Strabopidae is the only

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 the order Strabopida, an extinct group of

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s known from the

Cambrian 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 Ordov ...

period. The order Strabopida contains three valid genera and one doubtful, '' Khankaspis'', ''

Paleomerus ''Paleomerus'' is a genus of strabopid, a group of extinct arthropods. It has been found in deposits from the Cambrian period (geology), period (Atdabanian Epoch (geology), epoch). It is classified in the Family (biology), family Strabopidae of ...

'', '' Parapaleomerus'' and '' Strabops'', all assigned in the family Strabopidae. Previously, it also contained the genera '' Caryon'', '' Lemoneites'' and '' Neostrabops'', but after a study, they were reassigned as a

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 ...

, a glyptocystitid

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

and a

cheloniellid Cheloniellida is a taxon (usually referred to as an order) of extinct Paleozoic arthropods. As of 2018,Wendruff, Andrew James, et al. "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA." BioRxiv (2018) ...

arthropod, respectively. Paleomeridae was also previously included as a family, but after the discovery of a fourth ''Paleomerus''

specimen Specimen may refer to: Science and technology * Sample (material), a limited quantity of something which is intended to be similar to and represent a larger amount * Biological specimen or biospecimen, an organic specimen held by a biorepository f ...

and its study, Paleomeridae was synonymized into Strabopidae. Although often suggested to be closely related to aglaspidids due to their similar morphology, their exact phylogenetic placement is uncertain due to the poor preservation of all known strabopids. A 2013 attempt to place them in a phylogenetic analysis recovered them as part of a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...

with

megacheira Megacheira ("great hands", also historically great appendage arthropods) is an extinct class of predatory arthropods defined by their possession of spined "great appendages". Their taxonomic position is controversial, with studies either consider ...

ns, marrellomorphs,

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s and

artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

ns. Still, strabopids are often classified as part of the clade

Arachnomorpha Arachnomorpha is a proposed subdivision or clade of Arthropoda, comprising the group formed by the trilobites and their close relatives (Artiopoda), Megacheira (which may be paraphyletic) and chelicerates. Under this proposed classification sche ...

. In fact, Dunlop & Selden (1998) defined ''Paleomerus'' as "perhaps the best model of a primitive arachnomorph".

References

Strabopida Prehistoric arthropod families Controversial taxa {{paleo-arthropod-stub