INTRODUCTION—DIVISION I. ECARDINES—EXTERNAL CHARACTERS—INTERNAL CHARACTERS—DIVISION II. TESTICARDINES—EXTERNAL CHARACTERS—INTERNAL CHARACTERS—SYNOPSIS OF FAMILIES—STRATIGRAPHICAL DISTRIBUTION—PHYLOGENY AND ONTOGENY

Introduction

The wide distribution and vast abundance of the Brachiopoda throughout the whole series of geological formations make this group of especial importance to the student of the past history of the earth; and the zoologist must always regard the fossil forms with peculiar interest, because they not only largely outnumber the living representatives, but comprise numerous extinct genera, and even families, exhibiting types of structure and characters entirely absent in the modern members of the group. It is a most fortunate circumstance that the excellent state of preservation in which we frequently find them, and the immense amount of material at our disposal, enable us to determine with accuracy and certainty the internal characters of the shells in the great majority of cases. But it is only since the beginning of the present century that our knowledge of the anatomy of the soft parts of the living animal has rendered any tracing of homologies possible. In the case of features in fossil extinct types the interpretation must be to some extent doubtful. Barrande, Clarke, Davidson, Hall, King, Oehlert, Waagen, de Verneuil, and a host of other workers have contributed to the information which we now possess; and their works must be consulted for details of the subject.[428]

Since all Brachiopods are inhabitants of the sea, the geologist at once recognises as a marine deposit any bed which contains their remains. Under favourable conditions they swarmed in the seas of Palaeozoic and Mesozoic times. Beds of limestone are frequently almost entirely composed of their shells, as, for instance, some of the Devonian limestones of Bohemia. Often they give the facies to the fauna and outnumber in species and individuals all the other organisms of the period. The Ungulite Sandstone (Cambrian) of Russia and the Productus Limestone of the Salt Range in India of Carboniferous and Permian age are well-known examples.

Many species seem to have been gregarious in habit; thus Productus giganteus of the Carboniferous Limestone may generally be found in crowded masses, as in some localities in Yorkshire.

The fact that certain species of Brachiopods characterise definite stratigraphical horizons or “zones” gives them occasionally an importance equal to that of Graptolites; for instance, the Ecardinate species Trematis corona marks a set of beds in the Ordovician, and the isolated Stringocephalus Burtini is restricted to the upper part of the Middle Devonian, giving to the limestone on that horizon its distinctive name. It is noteworthy also how certain species affect a sandy and others a calcareous sea-bottom, so that beds of the same age show differences in their Brachiopod fauna owing to a dissimilar lithological composition.

While few of the recent Brachiopods reach a large size, some of the extinct species measure several inches in breadth, but the great Productus giganteus attained the width of even a foot.

The bright colours of the shells of the living animals are not generally preserved amongst the fossil species from the older rocks; yet in a Carboniferous Terebratula we can even now detect the purple bands in some specimens, and a Cretaceous Rhynchonella similarly exhibits its original colour.

The Brachiopoda are evidently a group in its decline, as the geological record shows; but they date back from the earliest known fossiliferous rocks, in which the Ecardinate division is alone represented. As we ascend through the stratigraphical series the number and variety of genera and species belonging to both divisions rapidly increase until in the united Ordovician and Silurian there are nearly 2000 species and about 70 genera. From this point of maximum development down to the present day there is a gradual decrease in numbers.

According to Davidson, at least 17 Upper Tertiary species are still living on our sea-bottoms; and many recent Mediterranean forms occur in the Pliocene rocks of the islands and shores of that sea, and in the Crags of East Anglia.