The early Cambrian Emu Bay Shale radiodonts revisited: morphology and systematics

Abstract

Two species of Radiodonta (stem-group Euarthropoda) from the Emu Bay Shale (Cambrian Series 2, Stage 4), Kangaroo Island, South Australia, are revised based on new field collections and insights from recent phylogenetic analyses and advances in knowledge of radiodonts globally. Anomalocaris briggsi Nedin, 1995, the most common Emu Bay Shale radiodont, is designated the type species of a new monotypic genus of Tamisiocarididae, Echidnacaris gen. nov. The less common species, previously identified as Anomalocaris aff. canadensis Whiteaves, 1892, is formally named Anomalocaris daleyae sp. nov. Oral cones are assigned to both Echidnacaris briggsi comb. nov. and A. daleyae based on that of the latter species being found in association with pairs of frontal appendages. The Echidnacaris briggsi oral cone is the best preserved for the family Tamisiocarididae; it is triradial, with three large plates and a more pervasive ornament of nodes than in any other known radiodont. Shared characters of the Echidnacaris and Anomalocaris oral cones add support for a sister group relationship between Tamisiocarididae and Anomalocarididae. Unique eye characters documented in E. briggsi, such as being sessile and encircled by an eye sclerite, are unknown in the other tamisiocaridids, Tamisiocaris and Houcaris, and are tentatively regarded as diagnostic for Echidnacaris. An ovate head element resembling that of Tamisiocaris borealis is assigned to E. briggsi, informed by the sister group relationship between these taxa. Isolated radiodont body flaps and sets of setal blades in the Emu Bay Shale cannot be confidently assigned to a species, although relative abundance suggests that many or most are likely E. briggsi. The inner attachment margin of the body flaps is sharply defined and may represent a suture at which flaps are shed in moulting.

http://zoobank.org/urn:lsid:zoobank.org:pub:AEFDB294-AE8F-426D-9805-FC701798A986

Keywords:

• Radiodonta
• Anomalocarididae
• Tamisiocarididae
• Australia
• Gondwana

Introduction

Since the first description of soft-bodied fossils in the late 1970s (Glaessner, 1979), the lower Cambrian (Series 2, Stage 4) Emu Bay Shale (EBS) Konservat-Lagerstätte on the north-east coast of Kangaroo Island, South Australia, has yielded a wealth of knowledge on early animal anatomy, diversity, evolution, ecology and preservation (Paterson et al., 2016). Among the many key discoveries are the remains of radiodonts – a clade of stem-group euarthropods possessing a pair of spinose, arthrodized frontal appendages, large compound eyes, a ventrally positioned oral cone, and a series of segmental body flaps used for swimming (Collins, 1996; Daley, 2013; Moysiuk & Caron, 2022; Potin & Daley, 2023; Zeng et al., 2023).

EBS radiodonts have not only expanded the diversity and biogeographic distribution of this clade but have also played a critical role in deciphering aspects of early euarthropod evolution and ecology (Paterson et al., 2011, 2016, 2020). Although initially reported by Nedin (1992), the first EBS radiodont fossils were described and illustrated by McHenry and Yates (1993), attributing fragmentary frontal appendages and an oral cone to Anomalocaris. Subsequently, Nedin (1995) formally assigned new specimens of the same type of frontal appendage to a new species, Anomalocaris briggsi. In this same publication, Nedin (1995) also documented a second, unnamed species of Anomalocaris, noting a close similarity to the type species, A. canadensis Whiteaves, 1892. Daley, Paterson et al. (2013) largely followed the previous taxonomic assignments of McHenry and Yates (1993) and Nedin (1995) in recognizing two species of Anomalocaris, and also documented disarticulated body flaps and setal blades, but these specimens were not assigned to specific taxa.

Until the discovery of EBS compound eye fossils (Lee et al., 2011; Paterson et al., 2011, 2020), the optics of radiodonts were essentially unknown, despite previous studies documenting a pair of large, stalked eyes in select species (e.g. Chen et al., 1994; Collins, 1996; Daley et al., 2009; Whittington & Briggs, 1985). The initial report on isolated eyes from an unknown EBS arthropod (Lee et al., 2011) were recently shown to be smaller (7–9 mm) specimens of a radiodont compound eye that can reach 4 cm in diameter and possess >13,000 ommatidia, including a dorsally oriented ‘acute zone’ of enlarged lenses (Paterson et al., 2020). This sessile (non-stalked) eye type was attributed to ‘Anomalocaris’ briggsi, which is interpreted as having inhabited dark environments (Paterson et al., 2020). A second type of compound eye was originally identified as belonging to Anomalocaris but was not assigned to either of the two known species from the EBS (Paterson et al., 2011). Morphological and phylogenetic evidence, in addition to new relative abundance data on frontal appendages and eyes from the EBS, allowed for this stalked form (with >24,000 lenses per eye) to be attributed to Anomalocaris aff. canadensis (Paterson et al., 2020).

The EBS radiodonts have also featured in several studies on radiodont feeding ecology. For example, ‘A.’ briggsi is often interpreted as a suspension feeder (Daley, Paterson et al., 2013; Jiao et al., 2021; Lerosey-Aubril & Pates, 2018; Paterson et al., 2020; Pates & Daley, 2019; Potin & Daley, 2023; Vinther et al., 2014), primarily based on its frontal appendages having multiple, needle-like auxiliary spines fringing each elongate endite. The second EBS species (A. aff. canadensis; sensu Paterson et al., 2020) was considered by Nedin (1999) to be the main predator of trilobites and other benthic euarthropods in this deposit. Although Daley, Paterson et al. (2013) did not entirely dismiss this taxon as a possible durophage, they suggested that the largest trilobite species from the EBS – now referred to as Redlichia rex Holmes, Paterson and García-Bellido, 2019 (see Holmes et al., 2020) – was most likely responsible for producing many of the trilobite injuries and large, sclerite-rich coprolites found in this deposit. This suggestion has now been largely substantiated in recent studies on the anatomy and biomechanics of R. rex biramous appendages (Bicknell et al., 2021; Holmes et al., 2020) and detailed analyses of EBS trilobite injuries (Bicknell et al., 2022, 2023).

A surge of new taxa and numerous phylogenetic analyses over the past decade underpin a phylogenetically informed classification of radiodonts into four families, but the membership and interrelationships of these are still in a state of flux (Caron & Moysiuk, 2021; Cong et al., 2014, 2018; Guo et al., 2019; Jiao et al., 2021; Lerosey-Aubril & Pates, 2018; J. Liu et al., 2018; Moysiuk & Caron, 2019, 2021, 2022; Pates et al., 2019, 2021; Potin & Daley, 2023; Sun et al., 2020; Van Roy et al., 2015; Vinther et al., 2014; Wang et al., 2013; Wu, Ma et al., 2021; Wu, Fu et al., 2021; Zeng et al., 2023). For example, certain species (or select specimens) previously assigned to Anomalocaris have recently been placed in new genera (e.g. Pates et al., 2021; Wu, Ma et al., 2021; Wu, Fu et al., 2021; Zeng et al., 2023; Zhang et al., 2023), but ‘Anomalocaris’ briggsi, which is regularly resolved as the sister taxon to Tamisiocaris borealis within the Tamisiocarididae (Caron & Moysiuk, 2021; Cong et al., 2014; Lerosey-Aubril & Pates, 2018; J. Liu et al., 2018; Moysiuk & Caron, 2019, 2021; Van Roy et al., 2015; Vinther et al., 2014; Zeng et al., 2023), still requires generic reassignment. It is therefore timely that the taxonomy of EBS species (Daley, Paterson et al., 2013; Nedin, 1995) is re-evaluated in light of this plethora of new information on radiodonts.

Here we document new material of frontal appendages, head elements, oral cones, body flaps, and setal blades, with a particular focus on key specimens collected over 13 field seasons since the last systematic study on EBS radiodonts by Daley, Paterson et al. (2013). The novel discovery of two types of oral cones, including specimens in association with frontal appendage pairs, has permitted species identifications of these mouthparts and enhanced our morphological knowledge of both EBS taxa, thus enabling the establishment of a new tamisiocaridid genus, Echidnacaris gen. nov., the type of which is E. briggsi (Nedin, 1995), and a new anomalocaridid species, Anomalocaris daleyae sp. nov.

Materials and methods

Materials

Specimens were sourced from the lower part of the EBS (Cambrian Series 2, Stage 4; Pararaia janeae Zone) in the Big Gully area to the east of Emu Bay, Kangaroo Island, South Australia – see García-Bellido et al. (2009), Gehling et al. (2011), and Paterson et al. (2016) for detailed locality and stratigraphic information. Specimens are housed in the South Australian Museum, Adelaide palaeontological collection (prefix SAMA P). All specimens illustrated herein were collected from a c. 2 m thick, predominantly mudstone interval (levels 9.7 to 11.6) within Buck Quarry (coordinates: 35°34′25ʺ S, 137°34′36ʺ E), with the exception of the holotype of Echidnacaris briggsi (SAMA P40180) from the shoreline locality to the east of the mouth of Big Gully, two E. briggsi oral cones (SAMA P52881 and P55650 from levels 10.1 and 11.0, respectively, in Daily Quarry; coordinates: 35°34′24ʺ S, 137°34′36ʺ E), and an Anomalocaris daleyae oral cone (SAMA P54874) from level 10.7 in Daily Quarry.

Illustration

Specimens were photographed under normal (fibre optic) lighting conditions using a Canon EOS 50D digital camera with a Canon Compact Macro Lens EF 50 mm and MP-E 65 mm 1–5 × macro lens using Canon EOS Utility 2 capture software. Lighting of specimens was consistently from the upper left, except where indicated in the figure captions. Interpretive drawings were made with an Olympus SZX10 microscope through a camera lucida attachment. Figures were composed on an Apple iMac with Adobe Photoshop 2023.

Terminology

Descriptive terms for frontal appendages largely follow Pates et al. (2019, 2021), Wu, Ma et al. (2021) and Wu et al. (2022). The ‘base’ refers to the proximal peduncular portion of the frontal appendage (sensu Wu, Ma et al., 2021; also called a ‘shaft’ sensu Pates et al., 2019). The ‘claw’ refers to the series of articulated podomeres distal to the base (= ‘distal articulated region’ sensu Pates et al., 2019), typically bearing large endites (sensu Wu, Ma et al., 2021). Cp1–N refer to podomeres 1 to N of the ‘claw’, with numbering reflecting the proximal-distal axis (sensu Wu, Ma et al., 2021). En1–N refer only to the endites on the claw podomeres (Cp1–N, respectively). ‘Auxiliary spines’ refer to the small spines located on the anterior and/or posterior margins of endites (sensu Wu, Ma et al., 2021; Wu et al., 2022). The term ‘spinules’ follows Pates et al. (2021), referring to the thin, needle-like auxiliary spines located on the basal portion of endites (= ‘basal spines’ sensu Daley, Paterson et al., 2013). ‘Dorsal spines’ are the typically large, curved spines situated at the dorso-distal corner of claw podomeres (sensu Pates et al., 2019). ‘Terminal spines’ are located on the distal-most podomere (sensu Wu, Ma et al., 2021; Wu et al., 2022). Oral cone terminology follows Daley and Bergström (2012) and Zeng et al. (2018).

Phylogenetic analysis

In order to assign the two EBS radiodont species to genera and suprageneric groupings informed by phylogeny, their codings were updated in the 179-character radiodont matrix of Zeng et al. (2023) (see Supplemental material). One new character was added (character 180). The taxonomic sampling in that analysis is retained herein, apart from the deletion of a species therein called Radiodont C, which was coded by Zeng et al. (2023) from body parts that have not yet been published but are documented in a thesis (Wu, 2021). We filled previously unknown cells for Anomalocaris daleyae (characters 17, 51, 77, 83–88, 92, 93, 98, 101) and Echidnacaris briggsi (characters 21, 26, 31–34, 46–56, 102) based on data from new specimens figured herein. Stanleycaris hirpex codings were updated using new morphological information in Moysiuk and Caron (2022).

Several characters in the Zeng et al. (2023) matrix are uninformative, the codings exclusively being inapplicable (-), unknown (?) or state 1, without any terminals coded as state 0. We infer that this resulted from the characters being extracted from a previous data set that coded for many more euarthropods (Zeng et al., 2020), with uninformative characters or character states that are included but uncoded by Zeng et al. (2023) not being deleted. We have retained these uninformative characters to preserve the intactness of the character numbering for reference to the character list in Zeng et al. (2023). We coded state 0 for one of the previously uninformative characters (character 4: Main body, arthrodization) to specify that, where known, radiodonts are non-arthrodized.

The data were analysed with TNT v. 1.5 (Goloboff & Catalano, 2016) under equal and implied character weights. Multistate characters were treated as unordered. One multistate character with polymorphism (of the form ‘0 or 1, but not 2’) was treated as either of the two observed attributes, as implemented by NONA and PeeWee. Analysis with equal weights explored both Traditional and New Technology search strategies. The Traditional search applied 10,000 stepwise addition sequences, saving 50 trees per iteration, with tree bisection reconnection (TBR) branch swapping. Implied weighting used the basic setting, employing the same Traditional search strategy as for equal weights, exploring all integer values of concavity constants from k = 3 to k = 10. Branch support was quantified by jackknife frequencies for the equal weights analysis (heuristic search with 36% character deletion; 1000 replicates) and by symmetrical resampling for implied weights (heuristic search with 33% change probability; 1000 replicates), measured by Group present/Contradicted (G/C) values (Goloboff et al., 2003).

The analysis with equal character weights recovered nine shortest cladograms of 306 steps (consistency index: 0.64; retention index: 0.86), the strict consensus of which is depicted in Figure 1A. All analyses under implied character weights recovered a single best-fit cladogram (Fig. 1B), although with minor variation between the eight different concavity constants. Relationships of the two EBS radiodont species are discussed under the Remarks sections for Anomalocaris daleyae and Echidnacaris, below.

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).

Systematic palaeontology

Total group Euarthropoda Lankester, 1904

Order Radiodonta Collins, 1996

Family Anomalocarididae Raymond, 1935

Genus Anomalocaris Whiteaves, 1892

Type species

Anomalocaris canadensis Whiteaves, 1892; Stephen Formation (Miaolingian, Wuliuan), British Columbia, Canada.

Anomalocaris daleyae sp. nov.

(Figs 2–7A, B)

Figure 2. Anomalocaris daleyae sp. nov. Holotype SAMA P51398a. Paired frontal appendages and oral cone. A, photograph. B, camera lucida drawing (grey lines correspond to cuticle wrinkles). Abbreviations: BEn, base endite; Cp1–Cp9, claw podomeres 1–9; ds, dorsal spine; En1–9, endites of claw podomeres 1–9; ts, terminal spine on Cp13. Scale bars: 10 mm.

1995 Anomalocaris sp.; Nedin: 33–36, figs 2, 3B.

Figure 3. Anomalocaris daleyae sp. nov. Holotype SAMA P51398a. Details of frontal appendage and overview of oral cone. A, endite 1 (En1). B, endite 3 (En3). C, endite 5 (En5). D, distal part of frontal appendage. E, oral cone. Arrowheads in A–C indicate auxiliary spines, anterior to left, posterior to right. Abbreviations: Cp9–Cp12, claw podomeres 9–12; ds, dorsal spine; ts, terminal spine on Cp13. Scale bars: A–D = 2 mm; E = 5 mm.

1997 Anomalocaris sp.; Nedin: 134.

1999 Anomalocaris sp.; Nedin: 989.

2006 Anomalocaris sp.; Paterson and Jago: 43.

2011 Anomalocaris sp. nov.; Paterson, García-Bellido, Lee, Brock, Jago, and Edgecombe: 237–240, figs 1, 2, SI fig. 1a–e.

2013 Anomalocaris cf. canadensis Whiteaves; Daley, Paterson, Edgecombe, García-Bellido, and Jago: 973, 975–977, fig. 3.

2014 Anomalocaris sp. Emu Bay; Vinther, Stein, Longrich, and Harper: 498.

2014 Anomalocaris cf. canadensis Whiteaves; Lerosey-Aubril, Hegna, Babcock, Bonino, and Kier: 274–275.

2014 Anomalocaris cf. canadensis Whiteaves; Cong, Ma, Hou, Edgecombe, and Strausfeld: ext. data fig. 4.

Figure 4. Anomalocaris daleyae sp. nov. Paratype SAMA P54844a, b. Paired frontal appendages and partial oral cone. A, B, P54844a. Photograph and camera lucida drawing, respectively. C, D, P54844b. Photograph and camera lucida drawing, respectively. Abbreviations: BEn, base endite; Cp1–Cp11, claw podomeres 1–11; ds, dorsal spine; En1–7, endites of claw podomeres 1–7; oc, oral cone. Scale bars: 10 mm.

2015 Anomalocaris cf. canadensis Whiteaves; Van Roy, Daley, and Briggs: ext. data fig. 10.

2016 Anomalocaris cf. canadensis Whiteaves; Paterson, García-Bellido, Jago, Gehling, Lee, and Edgecombe: 3–5, figs 3j, k, 4c, d.

2016 Anomalocaris cf. canadensis Whiteaves; Jago, García-Bellido, and Gehling: 549.

2017 Anomalocaris cf. canadensis Whiteaves; Pates and Daley: 468.

2018 Anomalocaris sp. A (Emu Bay); Lerosey-Aubril and Pates: 4.

2019 Anomalocaris cf. canadensis Whiteaves; Guo, Pates, Cong, Daley, Edgecombe, Chen, and Hou: 105–106.

2020 Anomalocaris aff. canadensis Whiteaves; Paterson, Edgecombe, and García-Bellido: 1, 2, 5, 6–8, figs 4, 5C, D.

Figure 5. Anomalocaris daleyae sp. nov. Paratype SAMA P55619b. Frontal appendage. A, B, photograph and camera lucida drawing, respectively. C, endite 5 (En5). D, endite 7 (En7). Arrowheads in C, D indicate auxiliary spines; anterior to left, posterior to right. Abbreviations: Cp1–Cp11, claw podomeres 1–11; ds, dorsal spine; En1–9, endites of claw podomeres 1–9. Scale bars: A, B = 10 mm; C, D = 3 mm.

2021 Anomalocaris cf. canadensis Whiteaves; Pates, Daley, Edgecombe, Cong, and Lieberman: 258.

2021 Anomalocaris cf. canadensis Whiteaves; Wu, Ma, Lin, Sun, Zhang, and Fu: 3, 6–10.

2021 Anomalocaris cf. canadensis Whiteaves; Wu, Fu, Ma, Lin, Sun, and Zhang: 215.

2021 Anomalocaris cf. canadensis Whiteaves; Jiao, Lerosey-Aubril, Ortega-Hernández, Yang, Lan, and Zhang: 269, 271.

2022 Anomalocaris aff. canadensis Whiteaves; Bicknell, Holmes, Pates, García-Bellido, and Paterson: 13.

2023 Anomalocaris aff. canadensis Whiteaves; Zeng, Zhao, and Zhu: 14, 15.

2023 Anomalocaris aff. canadensis Whiteaves; Potin and Daley: 11, 16, fig. 6F–H.

Figure 6. Anomalocaris daleyae sp. nov. A, B, paratype SAMA P15374b. Frontal appendage. A, overview. B, detail of proximal and distal parts of enrolled appendage; arrowheads indicate auxiliary spines. C, SAMA P54915. Distal part of frontal appendage, showing four dorsal spines. D, E, SAMA P54874a, b, respectively. Oral cone. Abbreviations: BEn, base endite; Cp1–Cp10, claw podomeres 1–10; ds, dorsal spine; En1, endite of claw podomere 1. Scale bars: A = 10 mm; B–E = 5 mm.

Diagnosis

Base endite of frontal appendage 80% height of associated podomere; pair of tiny auxiliary spines on base endite. Proximal portion of claw endites 1, 3, 5 and 7 relatively robust; En1 with three pairs of auxiliary spines; En3 with two pairs of auxiliary spines; En5 and 7 with two posterior auxiliary spines. Cp 9–13 with well-developed dorsal spines; Cp13 with terminal spine of roughly equal length to dorsal spine.

Derivation of name

For Allison Daley, in recognition of her impressive work on radiodonts, including Emu Bay Shale taxa.

Holotype

SAMA P51398; pair of frontal appendages and associated oral cone (Figs 2, 3) from level 10.9 in Buck Quarry (35°34′25ʺ S, 137°34′36ʺ E), Big Gully, Kangaroo Island, South Australia. Previously illustrated by Paterson et al. (2016, fig. 3j, k).

Paratypes

Frontal appendages: SAMA P15374 (Fig. 6A, B; En1 previously illustrated by Daley, Paterson et al. 2013, fig. 3E), P40807 (previously illustrated by Nedin, 1995, fig. 2A and Daley, Paterson et al., 2013, fig. 3A–D), P42037 (previously illustrated by Daley, Paterson et al., 2013, fig. 3F–H), P54844 (Fig. 4; pair of frontal appendages and associated oral cone), and P55619 (Fig. 5).

Additional material

Frontal appendages: SAMA P54915 (Fig. 6C), in addition to other specimens listed by Daley, Paterson et al. (2013, supp. information) and Paterson et al. (2020, table S4) as ‘cf. canadensis’ or ‘aff. canadensis’, respectively. Oral cones: SAMA P54874 (Fig. 6D, E), in addition to P54255 and P54799. Compound eye specimens are listed by Paterson et al. (2020, table S3) as ‘Anomalocaris-type’.

Description

Frontal appendages are ≤183 mm in length along the dorsal margin (Daley, Paterson et al., 2013), consisting of a non-segmented base and a claw of 13 podomeres (Cp1–13). The single podomere of the base is rectangular (length:height is ∼1.8) and at least twice as long as the adjacent podomere (Cp1), without a conspicuous dorsal kink at the articulation of the base and Cp1 (Figs 2–4). The base endite located at the distal corner is relatively long (length ∼80% the height of the associated podomere) and has a single, tiny pair of auxiliary spines situated about two-thirds of the way from its base (Figs 2, 6A, B). Claw podomeres have a trapezoidal outline in lateral view (observed Cp1–Cp11 length:height ratios range from ∼0.6 to 0.7), with dorsal margins longer than ventral margins, and the overall size of podomeres gradually decreasing distally. The proximal margin of each podomere is relatively straight, and the distal margin is slightly sinuous. Arthrodial membrane is preserved as recessive triangular regions between podomeres in outstretched frontal appendages (e.g. Fig. 2). Claw endites are paired (e.g. Figs 2, 5) and alternate in length to at least En9, with odd-numbered endites (En1, En3, and so on) being longer than adjacent, even-numbered endites; all endites project anteriorly to varying degrees, with the angle between the endite and the anteroventral margin of the podomere becoming more acute towards the distal end of the claw, shifting abruptly at En9. En1 is enlarged, slightly curved anteriorly, length is ∼95% the height of the podomere, and bears three spikey auxiliary spines on both the anterior and posterior margins arranged in pairs, with each pair becoming longer towards the ventral tip of the endite (Figs 2, 3A, 4A, B, 6A, B; Daley, Paterson et al., 2013, fig. 3F–H). En2 length is ∼50% the height of the podomere, with one small pair of auxiliary spines (Figs 2, 4A, B). En3 is stout, slightly curved anteriorly, length is ∼75% the height of the podomere, bearing two pairs of auxiliary spines (one variably unexposed on either anterior or posterior margin), with each pair becoming longer towards the ventral tip of the endite (Figs 2, 3B, 4, 5; Daley, Paterson et al., 2013, fig. 3D). En4 length is ∼40% the height of the podomere, with one small pair of auxiliary spines (Figs 2, 4). En5 and En7 are stout, slightly curved anteriorly, lengths are ∼60–65% the height of the podomeres, each bearing two auxiliary spines on the posterior margin and one auxiliary spine on the anterior margin (Figs 2, 4, 5). En6 and En8 lengths are ∼25–40% the height of the podomeres, with no apparent auxiliary spines (Figs 2, 5), or with a single pair of auxiliary spines on En6 in SAMA P54844 (Fig. 4). En9 length is ∼70% the height of the podomere, with the angle between the endite and the ventral margin of the podomere being ∼40°, and without auxiliary spines (Figs 2, 5; Daley, Paterson et al., 2013, fig. 3A, B). En10 and En11 are poorly preserved in available specimens. Dorsal spines are present on Cp7–13, with those on Cp7 and Cp8 being very short, and those on Cp9–13 being very long (i.e. of equal or greater length than the dorsal margin of the associated podomere) and have a sickle-blade-like outline (Figs 2, 3D, 4, 5A, B, 6A–C; Daley, Paterson et al., 2013, fig. 3A, B, G, H). The distal-most podomere (Cp13) largely consists of the dorsal spine paired with an elongate terminal spine of roughly equal length, the two diverging at ∼35° (Figs 2, 3D).

The compound eyes of Anomalocaris daleyae sp. nov. have been previously described in detail by Paterson et al. (2011, 2020) and this will not be reiterated here.

The oral cone is sub-circular in outline, reaching a diameter of ∼50 mm in available specimens. It has a triradial symmetry, with three large plates separated by medium- and small-sized plates, and a small central opening. The large plates are oriented ∼110–125° from each other, of roughly equal size, and have subparallel margins (Figs 2, 3E); one large plate appears to align with the anterior-posterior axis of the head based on two assemblages of paired frontal appendages and the oral cone apparently in situ (Figs 2, 4A, B). Medium- and small-sized plates tend to alternate and exhibit folds in their outer portions towards the oral cone margin, with up to eight medium plates in each sector (Figs 2, 3E, 6D, E). Teeth on the inner margins of plates are not clearly preserved in the available specimens. The large and medium-sized plates exhibit nodes that tend to be clustered on the inner portions of the plates, with as many as 12 nodes on large plates (Figs 2, 3E, 6D, E). The nodes have an asymmetrical surface profile, with the raised tips directed towards the centre of the oral cone. The remainder of the oral cone surface appears to be smooth. The somewhat distorted preservation of specimen SAMA P54874 (Fig. 6D, E) suggests that the oral cone was a pliable structure. Based on the holotype (SAMA P51398, Fig. 2), the ratio of frontal appendage length to oral cone diameter is about 5:1.

Remarks

The following discussion considers phylogenetic evidence for the generic assignment of Anomalocaris daleyae sp. nov., followed by a species-level comparison with A. canadensis from the Burgess Shale, with which it has regularly been compared.

Parsimony analysis of our modified version of the Zeng et al. (2023) matrix under equal weights (Fig. 1A) unites Anomalocaris daleyae in an unresolved trichotomy with the type species of Anomalocaris, A. canadensis, and the type species of Ramskoeldia, R. platyacantha. As the oral cone is undescribed for R. platyacantha and body flaps are not assigned to A. daleyae, this grouping is based on detailed similarities in the frontal appendages. Implied weighting finds the interrelationships of the three species in this clade to be sensitive to differing concavity constants: k = 3, 6 and 7 recover a clade of A. daleyae and R. platyacantha (Fig. 1B), k = 4, 5, 9 and 10 group A. canadensis and R. platyacantha, and k = 8 unites A. daleyae and A. canadensis. These unstable alternatives receive almost no support under symmetrical resampling.

The characters allying Anomalocaris daleyae and R. platyacantha to the exclusion of A. canadensis under some concavity constants involve the greater number of auxiliary spines on claw endites in the former two species. The character matrix of Zeng et al. (2023) is quantitatively dominated by frontal appendage characters, which are coded independently for different podomeres or regions along the proximodistal axis of the appendage. This allows a large character sample to be amassed, but predicts the independence of codings for each podomere or region. However, the form of endites, plus numbers and morphologies of anterior and posterior auxiliary spines, exhibit a high degree of covariation along the frontal appendage.

Our classification of A. daleyae as Anomalocaris rather than Ramskoeldia draws on the near identical tripartite, tuberculate, furrowed oral cones of A. daleyae and A. canadensis. Ramskoeldia platyacantha, Ramskoeldia consimilis and Amplectobelua symbrachiata differ from other radiodonts in possessing smooth plates, tuberculate plates and gnathobase-like structures – with shared features of the teeth and scales – on each of the segments with reduced flaps in an intermediate region between the head and trunk (new character 180 herein) (Cong et al., 2017, 2018). These likely constitute the exclusive mouthparts for Amplectobelua symbrachiata, which is known from numerous assemblages of paired frontal appendages, head sclerites, associated smooth plates, tuberculate plates and gnathobase-like structures, and body flaps, none of which have an oral cone (Cong et al., 2017). Based on the detailed similarities in the smooth plates, tuberculate plates and gnathobase-like structures in Ramskoeldia and Amplectobelua, it cannot presently be excluded that an absence of an oral cone in the latter may pertain to the former as well; the smaller number of body assemblages of the two Ramskoeldia species conform to those of A. symbrachiata in apparently lacking an oral cone.

The historical classification of Anomalocaris daleyae (see synonymy above) reflects consistent comparison and perceived affinities with A. canadensis. Nedin (1995) classified the EBS species under open nomenclature as Anomalocaris sp. but considered Chengjiang frontal appendages previously misassigned to A. canadensis by Hou and Bergström (1991) to be conspecific. The Chengjiang specimens were later identified by Hou et al. (1995) as A. saron (now Houcaris saron, following Wu, Fu et al., 2021) and Anomalocaris sp. Subsequent classification of the EBS Anomalocaris species as A. cf. canadensis and then A. aff. canadensis has reinforced the hypothesis that a distinct EBS species is closely comparable to A. canadensis. The presence of extra pairs of auxiliary spines on some claw endites was cited by Daley, Paterson et al. (2013) as the sole difference between the Australian and Canadian frontal appendages, but variability in auxiliary spine numbers in the EBS material led them to keep the species under open nomenclature. With the larger sample now available, we identify characters of the frontal appendages that permit the diagnosis of A. daleyae as distinct from A. canadensis, but despite the exceptional preservation in both biotas, we cannot identify consistent diagnostic differences in the oral cones. The most reliable characters for distinguishing A. daleyae from A. canadensis are the much longer base endite (80% the height of the associated podomere, vs 40% in A. canadensis), the presence of auxiliary spines on the base endite (one pair vs none), the more robust basal portions of the claw endites on odd-numbered podomeres, three pairs of auxiliary spines on En1 (vs one pair or rarely two; see Zeng et al., 2023, supplementary fig. 1d, f), two pairs of auxiliary spines on En3 (vs one pair), two posterior auxiliary spines on En5 and En7 (vs one spine), and a terminal spine of roughly equal length to the dorsal spine on Cp13 (vs a terminal spine less than half the length of the dorsal spine).

Family Tamisiocarididae Pates and Daley, 2019

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).

Echidnacaris briggsi (Nedin, 1995)

(Figs 7C, D–16)

Figure 7. Reconstructions of Emu Bay Shale radiodont frontal appendages. A, B, Anomalocaris daleyae sp. nov. A, entire appendage. B, oblique cross-section of first claw podomere (Cp1) showing enlarged paired endites, each bearing three pairs of auxiliary spines. C, D, Echidnacaris briggsi (Nedin, 1995). C, entire appendage. D, oblique cross-section of typical claw podomere in middle portion of appendage showing paired endites bearing both auxiliary spines and spinules.

1992 Anomalocaris sp.; Nedin: 221.

1993 Anomalocaris sp.; McHenry and Yates: 77–81, 84–85, figs 2–8.

Figure 8. Echidnacaris briggsi (Nedin, 1995) comb. nov. Holotype SAMA P40180a, b. Frontal appendage. A, overview of part. B, detail of distal end of appendage, including podomeres 12 and 13, and En11. C, camera lucida drawing (incorporating information from counterpart). D, overview of counterpart (flipped horizontally to facilitate comparison). Abbreviations: BEn, base endite; Cp2—Cp12, claw podomeres 2–12; ds, dorsal spine; En1–11, endites of claw podomeres 1–11; sp, spinules; ts, terminal spines on Cp13. Scale bars: A, C, D = 20 mm; B = 5 mm.

1995 Anomalocaris briggsi Nedin: 31–35, figs 1, 3A.

1997 Anomalocaris briggsi Nedin; Nedin: 134.

1999 Anomalocaris briggsi Nedin; Nedin: 989.

2002 Anomalocaris briggsi Nedin; Hagadorn: 98.

2006 Anomalocaris briggsi Nedin; Van Roy and Tetlie: 240, 244, fig. 1A.

2006 Anomalocaris briggsi Nedin; Paterson and Jago: 43.

2008 Anomalocaris briggsi Nedin; Hendricks, Lieberman, and Stigall: table 1.

2008 Anomalocaris briggsi Nedin; Paterson, Jago, Gehling, García-Bellido, Edgecombe, and Lee: 320.

2009 Anomalocaris briggsi Nedin; García-Bellido, Paterson, Edgecombe, Jago, Gehling, and Lee: 1221.

2010 Anomalocaris briggsi Nedin; Daley and Peel: 354.

2011 Anomalocaris briggsi Nedin; Jago and Cooper: 239.

2011 Complex arthropod eyes; Lee, Jago, García-Bellido, Edgecombe, Gehling, and Paterson: 631, figs 1a–c, 2, SI figs 1–3.

2011 Anomalocaris briggsi Nedin; Paterson, García-Bellido, Lee, Brock, Jago, and Edgecombe: 239, SI fig. 2.

2012 Anomalocaris briggsi Nedin; Jago, Gehling, Paterson, Brock, and Zang: 252.

2013 Anomalocaris briggsi Nedin; Q. Liu: 339, 340.

2013 Anomalocaris briggsi Nedin; Daley, Budd, and Caron: 782.

2013 Anomalocaris briggsi Nedin; Daley, Paterson, Edgecombe, García-Bellido, and Jago: 972–973, figs 1, 2, 4.

2014 Anomalocaris briggsi Nedin; Vinther, Stein, Longrich, and Harper: 497, 498, ext. data fig. 6b.

2014 Anomalocaris briggsi Nedin; Daley and Edgecombe: 90.

2014 Anomalocaris briggsi Nedin; Lerosey-Aubril, Hegna, Babcock, Bonino, and Kier: 275, fig. 3.

2014 Anomalocaris briggsi Nedin; Cong, Ma, Hou, Edgecombe, and Strausfeld: ext. data fig. 4.

2015 Anomalocaris briggsi Nedin; Van Roy, Daley, and Briggs: ext. data fig. 10.

2016 Anomalocaris briggsi Nedin; Paterson, García-Bellido, Jago, Gehling, Lee, and Edgecombe: 3, 5, fig. 4a, b.

2016 Anomalocaris briggsi Nedin; Jago, García-Bellido, and Gehling: 549.

2017 Anomalocaris briggsi Nedin; Zeng, Zhao, Yin, and Zhu: 2.

2017 Anomalocaris briggsi Nedin; Pates and Daley: 468.

2018 Anomalocaris briggsi Nedin; Bicknell and Paterson: 769.

2018 Anomalocaris briggsi Nedin; Cong, Edgecombe, Daley, Guo, Pates, and Hou: 615, 619.

2018 Anomalocaris briggsi Nedin; Lerosey-Aubril and Pates: 4, 5, fig. 3b.

2019 Anomalocaris briggsi Nedin; Guo, Pates, Cong, Daley, Edgecombe, Chen, and Hou: 100, 105, 107.

2019 Anomalocaris briggsi Nedin; Pates, Daley, and Butterfield: 3, 4, 8.

2019 Anomalocaris briggsi Nedin; Pates and Daley: 1235, 1237, 1239, 1240, 1242, fig. 6e.

2019 ‘Anomalocaris’ briggsi Nedin; Moysiuk and Caron: 8.

2020 ‘Anomalocaris’ briggsi Nedin; Paterson, Edgecombe, and García-Bellido: 2–8, figs 1, 2, 5A, B.

2021 Anomalocaris briggsi Nedin; Pates, Daley, Edgecombe, Cong, and Lieberman: 237, 252.

2021 Anomalocaris briggsi Nedin; Wu, Fu, Ma, Lin, Sun, and Zhang: 209–211, 216–219, fig. 4e.

2021 ‘Anomalocaris’ briggsi Nedin; Jiao, Pates, Lerosey-Aubril, Ortega-Hernández, Yang, Lan, and Zhang: 263, 267–271.

2021 Anomalocaris briggsi Nedin; Moysiuk and Caron: 718.

2021 ‘Anomalocaris’ briggsi Nedin; Caron and Moysiuk: 13–14.

2022 ‘Anomalocaris’ briggsi Nedin; Wu, Pates, Ma, Lin, Wu, Zhang, and Fu: 9.

2023 Anomalocaris briggsi Nedin; Zeng, Zhao, and Zhu: 14, 15.

2023 ‘Anomalocaris’ briggsi Nedin; Potin and Daley: 8, 11, 12, 18, figs 5C, 6I–K.

Holotype

SAMA P40180; frontal appendage (Fig. 8). Originally referred to as AUGD 1046-630 by Nedin (1995, fig. 1A).

Paratype

SAMA P40763; frontal appendage. Originally referred to as AUGD 1046-335 (Nedin, 1995, fig. 1C).

Additional material

Frontal appendages: SAMA P48371 (Fig. 12C, D), P48975 (Fig. 10), P49151 (Fig. 12A, B), P54790 (Fig. 11), P54876 (Fig. 9), in addition to other specimens listed by Daley, Paterson et al. (2013, suppl. information) and Paterson et al. (2020, table S4) as Anomalocaris briggsi or ‘A.’ briggsi, respectively. Head elements: SAMA P45911 (Fig. 13B, C), P51380 (Fig. 13A). Oral cones: SAMA P48195 (Fig. 14D), P52881 (Fig. 15C, D), P55433 (Fig. 16D, E), P55600 (Fig. 16A, B), P55646 (Fig. 14C), P55650 (Fig. 16C), P57415 (Fig. 15A, B), P57418 (Fig. 14A, B), in addition to SAMA P31956 (previously illustrated by McHenry & Yates, 1993, fig. 8 and Daley, Paterson et al., 2013, fig. 4), P48319, P48343, P48373, P49777, P52899, and P54231. Compound eye specimens are listed by Paterson et al. (2020, table S3) as ‘Acute zone-type’.

Figure 9. Echidnacaris briggsi (Nedin, 1995) comb. nov. SAMA P54876. Frontal appendage. A, B, P54876a, overview. Photograph and camera lucida drawing, respectively. C, P54876b, detail of counterpart, showing well-preserved endites (En2–8) with auxiliary spines in epirelief. Abbreviations: BEn, base endite; Cp1—Cp11, claw podomeres 1–11; En1–11, endites of claw podomeres 1–11; sp, spinules. Scale bars: A, B = 20 mm; C = 10 mm.

Diagnosis

As for the genus, by monotypy.

Description

Frontal appendages are ≤175 mm in length along the dorsal margin, consisting of a non-segmented base and a claw of 13 podomeres (Cp1–13). The single podomere of the base appears bilobate in the proximal region, with the holotype (SAMA P40180) and SAMA P48975 showing a notch in the proximal margin that separates a large dorsal lobe from a smaller ventral lobe (Figs 8, 10); in SAMA P54876 (Fig. 9), only the dorsal lobe is preserved, which has a straighter dorsal margin. The base is at least four times as long as the adjacent podomere (Cp1), with a subtle dorsal kink at the articulation of the base and Cp1 (Fig. 10). The base endite located at the distal corner is relatively long (length ∼95–100% the height of the distal-most portion of the associated podomere), oriented posteriorly, with at least seven evenly spaced anterior auxiliary spines along its length and a single posterior auxiliary spine situated near the tip of the endite (Figs 8–10). Claw podomeres have a sub-rectangular outline in lateral view, with dorsal margins slightly longer than ventral margins, and the overall size of podomeres gradually decreasing distally; observed length:height ratios are ∼0.5–0.8 for Cp1–8, ∼1.0 for Cp9, ∼1.2 for Cp10, ∼1.8–2.4 for Cp11–12, and ∼1.3 for Cp13. The proximal and distal margins of podomeres Cp1 to Cp12 are straight to slightly curved. Arthrodial membrane is occasionally preserved as narrow recessive triangular regions between podomeres (e.g. Figs 8, 10, 11). Claw endites are paired (e.g. Figs 8, 12; Daley, Paterson et al. 2013, fig. 1C, D), straight to slightly curved, oriented posteriorly, and of approximately equal length up to En11; observed ratios of endite length to associated podomere height range from ∼1.2 (En1:Cp1) to ∼4.4 (En9:Cp9), then decrease to ∼3.1 (En11:Cp11) (Figs 8, 11). En1 bears at least eight evenly spaced anterior auxiliary spines along its length and one posterior auxiliary spine near the tip of the endite (Figs 8–10). En2 has at least eight evenly spaced anterior auxiliary spines, but only one posterior auxiliary spine has been observed in available specimens (Figs 8–10). En3 to En11 each have several evenly spaced auxiliary spines on both margins, with up to 13 and seven observed on the anterior and posterior margins, respectively (Figs 8, 9, 11, 12A, B; Daley, Paterson et al., 2013, fig. 1C, D). En12 length is ∼1.4 times the height of the associated podomere, with the angle between the endite and the ventral margin of the podomere being ∼30°, possibly with posterior auxiliary spines (Figs 8, 11). En13 length is ∼1.4 times the height of the associated podomere, possibly with posterior auxiliary spines (Figs 8, 11). Spinules extend from the proximoventral margin of the podomere onto the basal portion of the posterior margin of endites on En1 to En11; with up to 10 spinules per podomere (Figs 8–11, 12B, C). Dorsal spines are present on Cp11–13 and are slightly curved, with the one on Cp11 being short (∼25% the remainder of the dorsal margin of the podomere; Fig. 11), and those on Cp12 and Cp13 being very long (i.e. of length equal to or greater than the dorsal margin of the associated podomere) (Figs 8, 11).

Figure 10. Echidnacaris briggsi (Nedin, 1995) comb. nov. SAMA P48975a. Frontal appendage. A, B, photograph and camera lucida drawing, respectively. C, detail of BEn and En1. Abbreviations: BEn, base endite; Cp1–Cp9, claw podomeres 1–9; En1–5, endites of claw podomeres 1–5; sp, spinules. Scale bars: A, B = 20 mm; C = 10 mm.

Figure 11. Echidnacaris briggsi (Nedin, 1995) comb. nov. SAMA P54790a. Frontal appendage. A, B, photograph and camera lucida drawing, respectively. Abbreviations: Cp5—Cp13, claw podomeres 5–13; ds, dorsal spine; En5–13, endites of claw podomeres 5–13; sp, spinules. Scale bars: 10 mm.

Figure 12. Echidnacaris briggsi (Nedin, 1995) comb. nov. frontal appendages. A, B, SAMA P49151b. C, D, SAMA P48371a. Scale bars: A = 20 mm; B, D = 5 mm; C = 10 mm.

Head elements are represented by two isolated ovate sclerites that are gently convex (sag., tr.): SAMA P51380 (83.2 mm long, 61.1 mm wide; Fig. 13A); and SAMA P45911 (85.2 mm long, c. 54 mm wide; Fig. 13B, C). The inferred anterior margin forms a rounded apex. A narrow marginal rim is defined around the circumference. A few sub-marginal concentric folds are observed in both specimens; the surface is otherwise smooth.

Figure 13. Echidnacaris briggsi (Nedin, 1995) comb. nov. head elements. A, SAMA P51380a. B, C, SAMA P45911b, a, respectively (light in C is from the top). Arrowheads in A and C indicate narrow marginal rim. Scale bars: 10 mm.

The compound eyes of Echidnacaris briggsi have been previously described in detail by Lee et al. (2011) and Paterson et al. (2020) and this will not be reiterated here.

The oral cone is sub-circular in outline, reaching a diameter of ∼77 mm in available specimens. It has a triradial symmetry, with three large plates separated by eight or nine medium-sized plates of variable width, and a small central opening into which the large plates sometimes project. The large plates are oriented ∼100° and 125–130° from each other, are of sub-equal size, and have gently convex lateral margins, conferring greatest width at their midlength (Figs 14A–C, 15A, B). Medium-sized plates have an anastomosing fold in their outer portions towards the oral cone margin (Figs 14A, B, 15A, B). Three (Fig. 15D) or four (Daley, Paterson et al., 2013, fig. 4) teeth are present on the inner margins of large plates; two teeth are observed on the inner margin of a medium-sized plate adjacent to a large plate in SAMA P55650 (Fig. 16C). The large- and medium-sized plates bear relatively numerous nodes on their inner halves, with as many as 30 nodes on large plates (Fig. 14); on large plates, larger nodes are concentrated inwards of small nodes; the medium-sized plates bear more uniformly sized small nodes. The nodes have an asymmetrical surface profile, with the raised tips directed towards the centre of the oral cone. The outer half to third of the oral cone surface appears to be smooth in mature specimens. Folding of specimen SAMA P55600 (Fig. 16A, B) shows that the oral cone was pliable. A small oral cone (∼10 mm in diameter; Fig. 16D, E) shows that the large plates are of a similar size to the medium plates (in contrast to mature specimens), suggesting allometric growth of large plates throughout ontogeny; this specimen also exhibits more pervasive tuberculation, especially near the outer margins of all plates.

Figure 14. Echidnacaris briggsi (Nedin, 1995) comb. nov. oral cones. A, B, SAMA P57418. Photograph and camera lucida drawing, respectively. C, SAMA P55646a. D, SAMA P48195. Scale bars: 10 mm.

Figure 15. Echidnacaris briggsi (Nedin, 1995) comb. nov. oral cones. A, SAMA P57415a. B, SAMA P47415b. C, D, SAMA P52881a. C, overview. D, detail of teeth (arrowheads) at inner margin of a large plate (arrowhead in C). Scale bars: A–C = 10 mm; D = 1 mm.

Figure 16. Echidnacaris briggsi (Nedin, 1995) comb. nov. oral cones. A, B, SAMA P55600. Deformed oral cone. A, SAMA P55600a. B, SAMA P55600b (light from upper right). C, SAMA P55650a. Detail of teeth (arrowheads) on inner margin of large and medium-sized plates. D, E, SAMA P55433. Smallest known oral cone. D, SAMA P55433a. E, SAMA P55433b. Scale bars: A, B = 10 mm; C = 5 mm; D, E = 2 mm.

Remarks

The synonymy of Echidnacaris briggsi above indicates the recognition that its classification as Anomalocaris was inappropriate, as phylogenetic analyses consistently allied it with Tamisiocaris rather than with Anomalocaris canadensis, and it has been classified in Tamisiocarididae rather than Anomalocarididae since that family was established (Pates & Daley, 2019). Assignment to ‘Anomalocaris’ in recent works reflected the need for generic reassignment, formalized herein with the naming of Echidnacaris. Affinities of E. briggsi with Tamisiocaris rather than Anomalocaris were first signalled by frontal appendages (Vinther et al., 2014), and striking differences between E. briggsi and Anomalocaris were revealed in eye morphology (Paterson et al., 2020). The diagnosis and description of E. briggsi add more characters for distinguishing this species from Anomalocaris, drawing on the oral cone, eyes and head elements.

The assignment of the pervasively tuberculate oral cone to E. briggsi is largely based on it being the only other morphotype known from the EBS, with the A. daleyae oral cone identified by direct association with frontal appendages. This is further reinforced by relative abundance data in the EBS, with the tuberculate form being far more common (15 isolated oral cones) than specimens of A. daleyae (five oral cones, including appendage associations), echoing frontal appendage counts of both species (see Paterson et al., 2020).

Body flaps (described below) cannot be confidently assigned to either of the two EBS species, but based on relative abundance at horizons in which Echidnacaris briggsi is the more common of the two species, much of the body flap material is likely to be that species. The body flaps are similar in outline and in the arrangement of transverse lines to those of the only tamisiocaridid with well-known body flaps, Houcaris saron (Hou et al., 1995, figs 4, 5). Below, we interpret a sharply delimited inner attachment margin of the flaps as representing a suture. If this is correct and these flaps are those of E. briggsi, this is another potential autapomorphy of the genus and/or species, if not shared with other tamisiocaridids.

Emu Bay Shale radiodont body flaps and setal blades

Isolated body flaps were described in detail by Daley, Paterson et al. (2013), so description herein is limited to novel observations. The largest body flap (SAMA P50326; Fig. 17A) has a width of 113.4 mm (vs 73 mm in material available in 2013). A maximum of 14 transverse lines are observed (Fig. 17D), compared to 12 in previously published material (Daley, Paterson et al., 2013). SAMA P57407 is an isolated flap with the usual sub-triangular outline, showing transverse lines distally on the anterior half of the flap (Fig. 17C). A nearly straight inner margin of attachment noted in a previous description of EBS body flaps (Daley, Paterson et al., 2013, p. 979) is especially sharply defined in this specimen (arrowhead, Fig. 17C). Isolated flap SAMA P48154 also depicts a sharply defined margin of attachment; it is curved (where well preserved) but is incomplete (Fig. 17D). The sharp definition of this attachment margin suggests that flaps attached to the body along a suture. If such sutures were ecdysial lines, this may account for the abundance of isolated body flaps in the EBS, with flaps being shed in the moult. An isolated hurdiid flap from the mid-Cambrian (Miaolingian, Wuliuan) Spence Shale appears to show a similar inner margin (Pates et al., 2018, figs 4.6, 4.7), suggesting that this feature may be more widespread across radiodonts. Some EBS specimens also show a region of three-dimensional, roughly textured mineralization of variable extent on the posterior portion of the flap (e.g. Fig. 17A, B; Paterson et al., 2011, SI fig. 1f), which may represent labile tissues.

Figure 17. Unassigned Emu Bay Shale radiodont body flaps. A, SAMA P50326a. Largest known body flap. B, SAMA P55654b. C, SAMA P57407a. Flap with straight margin of articulation (arrowhead). D, SAMA P48154a, flap with 14 transverse lines and sharply defined margin of attachment (arrowhead). Scale bars: A = 20 mm; B–D = 10 mm.

Well-preserved sets of setal blades in which the free margin exposes splayed blades demonstrate that the blades are lanceolate, abruptly tapering near their pointed tips (Fig. 18A, B, D). The surface of the blades is variably smooth or depicts a few closely spaced parallel furrows. The latter are preserved along the entire length of the setal blades in SAMA P50287, in which the width of the lanceolate blades is indicated at their distal tips, with each blade incised by two or three furrows (Fig. 18C).

Figure 18. Unassigned Emu Bay Shale radiodont setal blades. A, B, SAMA P54822. Body flap and setal blades. A, SAMA P54822a. B, camera lucida drawing incorporating information from counterpart SAMA P54822b. C, SAMA P50287. D, SAMA P43611a. Scale bars: A, B, D = 10 mm; C = 5 mm.

Conclusions

Thirty years of research on EBS radiodonts began with a few specimens assigned to Anomalocaris (McHenry & Yates, 1993; Nedin, 1995), at a time when few species were known globally, and all radiodonts were classified as that genus. Ten years ago, when a greatly expanded collection was studied (Daley, Paterson et al., 2013), the EBS species became known from additional body parts, providing novel details of body flaps, but classification as two species of Anomalocaris remained the state of play. Radiodont classification was subsequently transformed by phylogeny (Vinther et al., 2014), as well as discoveries of new taxa and morphologies from Cambrian Konservat-Lagerstätten around the world, notably in China and North America. Radiodonta is now represented by 26 genera, including Echidnacaris gen. nov., grouped into four families (Potin & Daley, 2023). The EBS uniquely contributes to our understanding of radiodont form and function through insights into the compound eyes (Paterson et al., 2020), with eye characters informing the current classification. Knowledge of EBS radiodonts now draws on oral cones, head elements, better preserved frontal appendages, body flaps and setal blades than were available in previous studies, and more associations of body parts. Phylogenetic analysis, including most of the known radiodonts worldwide (Zeng et al., 2023), shows that the two EBS species have close affinities with Laurentian taxa: an anomalocaridid (Anomalocaris daleyae) allied to the type species of Anomalocaris from the Burgess Shale, and a tamisiocaridid (Echidnacaris briggsi) allied to a species from Sirius Passet in Greenland.

Associate Editor: Paul Barrett

Acknowledgements

We thank our previous collaborators and volunteers in Emu Bay Shale research: Ronda Atkinson, Marissa Betts, Russell Bicknell, Mary-Anne Binnie, Glenn Brock, Aaron Camens, Allison Daley, Harriet Drage, Ellie Ellis, Robert Gaines, Jim Gehling, Mike Gemmell, James Holmes, Trevor and Carol Ireland, Jim Jago, Katrina Kenny, Pierre Kruse, John Laurie, Mike Lee, Ben McHenry, Mike Mills, Javier Ortega-Hernández, Lily Reid, Dennis Rice and Natalie Schroeder. Financial and logistical assistance was provided by the South Australian Museum and SeaLink. We are appreciative to landowners Paul and Carmen Buck for generously allowing access to the field site. Reviews by Stephen Pates and an anonymous referee helped to refine the manuscript.

Supplemental material

The supplemental material (Appendix 1 – character list; Appendix 2 – taxon/character matrix) for this article can be accessed here: https://doi.org/10.1080/14772019.2023.2225066.

Additional information

Funding

This research was supported by grants from the Australian Research Council (LP0774959 and FT120100770 to JRP, FT130101329 to DCGB, and DP200102005 to JRP and GDE).