Giraffes are one of the world’s most iconic animals. With their two metre long necks and leopard-like patterning they is unmistakable. Today there are only two extant members of the family Giraffidae, the giraffe and the okapi and they couldn’t be more different. Although both have only six vertebrae in their necks in the giraffes these have become incredibly elongated, up to thirty centimetres long in some cases. This lengthening has also led to a change in the shape and structure of these vertebrae to accommodate the adaptations to muscles, blood vessels and nerves necessary to prevent the neck collapsing under its own weight or starving the now distant brain of oxygen. This means modern giraffes are highly specialised but how did this process evolve? Was it a sudden mutation that saw their necks leap in size, or was it a gradual progression? A partial answer to that question came earlier this month (November 2015) in the form of a fossil giraffid called Samotherium.
Diagram showing the difference between a modern giraffe’s neck vertebrae (a), Samotherium major (b) and a modern okapi (c). Source: Royal Society Open Science journal
Samotherium roamed the grasslands of Africa and Eurasia around 7 million years ago. It superficially resembled an okapi but it also had several giraffe-like features such as two prominent ossicones (horn-like projections) on its skull and the long, slender legs that make their modern relatives seem strangely spindly. Although this remarkable animal has been known from the fossil record for over a century this new analysis of both the size and shape of its neck bones allowed the researchers to build an incredibly detailed picture of its neck’s structure. They were then able to compare this to the bones of modern giraffes and okapis. Samotherium‘s neck isn’t as long as a giraffe’s but it is much longer then an okapi’s and in fact it this new study shows that the size of the vertebrae are almost exactly intermediate in length between the two, but that isn’t its only apparently transitional feature. The shape and structure of the neck vertebrae, including the angles between the bones, also shows a mosaic of okapi-like and giraffe-like features. This suggests Samotherium represents an intermediary form, part way between modern giraffe with their incredible specialisation and an ancestral, okapi-like animal with a much shorter neck similar to that seen in other hoofed mammals.
Why giraffes evolved these long necks is still something of a mystery but this new analysis at least shows that it happened slowly, with a gradual progression from short necked forms to longer ones. Samotherium is a fascinating example of a transitional fossil and it has plugged a significant gap in our understanding of giraffe evolution.
Reference: Danowitz, M., et al. 2015. The cervical anatomy of Samotherium, an intermediate-necked giraffid. Royal Society Open Science. DOI: 10.1098/rsos.150521
Featured Image: Wikimedia Commons
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