Dinocarida (Order: Radiodonta, stem group arthropods)
Amplectobelua is an anomalocaridid. Anomalocaridids have been variously regarded as basal stem-lineage euarthropods (e.g. Daley et al., 2009), basal members of the arthropod group Chelicerata (e.g. Chen et al., 2004), and as a sister group to the arthropods (e.g. Hou et al., 2006).
Daley and Budd
Amplectobelua – from the Latin amplecto, “embrace,” and belua, “monster.”
stephenensis – from Mount Stephen (3,199 m), the mountain peak in Yoho National Park from which the specimens were collected. Named in 1886 for George Stephen, the first president of the Canadian Pacific Railway.
Holotype –ROM59492 in the Royal Ontario Museum, Toronto, Canada.
Burgess Shale and vicinity: none.
Other deposits: Amplectobelua symbrachiata from the Chengjiang Fauna in China.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Tulip Beds (S7) on Mount Stephen.
Brief history of research:
The Chengjiang specimens of Amplectobelua were first described as “anomalocaridid animal 2” in Chen et al. (1994) and given a formal designation as Amplectobelua symbrachiata by Hou et al. (1995). The Burgess Shale genus Amplectobelua stephenensis was described by Daley and Budd (2010) from six specimens of isolated appendages in the Royal Ontario Museum collections.
Amplectobelua stephenensis is known from isolated appendages that have twelve segments, with paired ventral spines on the second to ninth segments. The appendages range in size from 2.8 cm to 5.1 cm. The nearest podomere has a pair of large spines that extend most of the length of the whole appendage. There are also paired dorsal spines on the three furthest segments, which are long and curved towards the end of the appendage. No full-body specimens of A. stephenensis have yet been found, but it may have a similar morphology to A. symbrachiata and Anomalocaris, with wide swimming flaps on a dorsoventrally flattened body and a head with circular mouth parts and eyes on stalks (Chen et al., 1994).
Six specimens of Amplectobelua have been described from a single locality, the Tulip Beds (S7), on Mount Stephen.
Amplectobelua is considered a predator, based on the morphology of its frontal appendage. The wide range of mobility exhibited in the many-segmented appendage and the paired ventral spines means that they would have been ideal for gripping and manipulating prey items. The distal podomeres could be used to grasp prey with a scissor-like motion when brought into opposition against the proximal endites. Like other anomalocaridids, Amplectobelua has a streamlined body and would swim through the water column by undulating its lateral flaps to propel itself forward.