Unranked clade Megacheira? (stem group arthropods)
Alalcomenaeus is closely related to Leanchoilia, and together with Yohoia and some Chengjiang taxa, they form the “great appendage” arthropods, or Megacheira (Hou and Bergström, 1997; Wills et al., 1998; Cotton and Braddy, 2004). The phylogenetic placement of the megacheirans is uncertain and they are considered to be either stem-lineage chelicerates (Chen et al., 2004; Edgecombe, 2010) or upper stem-lineage euarthropods (Budd, 2002).
Alalcomenaeus – from the Greek Alalcomenae, the birthplace of the goddess Athena.
cambricus – of Cambrian age, derived originally from Cambria in Wales.
Holotype – USNM 155658 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: Possibly one species from the Lower Cambrian Chengjiang biota, China, described originally as Alalcomenaeus? illecebrosus (referred to as Leanchoilia illecebrosa by Hou and Bergström, 1997).
Middle Cambrian, Glossopleura Zone and Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Collins Quarry on Mount Stephen, and the Walcott and Raymond Quarries on Fossil Ridge.
Brief history of research:
Simonetta (1970) described this organism on the basis of a handful of specimens originally collected by Walcott on Fossil Ridge. The taxon was redescribed by Whittington (1981), who accepted only two of Simonetta's original specimens as Alalcomenaeus. The recovery of over 300 new specimens from Collins Quarry on Mount Stephen allowed for a more detailed description of the taxon (Briggs and Collins, 1999).
The largest specimens of Alalcomenaeus reach about 6 cm in length, excluding the anterior great appendages. The head has a trapezoidal head shield, a pair of ventral pedunculate eyes, three (or possibly five) median eyes, one pair of large frontal appendages (the “great appendage”) and two pairs of smaller biramous appendages. Each great appendage has three long whip-like extensions. The outer branch of the biramous limb is flap-like and the inner branch bears spines. The trunk is composed of eleven segments, each bearing a pair of biramous appendages, and the paddle-like tail is fringed with short, sharp spines.
Just a few dozen specimens of Alalcomenaeus have been found in the Burgess Shale at several horizons on Fossil Ridge. Alalcomenaeus represents 0.04% of the Walcott Quarry community (Caron and Jackson, 2008). It is more common in the Glossopleura Zone on Mount Stephen, where over 300 specimens have been found in one single locality (Collins Quarry), but in general this species is rare.
The primary mode of locomotion was probably swimming, powered both by the wave-like fanning of its lateral flaps and flicks of its tail. The outer leg branches also served as gills. The spiny inner branches are not well suited to walking, but might have been used to macerate food. The long filaments of the great appendages were probably sensory and suggest an active predatory habit, consistent with the large, downward-facing stalked eyes and spinose inner leg branches. Alalcomenaeus most likely hunted organisms that lived either in or on the sediment.