Presence of the extinct sawfish, Onchosaurus (Neoselachii, Sclerorhynchiformes) in the Late Cretaceous of Peru with a review of the genus

摘要

In this paper we report the first Peruvian record of the extinct giant sclerorhynchiform sawfish, Onchosaurus pharao. The single specimen consists of a characteristic rostral spine, which was recovered from nodular limestone in the West-Peruvian Trough of the central Andean Basin, close to the city of Cajamarca. The fossil-bearing strata belong to the lower part of the Celendin Formation, which is of middle-late Coniacian age. This specimen is the first Coniacian record of Onchosaurus pharao and constitutes the third known record of this sclerorhynchiform taxon in South America filling previous gaps in the stratigraphic and geographic distribution of this species. A review of the distribution patterns of both species assigned to Onchosaurus reveals that the genus originated in circum-Equatorial, tropical waters and subsequently dispersed along different pathways south- and northwards. Based on its palaeogeographic distribution and the size of the rostral spines, both Onchosaurus spp. are considered large, bottom-dwelling sclerorhynchiforms primarily inhabiting shallow marine, near-coastal environments. The palaeogeographic distribution nevertheless indicates that they were powerful swimmers and able to cross wide, open marine distances. The reason for the disappearance of Onchosaurus in the Campanian remains unknown. In the early Maastrichtian, Dalpiazia seemingly replaced Onchosaurus ecologically. However, we hypothesize that the different histologies of rostral spines of Onchosaurus and Dalpiazia are of minor taxonomic importance. Dalpiazia thus might represent a derived onchosaur rather than a distinct taxon.