Amplectobelua stephenensis (ROM 59492) – Holotype. Individual claw. Specimen length = 51 mm. Specimen wet – polarized light. Tulip Beds (S7) on Mount Stephen.
© Royal Ontario Museum. Photo: Jean-Bernard Caron
Amplectobelua is a member of Radiodonta, the group of arthropods which also includes the more famous Anomalocaris (Collins 1996). The appendages bear a pair of greatly enlarged spines near their bases which form a pincer-like apparatus with the appendage tip. This provides a link with better preserved fossils from China (Chen et al. 1994; Liu et al. 2018), demonstrating membership to the genus Amplectobelua (Hou et al. 1995) of the Family Amplectobeluidae (Vinther et al. 2014).
Amplectobelua – from the Latin amplectus, “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.
Burgess Shale and vicinity: none.
Other deposits: Amplectobelua cf. stephenensis is known from the Wheeler Formation of Utah (Lerosey-Aubril et al. 2020). A second species, A. symbrachiata, is known from the Chengjiang Fauna in China (Hou et al. 1995) and possibly from the Kinzers Formation of Pennsylvania (Pates and Daley 2018).
The Tulip Beds (S7) on Mount Stephen.
The specimens of Amplectobelua from the Chinese Chengjiang deposits were first described as “anomalocaridid animal 2” (Chen et al. 1994) and given a formal designation as Amplectobelua symbrachiata by Hou et al. (1995). The Burgess Shale species A. stephenensis was described by Daley and Budd (2010) from six isolated appendages in the Royal Ontario Museum collections. Several papers have subsequently discussed the functional morphology of the appendages of Amplectobelua based on comparisons with modern arthropods and 3D modeling (Liu et al. 2018; de Vivo et al. 2021) as well as the homology of different appendage regions (Moysiuk and Caron 2021). Other discoveries in China have yielded new anatomical information about the genus, particularly related to the feeding apparatus which has been argued to include spinous plates associated with three anterior body segments in addition to the circlet of oral plates shared with other radiodontans (Cong et al. 2017, but see Moysiuk and Caron 2021).
Amplectobelua stephenensis is known only from isolated appendages that have thirteen segments including a hooked terminal spine (Moysiuk and Caron 2021). The segment nearest to the body has a pair of thick spines nearly as long as the whole appendage which are directed at an angle towards the tip of the appendage, forming a pincer. Segments 2 to 9 have tiny paired inner spines. The appendages range in size from 2.8 cm to 5.1 cm (Daley and Budd 2010). There are also paired outer 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 had a similar morphology to A. symbrachiata and Anomalocaris, with wide swimming flaps on a dorsoventrally flattened body and a head with 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 pincer-like, many-segmented appendage would have been ideal for gripping and manipulating prey items (de Vivo et al. 2021). The distal podomeres could be used to grasp prey with a scissor-like motion when brought into opposition against the proximal endites (Liu et al. 2018). Like other anomalocaridids, Amplectobelua has a streamlined body and would swim through the water column by undulating its lateral flaps to propel itself forward (Usami 2006).
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