Fossil Crabs

The decapods or "ten footed" crustaceans, an Order including crayfish, crabs, lobsters, prawns and shrimp are known from the Permian Period. However true crabs evolved some 150 million years ago in the Jurassic Period. There are hundreds of Species of fossil crabs known, many  have modern relatives and some are only known in the fossil record.

Fossil Raninids along with Longusorbis cuniculosus are known from the western Canada.

The general structure of crabs has not changed much since the first appearance in the fossil record.  Modern crabs still show the same variations of limb to carapace proportions and ornamentation found in the fossil record. Fossil crabs even seem to have been plagued by the same things as there modern cousins. Evidence of Saculina, a parasite that in effect castrates the male has been found in a crab from the Miocene. Even 20 million years ago, parasites were specialized enough to make a male crab act like a female so it would care for the parasites continuing broods. A number of fossil crabs have also shown evidence of a parasitic bopyrid isopod. Unlike saculina, a shelless barnacle, the isopod also attacks the female causing reproductive death as it leeches nutrients from the crab.

Extant Dungeness crabs can live up to 10 years and as young crabs they may moult as often as every 2 months. Upon maturity, at about the 3 year mark the moults decline to about once a year. Exuvia, or crab moults are common on beaches at certain times of the year, Females in  spring and Males in the late summer. When the crab backs out of the exuvia it leaves behind everything external, even it's old gills. If the carapace were to drop back into it's original position, and the moult is preserved as a fossil it would be almost indistinguishable from a dead crab. The only way to tell fossil exuvia from a dead crab is the moult line, or  the seam where the carapace joins the lower part of the crab is broken. Most females moult shortly after mating, never to mate again she generally will not grow much bigger either; there are exceptions though such as the Fiddler Crab which will mate regularly throught it's short life. The male can mate again and although the moulting is less frequent it will grow bigger. The crusher claw often grows proportionally bigger slowly changing shape, this is why older males of some species are much larger than the females with larger and different shaped claws than other males. Older crabs can die when moulting because the have lost flexibility and get stuck in the exuvia.  All crabs will attempt to protect themselves right after moulting, burying themselves in sand or hiding, hermit crabs (not true crabs) will moult inside there habitat shell throwing out the moult. Burrowing crabs will moult inside the protection of there borrows and some modern species have been shown to close off the entrances before moulting.

 

This Avitelmessus grapsoideus is from a site that unfortunately is lost to science like so many others.

Today's littoral zone, or from the sea shore to the edge of the continental shelf is rich in crabs. They are are also common in the Tertiary and Upper Cretaceous littoral zones, however crabs are less common in the Lower Cretaceous and rarer still in the Jurassic.  Crabs living on rocky or turbulent shores have little chance of anything more than fragmentary preservation, while crabs living  on softer substrates such as sand or clay are more common in the fossil record.  Burrowing crabs from the littoral zone are not  common in fossil record,  as most  do not create complicated lasting burrows but are more likely to dig in for temporary concealment.  Even with modern crab burrows for examples it is difficult to make an accurate identification as to the owner of a fossil burrow.  Fiddler crabs are known burrowers and the male and female burrows can vary in shape and depending on the need. A simple "hide" burrow is straight down but not deep,  a home burrow can be up to a meter ending in a small room, some with more than one level and they may even be interconnected.  Some tunnelling crabs create complex networks of passages covering large areas.  The only way to confirm the owner of a burrow is to find more than one burrow with the same species of owner entombed in each.  It should also be considered that infill is made of whatever is washed into a burrow, so it cannot be completely trusted as it could contain almost anything from the surrounding environment.

The fossilization process of crabs has a do with the crab as well as the environment. The chitin or the exoskeleton of a crab is subject to slow bacterial decay. The quality of preservation can be effected by rapid burial , anaerobic environment and absence of scavengers. Crabs from the intertidal zone where sediments are continually reworked are less likely to be preserved in completeness as the carcass disarticulates relatively quickly leaving only the heavily calcified claws recognizable. However you look at it a crab exoskeletons are amazingly complex structures that fortunately preserve very well...sometimes. 

 

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