Skull Development

Extant rhinos and T. major have similar ontogenetic patterns in dorsal cranial developmental changes. In Age Classes 1-2, all species have rounded crania except for C.

simum, which has a lengthened and angular cranium. All species undergo distinct cranial

lengthening in Age Classes 3-5, making crania easily distinguishable to species. Co-occurring with cranial lengthening is occipital crest pronouncement and temporal line appearance. The temporal lines become visible early in T. major and R. unicornis (Age Classes 3-4) and late in C.

simum and D. bicornis (Age Class 6).

In Age Class 5, the occiput and frontal plate begin to widen and develop angularly into adult morphologies, and different developmental patterns follow. The Asian rhino crania widen more proportionately, while African rhino crania widen with continued lengthening. Through Age Classes 3-6, T. major has morphological similarities with both Asian rhinos (rounded occiput) and African rhinos (frontal plate). Teleoceras major also has distinct characters early in ontogeny that are not found in extant rhinos, such as laterally flared zygomatic arches and short, pointed nasals; these characters are present in T. major through ontogeny.

In all species, nasals, frontal plate, and occiput continue to change in Age Classes 7-9, and in Age Class 10 adult morphology is reached with little change afterward. Morphological changes in Age Classes 10-15 are slight exaggerations to the nasals, zygomatic arches, frontal plate, and occiput. The occipital and temporal crests also become more pronounced through Age Classes 10-15.

Cranial suture development is similar across rhino species. Parietal-occipital and frontal- parietal sutures close in Age Classes 6-8. Facial sutures close in Age Classes 10-12. Nasal- frontal and jugal-zygoma sutures disappear in Age Classes 10-12 in extant rhinos, but these

sutures were not closed in T. major specimens. This apparent lack of suture closure is likely due to fracturing of peripheral fossil bones common in T. major specimens.

Dorsal cranial ontogenetic changes in extant rhinos and T. major are similar to those described in Chilotherium wimani by Deng (2001). Deng (2001) described similar occipital and parietal shape changes, occipital and temporal crest pronouncement timings, and suture closings in C. wimani. Unfortunately, descriptions and depictions of the lateral skull were not provided for a comparison of characters related to feeding ecology used in this study.

Comparisons of the lateral skull suggest that development of primary muscle attachment areas (mandibular angle, zygomatic arch, and occiput) and dentition area (mandibular body) are distinct among species. Early in ontogeny, T. major stands out with its enlarged mandibular angle and zygomatic arch. These masseter attachment sites in T. major continually expand through ontogeny, remaining significantly larger than in other species. All species appear to have considerable increases in jugal broadness in Age Classes 5-7, and the mandibular angle appears to change consistently across species. Although R. unicornis reaches mandibular angle morphology similar to T. major in adulthood, T. major has the most developed angle and zygomatic arch earliest in ontogeny.

Ceratotherium simum also stands out with its enlarged and posteriorly oriented occiput

early in ontogeny. The occiput in C. simum expands through ontogeny, remaining larger and more posteriorly oriented than the other species. Rhinoceros unicornis and D. bicornis have changes in occipital height and angle beginning in Age Class 3. In T. major, the occipital angle does not begin to change until Age Classes 4-5, and occipital height does not noticeably change until after Age Class 5.

The deep mandibular bodies of C. simum and T. major appear to follow different developmental patterns compared to the shallower mandibular bodies of R. unicornis, D.

sumatrensis, and D. bicornis. In C. simum, the mandibular body is deep early in ontogeny, but

other species including T. major have thin mandibular bodies at similar ages. Ceratotherium

simum and other extant rhinos have consistent increases in body depth through ontogeny, but T. major reaches the deep body proportions of C. simum around Age Class 10, which suggests

different mandibular body developmental patterns in T. major and C. simum.

Despite early developmental differences in the mandibular angle, zygomatic arch, occiput, and mandibular body, all species show similar exaggerations in robustness in Age Classes 10-15. Adulthood is thus characterized by increases to species-specific morphologies achieved earlier in ontogeny. Perhaps having larger attachment areas and more developed muscles early in ontogeny are a selective advantage for the mastication of coarse vegetation. 5.2.5 Thin-Plate Splines

The thin-plate splines depicted developmental shape differences related to muscle

attachment areas, dentition, and horns. The most similar ontogenetic change across all species is consistent posterior and ventral occipital contraction. Similar contraction seems counterintuitive as species develop large occiputs with differing morphologies and orientations. However, braincase develops with negative allometry in mammals (Emerson and Bramble 1993), which is thus demonstrated in the thin-plate splines. Another similarity is consistent mandibular ramus expansion in all species. Rhinoceros unicornis, however, does have stronger expansion, which is explained by its small ramus early in ontogeny and its large ramus in adulthood. All species demonstrate zygomatic arch expansion through ontogeny except for C. simum. Ceratotherium

simum has zygomatic expansion only in adulthood as C. simum has a long zygomatic throughout

ontogeny.

Nasal, maxilla, and mandibular body developments exhibit the greatest differences among species. Nasal expansion is consistent through ontogeny in the species, but the degree of expansion varies among rhinos. African rhinos have weak nasal expansion, and Asian rhinos and T. major have strong nasal expansion. Weaker nasal expansion in the African rhinos is likely related to their large nasal size early in ontogeny. Maxilla and mandibular body expansion through ontogeny also is present in all species, but C. simum and T. major have weaker and stronger expansions, respectively. These maxilla and mandibular body differences in C. simum and T. major are related to the areas for dental housing, being deep in C. simum throughout ontogeny and becoming deep in T. major in adulthood.