Evolution & Systematics

During the Cretaceous period decapod species were highly diverse with most forming an association with coral, this diversification was caused by the expansion of reef environments in the late Jurassic (Klompmaker, 2013). This is similar to what we see today with most decapods found in the environment of a coral reef (Klompmaker, 2013).  The knowledge of the early Mesozoic crabs however, is poor due to the limited preservation potential of early decapods with the carapace and appendages not often found together(Krobicki & Zanton, 2008).

The first evidence of brachyuran species appeared in the fossil record around 200 million years ago, in the Mesozoic Jurassic period (Garth& Abbott, 1980). It has been suggested that the brachyurans or the true crabs, originated from the extant family known as Propsopidae (Krobicki & Zanton, 2008). Some of the oldest known species of this family are believed to have lived on a silty sea floor and in shallow warm waters within organic buildups or shelly accumulations (Krobicki & Zanton, 2008).

Throughout the fossil history the general trend in evolution of the Brachyurans sees a reduction in body length (Garth & Abbott, 1980). We see fusion of the head and thorax to form a cephalothorax and covered by the hard exterior carapace. Also noticeable is the reduction in abdomen length, hence why the group is called Brachyura or ‘short tailed (Garth & Abbott, 1980).

A recent ichnology trace fossil study (Semeniuk & Unno, 2014) was conducted on the Western Australian solider crab species Mictyris occidentalis. Whilst the information is not relevant to M. longicarpus history, the idea of looking for trace fossils in the form of burrows and pellets is an idea that could be used in the future if the situation arises for M. longicarpus. These types of fossils helped researchers gather a greater understanding of the evolution of certain behaviours as well as a better understanding of population structure, ecology and the age of populations (Semenjuk & Unno, 2014).