Exceptional multifunctionality in the feeding apparatus of a mid-Cambrian radiodont

Abstract

Radiodonts (stem Euarthropoda) were ecologically diverse, but species generally displayed limited functional specialization of appendages along the body axis compared with crown group euarthropods. This is puzzling, because such functional specialization is considered to have been an important driver of euarthropod ecological diversification. One way to circumvent this constraint could have been the functional specialization of different parts of the frontal appendages, known to have been ecologically important in radiodonts. This hypothesis has yet to be tested explicitly. Here we redescribe the poorly known mid-Cambrian hurdiid radiodont Stanleycaris hirpex from the Burgess Shale (Stephen Formation) and quantitatively assess functional specialization of the frontal appendages of stem euarthropods. The appendages of Stanleycaris are composed of 14 podomeres, variously differentiated by their possession of pectinate endites, mono- to trifurcate medial gnathites, and outer spines. The oral cone is tetraradially organized and can be uniquely distinguished from those of other hurdiids by the presence of 28 rather than 32 smooth tridentate plates. Our phylogenetic analysis finds Stanleycaris in a grade of hurdiids retaining plesiomorphic raptorial appendicular functionality alongside derived adaptations for sweep feeding and large, bilaterally opposed gnathites. We conclude that the latter performed a masticatory function, convergent with gnathal structures like mandibles in various panarthropods. Taken together, Stanleycaris and similar hurdiids provide an extreme example of the evolution of division of labor within the appendage of a stem euarthropod and suggest that this innovation may have facilitated the functional transition, from raptorial to sweep feeding, at the origin of the hurdiid clade.

Furongian (Jiangshanian) occurrences of radiodonts in Poland and South China and the fossil record of the Hurdiidae

“…Our understanding of the morphological diversity of radiodont oral cones has considerably increased over the last 25 years, and families or even genera are now known to display unique combinations of circumoral features (Daley & Bergström, 2012;Zeng et al, 2018a;Liu et al, 2018;Sun, Zeng & Zhao, 2020a;Moysiuk & Caron, 2021). Our re-study shows that the Polish oral cone actually comprises 32 plates in total; it also considers some aspects of the morphology omitted in the original description that are necessary to re-assess the affinities of this fossil using modern standards.…”

“…1B for possible evidence of the contrary). Accordingly, we follow the terminology first introduced in the Hurdiidae diagnosis of Lerosey-Aubril & Pates (2018), and describe hurdiid frontal appendages as composed of: (1) a proximal region (or shaft/peduncle sensu Pates, Daley & Butterfield, 2019) typically composed of rarely more than one podomere (but see Lerosey-Aubril & Pates, 2018;Moysiuk & Caron, 2021), and usually associated with a single (absent in Cambroraster, possibly two in Pahvantia) distally-located endite that differs in size (narrower and usually shorter), shape, or orientation from intermediate endites;…”

“…(2) an intermediate region of classically five or six podomeres (possibly up to eight podomeres in Cordaticaris; Sun, Zeng & Zhao, 2020b), which are associated with long (length of endite >height of corresponding podomere), typically laminiform endites; and (3) a distal region that includes up to six podomeres (e.g., Stanleycaris; Moysiuk & Caron, 2021) bearing much shorter, spiniform endites or no endites at all. In addition, when applied to the appendage: proximal and distal refer to the parts closest to or furthest from its insertion site on the body, respectively; ventral to the typically concave margin bearing the endites, and dorsal to the usually convex margin opposite to it.…”

“…The family includes the genera Aegirocassis, Buccaspinea, Cambroraster, Cordaticaris, Hurdia, Pahvantia, Peytoia, Stanleycaris, Ursulinacaris, and possibly Schinderhannes and Zhenghecaris (Pates et al, 2021a). The endites of the frontal appendage from Guole are laminiform, unlike in Ursulinacaris, and narrowly-spaced, unlike in both Stanleycaris and Ursulinacaris (Pates, Daley & Ortega-Hernández, 2017;Pates, Daley & Ortega-Hernández, 2018a;Pates, Daley & Butterfield, 2019;Moysiuk & Caron, 2021). They bear auxiliary spines, not setae like Aegirocassis or Pahvantia (Van Roy, Daley & Briggs, 2015;Lerosey-Aubril & Pates, 2018), and these spines are apparently all shorter than the width of the endite they project from, which markedly differs from the conditions observed in Buccaspinea (Pates et al, 2021a), Cambroraster (Moysiuk & Caron, 2019), Cordaticaris (Sun, Zeng & Zhao, 2020a), Schinderhannes (Kühl, Briggs & Rust, 2009), and to a lesser extent Hurdia (Daley, Budd & Caron, 2013).…”

“…Considering our present knowledge-the oral cones of tamisiocaridids and the peculiar radiodont Caryosyntrips are unknown-the Furongiame of the first ann specimen remains best assigned to the Hurdiidae and more specifically to the genus Peytoia, as originally tentatively proposed by Masiak & Żylińska (1994). Other representatives of this family, for which data on the oral cone are available, display 28 plates in total (Stanleycaris; Moysiuk & Caron, 2021), or nodes on the plates (Cordaticaris, Stanleycaris; Sun, Zeng & Zhao, 2020a;Moysiuk & Caron, 2021), or a subquadrate, rather than subrectangular central opening (e.g., Cambroraster, Hurdia; Zeng et al, 2018a;Sun, Zeng & Zhao, 2020a), features that do not characterize the Polish specimen. External nodes would be visible on an external mould, and a notable post-burial elongation of an originally square central opening would have resulted in the conspicuous elongation of some plates and the widening of others, which is not observed in the fossil.…”