analyses,
answering questions concerned with rate and disparity. The retrieved tree shows a well
resolved pattern of taxa with groupings that have been found previously. New taxa have
impacted the tree tremendously pushing the divergence between the stem-amniotes and stemamphibians backwards. The analysis of disparity reveals that phylogenetic diversification
resulted in higher degrees of morphological disparity among clades relative to the degrees of
within clade variation. Both the rate and disparity analyses indicate that early tetrapods show
a burst of evolution at the start of the lineage and also at the amphibian-amniote split. This
study has shed light upon how the early tetrapod group has evolved, revealing that these
organisms underwent a degree of rapid diversification at the beginning of the clade across
both analyses as well as creating a new tree for the group. The next step would be to add post
cranial characters to the dataset allowing an even more comprehensive look at the early
tetrapod radiation. Overall, my research contributes to appreciate the patterns and rates
underlying the early proliferation of tetrapods, which is a fundamental step to advance our
understanding of the subsequent diversification of modern tetrapods and their morphological
complexities.