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

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

genera of the Early and Middle

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

. 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 fos ...

and the Chengjiang biota. It is a member of the family Leanchoiliidae in the group Megacheira.

Morphology

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

—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 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 ...

("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'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...

''. 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 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 (), a large, diverse group of mainly aquatic arthrop ...

, and was placed in a clade with ''

Leanchoilia ''Leanchoilia'' is a megacheiran marine arthropod known from Cambrian deposits of the Burgess Shale in Canada and the Chengjiang biota of China. Description ''L. superlata'' was about long and had long, whip-like flagellae extending from i ...

'', '' Actaeus'' and '' Yohoia''. 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, Habelia, Sarotrocercus ''and ''

Sidneyia ''Sidneyia'' is an extinct marine arthropod known from fossils found from the Early to the Mid Cambrian of China and the Mid Cambrian Burgess Shale of British Columbia, Canada. Description Sidneyia size comparison.svg, Size comparison of ''Si ...

'', somewhere within the arachnomorpha. 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 Megacheira.

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 and from

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

, giving it a long stratigraphic range. Including 596 juveniles, 618 specimens of ''Alalcomenaeus'' are known from the Greater Phyllopod bed, where they comprise 1.2% of the community.

References

External links

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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