(Redirected from Anomalocaridid)

Radiodonta is an order of stem-group arthropods that was successful worldwide during the Cambrian period, and included the earliest large predators known. They may be referred to as radiodonts,[1][2][3] radiodontans,[4][5] radiodontids,[6] anomalocarids,[7] or anomalocaridids,[8][9][10] although the latter originally refer to the family Anomalocarididae, which previously included all species of this order but is now restricted to only a few species.[7] Some of the most famous species of radiodonts are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, Peytoia nathorsti, and Amplectobelua symbrachiata, the Ordovician Aegirocassis benmoulai and the Devonian Schinderhannes bartelsi.

Temporal range: Cambrian Stage 3 - Early Devonian, 521–400 Ma
20191201 Radiodonta Amplectobelua Anomalocaris Aegirocassis Lyrarapax Peytoia Laggania Hurdia.png
Amplectobelua symbrachiata, Anomalocaris canadensis, Aegirocassis benmoulai, Peytoia nathorsti, Lyrarapax unguispinus, and Hurdia victoria
Scientific classification
Collins, 1996


The name Radiodonta (Latin for radius "spoke of a wheel" and Greek for odoús "tooth") refers to the radial arrangement of tooth plates (oral cone) surrounding the mouth,[6] although these features are suggested to be absent in some radiodont species.[4][1]


The original diagnosis of order Radiodonta in 1996 is as follows:[6]

Radiodontids are bilaterally symmetrical, elongate arthropods with a nonmineralized cuticle typically most robust in the jaws and claws. The body is subdivided into two tagmata, much like the prosoma and opisthosoma of chelicerate arthropods. Typically, the front part shows no external segmentation, bears one pair of preoral claws, a pair of prominent eyes, and ventral jaws with radiating teeth. Some forms have additional rows of teeth and three or four postoral gnathobasic limb pairs. The trunk is metameric, typically with about 13 segments laterally developing imbricating lobes for swimming and gills for respiration, and may end in a prominent three-part tail. Some forms have gnathobasic trunk limbs.

In 2014, the clade Radiodonta was defined phylogenetically as a clade including any taxa closer to Anomalocaris canadensis than Paralithodes camtschaticus.[7] In 2019, it was redefined morphologically as animal bearing head carapace complex with central (H-) and lateral (P-) elements; outgrowths (endites) from frontal appendages bearing auxiliary spines; and reduced anterior flaps or bands of lamellae (setal blades) and strong tapering of body from anterior to posterior.[3]

Reconstruction of Aegirocassis benmoulai, the largest radiodont.


Reconstruction of Lyrarapax unguispinus. One of the smallest radiodonts.

Most radiodonts were significantly larger than other Cambrian fauna, with typical body lengths varying from 30 to 50 centimeters.[2] The largest described radiodont is Ordovician Aegirocassis benmoulai, which may have grown up to two meters long.[10][2] The smallest reported radiodont, a juvenile Lyrarapax unguispinus, was only about 18 millimeters.[11]

The body of a radiodont could be divided into two regions: head and trunk. The head is composed of only one body segment[12] known as the ocular somite, covered by sclerites (head carapace complex), bore arthropodized frontal appendages, ventral mouthparts (oral cone), and stalked compound eyes. The tapering trunk is composed of multiple body segments, each associated with pairs of flaps and gill-like structures (setal blades).[3]

Frontal appendageEdit

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 which have been referred to as 'claws', 'grasping appendages', 'feeding appendages', or 'great appendages' in previous studies (the last term is no longer used since the frontal appendages are now considered non-homologous to megacheiran great appendages.[12]). They are sclerotized and arthropodized (segmented), bearing ventral endites (spines) on most of their podomeres (segmental units), and the endites may bear additional rows of auxiliary spines on their anterior and posterior margins.[13][3] The frontal appendage consists of two regions: the shaft ('peduncle' in previous studies) and the distal articulated region.[13] A triangular region covered by soft cuticle (arthrodial membrane) may occur on the ventral side between podomeres 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 or 'great appendages' of other arthropods, which are deutocerebral (arose from post-ocular somite).[9][12] Since the morphology of the frontal appendages, especially those of the spines, always differs between species, it is one of the most important means of species identification.[13] In fact, many radiodonts are only known from a handful of fossilized frontal appendages.[14][13]

Oral coneEdit

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]

Head sclerites, eyes and trunkEdit

H-element of Hurdia victoria.
H-elements of various hurdiid radiodonts.
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,[11][3] an elongated terminal structure,[18] or a featureless blunt tip.[10]

Internal structuresEdit

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][20] 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.[21] Pairs of dorsal flaps, which make up a tail fan in some species, may help steering and/or stabilizing the animal during locomotion.[10][22] In Anomalocaris, morphology of the tail fan even suggests it could rapidly change its swimming direction efficiently.[23] 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.[24]


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][25] 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.[11] 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][26]

Frontal appendages of Amplectobelua, showing enlarged endites.

Oral cones of radiodonts may able to perform suction and/or biting ability.[15][25][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;[25] while the rigid tetraradial oral cones of Hurdia and Cambroraster with larger opening and additional tooth plates may capable to consume larger food item.[25][3]


Taxomomic affinitiesEdit


Priapulida   and relatives




Lobopodian grade

Siberiid lobopodians  






Summarized phylogeny between Radiodonta and other Ecdysozoan taxa.[27]

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).[27] This interpretation is supported by numerous arthropod-like features found on radiodonts, such as compound eyes,[16] digestive glands,[20] arthropodization (on frontal appendages),[27][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.[28] 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]

Taxa just basal to the radiodont and euarthropod branch are Pambdelurion, Kerygmachela and Opabinia, three radiodont-like dinocaridid genera usually referred as 'gilled lobopodians'.[27][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.[27] Taxa even basal to 'gilled lobopodians' are siberiids like Megadictyon and Jianshanopodia,[27] 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.[27]

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);[29] stem chelicerates alongside megacheirans a.k.a great appendage arthropods (based on the similarity between radiodont frontal appendages, megacheiran great appendages and chelicerae);[30] or Schinderhannes bartelsi, which resolved as a hurdiid radiodont in recent analyses,[27][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).[22] 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;[31] the megacheiran great appendages were considered to be deutocerebral,[32][33] 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.[27]


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]

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]




Paranomalocaris multisegmentalis 

Anomalocaris pennsylvanica

Anomalocaris sp. Balang

Anomalocaris canadensis 

Anomalocaris cf. canadensis Emu Bay

Anomalocaris cf. saron Pioche

Anomalocaris saron 


Anomalocaris kunmingensis

Amplectobelua stephenensis

Amplectobelua symbrachiata 

Lyrarapax unguispinus 


Anomalocaris briggsi 

Tamisiocaris borealis 


Hurdiid appendage Fezouata

Schinderhannes bartelsi 

cf. Peytoia Balang

Peytoia nathorsti 

Aegirocassis benmoulai 

Hurdia cf. victoria

Hurdia victoria 

Stanleycaris hirpex

Hurdia sp. Spence Shale

Hurdia sp. B Burgess


Species of Radiodonta
Species Describers Year Named Family Age Location Frontal Appendage
Cucumericrus decoratus Hou, Bergström, & Ahlberg 1995 Unknown
Caryosyntrips serratus Daley & Budd 2010 WuliuanDrumian   Canada   United States  
Caryosyntrips camurus Pates & Daley 2017 Wuliuan   Canada   United States  
Caryosyntrips durus Pates & Daley 2017 Drumian   United States  
Paranomalocaris multisegmentalis Wang, Huang, & Hu 2013   China  
Laminacaris chimera Guo, Pates, Cong, Daley, Edgecombe, Chen, & Hou 2018   China  
Anomalocaris canadensis Whiteaves 1892 Anomalocarididae  
Anomalocaris pennsylvanica Resser 1929  
Anomalocaris kunmingensis Wang, Huang, & Hu 2013
Anomalocaris magnabasis Pates, Daley, Edgecombe, Cong & Lieberman 2019  
Anomalocaris saron Hou, Bergström, & Ahlberg 1995  
Anomalocaris briggsi Nedin 1995 Tamisiocarididae   Australia  
Ramskoeldia platyacantha Cong, Edgecombe, Daley, Guo, Pates, & Hou 2018 Amplectobeluidae  
Ramskoeldia consimilis Cong, Edgecombe, Daley, Guo, Pates, & Hou 2018 Amplectobeluidae  
Lyrarapax unguispinus Cong, Ma, Hou, Edgecombe, & Strausfield 2014 Amplectobeluidae Cambrian Stage 3   China  
Lyrarapax trilobus Cong, Daley, Edgecombe, Hou, & Chen 2016 Amplectobeluidae Cambrian Stage 3   China  
Amplectobelua symbrachiata Hou, Bergström, & Ahlberg 1995 Amplectobeluidae  
Amplectobelua stephenensis Daley & Budd 2010 Amplectobeluidae  
Tamisiocaris borealis Daley & Peel 2010 Tamisiocarididae   Greenland  
Ursulinacaris grallae Pates, Daley & Butterfield 2019 Hurdiidae Wuliuan   Canada  
Schinderhannes bartelsi Kühl, Briggs, & Rust 2009 Hurdiidae Emsian   Germany
Stanleycaris hirpex Pates, Daley, & Ortega-Hernández 2018 Hurdiidae   Canada   United States
Peytoia nathorsti Walcott 1911 Hurdiidae  
Peytoia infercambriensis Lendzion 1975 Hurdiidae Cambrian Stage 3   Poland
Aegirocassis benmoulai Van Roy, Daley, & Briggs 2015 Hurdiidae Tremadocian   Morocco  
Hurdia victoria Walcott 1912 Hurdiidae WuliuanDrumian   Canada   Czechia  
Hurdia triangulata Walcott 1912 Hurdiidae Wuliuan   Canada  
Cambroraster falcatus Moysiuk & Caron 2019 Hurdiidae Wuliuan   Canada  
Pahvantia hastata Robison & Richards 1981 Hurdiidae Drumian   United States
Cordaticaris striatus Sun, Zeng, & Zhao 2020 Hurdiidae Drumian   Canada
Zhenghecaris shankouensis Vanner, Chen, Huang, Charbonnier, & Wang 2006 Hurdiidae   China Unknown


The history of radiodonts is complex. Incomplete specimens pertaining to different body parts of the same species had historically been interpreted as belonging to different species and even different phyla.[6][8] Prior to their recognition as a group, radiodont specimens had been assigned to five different phyla: Porifera, Cnidaria, Echinodermata, Annelida, and Arthropoda.[6]

The first known radiodont specimens were collected from the trilobite beds of Mount Stephen by Richard G. McConnell of the Geological Survey of Canada in 1886[6] or 1888.[35] These specimens were named Anomalocaris canadensis in 1892 by GSC paleontologist Joseph Whiteaves.[35] Whiteaves interpreted the specimens, now known to be isolated frontal appendages, as the abdomen of a phyllocarid crustacean. Additional radiodont specimens were described in 1911 by Charles Walcott. He interpreted an isolated oral cone, which he named Peytoia, as a jellyfish, and a poorly-preserved but relatively complete specimen, which he named Laggania, as a holothurian. In 1912 Walcott named Hurdia based on an isolated h-element, which he interpreted as the carapace of a crustacean. A Hurdia p-element was named Proboscicaris in 1962, and interpreted as the carapace of a bivalved arthropod.

The Geological Survey of Canada initiated a revision of Burgess Shale fossils in 1966, overseen by Cambridge University paleontologist Harry B. Whittington.[6] This revision would ultimately lead to the discovery of the complete radiodont body plan. In 1978, Simon Conway Morris recognized that the mouthparts of Laggania were Peytoia-like, but he interpreted this as evidence that it was a composite fossil made up of a Peytoia jellyfish and a sponge.[36] In 1979, Derek Briggs recognized that the fossils of Anomalocaris were appendages, not abdomens, but interpreted them as walking legs.[37] It was not until 1985 that the true nature of the fossils of Anomalocaris, Laggania, and Peytoia was recognized, and they were all assigned to a single genus, Anomalocaris. Subsequently it was recognized that Anomalocaris was a distinct form from the other two, resulting in a split into two genera, the latter of which was variously named Laggania and Peytoia until it was determined that Peytoia had priority. It was later recognized that some of the fossils assigned to these taxa belonged to another form, which was recognized as bearing a carapace made up of Hurdia and Proboscicaris elements. Finally, in 2009, these specimens were redescribed as Hurdia.[8] Radiodonta was originally viewed as containing a single family, Anomalocarididae, but it was divided into four families in 2014.[7]


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