292-16 From Feeding to Fossil Feces: Connecting Enigmatic Trace Fossils to a Likely tracemaker in the Late Cretaceous Niobrara Formation of Western Kansas

Abstract:

Exploration of the paleontology of the Smoky Hill member of the Niobrara Formation has regularly occurred since E. D. Cope’s 1870 expedition to western Kansas. Hattin’s 1982 description of the Smoky Hill Chalk marker units established useful stratigraphic references based on lithology, addressing complaints of the lack of stratigraphic data collected with vertebrate macrofossils. The description of trace fossils has lagged behind, with few mentions in literature. Here we describe a new ichnofossil with implications for paleoecology and biostratigraphy.

Unique trace fossils composed of angular bivalve fragments are found between Hattin’s marker units 4 and 6. This brief interval of deposition closes with the disappearance of the inoceramid Volviceramus involutus and the end of the Coniacian stage (~85.5 Ma). These trace fossils, here interpreted as coprolites, are strictly found in association with the rare and poorly known ram-snouted fish Martinichthys ziphoides. These fish, primarily found as robust isolated rostra, only occur in this interval in Kansas. We interpret the coprolites as traces of Martinichthys feeding activity.

CT scans, photographs, and thin sections of the coprolites were analyzed to study their composition. Thin sections were compared to sliced samples of Volviceramus, Pseudoperna, Platyceramus and barnacles.These analyses indicate that Marthichthys used a percussive, durophagous feeding strategy. Martinichthys, restricted here to M. ziphoides, likely preyed on Pseudoperna congesta oyster communities anchored to hardgrounds on the soft seafloor. These include large inoceramids, the rudist Durania maxima and encrusting fauna on sunken driftwood logs. Individually variable beveled rostral tips are interpreted as wear facets from feeding. There are no external indicators of stress fractures. We hypothesize an effective face-ramming behaviour for M. ziphoides that smashes encrusting oysters against the hardground, enabling a fish that completely lacks crushing dentition to exploit a hard-shelled prey. The resulting pulp is then ingested and processed through the fish. The relative abundance of coprolites compared to body fossils highlights their potential as biostratigraphic proxies for Martinichthys. Other durophagus Cretaceous fish were evaluated as a tracemaker; however, they have broader geographic and stratigraphic ranges and no co-occurrence with crushed shell coprolites. This research enhances our understanding of Late Cretaceous paleoecology, revealing a highly specialized trophic niche in the Niobrara. By describing these coprolites and assigning them to Martinichthys, this study highlights paleoecology significance and biostratigraphic value.

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