| The common ancestor of all tetrapods, or four-limbed vertebrates, originated
approximately 355 million years ago. This common ancestor would give rise to all major
terrestrial vertebrate groups today, including Amphibia, Reptilia, and Mammalia which would
come to dominate terrestrial ecosystems. The common ancestor of each of these three groups,
however, originated from tens of millions to hundreds of millions of years after their initial
divergence from the tetrapod common ancestor. The organisms visible to us today represent only
a fraction of the diversity that would have existed in the past, and our understanding of the
evolutionary history of these groups remains incomplete, in particular for their earliest ancestors.
Reptiles originate amongst this uncertainty sometime in the Late Carboniferous (~310 MA), yet
their crown divergence - charismatic organisms such as turtles, lizards, and dinosaurs - do not
appear until the latest Permian (~260 MA) according to both fossil and molecular data. This 50-
million-year long ghost lineage for the stem-lineage leading to crown reptiles has made
reconstructing their evolutionary assembly and even the interrelationships of living members
difficult, with recent debate concerning the evolutionary relationships of turtles.
It is during this ghost lineage that extinct reptiles undergo dramatic anatomical
transformations that distinguish living reptiles today from their distant amphibian and
mammalian cousins. These transformations include hypothesized origins of the ‘diapsid’ pattern
of temporal fenestration in the Carboniferous and for multiple origins of tympanic hearing among early crown reptiles in the latest Permian. Paleozoic stem-reptiles, in particular a group called ‘Parareptilia’ thought to be unrelated to crown-group reptiles, bear a suite of anatomical
features that conflict with current paradigms of early reptile evolution. These ‘parareptiles’ and
other groups of Paleozoic tetrapods bear latent anatomical information that may inform our
understanding of the timing and sequence of early reptile evolution, but remain woefully
understudied.
In this dissertation, I study the evolutionary origins of stem-reptiles by using state-of-theart imaging methods that allow to me describe anatomically conserved regions of the reptilian
skull. To polarize the Paleozoic origins of reptiles among other amniotes and the explosive
radiations of the reptile crown group in the Triassic, I studied the anatomy of all putative stemreptiles and close relatives such as ‘microsaurs’ (Chapter 1), stem-mammals (Chapter 2),
‘parareptiles’ (Chapter 3), neodiapsids (Chapter 4), and the oldest putative stem turtle (Chapter
5). This dissertation offers a synthesis of these anatomical observations, with chapters ordered
phylogenetically from plesiomorphic members of the amniote stem-lineage up to the origins of
turtles, to bridge the gap between historical anatomical studies and recent cladistic techniques.
My findings demonstrate that the reptile stem-lineage was diverse throughout the
Paleozoic and filled by former members of ‘Parareptilia’, providing anatomical evidence that the
tympanic fossa of crown reptiles originated in their middle Permian ancestors. This new scheme
for early reptile evolution provides robust evidence that turtles form the sister-group to
Archosauria (crocodiles and birds), forming a long awaiting consilience between molecular and
anatomical hypotheses of turtle origins.
Keywords: Paleontology, Amniota, Reptilia, Testudines, Paleozoic, Neuroanatomy. |