The Rhenopteridae are a family of

eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is l ...

s, 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, and mite ...

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

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 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 ...

. The family contains one of the earliest known eurypterids, '' Brachyopterus'', known from the

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...

(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 The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, ...

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'' 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 Stylonuridae is a family of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". The family is one of two families contained in the superfamily Stylonuroidea (along with Parastylonuridae), which in turn is o ...

and superfamily Hibbertopteroidea, rhenopterids retained primitive '' Hughmilleria''-type prosomal appendages II-IV, which are unsuited to sweep-feeding. The family is thus more likely to have adopted a

scavenging Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding ...

lifestyle.

Evolutionary history

The Rhenopterids first appeared during the

as one of the earliest and most basal groups of Stylonurine 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'') 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 '' Rhenopterus''). ''

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 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 '' Rhenopterus''. The Rhenopterinae was restricted to the Early to

Middle Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

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 within the

Eurypterina Eurypterina is one of two suborders of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". Eurypterine eurypterids are sometimes informally known as "swimming eurypterids". They are known from fossil deposi ...

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

'' Waterston, 1979 ** '' Leiopterella'' Lamsdell, Braddy, Loeffler, & Dineley, 2010 ** '' Rhenopterus'' Størmer, 1936

References

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