Alalcomenaeus
Encyclopedia
Alalcomenaeus is one of the most widespread and longest surviving arthropod
genera of the Early and Middle Cambrian
. Known from over 300 specimens in the Burgess shale
, and the Chengjiang, it bears great similarity to the opabiniids.
; and two more head appendages posterior to that. Like its body appendages, these were biramous — 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
("tail") which probably helped to propel the organism; this ended with long flat spikes in the plane of the tail fin.
. 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.
, Actaeus
and Yohoia
. It has more recently been placed with 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
, 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. The recently described Dicranocaris appears to be very closely related indeed to Alalcomenaeus.
, 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
, giving it a long stratigraphic range.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
genera of the Early and Middle Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
. Known from over 300 specimens in the Burgess shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
, and the Chengjiang, it bears great similarity to the opabiniids.
Morphology
Alalcomenaeus had three median eyes; two stalked, more lateral eyes; a triflagellate great appendageGreat appendage
Great appendages are large claw-like appendages which attach to the heads of the "great appendage arthropods", a group whose monophyly is debated, but which includes the anomalocaridids...
; and two more head appendages posterior to that. Like its body appendages, these were biramous — 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
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...
("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 AnomalocarisAnomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...
. 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 LeanchoiliaLeanchoilia
Leanchoilia is a four-eyed arachnomorph arthropod known from the Cambrian Burgess shale. It was about 5cm long and had long, whip-like feelers mounted on frontal arm-like appendages. Its guts are sometimes preserved in three dimensions....
, Actaeus
Actaeus (Trilobitoid)
Actaeus is a Cambrian organism with a resemblance to the great appendage arthropods. It is known from a single specimen recovered from the Burgess Shale of British Columbia, Canada. The specimen is over 6 cm long and has a body consisting of a head shield, 11 body tergites, and a terminal plate....
and Yohoia
Yohoia
Yohoia is a tiny, extinct animal 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 arachnomorphs, a group of arthropods that includes the chelicerates and trilobites. Their sizes range from 7 to 23 mm...
. It has more recently been placed with 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
Sanctacaris is a Middle Cambrian arthropod from the Burgess Shale of British Columbia. It was most famously regarded as a primitive chelicerate, a group which includes spiders and scorpions, although subsequent phylogenetic studies have not always supported this conclusion; it is best accommodated...
, Habelia
Habelia
Habelia is an extinct genus of arthropod from the Middle Cambrian. Its fossils have been found in the Burgess Shale in British Columbia, Canada. -Sources:* Invertebrate Palaeontology & Evolution by Euan Neilson Kerr Clarkson...
, Sarotrocercus
Sarotrocercus
Sarotrocercus is a small Cambrian organism known from Burgess shale-type localities, reaching a centimetre or two in length. It was pelagic and swam on its back...
and Sidneyia
Sidneyia
Sidneyia is an extinct arthropod known from fossils found in the Cambrian-age Burgess Shale formation of British Columbia. -General description:...
, 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. The recently described Dicranocaris appears to be very closely related indeed to Alalcomenaeus.
Distribution
Alalcomenaeus is very rare in the Walcott quarryWalcott Quarry
The Walcott Quarry is the most famous quarry of the Burgess Shale, bearing the Phyllopod beds. This lies at the base of the Walcott Quarry member, and three other quarries – the Raymond, UE and EZ – lie above it...
, so was described on the basis of half a dozen specimens. As other exposures of the Burgess shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
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
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, giving it a long stratigraphic range.