The Arctic Cretaceous revisited

One of the earliest posts here, way back in March 2014, was about an assemblage of high-Arctic Late Cretaceous coprolites that had been part of my graduate work. We did quite a bit with them (Chin et al. 2008), but there's always more that can be discovered. One of the things Chin et al. (2008) noticed was the rarity of body fossils for large potential coprolite producers. They broached the idea that the producers were only around part of the time, living elsewhere during the polar winter and only turning up to feed during the long sunny summer days. Duffy et al. (2026) takes this idea and runs with it.

You may recall that back in 2014 we discussed two major categories of coprolites, those with a dominantly greensand composition and those with a dominantly phosphatic composition. The greensand coprolites had various kinds of inclusions, including crustacean carapaces, bivalves, and squid hard bits* and such (interestingly without evidence of processing by teeth), suggesting consumers that were bottom-feeders, whereas the phosphatic coprolites were loaded with planktonic microfossils, suggesting filter feeding (or feeding on soft-bodied filter feeders) (Chin et al. 2008). The division is nuanced a bit more this time around, with an intermediate group with greensand embedded in phosphate. At the time, we could easily see that the phosphatic coprolites had spiral structures, as in shark excrement. As it turns out, at least some greensand coprolites also have internal tubular structures, although usually not as easy to spot (Duffy et al. 2026). What that means is that most of the coprolite producers therefore had spiral intestinal valves like sharks.

*I had the hardest time figuring out those squid-pen bits when I was working with the fossils. I thought they might be horseshoe crab tails.

There is one group that fits quite well for producing the whole range of coprolites while also not producing abundant body fossils: sturgeons. Sturgeons have spiral valves. The adults don't have teeth to shed, so they aren't leaving hundreds of potential fossils, and they aren't crushing or cutting prey. Rather, they are bottom-feeders that suck in prey items (and sediment). They are also big enough to produce the size range of greensand coprolites. Meanwhile, young sturgeons have small teeth (but they lose them), and they feed on zooplankton. Finally, there are sturgeons today that migrate from marine water to estuaries or shallow marine settings to feed on seasonal food blooms before spawning in freshwater (Duffy et al. 2026). They're about as perfect of "poopetrators"** as we could ask for.

**I still regret nothing! 

It's the migration part that particularly interested Duffy et al. Most behaviors are pretty darn difficult to fossilize clearly; or, if you want to get philosophical about it, all behaviors are reflected somewhere in anatomy, but most of them produce effects that are too subtle to pick out or are swamped by the effects of other behaviors. We can look for evidence for migration in certain large land animals such as mammoths by studying stable isotopes in bones. These animals were big enough to travel long distances (and thus drink water in places with different isotopic signatures, for example) and had nice big bones that allow sampling for time sequences. The Devon Island situation is not quite so convenient, but a few lines of evidence are suggestive (Duffy et al. 2026):

1. The microfossils in the coprolites suggest plankton blooms during long polar summer days, which is a pretty common high-latitude pattern; make hay when the sun shines, after all. Blooms of one kind of organism attracts populations of other organisms to consume them. This is a pretty simple way to establish a migratory pattern. (The flip side is that everything would clear out during the polar winter. Think of an Old West boom town, except for having annual booms and busts.) This is the strongest line of evidence in my mind.
2. There are lots of coprolites and not much skeletal evidence for what made them. This is interpreted as evidence for producers that only lived there part of the time. I think this is a bit weaker, as there are always going to be more turds than bodies, but it's worth noting.
3. Sturgeons, as likely producers for some large percentage of the coprolites, are known to be migratory today, and anatomically haven't changed much since the Late Cretaceous.
4. Finally, many of the vertebrates inhabiting the Western Interior Seaway and neighboring areas have distributions that are pretty darn cosmopolitan in this region, consistent with migratory patterns. Like the second point, I don't think this is as strong as the first, but again it's worth noting.

All in all, I like it, and I think it's great to learn new tricks from old turds. You never know what will turn up once you start looking at these humble fossils!

References

Chin, K., J. Bloch, A. Sweet, J.Tweet, J. Eberle, S. Cumbaa, J. Witkowski, and D. Harwood. 2008. Life in a temperate Polar sea: a unique taphonomic window on the structure of a Late Cretaceous Arctic marine ecosystem. Proceedings of the Royal Society B 275(1652): 2675–2685. doi: 10.1098/rspb.2008.0801.

Duffy, F., K. Chin, S. Cumbaa, and L. Wilson. 2026. Coprolite evidence for marine vertebrate migration in the warm Cretaceous Arctic. Historical Biology. doi: 10.1080/08912963.2026.2670771.