Selected Studies in Lower Paleozoic Ichnology

Abstract

<p>Despite the attention recently given to ichnology, intensive investigations have been mounted in only a limited number of critical areas. These include the zoological affinities of, and the paleobathymetric significance of soft-sediment ichnofossils. Such areas as establishing a workable nomenclatural system, the significance of Lower Paleozoic macroborings, and quantitative methods in ichnology, although important, have received considerably less mention. In order to examine these lesser known facets an integrated study of selected Lower Paleozoic ichnofossils was conducted.</p> <p>Part one involves the identification and significance of an extensive ichnofossil suite, comprising 27 different ichnogenera from the Middle Silurian Thorold Sandstone of southeastern Ontario and New York. These ichnofossils are indicative of the shallow-water, nearshore Skolithos and Cruziana ichnofacies, and can be divided into four distinctive assemblages. As well as providing information on the interpretation of the paleoenvironments, lateral variations in ethological categories prove valuable in delineating ancient storm deposits.</p> <p>Part two comprises a collection of individual investigations of significant Lower Paleozoic ichnofossil occurrences with particular attention paid to quantitative methods.</p> <p>1) Since ichnofossils are used not only by ichnologists but by vertebrate and invertebrate paleontologists, paleobotanists, stratigraphers, and sedimentologists, it is impertive to have a workable nomenclature system. One of the most abundant and important groups of ichnofossils are single-entranced, vertical burrows, which currently include many ill-defined and poorly understood forms. Through the examination of type specimens and an analysis of pertinent morphological characteristics, this diverse group can be referrable to only four distinct ichnogenera: Cylindrichnus, Monocraterion, Skolithos, and Stipsellus.</p> <p>2) Ichnology also encompasses structures produced biogenically in hard substrates. Such ichnofossils, although well studied in post-Paleozoic deposits, have received considerably less attention in Lower Paleozoic strata. The Silurian-Devonian disconformity in southern Ontario exhibits well-preserved macroborings of the ichnogenus Trypanites. Quantification of borings using shape parameters shows that boring morphology can be described accurately. Comparison with borings made by recent sipunculids indicates a high degree of similarity, suggesting that these examples of Trypanites were probably produced by sipunculids.</p> <p>3) A specimen of the ichnospecies Lingulichnus verticalis was recovered from the Lower Cambrian Bradore Formation of southern Labrador. The sample represents the only known Cambrian example of an in situ lingulid, and sheds light on the evolution of their ethology. Prior to this, it was assumed that Cambrian lingulids maintained a non-burrowing mode of life.</p> <p>4) Ichnology has progressed beyond the stage of mere description and it may now be possible to try to apply some quantitative concepts. Modern ecological methods to measure and interpret spatial distributions were applied to selected Lower Paleozoic vertical ichnofossils. Such analyses may prove valuable in interpreting a host of biotic interactions among ichnofossil assemblages.</p>