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 from the Neo-Latin ''echino''- ("spiny") and the Greek ''gnáthos'' ("jaw"), in reference to a spiny endognathary (used to handle food) appendage part of the fossil type material.

''Echinognathus'' is only known from fragmentary fossil material, consisting of body segments, an appendage used to handle food and possibly other body segments. The genus is distinguished from other eurypterids by the large number of elongated and curved spines, blade-like in life, on its limbs. Initially assumed to represent a species of ''Eurypterus'', these distinguishing features were quickly noticed and deemed important enough to designate ''Echinognathus'' as its own genus. With some additional fossil assigned to ''Echinognathus'' in the early 20th century, the genus was noted to be similar to ''Megalograptus'', another eurypterid with spiny limbs. In 1955, ''Echinognathus'' and ''Megalograptus'' were placed into their own taxonomic family of eurypterids, the Megalograptidae.

Based on the proportions of other eurypterids, ''Echinognathus'' would have been a medium-sized predator, reaching approximately 45 centimeters (17.7 in) in length. The spines on its limbs were presumably used for active prey capture, securing food and carrying it to the mouth. The fossils of ''Echinognathus'' are known from what was once marine environments, and it lived alongside a fauna including graptolites, brachiopods, cephalopods and trilobites.

Description

''Echinognathus'' is only known from fragmentary fossil remains. It was a medium-sized megalograptid eurypterid, reaching approximately 45 centimeters (17.7 in) in length. On account of being known from such fragmentary remains, most of the body of ''Echinognathus'' is unknown. Based on the related ''Megalograptus'', it is possible that the head was subquadrate (vaguely quadratic) in shape. ''Echinognathus'' was probably robustly built.

The most distinguishing feature apparent in the fossil material was the spines of the known endognathary (used to handle food) appendage. The limb was formed by eight or nine joints, most probably eight, six of which were equipped with large, curved and elongated spines. The spines, similar to but also distinct from those of the related ''Megalograptus'', were flattened, had a subtriangular intersection and were distinctly striated (had grooves) longitudinally. In life, they would have had a distinct blade-like appearance.

The body segments of ''Echinognathus'' were ornamented with prominent oblong scales, similar in shape to "raindrops running down a windowpane". The metastoma (a large plate located on the underside of the body) of ''Echinognathus'' was broad and cordate (heart-shaped) in shape, a feature similar to most eurypterids but differentiating it from ''Megalograptus'', which had a more unique metastoma.

History of research

The type material of ''Echinognathus clevelandi'' was first reported in February 1882 by Charles Doolittle Walcott. The fossils were recovered in deposits of Katian (Late Ordovician) age north of Utica, New York by William N. Cleveland, a friend of Walcott. At first, Walcott provisionally referred the fossils to the genus ''Eurypterus'', proposing the species name ''Eurypterus''? ''clevelandi'', the name honoring Cleveland. The fossils consisted of the remains of a large endognathary appendage equipped with intact fossil spines, and part of a body segment. Later that same year, Walcott formally described the fossils. Because comparisons with other known eurypterids yielded few similarities, Walcott named ''Echinognathus clevelandi'' as a new genus and species, the genus name referring to the spiny endognathary appendage. Etymologically, the name derives from the Neo-Latin ''echino''- ("spiny") and the Greek ''gnáthos'' ("jaw"). The main distinguishing feature of the ''Echinognathus'' fossils, as noted by Walcott, was the long and curved spines of the appendage.

Further fossils from the type locality of ''E. clevelandi'' were referred to ''Echinognathus'' in the early 20th century by John Mason Clarke and Rudolf Ruedemann (and later Ruedemann alone), who described several fragments of body segments, and mentioned a larger collection of unpublished fragments, in 1912 and 1926. Fragments of a spine, or possibly the telson (the final body segment, often in the shape of a spike), were referred to ''Echinognathus'' by Ruedemann in 1916. Ruedemann's spine/telson fragments were covered in scales and not striations (as the spines of the type specimen were), and as such he noted that they might either belong to a new, second species of ''Echinognathus'', or that the striations of the spines broke up into scales as ''Echinognathus'' matured. The spine was suggested by Kenneth E. Caster and Erik N. Kjellesvig-Waering in 1964 to instead belong to ''Megalograptus''.

Classification

In his original description of ''Echinognathus'', Walcott made no assessment of the relationship between the genus and other eurypterids. In Clarke's and Ruedemann's 1912 ''The Eurypterida of New York'', ''Echinognathus'' was tentatively associated with the genus '' Stylonurus'', particularly the subgenus '' Ctenopterus'' (later raised to a full, distinct, genus). The association was made since ''Echinognathus'' had so many paired spines on its appendage, most known eurypterids at the time, with the exception of ''Stylonurus'' and some others, only having one pair per segment. The continuous series of spines was noted to specifically be characteristic of ''Ctenopterus''. The blade-like spines and the striations on the spines were also noted to be characteristics also seen in ''Ctenopterus''. In ''The Eurypterida of New York'', August Foerste compared the ''Echinognathus'' fossils to those of ''Megalograptus'', also fragmentarily known at the time, and concluded that the two were likely closely related, if not congeneric. The discovery of more ''Megalograptus'' fossils, and more fragmentary fossils of ''Echinognathus'', later in the 20th century allowed for the two to be firmly established as distinct, but closely related genera.

In 1934, Leif Størmer classified ''Megalograptus'' and ''Echinognathus'', together with the genera ''Mixopterus'' and ''Carcinosoma'', into the family Carcinosomatidae. The taxonomy was amended by Erik N. Kjellesvig-Waering in Størmer's 1955 ''Treatise on Invertebrate Paleontology'', wherein ''Mixopterus'' was transferred to its own family, the Mixopteridae, and ''Megalograptus'' and ''Echinognathus'' were also placed within their own family, the Megalograptidae. Though the taxonomic position of this family has been contested historically, phylogenetic analyses support the Megalograptidae as grouped with the Carcinosomatidae and Mixopteridae in the superfamily Carcinosomatoidea. The cladogram below is simplified from the results of a 2015 phylogenetic analysis by James Lamsdell and colleagues, collapsed to only display the Carcinosomatoidea.

Paleoecology

''Echinognathus'' is known from marine deposits. The spines on the appendages were initially hypothesized by Walcott to have been related to the branchial (i. e. respiratory) system. Walcott noted that it was "not apparent" that they were used to secure food or carry it to the mouth of the animal, the only other viable hypothesis. Based on the related ''Megalograptus'', the second hypothesis, that the spines of ''Echinognathus'' were used for active prey capture and to move food to the mouth, is more likely. There were numerous other organisms present at the fossil site where the ''Echinognathus'' fossils were found, including graptolites '' Mastigograptus'', '' Geniculograptus'', '' Orthograptus'' and '' Climacograptus'', orthocerid cephalopod '' Geisonoceras'', lingulid brachiopod '' Leptolobus'', rhynchonellatan brachiopod '' Camarotoechia'' and trilobite ''Triarthrus''.

See also

* List of eurypterid genera
* Timeline of eurypterid research

References

{{Taxonbar, from=Q5332491

Carcinosomatoidea
Ordovician eurypterids
Eurypterids of North America
Fossil taxa described in 1882