Cambroraster falcatus, isolated H-element ROMIP 65316
With its single pair of jointed frontal appendages, lateral swimming flaps, and circular mouth structure, Cambroraster possesses all the hallmarks of Radiodonta, part of the stem group to the true arthropods which also includes the iconic Anomalocaris (Collins 1996). The frontal appendages with comb or rake-like inner spines are characteristic of the radiodont family Hurdiidae. Phylogenetic analysis found it to be closely related to Titanokorys from the Burgess Shale and Zenghecaris from the Chengjiang deposit, which share similarities in carapace shape and a large number of finely-spaced spines on the appendages (Caron and Moysiuk 2021).
Cambroraster – Cambro, for Cambrian; raster, for the rake-like morphology of the inner spines on the frontal appendages.
falcatus – meaning sickle-shaped, but more specifically in reference to the dorsal carapace’s resemblance to the fictional Millennium Falcon starship in the Star Wars franchise.
Burgess Shale and vicinity: None
Other deposits: Cambroraster sp. from the early Cambrian Chengjiang biota (Liu et al. 2020); Cambroraster cf. C. falcatus from the mid-Cambrian Mantou Formation of north China (Sun et al. 2020).
Marble Canyon and Mount Whymper / Tokumm Creek, Kootenay National Park, British Columbia.
Several specimens of Cambroraster were discovered at the Marble Canyon and North Tokumm sites in Kootenay National Park in 2014. Because of their distinctive shape, the head carapaces were nicknamed the “spaceship.” Isolated frontal appendages were initially tentatively assigned to the genus Hurdia (Caron et al. 2014). The affinities of Cambroraster were not well-understood until further finds of abundant material at North Tokumm in 2018. The genus and species were formally described in 2019 (Moysiuk and Caron 2019). 3D digital modeling of an appendage of Cambroraster found it to have the lowest potential degree of appendage articulation of any of the studied radiodontans (de Vivo et al. 2021).
The defining feature of Cambroraster falcatus is its large, horseshoe-shaped dorsal carapace. This carapace is rounded frontally and projects along the rear sides into elongate wing-like projections lined along their margins with small spines. The rear central part of the carapace extends into a bilobate projection. Between the lateral “wings” and central projection are deep notches that accommodate the elliptical eyes, which are directed upwards. On the underside, the head is protected by two additional plates, shaped like elongate paddles and joined together at the front by their narrow ends. A circular, tooth-lined jaw and a pair of jointed frontal appendages with five long, curving, strong rake-like inner spines are located on the underside, near the front of the head. The body is stout, shorter than the dorsal carapace, and composed of 11 segments bearing rows of stacked gill blades and short lateral swimming flaps plus four short tail blades.
Cambroraster is abundant in Kootenay National Park, being known from over 100 specimens. It is particularly abundant around the North Tokumm locality, and may occur by the dozens on certain bedding planes, suggesting gregarious mass moulting behaviour. Rarer remains are known from Marble Canyon and single, isolated carapace fragments are known from Mount Stephen and Mount Field.
Like other hurdiids, Cambroraster shows adaptations to sweep feeding. Specifically, the stout and rigid frontal appendages are ill-suited to grasping large, mobile prey (de Vivo et al. 2021). Instead, the rake-like inner spines on the appendages form a rigid, basket-like apparatus of spines surrounding the mouth. Sideways movements of the appendages could have disturbed the sediment, sifting out burrowing organisms, and transferring them to the mouth for further processing (Moysiuk and Caron 2019). Compared to related hurdiids like Hurdia and Stanleycaris, the particularly numerous and finely-spaced, strong, hooked secondary spines on the inner spines could have enabled capture of minute benthic organisms, although larger prey may also have been consumed. As one of the largest animals in the Marble Canyon and Tokumm communities, Cambroraster would have been near the top of the food chain. The broad dorsal carapace, upward facing eyes, and stubby body suggest it spent most of its time near the sea floor (Moysiuk and Caron 2019). As in other radiodontans, swimming was facilitated by undulation of the lateral flaps while respiration would have been accomplished primarily through the rows of gill blades on the body (Usami 2006; Daley et al. 2013).