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Fossil record and evolution


Origins

The origin of decapods is poorly understood, but a fossil group of crustaceans known as the Phyllocarida are thought to hold the key. These bear a resemblance to the Decapoda and Euphausiaceae, with a long abdomen bearing pleopods (although this is not quite as well developed) and a bivalved carapace covering the thorax. Like decapods, this carapace has a series of transverse grooves. Although Echinocaris, a Devonian phyllocarid, shows the post-cervical and cervical grooves, it lacks the branchio-cardiac groove (instead possessing an anterior groove [v]) and this is thought to indicate that it is more primitive than true decapods. But it is on its way there!

There is some difficulty in using carapace grooves only. Palaeopemphix was originally considered to belong to the Decapoda, with three transverse grooves clearly present. However, the carapace was weakly hinged, with a broad reflex in the surface (a doublure), and this excludes it from the Decapoda, placing it instead within the Phyllocarida. Clearly more work has to be done on groove identification before relationships among these early forms can be better understood.

The Earliest Decapod

The earliest decapod, Palaeopalaemon from the Devonian of the USA, lived 100 million years before the Mesozoic radiation of decapods. It is known from a very well preserved specimen (seen in various views, left) and all the major carapace grooves may be identified. In addition, it lacks exopod-bearing pereiopods and has five pairs of pereiopods.

Despite the very early occurence of Palaeopalaemon, it has rather advanced characteristics. For example, it has a reptant macrurous condition and, contradicting early hypotheses, the legs of this animal are differentiated, some being chelate, while others are achelate.

Trends in Decapod Evolution

Even though Palaeopalaemon was a crawler, it is still thought that the first decapods were swimmers. For swimmers, the escape mechanism is a powerful contraction of the abdomen, which results in a very rapid backward movement. However, as a more benthic lifestyle is adopted, the large abdomen (pleon) actually becomes a hindrance. There is therefore a reduction and loss of the pleopods, and the abdomen (while never actually lost) is greatly reduced and held under the body where it is used only in reproduction (as in the crabs). For some time the origin of crabs was unknown, but in the 1930s the missing link, Eocarcinus, showed that it must lie among the Pemphicidea, an extant group of lobsters.

Diagram showing the major changes in body shape through time, from top to bottom

When looking at the carapace grooves, many changes were associated with the increasing complexity of decapods throughout their evolution. As the walking legs of decapods become specialised (or at least differentiated) with the adaptation towards a benthic mode of life, the lower surface of the carapace had to expand. The carapace grooves then crowded together. As the thoracic somites also adapted to these changes, the increasing nonfunctional areas became reduced and this may have led to a posteriorly directed elongation of the branchio-cardiac groove. Finally, as the attractor epimeralis muscle became concentrated into a smaller space, the anterior and lateral margins of this groove became reduced or even disappeared completely.