Surusicaris elegans, holotype ROMIP 62976. Specimen dry – direct light (left column), dry – polarized light (right column).
Surusicaris is a close relative of Isoxys, as indicated by the type of carapace, eyes, and frontal pair of raptorial appendages (Aria & Caron, 2015). The presence of spines on the dorsal side of the frontal appendage is a character shared with radiodontans, such as Anomalocaris. Current evidence draws out a consensus among authors placing isoxyids as sister taxa to true arthropods (Edgecombe, 2020; Aria, 2022), although it is not clear whether Surusicaris and Isoxys are part of a single separate lineage (that is, form a monophyletic group).
Surusicaris – After Surus, “the Syrian,” which would have been the last elephant of Hannibal, with broad shields covering its sides and missing a tusk.
elegans – Referring to the delicate, laced appearance of the limbs.
Burgess Shale and vicinity: None
Other deposits: None
Marble Canyon, Kootenay National Park, British Columbia.
Along with Yawunik kootenayi (Aria, Caron & Gaines, 2015), Surusicaris elegans (Aria & Caron, 2015) is one of the first two new arthropods described from the Marble Canyon locality of the Burgess Shale. The original study was based on a single specimen from the original 2012 expedition. No other specimen has been confirmed so far, in the Burgess Shale or elsewhere. Surusicaris has remained a critical taxon in understanding the place of isoxyids in the transition to a euarthropod body plan (Fu et al., 2022; Aria, 2022).
Surusicaris elegans is about 15mm long and enclosed in a broad carapace made of two semi-circular valves, without spines. Only the posterior extremity of the body and tailpiece remain uncovered. The animal has a well-defined head composed of, at the front, a pair of large spherical eyes and a segmented predatory appendage, and, at the back of the head, under the carapace, three pairs of short limbs with a lobopod aspect. The frontal appendages show a complex ornament of spines on both the ventral and dorsal margins. The trunk limbs are clearly bipartite, forming two separate but similar branches. As for Isoxys, external segmentation of the trunk is not clearly visible. Inside the body, a bold, black trace runs alongside the gut and branches out inside one limb branch, showing similarities to hemolymphatic (“blood”) channels (Aria & Caron, 2015).
A single specimen from the Marble Canyon quarry.
The large lateral eyes and clawed frontal appendages suggest Surusicaris was an active predator, like its close relative Isoxys (Legg & Vannier, 2013). Trunk limbs lack strong functional modifications, but their lobate aspect in addition to their position underneath the carapace indicates that Surusicaris was mostly a swimmer (Aria & Caron, 2015). Although some authors implied a pelagic lifestyle (Vannier & Chen, 2000), isoxyids are commonly found among benthic/nektobenthic assemblages (Caron & Jackson, 2008) and possess general morphological characteristics of other nektobenthic Cambrian arthropods.