Apholidotus, a Highly Specialized New Actinopterygian from the Bear Gulch Limestone (Heath Formation, Serpukhovian, Upper Mississippian) of Montana, USA

A small, elongate actinopterygian, Apholidotus ossuous, new genus and species, is described from the marine Bear Gulch Member of the Heath Formation, Serpukhovian of Fergus County, Montana, USA. Apholidotus ossuous is found ubiquitously across the bay. The dermatocranium is reduced and cranial lateral line canals are independent of dermatocranial bones. The palate is braced against the parasphenoid and is only slightly attached to the hyoid arch. The neurocranium is postorbitally ossified as separate bones. The opercular cover consists only of branchiostegal rays. The pectoral girdle is slim and not anchored to the skull; the posteriorly extended scapula and coracoid articulate with five vertically oriented radials, which in turn support five vertical lepidotrichial fin rays. The body is scaleless and devoid of any traces of lateral line canals. The long, low dorsal fin and the caudal fin are supported by actinotrichia. There is a small, externally rounded but internally hemiheterocercal caudal. Discrete pelvic or anal fins are absent. Vertebrae are complex and regionalized; median dorsal and ventral apophyses stiffen the axis from the posterior abdomen to the caudal vertebrae. There are distinct dorsoventral bends between the head and the first two vertebral elements, and in the mid-abdominal region. Cranial structural and functional considerations, and comparisons with Recent fish, suggest that A. ossuous was primarily a suction feeder. Postcranial anatomy suggests that life posture in the water may have been vertical rather than horizontal. There are strong convergences between A. ossuous and modern heterocongrid eels as well as members of the modern teleostean Syngnathiformes. The osteology and its functional implications indicate that it is a derived actinopterygian in comparison to other basal or Paleozoic Osteichthyes, Sarcopterygii or Actinopterygii in many skeletal characters and that profound developmental deviations have occurred from fundamental osteichthyan patterns. These in turn render any tighter assignment of this new order's phylogenetic affinities impossible at this time.