Bulletin of the Mizunami Fossil Museum

Bulletin of the Mizunami Fossil Museum Vol. 50, No. 3 Research reports of the “Paleoparadoxiid Mizunami-Kamado specimen”

A paleoparadoxiid specimen from Kamado, Mizunami exhibits numerous bone fragment-like fossils rang-ing in diameter from a few millimeters to about 3 centimeters. These fossils are associated with various morphological features, such as rod-like arrangements depending on the preservation state, surface features rich in irregularities and nutrient foramina, and histological characteristics including a peripheral compact layer composed of calcified cartilage and a spongy interior composed of bone tissue. Based on these features, we preliminarily identify the segmented structure as mineralized tissue developed within the costal cartilage. Most of the isolated bone fragment-like fossils probably have belonged to mineralized costal cartilage judg-ing from the morphological and histological similarity to the elements of the rod-like aggregate. The struc-ture and mineralization process of rib cartilage in desmostylians, including the family Paleoparadoxiidae, remain unclear, and this collection of rib cartilage specimens is expected to serve as a novel source of infor-mation for clarifying the physiology, ecology, and phylogeny of desmostylians. Future research should fo-cus on further elucidating the details of rib cartilage fossils in the Mizunami-Kamado specimen and explor-ing the relationships between the pattern of rib cartilage mineralization and ecology or physiology by com-prehensively investigating the morphological features of rib cartilage in various mammalian lineages. This approach is anticipated to provide new biological insights into desmostylians.