Horseshoe crab trace fossils from the Upper Cretaceous Two Medicine Formation of Montana, USA, and a brief review of the xiphosurid ichnological record

Abstract

A locality in the Upper Cretaceous Two Medicine Formation of Montana preserves abundant and variable horseshoe crab tracks and trails of the ichnotaxon Kouphichnium isp. These specimens span six morphologies differing in track form and trail configuration. These differences likely reflect variations in track-maker locomotion and behavior, substrate consistency, epichnial versus hypichnial preservation, and undertrack versus true tracks. Several tracks preserve the first clear appendage impressions for an extinct horseshoe crab. This discovery adds new information to the fossil horseshoe crab diversity in the Cretaceous Period. Trackway dimensions, such as the external width across the pusher legs or of the prosomal drag mark, provide information on the track-maker size. Most trackways correspond with crabs 9–14 cm wide; the abundance but limited size range of the traces suggests the large assemblage corresponds to a mating aggregation. The trace fossil record of xiphosurids indicates that throughout their history, horseshoe crabs inhabited both marine and nonmarine settings. They were definitively present in freshwater habitats from the lower Carboniferous through at least the Paleogene. Horseshoe crab trace abundance is highest from the upper Carboniferous through the Jurassic and likely reflects two factors: true upper Carboniferous taxonomic diversity and a preponderance of suitable sites for trackway preservation in the Late Triassic and Jurassic. Cretaceous traces are uncommon, and this Two Medicine locality is the first occurrence of horseshoe crab traces in the Late Cretaceous worldwide. Overall, track abundance and diversity would seem to correspond well with the reported horseshoe crab body fossil record.