THE SAME PATHOLOGICAL DEFORMATION PATTERS WERE RECOGNIZED IN JAPANESE AMMONOIDS FROM DIFFERENT AGE AND DIFFERENT TAXA

Ammonoid pathologies directly record the life history of animals, for example, bite marks as predator-prey relationship, epifauna on a shell as parasitism, or repair marks as shell secretion process. Especially some anomalies of ammonoid are assumed to be the effect of endogenic factor(s). In other words, ammonoid pathologies reflect the metabolism of extinct animals. Accordingly, hitherto many researchers wrestled with ammonoid pathology concerning classification, the presumption of causes, and frequency of deformation in a fauna. In contrast, very little research concerning ammonoid pathology had been achieved in Japan while Japanese ammonoid fossils preserved their three-dimensional morphology and original aragonite shell structures.

We newly discovered two ammonoid specimens with pathological deformations from the Cretaceous, Japan; an Acanthoceratidae “Yubariceras” sp. from the Turonian, the Yezo Group, and a Collignoniceratidae Menabites sp. from the Campanian, the Izumi Group. We described these deformations and observed internal structures with computed tomography (CT) scanning with Nikon XT H 225 (210kV, 200 mA, voxel size: 0.05 mm) and Amira Software 2022.1 (Thermo Fisher Scientific).

“Yubariceras” sp. has a small cave on the venter and ribs incline toward that (forma aegra verticata). Simultaneously, all tubercles are dismissed completely (forma aegra cacopotycha). At -225° adapical from that cave, the location of the siphuncle shifts to the right (forma aegra juxtalobata). This degree corresponds to the body chamber length, and therefore it is obvious that when the siphuncle deformation occurred at the adapical end of the soft body, the outer-wall secretion was also disordered at the peristome simultaneously.

In the last whorl of Menabites sp., a keel and tubercles on the ventrolateral shoulder are lost (forma aegra cacoptycha). A pair of subsidence on both sides continues the periphery of the whorl and ribs incline to that subsidence (forma aegra verticata).

Previously, it was thought that forma aegra verticata was caused by mantle injury; however, our study shows this symptom co-occurred with the anomaly of tubercles and siphuncle formation. Our result suggests that the Ammonoid malfunction of the biomineralization control mechanism developed this multiple symptom.