Buried Treasure – Jordan Mallon

What is my least appreciated paper? That’s an easy one:

Mallon, J. C., and Evans, D. C. 2014. Taphonomy and habitat preference of North American pachycephalosaurids (Dinosauria, Ornithischia). Lethaia 47:567–578.

This is a paper that I co-wrote with my good friend and colleague, Dave Evans. It’s been published for nearly three years now, and has garnered only two citations—both of which are from Dave and me! Despite this, the paper effectively debunks a widely held meme that North American pachycephalosaurs were mountain dwellers, à la big-horned sheep. This is an idea that gets a lot of play in both the popular media and textbooks (including some that have come out even after our paper was published).

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See what I mean? The belief that North American pachycephalosaurs lived in the mountains is all over the place!

 The idea for the project came to mind shortly after I started my postdoc at the Canadian Museum of Nature in 2013. I was reading some papers by my predecessor here, Charlie Sternberg, and repeatedly came across this notion of his that North American pachycephalosaur skull domes tend to be well worn, “as if they had been rolled down a stream” (C. M. Sternberg. 1970. Comments on dinosaurian preservation in the Cretaceous of Alberta and Wyoming. National Museums of Canada Publications in Palaeontology 4:1–9). For Charlie, the implication was that these pachycephalosaurs must have lifted in upland—even intermontaine—environments, and not in the ancient coastal plain environments where their skull domes are typically found. Others have run with the idea since then.

But was Charlie right? Are these pachycephalosaur domes typically water-worn? No one had done the hard work of looking over the original fossil material to find out. Fortunately, most of the domes that Charlie collected were available for examination at the Canadian Museum of Nature. Dave and I further supplemented our dataset with domes from the Royal Ontario Museum and the Royal Tyrrell Museum of Palaeontology. In all, we had a respectable dataset of 187 domes, which is nothing to sneeze at (particularly if you’re a dinosaur palaeontologist). Without going into the nitty gritty of how we assessed dome wear (you can read the paper for that), suffice it to say that we found that domes were not typically worn. We also found that dome wear does not correlate with distance from their presumed origin in the Rocky Mountains, nor are pachycephalosaur remains relatively more abundant in intermontaine deposits, which we could expect if the critters lived there.

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North American pachycephalosaurs almost certainly lived in the ancient coastal flood plains where we find their skull domes today. Image credit: Brett Booth.

Dave and I took this to mean that pachycephalosaurs must’ve been living where we find their remains: in the low-lying coastal floodplains, alongside the more common hadrosaurids and ceratopsids. It’s an important first step in understanding things like dinosaur community ecology and beta diversity. It’s also a good reminder that taphonomic processes like erosion can actually inform our understanding of the habits of fossil organisms, and are not simply information-destroying by nature.

If anyone wants a copy of the paper (which is behind a paywall), please fire me off an email at jmallon AT mus-nature.ca.

 

12 Responses to “Buried Treasure – Jordan Mallon”


  1. 1 Tim Donovan 06/03/2017 at 11:21 am

    I believe it was Dodson who mentioned “an intense taphonomic bias” against preservation of pachycephalosaur skeletal remains as opposed to the more durable domes. Assuming they lived in the same area as other, better preserved taxa, why was that so? I gave some thought to this in a Dinosaur Home blog post.

    • 2 Jordan Mallon 06/03/2017 at 3:37 pm

      There’s a taphonomic bias against small-bodied dinosaurs (not just pachycephalosaurs) in most fluviatile, Upper Cretaceous rocks in North America. It’s only due to their dense skull domes that we know as much as we do about pachys during that interval.

  2. 3 Tim Donovan 06/03/2017 at 6:40 pm

    What about Pachycephalosaurus? Was the 4.5m, 450kg dinosaur small bodied? No doubt Dakotaraptor is known from more postcranial elements. I like Mclouglin’s view of dome function; as I wrote in the “Dinosaur Issues” post, it may explain the paucity of remains.

    • 4 Jordan Mallon 06/03/2017 at 8:05 pm

      Compared to the contemporaneous 12 m Edmontosaurus and Tyrannosaurus, the 9 m Triceratops, and the 7 m Ankylosaurus, yes. Pachycephalosaurus is also known mostly from skull domes.

  3. 5 Tim Donovan 06/03/2017 at 8:50 pm

    Pachycephalosaurus was small relative to big contemporaries but not so small according to the apparent definition on page 574 (<60kg). Yet it's still not better known from postcrania than smaller relatives. Even if domeheads weren't mainly upland dwellers, their habitat must've been generally unfavorable to preservation. McLoughlin's view might explain this.

    • 6 Jordan Mallon 06/03/2017 at 9:35 pm

      So far as I know, Pachycephalosaurus is actually better known from postcrania than earlier NA forms. Those postcrania just haven’t been described yet. I know of at least two of them. Can’t say as much for most other pachys.

  4. 7 Matthew McLain 07/03/2017 at 12:56 am

    Jordan,

    Are teeth of small dinosaurs any rarer than teeth of large dinosaurs in the Cretaceous fluvial deposits of western North America? I realize that Edmontosaurus has a ton of teeth, but it seems to me that small theropods are going to have roughly the same number of teeth as Tyrannosaurus (although there is obviously a huge difference in the size of the teeth). Could tooth abundance for ornithischians give us any hint into actual taxic abundance?

    • 8 Jordan Mallon 07/03/2017 at 3:12 pm

      I think so, Matthew. Small ornithischian teeth aren’t terribly abundant in microsites, which I suspect reflects on their relative paucity across the landscape. Of course, as you allude to, the bigger hadrosaurids and ceratopsids have many more teeth that are turning over a higher rates, and so they are going to contribute more teeth to those microsites (although whether these guys actually shed their teeth is another matter). What someone needs to do is consider relative abundance at those microsites in the context of tooth replacement rates. I suspect the small guys would still prove quite rare — maybe a result of competition with the young of the big megaherbivores.

  5. 9 woodpalaeoillustration 08/03/2017 at 3:15 pm

    A concise read! It does seem to be most parsimonious to assume that pachycephalosaurs lived in the environment where we find their remains. I can claim ownership of one of the highland pieces myself, the “Stygimoloch” at bottom left there, though I feel it’s worth stating that I was following a meme in a different sense – the piece was based closely on a photograph of alpine ibex licking salt from a rock face accompanied the caption “Alpine Ibexes climb nearly 90 degree angles to lick salt deposits of of mountainsides. They crave that mineral.” which had in many social media circles become a popular joke. I was already aware of the thin case for highland pachycephalosaurs when I illustrated the piece, though I’ll admit not having read Mallon, J. C., and Evans, D. C. 2014. until recently. I’d go so far as to say the illustration probably doesn’t even qualify as palaeoart so much as it does a bad visual joke featuring dinosaurs.

    Still, it does fit in right with the rest of them, and I’m glad it’s suitable to illustrate the point at hand! I didn’t even consider it as a textbook example of the memetic trends with little or no evidence behind them that are so prevalent in palaeoart at the time of drawing.

    It is, of course, a shame that we know little about several pachycephalosaur taxa (I had to draw several a few months back, it was agonising) but “they all just rolled down from mountains” is perhaps not the best explanation for their shoddy remains.

  6. 10 Tim Donovan 11/03/2017 at 11:13 am

    If the paucity of pachy (postcranial) remains can’t be attributed to a preference for upland environments, how else might it be explained?

    • 11 Jordan Mallon 13/03/2017 at 8:40 pm

      It’s explained in the paper I sent you. Taphonomic size bias related to preservation in fluvial environments.

  7. 12 Tim Donovan 15/03/2017 at 1:18 pm

    It is odd that Thescelosaurus, at 2.5-4.5m, is probably better represented by postcranial remains than the larger Pachycephalosaurus–about 4.5m.


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