This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Some of you will know that I’m putting together a giant textbook on the vertebrate fossil record... and, oh god, it isn’t easy. If you want sneak-peeks on how things are going, please consider supporting me at my patreon page. And if you’re wondering what the book might be like when it’s finished, here’s an excised section. For no particular reason, it’s on stenomyline camels, the so-called gazelle camels.
Several slender-limbed, small camelid taxa of the Oligocene and Miocene are grouped together in Stenomylinae: all known members of this group were evidently cursorial, gazelle-like camelids of grassland habitats, and they've often been termed 'gazelle camels'. Slender, pointed unguals indicate unguligrady (as opposed to the digitigrady present in modern camelids). Stenomyline dentition is distinctive since their teeth are especially elongate and more transversely narrow than those of many other camelids. The stenomyline rostrum is constricted by prominent, deep, anteroposteriorly elongate maxillary fossae, and they also possess long posterior processes on their premaxillae.
The cranium is short and deep in some stenomylines, with Stenomylus in particular having a tall, wide, rounded cranium relative to its snout. Partial fusion of the metapodials has been recognised as a diagnostic feature of this group (Whistler & Webb 2005). Very narrow jaw tips and closely packed incisors (Frick & Taylor 1968) suggest selective browsing of some sort, not the cropping of mouthfuls of grass. This is supported by studies of tooth microwear which indicate that stenomylines were so-called 'dirty browsers', eating leafy plants that grew in open environments and which were covered in notable amounts of grit (Semprebon & Rivals 2010). All stenomylines are fairly small camels, their skulls typically being c 20 cm long: Pseudolabis and Miotylopus are especially large with skulls c 25-30 cm long. Shoulder height in Stenomylus was c 60 cm.
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Honey et al. (1998) included Pseudolabis of the Oligocene and Lower Miocene (and hence the group name Pseudolabidinae) within Stenomylinae, identifying it as outside the ‘core’ stenomyline clade Stenomylini. Besides the six or so species of Stenomylus, the latter contains Blickomylus, Rakomylus and Wyomylus. Stenomylus is an animal of Arikareean and Hemingfordian faunas (that is, those of the Late Oligocene and Early Miocene). Blickomylus is exclusively Hemingfordian, and Rakomylus is from the Early Barstovian. Wyomylus from central Wyoming showed that stenomylines survived rather later than thought prior to its discovery: it comes from sediments imprecisely dated as being between late Barstovian and early Hemphillian and thus potentially extends the presence of the group into the Late Miocene (Cassiliano 2008). Miotylopus from the Oligocene and Lower Miocene has been suggested to be the sister-taxon of Stenomylini (Honey et al. 1998).
As goes the larger affinities of stenomylines within Camelidae, a possible sister-group relationship with the miolabines of the Miocene is suggested on the shared presence of an elongate rostrum, enlarged upper incisors, broad postorbital bar and other cranial characters (Whistler & Webb 2005). Stenomylines are advanced and specialised right from their earliest appearance and have been considered ‘precociously advanced’ (Prothero 1996). Their origins are mysterious and fossils that link the oldest forms with other camelids have yet to be described. UPDATE: Don Prothero reminds me of views very much alternative to the ones outlined here, and of as-yet-unpublished work on this issue. All of this means that I'll come back to the subject of stenomyline affinities in future.
Nothing in particular is known about stenomyline social lives or behaviour. It has been assumed that they were social like their extant relatives; confirmation for this comes from the discovery of mass death assemblages that clearly show how large herds were killed off by disaster events (like volcanic eruptions). Their gracile limbs and apparent specialisations for life in savannahs and similar habitats indicate that they escaped potential predators by sprinting, and it might be interesting to work out how speedy and how agile they were relative to other camelids - did they have a higher top speed, and could they leap or even pronk like the antelopes they are so frequently compared with?
As suggested by the montage above, fossil camels include a substantial amount of diversity, little of which has been written about outside of the technical literature. On that note, much of the camelid section has been finished for the big book I alluded to above. Again, please consider supporting me at patreon if you'd like to know more or just want to help me out.
For previous Tet Zoo articles on artiodactyls, see...
Why putting your hand in a peccary’s mouth is a really bad idea
Those giant killer pigs from hell aren’t pigs (entelodonts)
Refs - -
Cassiliano, M. 2008. A new genus and species of Stenomylinae (Camelidae, Artiodactyla) from the Moonstone Formation (late Barstovian–early Hemphillian) of central Wyoming. Rocky Mountain Geology 43, 41-110.
Frick, C. & Taylor, B. E. 1968. A generic review of the stenomyline camels. American Museum Novitates 2353, 1-51.
Honey, J. G., Harrison, J. A., Prothero, D. R. & Stevens, M. S. 1998. Camelidae. 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. 439-462.
Semprebon, G. M. & Rivals, F. 2010. Trends in the paleodietary habits of fossil camels from the Tertiary and Quaternary of North America. Palaeogeography, Palaeoclimatology, Palaeoecology 295 131–145.
Whistler, D. P. & Webb, S. D. 2005. New goatlike camelid from the Late Pliocene of Tecopa Lake Basin, California. Contributions in Science, Natural History Museum of Los Angeles County 503, 1-40.
Prothero, D. R. 1996. Camelidae. In Prothero, D. R. & Emry, R. J. (eds) The Terrestrial Eocene–Oligocene Transition in North America. Cambridge University Press, Cambridge, pp. 609-651.