''Trimerus'' is an extinct genus of trilobite in the family

Homalonotidae Homalonotidae is a family of trilobites that lived from the Ordovician to the Devonian. They are characterised by a shovel-like cephalon (arthropod anatomy), cephalon (head), and are closely related to the family Calymenidae. It contains the fol ...

. ''Trimerus'' is one of North America's largest trilobites, reaching over in length. It had a thorax composed of 13 segments with weak trilobation, a large subtriangular head terminating in an expanded rostral plate, a two-pronged hypostome, and a triangular

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 ...

. It is known from all continents except for

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. Its tiny

compound eyes A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distin ...

and the shovel-like anterior of the head suggests a burrowing lifestyle, and an exoskeleton marked with many small pores which, in life, probably housed hair-like sensory setae in life, allowed the trilobite to feel which portions of its body were covered with sediment.

Species

In his description of new Homalonotid species from

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

and

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, Andrew Sandford divided ''Trimerus'' species into multiple

subgenera In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...

to better categorize distinct populations. A number of species previously assigned to other genera were included, and a number of species previously assigned to ''Trimerus'' were excluded, variously reassigned to the trilobite genera '' Dipleura'' (''Dipleura dekayi'' has been frequently included within ''Trimerus''), '' Digonus'', '' Burmeisteria'', and '' Wenndorfia''. The following list is therefore provisional, and other sources may differ. A handful of species also remain undescribed. ''Trimerus (Trimerus)''
This group is known only from the

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

( Wenlock to

Ludlow Ludlow ( ) is a market town and civil parish in Shropshire (district), Shropshire, England. It is located south of Shrewsbury and north of Hereford, on the A49 road (Great Britain), A49 road which bypasses the town. The town is near the conf ...

). *

Type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

''Trimerus (Trimerus) delphinocephalus'' Green, 1832 (USA, Canada, UK) ** ''T. (Trimerus) johannis'' Salter, 1865 (UK) ** ''T. (Trimerus) cylindricus'' Salter, 1865 (UK) ** ''T. (Trimerus) harrisoni'' McCoy, 1876 ** ''T. (Trimerus) vomer'' Chapman, 1912 ** ''T. (Trimerus) flexuosus'' Benedetto and Martel (in Baldis et al., 1976) (Argentina) ''Trimerus (Edgillia)''
This group is known from the Late Silurian to the Early

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

. Named after E. D. Gill, for his contributions to Australian paleontology, including naming a number of Homalonotid trilobites. * Type species ''T. (Edgillia) kinglakensis'' Gill, 1949 (Australia) ** ''T. (Edgillia) vanuxemi'' Hall, 1859 (USA) ** ''T. (Edgillia?) major'' Whitfield, 1885 (USA) ** ''T. (Edgillia) mongolicus'' Tchernycheva, 1937 (Mongolia) ** ''T. (Edgillia) grandis'' Benedetto and Martel (in Baldis et al., 1976) (Argentina) ** ''T. (Edgillia) jelli'' Sandford, 2005 (Australia) ''Trimerus (Ramiotis)''
This group is known only from the Silurian. Named for Otis Rami, the son of the author. * Type species ''T. (Ramiotis) rickardsi'' Sandford, 2005 (Australia) ** ''T. (Ramiotis) permutus'' Tomczykowa, 1978 (nom. nov. for ''T. lobatus'' Tomczykowa, 1975) (Poland) ** ''T. (Ramiotis) dyaulax'' Thomas, 1977 (Saudi Arabia) ** ''T. (Ramiotis) salteri'' Morris, 1988 (nom. nov. for ''Homalonotus (Koenigia) ludensis'' Salter, 1865) (UK) ** ''T. (Ramiotis) iani'' Sandford, 2005 (Australia) ** ''T. (Ramiotis) otisi'' Sandford, 2005 (Australia) ** ''T. (Ramiotis) thomasi'' Sandford, 2005 (Australia) ** ''T. (Ramiotis) tomczykowae'' Sandford, 2005 (Australia) The following species have not been assigned to a subgenus. *''T.? acuminatus'' Tromelin and Lebesconte, 1856 *''T.? lehiri'' Barrois, 1886 *''T. perceensis'' Clarke, 1913 *''T. swartzi'' Ohern and Maynard, 1913 * ''T. stelmophorus'' Busch and Swartz, 1985

References

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