Variation among indricotheres (giant horse-rhinos)

Earlier we looked at the now heretical nesting of giant indricothere perissodactyls closer to horses than to living rhinos, their traditional relatives. We also touched on that subject here and here.

Also
there has been a movement (Lucas and Sobus 1989) to make many of the largest indricotheres congeneric. A look at the skulls (Figs. 1) suggests otherwise.

Figure 1. Indricothere skulls to scale along with horse and rhino skulls.

Figure 1. Indricothere skulls to scale along with horse and rhino skulls. Clearly the giant skulls, all indricotheres, are not congeneric. Aceratherium is more closely related to the extant horned rhino Ceratotherium.

Wikipedia reports
Indricotheriinae is a subfamily oHyracodontidae, a group of long-limbed, hornless rhinoceroses convergently similar to the sauropod dinosaurs that evolved in the Eocene epoch and continued through to the early Miocene.” By contrast, in the large reptile tree (LRT, 1012 taxa) Hyracodon nests at the base of extant rhinos, apart from the horse/indricothere branch.

Figure 2. GIF movie (3 frames) showing what is known of the skeletons of Baluchitherium and Indricotherium. Note the more horse-like morphology.

Figure 2. GIF movie (3 frames) showing what is known of the skeletons of Baluchitherium and Indricotherium. Note the more horse-like morphology. All reconstructions are chimaeras of known specimens. That doesn’t mean they are congeneric.

A new Pappaceras illustration
(Wood 1963; fig. 3) is more horse-like than others in having an orbit in the (probable) posterior half of the skull.

Figure 3. Pappaceras confluens A.M.N.H. No. 26660 and A.M.N.H. No. 26666 (mandible)

Figure 3. Pappaceras confluens A.M.N.H. No. 26660 and A.M.N.H. No. 26666 (mandible). With such posetriorly-placed eyes, this skull is more horse-like than other rhinos. 

The post-crania of Indricotherium
appears to include the only vertebral column known for this clade. IF so the vertebrae cannot be imagined as similar to that of a rhino (Fig. 2). And maybe, just maybe those indricothere limbs were covered with more gracile muscles and thinner skin, like those of a horse, not a rhino, tradition not withstanding. based on phylogenetic bracketing.

References
Chow M and Chiu C-S 1964. An Eocene giant rhinoceros. Vertebrata Palasiatica, 1964 (8): 264–268.
Forster-Cooper C 1911. LXXVIII.—Paraceratherium bugtiense, a new genus of Rhinocerotidae from the Bugti Hills of Baluchistan.—Preliminary notice. Annals and Magazine of Natural History Series 8. 8 (48): 711–716.
Forster-Cooper C 1924. On the skull and dentition of Paraceratherium bugtiense: A genus of aberrant rhinoceroses from the Lower Miocene Deposits of Dera Bugti. Philosophical Transactions of the Royal Society B: Biological Sciences. 212 (391–401): 369–394.
Granger W and Gregory WK 1935. A revised restoration of the skeleton of Baluchitherium, gigantic fossil rhinoceros of Central Asia. American Museum Novitates. 787: 1–3.
Lucas SG and Sobus JC 1989. The Systematics of Indricotheres”. In Prothero DR and Schoch RM eds. The Evolution of Perissodactyls. New York, New York & Oxford, England: Oxford University Press: 358–378. ISBN 978-0-19-506039-3.
Osborn HF 1923. Baluchitherium grangeri, a giant hornless rhinoceros from Mongolia. American Museum Novitates. 78: 1–15. PDF
Pilgrim GE 1910. Notices of new mammalian genera and species from the Tertiaries of India. Records of the Geological Survey of India. 40 (1): 63–71.
Wood HE 1963. A primitive rhinoceros from the Late Eocene of Mongolia. American Museum Novitates 2146:1-11.

wiki/Juxia
wiki/Paraceratherium

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