New paper: theropod bite marks on Morrison sauropod bones

Distal end of MWC 4011, an ischium of Apatosaurus louisae that got munched on by a large theropod, probably Allosaurus or Ceratosaurus. On display at Dinosaur Journey in Fruita, Colorado.

New paper out today in PeerJ:

Lei R, Tschopp E, Hendrickx C, Wedel MJ, Norell M, Hone DWE. 2023. Bite and tooth marks on sauropod dinosaurs from the Morrison Formation. PeerJ 11:e16327 http://doi.org/10.7717/peerj.16327

This one had a long gestation. The earliest trace I can find of it in my Gmail archive is this bit I sent Dave Hone back in February of 2015:

Sorry to not have gotten around to sending the sauropod bite mark stuff. I still have the note in my phone, I’ll get on it ASAP. 

I have no idea what earlier conversation that was referencing — wherever it happened, my end of it apparently wasn’t in Gmail. I also apparently did not follow through, because on April 26, 2018, Dave wrote to me, “I’m vaguely trying to resurrect a survey of sauropod bite marks,” referencing that 2015 message.

At that point I did actually kick into gear and started sending him photos and refs. Which is how, about a month later, he sent one of kindest messages I’ve ever received:

This is starting to get silly, you’ve already turned up more examples than I’ve managed and you’ve also provided papers and photos too! Bearing that in mind, it seems ridiculous not to formally invite you in on this — are you up for continuing to supply some Morrison sauropod bites?

At that point I was the third on the project, with Dave and Emanuel. Later Mark Norell, Christophe Hendrickx, and Roberto Lei would join us, with Christophe serving as our resident theropod tooth expert, and Roberto in particular doing a lot of the heavy lifting of turning our findings into a paper.

The rest of MWC 4011.

So what’s the upshot? For one, a few good-sized sauropod elements are bitten through, showing that at least some Morrison theropods were capable of inflicting real damage on big bones. So right off the bat we have a survivorship problem: in a collections-based survey like the one, we can only tally bite marks on bones that survived being bitten in good enough shape to be collected and identified as sauropod bones. Bones that were consumed by theropods, or shattered beyond the ability to be preserved, recognized, or collected, are not available to us.* In other words, we can only tally bones in the “Goldilocks zone” of being directly chomped on but not too much — careful bites that stripped meat from a bone without biting in are invisible, and so are bites so violent or forceful that they destroyed the bone. This is sort of like the osteological paradox in paleopathology (see this post), just applied to individual bones instead of individual animals.

*In a field-based study, it’s possible to partially offset this by collecting and analyzing everything, not just the identifiable bits. Julia McHugh and colleagues did exactly that in their “nugget bucket” study (McHugh et al. 2023), an IMHO brilliant follow-up to their papers on theropod feeding traces (Drumheller et al. 2020) and invertebrate feeding traces (McHugh et al. 2020) on dinosaur bones from the Mygatt-Moore Quarry. One reason I’m so happy that Julia is at Dinosaur Journey is that she keeps thinking of interesting stuff to do with that collection.

I’ve argued before that baby sauropods left few bones because most of them either grew up, or — vastly more commonly — got processed into theropod poop. I felt like that quip was coming back to haunt me in this project; I find it perversely difficult to think clearly about evidence that I never get to see!

MWC 861, a pubis of Apatosaurus louisae with an extensively bitten distal end. Definitely from the same quarry as MWC 4011 — the Mygatt-Moore Quarry, in far western Colorado — and possibly from the same individual. Also on display at Dinosaur Journey.

Interestingly, we found zero examples of healed bites on Morrison sauropod bones. So all of the bite marks we found were either from successful predation events, or scavenging. And in fact we didn’t find that many bitten sauropod bones, period. We found 68 Morrison sauropod bones with bite marks, out of the 600 or so that we actively surveyed. That’s about 11%, compared to 14% in later tyrannosaur-dominated faunas (Jacobsen 1998). But also, we found a lot of wear on the teeth of large Morrison theropods, which suggests that they were processing tough stuff, including bones.

We suspect that big Morrison theropods were primarily targeting juvenile and subadult sauropods, and scavenging dead adults when they could get them. We think that partly because younger sauropods must have been more numerous than adults (and maybe vastly more numerous), and partly because almost all predators prefer easy fights to difficult ones. As I wrote back when,

Even assuming that max-sized individuals were around – which may not always have been the case… – the theropods would have to walk right past a whole boatload of smaller, easier targets to get to them, ignoring winnable fights and achievable calories just to roll the dice in the most dangerous possible encounters.

Naturally Dave has explored a lot of these ideas in his previous papers, especially Hone and Rauhut (2010) — this new paper is basically a spiritual successor to that one. Dave has his own blog post up about the new paper, here. 

Allosaurus munching on a dead Galeamopus while a pair of ceratosaurs look on hungrily. Art courtesy of Davide Bonadonna (www.davidebonadonna.it)

Theropods primarily attacking small sauropods would explain the patterns that we see, better than any alternative we can think of. Of course the Morrison covers a lot of space and time, and animals do all kinds of weird stuff if you watch them long enough, including suicidal attacks on much larger prey. But if theropods were preferentially attacking adult sauropods, we’d expect to see at least some healed bite marks from failed attacks, and we’d also expect to see more bite marks, period. Somehow big Morrison theropods were managing to put a lot of wear on their teeth without leaving many tooth-marked sauropod bones behind, which seems like a big mismatch. The best explanation we can think of is that the theropods were accumulating that wear munching on juvenile sauropods (which we thought they were doing anyway), and consuming or destroying their bones in the process (which the theropods were well-equipped to do).

But even if we’re right, there’s a ton we don’t know yet. We struggled to match any of the bite marks that we found to specific theropod taxa. Taphonomy and collector bias are probably both big filters, especially for bones that were bitten through or shattered before fossilization. There are definitely important differences between quarries — for example, Mygatt-Moore has a ton of bitten bones, and the Carnegie Quarry at Dinosaur National Monument has almost none, and we don’t know why.

In sum, there’s a lot to do, with interesting, tractable, as-yet-undone projects surrounding this paper in a quantum fuzz like an electron shell. Hopefully other folks will get out there and start turning those potential projects into real ones.

MWC 4011, once more with feeling. And fiberglass. Photo by Brian Engh.

References

• Drumheller, S.K., McHugh, J.B., Kane, M., Riedel, A. and D’Amore, D.C. 2020. High frequencies of theropod bite marks provide evidence for feeding, scavenging, and possible cannibalism in a stressed Late Jurassic ecosystem. PLoS One 15(5) p.e0233115.
• Hone, D.W. and Rauhut, O.W. 2010. Feeding behaviour and bone utilization by theropod dinosaurs. Lethaia 43(2): pp.232-244.
• Jacobsen AR. 1998. Feeding behaviour of carnivorous dinosaurs as determined by tooth marks on dinosaur bones. Historical Biology 13:17–26. DOI 10.1080/08912969809386569.
• McHugh, J.B., Drumheller, S.K., Riedel, A. and Kane, M. 2020. Decomposition of dinosaurian remains inferred by invertebrate traces on vertebrate bone reveal new insights into Late Jurassic ecology, decay, and climate in western Colorado. PeerJ 8, p.e9510.
• Mchugh, J.B., Drumheller, S.K., Kane, M., Riedel, A. and Nestler, J.H. 2023. Assessing paleoecological data retention among disparate field collection regimes: a case study at the Mygatt-Moore Quarry (Morrison Formation). Palaios 38(5):233-239.