''Acmarhachis'' is a genus of
trilobite
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 ...
in the order
Agnostida
Agnostida is an order of arthropod 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 Artiopoda. They a ...
, 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., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...
) and 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 ...
) are of approximately the same size (or
isopygous) and outline. As in all Agnostina, ''Acmarhachis'' has two
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 ...
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 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 three segments. ...
. 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 (2013) 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 (New ...
. 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-dimensional or three-dimensional object. Ogive curves and surfaces are used in engineering, architecture and woodworking.
Etymology
The earliest use of the word ''ogive'' is found in the 13th c ...
to