I said in the previous pronghorn article that the modern pronghorn – Antilocapra americana – is but the tip of the phylogenetic iceberg, if you will; the only surviving member of a group that was previously far more diverse [the adjacent photo (from wikipedia) shows Ramoceros osborni. Yes, it really looked like that, read on].
As we’ll see here, fossil pronghorns encompassed a reasonable amount of diversity: there were kinds with deer-like pseudo-antlers as well as others that superficially resembled living African antelopes like kudu or nyala, there were tiny dwarf forms, and there were also some real oddballs, unlike anything before or since. All of this is very well known among palaeomammalogists, but as usual the information isn’t widely available outside of the technical literature. My aim here is to provide a very brief, introductory review to fossil pronghorns: for in-depth treatments, the reader is directed to Janis & Manning (1997), Heffelfinger et al. (2004) and Davis (2007).
Pronghorns (or antilocaprids) are an entirely North American group: for reasons that aren’t clear, they never invaded South America when the Panamanian land-bridge formed, nor did they migrate into Asia (so far as we know). It might be that the general restriction of pronghorns to relatively arid habitats slowed or prevented their spread, but this doesn’t explain everything. During most of the Miocene, several pronghorn taxa were contemporaneous: this was true during the Middle Miocene when woodlands were widespread, but the largest radiation of the group coincided with the spread of savannah habitats in the Late Miocene. During the late Hemphillian (Late Miocene), as many as seven pronghorn genera were contemporaneous (some of which contained several species), all of which were derived antilocaprine forms, often with complex horns [pronghorn phylogeny below, showing some of the main characters, from Janis & Manning (1998)].
‘Merycodontines’: moose-like palmation and crazy asymmetry
Several pronghorn taxa that are mostly restricted to the Early and Middle Miocene have historically been grouped together as the merycodontines. These lack the derived characters that unite the antilocaprine pronghorns, and are generally small pronghorns with branched, sometimes antler-like, horns in the males (females were hornless: a fact suggesting more solitary, more territorial habits than those of later, antilocaprine pronghorns). It’s generally agreed that ‘Merycodontinae’ is paraphyletic (Janis & Manning 1997), though note that (so far as I know), no detailed, comprehensive phylogenetic analysis of Antilocapridae has ever been carried out… I mean, other than Janis & Manning (1997). McKenna & Bell (1997) recognised a merycodontine group, but used the name Cosorycinae (named by Cope in 1887, it has priority over Merycodontinae Matthew, 1904).
Among the ‘merycodontines’ is Merriamoceros coronatus from the late Early Miocene and early Middle Miocene of California, Nevada, Nebraska and Montana [shown here; from Heffelfinger et al. (2004)]. Its short horns terminate in an elongate platform formed of many short, finger-like tines: a palmate morphology superficially resembling that of moose.
One of the most unusual pronghorns – in fact, one of the most unusual artiodactyls – also belongs to this region of the cladogram: Ramoceros from the Middle and early Late Miocene. Several species (and even subspecies) have been named, and a second ‘genus’ – Paramoceros – is nowadays included by most authors within Ramoceros. The main horn shaft is long, and three tines are present near the tip, forming a deer-like pseudo-antler [skull of R. ramosus, in posterior view, shown below]. Oh, and – – one horn (it might be either the left or the right) is about twice the length of the other one. If the asymmetry was variable intraspecifically (and, so far as I can, it is), does this mean that ‘left handers’ (viz, those with a long horn on the left) could only fight with ‘right handers (viz, those with a long horn on the right), or what? I haven’t ever seen a discussion of this in the literature, but that doesn’t mean it doesn’t exist.
The surface texture of ‘merycodontine’ horns suggests that they were skin-covered, rather than sheathed in horn.
Antilocaprine pronghorns: some bovid-like
The majority of pronghorns belong to Antilocaprinae, a clade characterised by a more posteriorly positioned orbit, reduction of the anterior tine on the horn and enlarged auditory bullae. The metapodials of antilocaprine are more elongate than those of other pronghorns, they all seem to have lacked the side toes that were present in more basal forms, and their premolars are proportionally larger and more molariform (all pronghorns have high-crowned teeth; evidence from tooth wear indicates that Miocene species relied more on grass than does the living species (Semprebon & Rivals 2007). It might seem paradoxical that, while the living species has the highest-crowned teeth of any pronghorn, only 12% or so of its diet is composed of grass. However, it’s been argued (e.g., Janis 1995) that it’s the soil and grit ingested with leaves – and not necessarily the abrasiveness of the leaves themselves – that controls hypsodonty in herbivorous mammals. It may therefore be that Antilocapra inhabits ‘dustier’ habitats than its fossil relatives).
Most antilocaprine lineages possessed branched horns where a prominent anterior tine gave the bony horn-core a forked appearance: however, the anterior tine was reduced or lost in some lineages and they then evolved long horns resembling those of gazelles or spiral-horned antelope. In Proantilocapra platycornea [shown here, from Barbour & Schultz (1934)], the anterior tine was absent and the horns were short, laterally compressed and with rounded tips. As suggested by its name, Proantilocapra was originally interpreted as close to the ancestry of Antilocapra (Barbour & Schultz 1934). The affinities of Antilocapra seem to lie elsewhere, however (Texoceros from the Late Miocene may be the closest extinct relative of Antilocapra).
The anterior tine was also lost in Osbornoceros osborni from the Late Miocene of New Mexico and Texas: this taxon possessed slender, posterolaterally twisted horns that look much like those of some gazelles or the Nyala Tragelaphus angasii [adjacent life restoration of O. osborni by Nobu Tamura, from wikipedia. The fossils only have a single horn-core, so I’m presuming that the artist has reconstructed hypothetical horny prongs by analogy with extant Antilocapra: read on].
Four-horned and six-horned antilocaprines
In some members of the antilocaprine clade Stockoceratini, the main shaft of the horn became reduced until it was virtually absent while, at the same time, the tines became enlarged. The result is that these pronghorns must have looked as if they had four (e.g., Hayoceros and Stockoceros) or even six (e.g., Hexameryx) horns, all emerging supraorbitally in a cluster. Webb’s (1973) reconstruction of Hexameryx simpsoni from the Pliocene of Florida is shown below. In the fossils of these animals, the horn tips have a different, smoother surface than the horn bases, suggesting that skin clothed the horn base and that keratin sheaths were restricted to the tips.
These enlarged tines are typically cylindrical in cross-section, but can be blade-like. To make things even weirder, in some stockoceratins (e.g., Hexobelomeryx), two of the three enlarged tines are so closely aligned that they must have been united in a single keratin sheath, though this was presumably forked like the underlying bone. Worth noting here (again) is the fact that Antilocapra has unbranched bony horn-cores but a branching keratin sheath: could the unbranched horn-cores of fossil pronghorns have supported branched keratin sheaths? We really don’t know, and this possibility means that the reconstructions you see here – like Webb’s rendition of Hexameryx – may well be conservative.
Another particularly remarkable group of pronghorns are the ilingoceratins (their formal name is Ilingoceratini). As in Proantilocapra and Osbornoceros, the tines became reduced in this group, and in the most ‘extreme’ ilingoceratins the main shaft of the horn twists laterally. This is most prominently developed in Ilingoceros [shown here, from Heffelfinger et al. (2004)] where two spiral ridges extend along the length of each horn such that they resemble the spiralling horns of kudus and other such antelopes. On naming I. alexandrae (two species of Ilingoceros are currently recognised), Merriam (1909) originally considered it to be what we would today call a tragelaphine bovid (the group that includes nyalas, kudus and elands). He later realised that it was an antilocaprid thanks to its high-crowned teeth and other features. The horns in at least some Ilingoceros specimens have forked tips, but the forks here don’t seem to be homologous to the tines present elsewhere among pronghorns.
I said earlier that some extinct pronghorns were tiny. Capromeryx of the Pliocene and Pleistocene – again, several species are known, and subspecies have even been named for some of them – first appears in the fossil record as an animal similar in size to the living Pronghorn (or, about 85% of it anyway). It then seems to have undergone a dwarfing trend, such that a species from the Late Pleistocene (namely C. minor, originally described from Rancho La Brea) was less than 60 cm tall at the shoulder [in the painting below – by Carl Buell, used with permission – Capromeryx runs past the legs of the giant camel Titanotylopus]. It might be presumed that these tiny pronghorns were secretive animals that lived like modern dik-diks or dwarf antelopes, and were not denizens of open-habitats like their larger cousins. But their remains are in fact found in what were once grassland habitats, albeit in places where there were clumps of trees.
And so ends our brief look at the diversity of fossil pronghorns. I hope you enjoyed it! More on pronghorns later – there is still much to look at.
For the previous article on pronghorns see…
And for previous Tet Zoo articles on artiodactyls see…
- Welcome…. to the world of sheep
- Return…. to the world of sheep
- Tet Zoo picture of the day # 23 (entelodonts)
- Traumatic anal intercourse with a pig
- It’s such a load of bull
- Duiker, rhymes with biker
- Sable antelopes and the miseducation of youth
- Giant killer pigs from hell
- The plasticity of deer
- Over 400 new mammal species have been named since 1993
- Stuffed megamammal week, day 1: Khama
- Stuffed megamammal week, day 2: Eland
- Stuffed megamammal week, day 3: Okapi
- Great Asian cattle
- Dromomerycids: discuss
- Death by lightning for giraffes, elephants, sheep and cows
- A close-up look at a Hairy babirusa (includes links to many other babirusa articles)
- Testing the flotation dynamics and swimming abilities of giraffes by way of computational analysis (includes links to other giraffe articles)
- A new angle for hippos (includes links to other hippo articles)
- A ‘consensus cladogram’ for artiodactyls
Refs – –
Barbour, E. H. & Schultz, C. B. 1934. A new antilocaprid and a new cervid from the late Tertiary of Nebraska. American Museum Novitates 734, 1-4.
Davis, E. B. 2007. Family Antilocapridae. In Prothero, D. R. & Foss, S. E. (ed) Evolution of Artiodactyls. Johns Hopkins University Press, pp. 227-240.
Heffelfinger, J. R., O’Gara, B. W., Janis, C. M. & Babb, R. 2004. A bestiary of ancestral antilocaprids. Proceedings of the 20th Biennial Pronghorn Workshop 20, 87-111.
Janis, C. M. 1995. Correlations between craniodental morphology and feeding behavior in ungulates: reciprocal illumination between living and fossil taxa. In Thomason, J. (ed) Functional Morphology in Vertebrate Paleontology. Cambridge University Press, pp. 76-98.
– . & Manning, E. 1998. Antilocapridae. In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 491-507.
McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press (New York).
Merriam, J. C. 1909. The occurrence of strepsicerine antelopes in the Tertiary of northwestern Nevada. University of California Publications, Bulletin of the Department of Geology 5, 319-330.
Semprebon, G. M. & Rivals, F. 2007. Was grass more prevalent in the pronghorn past? An assessment of the dietary adaptations of Miocene to Recent Antilocapridae (Mammalia: Artiodactyla). Palaeogeography, Palaeoclimatology, Palaeoecology 253, 332-347.
Webb, S. (1973). Pliocene Pronghorns of Florida Journal of Mammalogy, 54 (1) DOI: 10.2307/1378880