The caudal skeleton of pachycormiforms: Parallel evolution

Hypurals alone or the parhypural plus an indetermined number of hypurals form the hypural plate of pachy-cormiforms. An unusual series follows posterior to the series of unpaired and a few paired neural spines bearing membranous outgrowths. This series is formed by unpaired, median, thick "uroneurals" that extend posteriorly dorsal to the hypural plate, in between the hemitrichia. The "uroneurals" are interpreted here as expanded neural spines, some of them still bearing part of their arches, of the last preural vertebrae and ural vertebrae, whereas they were identified as uroneurals and the pachycormiforms placed within Teleostei sensu PATTERSON (1973). The new morphological evidence questions the teleostean synapomorphies proposed by PATTERSON (1977) as well as the inclusion of pachycormiforms and other basal "teleosts" within the Teleostei. The broadening of the neural spines, "uroneurals", and "epurals", the thickness of haemal arch and spines, the fusion of hypurals, the increase of number of caudal rays, the peculiar structure of rays and fulcra, and the almost symmetrical position of dorsal and ventral elements of the caudal skeleton with respect to the body axis are interpreted as features increasing the stiffness and propulsion abilities of the tail.