First report of paired ventral endites in a hurdiid radiodont

Abstract

Background: Radiodonta, large Palaeozoic nektonic predators, occupy a pivotal evolutionary position as stem-euarthropods and filled important ecological niches in early animal ecosystems. Analyses of the anatomy andphylogenetic affinity of these large nektonic animals have revealed the origins of the euarthropod compound eye andbiramous limb, and interpretations of their diverse feeding styles have placed various radiodont taxa as primaryconsumers and apex predators. Critical to our understanding of both radiodont evolution and ecology are the pairedfrontal appendages; however, the vast differences in frontal appendage morphology between and within differentradiodont families have made it difficult to identify the relative timings of character acquisitions for this body part.Results: Here we describe a new genus of hurdiid, Ursulinacaris, from the middle Cambrian (Miaolingian, Wuliuan) MountCap Formation (Northwest Territories, Canada) and Jangle Limestone (Nevada, USA). Ursulinacaris has the sameorganisation as other hurdiid frontal appendages, with elongate endites on the first five podomeres in the distalarticulated region and auxiliary spines on the distal margin of endites only. Unlike all other hurdiid genera, whichpossess a single row of elongated and blade-like ventral endites, this taxon uniquely bears paired slender endites.Conclusion: The blade-like endite morphology is shown to be a hurdiid autapomorphy. Two other frontal appendagecharacters known only in hurdiids, namely auxiliary spines on the distal margin of endites only, and elongate enditeson the first five podomeres in the distal articulated region only, predate this innovation.

Occurrence of the hurdiid radiodontCambrorasterin the middle Cambrian (Wuliuan) Mantou Formation of North China

“…In addition, Cambroraster represents the first discovery of hurdiid radiodonts in North China. This group has been found previously in Laurentia (e.g., Briggs et al, 2008;Daley et al, 2009Daley et al, , 2013Caron et al, 2010;Pates et al, 2017Pates et al, , 2018Pates et al, , 2019aLerosey-Aubril and Pates, 2018;Kimmig et al, 2019), South China (e.g., Liu, 2013;Zeng et al, 2017;Liu et al, 2020), West Gondwana (e.g., Van Roy and Briggs, 2011;Van Roy et al, 2015), and Baltica (e.g., Chlupác and Kordule, 2002;Daley and Legg, 2015). The discovery of a hurdiid in North China indicates that this group as a whole had a good dispersal ability, which could have contributed to its successful colonization in various ecological niches (Daley and Budd, 2010;Vinther et al, 2014;Van Roy et al, 2015;Lerosey-Aubril and Pates, 2018;Moysiuk and Caron, 2019).…”

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

“…The presence of five main bladelike endites is a key apomorphy for this group (Vinther et al 2014;Moysiuk and Caron 2019). The inclination of the proximalmost endite is similar to the condition in Hurdia (Pates et al 2019) and Peytoia (Fig. 5A-D).…”

“…4, Supplementary Video) reveals a more complex appendage structure than has been recognized in previous studies of Stanleycaris and provides the first detailed account of oral morphology (Caron et al 2010;. The combination of morphologies shared by other taxa emphasizes the evolutionary lability of the hurdiid appendage (Pates et al 2019), perhaps related to interspecific differences in feeding ecology (see "Functional Implications").…”

“…Note on Ursulinacaris.-Ursulinacaris was recently described as possessing narrow, paired endites. This contrasts with the condition in all other hurdiids, in which one set of endites is broad and the other has either been modified into medial gnathites (as in Stanleycaris) or lost altogether (e.g., Cambroraster) (Vinther et al 2014;Moysiuk and Caron 2019), and was suggested to provide a link with tamisiocaridids (Pates et al 2019). We note, however, that the structures interpreted as paired endites consist of lighter-colored, parallel marginal areas flanking a darker axial lineation, with no clear evidence of interspace.…”

“…Peytoia also share a robust and multisegmented distal portion of the appendage, resembling the raptorial ends of anomalocaridid and amplectobeluid appendages (Daley and Budd 2010;Moysiuk and Caron 2019). Other hurdiids with this characteristic include an unnamed hurdiid appendage from the Fezouata Formation (Van Roy and Briggs 2011) and Ursulinacaris grallae (Pates et al 2019). Comparisons with these forms are hindered by the limited material available, but see "Note on Ursulinacaris."…”