Strange Symmetries #21: Uneven Ungulates

Asymmetry is commonly seen in the headgear of modern even-toed ungulates, with natural genetic variation, developmental stress, and injuries during life sometimes causing very wonky-looking horns or antlers.

No living species have asymmetry as a standard trait, however – but some fossil ungulates did.

Ramoceros osborni was a relative of the modern pronghorn living during the mid-Miocene, about 13 million years ago, in open plain habitats of what is now the Midwest and Mountain states of the USA.

It was smaller than modern pronghorns, around 70cm tall at the shoulder (~2'4"), and males had long antler-like horns with three tines. Bizarrely, one of these horns was always at least twice the size of the other, with "left-horned" and "right-horned" individuals seeming to occur in equal numbers.

It's not clear how this asymmetry affected combat between males. Could they only properly lock horns with "opposite-sided" rivals, or did this uneven arrangement actually prevent physical fights and restrict them more to just visual displays?

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Meanwhile in China another Miocene ungulate known as Tsaidamotherium hedini also had strange headgear, with an enlarged right "horn" forming a helmet-like dome on top of its head. This species was featured here on the blog just year, so check out that post for more details about it.

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Pronghorns 4-26-23

Cape giraffe (Giraffa camelopardalis giraffa)

Tsaidamotherium hedini was a ruminant ungulate living around 11 million years ago during the late Miocene, in the northeastern part of the Tibetan Plateau in what is now Northwestern China. Although it's known only from partial skull remains it was probably similar in body size to a large sheep, about 80cm tall at the shoulder (2'7").

Since its discovery in the 1930s it's traditionally been classified as part of the muskox lineage, but in 2022 it was proposed to actually be a giraffoid very closely related to the newly-discovered Discokeryx.

Tsaidamotherium had some extremely unusual headgear, with highly asymmetrical "horns" (actually ossicones if was a giraffoid). The left one was small and positioned above the eye, while the right one was shifted back and towards the middle of the forehead, and was expanded out into a wide bony disk that would have supported a large helmet-like domed keratin covering.

Its skull also had a very large nasal cavity resembling that of the modern saiga antelope, suggesting it may have convergently evolved a similar sort of complex air-filtering snout to deal with dry cold air in its mountainous habitat.

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Maasai giraffe (Giraffa camelopardalis tippelskirchii)

Weird Heads Month #09: Butterfly Faces

The nose-forks and head-crests we saw last time weren't the only unusual headgear in ancient ruminants.

The giraffoids are represented today by just pronghorns, giraffes, and okapi, but in the past they were much more diverse, modifying their prongs and ossicones into multiple sets of horns, or into deer-like and moose-like antler shapes.  

And Prolibytherium was probably the most striking of the lot.

Two different species have been identified, with Prolibytherium magnieri here living in North Africa during the early-to-mid Miocene, about 17-16 million years ago. Its exact evolutionary relationships are uncertain but it was probably part of a group called climacoceratids, deer-like giraffoids which often had thorny branching ossicones that resembled antlers.

It stood around 1.2m tall at the shoulder (~4'), and exhibited dramatic sexual dimorphism – females had slender forked horn-like ossicones, while those of the males flared out into large wide flat shapes that resembled butterfly wings.

Heavy reinforcement in the bones of the back of the males' skulls helped to support all the extra weight of those huge ossicones, and if they actually used the structures to fight with each other then this may have also provided some protection or shock absorption.

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Okapi (Okapia johnstoni)

Rothschild's giraffe (Giraffa camelopardalis rothschildi)