Eggshell from the Horseshoe Canyon Formation

This week has been really exciting for me, because I just had a paper accepted! It’s been extra special because it’s the first paper to come out of my own fieldwork. Since 2015, a team of volunteers and I have been working in the Horseshoe Canyon Formation exposed near Drumheller. Finally, our work is bearing fruit!

[Edit: The paper is now officially out online, you can access it here: http://www.nrcresearchpress.com/doi/10.1139/cjes-2017-0273#.Wqba7ZPwaRt or here: http://www.nrcresearchpress.com/eprint/X8HfpTRzP5mkhRif2FMw/full ]

This project really owes itself to Eric Felber, a geologist and amateur palaeontologist who has spent decades prospecting the Red Deer River Valley. He found several sites with abundant troodontid teeth, and reported them to Phil Currie while he was still at the Tyrrell Museum. One of these sites (L2000, see Ryan et al. 1998) was worked intensively, and shed light on the association between troodontids and nestlings of other dinosaurs. Their work showed that troodontids were preying on hatchlings of other dinosaurs, and they hypothesized that there must have been nests of these other dinosaurs nearby. The other sites weren’t rich enough to justify more study, so they sat untouched for nearly 20 years.

FTS-2
The FTS-2 (Felber Troodontid Site # 2) locality. The site is about 50 m long, exposed along the East side of the Red Deer River.

In 2016, Eric joined our crew working near the Tolman Bridge, and convinced us to check out this site further south, near the Morrin Bridge. We had such good luck that in 2017, we decided to move campsites so that we could focus on the locality. One of the perplexing things we found in 2016 were some light-coloured eggshell fragments. Although we were initially excited, comparison to other eggshell from Alberta, which is usually black, made us think that these were just broken eggshell from modern birds. In 2017, we found more fragments, and something just didn’t seem right. We were collecting bulk samples anyways, and so we decided to investigate the eggshell a little bit more closely. We dug out fresh sediment that couldn’t have been contaminated by recent material, and broke it down using water, to reveal the fossils inside.

 

 

 

When the screenwashed samples produced more eggshell fragments, and in varying shades of tan and brown, we realized that we might have something more important than we first thought. We investigated further by thin-sectioning the eggshell, and examining it using a scanning electron microscope (SEM). The results confirmed our suspicions: this wasn’t modern bird eggshell, it was dinosaur eggshell! This was an important discovery, because in more than 130 years of looking, no one had ever found eggshell in the Horseshoe Canyon Formation.

Eggshell histo
A view of the eggshell in thin-section, ground down thin enough so that light can shine through the shell. The alternating dark and light columns are a feature that characterizes Prismatoolithus.

But that wasn’t even the best part. I’m a novice with eggshell, so I really had to get up to speed to understand how eggs and eggshell are classified, and what features are important to examine. During my reading, I kept coming across different eggshell features that distinguished an unusual eggshell type: Prismatoolithus. If we break down the name, we can get a sense of what makes these eggs unusual. “Oo” is Greek for ‘egg’, and “-lithus” is Greek for stone or rock, because the eggs are fossilized. The first part of the name, “Prism-“, refers to the prismatic shape of the eggshell crystals. Whereas most dinosaur eggs have continuous or radiating microstructure, Prismatoolithus has angular columns of blocky crystals. This is a feature that our eggshell demonstrates really clearly, so we were confident that the eggs belonged to Prismatoolithus. The exciting part? Fossils from Montana and southern Alberta show that Prismatoolithus belonged to troodontids!

 

 

 

This completely changes our understanding of the association between troodontids and nestlings of other dinosaurs. Although troodontids were raiding those nests, the sites we’ve found in the Horseshoe Canyon Formation aren’t hadrosaur or ceratopsian nests, they’re troodontid nests! This means that troodontids were bringing other dinosaurs’ babies back to their own nests, probably to feed their own young. This is just the first of many exciting projects about this site, which is shedding light on previously unknown parts of the Horseshoe Canyon Formation communities. Stay tuned for more as we continue to work on this fantastic locality!

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