''Alkenopterus'' is a genus of prehistoric

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 ...

classified as part of the family Onychopterellidae. The genus contains two species, ''A. brevitelson'' and ''A. burglahrensis'', both from the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

Description

Like the other onychopterellids, ''Alkenopterus'' was a small

. The largest

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

was ''A. brevitelson'', being long. The other species, ''A. burglahrensis'', represents in fact the smallest species of eurypterid known as far, only measuring . The

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 ...

(head) was large, with a subquadrate (almost square) to semielliptic (nearly elliptic),

horseshoe A horseshoe is a product designed to protect a horse hoof from wear. Shoes are attached on the palmar surface (ground side) of the hooves, usually nailed through the insensitive hoof wall that is anatomically akin to the human toenail, altho ...

-like outline. It was

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

ly surrounded by a broad and flat marginal rim that reached its posterior corners. 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 ...

(the

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 ...

part covering the prosoma) was rounded in the front. Its surface was somewhat inflated, being distinguished several narrow grooves and ridges, most of them

wrinkle A wrinkle, also known as a rhytid, is a fold, ridge or crease in an otherwise smooth surface, such as on skin or fabric. Skin wrinkles typically appear as a result of ageing processes such as glycation, habitual sleeping positions, loss of b ...

-like. The prominent

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

eyes An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

were placed in the center of the carapace. They were reniform (bean-shaped), with a strongly arcuate visual surface (a "half moon" in the eye). Between the eyes was located a

tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projectio ...

or node carrying the

ocelli A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...

(simple eye-like sensory

organs In a multicellular organism, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to a ...

). ''A. brevitelson'' also had between the eyes, behind the tubercle, certain grooves resembling the reversed V-shaped structure found in some stylonurines. The

appendages 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 ...

(limbs) of ''Alkenopterus'' are not known in much detail. A pair of poorly preserved appendages of ''A. brevitelson'' representing the sixth (and last) pair of them is known. The

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, : ...

s (leg segments) were more or less rectangular and constant in width. In the end there was a

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), ...

about as long as the podomeres which was slightly curved, with a long

longitudinal Longitudinal is a geometric term of location which may refer to: * Longitude ** Line of longitude, also called a meridian * Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, ...

groove. The third to fifth appendages are also known, but they are not exceptionally preserved. Nevertheless, in all of them a

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

spine can be identified. Regarding ''A. burglahrensis'', only an appendage belonging to the sixth pair with five distal podomeres (podomeres that were not underneath the prosoma) is known. The seventh and eighth podomeres (and perhaps more of them) were somewhat prolonged outwards and flattened. They featured spine-like immovable spurs on their anterior margins. On the posterior margin of the seventh podomere was the movable spine-like podomere 7a, characteristic of the eurypterines. All the podomeres had similar proportions, except the distal spine which was slightly curved and probably had a pointed tip. 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 ...

(

abdomen The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...

) suffered a strong to moderate first order differentiation, that is, it was divided into a preabdomen (body segments 1 to 7) and a postabdomen (segments 8 to 12). The preabdomen had lateral convex margins and was quite short and broad, with the first

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 ...

(

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

half of the segment) being less wide than the subsequent ones. The postabdomen was narrow, had a constant width and did not have epimera (lateral "extensions" of the segment), like the preabdomen. The segments of the whole body were hardly distinguishable from each other. The integument of the body lacked

ornamentation An ornament is something used for decoration. Ornament may also refer to: Decoration * Ornament (art), any purely decorative element in architecture and the decorative arts * Ornamental turning * Biological ornament, a characteristic of animals ...

and was very smooth. The main difference between ''A. brevitelson'' and ''A. burglahrensis'' was the length of 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 ...

(the posteriormost division of the body). The one of ''A. brevitelson'' was short, measuring only in a long specimen. The

ratio In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

between the total body length and telson of this specimen is around 13.6. It was slightly subtriangular (almost triangular) and had a

median The median of a set of numbers is the value separating the higher half from the lower half of a Sample (statistics), data sample, a statistical population, population, or a probability distribution. For a data set, it may be thought of as the “ ...

"keel" (ridge), with an expanded anterior base articulated to the pretelson (segment that preceded the telson). In the other hand, the telson of ''A. burglahrensis'' was long, measuring in a specimen with in total, with a ratio significantly small of 5.8. It had an expanded anterior portion as well. The telson of both species had, however, the same styliform shape.

History of research

In 1974,

paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Leif Størmer Leif Størmer (1 July 1905 – 15 May 1979) was a Norway, Norwegian paleontologist and geologist. He was professor of historical geology at the University of Oslo from 1946 to 1975. His father was the mathematician Carl Størmer, and his son the ...

described two specimens of a new eurypterid. SMF VIII 150 (the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

) is a relatively complete and well preserved

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

with almost all the appendages missing, while SMF VIII 241 (the

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 ...

) is a smaller, little preserved and strongly telescoped (with segments overlapping each other, a defect product of the fossilization of the

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

) specimen. Both were collected in the

Nellenköpfchen Formation The Nellenköpfchen Formation is a Formation (geology), geologic formation in Germany. It preserves fossils dating back to the Devonian Period (geology), period. See also * List of fossiliferous stratigraphic units in Germany References

* ...

near the

municipality A municipality is usually a single administrative division having municipal corporation, corporate status and powers of self-government or jurisdiction as granted by national and regional laws to which it is subordinate. The term ''municipality' ...

of Alken in

Rhineland-Palatinate Rhineland-Palatinate ( , ; ; ; ) is a western state of Germany. It covers and has about 4.05 million residents. It is the ninth largest and sixth most populous of the sixteen states. Mainz is the capital and largest city. Other cities are ...

(then

West Germany West Germany was the common English name for the Federal Republic of Germany (FRG) from its formation on 23 May 1949 until German reunification, its reunification with East Germany on 3 October 1990. It is sometimes known as the Bonn Republi ...

). Currently, they are located in the

Naturmuseum Senckenberg The Naturmuseum Senckenberg () is a museum of natural history, located in Frankfurt am Main. It is the second-largest of its kind in Germany. In 2010, almost 517,000 people visited the museum, which is owned by the Senckenberg Nature Research S ...

. This new eurypterid was named ''Alkenopterus brevitelson'', with the generic name composed by Alken and the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

suffix πτερόν (''pteron'', "wing"), commonly used in eurypterids. On the other hand, 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 ...

''brevitelson'' derives from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

word ''brevis'' (short) and the Ancient Greek word τέλσον (literally "terminal", but here referring to the telson). Størmer also compared ''Alkenopterus'' with '' Drepanopterus'' and '' Moselopterus'', placing them in the stylonuroid

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 ...

Drepanopteridae doubtfully alongside '' Onychopterella''. In 2004, paleontologists Markus Poschmann and Odd Erik Tetlie described a series of new fossils found in the Nellenköpfchen Formation, in the center of the

Rhenish Massif The Rhenish Massif, Rhine Massif or Rhenish Uplands (, : 'Rhenish Slate Uplands') is a geologic massif in western Germany, eastern Belgium, Luxembourg and northeastern France. It is drained centrally, south to north by the river Rhine and a few ...

, Germany. Among them were two new specimens of ''A. brevitelson'' from Alken, 624-D (a well-preserved prosoma with remains of appendages) and 697-D (a fragmentary specimen with the prosoma, appendages, preabdomen, the last two postabdominal segments and telson). They also redescribed the holotype of ''A. brevitelson''. Further, a specimen of a new species of ''Alkenopterus'' was found in another locality in the Nellenköpfchen Formation, near

Burglahr Burglahr is a municipality in the district of Altenkirchen, in Rhineland-Palatinate, Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north ...

(in the same

state State most commonly refers to: * State (polity), a centralized political organization that regulates law and society within a territory **Sovereign state, a sovereign polity in international law, commonly referred to as a country **Nation state, a ...

as Alken). PWL 2002/5011 LS is almost complete and well-preserved but somewhat distorted; it is the only known find of this species. It was named ''A. burglahrensis'', the specific name coming from Burglahr due to the proximity of its type locality with this municipality. ''A. burglahrensis'' was very similar to ''A. brevitelson'', the former having a longer and broader telson and more strongly expanded distal podomeres in appendage VI than the latter. Poschmann and Tetlie claimed to find no evidence of a podomere 7a in the sixth appendage of ''Alkenopterus'', determining that it could no longer be classified as part of Drepanopteridae, thus erecting the new family Alkenopteridae for it. This family was not assigned to any superfamily due to the poor knowledge of the second to fourth appendages of its only genus, ''Alkenopterus''. Alkenopteridae was distinguished by the ''Drepanopterus''-type B (with no podomere 7a) fifth appendage and an "''Alkenopterus''-type" sixth appendage, this one having podomeres almost equal in length (except the distal spine) and moderately flattened. Both pairs of appendages lacked spines. The ''Alkenopterus''-type leg was introduced as a new standard type of non-spiniferous eurypterid appendage. ''Alkenopterus'' (as well as Drepanopteridae, now

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 ...

) would be subsequently included in the Stylonurina

suborder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...

for not possessing the podomere 7a. However, in 2014, Poschmann reexamined the holotype of ''A. burglahrensis'' by carefully removing some of the

matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...

of the fossil and using

light microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical, el ...

with a higher

magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...

. This resulted in the detection of a movable podomere 7a instead of a simple projection as previously thought. Therefore, Poschmann assigned it to the family Onychopterellidae, with whom it shared several characteristics. It is assumed that in the fossils of ''A. brevitelson'', this podomere was not preserved, but if this is not the case, ''A. brevitelson'' should be reassigned to Stylonurina and ''A. burglahrensis'' to a new onychopterellid genus.

References

{{Taxonbar, from=Q4727682 Onychopterelloidea Devonian eurypterids Fossil taxa described in 1974 Fossils of Germany Eurypterids of Europe