Revision of Climatius reticulatus Agassiz, 1844 (Acanthodii, Climatiidae), from the Lower Devonian of Scotland, based on new histological and morphological data

ABSTRACT

Disarticulated elements in a large, uncompressed regurgitate from Tillywhandland Quarry (Lochkovian), as well as serial sections of an articulated specimen, reveal the three-dimensional shape and structure of fin spines, scapulocoracoid and dermal plates, and the histological structure of dermal and endoskeletal hard tissues of the climatiid acanthodian Climatius reticulatus. Globular calcified cartilage is the only form of mineralization of the head endoskeleton, with the jaws preserved as double-layered globular calcified cartilage. Tooth whorls are borne on both the upper and lower jaws and comprise a vascularized bone base and tooth cusps composed of a vascular network and mesodentine, without a central pulp cavity. The short admedian spine is the only strongly laterally compressed spine; the anterior dorsal fin spine has a wide, splayed base. No evidence was found of large dermal plates between the pectoral fin spine and the median lorical plates, with the prepectoral pinnal plates with spines being the only large paired dermal components of the shoulder girdle. The anterior lorical plate, pinnal plates, and tesserae on the scapulocoracoid bear ornament comparable to the postbranchial apronic ornament in acanthothoracid placoderms. Branchiostegal plates have a thin, dense inner bone layer and an outer dentinous ornament layer but lack a middle osteodentine layer; other postcranial plates and fin spines have a smooth-surfaced inner layer of bone and a thick middle osteodentine layer. Body scales have a crown with areal growth zones formed of Stranggewebe and syncitial mesodentine on a cellular bone base.

ACKNOWLEDGMENTS

C.J.B. acknowledges the support of the Queensland Museum for providing basic support facilities and ESEM usage, and of an Australian Research Council Discovery Grant for 2010–2013. We thank M. Taylor and S. Walsh (NMS) and Z. Johanson (NHM) for allowing access to specimens in their collections, and T. Senden (ANU) for high-resolution X-ray tomography of QM F57177. We are particularly grateful to R. Jones for access to his collection, and his donation of material for our investigations. We would also like to thank M. Brazeau, M. M. Smith, and an anonymous reviewer, and editors, for their comments.

This is a contribution to IGCP 591: The Early to Middle Paleozoic Revolution, and IGCP 596: Climate change and biodiversity patterns in the mid-Paleozoic (Early Devonian to Late Carboniferous).