The earliest chimaeriform fish from the Carboniferous of Central Russia
The ancestry of chimaeriform chondrichthyan fishes can be traced back to the Late Triassic (∼220 Ma). To date, only one chimaeriform suborder, the Echinochimaeroidei, has been recognized from the Palaeozoic. The origin and evolution of the chimaeriforms has been a matter of debate for more than a hundred years. This problem is aggravated by the scarcity of holomorphic fossils; the bulk of the material includes only hard parts of the jaw apparatus and other skeletal elements. Here we describe the oldest record of the Chimaeriformes, represented by tooth plates from the lower Carboniferous deposits (∼338–332 Ma) of Russia, as Protochimaera mirabilis gen. et sp. nov., and establish a new family and suborder based upon their particular morphological and histological features. The newly described tooth plates demonstrate a continuous layer of varitubate dentine over their oral surfaces; beak-shaped mandibular plates have been formed by fusion of a labial component of cutaneous origin to two others formed orally. Three separate vascular systems enabled the growth of these histological components. The beak and wear facets suggest a grasping-cutting feeding action and are the earliest example of this feeding mode in fishes after the placoderm extinction at the end of the Devonian. Phylogenetic analysis of the dental characters places these stem chimaeriforms as a sister group to the Chimaeroidei.
