The Late Triassic dicynodont Placerias is probably one of the most famous dicynodonts, thanks largely to its prominent role in the first episode of the BBC television series Walking with Dinosaurs. In its onscreen debut, it was cast as an "endangered species", the last of its kind in the process of being usurped by the new, more advanced archosaurs.
Their final appearance quite literally had them walking out of the show in a desperate search for water, representing the last of the archaic "mammal-like reptiles" exiting the evolutionary stage while the plucky dinosaurs and mammal ancestors manage pulled through. The Placerias remain conspicuously absent when conditions improve again, and it's no coincidence that at the very end of the episode a herd of 'prosauropod' dinosaurs appear in their place, ushering in the age of the dinosaurs while the Placerias remain gone for good.
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| Placerias as it appeared in the 1999 BBC television series Walking with Dinosaurs, swaggering across the landscape. |
We now know that these ideas about dicynodont decline are flawed, if not wrong. At the time, Placerias was one of the few Late Triassic dicynodonts known at all, the only one outside of South America, and was among the youngest. Their low abundance and geographic restriction fed into a notion that the last dicynodonts were evolutionary relicts, the last gasp of a clade that was already well on its way out.
We now know that Late Triassic dicynodonts, the stahleckeriids, were actually still fairly widespread (even if there was usually only one species in any given ecosystem at a time), ranging across Pangaea from Europe, through North and South America, Morocco and all the way to South Africa! They were also still diversifying, with two subfamilies showing continued dispersal right up until their extinction. To say nothing of the anatomical innovations going on with Lisowicia. But this is supposed to be a post about Placerias, not the state of Late Triassic dicynodonts, so I'll stop here.
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| A Placerias on display at the Rainbow Forest Museum in Petrified Forest National Park from 2009. Note the characteristic large "tusks". Photo by the NPS (CC BY-SA 2.0). |
But this isn't the only thing Walking with Dinosaurs got wrong about
Placerias. In the show, it is prominently depicted with a pair of large, forward-pointing tusks. This is a pretty persistent image for
Placerias, and while Walking with Dinosaurs certainly wasn't the first to depict it like this, I do have to blame it for cementing this as the popular image for the animal (as well starting a trend of making it green, it's not just me seeing that right?).
In fact this is incorrect, the "tusks" on
Placerias aren't teeth at all, but actually long horn-like extensions of bone growing off of the caniniform process, i.e. they're part of its
skull. Savvy palaeoartists have gotten this right, covering the 'horns' (for a lack of better word) with, well, horn, much like the beak, as is suggested by their roughened bone texture.
Placerias does still have true
bona fide tusks, although they're nearly reduced to nubs and are completely hidden from view on the outside by the caniniform process itself. They're also only variably present, with some specimens lacking them altogether.
Regardless, the "bone-tusks" of Placerias are one of its defining characteristics, and every skeletal mount, television and book appearance depict it with them (including those that wrongly show the 'horns' as true tusks). Suffice to say it's easy to get the impression from pretty much every depiction ever that all Placerias had these 'horns'. But, according to what's actually known in the fossil record, this is not wholly representative of Placerias as it was in life.
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| Placerias looking very flabbergasted by this revelation, while the phytosaur Redondasaurus gasps in astonishment. Photo by Lee Ruk (CC BY-SA 2.0). |
Placerias was originally only known by, and was named from, a crushed humerus described in 1904. However, its big break came with the discovery of the aptly named Placerias Quarry in the Petrified National Forest of Arizona. The quarry is located in the well known Chinle Formation, which is spread over several southwestern states and spans many millions of years (it's a whole lotta sediment, in other words). The quarry contains over 1600 bones from at least 41 individual Placerias, and was meticulously described in 1965 by Charles L. Camp and Samuel P. Welles.
In their description, they noticed something unusual about the various maxillae (the upper jaw bone that housed the tusks) they were collecting. Of the maxillae they looked at, only around half of them bore the characteristic 'horn' we all know and love. The other half, however, either didn't have this 'horn' or only had a shorter, blunter 'horn' that was noticeably reduced by comparison.
This variation doesn't appear to correlate with size, so it's unlikely that this was due to ontogeny. Individual variation could be to blame, as occurs with the true tusks of some other dicynodonts, but the split appears so clear and evenly distributed that that doesn't seem to be the case. They concluded that this variation was perhaps sexual dimorphism, suggesting that the 'horn'-less maxillae may have belonged to females, while the males sported longer 'horns'. This suggestion has generally been upheld by later researchers as a reasonable explanation for the dimorphism.
Of course, there's no way to be sure what sex they each represent, if that's what it represents at all. Such suggestions can only be based on the assumption that male tetrapods are more likely to be the ones sporting exaggerated anatomical characteristics from sexual selection. Photos of these suggested "male" and "female" morphs by Christian Kammerer are shown below, also handily showing the real tusk that's usually hidden from view (below the thumb in the first image):
Regardless, the short-'horned' morphs exist and were seemingly present in half of the population. So why don't we see these "female" morph depicted more often, or rather, ever? To be clear, I'm not putting any blame on artists. This particular fact about Placerias biology isn't especially well known outside of specialists (or very heavily invested enthusiasts...ahem), and there's next to nothing available for artists to reference.
Indeed, the relevant literature on Placerias is difficult to access, including both Camp and Welles' monograph on the Placerias Quarry specimens and Timothy Rowe's 1979 publication on Placerias that went on to highlight this dimorphism and speculate on the function of the 'horns' (most of what I know was said in these publications comes from what has been repeated in later papers). It's not commonly reported outside of specialist literature either, so it's hardly a fact I'd expect people to have heard of in casual research.
It's also to be expected when palaeoartists have really only got one Placerias reference to go on, and that's the physical skeletal reconstructions. Because Placerias is mostly known from a bonebed, any complete skeletal reconstruction is a composite based on multiple individuals, including the skull. Most Placerias skulls are based on the composite reconstruction made by Camp and Welles in 1956—and later modified by Cox in 1965—which naturally was given the 'horns', as you would when you have an animal sporting a weird dimorphic feature.
As a brief aside, Camp and Welles'/Cox's composite reconstruction was vindicated by a nearly complete and articulated skull described in 2002. This skull was originally interpreted upside down, but when flipped the right way around looks strikingly similar to the composite reconstruction! Goes to show the veracity of Camp, Welles and Cox's work, so fortunately it seems unlikely that the rest of their composite reconstruction would be far off. Alas, the near complete skull unfortunately isn't as well preserved around the snout, so it's not exactly clear whether this was a "male" or "female" morph.
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As an artist, this speaks to me of the woes of flipping your canvas.
The original interpretation shown above, the flipped version compared with the reconstructed skull below. From Kammerer et al. (2013). |
With all this in mind, it's not really surprising this isn't more commonly depicted in palaeoart. Although I've got to say, I struggled to find any palaeoart that actually does at all. I carefully re-watched the first episode of WWD to see if I missed any variation in the models—nothing, and scouring the web for palaeoart and any other depictions turned up similar results.
The one exception might be the work of the inimitable Douglas Henderson, who has illustrated two scenes—cropped versions shown below—depicting Placerias that look as if they might include "female" morphs with short 'horns'. I'm not entirely sure in honesty, it could be down to angle, and most are tucked into the background, but it does look like some of them have been depicted with genuinely shorter 'horns' than the others to me.
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| Crops of Doug Henderson's works featuring what look like both long and short-'horned' Placerias. Do these individuals deliberately reference the "female" morph? With apologies to Douglas Henderson for shamelessly reposting (part of) his art. |
In any case, I wanted to contribute to the cause and drew up my own quick sketch the a few weeks back when #TriassicWeek was still going strong on Twitter. After a few kind pointers from Christian Kammerer, along with the photos posted above, I was able to make a few nips and tucks to land with the finalised sketch below:
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| Sketch of a "female" morph Placerias, front and centre. |
I originally sketched it with the little nubbin' tusks sticking out of the bottom of the caniniform process, although as explained to me by Kammerer it's evident that the tusks would still be hidden by the process in these individuals and so I covered them up.
This was intended to be a short post, so I'll cap it here with a message to all the palaeoartists out there who are ever considering reconstructing Placerias: draw the "female" morphs! This goes especially if you're illustrating a herd, but even for a solo picture I say mix things up a bit and go for the "female" morph. And if you're not thinking about drawing Placerias, do it anyway. The world could always use more Placerias art.
References
Bandyopadhyay, S. (1988). A Kannemeyeriid Dicynodont from the Middle Triassic Yerrapalli Formation. Philosophical Transactions of the Royal Society B: Biological Sciences, 320 (1198), 185–233.
Green, J. L. (2011). Bone and Dental Histology of Late Triassic Dicynodonts from North America. In: Chinsamy-Turan, A. (ed.), Forerunners of Mammals: Radiation Histology Biology. Bloomington: Indiana University Press, 178–196.
Sort of referenced but not really:
Camp, C. L., & Welles, S. P. (1956). Triassic dicynodont reptiles. Part I. The North American genus Placerias. Memoirs of the University of California, 13, 255–304.
Rowe, T. (1979). Placerias: an unusual reptile from the Chinle Formation. Plateau, 51 (4), 30-32.
