High incidence of cervical ribs indicates vulnerable condition in Late Pleistocene woolly rhinoceroses

PeerJ. 2017 Aug 29;5:e3684. doi: 10.7717/peerj.3684. eCollection 2017.

Abstract

Mammals as a rule have seven cervical vertebrae, a number that remains remarkably constant. Changes of this number are associated with major congenital abnormalities (pleiotropic effects) that are, at least in humans, strongly selected against. Recently, it was found that Late Pleistocene mammoths (Mammuthus primigenius) from the North Sea have an unusually high incidence of abnormal cervical vertebral numbers, approximately ten times higher than that of extant elephants. Abnormal numbers were due to the presence of large cervical ribs on the seventh vertebra, indicating a homeotic change from a cervical rib-less vertebra into a thoracic rib-bearing vertebra. The high incidence of cervical ribs indicates a vulnerable condition and is thought to be due to inbreeding and adverse conditions that may have impacted early pregnancies in declining populations. In this study we investigated the incidence of cervical ribs in another extinct Late Pleistocene megaherbivore from the North Sea and the Netherlands, the woolly rhinoceros (Coelodonta antiquitatis). We show that the incidence of abnormal cervical vertebral numbers in the woolly rhinoceros is unusually high for mammals (15,6%, n = 32) and much higher than in extant Rhinoceratidae (0%, n = 56). This indicates that woolly rhinoceros lived under vulnerable conditions, just like woolly mammoths. The vulnerable condition may well have contributed to their eventual extinction.

Keywords: Coelodonta; Developmental constraints; Extinction; Neck ribs; Neck vertebrae; North sea; Rhinoceratidae; Transitional vertebrae.

Grant support

This work was in part supported by Synthesys travel grants to F.G. to visit Royal Museum for Central Africa Tervuren, the Zoological Museum Copenhagen, the National Museum for Natural History in Paris, and the Natural History Museum of Stockholm (Grants BE-TAF-1649, DK-TAF-4886, FR-TAF-4104, SE-TAF-3009, respectively). There was no additional external funding received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.