Alokistocaridae is a
family
Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
of
ptychopariid trilobites
Trilobites (; meaning "three-lobed entities") are extinct marine arthropods that form the class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most successful of all early animals, ...
that lived from the
Botomian 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'', 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., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...
) is semicircular and has a well-defined border. The central raised area of the cephalon (or
glabella) 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 a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...
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 (: thoraces or thoraxes) or chest is a part of the anatomy of 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 di ...
), 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) is small, with few segments, and lacks a border. The surface is generally smooth.
References
Ptychoparioidea
Trilobite families
Cambrian trilobites
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