Alalcomenaeus Temporal range: | |
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Fossil holotype | |
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Life restoration | |
Scientific classification ![]() | |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | † Megacheira |
Order: | † Leanchoilida |
Family: | † Leanchoiliidae Simonetta & Delle Cave, 1975 |
Genus: | † Alalcomenaeus Simonetta, 1970 |
Species: | †A. cambricus |
Binomial name | |
†Alalcomenaeus cambricus Simonetta, 1970 |
Alalcomenaeus is one of the most widespread and longest-surviving arthropod genera of the Early and Middle Cambrian. [1] Known from over 300 specimens in the Burgess Shale and the Chengjiang biota. It is a member of the family Leanchoiliidae [2] in the group Megacheira.
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—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. [1] 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. [1]
The organism probably swam, wafting its outer flap-limbs in waves along its body to gain propulsion, in the manner of Anomalocaris . [1] 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. [1] 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. [1] 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. [1]
Alalcomenaeus was initially thought to fall in the stem group to the crustacea, and was placed in a clade with Leanchoilia , Actaeus and Yohoia . [1] It was subsequently suggested to be related to the opabiniids, mainly on the basis of its great appendage. [1] 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, [3] and Actaeus is often excluded from such works as it is poorly understood. [1] Currently, it is included within Megacheira. [4]
Alalcomenaeus is very rare in the Walcott quarry of the Burgess Shale, 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. [1] Including 596 juveniles, 618 specimens of Alalcomenaeus are known from the Greater Phyllopod bed, where they comprise 1.2% of the community. [5]