Echidnacaris, Paterson & Garćıa-Bellid & Edgecomb, 2023

Paterson, John R., Garc ́ ıa-Bellid, Diego C. & Edgecomb, Gregory D., 2023, The early Cambrian Emu Bay Shale radiodonts revisited: morphology and systematics, Journal of Systematic Palaeontology (2225066) 21 (1), pp. 1-29 : 12-15

Genus Echidnacaris gen. nov.

Type species. Anomalocaris briggsi Nedin, 1995 ; Emu Bay Shale (Cambrian Series 2, Stage 4), Kangaroo Island , South Australia .

Diagnosis. Frontal appendage having single podomere in base with proximal part expanded as dorsal and ventral lobes. Thirteen podomeres in claw. Claw endites straight to slightly curved, oriented posteriorly, and of approximately equal length up to En11; endite length c. 1.2 times height of associated podomere for Cp1, gradually increasing to c. 3.1 times for Cp11. Up to 13 anterior auxiliary spines and seven posterior auxiliary spines on claw endites. Spinules on En1 to En11, with up to 10 spinules per podomere. Large, ovate head element. Sessile eye with visual surface bounded by bilobate eye sclerite and marginal cuticular rim; lenses conspicuously enlarged in central part of visual surface. Oral cone with three large plates that widen slightly towards their midlength, each separated by eight or nine medium-sized plates with anastomosing furrows in outer part; inner half of all plates bearing relatively numerous nodes (19–30 on large plates), with larger nodes concentrated inwards of small nodes.

Derivation of name. After the Australian monotreme colloquially known as the echidna, which frequents the Big Gully site and possesses elongate quills (or ‘spines’) on its back that are reminiscent of the auxiliary spines and spinules on the endites of briggsi ; and ‘- caris ’, meaning ‘shrimp’ (Latin) – a common suffix for radiodonts.

Remarks. Some characters cited in the most recent revised diagnosis of Echidnacaris briggsi (see Daley, Paterson et al., 2013) are excluded from the diagnosis of Echidnacaris herein because they are now known to be shared with Tamisiocaris Daley and Peel, 2010 , following the redescription of its type and only species, T. borealis ( Vinther et al., 2014) . These include the frontal appendage being long and distally tapering, with endites longer than the height of the associated podomere, and the presence of spinules on the ventral part of the claw podomeres.

Coding previously unknown information for the oral cone and head element of Echidnacaris briggsi into the character matrix of Zeng et al. (2023) preserves the membership and interrelationships of Tamisiocarididae as shown in that study ( Fig. 1 ). A well-supported clade composed of Houcaris , Tamisiocaris and Echidnacaris (then ‘ Anomalocaris ’ briggsi ) corresponds to the scope of Tamisiocarididae as revised by Wu, Fu et al. (2021). The sister group relationship between Tamisiocaris and Echidnacaris receives strong node support (jackknife frequency 99% under equal weights; G/C ratio 94 under implied weights). This clade corresponds to the original scope of Tamisiocarididae as diagnosed by Pates and Daley (2019), who formalized a phylogenetic relationship first proposed by Vinther et al. (2014). The identification of Houcaris as the next closest relative of these taxa prompted a broadening of the scope of the family by Wu, Fu et al. (2021), who identified elongate endites much longer than the height of the associated podomere and multiple slender auxiliary spines as apomorphic characters of this broadened clade. Using the present data set, synapomorphies of Houcaris , Tamisiocaris and Echidnacaris (= Tamisiocarididae ) include needle-like auxiliary spines on claw endite 1 (En1) and other proximal claw endites (character 111, state 3; character 132, state 3, respectively), auxiliary spines of En1 and other proximal claw endites of close size to each other (character 114, state 1; character 135, state 1, respectively), and spinules around the root of En1 and other proximal claw endites (character 112, state 1; character 133, state 1, respectively).

Other recent phylogenies of Radiodonta have likewise recovered a sister-group relationship between Tamisiocaris and Echidnacaris ( Caron & Moysiuk, 2021; Lerosey-Aubril & Pates, 2018; Moysiuk & Caron, 2019, 2021, 2022). Synapomorphies of the two include an elongate and rod-like base endite and claw endite 1 (En1) (character 86, state 1; character 107, state 1, respectively), a posterior orientation of En1 and other proximal claw endites (character 105, state 1; character 128, state 1, respectively), pectinate auxiliary spines on En1 and other proximal claw endites (character 113, state 1; character 134, state 1, respectively), auxiliary spines on En1 much longer than the width of the associated endite (character 117, state 1), and roughly equal numbers of anterior and posterior auxiliary spines on proximal claw endites (character 144, state 0).

Echidnacaris briggsi provides new data on characters of the oral cone in Tamisiocarididae , otherwise known only from two incomplete specimens of Houcaris magnabasis (Pates, Daley, Edgecombe, Cong, and Lieberman, 2019) ( Pates et al., 2021, figs 6a, c, 7a–c). The triradial, strongly tuberculate and furrowed oral cone shares these character states with Anomalocarididae (e.g. Anomalocaris canadensis and A. daleyae ) and provides additional support to the hypothesis that anomalocaridids are the sister group of Tamisiocarididae . This grouping was recovered under implied weights in some parsimony analyses of Zeng et al. (2023) and is stable across concavity constants k = 3 to k = 10 using the present data set ( Fig. 1 ), albeit with low node support. Compared with other triradial oral cones, that of Echidnacaris has especially wide, barrel-shaped large plates, more consistent medium-sized plates (rather than numerous small plates amidst medium-sized plates), and more pervasive tuberculation. These characters readily allow the E. briggsi oral cone to be distinguished from that of the co-occurring Anomalocaris daleyae .

Two isolated radiodont head elements are assigned herein to Echidnacaris briggsi based on their similarity in shape to a head element preserved in association with a pair of frontal appendages of its closest relative, Tamisiocaris borealis ( Vinther et al., 2014, extended data fig. 4a, b, d). Both are ovate, with a rounded apex, and are presumed to be dorsal median coverings of the head. In addition to these shared characters of tamisiocaridids as a basis for assigning EBS head elements, the co-occurring Anomalocaris daleyae would instead be expected to possess an oval plate with a broad marginal rim, based on comparison to the dorsal carapace of A. canadensis ( Daley & Edgecombe, 2014) .

We have included oral cone and eye characters in the diagnosis of Echidnacaris despite the lack of data for them in Tamisiocaris and relatively poor preservation in Houcaris . The sessile eye of E. briggsi is unique for Radiodonta as a whole ( Paterson et al., 2020), other taxa having stalked eyes, but new data from Tamisiocaris in particular might show that some of the apparent autapomorphies of the eyes of E. briggsi are shared by other tamisiocaridids. Given that Tamisiocaris and Echidnacaris are both presently monotypic and their sister group relationship is strongly supported, the argument could be made that E. briggsi should be classified as a second species of Tamisiocaris . We chose to classify them separately at the generic level to reflect substantial differences in characters widely applied in generic diagnoses of radiodonts, such as numbers of claw podomeres (17 in Tamisiocaris vs 13 in Echidnacaris ), together with lengths of claw endites and the number of auxiliary spines borne by them (both much greater in Tamisiocaris ).

Figures

Figure 1. Radiodont phylogeny. A, strict consensus of nine shortest cladograms under equal character weights; numbers at nodes are jackknife frequencies>50%. B, single best fit cladogram under implied weights (k= 3); numbers at nodes are G/C values>50%. Colours indicate clades: Euarthropoda (yellow), Hurdiidae (purple), Amplectobeluidae (blue), Anomalocarididae (green), and Tamisiocarididae sensu stricto (pink).