Crinoids
Crinoid's scheme
Crinoids are marine animals that make up the class Crinoidea, one of the classes of the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins and sea cucumbers. Those crinoids which, in their adult form, are attached to the sea bottom by a stalk are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, being members of the largest crinoid order, Comatulida.
Adult crinoids are characterised by having the mouth located on the upper surface. This is surrounded by feeding arms, and is linked to a U-shaped gut, with the anus being located on the oral disc near the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognised, in most crinoids the five arms are subdivided into ten or more. These have feathery pinnules and are spread wide to gather planktonic particles from the water. At some stage in their lives, most crinoids have a stem used to attach themselves to the substrate, but many live attached only as juveniles and become free-swimming as adults.
There are only about 600 living species of crinoid, but the class was much more abundant and diverse in the past. Some thick limestone beds dating to the mid-Paleozoic to Jurassic eras are almost entirely made up of disarticulated crinoid fragments.
Fossils
A fossil (from Classical Latin: fossilis, literally 'obtained by digging') is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.
Palaeontology is the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils. The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystallization, adpression, carbonization, and bioimmuration.
Fossils vary in size from one-micrometre (1 µm) bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites). These types of fossils are called trace fossils or ichnofossils, as opposed to body fossils. Some fossils are biochemical and are called chemofossils or biosignatures.
ref: wikipedia
|