Uintatheres: Long lost bones reappear as horns and ridges

Get ready for some more heresy.
We’ve talked about the reappearance of long lost bones in various taxa, from digit zero on the hand of Limusaurus and Chauna, to the postorbital in primates like Archicebus. Among the many cranial bones lost from the surface in most mammals are the septomaxilla (the portion of the lacrimal contacting the naris), the prefrontal, the postfrontal, the postorbital and the supratemporal. All these can be seen on the surface of the skull in basal synapsids like Vaughnictis.

Adding the small uintathere
Bathyopsis fissidens (Fig. 1) to the large reptile tree (LRT, 1379 taxa) forced a review of the skull bones of Uintatherium, one of the bizarrely horned uintatheres of the Paleocene and Eocene (Fig. 1).

Yes, I’m learning as I go.
And yes, sometimes I find things that have been traditionally overlooked. And yes, I could be wrong. This needs to be looked at ‘with new eyes’  by several other workers.

Figure 1. Bathyopsis, xx and Uintatherium to the same fang-occiput length showing the reappearance of several bones that don't appear in most mammals. See text for details.

Figure 1. Bathyopsis, Elachoceras and Uintatherium to the same fang-occiput length showing the reappearance of several bones that don’t appear in most mammals. See text for details.

Some bones reappear in uintatheres that most mammals don’t have:

  1. The lacrimal originally extended to the naris, but the maxilla rose to meet the nasal and cover up the anterior lacrimal. In uinitatheres the typically hidden middle portion of the lacrimal rises to the surface and beyond between the nasal and maxilla to form the anterior horn.
  2. The prefrontal reappears as a ridge in uintatheres over the orbit.
  3. The postfrontal reappears in uintatheres over the posterior orbit and rises to form the anterior of the posterior horn.
  4. The postorbital reappears in uintatheres first flat and atop the skull, then atop the posterior horn.
  5. The supratemporal reappears in uintatheres along the posterior base of the posterior horn forming a lateral cranial ridge that other mammals don’t have.
Figure 2. Uintatherium skull with bones colored and labeled. Several of these bones have not been seen on mammal skulls, but once appeared on basal synapsid skulls.

Figure 2. Uintatherium skull with bones colored and labeled. Several of these bones have not been seen on mammal skulls, but once appeared on basal synapsid skulls. The hornier skulls are larger (Fig. 3).

Unitatherium anceps (Leidy 1872, Eocene 37 mya, 4m long, 1.7 m at shoulder) was a bulky basal ungulate and a sister to Coryphodon. Uintatherium had six bony processes over the skull. Wikipedia identifies it only as an herbivorous mammal. The LRT nests it with Arsinoitherium, Phenacodus and Gobiatherium. The tooth arcade of Uintatherium is less complete. The canines enlarge to saber teeth and a diastema appears with the loss of the anterior premolars. The pelvis widens. Five fingers and toes are retained.

The ‘horns’ in Giraffa, the giraffe,
are not homologous, nor analogous. The ossicone in Giraffa is a new ossification that sits atop the cranial bones with no reappearances of long lost bones detected.

Figure 3. Uintathere evolution from Wheeler 1961. Colors and eyeballs added.

Figure 3. Uintathere evolution from Wheeler 1961. Colors and eyeballs added.

Discoveries like this
require confirmation or refutation in the form of precise data. I’m looking forward to other workers re-examining the bone sutures of uintatheres with these new insights and possibilities in mind. If this is confirmed, remember, you heard it here first.

References
Cope ED 1881. On the Vertebrata of the Wind River Eocene beds of Wyoming. Bulletin of the United States Geological and Geographical Survey 6(1):183-202
Leidy J 1873. Contribution to the extinct vertebrate fauna of the Western Territories. Geological Survey of the Territories 1.
Osborn HF 1913. The skull of Bathyopsis, Wind River uintathere. Bulletin of the American Museum of Natural History 22:417–420 + plates.
Owen R 1845. Odontography; a treatise on the comparative anatomy of the teeth. Hippolyte Bailliere, London, 655pp.
Uhen MD and Gingerich PD 1995. Evolution of Coryphodon (Mammalia, Pantodonta) in the Late Paleocene and Early Eocene of Northwestern Wyoming. Contributions from the Museum of Paleontology, University of Michigan. 29 (10): 259–89.
Wheeler WH 1961. Revision of the Uintatheres. Peabody Museum of Natural History, Yale University bulletin 14: 93 pp. + plates.

wiki/Coryphodon
wiki/Uintatherium
wiki/Bathyopsis

wiki/Uintatheriidae