Merostomata is the name given to a grouping of the extinct Eurypterida (sea scorpions) and the Xiphosura

Xiphosura

Xiphosura is an order of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs...

 (horseshoe crabs). The term was originally used by James Dwight Dana

James Dwight Dana

James Dwight Dana was an American geologist, mineralogist and zoologist. He made pioneering studies of mountain-building, volcanic activity, and the origin and structure of continents and oceans around the world.-Early life and career:...

 to refer to Xiphosura only, but was emended by Henry Woodward

Henry Woodward (geologist)

Henry Bolingbroke Woodward was an English geologist.He became assistant in the geological department of the British Museum in 1858, and in 1880 keeper of that department. He became FRS in 1873, LL.D in 1878, president of the Geological Society of London...

 to cover both groups.

Etymology

The name "Merostomata" derives from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 roots

Root (linguistics)

The root word is the primary lexical unit of a word, and of a word family , which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents....

  (, "thigh") and (, "mouth"), in reference to the animals possession of appendages which are mouthparts at their proximal end, but swimming legs at their distal end.

History

The scientific consensus at the beginning of the twentieth century was that these two marine groups were closely related, and only more distantly related to the terrestrial Arachnid

Arachnid

Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...

a. More recent analyses suggest that the grouping Merostomata is not monophyletic

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

, with Xiphosura being basal to a clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 comprising Eurypterida and Arachnida. It is estimated that the Xiphosura diverged from the Arachnida .

The shared features of the two groups traditionally grouped in the Merostomata are now thought to be retentions of primitive conditions (symplesiomorphies

Symplesiomorphy

In cladistics, a symplesiomorphy or symplesiomorphic character is a trait which is shared between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration...

), and thus it has been recommended that the name Merostomata be abandoned.