A new study of woolly mammoth fossils indicated that they suffered from high-rates of birth defects involving an extra cervical rib, according to LiveScience.

Natural History Museum of Rotterdam and Utrecht University researchers discovered that most of the remains of a number of mammoths found near the North Sea from the late Pleistocene period had more than the average number of ribs.

The extra cervical rib is located along their neck vertebrae, according to LiveScience.

For the study, the researchers looked at the collections of the Natural History Museum of Rotterdam and the Naturalis Biodiversity Center in Leiden, Netherlands.

They examined the mammoth fossils and discovered 16 neck vertebrae, six pieces of the sixth cervical vertebrae and 10 of the seventh cervical vertebrae, according to LiveScience.They then analyzed nine of those and determined that three were once attached to the cervical ribs.

Despite only being able to examine a small sample of bones, the results surprised the researchers, especially when they compared it to the rate of the same birth defects in modern elephants. They reported that the rate in ancient mammoths is 10 times higher.

The high rate of birth defects in mammoths was likely caused by inbreeding, according to the researchers.

"The high incidence and large size of the cervical ribs indicates a strong vulnerability, given the association of cervical ribs with diseases and congenital abnormalities in mammals," said study researcher Jelle Reumer, a paleontologist, and his colleagues in a statement to LiveScience. "The vulnerable condition may well have contributed to the eventual extinction of the woolly mammoths."

Research was published in the March 25 issue of PeerJ.

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