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Friday, 20 June 2014

Oldest Aurochs skull found: huge horns, phylogeographic implications

Martinez-Navarro et al. 2014 present a partially preserved cranium with complete horn cores of the oldest aurochs specimen known hitherto, dated back ~700.000 years and found in Tunisia. Not only is this a quite large specimen (keep in mind that geologically young aurochs tended to be larger than most Holocene anyway) but its horn cores are huge, probably the largest known yet. They measure 112,0-111,5 cm in outer length and lateral span about 150 cm (for comparison, the horns of the Wörth bull Aretto, which are already large horned cattle, spanned about 100 cm). Since the keratinous sheath adds 25-10% length to the horn, the horns of this specimen must have been around 130 cm long, which is very impressing. Considering its age, I am not sure if it should be classified as B. p. africanus or simply as basal member of the species before any subspeciation took place.
The recently discovered oldest aurochs skull known to date
A common conception was that the clade Bos is of (Eur-)Asian origin because a number of extinct possible predecessor genera are located there (f.e. Leptobos) and all living members of the clade live there, and further it was suggested that the Aurochs itself was of Indian origin because the earliest previously known remains are from India. An alternate hypothesis that is increasingly well supported is that Bos originated in Africa since its cranial anatomy is more derived than that of Bison and Leptobos and shares more characteristics with the late Pliocene-early Pleistocene Pelorovis (no, not “Pelorovisantiquus which actually should be classified as Syncerus antiquus, but P. olduvaiensis and P./Bos turkanensis) [1]. And in fact, the oldest true Bos members, B. turkanensis (which can be interpreted as an intermediate form between Pelorovis and Bos, and is mutually assigned to both taxa) and the very long horned B. buiaensis, were inhabitants of North-eastern Africa.

Bos buiaensis
Pelorovis olduvaiensis

The fact that this oldest aurochs specimen known up to date yet is from North Africa does not necessarily imply the species evolved there. But I do see a certain continuity between P. olduvaiensis and this basal aurochs specimen: 1) The angle between the horns and the snout is much sharper than in many later European aurochs specimen, what is “suspicious” considering that this specimen is at the root of the species and that those of Pelorovis are oriented almost parallel to the snout. 2) The horns of Pleistocene aurochs are more dorsoventrally compressed than in Holocene aurochs/cattle, and even more so in this specimen (~14 cm vs. ~9,3 cm in diameter). Those of Pelorovis are very dorsoventrally compressed. 3) The skull of this definitely male specimen is comparably narrow, and the skull of Pelorovis is also very narrow.
So the postulated African origin of Bos and Bos primigenius seems plausible to me as well. Bos buiaensis has been suggested as the ancestor of the aurochs, but the fact that its horns are considerably more outwards-ranging than in both Pelorovis and B. primigenius makes that unparsimonious. My personal opinion is that it perhaps might be a sister species and both might have descended from B. turkanensis. Another interesting question is whether the common ancestor of the Kouprey-Banteng-Gaur clade was either B. turkanensis, another species or even basal aurochs (although I think the aurochs might be too young to be a probable ancestor of this clade). So the aurochs must have spread from the north of Africa to the Near East to all over Eurasia. Interestingly, the Indian aurochs B. p. namadicus is reported having longer and more wide-ranging horns than other aurochs populations. Perhaps this indicates hybridization with B. acutifrons, an Asian species that was similarly long-horned as B. buiaensis, which died out shortly after the aurochs arrived in that region. Genetics do indicate that a lot of hybridization within Bos and also with Bison took place – Yaks, now recognized to be members of the Bison clade [2], were introgressed by Bos; Gaurs and Koupreys introgressed the Banteng [1], and the Wisent probably is a hybrid of Bos and Bison [2].
The subject gets even more interesting (and more speculative) when you look at the pinned V-shaped dorsal edge of the frontal bone between the horns of some Zebus, which pulls the horns closely together and upwards, giving them an antelope-like appearance. You will see that P. olduvaiensis has the same condition. Could that be caused by the same genetic background inherited by this species that was reactivated by the altering of the genome through domestication? This is only a wild speculation of mine.

Combining all that, I did a little cladogram of my idea on the phylogeny of Bos – without having done a phylogenetic analysis, but based on the molecular and morphologic data cited and outlined here and the rough guess that the Kouprey is the sister species of Gaur + Banteng. Do not take this phylogeny for granted.

P. olduvaiensis àB. turkanensis + --- + -- †B. buiaensis
                                                             `        `
                                                                `        ` --- † B. acutifrons
                                                                   `
                                                                      `--+ --- B. primigenius
                                                                             `
                                                                                `--+ --- † B. sauveli
                                                                                     `
                                                                                       `--+ --- B. javanicus
                                                                                             `
                                                                                                ` --- B. gaurus


I have to say I can’t wait to illustrate a life restoration of B. buiaensis, B. p. africanus and B. p. namadicus and to write a post on them and their paleoenvironment.

I wondered why the early members of Bos had these large horns. Their size is too extreme to serve for defence only, but could be the result of sexual selection for display like in other bovids like caprines [3], although the horns of the early aurochs surely were formidable weapons too. Thermoregulation might be another explanation, since horn cores are highly vascularized bones and might serve as a tool to get rid of excessive heat. And no, I do not think that the large horns of early African aurochs and Watussi cattle are more than a coincident unless there are some conclusive hints for it. The fact that early Bos had more forwards-pointing horns than later ones, including North African Aurochs, might indicate that Iberian landraces had some africanus introgression considering that they mostly come from Africa (more on that later), but I do not think that because taurine cattle probably arrived in NA after the African aurochs died out.


Literature

[1] Martínez-Navarro, B., Karoui-Yaakoub., N., Oms, O. et al., "The early Middle Pleistocene archeopaleontological site of Wadi Sarrat (Tunisia) and the earliest record of Bos primigenius", Quaternary Science Reviews (2014).
[2] Maternal and Paternal Lineages in Cross-Breeding Bovine Species. Has Wisent a Hybrid Origin? Edward L. C. Verkaar,* Isaa ̈c J. Nijman,* Maurice Beeke,* Eline Hanekamp,* and Johannes A. Lenstra 
[3] Hans-Peter Uerpmann: Der Rückzucht-Auerochse und sein ausgestorbenes Vorbild.



1 comment:

  1. Fascinating. Your conclusion about hornsize being mostly for display sounds very logical. Especiallly when you consider that fighting cattle, that are only selected on their fighting capabilities, have quite small horns.

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