''Acmarhachis'' is a genus of

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 in the order

Agnostida Agnostida are an order of extinct arthropods which have classically been seen as a group of highly modified trilobites, though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the Artiopod ...

, which lived in what are now Australia (Queensland, Tasmania), Canada (British Columbia, Newfoundland, Northwest Territories), China (Anhui), Kazakhstan, Russia (Kharaulakh), and the US (Alaska, Alabama, Nevada, Maryland, Vermont). It was described by Resser in 1938, and the type species is ''Acmarhachis typicalis''.

Distribution

* ''A. typicalis'' occurs in the Cambrian of Kazakhstan (Kyrshabakty River, ''Lejopyge laevigata''-trilobite zone, Zhumabai Formation, Paibian and Cambrian Stage 9, 43.5° N, 70.0° E). * ''A. whittingtoni'' is known from the Upper Cambrian of Canada (Hickey Cove, ''Dunderbergia''-trilobite zone, Downes Point Member, Shallow Bay Formation, Paibian, Newfoundland, 49.8° N, 57.9° W). ''cited in''

Description

Like all Agnostida, ''Acmarhachis'' is diminutive and 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 ...

) and 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 ...

) are of approximately the same size (or isopygous) and outline. As in all Agnostina, ''Acmarhachis'' has two

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 ...

segments. When Agnostina are enrolled, a tiny opening between the cephalon and the thorax becomes visible (the cephalothoracic aperture) that uniquely distinguishes this group from the

Eodiscina Eodiscina is trilobite suborder. The Eodiscina first developed near the end of the Cambrian, Lower Cambrian period (late Atdabanian) and became extinct at the end of the Middle Cambrian. Species are tiny to small, and have a thorax of two or thre ...

. A feature shared with all Agnostoidea is that the frontal lobe of the central raised area of the cephalon (or glabella) is not laterally expanded compared to the backward lobe, like in the

Condylopygoidea The Condylopygidae RAYMOND, P. E., 1913: Some changes in the names of genera of trilobites. ''The Ottawa Naturalist'', 26: 137−142. are a family of small trilobites that lived during the Middle Cambrian, and found in Canada (Newfoundland and No ...

. In ''Acmarhachis'' the furrow at midline between the front of the glabella and the border is weak or absent. The glabella has a long

ogival An ogive ( ) is the roundly tapered end of a two- or three-dimensional object. Ogive curves and surfaces are used in engineering, architecture, woodworking, and ballistics. Etymology The French Orientalist Georges Séraphin Colin gives as ...

to subquadrate front lobe, and the furrow defining it at its back is straight. The back lobe of the glabella is accompanied by two triangular basal lobes of moderate to large size. On the pygidium the axis or rhachis reaches the border. It is divided along its length in three lobes. It is shaped a bit like an

amphora An amphora (; ; English ) is a type of container with a pointed bottom and characteristic shape and size which fit tightly (and therefore safely) against each other in storage rooms and packages, tied together with rope and delivered by land ...

, the back pointed and strongly constricted across the middle lobe. The middle lobe also carries a node where it contacts the posterior lobe.

References

Agnostidae Agnostida genera Cambrian trilobites of Australia Fossils of British Columbia Fossils of Kazakhstan Cambrian trilobites of North America Cambrian trilobites of Asia Paleontology in the Northwest Territories Paleozoic life of Newfoundland and Labrador Paleozoic life of Yukon Cambrian genus extinctions {{agnostida-stub