Hematitida is a group of

coleoid Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose ...

cephalopods known from the early

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

Period. They are the oldest definite coleoids, although there are controversial claims for even older coleoids from the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

. Fossil hematitidans have so far been found only in

Arkansas Arkansas ( ) is a landlocked state in the South Central United States. It is bordered by Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, and Texas and Oklahoma to the west. Its name is from the Osage ...

and

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

of the United States. The only family described so far is Hematitidae.

Characteristics

Some features shared by hematitidans include * a short rostrum, made of

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...

and organic material, and bearing ridges * a short

living chamber The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods In gastropods, the ...

, only 1.5 to 2 times the length of the last chamber of the

phragmocone The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae. In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camarae are linked by a ...

* a spherical

protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called ...

* a narrow, straight

* a multi-layered conotheca - the outer wall of the phragmocone * a narrow, ventral

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and ...

Classification

The classification for this group comes from Doguzhaeva ''et al.'' 2003 Order Hematitida Doguzhaeva, Mapes, & Mutvei, 2002 * Family Hematitidae Gustomesov 1976 ** ''Hematites'' Flower & Gordon 1959 - type genus ** ''Bactritimimus'' Flower & Gordon, 1959 ** ''Paleoconus'' Flower & Gordon, 1959

References

{{Taxonbar, from=Q16982961 Cephalopod orders Belemnoidea