Mammal‐bearing gastric pellets potentially attributable to Troodonformosus at the Cretaceous Egg Mountain locality, Two Medicine Formation, Montana, USA

Abstract

Fossil gastric pellets (regurgitalites) have distinct taphonomic characteristics that facilitate inferences of behavioural ecology in deep time, despite their rarity in the fossil record. Using the taphonomic patterns of both extant and fossil small mammals from more recent geologic deposits as a guide, we assess the taphonomy of three unusual multi‐individual aggregates of mammal skeletons from palaeosols at Egg Mountain, a dinosaur nesting locality from the Upper Cretaceous Two Medicine Formation, Montana, USA. One aggregate consists of two individuals of the multituberculate Filikomys primaevus. This specimen is characterized by brecciated crania, articulated postcrania, and an absence of digestive markings, all suggestive of a non‐predatory origin. Two additional aggregates consist of 3 and 11 individuals, respectively, primarily of the marsupialiform Alphadon halleyi. High proportions of crania and indigestible elements (e.g. teeth), extensive disarticulation and breakage, digestive corrosion patterns, and the absence of a phosphatic ground mass are indicative of regurgitalites and align with features of extant prey in diurnal raptor gastric pellets. We interpret these specimens as the oldest known mammal‐bearing regurgitalites. The discrepancy in taphonomic features implies behavioural separation between the two mammalian taxa at the locality. Abundant shed teeth and nesting evidence at the locality favours the non‐avian theropod Troodon formosus as the predator responsible for the regurgitalites, congruent with previous inferences of a small‐bodied prey diet, manipulation of prey during feeding, heightened metabolic processes, and potential nocturnality for this taxon.

An extraordinary fossil captures the struggle for existence during the Mesozoic

“…Dinosaurs and mammals lived alongside one another throughout most of the Mesozoic, but direct fossil evidence for their interaction is rare 1 – 3 , 33 , 33 , 35 . An extraordinary new fossil (WZSSM VF000011) from the Lujiatun Member of the Lower Cretaceous Yixian Formation in China preserves a mammal ( Repenomamus robustus ) and dinosaur ( Psittacosaurus lujiatunensis ) in close association.…”

Tough to digest: first record of Teleosauroidea (Thalattosuchia) in a regurgitalite from the Upper Jurassic of north‐eastern Italy

“…2020; Freimuth et al . 2021). In this way, the aeolodontine in MGP‐PD 27566 represents a node in the trophic web of the Belluno area in the Late Jurassic of the western Tethys.…”

Generalist diet of Microraptor zhaoianus included mammals

“…As yet the only pellets found in association with a theropod are for Anchiornis (Zheng et al, 2018), most recently recovered as a basally branching avialan (Pei et al, 2020). Recently, Freimuth et al (2021) described pellets bearing the remains of mammals and tentatively referred them to Troodon . Thus despite multiple examples of gut contents, it is difficult to reach firm conclusions about diet.…”

“…Evidence pertaining to carnivore-consumed interactions may take such forms as bite traces on bones (e.g., Hone & Tanke, 2015), shed teeth from feeding (e.g., Maxwell & Ostrom, 1995), coprolites (e.g., Chin et al, 1998) or pellets (e.g., Freimuth et al, 2021) containing identifiable bones and, most importantly, gut contents (e.g., Dal Sasso & Maganuco, 2011). Bite traces from carnivorous dinosaurs left on the bones of other animals are generally not common (e.g., Jacobsen, 1998; Fiorillo, 1991) and, except on rare occasions (e.g., Currie & Jacobsen, 1995; Hone & Watabe, 2010), it is difficult to be specific about the taxonomic identity of even one of the individuals involved in the interaction (Chin, 2012).…”