''Archaeocaris'' (meaning "ancient shrimp") is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

mantis shrimp Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 340 million years ago. Mantis shrimp typically grow to around in length ...

that lived in North America during the

Early Carboniferous Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

period. Though it was placed as a member of the family Perimecturidae until 2008, it is currently deemed the only genus in the family Archaeocarididae, and contains two species. The type species, ''A. vermiformis'', was described by

Fielding Bradford Meek Fielding Bradford Meek (December 10, 1817 – December 22, 1876) was an American geologist and a paleontologist who specialized in the invertebrates. Biography The son of a lawyer, he was born in Madison, Indiana. In early life he was in ...

in 1872 from specimens collected at the base of the Waverly Group in

Kentucky Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virgini ...

. A second species, ''A. graffhami'', was named by Harold Kelly Brooks in 1962 based on a fossil found in the

Caney Shale Shale gas in the United States is an available source of unconventional natural gas. Led by new applications of hydraulic fracturing technology and horizontal drilling, development of new sources of shale gas has offset declines in production fro ...

of Oklahoma, with additional remains later found in the Pilot Shale of

Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...

. Members of this genus were small animals, with ''A. vermiformis'' reaching a length of and the largest known ''A. graffhami'' growing long. A

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 und ...

covers the head and part of the

thorax The thorax or chest is a part of the anatomy of humans, 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 divisions of the ...

, extending further back at the sides than on the top. The

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the tors ...

is made up of six segments and a circular or ovular cross section. The

telson The telson () is the posterior-most 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 accou ...

is ovoid, with broad

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 at its sides. ''A. vermiformis'' has proportionally larger

mandibles In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

than ''A. graffhami'', and furrowed abdominal

sclerite A sclerite ( Greek , ', meaning " hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonl ...

s as opposed to the smooth sclerites of the latter species. Like other mantis shrimps, ''Archaeocaris'' was a

carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other ...

with

raptorial The term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod's foreleg that make it function for the grasping of prey while it is consumed, where the gripping surfaces are formed from the o ...

thoracic appendages. These are lined with conical spikes on the penultimate segment and would have been used to grasp prey.

History of discovery

''Archaeocaris'' was named in 1872 by American paleontologist

, with ''A. vermiformis'' as the type and only known species at the time. The generic name combines the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

words (, meaning "ancient") and (, meaning "shrimp"), in reference to its ancient age. Meek studied several specimens of the animal preserved in gray

phosphatic In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

concretions found at the base of the Waverly Group near

Danville, Kentucky Danville is a home rule-class city in Boyle County, Kentucky, United States. It is the seat of its county. The population was 17,236 at the 2020 Census. Danville is the principal city of the Danville Micropolitan Statistical Area, which include ...

, which include the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

and seven

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 nor a syntype). O ...

s. These fossils are deposited in the collection of the

US National Museum The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7. ...

. Meek believed these remains were too imperfectly preserved to determine the animal's classification, though a possible relationship with the modern

Cumacea Cumacea is an order of small marine crustaceans of the superorder Peracarida, occasionally called hooded shrimp or comma shrimp. Their unique appearance and uniform body plan makes them easy to distinguish from other crustaceans. They live in sof ...

is proposed. In 1897, Prussian-born American zoologist

Arnold Edward Ortmann Arnold Edward Ortmann (April 8, 1863 – January 3, 1927) was a Prussian-born United States naturalist and zoologist who specialized in malacology. Biography Ortmann was born in Magdeburg, Prussia on April 8, 1863. A student of Ernst Haeckel, h ...

analysed an additional 27 fossils representing 18 individuals, which he determined to belong to the same species named by Meek. These specimens, kept in the collections of

Princeton University Princeton University is a private research university in Princeton, New Jersey. Founded in 1746 in Elizabeth as the College of New Jersey, Princeton is the fourth-oldest institution of higher education in the United States and one of the n ...

, were found in

Boyle County, Kentucky Boyle County is a county located in the U.S. state of Kentucky. As of the 2020 census, the population was 30,614. Its county seat is Danville. The county was formed in 1842 and named for John Boyle (1774–1835), a U.S. Representative, ch ...

, at or near the same locality where Meek's specimens originate. Ortmann concluded that ''A. vermiformis'' was so similar to known fossils of ''

Crangopsis ''Crangopsis'' is an extinct genus of crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepo ...

'' that it should not be placed in a separate genus, and thus renamed the species as ''Crangopsis vermiformis'', rendering ''Archaeocaris'' a junior synonym of ''Crangopsis''. Further analysis by American paleontologist Harold Kelly Brooks in 1962 found that the aforementioned fossils show clear features of

s, and that the species represented by these remains is unrelated to ''Crangopsis'', thus the genus ''Archaeocaris'' was revalidated. In addition to the type species, Brooks described a second species of ''Archaeocaris'' which he named ''A. graffhami'', based on a single specimen collected from the

Pontotoc County, Oklahoma Pontotoc County is in the south central part of Oklahoma. As of the 2010 census, the population was 37,492. Its county seat is Ada. The county was created at statehood from part of the Chickasaw Nation in Indian Territory. It was named for a ...

. 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 ...

honors Allen Graffham, who found this specimen. While this species was initially known from only one specimen, additional fossils of ''A. graffhami'' would later be found in the upper Pilot Shale on Bactrian Mountain of the

Pahranagat Range The Pahranagat Range is a mountain range in Lincoln County, Nevada. References See also * Pahranagat Valley The Pahranagat Valley is a Tonopah Basin landform in Lincoln County, Nevada. The more fertile part of Pahranagat Valley is a narro ...

, Nevada, and described in 1979 by American paleontologist

Frederick Schram Frederick Robert Schram (born August 11, 1943, in Chicago, Illinois) is an American palaeontologist and carcinologist. He received his B.S. in biology from Loyola University Chicago in 1965, and a Ph.D. on palaeozoology from the University of Ch ...

. Furthermore, Brooks reclassified '' Perimecturus fraiponti'' (described by Belgian paleontologist

Victor van Straelen Victor van Straelen (14 June 1889 – 29 February 1964) was a Belgian conservationist, palaeontologist and carcinologist. Van Straelen was born in Antwerp on 14 June 1889, and worked chiefly as a palaeontologist until his retirement in 1954. H ...

in 1932) as a species of ''Archaeocaris'' in his 1962 publication. However, this species was reassigned again to '' Tyrannophontes'' in 1984 and '' Gorgonophontes'' in 2004 (which it is currently placed in), leaving only ''A. vermiformis'' and ''A. graffhami'' as valid species of ''Archaeocaris''.

Description

A small

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

, specimens of ''Archaeocaris vermiformis'' range from in length (not counting the

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...

), with half of the known individuals measuring long. ''A. graffhami'' is the larger of the two species, with the biggest specimen measuring about long. The

cephalothorax 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 ''ceph ...

makes up a third of the body length. The

is smooth, covering the head and part of the

. It is fused to the frontmost thoracic segments and covers them entirely, while wing-like projections extend from the side of the carapace to the border of the thorax and

. As a result, the carapace covers the sides of the last three thoracic segments while leaving the top of them exposed. The

are well

sclerotised Sclerotin is a component of the cuticle of various Arthropoda, most familiarly insects. It is formed by cross-linking members of particular classes of protein molecules, a biochemical process called sclerotization, a form of tanning in which qu ...

, and are proportionally smaller in ''A. graffhami'' than in ''A. vermiformis''; the largest ''A. graffhami'' specimen has mandibles measuring 1.7% the length of its body, while ''A. vermiformis'' has mandibles about 2.3% the body length. The rostrum is triangular and plate-like, with a length equal to its width at the base. The second to fifth pairs of thoracic appendages were

, allowing the animal to grasp prey. Because these appendages are clustered together in the fossils, study of their structure is difficult. They fold into a Z-shape, with the

ischium The ischium () form ...

pointing forwards while the segment after it points backwards, and the

dactylus The dactylus is the tip region of the tentacular club of cephalopods and of the leg of some crustaceans (see arthropod leg). In cephalopods, the dactylus is narrow and often characterized by the asymmetrical placement of suckers (i.e., the ven ...

(final segment) points forwards again. Conical spikes are present on the propodus (penultimate segment), opposing the sharp dactylus to form a set of grasping claws. The abdomen has an ovular or circular cross section, and is made up of six segments. It makes up most of the animal's length and is around twice as long as the cephalothorax. The bottom margins of the

s (hardened plates) on each abdominal segment is straight and horizontal, aside from a small upwards curvature at the front. While the abdominal sclerites of ''A. vermiformis'' have marked furrows, those of ''A. graffhami'' are undecorated. The telson is smooth and ovoid in shape, with no indication of it narrowing into a spike like in ''

Perimecturus ''Perimecturus'' is an extinct genus of mantis shrimp that lived during the Early Carboniferous period in what is now Scotland and the United States. The first known specimens were collected near the River Esk, Dumfries and Galloway, River Esk in ...

'', though it does become more pointed towards the end. Small protrusions known as caudal furcae are present on the telson of ''A. vermiformis'', giving it a forked appearance. The

s can be seen as broad, blade-like lobes.

Classification

When it was first described, the classification of ''Archaeocaris'' was unclear. Relationships with modern crustaceans were initially proposed, with Meek (1872) and Ortmann (1897) suggesting the genus had affinities with

and

Mysidacea The Mysidacea is a group of shrimp-like crustaceans in the superorder Peracarida, comprising the two extant orders Mysida and Lophogastrida and the prehistoric Pygocephalomorpha The order Pygocephalomorpha is an extinct group of peracarid cr ...

respectively. However, both authors overlooked that the animal had features distinctive of mantis shrimps. These were first noticed by Brooks (1962), who recognized ''Archaeocaris'' to be an early mantis shrimp and placed it in the family Perimecturidae, believing it was a close relative of ''

''. Brooks assigned this family to the order Palaeostomatopoda (now delisted as a

suborder Order ( la, ordo) 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 ...

and named Palaeostomatopodea). As cladistic analyses grew widespread in usage, it became clear that the palaeostomatopods are a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

grouping, as first discovered by Jenner ''et al.'' (1998). This finding was then further confirmed by Schram (2007). In addition, both studies found that ''Archaeocaris'' is an early-diverging lineage not closely related to ''Perimecturus''. Therefore, the genus was moved to a separate family named Archaeocarididae in 2008. This family is

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 "unispec ...

, with ''Archaeocaris'' as its type and only genus, and is placed within Palaeostomatopodea (which is still used in a paraphyletic sense for the convenience of referring to the

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Definition An evolutionary grade is a group of s ...

). Several studies including Jenner ''et al.'' (1998), Schram (2007), Haug ''et al.'' (2010) and Smith ''et al.'' (2023) have conducted

phylogenetic analyses In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

on fossil mantis shrimps, and have all recovered ''Archaeocaris'' as a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic ...

genus and the earliest-diverging lineage within the order

Stomatopoda Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 340 million years ago. Mantis shrimp typically grow to around in length ...

, placing it as a

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to all other members of the order. The results of the analysis from Smith ''et al.'' (2023) are displayed below:

Palaeoenvironment

Unlike other known mantis shrimps of the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

(which lived in

shallow marine Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this ...

brackish 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 estuari ...

-freshwater environments), both species of ''Archaeocaris'' are believed to have inhabited deep water in open seas. There is some overlap in the chronological ranges of the two species, but they occur in different geographic areas and thus would not have coexisted with each other. All known remains of ''A. vermiformis'' were found in Boyle County, Kentucky and originate from deposits at the base of the Waverly Group, dating to the

Tournaisian The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from Ma to Ma. It is preceded by the Famennian (the uppermost stag ...

stage of the Carboniferous period (

Kinderhookian The Mississippian ( , also known as Lower Carboniferous or Early Carboniferous) is a subperiod in the geologic timescale or a subsystem of the geologic record. It is the earlier of two subperiods of the Carboniferous period lasting from roughl ...

stage in the North American regional series, approximately 358.9 to 348 million years ago). The deposits in which this species is found has also preserved fossils of brachiopods (such as ''

Lingula Lingula is Latin for "little tongue". It can stand for: * ''Lingula'' (brachiopod), a brachiopod genus of the family Lingulidae, which is among the few brachiopods surviving today but also known from fossils over 500 million years old * Lingala la ...

'', ''

Productus ''Productus subaculeatus'' is an extinct species of brachiopods. Its fossils are present in the Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, ...

'' and ''

Spirifer ''Spirifer'' is a genus of marine brachiopods belonging to the order Spiriferida and family Spiriferidae. Species belonging to the genus lived from the Middle Ordovician ( Sandbian) through to the Middle Triassic (Carnian) with a global distri ...

''), bivalves (such as ''

Aviculopecten ''Aviculopecten'' is an extinct genus of bivalve mollusc that lived from the Early Devonian to the Late Triassic in Asia, Australia, Europe, North America, and South America.Palaeoneilo'' and ''

Schizodus Schizodus is an extinct genus of shallow marine clams. It lived from the Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the begin ...

''), bryozoans (such as ''

Fenestella Fenestella (c. 52 BC – c. AD 19) was a Roman historian and encyclopaedic writer. Biography He flourished in the reign of Tiberius. According to Jerome, he lived from 52 BC to AD 19 (according to others 35 BC – AD 36).Pliny the Elder, '' Natur ...

''), conulariids (such as '' Conularia'') and crustaceans (such as ''

Palaeopalaemon ''Palaeopalaemon'' is an extinct genus of the oldest lobster-like aquatic decapod crustaceans, containing the species ''Palaeopalaemon newberryi''. References {{Taxonbar, from1=Q98077933, from2=Q107155530 Decapods Prehistoric crustacean ge ...

'' and ''

Ceratiocaris ''Ceratiocaris'' is a genus of paleozoic phyllocarid crustaceans whose fossils are found in marine strata from the Upper Ordovician until the genus' extinction during the Silurian. They are typified by eight short thoracic segments, seven longe ...

''). This assemblage is indicative of 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 ancient Greek, βένθος (bénthos), meaning "t ...

level community in an open marine environment. The species ''A. graffhami'' has a wider chronological range, with the geologically oldest specimens originating from the upper Pilot Shale of Nevada and dating back to the earliest Kinderhookian (around 358.9 million years ago) of the Tournaisian stage. Remains of

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...

s, and

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, ...

s are also known from this site. In addition, the type specimen of ''A. graffhami'' was collected from the

in Oklahoma, a younger site which dates to the late

Viséan The Visean, Viséan or Visian is an age in the ICS geologic timescale or a stage in the stratigraphic column. It is the second stage of the Mississippian, the lower subsystem of the Carboniferous. The Visean lasted from to Ma. It follows the ...

stage (latest

Meramecian The Meramecian or Maramec stage is a sequence of Mississippian rocks in the Mississippi River Valley. It is named for the Meramec River in Missouri. Members Included are the St. Louis Limestone, Ste. Genevieve Limestone, Salem Limestone, and the ...

or earliest Chesteran stages in the North American regional series, as recently as 330.9 million years ago), making it the youngest known record of the genus. Here, remains of the species are found alongside those of the brachiopod ''

Linoproductus ''Linoproductus'' is an extinct genus of brachiopod belonging to the order Productida and family Linoproductidae. Specimens have been found in Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans ...

'', the bivalve '' Caneyella'', several

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...

s and numerous

conodont Conodonts ( Greek ''kōnos'', " cone", + ''odont'', " tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, w ...

s. In both the Pilot Shale and Caney Shale, the associated fossil assemblage is largely made up of species typically found in the bottom communities of open deep water, suggesting this was the preferred habitat of ''A. graffhami''.

References

External links

{{Taxonbar, from=Q124958811 Carboniferous animals of North America Stomatopoda Fossil taxa described in 1872 Prehistoric crustacean genera Prehistoric mantis shrimps