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It is claimed to be the oldest animal fossil, being found in rock aged between 760 and 550 million years ago. The genus was named after the Otavi Group in Namibia in which the fossils were found. The oldest fossils are from the Tonian period, before the Cryogenian glaciations, but the latest found were from the Nama Group rocks, which are from ...
Pleurotomariidae, common name the "slit snails", is a family of large marine gastropods in the superfamily Pleurotomarioidea of the subclass Vetigastropoda. [1] This family is a very ancient lineage; there were numerous species in the geological past. The genus includes several hundred fossil forms, mostly Paleozoic.
Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Another term, "zone fossil", is used when the fossil has all the characters stated above except wide geographical distribution; thus, they correlate the surrounding rock to a biozone rather than a specific time period.
Volborthella, some fossils of which predate , was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are.
This list of marine gastropod genera in the fossil record is an attempt to list all the genera of sea snails or marine gastropod mollusks which have been found in the fossil record. Nearly all of these are genera of shelled forms, since it is relatively rare for gastropods without a shell ( sea slugs ) to leave any recognizable traces.
[11] [12] The total number of living species of freshwater snails is about 4,000. [13] Recently extinct species of gastropods (extinct since 1500) number 444, 18 species are now extinct in the wild (but still exist in captivity), and 69 species are "possibly extinct". [14] The number of prehistoric (fossil) species of gastropods is at least ...
Ammonoids are excellent index fossils, and they have been frequently used to link rock layers in which a particular species or genus is found to specific geologic time periods. Their fossil shells usually take the form of planispirals , although some helically spiraled and nonspiraled forms (known as heteromorphs ) have been found, primarily ...
The oldest fossil cerionid is C. acherontis from the Upper Cretaceous Hell Creek Formation, in Montana, northwestern USA. [5] The second oldest record is the genus Brasilennea from the Brazilian Paleocene Itaboraí Basin , in Rio de Janeiro.