Systematic, stratigraphic, geographic and paleoecological distribution of the Late Cretaceous shark genus Ptychodus within the Western Interior Seaway.

摘要

This is the first comprehensive review of the Cretaceous elasmobranch family Ptychodontidae and the first 3 dimensional morphometric analyses regarding the variability in each species and tooth position of individual teeth within the dentition. The Ptychodontidae consists of four genera, Hylaeobatis, Heteroptychodus, Paraptychodus n. gen. and Ptychodus. Hylaeobatis, Heteroptychodus and Paraptychodus n. gen. are considered successive sister taxa to Ptychodus. Based on stratigraphic occurrence, modification in dental morphology within the family illustrates continuous specialization. Dental characteristics indicate that the high crowned species of Ptychodus arose repeatedly and separately from low crowned sister species. Skeletal elements associated with Ptychodus include, dermal denticles, circular vertebral centra, plesoid pectoral fin and a dorsal fin, suggesting Ptychodus has a spindle shaped body. Its co-occurrence with large lamniform shark taxa in contemporaneous deposits suggests that Ptychodus were large bodied predators capable of moving at fast speeds that occupied the apex of the food chain. The use of 3 dimensional morphometric data capture and statistical analysis demonstrates that it is possible to determine the exact placement of an individual tooth specimen within a dentition that can contain up to ten tooth files. Based on this data the systematic position of the family Ptychodontidae should be placed within its own order, the Ptychodontiformes at the base of the SuperOrder Galea.