THE HISTOLOGICAL ONTOGENY OF ORNAMENTED TURTLES FROM THE KAIPAROWITS FORMATION OF UTAH

In the Late Cretaceous Kaiparowits Formation of Utah, upwards of 14 species of turtles can be found. Because specimens are often fragmentary, surficial ornamentation is frequently used to generically identify shell material. Despite this, ontogenetic and phylogenetic variation is still poorly understood on a histological level, especially in the Kaiparowits formation. To remedy this, seven Kaiparowits turtle genera from multiple ontogenetic stages have been described herein to understand how the shell grows, and how this process varies taxonomically.

Consistent with previous studies, the relative thickness and vascularization of cortices appear heavily influenced by ecology. Genera that are inferred to be terrestrial (Basilemys) exhibit relatively thick and avascular cortices, whereas aquatic (Adocus) and semiaquatic (trionychid) genera exhibit relatively thin, vascularized cortices.

We propose two mechanisms of ornament deposition. Trionychids rapidly deposit a basal layer, then continue laminar deposition that is consistent with existing reptilian growth models. Adocusians remodel the external surface of the previous ornamentation and deposit laminar tissue that forms the ornamentation on top of it. As a result, resorption lines are present in genera where lateral migration of the ornamentation exceeds radial growth.

The lower external cortex of the trionychids consists of structural collagen fiber bundles in alternating layers that resemble plywood. During ontogeny, the plywood-like structure expands externally, with additional layers being added along the distal margins of the shell. In contrast, the adocusian lower external cortex consists of a dense mat of secondary osteons that frames the medullary cavity.

All examined taxa exhibit externally-expanding medullary cavities with canals that preferentially remodel ISF bundles between LAGs. The internal cortex of all genera is laminar and largely unaltered, save some radially decreasing primary vascularization in the trionychids that is the result of early growth.

In short, the external surface of the shell is highly diagnostic, but the histological processes that create it are not. In some cases, ecological signals can override phylogenetic signals entirely.