Eurypterus - WikiHQ

Eurypterus ( ) is an extinct genus of eurypterid, a group of organisms commonly called "sea scorpions". The genus lived during the Silurian period, from around 432 to 418 million years ago. Eurypterus is by far the most well-studied and well-known eurypterid. Eurypterus fossil specimens probably represent more than 95% of all known eurypterid specimens.

There are fifteen species belonging to the genus Eurypterus, the most common of which is E. remipes, the first eurypterid fossil discovered and the state fossil of New York.

Members of Eurypterus averaged at about in length, but the largest individual discovered was estimated to be long. They all possessed spine-bearing appendages and a large paddle they used for swimming. They were generalist species, equally likely to engage in predation or scavenging.

Discovery

The first fossil of Eurypterus was found in 1818 by S. L. Mitchill, a fossil collector. It was recovered from the Bertie Formation of New York (near Westmoreland, Oneida County). Mitchill interpreted the appendages on the carapace as barbels arising from the mouth.

It was only after seven years, in 1825, that the American zoologist James Ellsworth De Kay identified the fossil correctly as an arthropod. He named it Eurypterus remipes and established the genus Eurypterus in the process. The genus name comes from Ancient Greek εὐρύς (eurús), meaning "wide", and πτερόν (pterón), meaning "wing", referring to their paddle-shaped legs.

Description

left|thumb|Comparison of an average adult human male () with the average E. remipes length (around ) and the largest known Eurypterus lacustris fossil ().

The largest arthropods to have ever existed were eurypterids. The largest known species (Jaekelopterus rhenaniae) reached up to in length, about the size of a crocodile. Species of Eurypterus, however, were much smaller.

E. remipes are usually between in length. E. lacustris average at larger sizes at in length. However, a single telson (the posteriormost division of the body) of a specimen of this species reaches this length, being long and indicating a specimen of of length, and that is the largest specimen ever described in literature. In the introduction page of E. remipes in website of University of Texas at Austin says that the largest specimen ever found was long, currently on display at the Paleontological Research Institution of New York. However, the text section describes the group eurypterid itself rather than Eurypterus, so it is not possible to determine in context whether the long specimen is actually from E. remipes or another eurypterid.

Eurypterus fossils often occur in similar sizes in a given area. This may be a result of the fossils being "sorted" into windrows as they were being deposited in shallow waters by storms and wave action.

The Eurypterus body is broadly divided into two parts: the prosoma and the opisthosoma (in turn divided into the mesosoma and the metasoma).

The prosoma is the forward part of the body, it is actually composed of six segments fused together to form the head and the thorax. They also possessed two smaller light-sensitive simple eyes (the median ocelli) near the center of the carapace on a small elevation (known as the ocellar mound). Underneath the carapace is the mouth and six appendages, usually referred to in Roman numerals I-VI. Each appendage in turn is composed of nine segments (known as podomeres) labeled in Arabic numerals 1–9. The first segments which connect the appendages to the body are known as the coxa (plural coxae). It is further subdivided in two ways.

Based on the width and structure of each segment, they can be divided into the broad preabdomen (segments 1 to 7) and the narrow postabdomen (segments 8 to 12). The preabdomen is the broader segments of the anterior portion of the opisthosoma while the postabdomen are the last five segments of the Eurypterus body. Each of the segments of the postabdomen contain lateral flattened protrusions known as the epimera with the exception of the last needle-like (styliform) part of the body known as the telson. The segment immediately preceding the telson (which also has the largest epimera of the postabdomen) is known as the pretelson. The mesosoma contains the gills and reproductive organs of Eurypterus. Its ventral segments are overlaid by appendage-derived plates known as Blattfüsse (singular Blattfuss, German for "sheet foot"). Protected within which are the branchial chambers which contain the respiratory organs of Eurypterus.

Eurypterus are sexually dimorphic. On the bottom side of the first two segments of the mesosoma are central appendages used for reproduction. In females, they are long and narrow. In the males they are very short.<!--CONFUSING SOURCE: The segment they originate from is also covered by large plates, which like the Blattfüsse, might also have originated from appendages. Eurypterus is the only genus in Eurypteridae, which in turn is the only genus in the superfamily Eurypteroidea. Until recently, eurypterids (e.g. order Eurypterida) were thought to belong to the class Merostomata along with order Xiphosura. It is now believed that eurypterids are a sister group to Arachnida, closer to scorpions and spiders than to horseshoe crabs.

Eurypterus was the first recognized taxon of eurypterids and is the most common. As a consequence, nearly every remotely similar eurypterid in the 19th century was classified under the genus (except for the distinctive members of the family Pterygotidae and Stylonuridae). The genus was eventually split into several genera as the science of taxonomy developed.

thumb|Painting painted in 1912 by [[Charles R. Knight depicting various eurypterids discovered in New York. The painting includes Dolichopterus, Eusarcana, Stylonurus, Eurypterus and Hughmilleria. ]]

The following is the phylogenetic tree of Eurypterus based on phylogenetic studies by O. Erik Tetlie in 2006. Some species are not represented.

An alternative hypothesis for Eurypterus swimming behavior is that individuals were capable of underwater flying (or subaqueous flight), in which the sinuous motions and shape of the paddles themselves acting as hydrofoils are enough to generate lift. This type is similar to that found in sea turtles and sea lions. It has a relatively slower acceleration rate than the rowing type, especially since adults have proportionally smaller paddles than juveniles. But since the larger sizes of adults mean a higher drag coefficient, using this type of propulsion is more energy-efficient.

thumb|right|Rowing in Eurypterus.

Trace fossil evidence indicates that Eurypterus employed a rowing stroke when in close proximity to the seafloor. Arcuites bertiensis is an ichnospecies that includes a pair of crescent-shaped impressions and a short medial drag, and it has been found in upper Silurian eurypterid Lagerstatten in Ontario and Pennsylvania. This trace fossil is very similar to traces made by modern aquatic swimming insects that row such as water boatmen, and is considered to have been made by juvenile to adult-sized eurypterids while swimming in very shallow nearshore marine environments. The morphology of A. bertiensis suggests that Eurypterus had the ability to move its swimming appendages in both the horizontal and vertical plane.

Eurypterus did not swim to hunt, rather they simply swam in order to move from one feeding site to another quickly. Most of the time they walked on the substrate with their legs (including their swimming leg). They were generalist species, equally likely to engage in predation or scavenging. They hunted small soft-bodied invertebrates like worms. They utilized the mass of spines on their front appendages to both kill and hold them while they used their chelicerae to rip off pieces small enough to swallow. Young individuals may also have fallen prey to cannibalism by larger adults.

Eurypterus were most probably marine animals, as their remains are mostly found in intertidal shallow environments. The concentrations of Eurypterus fossils in certain sites has been interpreted to be a result of mass mating and molting behavior. Juveniles were likely to have inhabited nearshore hypersaline environments, safer from predators, and moved to deeper waters as they grew older and larger. Adults that reach sexual maturity would then migrate en masse to shore areas in order to mate, lay eggs, and molt. Activities that would have made them more vulnerable to predators. This could also explain why the vast majority of fossils found in such sites are molts and not of actual animals. The same behavior can be seen in modern horseshoe crabs.

Ontogeny

Juvenile Eurypterus differed from adults in several ways. Their carapaces were narrower and longer (parabolic) in contrast to the trapezoidal carapaces of adults. The eyes are aligned almost laterally but move to a more anterior location during growth. The preabdomen also lengthened, increasing the overall length of the opisthosoma. The swimming legs also became narrower and the telsons shorter and broader (though in E. tetragonophthalmus and E. henningsmoeni the telsons changed from being angular in juveniles to larger and more rounded in adults). All these changes are believed to be a result of the respiratory and reproductive requirements of adults.

Paleoecology

thumb|left|Model in the [[Smithsonian National Museum of Natural History Hall of Fossils.]]

Members of Eurypterus existed for a relatively short time, yet they are the most abundant eurypterids found today.

The ancestors of Eurypterus were believed to have originated from Baltica (eastern Laurussia, modern western Eurasia) based on the earliest recorded fossils. During the Silurian, they spread to Laurentia (western Laurussia, modern North America) when the two continents began to collide. They rapidly colonized the continent as invasive species, becoming the most dominant eurypterid in the region. This accounts for why they are the most commonly found genus of eurypterids today. Eurypterus (and other members of Eurypteroidea), however, were unable to cross vast expanses of oceans between the two supercontinents during the Silurian. Their range were thus limited to the coastlines and the large, shallow, and hypersaline inland seas of Laurussia.]]

They are now only known from fossils from North America, Europe, and northwestern Asia, cratons that were the former components of Laurussia. While three species of Eurypterus were purportedly discovered in China in 1957, the evidence of them belonging to the genus (or if they were even eurypterids at all) is nonexistent. No other traces of Eurypterus in modern continents from Gondwana are currently known.

Eurypterus are very common fossils in their regions of occurrence, millions of specimens are possible in a given area, though access to the rock formations may be difficult. Most fossil eurypterids are the disjointed shed exoskeleton (known as exuviae) of individuals after molting (ecdysis). Some are complete but are most probably exuviae as well. Fossils of the actual remains of eurypterids (i.e. their carcasses) are relatively rare.

References

External links

Eurypterid.co.uk maintained by James Lamsdell
Eurypterids.net maintained by Samuel J. Ciurca, Jr.
Fossil biomechanics