Megalograptidae are a family of

eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period, 467.3 Myr, mil ...

s, an extinct group of

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

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 commonly known as "sea scorpions". The megalograptids were likely the first major successful group of eurypterids, evidenced by a

Late Ordovician The Late Ordovician is the third and final epoch of the Ordovician period, lasting million years and spanning from around 458.2 to 443.1 million years ago. The rocks associated with this epoch are referred to as the Upper Ordovician Series. At ...

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 Geology of North America, 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 ...

with the exception of the large '' Pentecopterus'' 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 hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...

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'' and the Eurypteroidea) because they lack the

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

of all more derived swimming eurypterines; namely, the modified distal margin of the sixth podomere of the swimming leg. Though such a position has never been recovered in

phylogenetic analyses In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as Computational phylogenetics, phylogenetic inference. It infers the relationship among organ ...

and is primarily based on this missing characteristic (along with other primitive characters), the clade might instead be placed between the Eurypteroidea and the Mixopteroidea. Indeed, 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 ...

Genera

Family Megalograptidae Caster & Kjellesvig-Waering, 1955 *'' Megalograptus'' Miller, 1874 *'' Echinognathus'' Walcott, 1882 *'' Pentecopterus'' Lamsdell et al., 2015

References

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

Description

Classification

Genera

See also

References