Ophiomorpha irregulaire, Mesozoic trace fossil that is either well understood but rare in outcrop or poorly understood but common in core

Abstract

The distinctive trace fossil, Ophiomorpha irregulaire, differs from other ichnospecies of Ophiomorpha in the so-called “meander maze” configuration of its horizontal course, and by the possession of unevenly distributed, elongated, tapering wall-lining pellets. The ichnospecies has been described, without doubt, from only two outcrop localities, both Late Cretaceous in age and both lying within the Cretaceous Western Interior Seaway. Only a single, almost convincing example of the trace fossil has been reported elsewhere, in a Lower Jurassic outcrop in Italy. Apart from this last occurrence, the distribution of O. irregulaire in outcrop suggests that the trace fossil has a restricted time span and geographical range. In contrast, O. irregulaire is identified more commonly in cores, and these occurrences show no clear geographical restrictions and are reported in sediments of Jurassic as well as Cretaceous age. Outcrop exposures of Jurassic and Cretaceous sediments have been extensively studied by ichnologists and if the large, eye-catching O. irregulaire were as common as is suggested by core material, then the trace fossil would not have escaped notice in outcrop. Identification of the trace fossil in core is generally made on the basis of two-dimensional, vertical sections. The characteristic meander maze form cannot be seen in this view, and the identification is thus based almost entirely on the special shape of the lining pellets. As is well known, the identification of large trace fossils in the restricted format of core lacks the security of extensively exposed trace fossils in outcrop. The discrepancy between age and geographic distribution of O. irregulaire in outcrop and core suggests that some misidentification is occurring in core analysis.