Have we overlooked how remarkable Equus really is, as a genus?

(writing in progress)

Please see https://www.frontiersin.org/articles/10.3389/fevo.2019.00235/full and https://www.frontiersin.org/articles/10.3389/fevo.2019.00343/full.

The equids of Patagonia and elsewhere in southern South America, formerly classified as different genera, may belong to the widespread genus Equus.

It is possible that Equus not only reached the Pampas of Argentina, but differentiated there into at least two distinct species (one currently known as Hippidion saldiasi (https://link.springer.com/article/10.1007/s00239-008-9100-x) and the other currently known as Amerhippus neogeus, https://en.wikipedia.org/wiki/Amerhippus).

If so, it would show how paradoxical Equus is, in the following sense.
 
On the one hand, all equids are ‘much the same’. Even a child can see the affinity among zebras, horses, asses, and donkeys. Anyone with the least familiarity with zebras knows that, in a sense, zebras are just ‘striped horses’ or ‘striped asses’.
 
But this impression of remarkable morphological and behavioural homogeneity is, I suggest, misleading.
 
For there is, in a different sense, a pattern in which Equus is perhaps the most heterogeneous of all ungulate genera.
 
Everyone knows that zebra striping is both extreme and puzzling, but what seems to have been underplayed is the realisation of how extreme it is for a single genus to differentiate in this way, into plain-coloured forms on the one hand and bizarrely striped forms on the other.

Even the categorical transformation, ontologically, from camouflage colouration to plain colouration in tapirs, as they grow from infant to adult, does not really prepare one for the finding that a zebra is at once just a ‘dressed-up horse’ and ‘a horse dressed beyond anyone’s wildest imagination.

The heterogeneity of Equus, in this sense, strains credulity.
 
Now this seems relevant to the extension of Equus across such an enormous geographical area. I emphasis: for Equus to live in the Pampas would be remarkable enough, as a far-flung area, even if the rest of the fauna of the Pampas replicated that in the Old World by means of evolutionary convergence.

But this greatly understates the situation, because the Pampas was inhabited by a community almost incomparably different from that in which zebras have always lived in Africa.

Where in the Old World have there ever been counterparts for the various autochthonous quasi-ungulates of South America, which are so odd that even with fossil material no artist really knows how to reconstruct them in paintings? The giant sloths? The glyptodons? The giant rodents? The marsh-vicugna? The remarkable lack of ruminants despite the fact that South America was invaded by deer?
 
Just consider the following sketch, in the broadest of strokes, of how different the communities have been, of which Equus has been an important part.
 
In central Eurasia, Equus dominated the ungulate community. As far as we know (and information is hard to find on this), E. ferus may have been the only large ungulate in extensive treeless grasslands, frosty in winter and lush and nutritious in summer.

Sure, there were aurochs, bison, saiga, and gazelles around and about, but we don’t seem to have any reason to believe that E. ferus actually coexisted with these, at least in the richest habitat in Eurasia, i.e. the treeless grasslands which have proven so valuable for farming.
 
In Africa, Equus somehow manages to fit into the most diverse communities of ruminants on Earth.

It would seem that the antelopes and other bovids have filled every conceivable niche, and yet there we have Equus, somehow finding not only a niche for itself but a prominent one. It is in order to accomplish this remarkable integration that, I would argue, zebras have had to evolve their bizarre striping.
 
And the, it would seem, we have the Pampas (in less extreme form, the various other treeless grasslands of the American supercontinent). Here all bovids are absent and even deer are marginalised ecologically.

There are precious few true ungulates, although proboscideans are important. The rapid metabolism of Equus contrasts with the slow metabolism of various ‘edentates’, which seem to represent a different world.

What did Equus look like in this situation, in order to insinuate itself into such a far-fetched community?

Which unimaginable permutation, analogous to striping but not along the same lines, is out there in our conceptual space, waiting for us to bring it to consciousness?
 
What I’m hinting at is that the puzzle of zebra striping may be solvable, but that could just be the beginning – for Equus is at once so familiar and so other-worldly in its biogeography, morphology and ecology.
 
(writing in progress)

Posted on September 13, 2022 08:14 PM by milewski