Agmata is a proposed extinct

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

of small

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s with a

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

conical shell. They were originally thought to be

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s or

annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

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. The phylum's name comes from the Greek word for "fragments", referring to these fine fragments and grains of detritus. It was proposed by the paleontologist and geologist (1928–2006) in 1977 to house the agglutinating Early Cambrian fossils ''

Salterella ''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 (with ...

'' and ''

Volborthella ''Volborthella'' is an animal of uncertain classification, whose fossils pre-date . It has been considered for a period a cephalopod. However discoveries of more detailed fossils showed that ''Volborthella''’s small, conical shell was not secre ...

'', with the

Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...

'' Ellisell yochelsoni'' later included. The poorly known Middle Cambrian fossil '' Vologdinella'' was also considered for inclusion, as it has superficial resemblance to the Agmata, but was later excluded from the group. Currently, the phylum contains only one family,

Salterellidae 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 ...

; a second family, Volborthellidae, was originally included but later became a synonym of the former. No orders, classes or superfamilies are used within the phylum, despite the order "Volborthellida" being previously proposed for ''Volborthella'' before the phylum's own proposal. The reasoning for this was that taxa of these ranks were not seen as necessary in a phylum with very few genera. The genera within the group are clearly different: ''Salterella'' had a pointed shell made of calcium carbonate with a thin outer layer and a thick inner layer, and secreted a calcium carbonate cement to hold its grains in place. ''Volborthella'' was the older genus, had a blunter shell with a shallower opening, and cemented grains in place with organic material that also may have formed the outside surface of the shell. Fossils are found in large numbers in some areas. Paleontologists have offered several different ideas of how these animals lived: filter-feeding with the points of the shells embedded in the substrate, grazing actively like snails, or lying on their sides on the substrate. ''Volborthella'' is found in silt and clay deposits, and apparently lived on tidal mudflats. Attempts to reconcile these genera as members of any other group have been rejected due to basic differences in structure, but not all paleontologists accept them as a phylum; Jones (2007) considers the shell an agglutinating test parallel to that of

foraminifers Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an ...

. Yochelson considered Agmata to be complex multicellular animals.

References

Cambrian fossil record Animal phyla Controversial taxa {{Cambrian-animal-stub