Gluteus minimus

“Horse collars” are for balancing? The function of an enigmatic Devonian fossil re-examined with 3D visualization

Thomas A. Hegna and Daniel Snyder

   Gluteus minimus is a small enigmatic fossil with a composition largely of apatite. Descriptively referred to as “Horse Collars”, they are known only from the Devonian of Iowa. The type specimen was re-examined using micro-CT scan technology. Internal visualization revealed a high degree of ordered layering and a lack of vascularization. The fossils have a high degree of bilateral similarity, but are not truly bilaterally symmetrical.

   We suggest that Gluteus was part of a balancing organ (i.e. otolith or statolith) in some as yet unidentified free swimming animal, based on the presence of a single lateral primordium or core and nonisometric growth layers. The homology of the primordium cannot be resolved conclusively because a number of groups of modern animals have similar mineralized balancing organs. Living animals that have relatively large stato/otoliths (millimeter- to centimeter-scale) with ordered layering include fish and cephalopods. However, "balancing stones" in these taxa are usually calcium carbonate and show strong lateralization. The observed presence of phosphate and fluorine in Gluteus is explained by diagenetic replacing—the same pattern is exhibited in gastropods from the same locality. Additionally, there are living counter examples to bilateral symmetry between lateralized otoliths, notably in pleuronectiform flatfish (i.e. flounders) and deepwater lanternfish (i.e. anglerfish).

10th North American Paleontological Convention abstract