Palæos:		Unit 530: Altungulata
The Vertebrates		Overview

Altungulata: Overview

UNGULATOMORPHA 
|--CETARTIODACTYLA
`--Altungulata
   |--Hilalia  
   `--+--+--Perissodactyla 
      |  `--Tubulidentata 
      `--Paenungulata 
         |--Hyracoidea 
         `--+--Sirenia 
            `--Proboscidea 

Introduction

The altungulates are the "odd-toed" ungulates. The group also takes in some oddballs that don't fit well elsewhere in the scheme and may not belong here, either. The horses and elephants are certainly closely related. Less certain are manatees, aardvarks, and hyraxes. Finally, some basal members of the group, such as Hilalia from the Eocene of Turkey, are too poorly known to place with much confidence. As ungulates go, this is the ne'er-do-well branch of the family. Their biological diversity is astonishing. However, this may be the result of having been forced into odd, marginal environments by competition from their generally more succesful kin: the pigs, deer, and whales of the eparctocyonan clan.  

The Origin of the Altungulates -- and Some Anatomy

As a whole, the ungulates are the big, fast herbivores of the Cenozoic. This is the same role which was played by the ornithopods before them, and by various therapsid groups at times in the Late Paleozoic and Early Mesozoic. The altungulates got off to a late start, perhaps in the Late Paleocene, but arguably continued that trend to become bigger and faster -- but not both at the same time. The key innovations of the altungulates do seem to be related to further increases in mobility. The clavicle is completely absent, finally disposing of that awkward lock between the front limbs which was inherited from the first tetrapods. The astragalus (one of the main ankle bones) is shortened, sacrificing flexibility for the sake of strengthening the leg as a whole. The weight is concentrated on the middle of the foot, rather than being balanced between the two sides. These sorts of adaptations allow the evolution of extremely fast animals, like horses; or to massive animals with remarkable turning ability, like rhinos, elephants, and even manatees. The hind limbs can take a huge amount of pounding.  The front limbs move more independently and allow changes in direction without loss of balance. On the other hand, these adaptations do not favor jumping ability or rapid postural changes. That sort of behavior requires a different sort of springy-ness, close limb coordination, and superior balance. Try a standing jump or rapid squat with your arms held out at different angles, or just standing on the edges of your feet, and you'll see the problem. An altungulate is a bit like a human on skiis.  With practice, speed and agility are very good at any mass; the limiting factors are balance and posture.

Phylogeny of the Altungulates

This is an awkward issue. In essence, the altungulates diverged into two main clades, the perrisodactyls (horses) and paenungulates (elephants and manatees). But where the hyraxes and aardvarks go is anyone's guess. Since we have not delved far into these top branches of the vertebrate tree, we will defer for discussion for now.

Altungulate Issues

As mentioned earlier, one significant issue is whether this is really a natural group. The elephants and horses surely belong together, but the rest are up for grabs. This is also an area in which the molecular and morphological researchers are very far apart, with the former denying that the altungulates are a clade at all. 

The gradual replacement of altungulate groups with terrestrial artiodactyls (pigs, goats, cattle, deer), may relate to long-term changes in climate and vegetation. Some of this relates probably to the unique digestive apparatus of the artiodactyls. But what exactly are the environmental factors responsible for the relative failure of the altungulates?  -- ATW 020309

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