Acritolepidae, Valiukevičius & Burrow, 2005

Valiukevičius, Juozas & Burrow, Carole J., 2005, Diversity of tissues in acanthodians with Nostolepis-type histological structure, Acta Palaeontologica Polonica 50 (3), pp. 635-649 : 644

Family Acritolepidae nov.

Comment.—We erect the family Acritolepidae nov. for the taxa of the fourth histological group.

Type genus: Acritolepis Valiukevičius, 2003a .

Genera included: Acritolepis , Monospina Valiukevičius, 2003b , and Pechoralepis gen. nov. (= Nostolepis , partim).

Diagnosis.—Moderately deep−bodied acanthodians with dorsal spines each with 12 longitudinal ribs and long pectoral spines each with 16 ribs; insertion position of posterior dorsal spine opposite the anal spine; no ventral (prepelvic) spines; two pairs of tuberculated, wide−based prepectoral spines that are composed of a thick layer of highly vascularized acellular bone surrounding a central cavity, and cellular bone and mesodentine forming the ribs; scapula with slender shaft having a semicircular cross−section; jawbones with large triangular tooth cusps and smaller secondary cusps along the main lateral ridge and striated denticles on the mesial ledge; small palatine teeth composed of trabecular dentine; body scales with a simple spine−like shape, or with short subparallel or stout radial ridges which may form a “posterior starlet” converging into a pair of symmetric neck ridges; scale and tessera crowns composed of bone−like odontocytic mesodentine, sometimes with stranggewebe in the posterior parts of the crown growth zones; polygonal osteocyte cavities densely distributed in all growth lamellae, with little or no development of a “normal” mesodentinal network.

Discussion.—The histological structure of the prepectoral spines and the highly odontocytic crown mesodentine differ to those of all other acanthodians. The scales differ to those of the Tchunacanthidae in shape and base/crown proportions, coordinated growth of crown and base (in Tchunacanthidae , the base continued to grow after the crown stopped), and in the shape and abundance of osteocytes or odontocytes (which are rare, small and rounded in Tchunacanthidae ). The acritolepids resemble the ischnacanthiforms in having dentigerous jaw bones and palatine teeth, but differ in having prepectoral spines and in the histological structure of all dermal elements.