Megalograptidae 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, and mite ...

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 megalograptids were likely the first major successful group of eurypterids, evidenced by a

Late 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. The ...

radiation. All known members of the Megalograptidea are from the

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...

with the exception of the large ''

Pentecopterus ''Pentecopterus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils have been registered from the Darriwilian age of the Middle Ordovician period, as early as 467.3 million years ago. The genus contains only one species, ...

'' from the Middle Ordovician.

Description

Megalograptids are characterized by large exoskeletons with ovate to triangular scales. The prosoma (head) is subquadrate, with a tonguelike anterior process bearing marginal spines, and compound eyes on the top front of the head. The chelicerae (claws in front of the mouth) are small and short. The first and third pairs of walking legs are short, with diverging or closely spaced spines. The second pair of walking legs is enormously developed, with long paired spines. The fourth pair of walking legs are nearly spineless. The preabdomen, the front portion of the body, is narrow with axial furrows, while the postabdomen is moderately narrow with broad, flat and curved appendages on the last body segment. The

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...

is short and lanceolate. 1955. Merostomata. ''Treatise on Invertebrate Paleontology, Part P Arthropoda 2, Chelicerata'', P36.

Classification

The Megalograptidae are thought to be relatively primitive (between the genus ''

Onychopterella ''Onychopterella'' ( , from Ancient Greek: ὄνῠξ (''ónyx''), "claw", and πτερόν (''pteron''), "wing") is a genus of predatory eurypterid ("sea scorpion"), an extinct group of aquatic arthropods. Fossils of ''Onychopterella'' have ...

'' and the

Eurypteroidea Eurypteroidea are an extinct superfamily of eurypterids. It contains three families ( Dolichopteridae, Eurypteridae, Strobilopteridae) and two genera of uncertain classification, '' Paraeurypterus'' and '' Pentlandopterus''. Description Eurypte ...

) because they lack the synapomorphy of all more derived swimming eurypterines, the modified distal margin of the sixth podomere of the swimming leg. Though such a position has never been recovered in phylogenetic analyses and is primarily based on the lack of said characteristic and other traits that are thought to be primitive. The clade might instead belong between the Eurypteroidea and the

Mixopteroidea Carcinosomatoidea is an extinct superfamily of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". It is one of the superfamilies classified as part of the suborder Eurypterina. Some carcinosomatoid genera ...

since the sixth podomere in the swimming leg resembles the reduced podomere found in the

Mixopteridae The Mixopteridae are 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 Carcinosomatoidea (along with Carcinosomatidae), which in t ...

Genera

Family Megalograptidae Caster & Kjellesvig-Waering, 1955 *''

Megalograptus ''Megalograptus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Megalograptus'' have been recovered in deposits of Katian (Late Ordovician) age in North America. The genus contains five species: ''M. alveolatu ...

'' Miller, 1874 *''

Echinognathus ''Echinognathus'' is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of ''Echinognathus'', ''E. clevelandi'', is known from deposits of Late Ordovician age in the United States. The generic name is derived ...

'' Walcott, 1882 *''

'' Lamsdell et al., 2015

References

Carcinosomatoidea Ordovician arthropods Ordovician first appearances Ordovician extinctions Prehistoric arthropod families {{eurypterid-stub

Description

Classification

Genera

See also

References