The Habitat of the Eurypterida/Introduction
THE HABITAT OF THE EURYPTERIDA[1]
by
Marjorie O'Connell, A.M.
Curator in Palaeontology
Columbia University
INTRODUCTION
It has been the custom to consider that all fossils are the remains of marine organisms unless obvious and indisputable evidence of their fluviatile, lacustrine, or terrestrial habitat is available; a fossil found without any other associates has been held to be marine until proved to be otherwise, but never has the suggestion been made that such a fossil was fluviatile until proved to be marine. Yet such a suggestion would be most logical, and, as we shall see, it would be far more natural than the one usually made. The early fish have always been considered normally marine, though recent studies of the character of the sediments in which their remains occur has led many of the former advocates of the marine habitat to concede that the earliest fishes lived in non-marine waters, perhaps lacustrine, but most probably fluviatile. Similarly, limestone faunas were at one time referred without question to a marine origin, but we now know that limestones of purely marine organisms may be formed by eolian deposition, as in the case of the Miliolitic limestone of the Kathiawar Peninsula of Western India (Grabau, 87, 574).[2] There is thus no a priori reason for implicitly accepting the marine origin of all those rocks for which it has been claimed, nor for believing that all fossils found in the Palaeozoic rocks, with the exception of freshwater molluscs, plants, and insects, are the remains of marine plants or animals. Just as there is a growing tendency at the present time to recognize the importance of the wind and of rivers as agents of transportation and deposition in the past, so there is noticeable an awakening from the old belief that all fluviatile organisms began their life in the sea, and only after countless ages of evolution in that realm, migrated first into brackish water and then into the rivers.
The present paper deals with the habitat of a class of crustaceous animals widespread in the Palaeozoic and confined to it. The Euryterida belong to the subclass of the Merostomata in the class Acerata of the phylum Arthropoda. Their nearest relatives are the limulids and scorpions with which latter group they have been classed by certain authors.
While it is generally accepted that some eurypterids lived in fresh water, the majority of palaeontologists at the present time still maintain that the early periods of the racial history of these organisms were passed in marine waters and that it was only, indeed, after their acme in development had been reached that these merostomes, becoming at first euryhaline, finally forsook the sea altogether and lived in rivers and in brackish water bodies until they became extinct at the end of the Palaeozoic. The evidence set forth in support of this hypothesis is so plausible that many have been led to think that there is a large and convincing array of facts sufficient to furnish an indisputable proof that the eurypterids lived during at least a part of Palaeozoic time in marine waters. It was with the purpose of showing that such a proof was really non-existent, and that the observed facts can also, and perhaps more rationally be accounted for in another way, that the present paper was undertaken. The author proposes to formulate a few of the principles which must be borne in mind in considering such a problem, and to point out the inconsistency in the lines of argument generally given to prove that the eurypterids were originally marine organisms. After a review of all the evidence available, the attempt will be made to judge it impartially and to determine which interpretation is really called for by the facts. The first chapter contains a record of facts, without comment; they are the data from which deductions are later made and of which interpretations are offered. These facts include: A, the distribution of all known species of eurypterids throughout the world; B, the horizons in which the remains occur, with particular reference to the fades exhibited and to the exact stratigraphical position; C, the mode of occurrence of the remains, whether well preserved or fragmentary, whether a single fragment or a large number of individuals; and finally, D, the other fossils, if there are any, which occur associated with the eurypterids. These facts are all summarized in tables I-III on pp. 37–49 and in the list of faunal associates on pp. 84–91. The second chapter is a résume of the various opinions which have been held regarding the habitat of the Eurypterida. The next three chapters (III, IV and V) deal with the three chief lines of evidence which may be adduced to determine any fossil habitat, namely, the bionomic characters of the faunas, the lithogenesis of the formations in which the remains occur, and the type of migration and dispersal, marine or fluviatile, indicated by the relations existing between species and genera in synchronous faunas and by the phyletic relationships in successive horizons. In these three chapters general principles are first discussed and criteria are established for recognizing various types of habitats, sediments, and fossil faunas; the application of these criteria to the eurypterid problem is then given in detail. The conclusion reached by the author after the study of all available data and in the light of manifold theoretic considerations is that: the eurypterids throughout their entire phylogenetic history lived in the rivers.
Aside from the work done on the literature, a large amount of material has been studied, including hundreds of typical specimens of eurypterids, thin sections of some of the waterlimes, the collections of the rock types from the eurypterid-bearing horizons of Europe collected by Professor A. W. Grabau and now in the Palaeontological Museum of Columbia University; further, a number of the best sections in the field have been visited. When the present paper was nearly finished there appeared Clarke and Ruedemann's exhaustive Monograph on the Eurypterida of New York (39), which, with Woodward's Monograph of the British Fossil Crustacea, gives us the most illuminating and comprehensive work on the Eurypterida. Many important points in the ontogeny and phylogeny of the eurypterids are here set forth for the first time, and all of the North American species are described in great detail and figured in a volume of plates that surpass all former illustrations.
I wish to express my thanks to Dr. R. Ruedemann, who allowed me to study the large collection of New York eurypterids at the State Museum in Albany; to Dr. C. D. Walcott, who showed me the large Beltina fauna and the beautiful specimens of Limulava from the Middle Cambric Stephen shale of Canada in the Smithsonian Institution at Washington, D. C; and to Mr. McIntosh at the Museum of the Natural History Society, St. John, New Brunswick, for information about the age of the Little River Plant beds and for the privilege of being allowed to inspect the type material from those beds.
To the courtesy and helpfulness of Mr. Henry R. Howland, Superintendent of the Buffalo Society of Natural Sciences, I owe the opportunity of studying every specimen of eurypterid in the museum of that Society. Furthermore, Mr. Howland loaned me a number of specimens to describe, and I was thus able to show the existence of two species of a pulmonale gastropod, Hercynella, in the Bertie waterlime. It was because of Mr. Howland's interest in papers dealing with the geological problems of the Buffalo region that the present contribution appears in the Bulletin of the Buffalo Society of Natural Sciences.
With the fullest appreciation for the inspiration and guidance which I have received, I gladly acknowledge my indebtedness to Professor Amadeus W. Grabau. He was one of the first to advocate the fluviatile habitat of the eurypterids and one of my earliest geological recollections was of a discussion between him and a number of men who argued for the marine habitat, a discussion to which I listened with the utmost interest although I was then not in a position to weigh the evidence brought forward on either side. More than four years ago Professor Grabau suggested that I take up the problem, with the purpose of marshalling all of the available evidence in proof of an hypothesis which he had strong theoretic reasons for believing to be true. Throughout the work I have profitted by the helpful criticisms and keen suggestions of a man who has made such problems his specialty for twenty-five years, and without whose assistance this paper certainly could not have been written. The method of treatment which I have used is based upon the principles of interpretational geology expounded in the Palaeontological Laboratory of Columbia University, and with the hope that this paper shall not prove unworthy of the teachings there set forth, I informally dedicate it to the American School of Philosophic Geologists, among the leaders of whom Professor Grabau stands so preëminent.
- ↑ This paper was awarded the Walker First Prize by the Boston Society of Natural History in May, 1914.
- ↑ Throughout this paper numbers in parentheses will refer to the bibliography at the end, p. 257; the full-face type referring to the titles with the same number, the light-face numbers giving page reference in the particular article.