Alokistocaridae is a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of ptychopariid

trilobites Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

that lived from the

Botomian Cambrian Stage 4 is the still unnamed fourth stage of the Cambrian and the upper stage of Cambrian Series 2. It follows Cambrian Stage 3 and lies below the Wuliuan. The lower boundary has not been formally defined by the International Commission on ...

epoch of the Early Cambrian until the Late Cambrian. Alokistocarids were particle feeders and left small furrows which are occasionally preserved.Coppold, Murray and Wayne Powell (2006). ''A Geoscience Guide to the Burgess Shale'', p.54. The Burgess Shale Geoscience Foundation, Field, British Columbia. . Their remains are found worldwide. ''

Elrathia kingii ''Elrathia'' is a genus of trilobite belonging to Ptychopariacea known from the mid-Cambrian of Laurentia (North America). ''E. kingii'' is one of the most common trilobite fossils in the USA locally found in extremely high concentrations within ...

'', one of the most collected trilobites in the world, is a typical alokistocarid.

Description

Alokistocarids have an exoskeleton that is elongated ovate to inverted egg-shaped. The headshield (or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

) is semicircular and has a well-defined border. The central raised area of the cephalon (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

) is somewhat tapering forward, generally with 3 or 4 pairs of more or less distinct lateral furrows. The front of the glabella is rounded or truncate, and is separated from the border by a wide, moderately convex to flat (or rarely concave) so-called preglabellar field. Narrow ridges that connect the eyes with the glabella are well developed. The distance between the eye and the glabella (or palpebral lobe) is small. The fracture lines (or sutures) that in

moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...

separate the fixed from the free cheeks (fixigenae and librigenae) may converge or diverge in front of eyes. Behind the eyes they cut the posterior margin of the cephalon inside the inner bend of the genal spine (or opisthoparian sutures). The genal spines are short or moderately long. Alokistocarids have a relatively large articulating middle part of the body (or

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

), consisting of 12 to 19 segments. The thorax axis is moderately convex and sharply defined, while the areas lateral of the axis (or pleurae) are nearly flat with distinct grooves. The tailshield (or

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

) is small, with few segments, and lacks a border. The surface is generally smooth.

References

Ptychoparioidea Trilobite families Cambrian trilobites {{Ptychopariida-stub