FROM ROSTRA TO REAR: SMASHED SHELL COPROLITES FROM THE NIOBRARA CHALK WITH A LIKELY TRACEMAKER AND IMPLICATIONS FOR BIOSTRATIGRAPHY AND PALEOECOLOGY

Paleontological explorations of the Niobrara Formation of North America have been ongoing since the 1870s. Multiple collectors have scoured the Smoky Hill Chalk Member, describing hundreds of new species. Hattin’s 1982 description of marker units established stratigraphic references based on lithology, addressing myriad complaints of lack of stratigraphic data collected with vertebrate macrofossils going back to the late 19th century. The description of trace fossils has lagged behind, with few mentions in literature.

Trace fossils composed of crushed, angular fragments of bivalves have been long recognized by researchers in the Chalk. Recent extensive fieldwork shows these fossils are constrained to

Hattin’s marker unit 4 and 6. This brief interval of deposition concludes with the disappearance of the inoceramid Volviceramus involutus as well as the end of the Coniacian stage. These fossils, here interpreted as coprolites, are found strictly in stratigraphic association with the remains of the poorly known ram-snouted fish Martinichthys ziphoides. These fish, primarily found as robust isolated rostra, as well as the coprolites, have only been reported from this interval and because of this occurrence, we interpret the coprolites as traces of Martinichthys feeding activity.

Preliminary research indicate they are the result of a durophagous feeding strategy. Martinichthys, restricted here to M. ziphoides, likely preyed on oyster communities anchored to hardgrounds on the seafloor muds. Ubiquitous, individually variable beveled rostral tips are interpreted as wear facets that develop during feeding and are not seen on M. brevis. We hypothesize a face-ramming behavior restricted to M. ziphoides that smashes the encrusting oysters against the hardground, enabling a fish that lacks crushing dentition to exploit a hard-shelled food source. Other durophagous Cretaceous fish were evaluated as a tracemaker, however, they have a much larger geographic and stratigraphic range with no co-occurrence of similar coprolites. This research enhances our understanding of Late Cretaceous paleoecology, revealing a highly specialized trophic niche within the Niobrara ecosystem. This work highlights their significance in Late Cretaceous paleoecology and establishes their value as index fossils.