Oxford Geology Group logo

GEOLOGY | FOSSILS | key fossil groups

OSTRACoDS

Ostracods (formally called Ostracoda) take their name from the Greek ostrakon, which means ‘a shell’, and refers to the characteristic bi-valved carapace which is easily fossilised.

 

Ostracods are likely to have evolved by the end of the Cambrian Period and true fossil ostracods are found in early Ordovician Period rocks, later spreading and diversifying into deep oceanic as well as continental environments such as lakes and rivers.  The virtue of a long and rich fossil record and global distribution, make ostracods extremely useful for determining the age of the sedimentary rocks in which they are fossilised, as well as providing clues to the nature of the prevailing palaeo-environmental conditions during which the sediments were deposited.

 

Modern ostracods are globally widespread and diverse, inhabiting almost every kind of aquatic environment from the deep oceans to freshwater streams and ponds.  A few species can be found crawling around on land in moist habitats, such as wetland mosses.

Fossil ostracods are used by geoscientists to relatively dating of the rocks in which they are found and enable correlation to be made. Fossil ostracods also tell us about the environment in which the sediments accumulated, because different types of ostracod lived in different types of environment.

Bioluminescent is a feature of some species of Ostracod. It is said that during World War II, Japanese soldiers and sailors would keep cultures of glowing ostracods in bowls of water so that they could use the light to read their maps and instruments, while remaining concealed from opposing forces.

All species of ostracod can be placed within one of six orders. Three orders are known only from the fossil record, but species of the other three exist today, hundreds of millions of years after they first evolved.

Males of specific species of ostracods are not represented in the fossil record. It is thought that reproduction for them was by parthenogenesis. 

A senior geoscientist reported that he once sampled the guts of a 700 g brown trout that contained an estimated 150 000 ostracods, all of the species Heterocypris reptans. Being a keen fisherman, he made an artificial ‘fly’ to imitate this ostracod species and succeeded in catching seven brown trout.