KAREN CHIN, JOSEPH H. HARTMAN, BARRY ROTH (2008). Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana Lethaia DOI: 10.1111/j.1502-3931.2008.00131.x
In this post I wrote about a snail and a slug that were helping themselves some dog feces. Now it turns out that terrestrial gastropods may have been into coprophagy for quite a long time, at least since dinosaurs were around.
In a very interesting paper, Chin et al. report the discovery of approximately 77 million-year old dinosaur coprolites (fossilized dung) containing snail shells. These came from the Two Medicine Formation in northwestern Montana. More than 130 snail shells, intact or fragmented, were observed on or in 6 of the 15 coprolites at the location. The shell remains represented at least 7 fossil taxa of 4 terrestrial and 3 aquatic snails.
A piece of dinosaur coprolite (top) and a snail shell sticking out of one (bottom). From Fig. 2 in Chin et al.
As I discussed in my aforementioned post, land snails and slugs do eat feces, but the presence of snail shells in coprolites does not necessarily mean that the snails were feeding on the feces. The authors consider 2 possible scenarios to explain their findings: 1. the dinosaurs ate the snails; 2. the snails were eating the dinosaurs’ feces. To these I will add a 3rd possibility that Chin et al. mention only in passing: 3. the mixing of the snails and the dinosaur dung happened during the flooding event that apparently buried the dung.
The subject dinosaurs (tentatively identified as members of Maiasaura) certainly had a peculiar diet: their coprolites were composed mostly of "fragmented conifer wood". They may have been feeding on fungi growing on rotting wood and also ingesting some wood along with any snails that may have been present on the wood or the fungi. I am no expert on coprolites and so wouldn’t know if fungal remains in them could be identified. The authors, however, reject the possibility that the shells in the coprolites are the remains of ingested snails based on their contention that snail shells present in the dinosaurs’ food would likely have been crushed during chewing and the subsequent digestion. But I don’t think we can yet rule out that scenario. If the snails survived mastication more or less intact (only 30% of the coprolite shells were determined to be whole), they could also have survived digestion with minimal shell damage. For example, there is one report of freshwater mollusks surviving their fish predators’ digestive processes (Brown, 2007).
Chin et al. favor the 2nd scenario that the snails were eating the dinosaurs’ feces before they got fossilized along with their food. The problem I have with that explanation is the presence of both terrestrial and freshwater snails in the coprolites. The authors' explanation that first, the land snails were on the dung, then the area got flooded (implying that the land snails stayed on the dung underwater), and the freshwater snails came to feed on the dung and finally they all got fossilized seems a bit stretched.
I am leaning towards the 3rd explanation that all of the snails were deposited on the dinosaur feces as empty shells during a flooding event and got fossilized subsequently. Ancient coprophagous snails is, of course, a more intriguing interpretation and I hope more unburied feces will be found to clinch the case.