Why Equus ferus should not be used for the wild horse

The use of the binominal name Equus ferus for the wild horse is extremely common, and I used it myself in the past. However, this binomen is problematic and should not be used except in a list of synonyms of Equus caballus. 

My readers will know that I prefer to use Bos primigenius for the aurochs, thus the later-published binomen that is frequently used for the wildtype, while I treat the species of the horse (including the wild horse and its domesticated derivatives) under the name Equus caballus, which is usually used to refer to the domestic form only. This may seem inconsistent at first, but examining the problem at a deeper taxonomical level shows that it actually is not. 

I prefer Bos primigenius because this binomen has a holotype that is undoubtedly a wild specimen, the Haßleben aurochs skeleton, while Bos taurus lacks a holotype or lectotype. Type specimen are important because they clarify which species is referred to by the name. Which species is Bos primigenius? The species that the Haßleben skeleton belonged to, whatever a species definition you work with, no matter how narrow or wide your species definition is. Thus, the lack of a type specimen is problematic because it makes the situation far more ambiguous. 

As for the horse, neither Equus caballus nor Equus ferus have a type specimen. For Equus caballus, this is less problematic because it referred to domestic horses, while for Equus ferus it is a big problem because of synonymity. If two binominal names are synonymous, i.e. they refer to the same species, the rule of priority of the ICZN dictates that the name that was published first is the one to be used (the senior synonym), while the younger one is the junior synonym (as a side note, synonymity does not make the junior synonym invalid [as long as it is described accordingly to the rules of the ICZN], as it is often believed, it just means that it does not have priority and shall not be used). In the case of domestic animals, where the binomen that was erected for the wildtype exclusively often is the junior synonym, opinion 2027 of the ICZN allowed the use of the names for the wildtypes for the entire species. This includes Bos primigenius, but also Equus ferus. The ICZN explicitly assumed in opinion 2027 that Equus ferus is based on the ancestor of domestic horses, which they call “tarpan” in their document. For Bos primigenius, there is no problem following opinion 2027 because it is based on a specimen that was indeed a member of the wildtype of cattle, for Equus ferus there is indeed a problem. In order to see if Equus ferus is justifiable as the binomen of the wild progenitor of the domestic horse, we have to look at the original description of this name. 

I used Equus ferus in the past because, like everyone else, I did not check the original source and assumed that everything is alright with that name. This is the reason why it is that widely used as the name for the wild horse. For some reason, I assumed Equus ferus was based on fossil or subfossil remains of definite wild horses in Europe. When I checked the original source, I realized that this name should not be used for the wild horse. 

Equus ferus is introduced by Pieter Boddaert in his 1785 work Elenchus animalum. Boddaert gives a short description mentioning long ears, a short curly mane, a blackish grey mouse-coloured body and a short tail, and refers to the written description of alleged wild horses encountered by S. G. Gmelin in Voronezh in 1769. That is the original description of Equus ferus. The big problem is that it is far from clear what those horses spotted by Gmelin were. Whether the horses were wild, feral or hybrids of wild and feral horses. The description fits what other writers reported of free-roaming horses at that time and region. At least one member of the herd was a black domestic mare, as reported by Gmelin himself. It is generally unclear what the horses of that time and region were, and there never was a consensus among authors, be it contemporaneous ones or after the extinction of those populations in the 19^thcentury to this day. And in the lack of a type specimen, be it a holotype or a lectotype, it cannot be ascertained anymore. This case shows why type specimen are important. 

Therefore, there is no certainty that Equus ferus is based on a wild representative of the species that includes the domestic horse. If domestic animals and their wildtypes are regarded as separate species, the name to be used for the species of the wild horse would be Equus przewalskii because this binomen was the first one to be erected based on an undoubtedly wild specimen of the species. It has both a holotype and paratype which were wild Przewalski’s horses. I usually treat wild and domestic animals as members of the same species, and the first name erected for this species is undoubtedly Equus caballus by Linnaeus 1758. Linnaeus also erected a senior synonym for Bos primigenius, namely Bos taurus. However, in this case I prefer to follow opinion 2027 because the junior synonym conserved is based on a type specimen that was undoubtedly wild, while Equus ferus is based on a description which itself is based on a description of an encounter with horses of unknown status. Therefore, following opinion 2027 in the case of Equus ferus is not useful. For the same reason Equus ferus should not be used on subspecies level for the subspecies that was ancestral to the domestic horse. There is not the slightest bit of scientific evidence that the horses encountered by Gmelin in 1769 were members of the form that was domesticated 5000 years ago. For the same reason Equus ferus should not be used for Pleistocene wild horses. It should not be used for wild horses at all. If one wants to use Equus ferus solely to refer to the horses historically called “tarpan”, it is questionable if these populations deserve a subspecies status as it is unclear what those horses were. So neither Equus ferus nor Equus ferus ferus are taxonomically justified. 

This of course leaves the question what the subspecies that was the predecessor of the domestic horse should be called then. For this question to be answered, a rigorous assessment of the Holocene wild horse material that has been found in Eastern Europe would be necessary. It is well possible that someone already erected a taxon based on that material that would have priority. Whether or not a Pleistocene wild horse name (and there are a lot of those) is applicable to this form remains to be seen, this would have to be morphologically and genetically evaluated. When a subspecies is not yet scientifically described, it is referred to as species X ssp., in the case of the wild horse that was ancestral to the domestic horse it would be Equus caballus ssp. – a possible way to “rescue” Equus ferus is to assign undoubted Holocene European wild horse material to Equus ferus as a lectotype. As long as this has not been done formally, Equus ferus should not be used as the binominal name of the wild horse.