Finally the PBS Eons narrator spilled the beans
on why mammal workers traditionally considered Paraceratherium (Fig. 1) to be a giant hornless rhino. It was (as I suspected, but never learned from emails to rhino experts) the shape of the molar cusps. That issue is addressed below.
Figure 1. Equus the horse shares many traits with Paraceratherium, the giant rhino?/horse?
The following are my two comments yesterday
in the YouTube comments section:
Not a rhino! A giant three-toed horse. Molars are convergent between closely related horses and rhinos. Details here: https://pterosaurheresies.wordpress.com/2016/12/30/maybe-paraceratherium-is-really-a-giant-horse/ and here: https://pterosaurheresies.wordpress.com/2017/01/03/more-taxa-for-the-paraceratheriumgiant-horse-hypothesis/ and here: https://pterosaurheresies.wordpress.com/2018/11/11/aceratherium-vs-paraceratherium/
PS. Hyrachyus IS basal to rhinos. Pappaceras nests between two 3-toed horses (Mesohippus and Miohippus) and Juxia is the size and shape of a horse because it nests between Equus (the modern horse) and Paraceratherium. If you exclude horses from the analysis, sure Paraceratherium will nest with rhinos. When horses are added to the analysis Paraceratherium and Juxia nest with Equus. Don’t trust just one character like molar shape. Look at all the traits from nose to tail and then do your analysis. More details here: https://pterosaurheresies.wordpress.com/2017/04/27/indricotheres-horse-like-rhinos-or-rhino-like-horses/
At the base of the giant indricotheres
we find Pappaceras (Fig. 1). To that everyone agrees. Where the disagreements start is what came before Pappaceras? In the large reptile tree (LRT, 1012 taxa) it’s Miohippus (Fig.1 1) and Equus the extant horse.
Figure 1. Miohippus, Pappaceras and a freehand sketch of Pappaceraus, the latter two from Wang et al. 2017, but colorized here. Note the freehand sketch differs from the CT scan in several subtle and not so subtle ways.
Wang et al. 2017
bring new insight into the chewing mechanism in Pappaceras. They write: “The paraceratheriid Pappaceras is the earliest unequivocal rhinocerotoid genus to date, for which the osteological morphology is relatively unique compared to other perissodactyls. The reconstruction of the masticatory muscles suggests that Pappaceras meiomenus is strictly herbivorous, probably folivorous, with a primary component of vertical biting.”
Distinct from the LRT
The Wang et al. cladogram nests horses at the base of the Perissodactyla. Chalicotheres and brontotheres nest together without resolution. Indricotheres nest with rhinos as sisters to tapirs. The LRT does not support that tree topology at all. The Wang et al. cladogram also nests a perissodactyl outgroup taxon I had never heard of before: Cambaytherium. which I just added to the LRT and we’ll talk about tomorrow. It is not related to perissodactyls in the LRT.
The freehand drawing of Pappaceras
by Wang et al. (Fig. 1`) is distinct in certain subtle aspects from their CT scan (Fig. 1). It’s better to trace directly from photos. Or just use the CT scans. Freehand drawing, by its nature, emphasizes things that catch the eye and deemphasizes things that don’t catch the eye, but perhaps should.
Wang H-B, Bai B, Gong Y-X, Meng J and Wang Y-Q 2017. Reconstruction of the cranial musculature of the paraceratheriid rhinocerotoid Pappaceras meiomenus and inferences of its feeding and chewing habits. Acta Palaeontologica Polonica 62 (2): 259–271.