The number of cervical vertebrae in mammals is remarkably constant at seven, in contrast to other tetrapods, where this number varies considerably ( Leboucq, 1898 Schultz, 1961 Starck, 1979 Narita & Kuratani, 2005). The vulnerable condition may well have contributed to their eventual extinction. This indicates that woolly rhinoceros lived under vulnerable conditions, just like woolly mammoths. 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). 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). 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. 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. 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. Changes of this number are associated with major congenital abnormalities (pleiotropic effects) that are, at least in humans, strongly selected against. Mammals as a rule have seven cervical vertebrae, a number that remains remarkably constant. High incidence of cervical ribs indicates vulnerable condition in Late Pleistocene woolly rhinoceroses. Cite this article van der Geer AAE, Galis F. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited. 2 Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Zografou, Greece DOI 10.7717/peerj.3684 Published Accepted Received Academic Editor Virginia Abdala Subject Areas Ecology, Evolutionary Studies, Paleontology, Veterinary Medicine, Pathology Keywords Transitional vertebrae, Coelodonta, North sea, Developmental constraints, Neck vertebrae, Extinction, Neck ribs, Rhinoceratidae Copyright © 2017 Van der Geer and Galis Licence This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed.