A locked dinosaur mystery

So, one of the things you're supposed to do if you have a blog is to talk about your research when a paper you authored comes out. Okay, give me a minute. I mentioned back in the gut contents post that I did my thesis on what was found in the inside of a brachylophosaur. The first part, on the gut contents themselves, was published back in 2008 (Tweet et al. 2008). Finally, after about a decade of gnawing on the manuscript, comes the other part (Tweet et al. 2016).

When my advisor Karen Chin and I started looking at Leonardo, we had all sorts of clever ideas we wanted to subject it to. Differences in the plant material from the thoracic region to the lower abdomen? Heck, why not try stable isotopes? The mind reels at the possibilities. Leo, of course, was having none of it, which was apparently his protest at the involuntary posthumous nature of his participation. This is what comes of trusting duckbills.

They may gaze beseechingly with those empty eye sockets, but they'll get you every time. (oh, and forgive these images; some of them are scanned from photos, and the others are early generation digital cameras having to contend with depth of field issues.)

The great mass of plants turned out to be about the same throughout. However, there was something kind of odd about the gut material: it was laced with these tiny off-white markings. In the confines of the lab, under the trusty stereo microscope, they turned out to be hollow cylindrical features defined by thin whitish layers.

One hand sample of Leo guts plus little white tubules.

Our samples were loaded with these things. Some were in great shape, with well-defined white layers marked with fine parallel striations and nice distinct walls. Some were little more than smears. Some were exposed only in cross-section as little circular to ovoid voids on the edges of pieces. Some of them were split into parts and counterparts. Sometimes they ran over each other, the more recent feature overprinting the older feature with a lip of whitish material. Most unusually, some of them ran right next to each other, sharing a wall of the whitish stuff for several millimeters or more before parting ways. Practically all of the features were about 300 microns (about a third of a millimeter) across; some of the smeary features were substantially wider, but they were too poorly preserved to tell if they were large single features or smeared paired features.

Three or four traces, two of which share a wall near the top of the photo, a third being crossed by one of the pair before descending into the sample, and a fourth fragment of a trace in the lower right (probably the right-hand burrow of the pair, but potentially a different trace).

So what were they? If they were abiotic features, they were sure doing a great job of looking like regular biotic features. In isolation, root traces can do a lot of the things that the features show. However, we had numerous examples of the features, and none of them showed any indication of branching or tapering (where they look like they're branching from a distance is actually pairings). Also, if we had hundreds of root traces in the handful of gut samples, there should have been comparable root activity in other parts of the carcass. If that was the case, Leonardo would have been rooted to heck and gone instead of being the exquisite chunk of ex-dinosaur that it is. Fungal hyphae are much too small to fit the features. We were left with the possibility that we were looking at burrows.

There's nothing morphologically against these being burrows. They look like burrows, so they pass the basic sniff test. Many burrowing critters secrete mucus which can be lithified, and have bristly structures on the body that can leave striations. There was certainly plenty of stuff to eat, either from the plant matter in the carcass or from the carcass itself. You can even get the paired-burrow thing from the reproductive habits of many worm-like critters. Something comparable to a modern enchytraeids (you may know them as "whiteworms" or "potworms") is about the kind of thing that would fit reasonably well. We gave the little things the name Parvitubulites striatus, which means "tiny striated tubule". If I saw little structures like these in any old generic sedimentary rock, I'd probably think "burrow" and move on. It's the whole "inside the duckbill" thing that's the real puzzler.

Striae along the length of a burrow.

One odd thing is that there's only one kind of burrow. If you have a dead critter today, you've also probably got lots and lots of live critters from across the animal kingdom, of all shapes and sizes, leaving all kinds of evidence of their activities (most of which are destroyed, but that's another story). Leo, of course, did not suffer the typical taphonomic indignities of your average dead large animal. The presence of internal burrows implies that either only one type of diminutive animal found its way to the carcass, or that we're looking at an inside job (i.e., some parasites that were left in the lurch upon the decease of their meal ticket, or baby parasites that hatched after Leo bit the dust). Like any good locked-room mystery, you can argue either option.

References:

Tweet, J. S., K. Chin, D. R. Braman, and N. L. Murphy. 2008. Probable gut contents within a specimen of Brachylophosaurus canadensis (Dinosauria: Hadrosauridae) from the Upper Cretaceous Judith River Formation of Montana. Palaios 23(9):624–635.

Tweet, J., K. Chin, and A. A. Ekdale. 2016. Trace fossils of possible parasites inside the gut contents of a hadrosaurid dinosaur, Upper Cretaceous Judith River Formation, Montana. Journal of Paleontology 90(2):279–287.