THE PARADOX OF LARGE SCYLIORHINOID-TYPE SHARK VERTEBRAE IN THE UPPER CRETACEOUS OF NEW JERSEY

Carcharhiniform and lamniform sharks are thought to derive from a common ancestral galeomorph during the Middle Mesozoic. Scyliorhinoid-type vertebrae, typical of Mesozoic carcharhiniforms, found in the upper Cretaceous of New Jersey preserve anatomical characteristics, namely solid centra, analogous to vertebrae found in Cenozoic and modern carcharhiniforms. Conservative estimates of body size based on total length-centrum diameter regressions for modern carcharhiniforms and lamniforms indicates that some of these upper Cretaceous sharks with scyliorhinoid-type vertebrae achieved total lengths of 6 meters or more. Paradoxically, a rich fossil record of shark teeth from the upper Cretaceous of New Jersey and elsewhere in North America lacks any firm evidence for large-bodied ancestral carcharhiniforms. Although large lamniforms have left an abundant tooth record in the New Jersey upper Cretaceous, the entire Atlantic Coastal Plain carcharhiniform tooth record is one of small-bodied genera (e.g., Galeorhinus, Palaeogaleus) unlikely to have possessed such large scyliorhinoid-type vertebrae. However, one extinct family of large upper Cretaceous lamniform sharks, the Anacoracidae, contains species with both lamniform teeth and scyliorhinoid-type vertebral morphology. The co-occurrence of these features in the same animals suggests that solid centra should not be regarded as a synapomorphic character for separating groups of galeomorphs, and that an anacoracid may be responsible for the large New Jersey scyliorhinoid-type vertebrae. Extinction across the Cretaceous-Tertiary boundary of large lamniforms with scyliorhinoid-type vertebrae may explain the absence of this shared anatomical characteristic during the Cenozoic.