Fusion of caudal vertebrae in Late Jurassic sauropods

Abstract

ABSTRACT Fusion of caudal vertebrae in the sauropod dinosaurs Apatosaurus, Diplodocus, and Camarasaurus is explained as the result of ossification of ligaments spanning consecutive centra. The ossification does not involve the intervertebral space or the annulus fibrosus of the disc and therefore represents true bridging. This is verified by not only the gross external appearance, but also by the use of CAT scans and longitudinal section of a fused pair of Diplodocus caudal vertebrae. Fusion of caudal vertebrae in Camarasaurus is reported here for the first time. It is concluded that the fusion is the result of the physiological phenomenon called diffuse idiopathic skeletal hyperostosis (DISH) and therefore is not a pathological response or due to a physical injury. Various proposed hypotheses to explain the function of the fused caudals, such as defense and postural support, are considered untenable. Fusion of caudal vertebrae probably aided in keeping the tail elevated and undoubtedly served a functional role, though the specific nature of that function is presently unknown. Survey of North American specimens indicates that fusion of caudal vertebrae occurs in 50% of the individuals of Apatosaurus and Diplodocus in which the requisite portion of the tail is preserved, whereas this figure for Camarasaurus is about 25%. For this reason it is suggested that fusion of caudal vertebrae may be a sexually dimorphic feature.