Article: Astraspis – the anatomy and histology of an Ordovician fish
A newly discovered specimen of Astraspis desiderata Walcott from the mid Ordovician Harding Sandstone Formation of Colorado, USA, is described and represents the third, and most complete, articulated specimen of the taxon recorded to date. Astraspis is a pteraspidomorph agnathan with a dorsal headshield formed from polygonal tesserae, and a ventral shield composed of more irregular tesserae. The tesserae are surmounted by star-shaped or smooth, round or ovate tubercles. Well-developed orbits lie directly anterior to a series of eight uncovered, branchial openings which are arranged in a postero-ventrally slanting line. Sensory grooves flank a pineal region on the dorsal headshield. The tail is made up of large rhomboid scales, but the structure of the tail termination remains unknown. Histologically, the hard tissues of A. desiderata comprise a basal component of spongy aspidin with large trabeculae and large, irregular soft tissue spaces. Individual tubercles overlie the aspidin and are composed of dentine with an enameloid cap separated by a junctional zone. The dentine is characteristic of Astraspis and contains a high density of parallel, fine calibre tapering tubules. Pycnaspis splendens Orvig is considered to be a junior synonym of Astraspis desiderata and the genus is monotypic. Astraspis has few derived characters in common with other agnathan taxa and is here placed within the monotypic Astraspida, as the sister group of Arandaspida + Heterostraci.