The histology of the Upper Silurian osteostracan Timanaspis kossovoii Obruchev (Agnatha) from North Timan, Russia

Abstract

AbstractThe fine structure of the exoskeleton of Timanaspis kossovoii Obruchev 1962 was investigated based on new well‐preserved material. The remains come from the lower part of the Eptarma Formation (Upper Silurian). Timanaspis kossovoii was referred by Obruchev to the family Tremataspididae and most authors agree with the placement of Timanaspis among tremataspidid osteostracans. Histological study of the shield of T. kossovoii has revealed the presence of unusual features in the exoskeletal structure. The smooth and shiny surface of the ventral side of the shield of T. kossovoii is similar to that of species of Tremataspis, but thin sections show differences in histological composition. The exoskeleton of T. kossovoii consists mainly of dense bony tissue in the middle layer and a laminated basal layer. No osteocyte cavities of typical form and size were found in the middle layer. The upper parts of the elongated tubercles of the dorsal side of the shield are formed by a well‐developed dentine‐like tissue; the arrangement of numerous tubules within this tissue is far more regular than in the mesodentine of Tremataspis. No traces of perforated septa, pore fields or a polygonal model were detected in the structure of the exoskeleton of Timanaspis. We conclude that Timanaspis can be differentiated from conventional tremataspidids by histological features and refer it to the new family Timanaspididae. Moreover, the Timanaspis exoskeleton provides evidence that an acellular condition of bony tissue does not appear to be characteristic of only Late Devonian osteostracans.