Chapter XII: Crustacea of the Past

Since the acceptance by naturalists of the theory of Evolution as indicating the mode of origin of the various forms of life now existing, one of the chief lines of biological investigation has had for its object the reconstruction of the pedigree (or, as it is called, the "phylogeny") of the larger groups of the animal and vegetable kingdoms. In attempting to do this, there are three main sources from which evidence may be drawn. The results of Comparative Anatomy enable us to decide with more or less confidence as to the degrees of relationship between the groups of organisms, and to distinguish between the more primitive and the more specialized; the study of Embryology is, at least, an indispensable adjunct to Comparative Anatomy, even if it does not, as was once supposed, give us an actual recapitulation of ancestral history; and, finally, the study of Fossil Remains holds out the hope that we may be able to find the ancestral types themselves.

It is clear that evidence from the last-named source, when it is available, is the most important of all, since the order of succession of the various types is given by that of the rock strata in which they occur, and we can be quite certain that we are dealing, if not with the actual ancestors, at least with the forerunners of existing species. The "imperfection of the geological record," however, is so great that the organisms preserved in the fossil state represent only an insignificant part of the whole number of organisms that have lived on the globe since life began; and it is not surprising, therefore, that in many groups the study of fossils has hitherto afforded little help towards the working out of their genealogical history. Thus, among Crustacea there are many important groups such as the Copepoda, which are entirely unknown as fossils, their small and delicate bodies being ill adapted for preservation, although there is every reason to suppose that they are a very primitive and very ancient group. In many fossil Crustacea only the hard shell or carapace has been preserved, the appendages being lost or represented only by indecipherable fragments, and in some cases it is hardly possible to guess at the affinities of the animals. Further, several important groups are already represented in some of the oldest of the fossil-bearing rocks at present known, and the differentiation of these groups must have taken place in the dark ages before the record of the fossils begins. In spite of these disadvantages, however, the study of fossil Crustacea does throw considerable light on the evolution of the group, and it is likely that interesting results in this direction await future investigations.

In the earliest fossiliferous rocks the most abundant and important Arthropods are the Trilobites (Fig. 81), an extinct group which appears to have been related to the primitive Crustacea. The name Trilobite refers to the three-lobed form of the body when seen from the dorsal side, most species having a pair of grooves running lengthwise which divide off a middle lobe containing the principal organs of the body from two lateral "pleural" expansions covering the limbs. The head-shield shows indications of being composed of five segments, and bears a pair of sessile compound eyes. It is followed by a number (up to twenty-six) of free somites, and the body ends in a tail-shield, or "pygidium," which is often plainly composed of several somites fused together. Although Trilobites are among the commonest and most familiar of fossils in the older rocks, the nature of their appendages remained quite unknown until within recent years, when specimens of several species showing the structure of the limbs and under-side of the body were discovered in America. From these it appears that the head bore in front a pair of long thread-like antennæ and four pairs of two-branched appendages, each with a jaw process, or "gnathobase," turned towards the mouth, which is covered below by a large anterior lip, or "hypostome." It seems probable that the five pairs of head-appendages correspond respectively to the antennules, antennæ, mandibles, maxillulæ, and maxillæ, of Crustacea; but the second pair appear to have acted as jaws, retaining the gnathobase which, among Crustacea, is only hinted at by the hooked spine on the antenna of the nauplius larva.

Each of the free somites and of those forming the tail-shield bears a pair of two-branched appendages, not differing greatly from the posterior appendages of the head, but becoming smaller and more flattened towards the hinder end of the body. The numerous genera and species of Trilobites present great differences in the form and ornamentation of the dorsal surface of the body, and it is probable that considerable differences may also have existed in the structure of the limbs, which are only known in two or three species. Some Trilobites are among the most ancient of known fossils, being found in rocks of the Lower Cambrian epoch. The group reaches its maximum development in the Ordovician, and the number of the species and size of the individuals gradually diminish through the Silurian and Devonian till they become extinct at the close of the Carboniferous epoch, except for a single species found in rocks of Permian age in America.

Although zoologists are not all agreed as to the precise systematic place to be assigned to the Trilobites, there can be little doubt that they were related more or less closely to the most primitive Crustacea, and they are of special interest as preserving for us the stage in which the second pair of appendages were still used as biting jaws, and had not moved forwards in front of the mouth to become antennæ, as in all living Crustacea.

Contemporary with some of the earliest Trilobites, however, are undoubted Crustacea, which, so far as we know their structure, are not very different from types now living. In the Cambrian epoch the Branchiopoda appear to be represented by _Protocaris_, which in its general form resembles _Apus_; and there are a variety of genera and species of Ostracoda, although, since their shells alone are preserved, it is not possible to determine their exact relations to existing forms. In the succeeding Ordovician and Silurian epochs we first meet with the remains of Barnacles, and it is interesting to note that some of them are referred to the genera _Pollicipes_ and _Scalpellum_, which are represented by numerous species in the seas of the present day. Along with these, however, are some strange-looking forms (_Turrilepas_, etc.), having the body covered with rows of overlapping plates. If these are really Cirripedes, they must have differed considerably in structure from the more modern types.

The Malacostraca are more interesting from the point of view of palæontology than the other subclasses of Crustacea, since the evolution of the group appears to have taken place within the period covered by the fossil records, and it is possible to trace the course of that evolution--at least, in its broad outlines. It has already been pointed out that the most primitive of existing Malacostraca are the Phyllocarida (_Nebalia_ and its allies), which are in several respects intermediate between the higher Malacostraca and the Branchiopoda; and it is interesting to find that fossils apparently belonging to the Phyllocarida are found far earlier than any of the other Malacostraca. In the Cambrian, and more abundantly in the Ordovician and Silurian, there are found Crustacea (Fig. 82) that resemble _Nebalia_ in having a large bivalved carapace, with a movable beak-like plate in front, a projecting abdomen without conspicuous limbs, and a pair of large spines at the sides of the telson. Unfortunately, we have almost no knowledge of the structure of the limbs; but it can hardly be doubted that these very ancient Crustacea were allied to the existing Phyllocarida, and that they included the forerunners of the higher Malacostraca.

_a_, Traces of antennules; _m_, possibly mandibles; _r_, rostral plate]

It is in the Carboniferous epoch, in all probability, that we must look for the origin of most of the existing orders of Malacostraca. In the rocks of this age in different parts of the world there have been found a number of undoubted Malacostraca, nearly all of the shrimp-like form which there is good reason to believe to be a primitive characteristic. Some of these (_Pygocephalus_--Fig. 83) have recently been shown to possess a brood-pouch formed of overlapping plates on the under-side of the thorax, and thus resemble the existing Mysidacea, which stand at the base of the Peracaridan series of orders. Others have a pair of strong side-spines near the tip of the telson, and in other ways resemble the recent Euphausiacea, so that they may have been primitive members of the Eucaridan series.

_c.gr._, "Cervical groove," marking off the first thoracic somite; ii-viii, the remaining thoracic somites; 1-6, the abdominal somites]

Among the Crustacea of the Carboniferous and Permian epochs, there are a number of forms of which the affinities were until recently quite obscure. They have two-branched antennules, a scale-like exopodite on the antenna, and the last pair of appendages (uropods) form, with the telson, a tail-fan. In these points they resemble the shrimp-like forms, but there is no carapace, and all the somites of the thorax are distinct, so that the form of the body is rather that of an Amphipod or Isopod. On the discovery of the remarkable Crustacean _Anaspides_ (Fig. 84), which lives in fresh-water pools in the mountains of Tasmania, it was pointed out that it agreed with the fossil genera _Uronectes_, _Palæocaris_, and their allies, in those very characters in which they differed from all other Crustacea, and that it must be regarded as a surviving representative of the ancient group to which the name of Syncarida had been given. The more recent discoveries of living forms, _Paranaspides_ from the Great Lake of Tasmania and _Koonunga_ from fresh-water pools near Melbourne, and of the fossil _Præanaspides_ (Fig. 85) from the Coal-measures of Derbyshire, have tended to support this conclusion. There can be little doubt that the Syncarida arose during the Carboniferous epoch (or earlier) from primitive shrimp-like forms which lost the carapace; but, after flourishing for a relatively brief period, the group dwindled away, although a few survivors have lingered on, like so many other "living fossils," in the isolated Australian region.

It must be pointed out that, in spite of the resemblance of the body of _Anaspides_ to that of an Amphipod, the Syncarida can have had no close relation to the origin of the Isopoda and Amphipoda. These have also been derived from a shrimp-like type, but their possession of a brood-pouch, among other characters, shows that they are linked on to the Mysidacea, and must have arisen from some primitive member of that group, like _Pygocephalus_. Although palæontology as yet gives little help in tracing the course of their evolution, we can imagine what the intermediate links must have been like by comparison with the living Cumacea and Tanaidacea.

It is possible, indeed, that the divergence of the Isopod line of descent from that of the Mysidacea took place earlier than the Carboniferous epoch, for there has recently been discovered in rocks of Devonian age in Ireland a single specimen of a fossil, to which the name of _Oxyuropoda_ has been given, which has every appearance of being an Isopod. At all events, undoubted Isopods make their appearance in rocks of the Secondary Period, and some of those from the Jurassic epoch are not very different in general form from types still existing.

Some of the Carboniferous shrimp-like Crustacea present characters which seem to point in the direction of the Stomatopoda, and fossils which clearly belong to that group are found in Jurassic and later deposits. In the Cretaceous epoch there were Stomatopoda resembling modern types so closely that they have been referred to the existing genus _Squilla_. We are even able to say that they resembled the living Stomatopoda in their mode of development, for larvæ of the type known as _Erichthus_ have been recognized in rocks of Cretaceous age from Lebanon. This is a striking example of the way in which, by a fortunate accident as it were, organisms apparently ill-adapted for fossilization may occasionally be preserved.

Of the Decapoda the geological history is tolerably full, and it is possible to trace in its broad outlines the course of evolution of the various suborders. Here again it is likely that the beginnings of the group are to be sought for in the Carboniferous epoch, and some of the obscure shrimp-like forms of that age show hints of an affinity with the Decapods. In the Triassic epoch, however, and more abundantly in the succeeding Jurassic, a number of types are found which seem to include primitive representatives of several of the existing groups of Natantia and Reptantia. It is noteworthy that among them are some forms (_Æger_, etc.) resembling the existing Stenopidea, a tribe which in some respects is intermediate between these two suborders. In the Stenopidea the first three pairs of legs bear pincer-claws, as in the Lobster, but the third pair is much the largest; and _Æger_ agrees with them in this unusual character, though there is little else, in what is known of its structure, to help to determine its affinities.

The tribe Penæidea, which occupies in many respects a primitive place among the Natantia, is abundantly represented in the Jurassic epoch, especially in the lithographic stone (Upper Jurassic) of Solenhofen, and by somewhat doubtful specimens from the earlier Trias. All these agree in having the first three pairs of legs with pincer-claws, and not differing greatly in size. Some of the Jurassic and later fossils are of so modern a type that they have been referred to the existing genus _Penæus_.

The Upper Jurassic rocks also preserve the earliest undoubted specimens of true Prawns of the tribe Caridea, and some of these show swimming branches (exopodites) on the thoracic legs, so that they were probably related to the primitive family Acanthephyridæ, of which the existing members are found in the deep sea. It is possible, however, that Caridea were already in existence far earlier, for some of the obscure Carboniferous forms seem to have the broadened side-plates of the second abdominal somite, which, so far as we know, are exclusively characteristic of that tribe.

The Reptantia, forming the other large division of the Decapoda, also had their origin at least as early as the Triassic epoch, where representatives of the tribes Eryonidea and Scyllaridea are found. The history of the Eryonidea has already been discussed (p. 133) in dealing with the deep-sea Crustacea. The oldest representatives of the Scyllaridea belong to a family (Glyphæidæ) now wholly extinct, and are in many respects more primitive and lobster-like than any of the living Spiny Lobsters and their allies (Palinuridæ and Scyllaridæ). Forms with greatly thickened antennæ, indicating a transition to the Palinuridæ, begin to appear in the Jurassic; and in the later Cretaceous a genus, _Podocrates_, occurs which is hardly to be distinguished from _Linuparus_, now living in Japanese seas. The Scyllaridæ have the antennæ modified into broad shovel-like plates, and perhaps take their origin from _Cancrinus_, in the Solenhofen lithographic stone (Jurassic), which has broad and apparently flattened antennæ. True Scyllaridæ are certainly found in Cretaceous deposits, and some, from the Upper Chalk, are even referred to the existing genus _Scyllarus_.

The Anomura are almost unknown as fossils, but the true Crabs, or Brachyura, are abundantly represented. They first appear about the middle of the Jurassic epoch, and, as already pointed out, the earliest forms (Prosoponidæ) are referred to the Dromiacea, and appear to be closely related to the primitive Homolodromiidæ now living in the deep sea (p. 134). One of the oldest, and at the same time one of the most completely known, is _Protocarcinus_, from the Great Oolite of Wiltshire, which is preserved (in the only known specimen) with the abdomen partly extended, possibly indicating that the abdomen was less closely doubled under the body than in modern Crabs.

The next group of Crabs to appear are the Oxystomata, which are found from the middle of the Cretaceous epoch onwards. The Brachyrhyncha perhaps begin to appear about the same time, but the affinities of the earlier types are doubtful, and it is only in the Tertiary that they become abundant and unmistakable. Several living genera, such as _Cancer_, date back to the Eocene. The Spider Crabs (Oxyrhyncha) are rare as fossils, and the earliest specimens are found near the beginning of the Tertiary.