PBS Eons video on Paraceratherium, the giant horse

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/

wiki/Juxia
wiki/Paraceratherium
wki/Indricotheriinae

Aceratherium vs. Paraceratherium

Aceratherium is a hornless rhino (Figs 2-4).
Paraceratherium is a GIANT hornless horse (Fig. 1). Even so, the two are similar enough that that latter was named for the former. Thereafter Paraceratherium became known as a rhino.

Figure 1. Equus the horse shares many traits with Paraceratherium, the giant three-toed horse.

However,
and as we learned earlier by testing prior assumptions in the large reptile tree (LRT, 1318 taxa, subset Fig. 5), Aceratherium nested between rhinos and brontotheres. Paraceratherium nested with other large three-toed horses.

Fig. 2 Aceratherium skeletal mount. This hornless rhino is transitional to brontotheres, not indricotheres (= paraceratheres) in the LRT.

Even so,
the convergence is impressive! No wonder earlier workers named the one for the other.

Figure 3. Aceratherium acutum skull drawing and fossil.

Convergence is rampant within the LRT.
For example, we’ve seen mysticetes and odontocetes converge so much we call them all ‘whales’ or ‘cetaceans‘, two terms that need to be dumped in favor of something more in keeping with their phylogenetic nestings. The same has happened with Aceratherium and Paraceratherium. The details of their skeletal traits distinguish them. You can examine those traits in a MacClade file by request.

Figure 4. Indricothere skulls to scale along with horse and rhino skulls.

Perhaps this is just one more instance of paleontology
turning a blind eye toward testing a wider gamut of taxa to validate prior hypotheses… or invalidate them. That’s why the LRT is here: to test prior hypotheses.

Figure 5. Various ungulates and kin subset of the LRT. Here Aceratherium, a hornless rhino, does not nest with Paraceratherium, a giant three-toed horse.

Shifting all the paraceratheres
over to the aceratheres adds 21 steps to the LRT.

Aceratherium incisivum (Kaup 1832; originally Rhinoceros incisivum, Cuvier 1822; Miocene; 2.3m long) nests with short-legged Metamynodon and shares with it long anterior dentary teeth, a straight jugal and a short nasal. Aceratherium lacks an upper canine.

Figure 6. GIF movie (3 frames) showing what is known of the skeletons of Baluchitherium and Indricotherium. Note the more horse-like morphology.

Paraceratherium transouralicum  (P. bugtiense holotype, Pilgrim 1908; Baluchitherium, Osborn 1923; late Oligocene, 34-23mya; 4.8m shoulder height, 7.4m long) was long considered a giant hornless rhinoceros, but here nests with the horse, Equus. They share a long neck, straight ventral dentary and the retention of premaxillary teeth, among other traits. Paraceratherium retains three toes, as in ancestral horse/rhinos like Heptodon and Hyracotherium.

Figure 7. Mesohippus, the last common ancestor in the LRT to horses and indricotheres.

References
Chow M and Chiu C-S 1964. An Eocene giant rhinoceros. Vertebrata Palasiatica, 1964 (8): 264–268.
Cuvier G 1822a. Recherches sur les ossements fossiles. Tome second, G. Doufor et d’Ocagne éd., Paris, – (1822b). Tome troisième, – (1824). Tome cinquième.
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. doi
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.
Kaup J 1832. Über Rhinoceros incisivus Cuv., und eine neue Art, Rhinoceros schleier-macheri, Isis von Oken, Jahrgang1832 (8: 898-904.
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
wki/Indricotheriinae
wiki/Metamynodon
wiki/Aceratherium

Horse fingers

Here’s a paper that recovers overlooked data.
So it parallels what is done here.

Horses have only one finger and one toe,
right?

Now horses have portions of all five toes… extending to the hooves
(Solounias et al. 2017). And everybody overlooked that, until now (Figs. 1, 2).

Figure 1. Horse fingers extend to the hoof, separated from metacarpals by unossified tissue. Image from Solounias et al. 2017.

From the abstract:
“We revisit digit reduction in the horse and propose that all five digits are partially present in the modern adult forelimb.”

Figure 2. The modern horse, Equus, and extinct horse, Mesohippus manus. Colors over diagrams are from the original diagrams. Colors over proximal metacarpals added here. Image from Solounias et al. 2017.

Vestiges count.
Even when fused, bones are still there.

References
Solounias, et al. (6 co-authors) 2017. The evolution and anatomy of the horse manus with an emphasis on digit reduction. Royal Society Open Science 5:17182. http://dx.doi.org/10.1098/rsos.171782

 

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. 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 of Hyracodontidae, 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. 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). 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

Maybe horses are just tall, skinny, hornless rhinos…

Short one today,
With yesterday’s addition of two more basal rhinos to the large reptile tree (LRT 1010 taxa) maybe it’s time to change our thinking from ‘either horse or rhino’ to ‘horses are a type of rhino’. We know they are related. Maybe they are more intimately related than we first thought. That would make indricotheres like Paraceratherium giant hornless rhinos again, if you prefer it that way.

Figure 1. Subset of the LRT with the addition of Metamynodon and Amynodon, two former rhinos.

Even though,
in the LRT (subset Fig. 1) fewer taxa intervene at present between indricotheres and horses than indricotheres and extant rhinos, like Ceratotherium. Sort of like, you know, birds are a type of dinosaur. It just takes some getting used to – creating a new mental paradigm following the present data without excluding pertinent taxa.

 

Two more odd ‘hornless rhinos’ nest in the LRT

Updated October 31, 2021
with the addition of more taxa and an adjustment to the scoring.

First a little backstory
Earlier, Paracerathierium and Juxia, two traditional hornless rhinos, nested with three-toed horses in the large reptile tree (LRT, 1009 taxa then, 1978 taxa now, Fig. 3).

Figure 1. Metamynodon nests with Eotitanops. It had large fangs and a bulky body like a hippo.

Today
the giant hippo-like traditional rhino, Metamynodon planifroms (Scott and Osborn 1867; Early Eocene; 4m long; Fig. 1), nests with another smaller, hornless rhino, Amynodon (Fig. 2).

Figure 2. Amynodon was formerly linked to Metamynodon as a basal rhino, but here nests with Mesohippus.

Both are derived from Hyracotherium.

Amynodon advenus (Marsh 1877; 1m in length; Oligocene-Eocene, 40-23 mya) was originally considered an aquatic rhino. The long neck and other traits are more horse-like than rhino-like. Manual digit 5 was retained. The skull was deeper as in basal forms like Hyracotherium.

Figure 3. Subset of the LRT focusing on Ungulata and Perissodactyla.

Traditional cladograms
nest horses separate from tapirs + rhinos. The LRT nests horses with rhinos both derived from a sister to Hyracotherium and a sister to tapirs + chalicotheres. Traditional cladograms also avoid mixing brontotheres, horses and rhinos, like the LRT does here (Fig. 3).

References
Marsh OC 1877. Notice of some new vertebrate fossils. American Journal of Arts and Sciences 14:249-256
Scott WB and Osborn HF 1887. Preliminary account of the fossil mammals from the White River formation contained in the Museum of Comparative Zoology. Bulletin of the Museum of Comparative Zoölogy at Harvard College 13(5):151-171.

wiki/Mesohippus
wiki/Amynodontidae
wiki/Metamynodon

Indricotheres: horse-like rhinos? Or rhino-like horses?

Earlier the large reptile tree (LRT, 993 taxa) nested the traditional giant rhinos, Paraceratherium (Fig. 1) and Juxia, with the horses Equus (one toe, Fig. 1) and Mesohippus (three toes) based on several traits lacking in Ceratotherium, the extant white rhinoceros. This heretical hypothesis of relationships is not in the academic literature. Evidently dental details separate horses from rhinos. The LRT employs very few dental traits.

So today’s quandary involves convergence.
Do we trust the teeth that say indricotheres are rhinos?
Or are the teeth convergent?
Do we trust the rest of thel traits that say indricotheres are three-toed horses?
Or are all the non-dental traits convergent?

Adding taxa
has solved issues like this in the past. Today we add the basal rhino, Hyrachyus. We add a taxon considerd basal to the indricotheres, Pappaceras. And we add a basal horse, Miohippus (Fig. 3) hoping for resolution and consensus.

Figure 1. Equus the horse shares many traits with Paraceratherium. So is this a giant rhino-like horse? Or horse-like rhino?

First let’s go back to the literature…
I wrote to Dr. Don Prothero, a renown expert in rhinos, asking if he knew of any phylogenetic analysis that included both horses and rhinos and their last common ancestor. Since his answer did not include the title of any such paper, I went to his biography and found a few likely resources. Unfortunately the best of them (see below) predate the advent of software in phylogenetic analysis and often rhinos are tested separate from horses.

Prothero, Manning and Hanson 1986
describe a situation in which rhinos had been little studied. They report, “We have tried to straighten out some of the problems in rhinoceros relationships.” They open with a history of the study. “Osborn 1898 was the first to recognize our modern division of rhinocerotoids into three distinct families: Hyracodontidae (including giant indricotheres), Amynodontidae and Rhinocerotidae.” They presented a new hypothesis of relationships with a de-emphasis on dental traits. They employed Hyrachyus as their outgroup and did not include any horses. Their tree recovered Forstercooperia and Pappaceras (Fig. 2) as sisters to the indricothere clade and that clade a sister to the clade that included Hyracodon.

As a side note
Embolotherium, the titanothere currently nesting with the extant white rhino, did not make the rhino list of taxa.

Figure 2. Pappaceras is one of the closest known sisters to the indricotheres according to rhino cladograms. It nests between Mesohippus and Miohippus in the LRT (Fig. 3).

Basal to the indricotheres,
The Prothero, Manning and Hanson 1986 cladogram recovered:

1. Rhinocerotoidea  diagnosed on the basis of dental traits plus a cylindrical odontoid process on the atlas and a fused intervertebral canal on the atlas.
2. Hyracodontidae had a tridactyl manus, long limbs and metapodials, laterally compressed carpus and tarsus.
3. Paraceratheriinae (= Indricotheriinae) had a large size, enlarged lower P1; upper M3 metacone reduced, conical incisors.

When I add
Hyrachyus to the LRT it nests basal to the rhinos confirming the hypotheses of prior workers. Surprisingly Mesohippus did not nest with Miohippus and Equus in the LRT. Furthermore, Pappaceras did not nest with Juxia and Paraceratherium in the LRT. Close… but the clades did not separate into rhinos and horses… and indricotheres did not nest with Hyracodon and the rhinos.

Figure 3. Subset of the LRT focusing on the hyracotheres, including horses and rhinos. Traditional rhinos are in light green. Traditional horses are in tan. Is this the result of convergence or taxon inclusion?

Prothero et al. 1988 reported
“Hyraxes are closer to perissodactyls than to elephants and sirenians. Perissodactyls are closer to whales and elephants than they are to artiodactyls.” The LRT does not support those findings.

 

re: Miohippus
Wikipedia reports, “Miohippus also had a variable extra crest on its upper molars, which gave it a larger surface area for chewing tougher forage. This would become a typical characteristic of the teeth of later equine species.” 

Miohippus skull and skeleton. Is this a sister to the ancestor of horses and indricotheres? Compare the foot the that of Paraceratherium (Fig. 1).

And…
“Miohippus had two forms, one of which adjusted to the life in forests, while the other remained suited to life on prairies. The forest form led to the birth of Kalobatippus (or Miohippus intermedius), whose second and fourth finger again elongated for travel on the softer primeval forest grounds. The Kalobatippus managed to relocate to Asia via the Bering Strait land bridge,”

That’s all it takes
|to start a new clade. Wikipedia reports, “Kalobatippus ate leaves and was characterized by unusually long legs.”

Summary
Let’s consider the possibility that indricotheres could be horses with rhino-like teeth, perhaps due to a similar herbivorous diet. Perhaps this result will stimulated further interest in the subject.

References
Marsh OC 1874. Notice of new equine mammals from the Tertiary formation. American Journal of Science 7(39):247-258.
Prothero DR, Manning E and Hanson CB 1986. The phylogeny of the Rhinocerotoidea (Mammalia, Perissodactyla). Zoological Journal of the Linnean Society 87:341-366.
Prothero DR, Manning EM and Fischer M 1988. The phylogeny of the ungulates Pp. 201-234 in The phylogeny and classification of the tetrapods, volume 2: Mammals (ed. Benton MJ) Systematics Association special volume 35B:
Wang H, Bai B, Meng J and Want Y-Q 2016. Earliest known unequivocal rhinocerotoid sheds new light on the origin of Giant Rhinos and phylogeny of early rhinocerotoids. Nature Scientific Reports 2016.
Wood H E 1963. A primitive rhinoceros from the Late Eocene of Mongolia. American Museum Noviates 2146:1–11.

wiki/Miohippus

More taxa for the ‘Paraceratherium=giant horse’ hypothesis

Figure 1. Adding a basal horse, Mesohippus, and a basal indricothere, Juxia, to the large reptile tree keeps indricotheres in the horse clade.

When you create a heresy
by breaking with decades of tradition, it’s always a good idea to test your hypothesis of interrelationships by adding pertinent taxa.

A few days ago Paraceratherium, a derived and gigantic indricothere, was nested in the large reptile tree (LRT, Fig. 1) with Equus, the extant horse. Tradition dictated that it should have nested with Ceratotherium, the extant white rhino – IF Paraceratherium was indeed a giant hornless rhino. But it wasn’t then and it isn’t now, based on current data. (One must always be willing to accept better data that busts up your favorite discoveries).

Today
Juxia (Chow and Chiu, 1964), a horse-sized indricothere with more premaxillary teeth and Mesohippus (Marsh 1875) a primitive three-toed horse, are added to the LRT (subset Fig. 1) to test the prior nesting. Mesohippus is known from a dozen species. Juxia is known form a nearly complete skeleton and a less-complete referred specimen. It is one of the smaller and more primitive indricotheres.\

Figure 2. A selection of horse and indricothere skulls to scale. No other taxa are more closely related to these than each is to each other.

Everyone knows indricotheres are supposed to be
giant hornless rhinos. But in the LRT they continue to nest with horses. The Bootstrap numbers (Fig. 1) are strongly supportive. And its pretty obvious when you get them together (Fig. 2). Both Juxia and Paraceratherium (Figs. 1, 4) look like giant three-toed horses, because that’s what they are. Not sure why this was never noticed before. Let me know if you know of any prior literature on this hypothesis of relationships.

Figure 3. In the LRT Mesohippus nests basal to horses and indricotheres (see figure 4).

Figure 4. Equus and Paraceratherium nest together on the LRT.

References
Chow M and Chiu C-S 1964. An Eocene giant rhinoceros. Vertebrata Palasiatica, 1964 (8): 264–268.
Marsh OC 1875. Notice of new Tertiary mammals, IV. American Journal of Science 9(51):239-250.

wiki/Paraceratherium
wiki/Juxia
wiki/Mesohippus