Woolly rhinos may have grown strange extra ribs before going extinct

As time ran out for the woolly rhino, strange things happened. Before going extinct, some of the beasts faced an unusually high risk of growing bizarre ribs in their neck, a new study suggests. Those misplaced ribs might have signaled the animals’ impending demise.

Scientists examined neck bones from 32 woolly rhinos and found indented spots on five of them where ribs had once attached to the seventh cervical vertebra, the lowermost bone in the neck. That amounts to strange cervical ribs on about 16 percent of the creatures. For comparison, 56 specimens of the same vertebra from modern rhino skeletons had no such spots, says Frietson Galis, an evolutionary biologist at the Naturalis Biodiversity Center in Leiden, the Netherlands. Galis and paleontologist Alexandra van der Geer, also at Naturalis, report the findings August 29 in PeerJ.

Found in what is now the North Sea and in adjacent Dutch deltas and coastal areas, the bones date from about 35,000 to 115,000 years ago, a time of changing climate and ecosystems. The woolly rhino, Coelodonta antiquitatis, probably disappeared from Western Europe sometime not long after roughly 35,000 years ago, although populations to the east survived longer.

Extra bits of rib sticking off a neck bone might not have been particularly harmful to the rhinos. But the anomalies could signal that more disruptive mutations were cropping up in a shrinking, inbreeding population as it slid toward extinction, Galis says. The basics of neck vertebrae are set early in a mammal embryo’s development, along with other crucial matters such as head-tail and front-back orientations, and mutations that affect neck bones may be too disruptive to many embryos for them to survive to birth. Cervical ribs have been associated with childhood cancer and congenital abnormalities in animals that do survive beyond birth.

Though the researchers didn’t match any ribs with the oddly shaped vertebral specimens, the indented spots on the neck bones look like those on vertebrae that support the rib cage. Plus, in 2014, Galis and other colleagues found similar evidence of high inci­­dence of cervical ribs in woolly mammoths from around the same place.

Still, a strong test of the proposed link between neck ribs and woolly rhinos’ impending doom may need more fossils. For instance, in the current study, “there are no comparisons [of neck vertebrae] through time,” says Johannes Müller, a paleozoologist at Humboldt University of Berlin and the Museum of Natural History, also in Berlin. It would be helpful to know how often the ribs show up in woolly rhinos from a time when their populations were flourishing, he says.