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

Masateru Shibata,David J Varricchio

doi:10.1017/jpa.2020.16

Abstract

AbstractA 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.

Highlights

Horseshoe crabs of the Limulidae survive today in areas along the East Coast of North America, western Japan, and restricted regions of East and Southeast Asia (Sekiguchi, 1999)
The fossil record of the Xiphosurida extends into the Paleozoic, the general bauplan of the order has undergone very little change, and Limulus has been proposed as a stabilomorph (Kin and Błaż ejowski, 2014; but see Lamsdell, 2016, for a different view)
Fossil horseshoe crabs occupied a variety of environments, including lakes, rivers, and marine habitats (e.g., Reeside and Harris, 1952; Schram, 1979; Babcock et al, 2000; Lamsdell et al, 2013; Lamsdell, 2016; Błaż ejowski et al, 2017)

Summary

Introduction

Horseshoe crabs of the Limulidae survive today in areas along the East Coast of North America, western Japan, and restricted regions of East and Southeast Asia (Sekiguchi, 1999). Except for one species in Southeast Asia, extant horseshoe crabs are limited to strictly marine settings (Sekiguchi, 1999). Despite differences among extant species, horseshoe crabs generally live in coastal bays with mud flats in Confusion over horseshoe crab trackways has included their misinterpretation as footprints of pterosaurs (Oppel, 1862; Winkler, 1886), Archaeopteryx (Nopsca, 1923), and amphibians (Willard, 1935). Abel (1935) first recognized horseshoe crab traces in the fossil record, and Caster (1938) used experimental work to reinterpret the amphibian tracks of Willard (1935) as having a limulid origin. The type species for horseshoe crab tracks, Ichnites lithographicus, was originally described by Oppel (1862) as pterosaur tracks. Nopcsa (1923) reassigned this ichnospecies to the ichnogenus Kouphichnium and as representative of Archaeopteryx tracks. Caster (1944) compiled previously described limulid-like tracks (e.g., Willard, 1935) and concluded

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Discussion

Conclusion