''Pentecopterus'' is a genus 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 ...
, an extinct group of aquatic
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. Fossils have been registered from the
Darriwilian
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species ' ...
age of the
Middle Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period Ma (milli ...
period, as early as 467.3 million years ago. The genus contains only one species, ''P. decorahensis'', that is the oldest known eurypterid, surpassing other Ordovician eurypterids, such as ''
Brachypterus'', in age by almost 9 million years. The
generic name derives from the
penteconter
The penteconter (alt. spelling pentekonter, pentaconter, pentecontor or pentekontor; , ''pentēkónteros'', "fifty-oared"), plural penteconters, was an ancient Greek galley in use since the archaic period.
In an alternative meaning, the term ...
, a warship from
ancient Greece
Ancient Greece () was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity (), that comprised a loose collection of culturally and linguistically r ...
, and the suffix -''pterus'', which means "wing" and is often used in other genus of eurypterids. The
specific name Specific name may refer to:
* in Database management systems, a system-assigned name that is unique within a particular database
In taxonomy, either of these two meanings, each with its own set of rules:
* Specific name (botany), the two-part (bino ...
refers to
Decorah, Iowa
Decorah is the largest city in and county seat of Winneshiek County, Iowa, Winneshiek County, Iowa, United States. The population was 7,587 at the time of the 2020 United States census, 2020 census. Decorah is located at the intersection of Iowa ...
, where ''Pentecopterus'' was discovered.
The genus is classified as part of the
Megalograptidae
Megalograptidae are a family of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions".
The megalograptids were likely the first major successful group of eurypterids, evidenced by a Late Ordovician radiation. ...
family
Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
of eurypterids, a family differentiated from other eurypterids by the possession of two or more pairs of
spine
Spine or spinal may refer to:
Science Biology
* Spinal column, also known as the backbone
* Dendritic spine, a small membranous protrusion from a neuron's dendrite
* Thorns, spines, and prickles, needle-like structures in plants
* Spine (zoology), ...
s per
podomere
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, : ...
on
prosoma
The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...
l
appendage
An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...
IV, a reduction of almost all spines and the large
exoskeleton
An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...
s with ovate to triangular scales.
It is estimated that ''Pentecopterus'' had a length of , turning it into one of the largest
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 or arthropods ever discovered. However, ''Pentecopterus'' was overtaken by other eurypterids such as ''
Jaekelopterus rhenaniae
''Jaekelopterus'' is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Jaekelopterus'' have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: t ...
'', the largest known arthropod with .
A replica of the eurypterid was created for the
National Geographic
''National Geographic'' (formerly ''The National Geographic Magazine'', sometimes branded as ''Nat Geo'') is an American monthly magazine published by National Geographic Partners. The magazine was founded in 1888 as a scholarly journal, nine ...
's "''
The Strange Truth''" program. The project had the help of James Lamsdell, one of the paleontologists who contributed to the description of ''Pentecopterus'', who provided information and two illustrations. When it was finished, it was taken to the impact site where the original fossils were discovered.
Description

''Pentecopterus'' is among the largest known arthropods, with the largest specimens having an estimated length of . With this size, it exceeds all other genera of the
Megalograptidae
Megalograptidae are a family of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions".
The megalograptids were likely the first major successful group of eurypterids, evidenced by a Late Ordovician radiation. ...
. While fragmentary
tergite
A ''tergum'' (Latin for "the back"; : ''terga'', associated adjective tergal) is the Anatomical terms of location#Dorsal and ventral, dorsal ('upper') portion of an arthropod segment other than the head. The Anatomical terms of location#Anterior ...
s have been suggested to indicate a length of for ''
Megalograptus shideleri'', they actually indicate a length of at most .
Most ''Pentecopterus'' specimens would have had a total length of .
The large amount of fragmentary specimens recovered of ''Pentecopterus'', including juveniles and
exuviae
In biology, exuviae are the remains of an exoskeleton and related structures that are left after ecdysozoans (including insects, crustaceans and arachnids) have molted. The exuviae of an animal can be important to biologists as they can often be ...
specimens, have allowed an almost complete description of the external morphology. ''Pentecopterus'' is diagnosed as a megalograptid retaining a single pair of spines on the third podomere of the third prosomal appendages, a short appendage V with a serrated distal margin of podomeres; prosomal ventral plates widening anteriorly, posterolateral pretelson lacking expansion and
xiphos
The ''xiphos'' ( ; plural ''xiphe'', ) is a double-edged, one-handed Iron Age straight shortsword used by the ancient Greeks. It was a secondary battlefield weapon for the Greek armies after the dory or javelin. The classic blade was generall ...
-like shaped telson, with a margin laterally ornamented with scales. The prosomal ventral plate is of ''
Erieopterus''-type, that is, it consists of a single plate that covers the anterior and lateral portion of the ventral carapace. The appendages are attached to the soft ventral integument of the plate. The general outline of the
carapace
A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...
was quadrate with an elongate trapezoidal outline.
History of research
Most of the fossils of the
Winneshiek Shale
The Winneshiek Shale (originally the Winneshiek Lagerstätte) is a Middle Ordovician (Darriwilian-age) geological formation in Iowa. The formation is restricted to the Decorah crater, an impact crater near Decorah, Iowa. Despite only being discov ...
of Decorah, where the fossils of ''Pentecopterus'' have been found, were recovered in 2010 from the upper 4 m section of the aforementioned site in
Iowa
Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...
. Other samples were collected from blocks eroded during flooding, which are assumed to have been sourced from the uppermost 2–3 m. The total number of fossils found is over 5,000, of which around 6.6% belong to eurypterids. Among the material eurypterid is the holotype of ''Pentecopterus'', SUI 139941, which consists of a prosomal ventral plate and proximal podomeres of prosomal appendage II, in addition to multiple
paratype
In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...
s and additional material, all housed in the
University of Iowa
The University of Iowa (U of I, UIowa, or Iowa) is a public university, public research university in Iowa City, Iowa, United States. Founded in 1847, it is the oldest and largest university in the state. The University of Iowa is organized int ...
Paleontology Repository.
The genus was named by James C. Lamsdell, Derek E. G. Briggs, Huaibao P. Liu, Brian J. Witzke and Robert M. McKay in 2015 and the
type species
In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
is ''Pentecopterus decorahensis''. It is the only eurypterid discovered in Iowa other than ''
Mycterops whitei'',
a species whose assignment to ''
Mycterops'' is doubtful. The genus is named after the
penteconter
The penteconter (alt. spelling pentekonter, pentaconter, pentecontor or pentekontor; , ''pentēkónteros'', "fifty-oared"), plural penteconters, was an ancient Greek galley in use since the archaic period.
In an alternative meaning, the term ...
, an early
galley
A galley is a type of ship optimised for propulsion by oars. Galleys were historically used for naval warfare, warfare, Maritime transport, trade, and piracy mostly in the seas surrounding Europe. It developed in the Mediterranean world during ...
(ship) from ancient Greece and one of the first true warships, due to some similarities in shape and that the taxon represents an early predator. The suffix -''pterus'' means "wing" and is commonly applied to eurypterid genera. The specific name ''decorahensis'' refers to Decorah,
Iowa
Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...
, where the fossils were discovered.
Classification

''Pentecopterus'' is classified within the family
Megalograptidae
Megalograptidae are a family of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions".
The megalograptids were likely the first major successful group of eurypterids, evidenced by a Late Ordovician radiation. ...
in the
monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...
superfamily
Megalograptoidea. ''Pentecopterus'' shares with the other
megalograptids a series of characteristics such as the randomly-oriented armature on the swollen podomeres of appendage IV and the narrow gnathobase bearing multiple rows of small teeth on the
coxa of appendage V. It also shares with ''Megalograptus'' the rows of enlarged scales running down the center of the
opisthosoma
The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma ( cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects ...
l tergites, however, these differ from those of ''Megalograptus'' because they are not situated on pronounced ridges. It is separated from the other megalograptids by the presence of only one pair of spines on the third podomere of appendage III, a single terminal spine on each prosomal appendage, and the absence of dense cuticular ornamentation. ''Pentecopterus'' also lacks the cercal blades that occur in ''Megalograptus'', where they have been interpreted as functioning as a biological rudder, like the pterygotid telson. This suggests that ''Pentecopterus'' was less able to swim than ''Megalograptus''. Further, ''Pentecopterus'' has some features unique among the eurypterids, notably the shape of its carapace and the unusual shape of the sixth podomere of appendage VI. ''Pentecopterus'' also possesses lateral scales on 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 ...
, something otherwise only seen in
pterygotid
Pterygotidae (the name deriving from the type genus ''Pterygotus'', meaning "winged one") is a family of eurypterids, an extinct group of aquatic arthropods. They were members of the superfamily Pterygotioidea. Pterygotids were the largest known ...
eurypterids. This feature likely arose through
convergent evolution
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
.
The cladogram below is simplified from a study by Lamsdell ''et al.'' (2015),
showing the
phylogenetic
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
positions of the genera within Megalograptidae.
Paleoecology

''Pentecopterus'' is known from Middle Ordovician deposits in the Winneshiek Shale of Iowa dated to the Darriwilian (467.3 mya). The Winneshiek Shale is an 18–27 m thick greenish brown to dark grey laminated sandy
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
that crops out only in one locality which is mostly submerged by the
Upper Iowa River
The Upper Iowa River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed May 13, 2011 tributary of the Mississippi River in the upper Midwest of the United States.
Its headwaters rise ...
near Decorah. Bore hole data indicate that the total thickness of the Winneshiek Shale is about 18 m at the outcrop locality, but only the upper 4 m was systematically collected during the excavation. In these 4 m, most of the ''Pentecopterus'' fossils were collected. This shale is confined to a circular basin about 5.6 km in diameter in the Decorah area that probably originated from a meteorite impact.
Palaeogeographic and
paleoenvironmental studies suggest that this
crater
A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...
was located in marginal to nearshore marine conditions with little oxygen, possibly in a
brackish water
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...
environment within the tropical southern
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 ...
. The Winneshiek fauna is very different from the shelly marine fossil fauna, which indicates that this environment was inhospitable to the typical marine taxa. This fauna was composed of
conodont
Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...
s, arthropods, possible
jawless fish
Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...
es and
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
.
See also
*
List of eurypterid genera
This list of eurypterid genera is a comprehensive listing of all genera that have ever been included in the order Eurypterida, excluding purely vernacular terms. The list includes all commonly accepted genera, but also genera that are now consider ...
References
{{Taxonbar, from=Q20900357
Carcinosomatoidea
Ordovician arthropods of North America
Ordovician eurypterids
Eurypterids of North America