Equus hydruntinus, the enigmatic European wild ass

After finally having done a post on Bubalus murrensis, it’s time to look at the extinct European wild ass, Equus hydruntinus. This enigmatic taxon once roamed Europe during the Pleistocene until the a part of the Holocene, and its presence is therefore interesting for “European rewilding”. This post is going to investigate its systematic position, time and range, looks and reasons for extinction. And of course which implications it has for the (re)introduction of megafauna here in Europe.

But before that, there is some (paleo-)equine terminology to learn:

caballines: anything closer to horses than to other living equines (or: domestic horses opposed to the Przewalski’s horse)

hemiones: E. hemionus plus E. kiang, which form a clade

stenonines:  Equus stenonis plus similar equine remains from the Plio-Pleistocene.

asinines: anything closer to the African wild ass E. asinus than to other living equines.

Skull of E. hydruntinus from Crimea, taken from [4]

Equus hydruntinus was a small equine with a gracile and cursorial build. It has been difficult to classify it for a long time. The remains were only sparse and mostly fragmentary, and it seemed that it showed a mosaic of characters [1;2]. The elements used for a phylogenetic classification based on skeletal morphology were mostly limb elements, tooth and – after two almost complete crania were found [3,4] – also skull proportions. Because of this morphological mosaic of characters, it has been variously classified as a donkey relative, a zebra relative, hemione and stenonine [1] since it shares characters with most Equus clades with the exception of horses.

The similarity to stenonine horses was based tooth characteristics [1], while its leg bones and skull morphology was reminiscent of hemiones and asinines [1]. A dental similarity with hemiones has also been pointed out, but also that there seems to be a certain degree of polymorphism within equines [5], making this feature less useful for evaluating systematic positions.

Nevertheless, there is a general agreement that morphological characters suggest that Equus hydruntinus was a distinct species, differing from but also resembling its Equus relatives in a number of features [5]. What is important to note is that hydruntinus was sympatric with other of species/subspecies in a number of regions, such as caballines, asinines, E. h. hemionus and E. h. hemippus [5]. It is out of question that hydruntinus was a horse due to the lack of morphological similarity. Its coexistence with sp. hemionus subspecies sheds doubts on the hypothesis that hydruntinus was a member of this species because they would interbreed and merge together, even if they occupy different ecologic niches. You will find no example of two subspecies of a species inhabiting one and the same biotope. The idea that hydruntinus occupied a different ecological niche is supported by its small size and different tooth morphology. Even within the clade of hemionus + kiang, hydruntinus seems to be kind of a patchwork of characters: the metapodials are as long as in the Kulan, the teeth and protocones are as small as in the Syrian wild ass.

However, since osteomorphologic characters alone are not ideal to create a solid hypothesis on hydruntinus’ identity due to intraspecific variation and the scarcity of the fossil/subfossil material. Therefore, genetic studies are needed to help to clarify.

Orlando et al. 2006 and 2009 did such studies using aDNA of the mitochondria. They found Equus hydruntinus showing no affinity to the Burchell’s zebra and asses (2006)[3] to be nested within Equus hemionus (2009)[2]. Together they share a 28-bp deletion in the mitochondrial HVR-1 gene [2]. They conclude that the hydruntinus therefore is a subspecies of Equus hemionus. On the other hand, kulan/onager and kiang are regarded as separate species because of different coat colour, morphology and karyotype yet show poor mitochondrial differentiation [2] (the coat colour and karyotype of hydruntinus are not even accessible to us). Another issue is that also the Grevy zebra is nested within the hemionus group (yet not showing the 28-bp deletion). Therefore Orlando et al. suggest that nuclear loci are needed to further clarify the relationships of E. hydruntinus.

Possible depiction of E. hydruntinus from the Chauvet cave (Bottom)

What we know from skeletal remains, including relatively recent finds of a nearly complete skeleton from Iran and two well-preserved crania from Crimea [3], E. hydruntinus was a small equine (smaller even than hemionus and hemippus) with gracile and cursorial legs. The body was short and the backline, typically for wild asses, straight or slightly convex. The muzzle was comparably short in the well-preserved crania – this has been interpreted as an adaption to cold climate [3], but I consider that premature because this based on pretty scanty data and equines have much cranial variation [3]. As an ass, it very likely had long ears, a short mane and its tail only had a short tassel on its end. But apart from that, we do not know much about its outer appearance because it is not nearly as often represented in coloured paintings as aurochs and wild horses are. There is one coloured painting in Lascaux that may or not may be a wild ass, the equine next to it is undefinable. Assuming it is E. hydruntinus, it might implicate it was of a dark brown colour. But this is nothing but a weak hint, the European wild ass might have had any colour nuance displayed by modern equines with a bay dun colour. There is another possible depiction of E. hydruntinus at Chauvet that shows a panel of wild horses plus an equine at the bottom which might be an ass because of the different, ass-like head shape and possibly long ears. In this case we might infer that E. hydruntinus was of a darker colour with a clearly recognizalbe muzzle ring. Or maybe it was another species of ass. Or maybe not an ass at all. The data is simply not sufficient.

Obviously E. hydruntinus is morphologically and genetically distinct enough to distinguish it from other members of the genus and to reconstruct a geographic and geologic range. The species seemingly had been existing since the Middle Pleistocene. In Europe, the  main body of its geographic range was Southern Europe, which also was a refuge for this and many other species during the last glacial maximum [3]. Its northwards range during the Pleistocene was restricted only by the ice sheaths. It went up was much north as UK or Germany [3] in the past. Findings in Siberia that have been previously assigned to hydruntinus in fact turned out to be related to Equus sussenbornensis, a more basal species from Germany, with which it forms the now extinct group of the sussemiones. Eastwards, its range reached as far as Iran[3], where it coexisted with E. h. hemionius but also Syria [3], where its range overlapped with the Syrian wild ass E. h. hemippus.  

The European wild ass died out comparably recently. It occurred in the subfossil record until the Iron Ages. For example, there is a solid record of the species in Neusiedl in the east of Austria (Pucher 1991 [6], Erich Pucher in personal communication 2012). The hypothesis that Equus hydruntinus is the mysterious “zebro” of the Iberian peninsular in historic time has always been controversial. For once, the “zebro” was described as an equine clear striping, while hemiones – which are most likely the closest relative of hydruntinus – do not show striped patterns, only a very faint shoulder streak in some cases. The skeleton of the alleged last zebro from the 17^th century turned out to be a donkey [2]. So there are no remains that suggest it survived beyond the Iron age.

The sites where it was found suggest it was an animal of dry and open areas, like other asses. In fact even the location in Austria where it was found, Neusiedl, is sometimes described as a “small exclave of the Puszta”.

The fact that Equus hydruntinus was not as prevalent in prehistoric art as other mammals were might implicate that it was not that spectacular and important to humans and not all too numerous. The last point is indeed supported by fossil and subfossil evidence to a certain degree. By the Holocene, its range was apparently highly fragmented [3] Probably this was originally the result of ecologic changes at the end of the last glacial [7], but it was probably human hunting and habitat loss that made it a true extinction process. Although it seems that it was not one of the most important game species for humans, there is evidence from several sites that this species was seized by man [7]. Particularly the fact that the population seemingly began to drop from the Neolithic onwards is indicative of human influence [7]. Therefore it is likely that the European wild ass would have recovered from the ecological changes that occurred by the end of the last glacial if there had not been anthropogenic factors, similar to the case of giant deer [7] or the woolly mammoth. So it might still inhabit at least some regions of Europe, particularly the dry and open ones. I cannot speak for the Near and Middle East.

So my view on wild asses in Europe is similar to that on water buffalo. I think it is O.K. to use a closely related and ecologically probably similar species to replace hydruntinus in some regions, be it kulan or onager (I favour kulan). But European reserves probably would do well without them as well. Whether it would be an ecologic benefit I cannot say, but it probably would do no harm either. But a claim that it is a “necessary keystone species” would be, in my opinion, baseless. Regions that might be best-suited for wild asses might be the Balkan, Eastern European steppe-like environments such as the Hungarian Puszta, and probably also the Italian and Iberian peninsular.

Literature

1. A. Burke, V. Eisenmann, G.K. Ambler: The systematic position of Equus hydruntinus, an extinct species of Pleistocene equid. 2003.

2. L. Orlando et al.: Revising the recent evolutionary history of Equids using ancient DNA. 2009.

3. L. Orlando et al.: Geographic distribution of an extinct equid (Equus hydruntinus: Mammalia, Equidae) revealed by morphological and genetical analyses of fossils. 2006.

4. E.N. van Asperen, K. Stefaniak, I. Proskurnyak, B. Ridush: Equids from the Emine-Bair-Khosar Cave (Crimea, Ukraine): co-occurrence of the stenoid Equus hydruntinus and the caballoid E. ferus latipes on skull and postcranial remains. 2012.

5. E.-M. Geigl, T. Grange: Eurasian wild asses in time and space: Morphological versus genetical diversity. 2012.

6. E. Pucher: Erstnachweis des Europäischen Wildesels (Equus hydruntinus, Regalia, 1907) im Holozän Österreichs. 1991.

7. J.J. Crees, S.T. Turvey: Holocene extinction dynamic of Equus hydruntinus, a late-surviving European megafaunal mammal. 2014.