Rhinoceros Giants: The Paleobiology of Indricotheres

6. Building a Giant

Building a Giant Dry Bones Now that we have thoroughly explored the geology of the regions that produce indricothere fossils and their evolutionary roots within the rhinocerotoids, let us look more closely at the monsters that still hold the record for the largest land mammal that ever lived. The most impressive part of the animal is the business end: its immense skull (Figs. 5.1B, 6.1). The nearly complete presumed male skull from Mongolia (American Museum of Natural History, or AMNH 18650) is a truly awesome sight all by itself. It measures about 1.3 meters (52 inches, or almost 5 feet) long, according to Granger and Gregory (1936), 33 cm tall at the back of the skull, 33 cm wide at the base of the back of the skull, and is about 61 cm (about 2 feet) wide across the skull at the zygomatic arches. Another partial skull from Mongolia , AMNH 26165, is even larger than this specimen, measuring about 35 cm wide at the base of the back of the skull and 35.5 cm tall at the back of the skull. An even larger partial skull from Mongolia, AMNH 26167, measures 36.5 cm at the back of the skull base and 38 cm tall at the back of the skull. The presumed female skulls from Dera Bugti (Fig. 5.1A) are almost as large in most of these dimensions (according to measurements in Forster Cooper, 1923a, 1923b), so far as can be determined from their less complete preservation and subsequent deformation and distortion. The next thing that you notice about the skull (Fig. 6.1), besides its immense size, is the large pair of conical upper tusks that point downward and the pair of short conical lower tusks that point forward. As mentioned in Chapter 4 (Figs. 4.7, 4.13), these teeth are unique among all known rhinos and only occur in Urtinotherium and Paraceratherium (with all its junior synonyms). These tusks occur at the end of an elongated snout, with a large gap (diastema) between them and the cheek tooth row. Unlike most other groups of primitive rhinos, indricotheres have lost all their remaining front incisors as well as the canines that would normally erupt right behind the incisor tusks (Figs. 4.7, 4.13). Osborn (1923a, p. 6) argued that these tusks were mainly for defensive purposes, but Granger and Gregory (1936, p. 2) suggested that “their primary function was to assist in the sudden jerking loose of shrubs by downward movements of the head and neck, since they are well placed to act thus as picks and levers, while the skull is braced to resist such stresses through its strong rostrum, down-curved zygomata and greatly emphasized basi-occipital eminence.” The next most striking feature is the top of the skull, with the long, smooth domed forehead and no trace in any specimen of a roughened area that would serve as the attachment point for a horn. The bones above the nasal region are long and delicate, and the opening for the nasal incision goes far back into the skull. This enlarged nasal incision is usually a mark of some kind of trunk or proboscis in living mammals, such as tapirs and elephants. Most reconstructions of indricotheres ignore this feature, but they should show a very long prehensile lip, as occurs in living black and Indian rhinos with a shallower nasal incision, and possibly even a short proboscis as well. Such a proboscis is extremely useful to leaf-eaters (browsers) like tapirs, which use them to wrap around branches as they strip off the leaves with their front teeth. Figure 6.2 shows the head muscles and snout-lips reconstruction of Juxia by Qiu and Wang (2007). This creature was much smaller with a much smaller nasal incision, so it has a normal rhino upper lip. A large indricothere would have had an even longer, more proboscislike snout. 88 . Rhinoceros Giants Figure 6.1. Skull usually called "Indricotherium" (originally called "Baluchitherium grangeri," but now Paraceratherium transouralicum) from the Hsanda Gol Formation, Mongolia (AMNH 18650), showing its immense size compared to preparator Otto Falkenbach, who reconstructed it from many fragmentary parts. (Image number 310387; courtesy American Museum of Natural History Library.) Putshkov and Kulczicki (1995) and Gromova (1959) speculated that indricothere tusks were primarily used to break twigs and strip bark, as well as to bend higher branches. They also...