Aenigmastacus
Encyclopedia
Aenigmastacus crandalli is a species
of fossil
freshwater crayfish
. It was found in Eocene
lake deposits in British Columbia
, and was described in 2011. It is the first member of the Gondwana
-distributed family Parastacidae
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 3 centimetre. On some specimens, details of the internal anatomy can be seen due to the exceptional preservation.
, Canada
, at . The McAbee beds comprise a 30 metre thickness of siliceous sediments within a sequence dominated by volcaniclastic rocks. They are belived to have 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 Ypresian
(Lower Eocene). The deposition of a silica-rich sediment deriving from diatom
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, which may have aided the preservation of fossils.
Twelve "nearly complete" specimens of Aenigmastacus have been recovered, with a total body length of 3–5 cm (1.2–2 ). The carapace
is thin, but the form of the rostrum
is not known. The antennae
are as long as the carapace, but the eye
s have not been preserved on any of the known specimens. The first pair of pereiopods are larger than the others, and are armed with a nearly symmetrical pair of chelae
(claws). The pleon
(abdomen) is generally smooth, and ends in a broad, slightly tapering telson
with no diaeresis. The specimens of Aenigmastacus show exceptional preservation. In some specimens, dark areas formed of fluorapatite
can be seen, which appear to represent soft tissue
in a pattern congruent with the endophragmal skeleton of extant crayfishes. The trace of the alimentary tract is also visible in some specimens.
by Rodney Feldmann, Carrie Schweitzer and John Leahy in 2011. The genus name, Aenigmastacus, derives from the Latin
, meaning "inexplicable", and the name of the genus Astacus
, which is frequently used as a suffix for genus names of crayfish. The specific epithet crandalli commemorates the crayfish expert Keith Crandall of Brigham Young University
.
The chief distinction between the Northern Hemisphere superfamily Astacoidea and the Southern Hemisphere superfamily Parastacoidea is usually the form of the genitalia, 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, Martin Glaessner
and Horton H. Hobbs, Jr
. While no diaeresis is visible on the telson of Aenigmastacus, a diaeresis on the uropods, 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.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
freshwater crayfish
Crayfish
Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobsters, to which they are related...
. It was found in Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
lake deposits in British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, and was described in 2011. It is the first member of the Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
-distributed family Parastacidae
Parastacidae
Parastacidae is 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 taxa also in Antarctica.Three genera are to be found in Chile,...
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 3 centimetre. On some specimens, details of the internal anatomy can be seen due to the exceptional preservation.
Origin and description
Aenigmastacus was discovered in the McAbee Fossil Beds of British ColumbiaBritish Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, at . The McAbee beds comprise a 30 metre thickness of siliceous sediments within a sequence dominated by volcaniclastic rocks. They are belived to have 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 Ypresian
Ypresian
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between and , is preceded by the Thanetian age and is followed by the Eocene Lutetian age....
(Lower Eocene). The deposition of a silica-rich sediment deriving from diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
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, which may have aided the preservation of fossils.
Twelve "nearly complete" specimens of Aenigmastacus have been recovered, with a total body length of 3–5 cm (1.2–2 ). The carapace
Carapace
A carapace is a dorsal 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 underside is called the plastron.-Crustaceans:In crustaceans, the...
is thin, but the form of the rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
is not known. The antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
are as long as the carapace, but the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s have not been preserved on any of the known specimens. The first pair of pereiopods are larger than the others, and are armed with a nearly symmetrical pair of chelae
Chela (organ)
A chela is a pincer-like organ terminating certain limbs of some arthropods. The name comes from Greek through New Latin . The plural form is chelae. Legs bearing a chela are called chelipeds. Another name is claw because most chelae are curved and have a sharp point like a claw....
(claws). The pleon
Pleon
Pleon is a Europe based Public Relations and communications consultancy. It is part of the Brodeur Pleon Worldwide network and a subsidiary of BBDO Worldwide. Pleon belongs to the Omnicom Group. Pleon is headquartered in Düsseldorf, Germany.- Overview :...
(abdomen) is generally smooth, and ends in a broad, slightly tapering telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
with no diaeresis. The specimens of Aenigmastacus show exceptional preservation. In some specimens, dark areas formed of fluorapatite
Fluorapatite
Fluorapatite, often with the alternate spelling of fluoroapatite, is a mineral with the formula Ca53F . Fluorapatite is a hard crystalline solid. Although samples can have various color , the pure mineral is colorless as expected for a material lacking transition metals...
can be seen, which appear to represent soft tissue
Soft tissue
In anatomy, the term soft tissue refers to tissues that connect, support, or surround other structures and organs of the body, not being bone. Soft tissue includes tendons, ligaments, fascia, skin, fibrous tissues, fat, and synovial membranes , and muscles, nerves and blood vessels .It is sometimes...
in a pattern congruent with the endophragmal skeleton of extant crayfishes. The trace of the alimentary tract is also visible in some specimens.
Taxonomy
Aenigmastacus crandalli was described as a new genus and species in the Journal of Crustacean BiologyJournal of Crustacean Biology
The Journal of Crustacean Biology is a quarterly peer-reviewed scientific journal in the field of carcinology . It is produced by The Crustacean Society and, since 2005, the editor-in-chief has been Frederick Schram. According to the Journal Citation Reports, its 2009 impact factor was 0.974....
by Rodney Feldmann, Carrie Schweitzer and John Leahy in 2011. The genus name, Aenigmastacus, derives from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
, meaning "inexplicable", and the name of the genus Astacus
Astacus
Astacus is a genus of crayfish found in Europe and western Asia, comprising three extant and four extinct species....
, which is frequently used as a suffix for genus names of crayfish. The specific epithet crandalli commemorates the crayfish expert Keith Crandall of Brigham Young University
Brigham Young University
Brigham Young University is a private university located in Provo, Utah. It is owned and operated by The Church of Jesus Christ of Latter-day Saints , and is the United States' largest religious university and third-largest private university.Approximately 98% of the university's 34,000 students...
.
The chief distinction between the Northern Hemisphere superfamily Astacoidea and the Southern Hemisphere superfamily Parastacoidea is usually the form of the genitalia, 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, Martin Glaessner
Martin Glaessner
Martin Fritz Glaessner AM was a geologist and palaeontologist. Born and educated in Austro-Hungarian Empire, he spent the majority of his life in working for oil companies in Russia, and studying the geology of the South Pacific in Australia...
and Horton H. Hobbs, Jr
Horton H. Hobbs, Jr.
Horton Holcombe Hobbs, Jr. was an American taxonomist and carcinologist, specialising in freshwater decapods. He was also a capable artist, musician, cook and botanist....
. While no diaeresis is visible on the telson of Aenigmastacus, a diaeresis on the uropods, 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.