Brian Switek Musings on Evolution, the Fossil Record, and the History of Science

Prehistoric mammal Prolibytherium had a “butterfly face”

Exactly what Prolibytherium magnieri was, no one is quite sure. Since the time it was described in 1961, the 17-16.5 million year old mammal from Egypt and Libya has been closely allied with prehistoric relatives of deer (Palaeomerycidae), ancient giraffes (such as Sivatherium), and a motley group of giraffe cousins (Climacoceratidae). Many experts now agree that it probably belonged to the latter group, but it was so bizarre that classifying it has been an extremely frustrating task.

What made Prolibytherium so strange were the huge structures growing out of its face. These bony appendages - which would have been covered by skin and hair in life - formed a butterfly-shaped plate which made Prolibytherium look like something Dougal Dixon might cook up for one of his speculative zoology books. Two bony support ribs stuck out towards the front of the face, and two more jutted out towards the back of the head, with a "web" of bone growing out from these structures to create the impressive ornament.

That Prolibytherium sported such hardware has been known for quite some time, but similar fossils found in the same deposits presented paleontologists with a petrified puzzle. These other fossils showed some similarities to the structure of the Prolibytherium appendages, but they were nowhere near as wide. Over the years scientists have hypothesized that this second kind of ornament might represent a member of the Palaeomerycidae, a juvenile or female Prolibytherium, or even the potential ancestor of Prolibytherium. Without more complete fossils, it was difficult to know which hypothesis was correct, but a short communication just published in the Journal of Vertebrate Paleontology reports that the mystery has been solved.

When the scientists behind the new study placed the complete Prolibytherium facial ornament next to the the smaller, cylindrical structures, the mystery ornaments looked very much like the supporting rods of the Prolibytherium cranial appendage. More than that, the structures were almost an exact match in terms of bone structure and their placement on the skull - the two types of ornaments belonged to the same type of animal. Being that there was no indication that a second species of Prolibytherium was present at the sites, the scientists propose that the second ornament type represents a female animal (since the degree of bone development on them indicates that the animals had been adults).

In effect, female Prolibytherium carried around the spiky framework which was the basis for the expanded, male ornaments. If the hypothesis that Prolibytherium belongs among the already strange Climacoceratidae, then it would even more of an oddball as the first known member of this group to exhibit such a distinctive degree of difference between the sexes.

Naturally, such conspicuous ornaments - both among males and females - beg the question of what they could have been used for. In living bovids, species in which males and females both have horns often use them for defense, but in these cases there is a decreased difference between the sexes. (The horns have to stay functional, so there is a limit to how far sexual selection can push them.) Clearly this pattern does not fit Prolibytherium, especially since the skin-covered wings of the males do not look like they would have been effective defensive weapons. Instead, it may be that the structures were primarily used for display or species recognition. Sexual selection - be it female choice or competition between males - appears to have created the two different types of ornaments in Prolibytherium, but the fact that females have conspicuous facial ornaments raises questions about how such structures evolved in the first place - the present function of a structure does not always explain its origin. The discovery made in the new report is fascinating, but Prolibytherium remains an enigmatic animal.

cranial wings among the males seem to beg the question of "What were they used for?" That is another mystery. Horns, antlers, and other such appendages are used for a variety of functions among mammals, not all of them mutually exclusive. In the case of Prolibytherium, it has been proposed that males could have used their cranial wings to wrestle with other males, but it is difficult to imagine what such conflicts would have been like. It may be that they evolved as display structures, perhaps as a result of female choice or competition between males for mates (the male with the biggest head wings, wins). Since Prolibytherium is long gone and there is no living animal quite like it, we may never know,

Sanchez, I., Quiralte, V., Morales, J., Azanza, B., & Pickford, M. (2010). Sexual dimorphism of the frontal appendages of the early Miocene African pecoran Prolibytherium Arambourg, 1961 (Mammalia, Ruminantia) Journal of Vertebrate Paleontology, 30 (4), 1306-1310 DOI: 10.1080/02724634.2010.483555