Stanleycaris is an extinct, monotypic genus of hurdiid radiodont from the middle Cambrian (Miaolingian). The type species is Stanleycaris hirpex. Stanleycaris was described from the Stephen Formation near the Stanley Glacier[1] and Burgess Shale locality of Canada,[2] as well as Wheeler Formation of United States.[3] The genus was characterized by the rake-like frontal appendages with robust inner spines.[4][2]
Stanleycaris was originally described only from frontal appendages and oral cone.[3] However, in 2022, 268 specimens of Stanleycaris, many of which were complete, were studied, making Stanleycaris a well documented radiodont.[2] Stanleycaris had three eyes, a bizarre configuration previously unknown among other radiodont genera; yet this head anatomy supports early differentiation among arthropod head and trunk segmentation.[2]
The original description of the taxon appeared in an online supplement to the article published by Jean-Bernard Caron, Robert R. Gaines, M. Gabriela Mángano, Michael Streng and Allison C. Daley in 2010.[1] That description did not satisfy of the requirements of the International Code of Zoological Nomenclature, as the Code did not accept taxa named in electronic publications as validly named until 2012;[3] the name was eventually validated by Pates, Daley & Ortega-Hernández (2018).[5]
Its generic name means "Crab of Stanley Glacier"; hirpex, L. "large rake", reflects the rake-like nature of its spiny frontal appendages.[1]
Stanleycaris was a small radiodont, with whole body specimen measured around 1 to 8.3 centimetres (0.39 to 3.27 in), excluding the tail. Even based on the largest, 3 centimetres (1.2 in)-long isolated frontal appendage, the upper body length was thought to be less than 20 centimetres (7.9 in) long.[2] Unlike most hurdiids with large head and broad neck region, the body of Stanleycaris was streamlined like those of anomalocaridids and amplectobeluids.[2]
The small head occupies about 15% of the total body length. Each lateral compound eye was estimated to have around 1000 ommatidia. In addition of a pair of stalked lateral eyes, a third, large median eye was located behind its preocular sclerite (H-element). Similar structures were evident in the fossils of Peytoia and Lyrarapax, suggesting these genera possibly had a median eye too.[2] Contrary to the agreement around mid and late 2010s (based on the discovery of Lyrarapax since 2014) suggest radiodonts had only protocerebrum on their cerebral ganglion,[6] The neuroanatomical evidence of Stanleycaris suggests radiodonts have both protocerebrum and deutocerebrum, and circumesophageal connective that surround digestive system between them.[2] Unlike other radiodonts, the paired lateral sclerites (P-element) are not evident in any Stanleycaris specimens, suggesting they were exclusively absent in this genus.[2]