HIDDEN HYPOSTOMES: HOW COVERT PRESERVATION IN AGNOSTOIDS INCREASES INFORMATION ON THE VENTRAL MOUTHPLATE

The hypostome, critical to interpreting feeding mode for extinct arthropods, is rarely found in agnostoids. We expand available material by studying new features: partially preserved hypostomes, often represented by fragments of anterior and posterior wings clinging to the ventral side of the carapace; depressions in the dorsal carapace, here termed collapse structures; and paired euhedral pyrite crystals on the dorsal carapace. We suggest that collapse structures formed from crushing of the carapace into the space between the upturned wings of the underlying hypostome, and that euhedral pyrite crystals formed where hypostomal wings met the dorsal carapace.

Partial hypostomes were detected by 3D imaging of Middle Cambrian agnostoids preserved ventral-side-up. Areal measurements of all hypostomes demonstrate that they were more posteriorly positioned than reported elsewhere: an average of 78 percent of the preserved hypostomal area is posterior to the transglabellar furrow.We evaluate the likelihood that collapse structures reflect the presence, or prior presence, of underlying hypostomes using landmarking techniques to compare locations of centroids of dorsal collapse structures with centroids of known ventral hypostomes. We evaluate the likelihood that pairs of euhedral pyrite crystals reflected the original positions of wing tips by comparing distances between paired crystals, scaled for overall size, with distances between similarly-scaled wing tips in preserved hypostomes.

From a field sample of 211 specimens in which only two hypostomes could be previously identified, these techniques yielded at least 30 hypostomes or related features. Discrepancies in reported hypostomal positions, especially with enrolled specimens and Late Cambrian taxa, may reflect: variation between taxa; evolutionary trends; flexibility and range in motion of the feeding apparatus; hypostomal position forced by enrolled or extended posture; or taphonomy. In the study material used here, however, minimal angular displacement of hypostomal structures, coupled with a bimodal distribution of amount of posterior displacement suggests that much of the discrepancy is due to flexibility of these natant hypostomes in life, or flexibility contributing to deformation during preservation in ventral-up position.