Xenusia (Order: Scleronychophora, stem group onychophorans)
Aysheaia is regarded as a member of the “lobopodans,” a group of vermiform Cambrian organisms possessing pairs of leg-like extensions of the body. The affinities of these animals are controversial; they have been placed at the base of a clade comprised of anomalocaridids and arthropods (Budd, 1996), or in a stem-group to modern onychophorans (Ramsköld and Chen, 1998).
Aysheaia – after the nearby Aysha peak (since renamed Ayesha peak) in the Wapta icefield (3,065 m); original meaning unknown.
pedunculata – from the Latin pedunculus, “foot.”
Holotype –USNM57655 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none
Other deposits: A.? prolata from the Middle Cambrian of Utah (Robison, 1985).
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Walcott originally described Aysheaia as an annelid worm (Walcott, 1911). It was later re-described as a velvet worm (or a close relative) (Brues, 1923; Hutchinson, 1930; Walcott, 1931; Walton, 1927), although it lacked features such as jaws and slime glands. Its position remains a subject of debate, with a position in a new phylum being mooted (Tiegs and Manton, 1958). A morphological reinterpretation based on photographs (Delle Cave and Simonetta, 1975) prompted a detailed re-study of the fossil specimens (Whittington, 1978), and relationships were suggested with the water bears (tardigrades) (Bergström, 1978). Aysheaia is now grouped with close relatives in the class Xenusia (Liu et al., 2008), lobopods that fall on the arthropod stem lineage (Budd, 1996, 1998; Whittington, 1978).
Aysheaia is a worm-like animal, 1 to 6 cm in length and about 5 mm broad, bearing ten pairs of clawed, spiny limbs on the lower part of its body. It did not have a separate head, but a mouth occupied the very front of the body, accompanied by a pair of appendages and a circlet of bumps (papillae). The animal had a soft, flexible, non-mineralized cuticle, which had a corrugated, accordion-like form. Each stubby limb had ten corrugations, some of which bore a spiny projection. A suite of claws also adorned the end of each stub-foot. A faint line running down the axis of the organism is interpreted as its gut.
Aysheaia is rare in the Walcott Quarry representing less than 0.04% of the specimens counted in the community (Caron and Jackson, 2008).
Aysheaia is frequently associated with the remains of sponges, and an ecological association has been posited. Whether Aysheaia used its spines to adhere to sponges while feeding on them, or whether it simply hid among sponges for protection from predators, is unclear.