''Alalcomenaeus'' is one of the most widespread and longest-surviving

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

genera of the Early and Middle Cambrian. Known from over 300 specimens in the

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fo ...

and the Chengjiang biota. it is a member of the group

Megacheira Megacheira ("great hands") is an extinct class of predatory arthropods that possessed a pair of great appendages, hence the class name as well as the common name "great appendage arthropods". Their taxonomic position is controversial, with stud ...

Morphology

''Alalcomenaeus'' had three median eyes; two stalked, more lateral eyes; a triflagellate

great appendage Great appendages are claw-like appendages which attach to the heads of the "great appendage arthropods", a name usually refers to Megacheira, a class of extinct arthropod characterized by a pair of "short-great appendages" bearing in front of the ...

; and two more head appendages posterior to that. Like its body appendages, these were

biramous 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, plu ...

—their inner branch was spiny, segmented, flexible and leg-like, while the outer portion had a large surface area and resembled a flap. ''Alalcomenaeus'' reached about 6 cm in length, although many smaller specimens are known. Its head was covered with a shield, and its eleven body segments were also covered with an exoskeleton. Its body terminated with a paddle-like

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 ...

("tail") which probably helped to propel the organism; this ended with long flat spikes in the plane of the tail fin.

Ecology

The organism probably swam, wafting its outer flap-limbs in waves along its body to gain propulsion, in the manner of ''

Anomalocaris ''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...

''. Its inner limb branches do not appear to be optimised for walking, although it is possible they helped the organism move along the sea floor. A variety of other functions have been suggested, such as clinging to algal fronds; they seem best suited to grabbing onto, and tearing up, other animals, suggesting that the organism was probably a scavenger. However, its large eyes and the long flagella on its great appendages, combined with its large feeding apparatus and the spines on its inner limb branches, are more consistent with a predatory lifestyle, and the most recent interpretation has it feeding on organisms that lived on or in the surface of the sea floor.

Affinity

''Alalcomenaeus'' was initially thought to fall in the stem group to the crustacea, and was placed in a clade with ''

Leanchoilia ''Leanchoilia'' is an megacheiran arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China. It was about long and had long, whip-like feelers mounted on frontal arm-like appendages. Its internal o ...

'', ''

Actaeus In Greek mythology, Actaeus (; Ancient Greek: Ἀκταῖος ''Ἀktaῖos'' means "coast-man"), also called Actaeon, was the first king of Attica, according to Pausanias. Family Actaeus was the father of Aglaurus, and father-in-law to Cecr ...

'' and ''

Yohoia ''Yohoia'' is an extinct genus of megacheiran arthropod from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. It has been placed among the arachnomorpha, a group of arthropods that in ...

''. It was subsequently suggested to be related to the opabiniids, mainly on the basis of its great appendage. More up to date analyses place it along with ''Leanchoilia'' and ''Yohoia'' as well as ''

Sanctacaris ''Sanctacaris'' is a Middle Cambrian arthropod from the Burgess Shale of British Columbia. It was most famously regarded as a stem-group chelicerate, a group which includes horseshoe crabs, spiders and scorpions, although subsequent phylogenetic ...

, Habelia, Sarotrocercus ''and ''

Sidneyia ''Sidneyia'' is an extinct arthropod known from fossils found from the Early Cambrian-age Maotianshan Shales to the Mid Cambrian Burgess Shale formation of British Columbia. 144 specimens of ''Sidneyia'' are known from the Greater Phyllopod be ...

'', somewhere within the

arachnomorph Arachnomorpha is a proposed subdivision or clade of Arthropoda, comprising the group formed by the trilobites and their close relatives (Artiopoda), Megacheira (which may be paraphyletic) and chelicerates. Under this proposed classification sche ...

a. The closeness of ''Sanctacaris'' and ''Leanchoilia'' has been supported by subsequent work, and ''Actaeus'' is often excluded from such works as it is poorly understood. Currently, it is included within

Distribution

''Alalcomenaeus'' is very rare in the Walcott quarry of the

, so was described on the basis of half a dozen specimens. As other exposures of the Burgess Shale were unearthed, it became apparent that the creature was, in fact, a dominant member of the fauna. It has also been discovered in the

Chengjiang Chengjiang (; earlier Tchinkiang) is a city located in Yuxi, Yunnan Province, China, just north of Fuxian Lake. Administrative divisions Chengjiang City has 2 subdistricts and 4 townships. ;2 subdistricts * Fenglu () * Longjie () ;4 towns C ...

and from

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

, giving it a long stratigraphic range. Including 596 juveniles, 618 specimens of ''Alalcomenaeus'' are known from the Greater

Phyllopod bed The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess Shale fossil ''Lagerstätte''. It was quarried by Charles Walcott from 1911–1917 (and later named Walcott Quarry), and was t ...

, where they comprise 1.2% of the community.

References

External links

*
Images
of fossils from the Chengjiang. {{Taxonbar, from=Q289293 Megacheira Prehistoric arthropod genera Burgess Shale fossils Cambrian genus extinctions Wheeler Shale Fossil taxa described in 1970 Cambrian arthropods