LEPTAENA (BRACHIOPODA) MORPHOLOGY ACROSS THE LATE ORDOVICIAN MASS EXTINCTION

The Late Ordovician mass extinction resulted in the loss of 85% of species. Despite this, the same major taxonomic groups were present before and after the extinction suggesting that the mass extinction was not ecologically severe. Other studies have shown that taxonomic continuity is accompanied by changes in morphology, indicating the mass extinction did in fact have some ecological effect. The aims of this study are twofold: to analyze shape variations in Leptaena as a means of understanding environmental and ecological change related to the mass extinction, and to develop new methodology for creating low-cost 3D morphological models of fossils.

We analyzed shape change in species of Leptaena from the Ordovician to the Devonian using 3D modeling, which more accurately reflects convex brachiopod shells than 2D morphometrics. To avoid costly imaging techniques, this study employs Structure-from-Motion (SfM), a low-cost photogrammetry technique. SfM uses a series of overlapping images with matching features that are identified and linked to generate high-resolution, 3D point cloud reconstructions of objects. The point cloud was used to analyze and compare the shapes of specimens by measuring distances between discrete features present on all specimens.

The analysis revealed that Leptaena species were medium-sized, triangular, and exhibited low relief in the Late Ordovician and larger, boxier, and exhibited greater relief in the Silurian. Morphologically different genera in the Silurian would have adaptations to the environment that varied from their Ordovician counterparts. Such adaptations in response to the extinction might have allowed Leptaena species to expand geographically and occupy broader niches, such that pre and post extinction communities were different. As such, the Late Ordovician mass extinction likely had a greater ecological impact than previously assumed.

Additionally, the use of SfM to create low-cost yet effective 3D models of brachiopods is a promising methodology to more accurately constrain shape and compare species across geologic time. The method has been successfully expanded to in situ 3D modeling of brachiopods in the field. Benefits of this method include modeling several brachiopod specimens at once, receiving instant data for processing, and geologic preservation.