Monoplacophora, meaning "bearing one plate", is a polyphyletic superclass of molluscs with a cap-like shell now living at the bottom of the deep sea. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record, and were thought to have become extinct 375 million years ago.
Nautiloids are a large and diverse group of marine cephalopods (Mollusca) belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus and Allonautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.
Bellerophon is a genus of extinct paleozoic marine molluscs of uncertain position in the family Bellerophontidae.
The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups. The fossil record of mollusks is relatively complete, and they are well represented in most fossil-bearing marine strata. Very early organisms which have dubiously been compared to molluscs include Kimberella and Odontogriphus.
Mark A. S. McMenamin is an American paleontologist and professor of geology at Mount Holyoke College. He has contributed to the study of the Cambrian explosion and the Ediacaran biota.
Scenella is an extinct genus of fossil invertebrate animal which is generally considered to be a mollusc; at various times it has been suggested that this genus belongs with the gastropods, the monoplacophorans, or the helcionellids, although no firm association with any of these classes has been established. An affinity with the hydrozoa has been considered, although some authors oppose this hypothesis. A gastropod affinity is defended on the basis of six pairs of internal muscle scars, whilst the serially-repeated nature of these scars suggests to other authors a monoplacophoran affinity. However the specimens showing this scarring have not been convincingly shown to belong to the genus Scenella. A similarity to the Ediacaran Ovatoscutum has also been drawn.
Canadia is a genus of extinct annelid worm present in Burgess Shale type Konservat-Lagerstätte. It is found in strata dating back to the Delamaran stage of the Middle Cambrian around 505 million years ago, during the time of the Cambrian explosion. It was about 3 centimeters in length. Charles Doolittle Walcott named Canadia in 1911 after Canada, the country from which its remains have been found. 28 specimens of Canadia are known from the Greater Phyllopod bed, where they comprise 0.05% of the community.
Knightoconus antarcticus is an extinct species of fossil monoplacophoran from the Cambrian Minaret Formation of Antarctica. It is thought to represent an ancestor to the cephalopods. It had a chambered conical shell, but lacked a siphuncle.
Matthevia is a genus of Cambrian molluscs, perhaps related to the chitons. It consists of repeated monoplacophoran-like shells; according to one hypothesis, chitons arose when these tall shells began to overlap over the generations. The tall element of the shell was retained and forms the tips of modern chiton plates. There are distinct head, 'centre', and tail valves, which occur approximately in the ratio 1:5:1 — suggesting a seven-plated configuration.
Volborthella is an animal of uncertain classification, whose fossils pre-date 530 million years ago. It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica), and that it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. This illusion was a result of the laminated texture of the organisms' tests. Therefore, Volborthella’s classification is now uncertain. It has been speculated that it may in fact represent a sclerite of a larger organism, on the basis of one specimen; however, it may be premature to accept this hypothesis, as the arrangement of sclerites producing this impression may have occurred by chance. The Ordovician scleritome-bearing Curviconophorus, as well as the Halwaxiids, lobopods and echinoderms, demonstrate the diversity of organisms which may produce a scleritome of this nature. The related Campitius was originally suggested to be part of a radula rather than a scleritome, but is now considered a synonym of Volborthella.
Stenothecidae is an extinct family of fossil univalved Cambrian molluscs which may be either gastropods or monoplacophorans.
The Kirengellids are a group of problematic Cambrian fossil shells of marine organisms. The shells bear a number of paired muscle scars on the inner surface of the valve.
Salterella is an enigmatic Cambrian genus with a small, conical, calcareous shell that appears to be septate, but is rather filled with stratified laminar deposits. The shell contains grains of sediment, which are obtained selectively by a manner also observed in foramanifera. The genus was established by Elkanah Billings in 1861, and was named after the English palaeontologist John William Salter.
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, and purported stem-group representatives present in the earliest Cambrian lagerstätten.
Olenecoceras is a genus of Middle Cambrian fossils that, once believed to be a cephalopod, has since been excluded from that group. The genus was established by Zakhar Grigor'evich Balashov in 1966, from the river Olenyok.
Agmata is a proposed extinct phylum of small animals with a calcareous conical shell. They were originally thought to be cephalopods or annelid worms. The living animals filled up to five-sixths of their shell with laminae, angled layers composed of grains of quartz or calcium carbonate detritus from the environment cemented together, with larger grains near the shell wall and smaller grains near the center. A very fine tube ran through the center of the shell. The grains may be of quartz or calcium carbonate, but are of specific shapes and materials that are rare in the surrounding rock. Though the body of the living animal is not preserved, it had to be able to find, choose, and retrieve rare grains from its environment to build the laminae.
Salterellidae is a family of enigmatic fossil genera from the Early to Middle Cambrian. It was originally created for the genus Salterella by Charles Doolittle Walcott, who placed it in the group Pteropoda. It was later placed in Agmata, a proposed extinct phylum by Ellis L. Yochelson which is accepted by some other authors.
Proaulopora is a Cambrian–Ordovician fossil genus of calcareous algae. It has been variously thought to belong to the green algae, red algae or cyanobacteria. It was originally established by the Russian paleontologist Aleksandr Grigoryevich Vologdin in 1937, for species known from the Lower Cambrian of the western Altai Mountains.
Ellisell is a Middle Cambrian genus of fossils from Denmark. It contains only one species, Ellisell yochelsoni. Both the genus and species are named after the paleontologist and geologist Ellis L. Yochelson (1928–2006), who had turned 60 at the time the fossils were first described. The genus was originally placed in the family Salterellidae of the phylum Agmata; this placement was rejected by Yochelson & Kisselev (2003), but was restored by Peel (2016). Ellisell is distinguished from Salterella by its slowly expanding conch and the resulting cylindrical apertural cavity, compared to the latter's more rapidly expanding conch and cone-shaped apertural cavity.
Omphalotrochus is an extinct taxonomic genus of Paleozoic gastropods. It is widely distributed in the upper Paleozoic and has been an important index fossil.
