''Archaeoniscus'' () is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of prehistoric

isopod Isopoda is an Order (biology), order of crustaceans. Members of this group are called isopods and include both Aquatic animal, aquatic species and Terrestrial animal, terrestrial species such as woodlice. All have rigid, segmented exoskeletons ...

s that first appeared during the

Bajocian In the geologic timescale, the Bajocian is an age and stage in the Middle Jurassic. It lasted from approximately 170.9 ±0.8 Ma to around 168.2 ±1.2 Ma (million years ago). The Bajocian Age succeeds the Aalenian Age and precedes the Bathonia ...

stage of the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

. It is a widespread genus with a paleogeographic distribution encompassing the continental margin environments of the central

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

and the western

Tethys Ocean The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...

. Fossils of ''Archaeoniscus'' suggest that this genus lived in diverse aquatic habitats, including the marine, paralic, and freshwater environments. While earlier descriptions suggested that it may have had an ectoparasitic association with fishes, some researchers argue that at least two species, ''A. aranguthyorum'' and ''A. coreaensis'', lived a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

free-living lifestyle based on morphological characteristics that are either unsuitable for or unrelated to parasitic behavior. Six species have been named: two species are known from the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

period rocks in

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, and the other four are known from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period rocks in Europe,

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

and

East Asia East Asia is a geocultural region of Asia. It includes China, Japan, Mongolia, North Korea, South Korea, and Taiwan, plus two special administrative regions of China, Hong Kong and Macau. The economies of Economy of China, China, Economy of Ja ...

. Of these species, the taxonomic position of the only

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

species, ''A. texanus'', is uncertain, for some researchers claim that this species should be neglected because of the significant morphological differences in comparison to the other species within the genus. Some specimens of the most completely known species, ''A. aranguthyorum'', suggest that the female individuals of this species possibly had brood pouches, indicating that the genus may have had

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

. An alternative interpretation suggests that the brood pouches may actually be the impressions of the

hindgut The hindgut (or epigaster) is the posterior ( caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal junct ...

and

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

. Additionally, two unnamed indeterminate species are known from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

rocks in

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

and

Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...

Discovery

Reverend

Peter Bellinger Brodie Peter Bellinger Brodie (1815 – 1 November 1897) was an English geologist and churchman, the son of the conveyancer Peter Bellinger Brodie and nephew of Sir Benjamin C. Brodie. He was born in London in 1815. While residing with his fathe ...

first discovered the crustacean fossils from the

Purbeck Group The Purbeck Group is an Upper Jurassic to Lower Cretaceous lithostratigraphy, lithostratigraphic group (stratigraphy), group (a sequence of rock strata) in south-east England. The name is derived from the district known as the Isle of Purbeck in ...

in the Vale of Wardour,

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

. He asked

Henri Milne-Edwards Henri Milne-Edwards (23 October 1800 – 29 July 1885) was a French zoologist. Biography Henri Milne-Edwards was the 27th child of William Edwards, an English planter and colonel of the militia in Jamaica and Elisabeth Vaux, a Frenchwoman. Hen ...

to examine some specimens and formally describe them. The specimens sent to Milne-Edwards measured in length, though other descriptions indicate their lengths were about . Brodie claimed that he also found larger specimens that are about the size of small

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s. Milne-Edwards identified that the specimens belong to an order

Isopoda Isopoda is an order of crustaceans. Members of this group are called isopods and include both aquatic species and terrestrial species such as woodlice. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed l ...

, and suggested that it may belong within the range of a family Cymothoidae, possibly an intermediate species between '' Serolis'' and '' Cymothoa''. In 1843, Milne-Edwards proposed a genus name ''Archaeoniscus'' and a species name ''A. brodiei'' in honor of the discoverer Rev. P. B. Brodie. From the same lithostratigraphic group, Charles Willcox collected various insect fossils and sent them to John O. Westwood. Upon his examination, Westwood found several isopod specimens which varied in size, being either equal to or larger than ''A. brodiei''. Westwood considered them to be a new species of ''Archaeoniscus'', so he designated these specimens under a new species name ''A. edwardsii'' in honor of Milne-Edwards. While ''A. brodiei'' is discovered from the

Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...

( Oxfordian-

Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...

) strata in England,

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

and

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 ...

, ''A. edwardsii'' is only discovered from the Early Cretaceous (Berriasian) strata in England, though both species may be synonymous. In 1918, Wilhelm Haack erected a monotypic family Archaeoniscidae and claimed that it is distinct from the suborder

Oniscidea Woodlice are terrestrial isopods in the suborder Oniscidea. Their name is derived from being often found in old wood, and from louse, a parasitic insect, although woodlice are neither parasitic nor insects. Woodlice evolved from marine isopods ...

based on the large

pleotelson 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 of the arthropod body, or an additional division that is not a true segment on account of ...

, the shapes of antennulae, antennae and

uropod Uropods are posterior appendages found on a wide variety of crustaceans. They typically have functions in locomotion. Definition Uropods are often defined as the appendages of the last body segment of a crustacean. An alternative definition sugge ...

s. In 1992, Robert W. Wieder and Rodney M. Feldmann examined various isopod fossils from

Austin Chalk The Austin Chalk is an upper Cretaceous geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas. The formation is made up of chalk and marl. Fossils The putative galloansera ...

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

, United States. Of these fossils, Wieder and Feldmann considered some specimens that are only known from posterior exuviae to belong to ''Archaeoniscus'' based on the raised axial node on the pleotelson which they claim to be distinct from almost every other known isopod species. Although they tentatively referred these specimens under a new species name, ''A. texanus'', they acknowledged that the taxonomic placement of these specimens is difficult; still, they considered this placement prudent enough and better than erecting a new genus name and establishing the taxonomic affinities for the specimens. The stratigraphic age of Austin Chalk is during the

(

Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by ...

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya ( million years ago) and 83.6 ± 0.7 m ...

), so ''A. texanus'' is the youngest known species of this genus. In 1998, Feldmann and his colleagues described a new species, ''A. aranguthyorum'', which was discovered from the Tlayúa Formation in

. In 2012, Park and his colleagues described a new freshwater-living species, ''A. coreaensis'', which was discovered from the

Jinju Formation The Jinju Formation (), also known as the Dongmyeong Formation () in some literature is an Early Cretaceous geologic Formation (geology), formation in South Korea.South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...

. Both species, ''A. aranguthyorum'' and ''A. coreaensis'', lived during the

(

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

). In 2014, a new species, ''A. italiensis'', was discovered from Plattenkalk Beds of Monte Fallano,

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

. Within the genus, ''A. italiensis'' is the oldest known species that lived during the

(

Bathonian In the geologic timescale the Bathonian is an age (geology), age and stage (stratigraphy), stage of the Middle Jurassic. It lasted from approximately 168.2 ±1.2 annum, Ma to around 165.3 ±1.1 Ma (million years ago). The Bathonian Age succeeds ...

). Unnamed

species have been discovered from the

Valanginian In the geologic timescale, the Valanginian is an age or stage of the Early or Lower Cretaceous. It spans between 137.05 ± 0.2 Ma and 132.6 ± 0.2 Ma (million years ago). The Valanginian Stage succeeds the Berriasian Stage of the Lower Cretac ...

Hauterivian The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous Epoch or a stage in the Lower Cretaceous Series. It spans the time between 132.6 ± 2 Ma and 125.77 (million years ago). The Hauterivian is preceded by the Valangi ...

stage rocks in Mexico and from the

stage rocks in

Description

''Archaeoniscus'' generally has an oval-shaped body which is dorsoventrally compressed. The lateral margins of the pereion (the

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...

of crustaceans) consist of broad, overlapping coxal plates with distinguishable sutures. The head is embedded into the first pereonite (the first segment of the pereion); it also possesses mandibles positioned transversely with a broad incisor process and has globular eyes on its lateral margin. The four anterior pleonites (parts of the

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 ...

of crustaceans called

pleon The anatomy of a decapod consists of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment – often called a somite – may possess one pair of appendages, although in various groups these ma ...

) are long and wide, while the fifth pleonite is relatively reduced and does not extend to the lateral margin. ''Archaeoniscus'' lacks marginal spines on its broad and semicircular

, the structure formed by the fusion of the last abdominal segment of an arthropod and its

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 unmodified ambulatory

pereiopod The anatomy of a decapod consists of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment – often called a somite – may possess one pair of appendages, although in various groups these m ...

s have distinct coxal dorsal articulations, which are distinguishable from modern cymothoids, suggesting that the genus was not suited for ectoparasitic behavior. The antennulae is large and multiarticulate, and the spine-like uropodal rami have a narrow, parallel-sided protopod. ''A. texanus'' has significant morphological differences in comparison to other species within this genus, including the presence of a long fifth pleonite, triangular epimeres, shorter pleotelson and more robust uropod.

Classification

''Archaeoniscus'', initially described as a possible cymothoid and now contained in its own monotypic family Archaeoniscidae, has an unclear phylogenetic position within the order

. This controversial classification may have occurred because the earlier studies have misinterpreted the anatomical characteristics of ''Archaeoniscus'', such as the reconstructions provided by Haack in 1918 and by Vega and his colleagues in 2005, and the morphological data of this genus is insufficient for a definitive suprafamilial classification. In 1969, Robert R. Hessler argued that some features of this genus resemble those of the suborder Oniscidea, but acknowledged that the features noted by Haack in 1918 does distinguish the genus from that suborder. Although it was commonly placed within the suborder Flabellifera, this view has been disputed since 1989 by Johann Wolfgang Wägele. In 1998, Feldmann and his colleagues claimed that possible brood pouches found in some specimens of ''A. aranguthyorum'' may indicate that ''Archaeoniscus'' might belong to the suborder

Sphaeromatidea Sphaeromatidea is a suborder of isopod crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea ...

; they assumed that the presence of brood pouch is a characteristic only observed in female sphaeromatid isopods. Following this suggestion, Vega and his colleagues in 2005 placed this genus within the superfamily Sphaeromatoidea of the suborder Sphaeromatidea. However, Brandt and his colleagues in 2003 did not consider the genus within the suborder Sphaeromatidea, but considered it to be an ancestral taxon of terrestrial isopods without specific reasoning behind their argument. In 2012, Park and his colleagues argued that Feldmann and his colleagues have mistaken the impressions of the

and

of ''A. aranguthyorum'' as possible brood pouches and that all female isopods have brood pouches, and thus the taxonomic position of this genus is still unclear. They also argued that ''A. texanus'' should be removed from the genus, as it significantly differs from other species, possessing flat limbs and five subequal pleonites with the fifth being the longest; in contrast, other species of ''Archaeoniscus'' possess narrow, rod-like limbs and only four free subequal pleonites with the fifth being reduced. Vega and his colleagues in 2019 also advocated the view that ''A. texanus'' is morphologically different from other species and that a new genus name should be erected for this species. They also considered that the latest occurrence of ''Archaeoniscus'' may more likely be the

stage of the

Paleobiology

As the earlier descriptions of this genus indicated a taxonomic association with cymothoids, Feldmann and his colleagues in 1998 considered that ''Archaeoniscus'' may have been

ectoparasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s on fishes just like modern cymothoids. Park and his colleagues in 2012 disagreed with this view, claiming that the morphological characteristics of ''A. aranguthyorum'' and ''A. coreaensis'' suggest otherwise. The researchers pointed out that the anterior limbs of ''Archaeoniscus'' are unmodified and ambulatory, contrary to those of modern cymothoids which resemble hooks. They also stated that the shape of the limbs and the flattened body resemble that of serolids or other sphaeromatids which live in

benthic zone The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

s. Their interpretation suggests that at least two species of ''Archaeoniscus'', ''A. aranguthyorum'' and ''A. coreaensis'', likely lived a benthic lifestyle without any parasitic association with fishes. Additionally, while Feldmann and his colleagues suggested that ''Archaeoniscus'' had

based on the possible existence of brood pouches, Park and his colleagues also disagreed with this claim, arguing that Feldmann and his colleagues misidentified the impressions of the hindgut and anus.

References

External links

* {{Taxonbar, from=Q4785512 Isopod genera Late Jurassic crustaceans Late Cretaceous crustaceans Middle Jurassic animals of Europe Late Jurassic animals of Europe Cretaceous arthropods of North America Early Cretaceous arthropods of Asia Bajocian genus first appearances Santonian genus extinctions Fossil taxa described in 1843 Cretaceous Mexico Cretaceous Egypt Jurassic England Jurassic France Cretaceous England Cretaceous Texas Cretaceous South Korea Jurassic Italy Bathonian genera Callovian genera Oxfordian genera Kimmeridgian genera Tithonian genera Valanginian genera Berriasian genera Hauterivian genera Barremian genera Aptian genera Albian genera Cenomanian genera Turonian genera Coniacian genera Fossils of Texas Fossils of England Fossils of France Fossils of Mexico Fossils of Italy Fossils of South Korea Early Cretaceous arthropods of Europe Jurassic arthropods of Europe Middle Jurassic crustaceans Early Cretaceous crustaceans Crustaceans described in 1843