 Reference Base  Notes on the lesser one-horned rhinoceros, Rhinoceros son... |
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Colbert, E.H., 1942. Notes on the lesser one-horned rhinoceros, Rhinoceros sondaicus, 2. The position of Rhinoceros sondaicus in the phylogeny of the genus Rhinoceros. American Museum Novitates 1207: 1-5, figs. 1-3
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| Subject: |
Taxonomy - Evolution |
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Fossil |
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The validity and the direction of the spezializations in Rhinoceros, shown in two successive stages by the modern species are corroborated by a study of the fossil material, particularly the evidence offered by the extinct genus Gaindatherium from the late Tertiary phases of the Siwalik series of India, described a few years ago by the present author (Colbert, 1934). When it was described, Gaindatherium was suggested as an ancestor to Rhinoceros, the comparison being between the fossil form and the Inclian animal. With a skull of Rhinoceros sondaicus at hand it can now be seen how nicely the Javan rhinoceros fits as a form intermediate in position between Gaindatherium and Rhinoceros unicornis.
Every character that sets Rhinoceros sondaicus off from Rhinoceros unicornis is expressed with greater emphasis in Gaindatherium,. This is conveniently shown when Gaindatherium is compared point by point with Rhinoceros sondaicus, using the numbers that were listed in the comparison, presented above, of the two living species.
Gaindatherium browni compared with Rhinoceros sondaicus
1. Gaindatherium is smaller and lighter than Rhinoceros sondaicus.
2. Less expansion of the nasals; horn boss pointed.
3. Ascending ramus lower.
4. Skull generally lower. (The occiput of Gaindatherium is vertical, a primitive character as compared with the forwardly inclined occiput of Rhinoceros)
5. Shallower saddle in cranial profile.
6. Zygomatic arch more angular at posterior termination.
7. Posterior margin of palate with small median projection.
11. Premaxillaries narrow. (Two incisor teeth, a primitive character.)
12. Teeth more brachyodont than in R. sondaicus
13. Ectoloph of cheek teeth sinuous.
14. Parastyle buttress prominent.
15. Neither crista nor crochet present.
It might be mentioned that in these characters Gaindatherium in turn is more or less intermediate between Rhinoceros sondaicus and certain mid-Tertiary rhinoceroses such as Caenopus or Subhyracodon. The reason that Gaindatherium is thought to be directly ancestral to Rhinoceros is its possession of certain characters mentioned in an earlier paragraph that distinguish the latter genus, specifically then presence of a single nasal horn and the consequent development of a saddle-shaped cranial profile, the presence of large, well-developed incisors and the closure of the external auditory meatus inferiorly by the fusion of the post-tympanic and post-glenoid processes.
At this place it might be well to consider briefly two Pleistocene species of Rhinoceros, Rhinoceros sinensis and Rhinoceros sivalensis, both known from considerable suites of materials from China and India, repectively.
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