Horn function in Arsinoitherium OR... the ArSUMOitherium Hypothesis™

Hello, 2017 painting of sheathed-horn Arsinoitherium zitteli. Time to see if we can figure out what those horns were for, other than looking regal in artwork.

Recently, I was in the Teylers Museum, Haarlem, for the opening of their sensational palaeoart-themed Dinomakers exhibit*, where a fine cast of the skull of the Palaeogene Afro-Arabian embrithopod Arsinoitherium zitteli is perched in the main fossil gallery. Arsinoitherium is a pretty darn fascinating mammal that we're all familiar with, yet we rarely give exclusive focus too. I assume this reflects its scientific vintage. Arsinoitherium was discovered well over a century ago and is now featured in so many books and museums that it's part of the popular palaeo furniture. Its size, fantastic cranial horns and status as the last of the embrithopods make it a remarkable and charismatic fossil species, but it just doesn't seem to be cutting it with the kids.

*If you're a regular reader and are in the Netherlands before June, you really want to check this out. It has heaps of original palaeoartworks, including many by classic artists - Hawkins, Knight, Burian, and some exceptional modern work from the Kennis brothers. I have stuff there too.

Today, Arsinoitherium is mostly discussed by palaeontologists documenting new Palaeogene faunas in Eocene-Oligocene sites of Africa and the Arabian Peninsula, but several fascinating behavioural and ecological revelations about this sometimes controversial mammal have also been published in recent years. It's increasingly apparent, for instance, that Arsinoitherium was anatomically conservative, the oldest members of the lineage being little different, morphologically speaking, to the youngest. Despite this, it was a very long-lived and widespread genus which must have been highly adaptable to demonstrate sustain such a broad geographic and stratigraphic range (Jacobs et al. 2005). It evidently lived in a range of habitats, the most surprising of which are upland regions well away from the coastlines and estuaries thought to be traditionally Arsinoitherium country (Kappelman et al. 2003). This adaptability occurs despite the unusual multifunctional dentition of Arsinoitherium being adapted to specialised browsing (Court 1992): perhaps it was more of a generalist than we realised.

More specific insights into Arsinoitherium palaeobiology have been proposed too. Studies of their ear anatomy, principally for their phylogenetic signature, have found adaptations to hear infrasound in the same manner as modern elephants (Benoit et al. 2013), and a compelling case is being built against the popular idea that Arsinoitherium was a hippo-like semi-aquatic animal. This proposal was founded on both functional and taphonomic grounds (e.g. Court 1993) but a suite of opposing data, including tooth wear, occurrences in relatively dry palaeoenvironments, the presence of graviportal limbs and details of bone chemistry, are now pointing to more terrestrial habits (e.g. Clementz et al. 2008; Sanders et al. 2010). Revisions to Arsinoitherium taxonomy are also shedding insights on behaviour. Two Arsinoitherium species are now recognised - the well known A. zitteli and the larger, relatively longer-legged A. giganteum - and prior taxonomic distinctions accounting for a third species are now interpreted as evidence of probable exaggerated sexual dimorphism in A. zitteli (Sanders et al. 2004, 2010). Behind the scenes, we're building a developed picture of what Arsinoitherium was like as a real animal, and not just a long-standing museum fixture.

One of my favourite images of Arsinoitherium is this 1907 piece by Charles Knight, published in Osborn (1907). Despite that awesome headgear, Arsinoitherium isn't often illustrated doing much else than standing around, so it's nice to see it having something to do. Hat-tip to Chris Manias for posting this image and making me aware of it.

But one aspect of Arsinoitherium palaeobiology that does not seem to have been discussed at length is horn function. Long-term readers may recall that this is not the first time I've mentioned these structures, as the life appearance of Arsinoitherium horns was the exclusive subject of a 2017 blog post. The take-home of that article is that we artists have probably been incorrect in generally restoring Arsinoitherium horns with facial skin. Rather, their horn surface texture, structure and growth mechanic is consistent with a bovid-like horny sheath. This is not a new idea, with sheathed horns being proposed by several authors (e.g. Andrews 1906; Sanders et al. 2010), but contradicted by others (e.g. Prothero and Schoch 2002; Rose 2006).

Assessing life appearance already tells us something about horn function as a covering of tough, insert tissue has some major biomechanical implications. Arsinoitherium horn cores are deceptively delicate on account of their hollow construction. Despite the skulls of these animals reaching over 80 cm long and their owners attaining masses of around two tonnes (Sanders et al. 2010), Arsinoitherium horn cores were constructed from bones just 5-10 mm thick. Without additional protection, such delicacy might prohibit antagonistic use and a more passive function would seem likely, such as visual communication or acoustic augmentation (sensu Benoit et al. 2013). But modern species show that a horny sheath over a hollow horn core creates an amazingly strong, impact-absorbing and bending resistant organ that can be used to bludgeon, wrestle and lance other animals or to forcefully modify the surrounding environment. The physics of this is pretty simple: the hollow bone core provides great bending resistance and reduces weight, while the horny sheath absorbs and dissipates shocks and impacts (Drake et al. 2016). We've seen this exact configuration evolving time and again across Tetrapoda, and its presence in Embrithopoda shouldn't be viewed as weird or improbable.

Partly restored left horn of the Teylers' Arsinoitherium zittelli skull showing the characteristic epidermal correlates (numerous oblique foramina and anastomosing blood vessels) for a bovid-like horn sheath. These horns are identical in texture to what you might see under a cow horn.

But while the horns themselves look formidable enough, their use would be limited without a substantial neck to support and wield the head. It's for this reason that I was pleased to see the Teylers' skull without any pesky postcrania obscuring details of its posterior face. The rearward aspects of animal skulls are often overlooked in favour of more spectacular anatomy but, if you're seriously interested in the functional morphology of fossil animal crania, you need to look at the occiput and other aspects of the posterior skull surface to assess the head/neck soft-tissues. These regions reveal much about neck muscle size and distribution, as well as something of head mobility via the shape of the occipital condyle. Even at a glance, you can often say something intelligent about how animals were wielding their heads by looking at the posterior skull.

I was thus greatly interested to see that the Teylers' posterior Arsinoitherium skull bore several features unfamiliar to me from other large animals. Here's what you can see of the back the skull as it stands in the Teylers gallery. I think there's some reconstruction in places but the skull of Arsinoitherium is completely known from several specimens, and any sculpting seems to be a faithful recreation of real anatomy.

Posterior view of the Teylers Museum Arinoitherium zitteli skull cast, as seen in January 2020.

And here's the same thing, more or less, as illustrated by Andrews (1906):

Image from Andrews (1906, courtesy Wikimedia), public domain.

The exciting parts here are not the unsurprisingly robust nature of the skull-neck junction or the general indications of expansive neck musculature. Nor even is it the substantially-sized occipital condyle that is almost as wide as the skull itself, and has a shape seemingly permitting more dorsoventral motion than lateral (a thought posited previously by Andrews, 1906). Rather, the interesting aspect is the unusual configuration of the bones surrounding the occipital condyle. Most animals, even species with large, heavy heads, have relatively flat occipital faces, but the medial dorsal region of the Arsinoitherium occiput is deeply recessed between two large protuberances which extend posteriorly almost as far as the occipital condyle. The dished medial region extends forward quite some way, projecting far over the braincase to form a deep depression in the skull roof (below). The neighbouring protuberances are prominent, posteriorly-directed outgrowths of the dorsal occipital margin (the superior nuchal line) which curve somewhat towards the skull midline, and are supported below by thick bony buttresses. I've looked for similar anatomy in a number of other large mammal skulls and, while my research isn't exhaustive enough to claim Arsinoitherium has an entirely unique posterior skull configuration, I'm happy to declare it unusual.

Dorsolateral view of the Arsinoiherium zittelli occiput, showing the large basin formed by the dorsal region and the two neighbouring projections. Note the complex surface and texturing, indicating scars and attachment sites of neck musculature.

Since seeing this, I've been wondering what it tells us about how Arsinoitherium neck tissues were arranged and what that might mean for horn function. I decided that a good place to start was a stab at reconstructing the muscles of the occipital region, which you can see below. A word of caution about this image: this is not a watertight study of Arsinoitherium specimens based on days and days of work, but more an attempt to get a basic understanding of what that peculiar anatomy represents if we assume conventional mammal occiput myology. I like to think it's not total garbage, but don't treat it as gospel either. I included the classic Gray's Anatomy human occiput illustration in there, scaled to the size of an average human adult (≈ 30 mm wide foramen magnum, apparently), to ram home how large the skulls of Arsinoitherium are.

My attempt to figure out what's going on at the back end of the Arsinoitherium skull. Skull outline after Andrews (1906), with some minor modification (including removal of the restored horn tips). That's an 'average' human occiput on the right, taken from Wikipedia (public domain). Does anyone else feel weirdly inadequate when looking at this image? I mean, I know it's not all about size and all, but still...

If my noodling on this is correct, then dorsomedically-anchoring muscles typically involved with elevating the neck (e.g. Semispinalis capitis, Trapezius) are now anchored partially on the skull dorsal surface, with the anteriormost located some distance forward of the occipital condyle. Such a configuration surely means that their contraction would not only elevate the neck (as expected) but also tip the head upwards to an unusual extent, and the increased distance between the occipital condyle and these muscles signifies a longer lever arm, and thus greater torque, on the head-neck joint. The lateral protuberances aren't quite in the right place for neck elevators however, and I initially wondered they were something to do with jaw musculature. Expanded temporalis muscles often create extended crests at the back of animal skulls but this is not the case in Arsinoitherium, where the temporal muscle housing clearly terminates well anterior to the occipital face. It seems more likely that these protuberances are something to do with laterally-placed skull-neck muscles - perhaps a set of considerably expanded obliquus capitis superior. These are muscles which run between the atlas (the first neck vertebra, a structure which is also huge in Arsinotherium) and the posterior skull to deliver fine control to head elevation and lateral rotation. But because the protuberances have migrated to somewhat overhang the atlas vertebra, the vertical action of these muscles was likely enhanced relative to other mammals. As with the muscles of the dished medial occipital region, this realignment of the oliquus capitis superior would likely see the head pitching up during contraction. Further large muscles are indicated by the broad mastoid and jugular processes, regions which anchor muscles that variably elevate, rotate and laterally flex the head and neck.

All being equal, it seems that the posterior Arsinoitherium skull wasn't just about supporting the head with a series of big, powerful muscles, but also specifically configured to enhance the extension of the craniocervical joint - in other words, to forcibly swinging the head upwards relative to the neck. Much of the rearrangement of the posterior skull seems to be geared towards this, both in terms of expanding muscle attachment area and also reorienting muscle vectors to better serve vertical head motion. This, of course, also fits well with observations that the occipital condyle is structured to facilitate more dorsoventral movement than lateral. I suspect we do not see an equivalent configuration in other mammal skulls because most heavy-headed mammals also possess expanded head-neck muscles anchored to withers (tall vertebral spines of the shoulder region). This configuration allows them to lift their heads and necks around using drawbridge-like actions whereas Arsinoitherium, which lacked significant withers (Andrews 1906), probably had to rely more on muscles localised around neck vertebrae to support and move its head.

Skull variation in Arsinoitherium, as figured in my previous blog post on Arsinoitherium. Note the changing posterior shape from the smallest to the largest skull.

What might all this mean for horn function? It goes without saying that enhanced adaptations for swinging a skull upwards could have a lot of functional implications when that skull is covered in horns. It seems reasonable to assume that Arsinoitherium could use this for a number of practical purposes, such as taking forceful swipes at predators or using its headgear to knock over trees and other vegetation to access certain food sources. However, it's noteworthy that the only the biggest (potentially male?) Arsinoitherium that have the most developed version of the complex occiput morphology outlined here (see image above), suggesting that enhanced head and horn motion was of principal use for big, mature animals concerned with territories, mates and other resource competition. Might this indicate that intraspecific combat, such as horn-locked wrestling matches with rival individuals, was an adaptive pressure here? I'm not aware of tests to see how well Arsinoitherium horns interlocked (as has been done for horned dinosaurs - see Farke 2004) but they certainly look like they'd slot between each other in a way that would allow for intraspecific wrestling, and powerful neck and head elevators would be useful to shove and unbalance opponents, deliver pointed jabs and parry incoming blows. If Arsinoitherium sheathed horns were as strong as those of modern mammals I suspect they could easily withstand the strain of such bouts, and their wide occipital condyle and cervical series would do well to resist the torsion incurred by wrestling activity. Of further significance is that Court (1993) noted that the limbs of Arsinoitherium were adapted for forceful retraction, a feature he assumed was useful for swimming. But a terrestrially wrestling Arsinoitherium would find that useful too, as powerful limb actions would push the body forward against a rival. I'm not sure these retractors would indicate running and charging behaviours however because, even with strong limb muscles, Arsinoitherium has stumpy distal bones ill-suited to rapid locomotion. In my mind, I'm visualising this hypothesis as four-horned sumo wrestling over jousting or fencing.

The idea of Arsinoitherium using its horns aggressively is not, of course, a radical or special insight - any three-year-old could make the same suggestion based on the observation that sharp, pointy bits of animals tend to be used in such ways. But I think it's neat that there might be an overlooked functional signature of this behaviour in the predicted tissues and structure of the horns as well as the morphology of the posterior skull, and suggest this might warrant further research. It seems to fit multiple aspects of Arsinoitherium functional morphology and chimes well with behaviour in large living herbivorous mammals: it's actually difficult to think of large mammals with substantial horns or tusks that don't use them for intraspecific fights. It'd be cool to see this investigated further, but that's beyond the scope of this article. For now, I'll leave you with my artistic take on what's clearly got to be called the Arsumoitherium Hypothesis - with a name like that, this idea has to be correct, right?

Two Arsinoitherium zitteli engaged in a wrestling bout - does antagonistic behaviour explain those powerful head extensors? Those neck humps aren't muscle, by the way, but rhino-like pads of neck tissue. Also, has anyone else rendered Arsinoitherium in this way? I can't find any other examples, but also refuse to believe that no-one has illustrated something similar since Arsinoitherium was described in 1903.

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References

• Andrews, C. W. (1906). A descriptive catalogue of the Tertiary Vertebrata of the Fayum. Publ. Brit. Mus. Nat. Hist. Land. XXXVII.
• Benoit, J., Merigeaud, S., & Tabuce, R. (2013). Homoplasy in the ear region of Tethytheria and the systematic position of Embrithopoda (Mammalia, Afrotheria). Geobios, 46(5), 357-370.
• Clementz, M. T., Holroyd, P. A., & Koch, P. L. (2008). Identifying aquatic habits of herbivorous mammals through stable isotope analysis. Palaios, 23(9), 574-585.
• Court, N. (1992). A unique form of dental bilophodonty and a functional interpretation of peculiarities in the masticatory system of Arsinoitherium (Mammalia, Embrithopoda). Historical Biology, 6(2), 91-111.
• Court, N. (1993). Morphology and functional anatomy of the postcranial skeleton in Arsinoitherium (Mammalia, Embrithopoda). Palaeontographica Abhandlungen A, 226, 125-169.
• Drake, A., Donahue, T. L. H., Stansloski, M., Fox, K., Wheatley, B. B., & Donahue, S. W. (2016). Horn and horn core trabecular bone of bighorn sheep rams absorbs impact energy and reduces brain cavity accelerations during high impact ramming of the skull. Acta Biomaterialia, 44, 41-50.
• Farke, A. A. (2004). Horn use in Triceratops (Dinosauria: Ceratopsidae): testing behavioral hypotheses using scale models. Palaeontologia Electronica, 7(1), 10p.
• Jacobs, B. F., Tabor, N., Feseha, M., Pan, A., Kappelman, J., Rasmussen, T., ... & Massini, J. L. G. (2005). Oligocene terrestrial strata of northwestern Ethiopia: a preliminary report on paleoenvironments and paleontology. Palaeontologia electronica [electronic resource]. Vol. 8, no. 1 (2005): 19 p.
• Kappelman, J., Rasmussen, D. T., Sanders, W. J., Feseha, M., Bown, T., Copeland, P., ... & Jacobs, B. (2003). Oligocene mammals from Ethiopia and faunal exchange between Afro-Arabia and Eurasia. Nature, 426(6966), 549-552.
• Osborn, H. F. (1907). Hunting the Ancestral Elephant in the Fayûm Desert: Discoveries of the Recent African Expeditions of the American Museum of Natural History. Century Company, October 1907, 815-835.
• Prothero, D. R., & Schoch, R. M. (2002). Horns, tusks, and flippers: the evolution of hoofed mammals. JHU Press.
• Rose, K. D. (2006). The beginning of the age of mammals. JHU Press.
• Sanders, W. J., Kappelman, J., & Rasmussen, D. T. (2004). New large-bodied mammals from the late Oligocene site of Chilga, Ethiopia. Acta Palaeontologica Polonica, 49(3), 365-392.
• Sanders, W.J., Rasmussen, D.T., & Kappelman, J. (2010). Embrithopoda. In: Werdelin, L., Sanders, W.J. (Eds.), Cenozoic mammals of Africa. The University of California Press, Berkeley, Los Angeles, London, pp. 115–122.

The horns of Arsinoitherium: covered in skin or augmented with keratin sheaths?

1.5 Arsinoitherium zitteli trotting about Eocene Egypt, looking a bit like they could be advertising farm products. But what's with those more elaborate than usual horns?

The horns of the giant, Egyptian, Oligocene afrotherian Arsinoitherium zitteli are probably a key factor in its status as one of the better known fossil mammals. Though perhaps not quite as popular as mammoths or sabre-toothed cats, this 3 m long, four-horned species has enough osteological charisma to warrant display in many museums as well as starring roles in books and films (including, cinema fans, narrowly missing out on an appearance in the 1933 King Kong). And unlike a fossil rhinocerotid (to which it is not at all related), Arsinoitherium doesn't need us to imagine the shape of its ornament in life: two enormous horns project over the end of the snout and another pair of smaller, sub-vertical horns grew above the eyes.

Recently, I painted a portrait of Arsinoitherium for an upcoming book project and, based on my understanding of epidermal osteological correlates, I threw a keratinous sheath over the entire horn set (below). This is not a typical reconstruction - Arsinoitherium has been reconstructed with 'regular' mammalian skin (perhaps better termed 'villose skin' - Hieronymus et al. 2009) on its horns for decades but, as we all know, popularity and longevity don't always equal 'credibility' when it comes to fossil animal reconstructions.

Arsinoitherium zitteli, sporting antelope-like horn sheaths.

Shortly after this image was shared online, Darren Naish, he of Tetrapod Zoology (and the upcoming TetZooCon meeting, which you should definitely attend if you're in the UK and reading this article), had a question: had I checked horns without keratinous sheaths, like deer antlers or giraffe ossicones? It turns out that these are the more typical artistic analogues for Arsinoitherium horns, and their reconstruction without a keratinous sheath reflects this interpretation. It wasn't a question I could easily answer because I'd zeroed in on a keratinous sheath quickly in my research for the image and, in a major palaeoart faux pas, hadn't given due consideration to other options. Simultaneously, neither of us could argue for any model of Arsinoitherium horn coverage confidently because no-one has looked into this in any detail. There are some ideas in the literature, but they are fleeting and conflicting (keratin sheaths - Anonymous 1903; Andrews 1906; Osborn 1907; or skin, Prothero and Schoch 2002; Rose 2006).

It's difficult to turn away a good palaeobiological mystery, and because I like to make sure my work is as credible as it can be, I followed this question up with more research. I reasoned that the structure, development and surface texture of the three major types of mammalian headgear - horns, ossicones and antlers - could be compared to Arsinoitherium horns to see which, if any, is the best match and indicator of life appearance. Looking into this has been very informative and might be of interest to fellow palaeoartists as well as those interested in cool fossil animals, so I thought I'd share my thoughts and process here. We'll start by looking at Arsinoitherium horns themselves, then move through modern potential analogues, and finally compare them at the end to see which model seems most apt.

Arsinoitherium horns: growth, structure and surface texture

PV M 8463, the most famous of all Arsinoitherium skulls, as illustrated in Andrews (1906). Note the dotted lines across the horns - they mark the end of the preserved skull and the start of reconstructed elements.

As noted above, Arsinoitherium has two pairs of horns: a larger anterior set, which grows out of the nasal bones and over the snout, and a smaller, second pair formed from the frontal bones, above the eyes. Both sets are highly conspicuous and dominate the skull, the weight of the anterior pair presumably accounting for the development of a bony bar between the nostrils in mature animals (Andrews 1906; Court 1992). Note that the Arsinoitherium horns we're used to seeing in museums are partly reconstructed and thus of limited use as reference material. Most exhibited skulls are based on NHMUK specimen PV M 8463 (above), a 'moderately sized' adult specimen (Osborn 1907) in which neither horn is complete (Anonymous 1903; Andrews 1906). This skull was among the earliest Arsinoitherium skulls collected from Egypt but was restored rapidly once it arrived back in London. A 1903 report describes how the skull was:

"...brought home by Dr. Andrews from Egypt, and after cleaning, strengthening, and the restoration of parts deficient on the left side by modelling from the right side, is now exhibited in the central hall of the Natural History Museum in Cromwell Road."

Anonymous, 1903, p. 530

The fact that some parts of the skull were in less than stellar shape is evident from this photo of PV M 8463 (from the NHM's data portal): note the variation in colour and texture, reflecting places of reconstruction against real bone. Thus, while the familiar Arsinoitherium museum skull is a useful reference for morphology, illustrations and descriptions in technical literature will be more informative for reconstructing their integument. I've based my assessment mostly on Charles Andrews (1906) monograph, as well as that of Court (1992).

Structure. Both horn pairs of Arsinoitherium are relatively simple in gross shape and maintain the same basic morphology throughout their lives (below), though the horns of mature animals are wider, taller and more pointed than those of juveniles. The figures presented in Andrews (1906) show an increase in anterior horn base length from 41.6% in the smallest specimen to over 56% in the largest. Both horn sets are hollow, with vast internal cavities being supported by sheets of trabecular bone. In some places the exterior bone walls are surprisingly thin, only 5-10 mm (Andrews 1906).

Arsinoitherium zitteli skull ontogeny. I wonder if the horns of the largest skull should be reconstructed as longer and taller, given their arcs in the completely known skulls and gentler tapering of other nasal horn specimens (e.g. Sanders et al. 2004). Skull drawn from Andrews (1906), skull measurements by me.

Surface texture. The base of the horns are marked by deep, broad and branching neurovascular channels running from the facial region onto the horns themselves. The horn shafts are rugose on account of many deep pits, grooves and branching channels aligned along their long axes (Andrews 1906; Sanders et al. 2004). The horn tips of young animals have an especially spongy texture at the tip, presumably reflecting growth of the horn core (Andrews 1906). These textures are not typical of the rest of the skull, which are of a more typical, smooth mammalian variety even in regions where skin was probably in close proximity to the bone (e.g. the zygomatic arch, over the braincase). This is an important distinction, implying that a different epidermal configuration - different skin types, in other words - was present on the horns compared to the rest of the skull.

Having learned something of the Arsinoitherium condition, let's take a look at how modern horns, antlers and ossicones compare...

Analogue 1. Bovid-style horns (keratinous sheaths over a bone core)

Bovid horns typify a widely used approach to cranial ornamentation and weaponry across Tetrapoda. They are perhaps the simplest approach to producing a sturdy cranial projection, being little more than a bony horn core covered in a hard keratinous sheath and are permanent feature in almost animals that bear them. The one exception is the pronghorn, which sheds its horn sheath annually (it also isn't a bovid). Biology, eh: can't we have one rule without an exception?

Bovid (bighorn sheep, Ovis canadensis) horn anatomy. From Drake et al. 2016.

Structure. Bovid-style horns are composed of a hollow bony core lined with trabeculae that strengthens an otherwise thin-walled structure (Drake et al. 2016). The bone portion only occupies the basal portion of the horn, anchoring ever-growing bands of keratin that grow from the bone-keratin interface, not at the horn tip (below). This means that the tip of the horn sheath is the oldest part of the structure and that the base of the sheath is the youngest. Because keratin sheaths are inert, dead and tough tissue, they cannot be remodelled once they are formed. This dictates that the growing bony core has to forever comply with the shape of the horn sheath and cannot change shape much over time. Size changes can be accommodated as wider and longer sheath layers can cover expanding horn cores, but it is not possible to form a more complex shape - say a branch or spur - at the tip of the horn. And before anyone mentions pronghorns: their horn branches are entirely soft-tissue: the bony core retains a simple shape.

Schematic bovid horn growth, adapted from an illustration in Goss (2012).

Surface texture. Deep, oblique foramina and branching neurovascular canals characterise the surface texture of bovid horn cores. This rugosity profile is most pronounced in younger animals, but is maintained to a lesser extent in adults - in many bovids, the horns never stop growing, they just slow down a great deal. This texture is not unique to horns but accompanies many structures with keratinous sheathing, including claws and beaks (e.g. Heironymus et al. 2009). A sharp lip and particularly deep rugosity can mark the transition from horn to facial skin.

Analogue 2. Giraffe ossicones (skin over ossified dermis)

Giraffes have awesome skulls with two - and often more - ossicones that are covered in the same skin as the rest of their faces (Davis 2011). Their approach to cranial ornamentation is restricted to the giraffe clade and their fossil relatives, but might be an apt model for aberrant extinct forms and is worth reviewing here. Clive Spinage (1968) provides an excellent overview of ossicone structure and development: the following is taken from his work.

Structure. Ossicones are low humps or columnar protuberances, continuous with the surrounding skull anatomy but formed from dermal ossifications, not outgrowths of skull bones. They eventually fuse with the skull in adult life but, unlike the underlying skull bones, ossicones are solid and very dense - they are described as being 'ivory-like' in compactness and hardness by Spinage (1968). Mature specimens show increasingly complex shapes including development of swollen tips on the frontoparietal 'horns', as well as hornlets and bosses across the major 'forehead' ossicone. Having an adaptable, living integument is essential to this process, as the ossicone covering needs to change shape to reflect the changing size and complexity of the underlying bone.

Giraffe skulls are full of sinuses, but they do not extend to their ossicones, which are extremely dense. From Spinage (1968).

Surface texture. Generally smooth with oblique foramina in juveniles and young adults, but increasingly gnarly in mature animals (more so in males). The continued ossification of dermal tissues produces a conspicuous pitting and 'flaky' rugosity profile that overgrows the surrounding skull bones and obscures the textures from earlier growth stages. In mature males, this rugosity can overgrow the entire upper surface of the skull and enhance the height and ornamentation of the ossicones considerably.

Young adult male giraffe skull by Wikimedia user Nikkimaria, CC BY-SA 3.0. Note the flaky, irregular textures of the ossicones and their complex shape: they are much more intricate and developed than those of less mature animals. There's room for more irregularity and texture on this skull, too: the skulls of old males look like they have cathedral spires growing from their faces.

Analogue 3. Deer antlers (bony projections atop cranial pedicles)

The familiarity of deer antlers allows us to forget what remarkable and unusual structures they are. Present almost universally in male deer (and in female reindeer), these elaborate, sometimes enormous structures are cast and regrown each year using a regenerative process that is the source of much anatomical and medical interest - no other mammal can regenerate such a complex appendage in this way, and the speed of the regeneration process is remarkable. Antlers are so unusual that they are only partly useful to our discussion here: we are primarily interested in antlers when they are covered in their velvet (specialised antler skin), as this is most comparable to the likely Arsinoitherium condition. Antler skin itself is interesting as, although it is continuous with the skin of the underlying pedicle, it lacks sweat glands and arrector pili (the tiny muscles that pull hair up or give us goosebumps) (Li and Suttie 2000). The antler pedicle (the permanent bony base) in contrast, is covered in the same type of skin as elsewhere on the body (Li and Suttie 2000).

A happy-looking moose (Alces alces) with his fuzzy antlers. Note the visible blood vessels on the underside of each palm. Photo by AlbertHerring, in public domain.

Structure. Both antlers and pedicles are solid, and antlers can - by virtue of growing at their tips - become more complex as they grow, developing from single spurs into networks of brows, tines and palms. As with giraffes, antler skin needs to be living and adaptable to facilitate this: a covering of inert keratin would preclude this form of growth.

More Alces antlers, this time without velvet. Note the long, branching channels. By Wikimedia user Nkansahrexford, CC-BY-4.0.

Surface texture. Antlers have variably developed rugosities consisting of conspicuous, long and branching channels impressed into smooth bone or around prominences and tubercles. These grooves are the impressions of blood and nervous networks that facilitated rapid antler growth. These textures are easily discerned even from a distance, and thus contrast with the texture of the pedicles, which are smoother and lined with relatively shallow, narrow and long impressions of vascular networks. It is unusual for hairy skin to leave such a significant osteological scar on underlying bone: typically, this form of epidermis leaves little to no remnant on skull bones (Hieronymus et al. 2009).

Arsinoitherium vs the analogues

Having looked at three major types of cranial projection in living animals, which - if any - is the best match the condition in Arsinoitherium? Giraffe ossicones are incomparable to Arsinoitherium horns in several aspects, perhaps the most significant being their increasing complexity and development of flaking bone textures in later life. Furthermore, the development of giraffe ossicones from bony growths in dermal tissues suggests a fundamentally different relationship between skull and dermis than of Arsinoitherium, where the bony horn component represents elongated skull bones. There's enough differences here to question whether giraffe ossicones are a good model for the life appearance of Arsinoitherium horns.

In being formed of polished, deeply vascularised bone, deer antlers are closer approximations of Arsinoitherium horns. However, there is so much weirdness associated with deer antler formation and tissues that they almost remove themselves from meaningful comparison to permanent skull horn cores. The fact that antler velvet, as hairy skin, is (to my knowledge) unique in leaving deep vascular channel impressions is a major issue here, implying that either antler bone is unusually susceptible to neurovascular imprinting (do they grow so fast that they grow around their blood vessels?) or that velvet is better at altering bone textures than other skin types. Both scenarios point to antlers having some endemic oddness about them, which complicates their use as a model for life appearance of non-antlered species.

All is not lost with the cervid data, however: antler pedicles are comparable to Arsinoitherium horns in being permanent outgrowths of bone, and they also have neurovascular impressions. However, these shallow grooves compare poorly to the deeper channels and pitting of Arsinoitherium horns. Indeed, there is little about antler pedicle texture to distinguish them from the surrounding skull bones, whereas the opposite is true for Arsinoitherium.

Our comparisons improve with the bovid horn condition, which seems to chime with the Arsinoitherium skull in many regards. Both are hollow outgrowths of skull bones supported by internal trabeculae; both have bone textures characterised by deep, bifurcating neurovascular channels as well as conspicuous longitudinal grooves and oblique foramina; and both maintain the same basic shape throughout growth - excepting some basic changes in base width and horn length. Further similarities include the development of particularly deep rugosties at the base of the horn cores, which is evident in at least large Arsinoitherium skulls (Andrews 1906). This interpretation is consistent with one of the longer (but still rather short, if we're honest) interpretations of the blood vessel impressions in Arsinoitherium:

"These channels evidently lodged blood-vessels which served for the conveyance of blood to or from the covering of the horn, and judging from the marked way in which both these vessels and those on the anterior face of the horns impress the bone, it seems probable that the covering was hard and of much, the same nature as that clothing the horn-cores of the cavicorn ruminants."

C. Andrews (1906), p. 7

So...

Of the three models looked at here, it seems the basic structure and textural package of bovid-like horns best matches what we see in Arsinoitherium. Moreover, unlike the antler or ossicone models, there's no obvious mismatches with this configuration: pretty much everything we would correlate to a bovid-like horn anatomy seems present on or in the Arsinoitherium skull. The idea that a keratinous sheath might have existed in Arsinoitherium might seem odd, but it is not that outlandish given the apparent ease through which keratinous sheaths evolve. This is, after all, the tissue which has covered just about every claw, hoof, nail, horn, cranial dome and beak that has ever existed, whereas ossicones and antlers seem like specialised, clade-restricted approaches to cranial projections. The functionality of hollow Arsinoitherium horns is further reason to suspect a horn sheath. Studies of bovid horns suggest hollow cores and keratin sheaths compliment each other biomechanically, optimising the horns for for impact dissipation (Drake et al. 2016 and references therein). Stripped of a keratinous sheath, we find that hollow horn cores are great at transmitting energy but are brittle and prone to buckling and fracturing under heavy loading. It's only with a tough, fracture resistant keratin sheath that these structures can avoid breaking under heavy use so, if Arsinoitherium employed its horns for anything vaguely physically demanding, they probably needed a keratinous sheath.

It's possible, of course, that these structures were just for show, but they do look like they had a function beyond display. It occurs to me as I write this that this scene recalls the painting from Ghostbusters II. I guess we'll call this guy 'Vigo'. 

Putting all this together, I feel the case for a keratinous sheath over the Arsinoitherium horn sets is reasonable, at least so far as it can be made with publicly available data. Aspects of morphology, growth, surface texture and - perhaps - functionality seem fully consistent with a bovid-like horn configuration, whereas other potential models are less comparable. From an artistic perspective, this is exciting: horn sheaths can be extremely elaborate structures and exaggerate the size of the horn core considerably, so Arsinoitherium might have been far more extravagant in life than we have previously imagined. I've tried to hint at this with my reconstructions - remember, this animal wasn't just a funny-faced rhinoceros!

But - before we go crazy with this - do remember that the core of this analysis - the interpretation of Arsinoitherium headgear - is entirely literature based. I've not seen original specimens nor even modern, high-res imagery of an unreconstructed skull (this wasn't for lack of trying - the literature on these animals needs updating). Thus, while I've tried to be as thorough as I can with my observations, and as cautious as I can with my interpretations, I might be ignorant of some important detail. Take everything here with an appropriate pinch of salt, and please chime in below if you can provide superior insight. There's clearly scope for a more detailed study on this topic and, given how unique the horns of Arsinoitherium are, there might be some interesting functional findings to emerge from further investigation.

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References

• Andrews, C. W. (1906). A descriptive catalogue of the Tertiary Vertebrata of the Fayum. Publ. Brit. Mus. Nat. Hist. Land. XXXVII.
• Anonymous. (1903). A New Egyptian Mammal (Arsinoitherium) from the Fayûm. (1903). Geological Magazine, 10(12), 529-532.
• Court, N. (1992). The skull of Arsinoitherium (Mammalia, Embrithopoda) and the higher order interrelationships of ungulates. Palaeovertebrata, 22(1), 1-43.
• Davis, E. B., Brakora, K. A., & Lee, A. H. (2011). Evolution of ruminant headgear: a review. Proceedings of the Royal Society of London B: Biological Sciences, 278(1720), 2857-2865.
• Drake, A., Donahue, T. L. H., Stansloski, M., Fox, K., Wheatley, B. B., & Donahue, S. W. (2016). Horn and horn core trabecular bone of bighorn sheep rams absorbs impact energy and reduces brain cavity accelerations during high impact ramming of the skull. Acta biomaterialia, 44, 41-50.
• Goss, R. J. (2012). Deer antlers: regeneration, function and evolution. Academic Press.
• Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
• Li, C., & Suttie, J. M. (2000). Histological studies of pedicle skin formation and its transformation to antler velvet in red deer (Cervus elaphus). The Anatomical Record, 260(1), 62-71.
• Osborn, H. F. (1907). Hunting the Ancestral Elephant in the Fayûm Desert: Discoveries of the Recent African Expeditions of the American Museum of Natural History. Century Company.
• Prothero, D. R., & Schoch, R. M. (2002). Horns, tusks, and flippers: the evolution of hoofed mammals. JHU press.
• Rose, K. D. (2006). The beginning of the age of mammals. JHU Press.
• Sanders, W. J., Kappelman, J., & Rasmussen, D. T. (2004). New large-bodied mammals from the late Oligocene site of Chilga, Ethiopia. Acta Palaeontologica Polonica, 49(3), 365-392.
• Spinage, C. A. (1968). Horns and other bony structures of the skull of the giraffe, and their functional significance. African Journal of Ecology, 6(1), 53-61.