The Rhenopteridae are a family of eurypterids, an extinct group of

chelicerate The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s commonly known as "sea scorpions". The family is the only family currently contained in the superfamily Rhenopteroidea, one of four superfamilies classified as part of the suborder Stylonurina. The family contains one of the earliest known eurypterids, '' Brachyopterus'', known from the Middle Ordovician (also the sister taxon to all other rhenopterids), and is the most primitive clade of stylonurine eurypterids. The last known members of the family went extinct during the Early Devonian.

Description

Rhenopterids were small, characterized by scattered tubercules and knobs on the outer surface of the exoskeleton. Their first two (or possibly three) pairs of walking legs had spines; the last two pairs were long and powerful, without spines. The prosoma (head) was subtrapezoidal, with arcuate compound eyes on parallel axes. The male genital appendages were short with two distal spines. The rhenopterids were the most primitive stylonurines and the family encompasses many previously enigmatic eurypterids, such as '' Brachyopterus'', '' Kiareopterus'' and ''

Rhenopterus ''Rhenopterus'' is an extinct prehistoric eurypterid. Fossils of ''Rhenopterus'' have been recovered from deposits of Lower Devonian age in Germany.Dunlop, J. A., Penney, D. & Jekel, D. 2015. A summary list of fossil spiders and their relatives. ...

'' itself, all united by a rounded posterior margin to the metastoma and prosomal appendage III bearing single fixed spines. Unlike the derived stylonurines of the family Stylonuridae and superfamily

Hibbertopteroidea Mycteropoidea is an extinct superfamily of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". It is one of four superfamilies classified as part of the suborder Stylonurina. Mycteropoids have been recovered ...

, rhenopterids retained primitive ''

Hughmilleria ''Hughmilleria'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Hughmilleria'' have been discovered in deposits of the Silurian age in China and the United States. Classified as part of the basal family Hughmille ...

''-type prosomal appendages II-IV, which are unsuited to sweep-feeding. The family is thus more likely to have adopted a scavenging lifestyle.

Evolutionary history

The Rhenopterids first appeared during the Middle Ordovician as one of the earliest and most basal groups of

Stylonurine Stylonurina is one of two suborders of eurypterids, a group of extinct arthropods commonly known as "sea scorpions". Members of the suborder are collectively and informally known as "stylonurine eurypterids" or "stylonurines". They are known from ...

eurypterids. Clear evolutionary trends can be observed in the carapaces of the rhenopterids. Basal rhenopterids possess broad carapaces narrowest at their base (as in '' Brachyopterus'' and ''

Brachyopterella ''Brachyopterella'' is a genus of prehistoric eurypterid classified as part of the Rhenopteridae family. Two species, both from the Silurian period, are known; ''B. pentagonalis'' from Norway and ''B. ritchei'' from Scotland.Dunlop, J. A., Penney ...

'') whilst more derived rhenopterids have broad carapaces narrowest at the front and the most derived members possess narrower carapaces narrowest at the front (as in ''

''). ''

Kiaeropterus ''Kiaeropterus'' is a genus of prehistoric eurypterid classified as part of the Rhenopteridae family. Two species, both from the Silurian period, are known; ''K. cyclophthalmus'' from Scotland and ''K. ruedemanni'' from Norway.Dunlop, J. A., P ...

'' is unusual in its morphology and might represent a

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleozoi ...

offshoot.

Systematics and genera

The Rhenopteroids are diagnosed as being stylonurines with the posterior margin of the metastomate being round. Rhenopterids are in turn rhenopteroids with single fixed spines on the prosomal appendage III and possess a short telson. The Appendages II-IV have short and fixed spines and V-VI are nonspiniferous. Since no currently known rhenopteroid challenge the diagnosis applied to the Rhenopteridae, all rhenopteroids are rhenopterids. Lamsdell, Braddy and Tetlie (2010) assigned one subfamily to the family, the Rhenopterinae, including the genera '' Alkenopterus'' and ''

''. The Rhenopterinae was restricted to the

Early Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

to Middle Devonian and were diagnosed as being rhenopterids with a non-spiniferous appendage IV and a caudal postabdomen. More recently, ''Alkenopterus'' has been reclassified as a member of the

Onychopterellidae Onychopterellidae are an extinct family of eurypterids. The family is the only family classified as part of the superfamily Onychopterelloidea. Genera included are ''Alkenopterus'', '' Onychopterella'' and '' Tylopterella''.Dunlop, J. A., Penney, ...

within the Eurypterina, making the subfamily defunct.Dunlop, J. A., Penney, D. & Jekel, D. 2015. A summary list of fossil spiders and their relatives. In World Spider Catalog. Natural History Museum Bern, online at http://wsc.nmbe.ch, version 16.0 http://www.wsc.nmbe.ch/resources/fossils/Fossils16.0.pdf (PDF). Superfamily Rhenopteroidea Størmer, 1951 * Family Rhenopteridae Størmer, 1951 ** ''

''Kjellesvig-Waering, 1966 ** '' Brachyopterus ''Størmer, 1951 ** ''

'' Waterston, 1979 ** ''

Leiopterella ''Leiopterella'' is a genus of prehistoric eurypterid of the family Rhenopteridae. It contains one species, ''Leiopterella tetliei'', from the Early Devonian of Nunavut, Canada.Dunlop, J. A., Penney, D. & Jekel, D. 2015. A summary list of foss ...

'' Lamsdell, Braddy, Loeffler, & Dineley, 2010 ** ''

'' Størmer, 1936

References

{{Taxonbar, from=Q1645821 Early Devonian extinctions Middle Ordovician first appearances Prehistoric arthropod families