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Friday, 17 November 2017

Can we predict the horn shapes of fossil animals? A thought experiment starring Triceratops

Triceratops horridus with some crazy long and curving brow horns. Just speculation, right? Surprisingly, maybe not...
For palaeoartists, animals with flamboyant headgear are among the most rewarding to render, but it's not only the bony aspects of their cranial ornaments that we have to pay attention too. Animal headgear is covered with various amounts of soft-tissue that, in extreme cases, can dramatically augment the shape of the underlying bony features. The headgear of living species has a spectrum of soft-tissue coverings from nothing at all (mature deer antlers), to relatively thin dermal tissues (giraffe ossicones), through to hard keratin sheaths that can add significant depth and length to a horn or crest (most other animal horns). This excellent breakdown of a bighorn sheep face by Aaron Drake of Colorado State University (uploaded by Simpleware Software Solutions) gives a pretty good idea of how much tissue extreme keratin sheaths can add to the underlying skull.

Not all horns are augmented to the extent seen in bighorn sheep, but even modestly proportioned keratin sheaths can add a lot of bulk, length and characteristic geometry to horn tissues. Thus, anyone hoping to accurately predict the appearance of ancient horned animals should want to predict the shape of their horn sheaths along with understanding the skull geometry. This isn't easy because, though incredibly tough and resistant, keratin sheaths are still prone to decay and rarely fossilise.

Researching horn growth for an upcoming book project has made me wonder if horn sheath shape might be more predictable than we've traditionally thought, however. Horn sheath growth mechanics are relatively simple, closely related to bone shape, and constrained by the properties of heavily keratinised tissues. They're also fairly universal across across tetrapods - the same processes that make a goat horn will make the enormous keratin sheath of a skimmer jaw, for instance. These properties might allow insights into sheath shape in fossil species even when the sheath is not preserved. So what aspects of horn sheath growth might allow this, and how could we transfer them to fossil animals?

Growing horn sheaths in living animals

Keratin sheaths are dead tissue with their only living components being the cells that synthesise the keratin at the horn core/sheath interface (e.g. at the inner surface of the horn soft-tissues, see diagram, below). Because no living tissue reaches the outer horn surface, they cannot grow by adding tissue to the tip. Rather, they grow by internal accumulation of keratin layers, each new deposit displacing the older sheath from the bony core. This creates a stack of keratin cones, with new cones growing at the base and causing the horn tissues to lengthen. Continuous internal deposition and displacement of old material is what creates the soft-tissue horn extension, as each new keratin layer shoves the older material a little further from the bony tip. This makes the tip of a keratin horn the oldest part of the sheath, and in many bovids the tips are many years old. Conversely, the youngest part of the horn tissues are located at the base. As we discussed in a recent post about the horns of Arsinoitherium, this growth mechanism binds the internal horn tissues in the overlying sheaths, limiting their ability to change size or shape. Changes in size or curvature can only be achieved by displacing the older horn layers, but complicating the horn shape - say, by branching the tip - is impossible unless the sheath is shed, pronghorn-style. The sheath itself can't be modified after deposition either, on account of no living tissue reaching it. Thus, old sheaths permanently maintain the size and geometry they were created with.

Stylised bovid horn growth, heavily modified from Goss (2012).
This growth mechanic presents three important points relevant to predicting the shape of fossil horn sheaths. The first is that sheath tissues are synthesised directly over the horn core, effectively making the internal sheath margin a cast of the bone at the time it grew. The second is that the shape of new keratin layers are constrained by the keratin sheaths that preceded them. They can't deviate too radically from the overlying horn shape and the horn core of the emerging layer should mostly nestle into margins of the older one. The third is that horn extensions are not simply exaggerations of their contemporary horn core, but a keratinous record of the horn history. Geometry exhibited by the earliest growth stages is maintained in the extending sheath regardless of later changes to the horn core morphology, and only periodic shedding or heavy abrasion are likely to alter this.

This being the case, could ontogenetic changes in horn cores provide insight into the sheath shape of fossil animals? If bone shape translates to keratin sheath shape, and sheath shape dictates the horn extension profile, then a growth series of bony horn anatomy may allow us to reconstruct horn keratin accumulations that are otherwise lost to decay. Horn core profiles give us a 'cast' of the inner sheath margin for that growth stage, and we can fit these into the margin of the preceding sheath layer (which, of course, can be deduced by the shape of a ontogenetically preceding horn core). Building a stack of nestled horn core profiles creates something akin the bovid horn diagram above and tells us something of how keratin layers were accumulated for that horn shape. The very tip of the horn sheath is lost to time because we cannot predict external appearance from horn core casts (they only represent the internal structure) but if the youngest animal in a growth series is suitably juvenile, we probably aren't missing much.

As proof of concept, I've taken the horncore outlines from the schematic bovid horn above and attempted to recreate the horn shape. Stacking them was achieved by simply eyeballing the margins, trying to fit the horn core outlines together as tightly as possible without their margins overlapping. Here's how it turned out...

I don't think that's too bad. It's not perfect, but it gives a pretty good idea what's going on with the actual horn. This method is very simple, but - as outlined earlier - keratin horns are simple, so we might not need a particularly complex method to predict their shape. But you're not here to talk about ram horns: what happens when we apply this idea to a fossil animal with a well-known growth series, and how do the results compare to our conventional means of reconstructing horn sheaths in fossil taxa?

Step forward, Triceratops

Triceratops growth series from Horner and Goodwin (2006). Both species of Triceratops are included here, but the generalities of this growth sequence are thought to apply to both. Say, that brow horn curvature looks pretty changeable - what would that mean for horn sheath shape?

The super-famous horned dinosaur Triceratops is a great animal to explore this idea with. It's known from dozens of specimens representing a range of ontogenetic stages, from small juveniles to giant adults (above, Horner and Goodwin 2006 - and no, the adults in question here are not Torosaurus). Like the horns of other ceratopsids, Triceratops brow horns have well-developed epidermal correlates for keratinous sheaths (oblique foramina and anastomosing neurovascular channels - Horner and Marshall 2002; Hieronymus et al. 2009) and these textures are present in the smallest known skulls, indicating that most or all their life was spent with sheathed brow horns (Goodwin et al. 2006). Confirmation of a horn sheath comes from poorly-preserved soft-tissues found on some Triceratops horns (Farke 2004; Happ 2010).

Triceratops skulls underwent pretty major changes as they grew, including complete reorientation and allometric scaling of the brow horns. In juveniles these curve backwards, but in big adults they arc forwards (Horner and Goodwin 2006). Typically, artists have assumed that the keratin sheaths covering these horns changed shape with them. Even pros, such as Greg Paul (2016), who have stressed that the keratin sheath should extend the horn shape, render the sheaths as more-or-less reflecting the underlying horn core of a given growth stage, without any hangovers from a previous iteration of horn shape. Whether intentional or not, the implication here is that the horn sheath was dynamic - capable of changing as the animal grew.

....just like this. Note how the brow horns of this Triceratops group are clearly changing shape as the animals increase in size, but that the keratin sheaths don't reflect any earlier horn history. Hmm. Say, do you know this image is on the front of my 2018 calendar?
The model outlined above conflicts with this traditional take, however. If we assume that the horn extension was composed of a series of retained keratin sheaths, and using Horner and Goodwin's (2006) ontogenetic sequence as a basis, the resultant horn shape is pretty surprising. Stacking horn cores in the juveniles sees those recurved shapes pushed off the horn core to extend and extenuate the curve strongly, to the point where the horn tip even points posteriorly at one stage (below). As the horn base tips forward on the approach to adulthood, these arcing tips rotate with them, creating a long, elaborate set of horns which curved twice: once at the tip, and again, but inversely, at the base. If the Triceratops in this model retained the full history of their horn sheaths into adulthood, the result would be pretty fantastic: very long horns where the tips pointed 90° away from the point of the horn core. Yowsers - that's quite different from our traditional 'just make it pointier' approach.

Stacking Triceratops horn cores, mimicking how living animal keratin sheaths grow, suggest the keratinous extension of the brow horns was strongly curved even in adult animals. As in the mock bovid horn above, the horn cores were stacked simply by trying to make them fit as neatly as possible.
Which is more likely: twirly horn sheaths or the more conventional, 'dynamic' sheaths? Where morphing horn sheaths immediately lose points is their requirement for the inert keratin horn tissues to react to each horn core shape, as well as for the horn sheath history to continuously disappear. Modern horn sheaths just don't grow like this: their extensions only exist because the old keratin tissues hang around, and we have to ask how the extending sheaths are created in our 'dynamic' sheath model. There are perhaps two ways we could attain morphing sheaths: the first is through continuous eradication of old sheath material, allowing new keratin to grow over the horn core without being obscured by previous sheath layers. This might have been achieved by Triceratops shedding and regrowing sheath extensions, or by abrading outer sheath tissues away. The second is that the horns weren't covered in one sheath but several interlocking plates, like the beaks of some birds, which might allow for jimmying and reconfiguration of the horn tissues through growth without adding lots of material to the end.

Let's consider shedding first. It's possible that at least some layers of Triceratops horns were shed because exfoliation is common on keratin sheaths in living species. For instance, puffins shed the outer layer of their beaks annually, and bovids exfoliate outer layers of their horns once or twice in their lifetimes (O'Gara and Matson 1975; Goss 2012). The fact that only a superficial layer of tissue is lost prevents the sheath being significantly altered however: exfoliation alone would probably not give us particularly 'dynamic' horn sheaths.

Constant reshaping of horn tissues might be plausible if Triceratops could regularly shed and regrow the horn sheath, as performed by pronghorns. Unfortunately, these mammals show us that detecting this growth mechanic in fossil species is challenging, however. Despite their unusual habit of regrowing an entirely new sheath each year, pronghorn horn cores have similar textures to those of animals with permanent sheathing (Janis et al. 1998). There are some differences, but they're subtle. O'Gara (1990) reported that pronghorn horn cores have annually variable properties, alternating between a spongy, relatively rounded horn core when the sheath is growing, and a smooth-textured, sharper horn core at peak sheath hardness (O'Gara 1990). It's pretty well established that dinosaur skeletons grade from spongy, rounded bones to smoother, sharpened bones as they aged, so perhaps variation in texture and shape of Triceratops horns that broke this pattern could indicate horn shedding - provided these differences could be distinguished from ontogenetic or intraspecific factors. I'm not aware of any evidence of this kind, despite the frequency in which Triceratops skull bone texture is commented, but I also don't know that anyone has specifically looked for this variation yet.

Lovely, lovely epidermal correlates on the skull of Triceratops prorsus illustrated in Hatcher (1907). Note that there's no divide between the correlates on the brow horn and surrounding skull - might we expect some sort of dividing sulcus if the horn sheath was routinely cast? From Wikimedia, uploaded by Biodiversity Heritage Library, CC BY 2.0.
A more illuminating insight may be that the correlates for Triceratops horn keratin are continuous with the epidermal correlates of the face (above). Horner and Marshall (2002) noted that the horn correlates for keratin sheathing extend over virtually the entire face - including the back of the frill (this is why so many Triceratops reconstructions have smooth 'face shields' nowadays). However, what's not seen on Triceratops horns is a boundary dividing the face sheath and a hypothetical temporary horn sheath, as might be expected where two keratinous sheaths meet (I'm assuming that the entire face shield wasn't shed annually either (palaeoartists: exfoliating/shedding Triceratops face - go!) - that's not a discussion I want to get into here).

A last, more arm-wavy point against horn shedding is that it is not at all common among living animals, possibly not even being present in some close pronghorn relatives (Janis et al. 1998). If Triceratops did shed its horns, it would be part of a club with very few members. This isn't a particularly scientific argument, but we have to concede that permanent horn sheaths are - by some way - far more common than ephemeral ones, and probably the 'default' condition for horned animals. Maybe we should assume permanence until there's good reason to think otherwise?

Could wear and abrasion create our morphing, dynamic horn sheaths in Triceratops? It's certainly true that keratin horns can be worn down, sometimes considerably. Bighorn sheep, for instance, can wear away years of horn growth in a behaviour known as 'brooming', but the results do not look like our palaeoart - in other words, they don't look like these sheep stuck their horns in a pencil sharpener. Nor do they echo the shape of the underlying skeleton. Instead, the ends are blunt, frayed and fractured (below). Any Triceratops that removed horn keratin through abrasion would presumably adopt a similarly 'sawn-off' appearance, and lack neat, pointed tips.

File:Desert Bighorn Sheep (8981484583).jpg
The broomed horns of a bighorn sheep (Ovis canadensis) - notice that they're heavily and deliberately worn at the tips, but they aren't shaped into fine points. From Wikimedia, uploaded by Lake Mead NRA Public Affairs, CC BY-SA 2.0.
Might a compound horn sheath be a route to horn sheath dynamism for Triceratops? Some readers may recall that we discussed compound keratin sheath covers last month and that they typically have deep grooves between abutting sheets. We don't see grooves of this nature on Triceratops skulls despite the very obvious rugosity profile created by the epidermal tissues, so I think we have to reject this hypothesis outright. The coverage of Triceratops horn core epidermal rugosities are pretty near identical to what we see on the horns of animals like cattle or goats, and I think we have to assume they indicate a similar, all-encompassing sheath morphology.

If Triceratops horns couldn't be renewed or take advantage of a more complex sheath arrangement, the likelihood of dynamic Triceratops horn sheaths is probably low. But does this idea of continuous sheath growth and twirly horns fare better under scrutiny? It seems to pass some basic tests, at least. The Triceratops brow horn outlines fit together pretty well with only a little displacement of the preceding horn layer, which is just what in see in modern horn growth, and the fact that their horn profiles don't change suddenly is consistent with them being perpetually constrained by layers of hard tissue. The predicted Triceratops sheath profile it is unexpected, but not beyond anything we see in living animals. And it scores points generally for being a simple model that is grounded in a well-understood aspect of living animal biology, in not needing to explain the loss of sheath tissue, and for factoring data we know is relevant to horn growth in living animals. I'm not saying this model is correct, but I am thinking that explains and fits our available data better than the dynamic sheath concept.

Of course, there are still lots of caveats. Remember that the model here is rough, being based on a generic Triceratops dataset and not the growth regime of a single species. The growth series outlined by Horner and Goodwin (2006) is a good general illustration of Triceratops growth, but results might vary if we restricted the data to a single species. My illustrations do not assume any exfoliation or tip abrasion, and we still don't have any idea what the external sheath morphology - including the presence of absence of ridges, spirals and bosses - might have been like. My attempt to stack the horn core profiles has also assumed minimal sheath thickness. If the sheath was thicker, the arcs of the horn could be stretched out over longer distances. So if you're buying this concept, remember that the horn shape proposed is only a general one - it's more in keeping with our understanding of sheath grow in modern animals, but it's still quite sketchy.


Perhaps the take-home message here is not, however, that Triceratops might have had loopy horns, but that there might be more to consider about fossil horn sheaths than we've assumed. Our discussion of dynamic horn sheaths does not just apply to Triceratops: artists take this approach with most horns and spikes in palaeoart, and it's clearly at odds with how most animals grow keratin sheaths today. But maybe this isn't just a topic for artists to ponder. There's potentially scope for a real study here and, seeing as fossil horn shape has a lot of functional significance, predicting sheath morphology would be a useful aid to predicting ancient behaviour. This needn't be restricted to horned dinosaurs, or even just horns, either: keratin sheaths on plates, spikes and so on grow in a similar way, and there's not reason this technique couldn't be used on other body parts, if validated. Moving this from food-for-thought-blog post to genuine science would require testing on modern species, perhaps through reconstructing living animal horns, to see how well it holds up. Recreating a schematic, 2D goat horn sheath using this method is fine, but real-world tests - especially using 3D horn casts, not just 2D drawings - might be more challenging. In the meantime, I'm curious to know what others think of all this - the comment field is open below...
"Hello, I'm Triceratops. I'll be your odd-looking concluding dinosaur reconstruction for this evening."

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