Among the tiniest animals that live independently today are members of the plankton which inhabit the seas everywhere. This zooplankton as they are more properly called is very important to the whole life of the sea because, together with the tiny plants upon which they feed, the phytoplankton, they provide the nourishment for all the other animals which live in the sea. Even giant whales feed upon plankton. Let’s know more about which animals are among the tiniest that have ever lived?
For the geologist, however, there are other groups of microscopic animals (this means that they can only be microscopic animals (this means that they can only be clearly seen with the aid of a microscope) that are more important. Firstly, there is a group of animals called ostracods which range in size from one millimeter to two centimeters so that they could be seen with the naked eye.
Ostracods belong to the arthropods which include insects and spiders. Do you remember where you have seen this word before? Remember that insects and spiders are arthropods, but which fossil group do they belong to?
Ostracods are different from all the other arthropods in that they have an external hard shell in the form of two parts or valves rather like that of the bivalves that we have already mentioned. The actual animal living inside its shell is not unlike a modern lobster. As it grows, the ostracod sheds its shell in a series of molt stages. Ostracods all live in water, but it may be in stagnant fresh water or in rivers or in the sea.
In fact, a single species can tolerate quite a wide range of conditions. They mostly live in the bed of the bed of the sea, river or pond and like the trilobites they are scavengers. The first ostracods can be found in rocks that are 550 million years old and representatives of the group can still be found today.
Another group of microscopic animals that are of special interest to the geologist is the foraminifers. They are members of the Protozoa, which all consist of only one living cell, and they are important because they produce a skeleton which can be fossilized. They are all marine animals, but different types may have lived on the sea floor and some may have been free-floating.
The skeletons were made in three different ways: there were some that were made of calcium carbonate, some of the material called chitin, and others which made the skeleton from tiny sand grains from the seabed. Foraminiferids can be found from Ordovician to recent times, although it has been suggested that some forms may have existed in rocks of Cambrian age.
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