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.