Giraffes and their necks

The standard story of how the giraffe got its long neck may not be as straightforward as we usually think of it (or teach it):

There is a large diversity of giraffe relatives in the fossil record, as the chart shows.
This chart is not an accurate depiction of the evolutionary history of giraffes, but is here displayed to show what the extinct presumed relatives and ancestors might have looked like.

For instance, the Siwalik giraffes were widespread in the Siwalik hills (outer Himalaya, India, Nepal and Pakistan), before they went extinct. Giraffokeryx punjabiensis was a short necked giraffe (once thought to belong to the genus Giraffa) that lived about 12 million years ago, and Giraffa sivalensis was a long necked giraffe that went extinct about two million years ago.  
The short-necked G. punjabiensis could easily be interpreted as an immediate ancestor of G. sivalensis, but recent studies show that they are on two different lineages. Indeed, it appears that G. sivalensis and G. camelopardalis are each other’s closest relatives (see abbreviated phylogenetic tree below).

A recent article entitled How Giraffes Became Winners by a Neck describes the complicated story of the evolution of the giraffe and its familiar long neck, based on research reported in the article Fossil evidence and stages of elongation of the Giraffa camelopardalis neck.

Surprisingly, the authors found that this group started off with already longish necks (as seen in the relatively narrow neck vertebrae in Pe and Cs in the figure below). Two lineages–one consisting of the okapi and the other containing Giraffokeryx punjabiensis, have shorter necks compared to Cs; necks in the other lineage appear to elongate, with much longer necks in Giraffa sivalensis and G. camalepardalis, which has the longest neck of all. 

Figure 5. Cladogram with geological age and dorsal view of C3 vertebrae of taxa evaluated. Pe, Prodremotherium elongatum; Cs, Canthumeryx sirtensis; Oj, Okapia johnstoni; Gp, Giraffokeryx punjabiensis; Sg, Sivatherium giganteum; Bm, Bramatherium megacephalum; Sm, Samotherium major; Pr, Palaeotragus rouenii; Ba, Bohlinia attica; Gs, Giraffa sivalensis; Gc, Giraffa camelopardalis. (a) A modified cladogram based on previously published cladograms by Hamilton [13] and Solounias [14], with the exclusion of species not evaluated in this study. Each clade terminates in a square point corresponding to the age of the respective taxon in millions of years (Ma). The dorsal view of a C3 vertebra for each taxon is demonstrated (excluding Bohlinia as there are no known C3 fossils). Each specimen is isometrically scaled so that all vertebrae are of equal length. (b) Silhouettes of O. johnstoni, S. major and G. camelopardalis (left to right) are provided to give a comprehensive image of a long-necked, short-necked and intermediate-necked individual. All the fossils in bold are found in the Siwalik Hills.

The story of the giraffe’s neck is not a smooth one — the neck started a bit long, in the early lineages such as Canthumeryx, became a bit short in lineages such as Giraffokeryx and relatives, then became long again (Samotherium), and longer and longer, ending up with the modern long-necked giraffe.

Wonder what will happen in the future…?










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