Radiodonta

Frontal appendages morphology of the radiodont families Anomalocarididae/Amplectobeluidae and Hurdiidae.

Frontal appendage of Anomalocaris canadensis.

Paired frontal appendages from a hurdiid radiodont.

The anterior structures on the head are a pair of frontal appendages. They may referred as 'claws', 'grasping appendages', 'feeding appendages', or 'great appendages' in previous studies, but recently the latter was no longer used as the frontal appendages were considered non-homologous to megacheiran great appendages.[12] They are scleritzed and segmented (arthropodized), bearing ventral spines (endites) on most of their segmental units (podomeres), and the endites may bore additional rows of spines (auxiliary spines) on their anterior and posterior margins.[13][3] The frontal appendage could divided into two regions: the shaft ('peduncle' in previous studies) and distal articulated region.[13] Triangular region covered by soft cuticle (arthrodial membrane), may occur between the ventral side of each podomere and provide flexibility.[14] Their pre-ocular and protocerebral origin suggest they are homologous to the primary antennae of Onychophora and the labrum of Euarthropoda (all arose from ocular somite), and not homologous with the chelicerae of Chelicerata nor the antennae and great appendages of other arthropods, which are deutocerebral (asore from post-ocular somite 1).[9][12] Since the morphology of the frontal appendages, especially those of the endites are always differ between species, they are one of the most important characters in species identification.[13] In fact, many radiodonts are only known from a handful of fossilized frontal appendages.[14][13]

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  Frontal appendages of Anomalocarididae, Amplectobeluidae, and possibly related species.
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  Frontal appendages of Tamisiocarididae (="Cetiocaridae").
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  Frontal appendages of Hurdiidae.

Oral cones of Anomalocaris, Peytoia and Hurdia

Oral cone of a Hurdiid radiodont previously misinterpreted as those of Anomalocaris canadensis.

The mouth is on the ventral side of the head, behind the attachment point of frontal appendages and is surrounded by a ring of tooth plates , forming the mouthpart known as oral cone ('jaws' in previous studies[6]). 3 or 4 tooth plates might enlarged, giving the oral cone a triradial (e.g. Anomalocaris) or tetraradial (e.g. Hurdiidae, Lyrarapax) appearance.[15][11] The inner margin of tooth plates have spikes facing towards the mouth opening. Additional rows of internal tooth plates may occur in some hurdiid genera.[8][3] Detail reconstruction of amplectobeluid oral cones are speculative, but they possibly did not present a typical radial arrangement.[4][1]

H-element of Hurdia victoria.

Nearly complete fossil of Anomalocaris canadensis (top) and Peytoia nathorsti (bottom), showing distinct overall appearance.

Three head sclerites (head carapace complex) forming by a central H-element (anterior sclerite or head shield) and a pair of P-elements (lateral sclerites), they cover the dorsal and laterovental surface of the animal's head respectively.[3] The P-elements may connect to each other as well as H-element by a narrow anterior extension (P-element neck or 'beak').[8][3] The head carapace complex are small and ovoid in Anomalocarididae and Amplectobeluidae,[4][3] but often enlarged in Hurdiidae.[3] The head bore two stalked compound eyes, which may have mobility,[16] and located between the gaps forming by the posterior regions of H-element and P-element.[8][3]

Anterior morphlogy of two generized Anomalocaris and Hurdiid radiodont. A: Dorsal view, B: Ventral view, Fa: Frontal appendage, He: H-element, Pe: P-element, Ey: Eye, Oc: Oral cone, Af: Anterior (neck) flap, Bf/Vf:, Ventral flap, Sb: Setal blade

Contrary to the original diagnosis, the division of body segments (segmental boundaries) can be visible externally[10][5][3] and no known member of Radiodonta (except the putative radiodont Cucumericrus[10][17]) is known to have pediform trunk appendages (legs).[18] The trunk have numerous body segments (somites), tapering from anterior to posterior, with the anterior 3 or 4 segments significantly constricted into a neck region.[3]

Ventral view of a generalized GLS-bearing radiodont, showing gnathobase‐like structures (GLSs) associated with reduced anterior flaps.

The trunk appendages were fin-like body flaps ('lateral flaps' or 'lobes' in some studies), usually one pair of ventral flap per body segment, each slightly overlapped on the one more anterior to it, but additional, non-overlapping set of small dorsal flaps may occur in some Hurdiid species.[10] The flaps may have numerous vein-like structures known as strengthening rays.[10][5][3] The flaps on the neck region, called anterior flaps or neck flaps, are significantly reduced. In some species, jaw-like feeding appendages called gnathobase-like structures (GLSs) arosed from each of the base of their reduced neck flaps.[4][1] Numerous elongated blade-like extension (lanceolate blades) arranged in a row, forming bands of gill-like structure known as setal blades or lamellae,[3] covered the dorsal surface of each body segment.[10] At least in Aegirocassis, each lanceolate blades are cover in wrinkles.[10]

The ventral flaps may be homologous to the endopod of the biramous limbs of euarthropods and lobopodous limbs (lobopods) of gilled lobopodians, and the dorsal flaps and setal blades may be homologous to the exopod and gill-bearing dorsal flaps of the former taxa.[19][10] The trunk may end either with 1 to 3 pairs of tail fan,[18][3] two long furcae,[20][3] an elongated terminal structure,[18] or a featureless blunt tip.[10]

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  Aegirocassis, Peytoia and Hurdia. Three hurdiid genera which bore dorsal flaps and enlarged head carapace complex.
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  Anomalocaris canadensis, an Anomalocaridid radiodont with small ovoid head carapace complex, 3 pairs of tail fan, and a terminal tailpiece.
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  Amplectobelua symbrachiata. An Amplectobeluid radiodont with gnathobase-like structures and two furcae.

Eyes (deep blue), brain (light blue) and digestive system (yellow) of a radiodont.

Traces of muscles, digestive system and nervous system were described from some radiodont fossils. Pairs of well-developed muscles which were connected to the ventral flaps, located at the lateral cavities of each body segment.[18][9] Between the lateral muscles is a sophiciated digestive system, formed by widening the foregut and hindgut, both connected by a narrow midgut associated with 6 pairs of gut divercula (digestive glands).[18][5][21] Compared to the three-segmented brain of euarthropod and two-segmented brain of onychophoran, the brain of radiodont is composed of only one brain segment originating from the ocular somite, the protocerebrum. Nerves of frontal appendages and compound eyes arose from the anterior and lateral regions of the brain, respectively.[9][12] Posterior to the brain was a pair of apparently unfused ventral nerve cord which ran through the animal's neck region.[9]

Radiodonts were interpreted as nektonic or nektobenthic animals, with their morphology suggesting an active swimming lifestyle. The muscular, overlapping ventral flaps may have propelled the animal through the water, possibly by moving in a wave-like formation resembling modern rays and cuttlefish.[22] Pairs of dorsal flaps, which make up a tail fan in some species, may help steering and/or stabilizing the animal during locomotion.[10][23] In Anomalocaris, morphology of the tail fan even suggests it could rapidly change its swimming direction efficiently.[24] Bands of setal blades with wrinkling lanceolate blades may increased the surface area, suggesting they were gills, providing the animal's respiratory function.[18][10] Abundance of the remains of scleritzed structures such as disarticulated frontal appendages and head carapace complex, suggest that mass moulting events may have occurred among radiodonts,[10][3] a behavior which also has been reported in some other Cambrian arthropods such as trilobites.[25]

Cambroraster falcatus, a species of sediment-sifting radiodont with enlarged, dome-shaped H-element.

Radiodonts had diverse feeding strategies, which could be categorized as raptorial predators, sediment sifters, or suspension, filter feeders.[2][26] For example, raptorial predators like Anomalocaris and Amplectobeluids might have been able to catch agile prey by using their raptorial frontal appendages, the latter even bore a robust endite for holding prey like a pincer.[17][14][4] With the smaller head carapace complex and large surface of arthrodial membranes, frontal appendages of these taxa had greater flexibility.[20] Stout frontal appendages of sediment sifters like Hurdia and Peytoia have serrated endites with mesial curvature, which could form a basket-like trap for raking through sediment and leading food items towards the well-developed oral cone.[3] Some species may have other features significantly specialized for a sediment sifting lifestyle, such as Cambroraster with its dome-like H-element similar to the carapace of a horseshoe crab.[3] Endites of frontal appendages from suspension/filter feeders like Tamisiocaris and Aegirocassis have flexible, densely-packed auxiliary spines, which could filter out organic components such as mesozooplankton and phytoplankton down to 0.5mm.[7][10] Frontal appendages of Caryosyntrips, which are unusual for radiodonts in having the direction of endite-bearing surfaces opposing one another and may have been able to manipulate and crush prey in a scissor-like slicing or grasping motion.[14][27]

Frontal appendages of Amplectobelua, showing enlarged endites.

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  Peytoia nathorsti, a sediment-sifting hurdiid radiodont with stout frontal appendages, closely associated with a tetraradial oral cone.
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  Frontal appendage of the suspension-feeding radiodont Tamisiocaris borealis, showing elongated endite with densely-packed auxiliary spines.
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  Paired frontal appendages of Caryosyntrips, showing opposed direction.

Oral cones of radiodonts may able to perform suction and/or biting ability.[15][26][3] Alongside frontal appendages, differentiation of oral cones between genera suggest preferences of different diet as well. For example, the triradial oral cone of Anomalocaris with irregular, tuberculated toothplates and smaller opening may adapted to small and active prey;[26] while the rigid tetraradial oral cones of Hurdia and Cambroraster with larger opening and additional tooth plates may capable to consume larger food item.[26][3]

Most analyses suggest that radiodonts are stem-group arthropods[8][7][9][10][2][3] and sister to deuteropoda, a clade including upper stem (e.g. fuxianhuiids and bivalved arthropods) and crown Euarthropoda (e.g. Artiopoda, Chelicerata and Mandibulata).[28] This interpretation is support by numerous arthropod-like features found on radiodonts, such as compound eyes,[16] digestive glands,[21] arthropodization (on frontal appendages),[28][3] trunk appendages forming by dorsal and ventral elements (precursor of arthropod biramus appendages),[10][3] cuticularized foregut and hindgut,[18] as well as protocerebral anterior sclerite.[29] The constricted neck region with feeding appendicular structures of some radiodont may also shed light on the origin of sophiciated arthropod head, which is form by the fusion of multiple anterior body segments.[4][12]

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  The 'gilled lobopodians' Opabinia (top), Pambdelurion (bottom left) and Kerygmachela (bottom right).
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  The siberiid lobopodians Siberion (upper left), Megadictyon (bottom center) and Jianshanopodia (upper right).

Taxa just basal to the radiodont and euarthropod branch are Pambdelurion, Kerygmachela and Opabinia, three radiodont-like dinocaridid genera usually referred as 'gilled lobopodians'.[28][10] They have body flaps, digestive glands and specialized frontal appendages like radiodont, but the frontal appendages are not arthropodized and they bore lobopod underneath each of their flaps.[10] Opabinia is more derived by having radiodont-like stalked eyes, tail fan, setal blades, and even euarthropod-like posterior mouth opening.[28] Taxa even basal to 'gilled lobopodians' are siberiids like Megadictyon and Jianshanopodia,[28] a group of lobopodians bore robust frontal appendages and digestive glands, but no body flaps. Such intermediate forms between lobopodian and radiodont/euarthropod suggest that the total-group arthropoda arose from a paraphyletic lobopodian grade, alongside the other two extant panarthropod phyla Tardigrada and Onychophora.[12]

Megacheira or 'great appendage arthropod', a class of possible stem-chelicerate previously thought to be radiodont's close relative.

Previous reconstruction of Schinderhannes, showing well-developod "tergites" which may actually soft like those of Opabinia, and "ventral biramus appendages" which may in fact present flap muscles and/or setal blades like those of other radiodonts.[28]

Previous studies may suggest radiodonts as a group other than stem-arthropods, such as cycloneuralian worms undergone convergent with arthropods (based on the cycloneuralian-like radial mouthparts);[30] stem chelicerates alongside megacheirans a.k.a great appendage arthropods (based on the similarity between radiodont frontal appendages, megacheiran great appendages and chelicerae);[31] or Schinderhannes bartelsi, which resolved as a hurdiid radiodont in recent analyses[28][7][10][2][3], as a species more closely related to euarthropods than other radiodonts (based on some putative euarthropod-like features found on Schinderhannes).[23] However, neither each of them were supported by later investigations. The radial mouthparts are not cycloneuralian-exclusive and more likely present result of convergent evolution or ecdysozoan plesimorphy, since they also have been found in panarthropods such as tardigrade and some lobopodians;[32] the megacheiran great appendages were considered to be deutocerebral,[33][34] which are non-homologous to the radiodont protocerebral frontal appendages;[9][12] putative euarthropod characters found on the single Schinderhannes fossil is questionable and may present other radiodont-like structures.[28]

Traditionally, Radiodonta included all taxa under the class Dinocaridida, and taxa currently included within Radiodonta have been placed within one family, Anomalocarididae,[8][17] hence the common name 'anomalocaridid' and it was still occasionally used to refer the whole order even after reclassification.[9][10] Recently, the basal dinocaridid genera Pambdelurion, Kerygmachela and Opabinia were placed outside of Radiondonta,[7][10][2][3] and most of the radiodont species were reclassified within three new families: Amplectobeluidae, Tamisiocarididae[2][3] (formerly Cetiocaridae[7]), and Hurdiidae. Including Anomalocarididae, the four recent radiodont families may form the clade Anomalocarida.[7]

Parapeytoia yunnanensis, a poorly-preserved megacheiran (B) previously misinterpreted as 'radiodont with legs' (A).[17]

The original description of the order Radiodonta included Anomalocaris, Laggania (later known as Peytoia), Hurdia, Proboscicaris, Amplectobelua, Cucumericrus, and Parapeytoia.[6] However, Proboscicaris is now regarded as a junior synonym of Hurdia, and Parapeytoia is considered to be a megacheiran.[8][18][10] The position of Cucumericrus within Radiodonta is unclear, as it was either unselected by phylogenetic analysis[3] or resolved in a polytomy with Radiodonta and Euarthropoda.[10]

Frontal appendages of Caryosyntrips, a genus suggest to be one of the most basal radiodont.

Frontal appendage of Anomalocaris briggsi. Despite the suggested generic name, this radiodont is unlikely to be a species of genus Anomalocaris based on phylogenetic analysis.

Under Radiodonta, Caryosyntrips is the basal-most genus alongside Cucumericrus (if included). The genus Anomalocaris always found to be non-monophyletic, usually with Anomalocaris kunmingensis and Anomalocaris briggsi resolved as a member of Amplectobeluidae and Tamisiocarididae respectively.[7][9][10][2][3] Interrelationship of Amplectobeluidae is uncertain, as the amplectobeluid affinities of Lyrarapax and Ramskoeldia were occasionally questioned.[1][3] Monophyly of the speciose family Hurdiidae is well-supported by several derived characters (e.g. distal articulated region of frontal appendage with proximal 5 podomeres bearing subequal endites[13][3]), with Tamisiocarididae often suggested to be its sister group.[7][10][2][3]

• Radiodonta
  • Cucumericrus
  • Caryosyntrips
  • Anomalocarida
    • Paranomalocaris (placed within Anomalocarididae by some studies.[20])
    • Laminacaris (placed within Amplectobeluidae by some studies.[2])
    • Anomalocarididae
      • Anomalocaris (some species may placed within the other families.[7][10])
    • Amplectobeluidae
      • Lyrarapax (position questioned by some studies.[1])
      • Amplectobelua
      • Ramskoeldia (position questioned by some studies.[3])
    • Tamisiocarididae
      • Tamisiocaris
      • Anomalocaris briggsi
    • Hurdiidae
      • Aegirocassis
      • Peytoia
      • Schinderhannes
      • Hurdia
      • Ursulinacaris
      • Stanleycaris
      • Pahvantia
      • Cambroraster
      • Zhenghecaris

The first in-depth phylogenetic analysis of Radiodonta was conducted by Vinther et al. in 2014, and it was expanded by Cong et al. later that year by the addition of Lyrarapax unguispinus.[9] The analysis was further modified in 2015 by Van Roy et al. with modified characters and the inclusion of Cucumericrus decoratus and Aegirocassis benmoulai.[10]