''Aenigmastacus crandalli'' is a

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), ...

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 ...

freshwater crayfish. It was found in early

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

Okanagan Highlands lake deposits in

British Columbia British Columbia is the westernmost Provinces and territories of Canada, province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that ...

, and was described in 2011. It is the first member of the

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

-distributed family

Parastacidae The Parastacidae are the family of freshwater crayfish found in the Southern Hemisphere. The family is a classic Gondwana-distributed taxon, with extant members in South America, Madagascar, Australia, New Zealand, and New Guinea, and extinct ta ...

to be found in the Northern Hemisphere, and is the only species in the genus ''Aenigmastacus''. Twelve specimens are known, with a total body length of . On some specimens, details of the internal anatomy can be seen due to the exceptional preservation.

Distribution

''Aenigmastacus'' was discovered in the Tranquille Formation's McAbee Fossil Beds of

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

, at . The McAbee beds comprise a thickness of

siliceous Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant ...

sediments within a sequence dominated by

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

rocks. They are formed from lake deposits, based on the presence of five families of freshwater fish and other terrestrial taxa, and the fossil-bearing sediments have been dated to the

Early Eocene In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

. The deposition of a silica-rich sediment deriving from

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

s indicates a calm environment with little influx of terrestrial silt. There is an alternation of dark and light layers, which may indicate a seasonal change, such as freezing over in winter. The bottom of the lake was probably

anoxic Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...

, which may have aided the preservation of fossils.

Description

Twelve "nearly complete" specimens of ''Aenigmastacus'' have been recovered, with total body lengths of between . 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 ...

is thin and the form of the

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * 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 ...

is not known. The antennae are as long as the carapace, but the

eye 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 ...

s have not been preserved on any of the known specimens. The first pair of

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 are larger than the others, and are armed with a nearly symmetrical pair of

chelae A chela ()also called a claw, nipper, or pinceris a pincer-shaped organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through Neo-Latin '. The plural form is chelae. Legs bearing a chela are called chelipeds ...

(claws). The

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 ...

(abdomen) is generally smooth, and ends in a broad, slightly tapering

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 ...

with no diaeresis. The specimens of ''Aenigmastacus'' show exceptional preservation. In some specimens, dark areas formed of

fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blu ...

can be seen, which appear to represent

soft tissue Soft tissue connective tissue, connects and surrounds or supports internal organs and bones, and includes muscle, tendons, ligaments, Adipose tissue, fat, fibrous tissue, Lymphatic vessel, lymph and blood vessels, fasciae, and synovial membranes.� ...

in a pattern congruent with the endophragmal skeleton of extant crayfishes. The trace of the

alimentary tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...

is also visible in some specimens.

Taxonomy

''Aenigmastacus crandalli'' was described as a new genus and species in the ''

Journal of Crustacean Biology The ''Journal of Crustacean Biology'' is a quarterly peer-reviewed scientific journal in the field of carcinology (crustacean research). It is published by The Crustacean Society and Oxford University Press (formerly by Brill Publishers and Alle ...

'' by Rodney Feldmann, Carrie Schweitzer and John Leahy in 2011. The genus name, ''Aenigmastacus'', 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 ...

', meaning " inexplicable", and the name of the genus ''

Astacus ''Astacus'' (from the Ancient Greek, Greek , ', meaning "lobster" or "crayfish") is a genus of crayfish found in Europe, comprising three Extant taxon, extant (living) species and three somewhat contested extinct species. These crayfish are foun ...

'', which is frequently used as a suffix for genus names of crayfish. The

specific epithet In Taxonomy (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...

''crandalli'' commemorates the crayfish expert Keith A. Crandall of

George Washington University The George Washington University (GW or GWU) is a Private university, private University charter#Federal, federally-chartered research university in Washington, D.C., United States. Originally named Columbian College, it was chartered in 1821 by ...

. The chief distinction between the Northern Hemisphere superfamily Astacoidea and the Southern Hemisphere superfamily Parastacoidea is usually the form of the

genitalia A sex organ, also known as a reproductive organ, is a part of an organism that is involved in sexual reproduction. Sex organs constitute the primary sex characteristics of an organism. Sex organs are responsible for producing and transporting ...

, but these have not been preserved in any known specimen of ''Aenigmastacus''. Feldmann ''et al.'' therefore relied on the form of the telson, which has a diaeresis (division into two halves) in Astacoidea, but none in Parastacoidea; this distinction was noted by

Thomas Henry Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist who specialized in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...

, Martin Glaessner and Horton H. Hobbs, Jr. While no diaeresis is visible on the telson of ''Aenigmastacus'', a diaeresis on the

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, which is seen in both superfamilies, is preserved in some specimens of ''Aenigmastacus'', suggesting that its apparent absence from the telson is not merely an artefact of preservation.

Biogeography

Fossils of freshwater crayfish are extremely rare. Only 13 species are known in total, and only two from North America, the Astacidae species '' Pacifastacus chenoderma'' and the Cambaridae species ''

Procambarus primaevus ''Procambarus'' is a genus of crayfish in the family Cambaridae, all native to North and Central America. It includes a number of troglobitic species, and the marbled crayfish (''marmorkrebs''), which is parthenogenetic. Originally described a ...

'', both of which belong to families still represented in North America. ''Aenigmastacus'', by contrast, belongs to the Southern Hemisphere family Parastacidae, and is that family's only Northern Hemisphere representative.

References

{{Taxonbar, from=Q4687834 Parastacidae Eocene crustaceans Freshwater crustaceans of North America Fossil taxa described in 2011 Monotypic decapod genera