Clarification of the Occipito-Vertebral Region of Arapaima gigas (Osteoglossomorpha: Osteoglossidae) through Developmental Osteology

The structure and composition of the ventral portion of the occipital region of the skull in Arapaima gigas (Osteoglossidae) is unique among teleostean fishes, and past comparative anatomical studies have variously interpreted it as containing only the basioccipital or the basioccipital fused to one or two vertebral centra. By studying an ontogenetic series, we show that the dominant element of the ventral occipital region of the skull in Arapaima is the first vertebral centrum and its greatly enlarged parapophyses. The parapophyses, which become fused to the centrum, extend anteriorly to suture to the lateral portions of the parasphenoid. In the adult, the anterior portion of the basioccipital is flattened, with a narrow ventrally directed keel of bone that is exposed ventrally only along the midline of the skull. Although a general enlargement of the anteriormost parapophyses appears to be a synapomorphy of the family Osteoglossidae, their arrangement in other osteoglossids does not closely resemble that described herein for Arapaima.