Stanleycaris hirpex (ROM 59944) – Holotype, part and counterpart. Individual claw. Specimen length = 29 mm. Specimen dry – polarized light. Stanley Glacier.
© ROYAL ONTARIO MUSEUM. PHOTOS: JEAN-BERNARD CARON
With its single pair of jointed frontal appendages, lateral swimming flaps, and a circular mouth structure, Stanleycaris 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 family Hurdiidae. However, other features like the small carapace, short head, and long, streamlined body probably represent ancestral radiodontan traits. Accordingly, Stanleycaris has been recovered as the sister species to all other hurdiid radiodontans in the most recent phylogenetic analysis (Moysiuk and Caron 2022).
Genus – From Stanley Glacier, 40 kilometres southeast of the Burgess Shale in Kootenay National Park, where the fossils come from and the Latin caris, meaning “shrimp.” The name Stanley was given after Frederick Arthur Stanley (1841-1908), Canada’s sixth Governor General.
species – from the Latin, hirpex, meaning “large rake,” in reference to the rake-like aspect of the appendage.
Burgess Shale and vicinity: none.
Other deposits: Stanleycaris sp. from the mid-Cambrian Wheeler Formation of Utah (Pates et al. 2017).
Stanley Glacier in Kootenay National Park and the Collins Quarry on Fossil Ridge, Yoho National Park. Smaller collections from the Walcott and Raymond Quarries on Fossil Ridge, Tokumm Creek, Mount Odaray, Mount Stephen, and Mount Field.
Isolated appendages and oral structures of this species were collected by the Royal Ontario Museum in 1996 from talus slopes at Stanley Glacier, but it was not until 2008, during a larger expedition, that additional specimens were discovered in their proper stratigraphic context. A description of this new genus and species soon followed (Caron et al. 2010). A single isolated appendage from the Wheeler Formation of Utah was described in 2017, representing the only possible occurrence reported outside of the Burgess Shale (Pates et al. 2017). Subsequently, it was noted that the original description of Stanleycaris did not satisfy the requirements of the International Code of Zoological Nomenclature due to its publication in an online-only repository prior to 2012 (Zhuravlev and Gámez Vintaned 2018). The name was ultimately validated by Pates, Daley, and Ortega-Hernández (2018). Moysiuk and Caron later redescribed the original type material, along with new specimens from Stanley Glacier and Tokumm Creek, in detail (2021). Most recently, a large collection of exceptional whole-body remains, mainly from ROM collections on Fossil Ridge in the 1980s and 90s, was recognized as belonging to the species, allowing the first full reconstruction (Moysiuk and Caron 2022). While there had been some debate as to whether radiodontans possessed externally developed body segmentation, the complete Stanleycaris material confirmed this, suggesting that traces of segmentation in other species are merely obscured due to preservation and weathering.
Stanleycaris has a short head and a relatively long body, the latter composed of 17 well-formed segments plus four filamentous tail blades. A small, oval, domed carapace covers the front of the head. The head bears a pair a pair of stalked compound lateral eyes and a large median eye situated behind the carapace. A pair of frontal appendages project from the head. Each has a series of curving, rake like inner spines, trident-shaped medial spines, and hooked outer spines. The mouth opening, on the underside of the head, is square and surrounded by an array of tooth-bearing plates. Each body segment has a row of gill blades along the underside and a pair of swimming flaps at the sides. The digestive tract includes a long foregut and a segmentally organized midgut. Elements of the nervous system of Stanleycaris are also preserved, including a two-segmented brain, visual processing centers in the eyes, and a pair of ventral nerve cords.
Collectively known from about 300 specimens, the species is most common at the Collins Quarry on Fossil Ridge and Stanley Glacier. It is rare at other localities.
The multitool-like frontal appendages of Stanleycaris could have enabled a variety of feeding functions. They are long and flexible, allowing grappling of prey with the hooked spines at their tips. The rake-like inner spines have been hypothesized to be adapted for sediment sifting, while trident-shaped medial spines would have opposed those on the opposite appendage, together acting like a jaw to tear and crush soft prey. Additional food processing would have been accomplished by the tooth-bearing plates around the mouth (Moysiuk and Caron 2019, 2021). The large compound eyes, particularly in juveniles, show that Stanleycaris was a visual hunter, but the relatively modest number of lenses (estimated at ~1000) suggest it may have been most active in low light conditions (Moysiuk and Caron 2022, 2023). 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). Stanleycaris has the most completely known growth series of any radiodontan, demonstrating that segments were added to the body during early development. Specimens of Stanleycaris showing shriveling of the body and flaps, disarticulation of the appendages and mouthparts, and loss of the top portion of the head have been interpreted as moult remains, providing direct evidence that Stanleycaris moulted periodically as it grew (Moysiuk and Caron 2022, 2023).
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