Rethinking the Evolution of Temporal Fenestrae in Turtles:
An Interactive Application for Comparative Anatomy & Phylogenetics
Where to do turtles come from? This question is among the oldest and most debated problems in vertebrate sys
tematics. A key factor in this debate is the pattern of temporal fenestrae (openings) in the skull, which has long been central to amniote evolutionary hypotheses.
Recently, there has been robust evidence supporting turtles as having evolved within the diapsid radiation, which includes all other living reptiles and birds. This group is defined by the presence of
two temporal fenestrae… but turtles are anapsid, they have NO temporal fenestrae. This requires that the anapsid skull in turtles is secondarily derived! How could that be?
‘Transitional’ fossils that support this theory were elusive until the Middle Permian reptile
Eunotosaurus africanus was re-examined in 2015. Eunotosaurus
exhibits features that help reconcile the gap between turtles and other reptiles, but how it does so is still misunderstood. Evolutionary thinking is vital to the life sciences, informing everything from model-based healthcare studies to agricultural practices that feed millions.
And yet, even scientific thinkers in the field can succumb to intuitive biases about the evolutionary process.
The solution to communicating the Eunotosaurus
findings was handled in two phases…
Phase 1: Novel Digital reconstruction
First described over a century ago but never modeled digitally until now, the Eunotosaurus skull was reconstructed in the tradition of other paleontological work. This model constitutes a scientific hypothesis about the antemortem shape of the animal, and can be used for public outreach and further scientific study.
Phase 2: didactic web application
A web-based application was designed with students in mind, using turtle evolution as a problem set for reviewing evolutionary concepts like tree-thinking. It is a valuable learning tool that can be incorporated into post-secondary biology curricula, and an accessible resource for contextualizing
Eunotosaurus in the broader ‘tree of life.’
Fossil Reconstruction
Fossils often undergo taphonomic deformation
, or the distortion of their shape as a result of myriad geological, chemical, and physical forces. Think breakage, compaction, shearing, and so forth. Identifying and recording these deformations is crucial to ensuring a successful reconstruction. Every decision in the process of reconstructing the antemortem shape, called
retrodeformation, of the animal hinges on the observations made at this stage.
This entailed first reading through the observations of this particular specimen, M777, in the primary literature (like Cope, 1892 and Bever et al.
2015), after which I added my own observations of the apparent distortions, as seen below.
Once identified, these deformations can be reversed using several techniques:
leverages and restores bilateral symmetry by reflecting elements across a midsagittal plane and averaging the positions of landmark features on either side.
restores broken fragments and rejoins disarticulated bones.
builds composite structures from unequally-preserved counterpart bones across the midsagittal plane.
corrects distortions that occur across the entire structure without breakage. Examples include warping, bending, or torsion.
attempts to restore missing or highly deformed portions of a fossil using information from specimens of the same or related taxon, or by estimating the morphology from preserved parts.
After restoring bilateral symmetry in Maxon Cinema4D, the digital fossil was brought into Pixologic Zbrush to restore its smooth contours, fill gaps and cracks, and finish the reconstruction of its nasal region and teeth.
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