Running With Arenahippus

No matter what you call them – Eohippus, Hyracotherium, or something else – most everyone is familiar with the diminutive “dawn horses” of the Eocene. Sure, dinosaurs get better press, but the connection of these small hoofed mammals to the modern horse (Equus) has turned them into minor paleontological celebrities. In fact, these horses have featured as evolutionary icons for over a century thanks to the perception – long ago proven wrong – that these unassuming creatures initiated a single-file evolutionary march to today’s beloved beast of burden.

Traditionally the relationship of the dawn horses to Equus has been represented by a straight line, either literally or through a series of graded transitional forms. Given this imagery it would be expected that horses traveled along a predetermined evolutionary trajectory of increasing “horsiness” through time – even if you lacked every transitional form between the two fossil bookends, you could predict what those forms would look like by modifying the ancestral form towards the modern type.

This type of thinking was more than scientific shorthand for museums exhibits and books. Given the fragmentary nature of many dawn horse skeletons, the anatomy of modern horses was often used to fill in the missing bits and pieces. After all, the prehistoric species had no closer relatives from which to draw from.

The paradox of the study of fossil horses is that paleontologists have long recognized that their evolution is best represented by a branching bush in which there were times of radiation which were subsequently cut back by extinction. The straight-line imagery is favored because it documents the graded steps of evolutionary change, but any time we create a strictly linear diagram we are by necessity leaving out the wider evolutionary context into which those animals fit. There was no internal trend forcing the dawn horses and their descendants upwards towards Equus, and a recently-discovered fossil skeleton underscores the point that early horses were quite distinct from their extant cousins.

Dawn horses are among the most common mammals in the early Eocene rock of Wyoming. The trouble is that they are mostly represented by teeth and isolated bones; despite being known to paleontologists for over a century, we still don’t have a very good idea of the precise anatomical details of each species. This has fed the taxonomic confusion which has surrounded these animals. The monikers Eohippus, Hyracotherium, and Protorohippus have all been applied to these animals, but in 2002 paleontologist David Froehlich undertook a revision of the known fossils in which he split many of these groups into new genera. (If correct, this means that there was a wide diversification of dawn horses rather than the traditional single root seen in many diagrams.) Among the revisions Froehlich proposed was taking the species Hyracotherium grangeri and placing it in a new genus, Arenahippus (making it Arenahippus grangeri).

I mention Arenahippus specifically because a team of paleontologists led by Aaron Wood have just described a nearly-complete skeleton of this 55.8 million year old horse. Part of its snout was crushed, and a few bits of the skeleton – such as the end of the tail – were missing, but otherwise the scientists had a complete skeleton to work with. (See the illustration above; the complete skeleton is on the left and the previous, incomplete view on the right.) Wood and colleagues refer the skeleton to Hyracotherium grangeri – preferring the old name as the mess of early horse taxonomy and systematics is still being worked out – but through the rest of this post I am going to use Froehlich’s proposed name, Arenahippus.

Taxonomic wrangling aside, the new specimen (UM 115547) is more dog-like than horse-like. It was not simply a tiny, multi-toed version of Equus. Instead this Arenahippus possessed a mix of characteristics which show that it was a much more flexible animal than many of its later relatives.

The vertebral column of Arenahippus provided some up-and-down flexibility, but also possessed characteristics which would have increased the stability of the back while running. The anatomy of the horse’s 17 thoracic vertebrae, for example, suggest that Arenahippus had a large splenius muscle which would have attached from the back of the head to the upper back. This would have kept the head stable while running, although the horse still would have had enough neck flexibility to move its head over a wide range while browsing on soft plants in the forest.

The shoulders, arms, hips, and legs of UM 115547 also hinted that Arenahippus was a fairly flexible animal. While the elbows and ankles of the horse were kept fairly rigid, the articulation of the upper arm and upper leg bones with the shoulders and hips (respectively) showed that Arenahippus was capable of a wider range of motion with its limbs than horses in which movement is restricted mostly to front-to-back motions. This finding was consistent with the proposed habitat of Arenahippus. It frequented closed, forested habitats where it ran over uneven ground. The increased flexibility in this horse’s limbs would have allowed it to maneuver through rocky, plant-tangled habitats.

Taken as a whole, the spine and limbs of Arenahippus exhibit adaptations for stability during running but a greater degree of limb flexibility than later horses. As pointed out by the authors of the new study, the later small horses Orohippus and Mesohippus possessed more adaptations for rigidity and stability during running, and these changes may have been attributable to a shift into more open environments. We should be wary of creating clean-and-neat scenarios, though, especially since the traditional view of horse evolution is that it is the story of small woodland browsers which became large grassland grazers, and information about shifts in paleoecology are key to investigating whether a certain anatomical change was attributable to a change in the environment. Given the successive evolutionary radiations of horses, any tidy story is almost certainly going to have exceptions to it, and the further discovery of complete dawn horses will help provide the context through which the radiation of other early horses can be better understood.

Images: Two skeletal reconstructions of Arenahippus (=Hyracotherium) grangeri. The skeleton on the right represents the traditional view, based upon other horses, while the one on the left was drawn from a recently-discovered, nearly-complete specimen from Wyoming. Modified from Wood et al., 2010.

References:

FROEHLICH, D. (2002). Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla) Zoological Journal of the Linnean Society, 134 (2), 141-256 DOI: 10.1046/j.1096-3642.2002.00005.x

Wood, A., Bebej, R., Manz, C., Begun, D., & Gingerich, P. (2010). Postcranial Functional Morphology of Hyracotherium (Equidae, Perissodactyla) and Locomotion in the Earliest Horses Journal of Mammalian Evolution DOI: 10.1007/s10914-010-9145-7

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