The Protoceratidae - slingshots and beanies

Here’s a little thought-experiment to try. Imagine a deer, or deer-like animal. Now give it fang-like tusks, like those of a muntjac, and prehensile lips. Now give it a pair of horns on its head. Give it another pair of horns on the tip of its nose. You can be creative with the horns if you want – merge them together, shape them into V or Y figures, or add an extra pair near the first, if you see fit. If you can imagine such an animal, you’ve got a pretty good idea of what a protoceratid looks like.

The Protoceratidae are a fascinating and unusual family of artiodactyl ungulates, noted for their baroque headgear. Unique to North America, they appeared in the mid-Eocene and disappeared by the start of the Pliocene, and were widespread, if uncommon, throughout their existence. Thirteen genera have been described to date.

As mentioned earlier, many protoceratids evolved bizarre horns. Some species had up to six horns, in a manner reminiscent of dinocerates. At least one species seemed to be wearing a propeller beanie. The later synthetoceratines carried the trend to the extreme, with a pair of horns on top of their head and another, often fused pair on the tip of their nose developing into an outlandish slingshot.


While the odd horns are a dead give-away for a protoceratid, the earlier species lacked horns altogether, so other characters are used for identification. Generally, the diagnosis for protoceratids is a short coronoid process on the mandible, a concave proximal side of sustentacular facet of astragalus, and strong upper molar lingual cingula (Prothero, 1998). The absence of a third metacarpal (the cannon bone) is also noteworthy. Many species also had enlarged canines much like those found in muntjacs, and advanced species lost their incisors entirely.

Painting of Protoceras by Heinrich Harder, from here.

Phylogeny and relationships

The phylogeny of protoceratids has been the subject of some dispute. When O. C. Marsh discovered Protoceras, the first known species, he assigned it to the giraffes; and, since then, protoceratids have been variously allied with giraffes, ruminants, and camels. The two best-supported hypotheses are that protoceratids are either a) the sister-group of tylopods or b) the sister-group of ruminants (Norris, 2000). It seems most likely that protoceratids are tylopods, whose closest relatives are the camelids.

Within Protoceratidae itself, subfamily “Protoceratinae” is used as a wastebasket for upper-level taxa. The subfamily Synthetoceratinae, within Protoceratinae, includes the later species with Y-shaped rostral horns, and it is further subdivided into the Kyptoceratini (with the rostral horn fused at the base, as in Syndyoceras) and the Synthetoceratini (with a long rostral horn, as in Synthetoceras).


A quick tour of the Protoceratidae

Leptotragulus, Leptoreodon, Poabromylus, Toromeryx, Heteromeryx, and Pseudoprotoceras are all basal protoceratids, without horns. A study of Leptotragulus, the earliest protoceratid, concluded that it already had many protoceratid specializations, and had evolved a strong skull and neck to support head-butting and other agonistic behavior (Norris, 2000). The known skull of Heteromeryx is hornless, but it might be a female specimen without horns. Pseudoprotoceras is also hornless, but, again, it has been suggested that the known specimens were all females; it also may be a "protoceratine".

Protoceras skeleton from here.

The “Protoceratinae” include the more famous horned species. Strong sexual dimorphism is known in Protoceras, whose males had no less than six horns – a pair on the maxilla, a pair on the orbit, and a pair on the parietals. Female Protoceras, on the other hand, retained only one pair of horns. The Protoceras type specimen was female, and as such caused some taxonomic confusion (Prothero, 1998). The closest genus to Protoceras is the equally six-horned Paratoceras, whose occipital horns fused at the base and spread out laterally into two flattened horns, likened to a propeller beanie.

All the remaining protoceratines are sorted into the Synthetoceratina, which itself divides into the tribes Kyptoceratini and Synthetoceratini. The Kyptoceratini includes two species. The easily-recognizable Syndyoceras had curving frontal horns and V-shaped rostral horns emerging directly above their base. Kyptoceras was the last known protoceratid, surviving into the Pliocene. The horns of Kyptoceras were similar to those of Syndyoceras, but both pairs were forward-tilting.


The Synthetoceratini includes most of the last and largest protoceratids. All three species had frontal horns and “slingshot” rostral horns on the end of a long projection. Lambdoceras, Prosynthetoceras, and Synthetoceras all followed this arrangement. Synthetoceras was the largest protoceratid, one of the best-known, and the latest-surviving species known until the discovery of Kyptoceras in 1981. Described at one point as a “near-unicorn” (Barnett, 1955), it is possibly the weirdest protoceratid, and often serves as a poster boy for the rest of the family.

Picture shows Syndyoceras (bottom) and Synthetoceras (top).

Paleobiology

With short limbs, protoceratids were probably poor runners, better-adapted for life in forests and woodland. The lack of incisors and the retracted nasals suggest the presence of a prehensile lip to pluck off plant matter. In all, they seem to have been rather like the extant moose. A study of the premaxilla showed that Synthetoceras was a grazer at ground-level (Solounias and Moelleken, 1993), but Lambdoceras had a narrow snout and probably browsed from small trees and shrubs.

The distinctive horns seem to have been well-suited for both visual display and intraspecific combat. With the wide variety of headgear available, protoceratids could identify their species easily and select mates with the best horns. Earlier horned species such as Protoceras probably displayed their horns sideways, while the slingshots of the synthetoceratines could only have been used in frontal display. Other characteristics show that the males could have bitten, butted, and neck-wrestled their way to dominance. While the horns were not suited for ramming at speed, they could have interlocked so as to minimize the chances of injury. As mentioned previously, even the most primitive species were already specializing for intraspecific combat (Norris, 2000).

Oh, and a final note to paleo-artists and paleontographers: how about painting less theropods and more protoceratids? There's precious little that shows up in a Google image search, you know. They do crop up from time to time, though. For starters, there's a Knight pic of Dinictis chasing Protoceras, and Zallinger's Age of Mammals mural has the "Big Three" protoceratids - Protoceras, Syndyoceras, and Synthetoceras.


References

Barnett, L. and the editors of Life. The World We Live In. 1955

Norris, C. A., 2000: The cranium of Leptotragulus, a hornless protoceratid (Artiodactyla: Protoceratidae) from the Middle Eocene of North America.
Journal of Vertebrate Paleontology: Vol. 20, #2, pp. 341-348

Prothero, Donald R. in Janis, Christine M.; Scott, Kathleen M.; and Jacobs, Louis L. Evolution of Tertiary Mammals of North America, Vol. 1. Cambridge University Press, 1998

Solounias, N., and Moelleken, S.M.C. Dietary Adaptation of Some Extinct Ruminants Determined by Premaxillary Shape. Journal of Mammalogy, Vol. 74, No. 4 (Nov., 1993), pp. 1059-1071