''Tricrepicephalus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of ptychopariid

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s of the

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 ...

Tricrepicephalidae with species of average size. Its species lived from 501 to 497 million years ago during the

Dresbachian The Dresbachian is a Maentwrogian regional stage of North America, lasting from 501 to 497 million years ago. It is part of the Upper Cambrian and is defined by four trilobite zones. It overlaps with the ICS-stages Guzhangian, Paibian and the ...

faunal stage In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by conven ...

of the late

Cambrian Period The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovici ...

. Fossils of ''Tricrepicephalus'' are widespread in Late Cambrian deposits in North America, but is also known from one location in South America. ''Tricrepicephalus'' has an inverted egg-shaped exoskeleton, with three characteristic pits in the fold that parallels the margin of the headshield just in front of the central raised area. The articulating middle part of the body has 12 segments and the tailshield carries two long, tubular, curved pygidial spines that are reminiscent of earwig's pincers that rise backwards from the plain of the body at approximately 30°.

Description

The outline of the

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

of ''Tricrepicephalus'' is an inverted egg shaped though almost ovate, widest at the tip of the genal spines and 1.2× as long as wide, not including the pygidial spines. 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 moderately vaulted. The well-defined central raised area (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 ...

) is convex, elongate tapering, with a rounded front, and has only one furrow, crossing it near the back of the cephalon, defining the occipital ring. A node may be present on the occipital ring. The remaining parts of the cephalon, called fixed and free cheeks (or fixigenae and librigenae) are upsloping. The fracture lines (or sutures) that in moulting separate the librigenae from the fixigenae are divergent just in front of the eyes, becoming parallel near the border furrow and slightly convergent at the margin. From the back of the eyes the sutures follow a lazy S-curve, diverging first more outward and near the margin more backward, cutting the posterior margin within the inner bend of the spine (or

opisthoparian 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, ...

sutures). Uniquely, the furrow parallel to the margin (or border furrow) has 2 not very noticeable pits in front of the glabella, and rarely a very faint 3rd median pit. There are no ridges that connect the eye to the glabella. The medium-sized eyes (over ⅓× the length of the glabella) are slightly behind the middle of the glabella. Genal angle ending in short spines extending backward about one thorax segment. The 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 ...

) has 12 segments. The axis is convex, and less than ½× as wide as each of the so-called pleurae to its sides. Segments pointed sideways with a rounded front (a shape called falcate). The tailshield (or

pygidium The pygidium (: 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 compos ...

) is about ½× as wide as the cephalon, almost twice as wide as long, excluding the two, widely spaced spines. These are at an angle of approximately 30° with the rest body. The axis in the pygidium is 1¼× longer than wide, with almost parallel sides, almost reaching the rear margin, with 3 or 4 axial rings; 3 sets of interpleural grooves and pleural furrows ending at distance of the margin. There is no furrow that would define a border in the pygidium.

Differences with ''Meteoraspis''

'' Meteoraspis'' has two equally prominent pits in the anterior border furrow, a much more vaulted cephalon, with short spines reaching to about the second thorax segment, 13 thorax segments and two flat, shark tooth shaped, widely spaced spines on the pygidium.

Reassigned species

* ''T. metra'' = '' Meteoraspis metra''

Distribution

* ''T. asiaticus'' occurs in the Upper Cambrian of China (Guzhangian, Blackwelderia zone, Chefu Formation, Yangweizhou-Jimachong) * ''T. arcuatus'' has been found in the Upper Cambrian of the United States (Merioneth, Warrior Formation, Highway No. 322 near Waddle, Bed 13.3, Centre County, Pennsylvania, 40.8° N, 77.9° W) * ''T. texanus'' is known from the Upper Cambrian of Canada (Dresbachian, Rabbitkettle Formation, southern Mackenzie Mountains, northwest Canada) and the United States (Dresbachian, ''Crepicephalus'' trilobite zone, Deadwood Formation, Whitewood Creek in Lead County, South Dakota, 44.3° N, 103.8° W). ''cited in'' * ''T. tripunctatus'' is present in the Upper Cambrian of the United States (Dresbachian, ''Crepicephalus'' trilobite zone, lower Member of the Deadwood Formation, Little Elk Creek in Lead County, South Dakota, 44.3° N, 103.8° W).

Ecology

''T. tripunctatus'' occurs together with trilobite species from the genera ''Coosia'', ''Crepicephalus'', ''Kingstonia'', ''Pseudagnostina'', and ''Coosina''.

References

{{Taxonbar, from=Q20720390 Ptychopariida genera Ptychopariida Cambrian trilobites Cambrian trilobites of North America Fossils of Georgia (U.S. state) Paleozoic life of the Northwest Territories Paleozoic life of Yukon Cambrian genus extinctions