Unranked clade Megacheira? (stem group arthropods)
Actaeus is thought to be closely related to Leanchoilia (Bruton and Whittington, 1981). If confirmed, Actaeus would belong to a clade of “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).
Actaeus – from the Greek Actaeus, the first king of Attica. Attica is an historical region of Greece which includes Athens today.
armatus – from the Greek armos, “joint,” and the Latin atus, “provided with.” The name refers to the large frontal appendages of this species.
Holotype – USNM 155597 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Simonetta (1970) described this organism on the basis of a single specimen originally collected by Walcott. Simonetta and Delle Cave (1975) provided the first reconstruction of this animal in a general monograph about Burgess Shale arthropods. The taxon was redescribed by Whittington (1981), and compared to Leanchoilia (Bruton and Whittington, 1983).
The single specimen reaches about 6.6 cm in length, excluding the anterior great appendages. The head has a trapezoidal head shield, a pair of large anterolateral pedunculate eyes, one pair of frontal appendages (the “great appendage”) and three pairs of smaller biramous appendages. The distal podomere of the great appendage has claws and two pairs of slim, flexible branches attached to the next podomere. The outer branch of the biramous limbs has a lobate shape fringed by lamellae and the inner branch is jointed and tapers distally. The trunk is composed of eleven segments, with the first eight showing clear lobes fringed by lamellae. The last segment terminates with a triangular and apparently smooth plate.
A single specimen is known of this species.
Like similar and better known forms such as Leanchoilia and Alalcomenaeus the primary mode of locomotion of Actaeus 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 long filaments of the great appendages were probably sensory and suggest an active predatory habit, consistent with the large stalked eyes. Actaeus most likely hunted organisms that lived either in or on the sediment.