Radiodonta

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

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 discouraged since the homology between frontal appendages and the original, megacheiran great appendages is questionable.[16][17]). They are sclerotized (hardened) 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.[18][3] The frontal appendage consists of two regions: the shaft ('peduncle',[2] 'base'[19] or 'promixal region'[2] in some studies) and the distal articulated region[18] (also referred to as 'claw'[19]). A triangular region covered by soft cuticle (arthrodial membrane) may occur on the ventral side between podomeres and provide flexibility.[20][11] 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 1).[9][16] 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.[18] In fact, many radiodonts are only known from a handful of fossilized frontal appendages.[20][18]

• 
  Frontal appendages of Anomalocarididae, Amplectobeluidae, and possibly related species
• 
  Frontal appendages of Tamisiocarididae
• 
  Frontal appendages of Hurdiidae

Oral cones of various radiodonts

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 be enlarged, giving the oral cone a triradial (e.g. Anomalocaris) or tetraradial (e.g. Hurdiidae, Lyrarapax) appearance.[21][12] 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 some amplectobeluid oral cones are speculative, but they possibly did not present a typical radial arrangement.[4][1]

Head sclerite complexes of various radiodonts

Three head sclerite (carapace) complex formed by a central H-element (anterior sclerite or head shield) and a pair of P-elements (lateral sclerites) cover the dorsal and laterovental surface of the animal's head.[3] The P-elements may connect to each other as well as the H-element by a narrow anterior extension (P-element neck or 'beak').[8][3] The head sclerites are small and ovoid in Anomalocarididae and Amplectobeluidae,[4][3] but often enlarged in Hurdiidae, corresponded to their distinct body shapes (streamlined in Anomalocarididae/Amplectobeluidae but often compact in Hurdiidae).[3] The head bore two stalked compound eyes, which may have had mobility,[22] and are located between the gaps formed by the posterior regions of the H-element and P-elements.[8][3]

Anterior region of two generized Anomalocaris and Hurdiid radiodont, showing distinct morphology 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][23]) is known to have pediform trunk appendages (legs).[24] The trunk has numerous body segments (somites), tapering from anterior to posterior, with the anterior three or four segments significantly constricted into a neck region.[3]

• 
  Variations of radiodont body flaps
• 
  The movement of radiodont flapping appendages
• 
  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 flaps per body segment, each slightly overlapping the one more anterior to it, but additional, non-overlapping sets of small dorsal flaps may occur in some Hurdiid species.[10] The flaps may have numerous vein-like structures (referred to as 'strengthening rays',[5] 'flap rays',[3] 'tranverse rods',[10] 'transverse lines'[25] or 'veins'[26]). The flaps on the neck region (referred to as 'reduced flaps',[4] 'neck flaps',[5] 'head flaps',[24] 'anterior flaps'[27] or 'differentiated flaps'[17]) are significantly reduced. In some species, jaw-like feeding appendages called gnathobase-like structures (GLSs) arose from each of the bases of their reduced neck flaps.[4][1] Numerous elongated blade-like extensions (referred to as lanceolate blades or lamellae[3]) arranged in a row, forming bands of gill-like structures known as setal blades, covered the dorsal surface of each body segment.[10] At least in Aegirocassis, each of the lanceolate blades are covered 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 exite and gill-bearing dorsal flaps of the former taxa.[28][10] The trunk may end either with a tail fan compose of 1 to 3 pairs of blades,[26][24][3] a pair of long furcae,[26][12][3] an elongated terminal structure,[24] or a featureless blunt tip.[10]

• 
  Stanleycaris, Hurdia, Aegirocassis, Peytoia and Cambroraster are all examples of hurdiid radiodonts. These were the most diverse and long lasting of the radiodont families, surviving from the Cambrian up until the Devonian.
• 
  Anomalocaris is a member of the anomalocarididae family, which at one point included all radiodonts, but now only includes a few genera such as Lenisicaris.
• 
  Amplectobelua and Lyrarapax are representatives of the amplectobeluidae which is a very inclusive family of mainly Chinese radiodonts.
• 
  Echidnacaris and Tamisiocaris are examples of the family tamisiocarididae which were exclusively suspension feeding radiodonts from the Cambrian.

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 were connected to the ventral flaps located at the lateral cavities of each body segment.[24][9] Between the lateral muscles is a sophisticated digestive system, formed by a widening of the foregut and hindgut, both connected by a narrow midgut associated with six pairs of gut divercula (digestive glands).[24][5][29] Compared to the three-segmented brains of euarthropods and two-segmented brains of onychophorans, the brain of radiodonts is composed of only one brain segment originating from the ocular somite, the protocerebrum. The nerves of the frontal appendages and compound eyes arose from the anterior and lateral regions of the brain.[9][16] Posterior to the brain was a pair of apparently unfused ventral nerve cords which ran through the animal's neck region.[9]

Paleoecological reconstruction of a group of Cambroraster swimming over a brine seep

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.[30][31] Pairs of dorsal flaps, which make up a tail fan in some species, may have helped steering and/or stabilizing the animal during locomotion.[10][32] In Anomalocaris, morphology of the tail fan even suggests it could rapidly change its swimming direction efficiently.[33] On the other hand, some hurdiids have features significantly specialized for a nektobenthic lifestyle, such as Cambroraster with its dome-like H-element similar to the carapace of a horseshoe crab.[3] Bands of setal blades with wrinkling lanceolate blades may have increased the surface area, suggesting they were gills, providing the animal's respiratory function.[24][10] Abundance of the remains of scleritzed structures such as disarticulated frontal appendages and head sclerite complexes, 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.[34]

Anomalocaris canadensis

Amplectobelua stephenensis

Caryosyntrips camurus

Tamisiocaris borealis

Peytoia nathorsti

Cambroraster falcatus

Suggested frontal appendage mobility and movement of various radiodonts[20][11]

Radiodonts had diverse feeding strategies, which could be categorized as raptorial predators, sediment sifters, or suspension, filter feeders.[2][35][11][36][37] 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.[23][20][4][11] With the smaller head carapace complex and large surface of arthrodial membranes, frontal appendages of these taxa had greater flexibility.[12] 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 passing food items towards the well-developed oral cone.[3][11] 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.[20][38]

Oral cones of radiodonts may have been used for suction and/or biting.[21][35][3] Together with the great variety of frontal appendages in different species of radiodonts, differentiation of oral cones between species suggests preferences of different diets as well.[35][11] For example, the triradial oral cone of Anomalocaris with irregular, tuberculated toothplates and a small opening may have been adapted to small and nektonic prey,[21][11] while the rigid tetraradial oral cones of Peytoia, Titanokorys, Hurdia, and one isolated oral cone attributed to Cambroraster with a larger opening and sometimes additional tooth plates may have been capable to consume larger food items relative to their body size and probably benthic or endobenthic prey.[21][35][3]

• 
  The opabiniids Opabinia (top) and Utaurora (bottom), were close relatives of the radiodonts
• 
  The radiodont/opabiniid-euarthropod intermediate Kylinxia, shares many of the characteristics found in both dinocaridids and euarthropods
• 
  A life reconstruction of the basal deuteropod Erratus, which helped reveal the evolution of arthropod trunk appendages
• 
  Mieridduryn is a dinocaridid panarthropod from the Middle Ordovician that shares traits with both opabiniids and radiodonts

Most phylogenetic analyses suggest that radiodonts, alongside opabiniids (Opabinia and Utaurora[40]), are stem-group arthropods just basal to deuteropoda,[39] a clade including upper stem (e.g. fuxianhuiids and bivalved arthropods) and crown Euarthropoda (e.g. Artiopoda, Chelicerata and Mandibulata).[8][41][42][43][44][45][7][9][10][2][3][27][17][36][37][46][47][40] This interpretation is supported by numerous arthropod groundplan found on radiodonts and opabiniids, such as stalked compound eyes,[22] digestive glands,[29] trunk appendages forming by dorsal and ventral elements (precursor of arthropod biramous appendages).[10][47] Compared to opabiniids, which possess posterior mouth opening and fused frontalmost appendages (comparable to euarthropod posterior-facing labrum/hypostome complex),[16][40] radiodonts on the other hand featured euarthropod-like dorsal sclerite (H-element) and arthropodization (frontal appendages) on their head regions,[48][16][40] alongside cuticularized gut termini.[24] The fact that both radiodonts and opabiniids lack exoskeleton on their trunk region suggests that the origin of compound eyes and arthropodization (segmented appendages) precede arthrodization (full set of trunk exoskeleton) in the arthropod stem lineage.[39][49][50] The constricted neck region with feeding appendicular structures of some radiodont may also shed light on the origin of the sophisticated arthropod head, which was formed by the fusion of multiple anterior body segments.[4][16] Basal deuteropods that possess a mixture of radiodont/opabiniid characters like Kylinxia and Erratus, may represent intermediate forms between radiodonts, opabiniids and other euarthropods.[17][47]

• 
  The siberiid lobopodians Siberion (upper left), Megadictyon (bottom center) and Jianshanopodia (upper right)
• 
  The 'gilled lobopodians' Pambdelurion (left) and Kerygmachela (right)
• 
  Megacheira or 'great appendage arthropods', a class of possible stem-chelicerate previously thought to be radiodont's close relative

Taxa just basal to the radiodont, opabiniid and euarthropod branch are 'gilled lobopodians' like Pambdelurion and Kerygmachela, which occasionally united under the class Dinocaridida with opabibiids and radiodonts.[51][52] They have body flaps, digestive glands, large (presumely compound) eyes and specialized frontal appendages like the former taxa, but their frontal appendages are not arthropodized nor fused, eyes sessile, gill-like structures less prominent, and certainlly bore lobopod underneath each of their flaps.[53][10][54][40] Taxa even basal to 'gilled lobopodians' are siberiids like Megadictyon and Jianshanopodia,[39] a group of lobopodians that 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.[55][39][16][56][49][50]

Previous studies may suggest radiodonts as a group other than stem-arthropods, such as a hitherto unknown phylum;[30] cycloneuralian worms undergone convergent with arthropods (based on the cycloneuralian-like radial mouthparts);[57][51] stem chelicerate euarthropods alongside megacheirans also known as great appendage arthropods (based on the similarity between radiodont frontal appendages, megacheiran great appendages and chelicerae);[58] or Schinderhannes bartelsi, which resolved as a hurdiid radiodont in recent analyses,[39][7][10][2][3][36][37] as a species more closely related to euarthropods than other radiodonts (based on some putative euarthropod-like features found on Schinderhannes).[32] 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;[59] radiodonts lacking definitive euarthropod features such as trunk tergites and multiple head appendages,[39] and the megacheiran great appendages were considered to be deutocerebral,[60][61] which could be non-homologous to the radiodont protocerebral frontal appendages;[9][16] putative euarthropod characters found on the single Schinderhannes fossil is questionable and may present other radiodont-like structures.[39]

Traditionally, all radiodont species have been placed within one family, Anomalocarididae,[6] hence the previous common name 'anomalocaridid'[23][8] and it was still occasionally used to refer the whole order even after reclassification.[9][10] Since the reassignment done by Vinther et al. 2014, most of the radiodont species were reclassified within three new families: Amplectobeluidae, Tamisiocarididae[2][3] (formerly Cetiocaridae[7]), and Hurdiidae.[7][10][2][3] Including Anomalocarididae, the four recent radiodont families may form the clade Anomalocarida.[7]

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][24][10] Due to the limited discovery, The position of Cucumericrus within Radiodonta is unclear, as it was either unselected by phylogenetic analysis[7][3][2][36][37] or resolved in a polytomy with Radiodonta and Euarthropoda.[10][12]

• 
  One of the poorly-known body parts (trunk appendage) of Cucumericrus decoratus; this species may not represent a true radiodont.
• 
  Frontal appendage of Echidnacaris briggsi, a tamisiocarid radiodont that was once suggested to belong to the Anomalocaris genus until its description in 2023.

The first in-depth phylogenetic analysis of Radiodonta was conducted by Vinther et al. in 2014,[7] followed by a handful of subsequest studies with more or less modified results.[9][10][2][12][3][36][37][40] In most analysis, Caryosyntrips is the basal-most genus, but resolved in a polytomy with other radiodonts and Euarthropoda (alongside Cucumericrus if included[10][12]). With the exclusion of Caryosyntrips and Cucumericrus, the monophyly of Radiodonta is widely supported,[7][9][10][2][12][3][36][37] with a few results suggest possible paraphyly (either the Anomalocarididae+Amplectobeluidae clade or Hurdiidae sister to Euarthropoda).[27][40] Putative synapomorphies of monophyletic Radiodonta including tripartite head sclerite complex and differentiated neck region.[3] The genus Anomalocaris in a broader sense always found to be polyphyletic, usually with "Anomalocaris" kunmingensis and "Anomalocaris" briggsi resolved as a member of Amplectobeluidae and Tamisiocarididae respectively.[7][9][10][2][3][36][37] Interrelationship of Amplectobeluidae is uncertain, as the amplectobeluid affinities of Lyrarapax and Ramskoeldia were occasionally questioned.[1][3][37] Monophyly of the speciose family Hurdiidae was recovered by most analysis and well-supported by several synapomorphies (e.g. distal articulated region of frontal appendage with proximal 5 podomeres bearing subequal endites[18][3]), with Tamisiocarididae often suggested to be its sister group.[7][10][2][3]

• Radiodonta
  •  ?Cucumericrus[23] (putative radiodont[62])
  • Caryosyntrips[20]
  • Anomalocarida
    • Paranomalocaris[63] (placed within Anomalocarididae by some studies.[12][64])
    • Laminacaris[65] (placed within Amplectobeluidae by some studies.[2])
    • Houcaris (either placed within Anomalocarididae,[10][12][40] Amplectobeluidae[7][2] or Tamisiocarididae.[14][36])
    • Innovatiocaris[66]
    • Anomalocarididae
      • Anomalocaris (in a broader sense, some species may placed within the other families.[7][10])
      • Lenisicaris[19]
    • Amplectobeluidae
      • Lyrarapax[9] (position questioned by some studies.[1])
      • Amplectobelua[23]
      • Ramskoeldia[1] (position questioned by some studies.[3])
      • Guanshancaris[64][67]
    • Tamisiocarididae
      • Tamisiocaris
      • Echidnacaris
      • Houcaris
    • Hurdiidae
      • Aegirocassisinae[68]
        • Aegirocassis[10]
        • Pseudoangustidontus
      • Peytoia
      • Schinderhannes
      • Hurdia
      • Stanleycaris[69]
      • Pahvantia[2]
      • Ursulinacaris[18]
      • Cambroraster[3]
      •  ?Zhenghecaris (putative radiodont[62])
      • Cordaticaris[70]
      • Buccaspinea[71]
      • Titanokorys[37]

Described species of Radiodonta

Species	Original description	Year named	Family	Age	Location	Frontal appendage	Head sclerite complex
Cucumericrus decoratus	Hou, Bergström, & Ahlberg	1995[23]	(unassigned)	Cambrian Stage 3	China	Unknown	Unknown
Caryosyntrips serratus	Daley & Budd	2010[20]	(unassigned)	Wuliuan–Drumian	Canada  United States		Unknown
Caryosyntrips camurus	Pates & Daley	2017[38]	(unassigned)	Wuliuan	Canada  United States		Incomplete[71]
Caryosyntrips durus	Pates & Daley	2017[38]	(unassigned)	Drumian	United States		Unknown
Paranomalocaris multisegmentalis	Wang, Huang, & Hu	2013[63]	Anomalocarididae?	Cambrian Stage 4	China		Unknown
Paranomalocaris simplex	Jiao, Pates, Lerosey-Aubril, Ortega-Hernandez, Yang, Lan, Zhang	2021[64]	Anomalocarididae?	Cambrian Stage 4	China		Unknown
Laminacaris chimera	Guo, Pates, Cong, Daley, Edgecombe, Chen, & Hou	2018[65]	(controversial)	Cambrian Stage 3	China		Unknown
Innovatiocaris maotianshanensis	Zeng, Zhao, Zhu	2022[66]	(unassigned)	Cambrian Stage 3	China		Incomplete[66]
Innovatiocaris? multispiniformis	Zeng, Zhao, Zhu	2022[66]	(unassigned)	Cambrian Stage 3	China		Unknown
Anomalocaris canadensis	Whiteaves	1892[72]	Anomalocarididae	Wuliuan	United States
Lenisicaris pennsylvanica (formerly Anomalocaris pennsylvanica)[19]	Resser	1929	Anomalocarididae	Cambrian Stage 3	United States		Unknown
Lenisicaris lupata	Wu, Ma, Lin, Sun, Zhang, & Fu	2021[19]	Anomalocarididae	Cambrian Stage 3	China		Unknown
Anomalocaris daleyae	Paterson, García-Bellidob & Edgecombe	2023	Anomalocarididae	Cambrian Stage 4	Australia		Unknown
Houcaris magnabasis (formerly Anomalocaris magnabasis)[14]	Pates, Daley, Edgecombe, Cong & Lieberman	2019	(controversial)	Cambrian Stage 4	United States		Unknown
Houcaris saron (formerly Anomalocaris saron)[14]	Hou, Bergström, & Ahlberg	1995	(controversial)	Cambrian Stage 3	China		Unknown
Echidnacaris briggsi[73]	Nedin	1995	Tamisiocarididae	Cambrian Stage 4	Australia		Unknown
Ramskoeldia platyacantha	Cong, Edgecombe, Daley, Guo, Pates, & Hou	2018[1]	Amplectobeluidae	Cambrian Stage 3	China		Incomplete[1]
Ramskoeldia consimilis	Cong, Edgecombe, Daley, Guo, Pates, & Hou	2018[1]	Amplectobeluidae	Cambrian Stage 3	China		Incomplete[1]
Lyrarapax unguispinus	Cong, Ma, Hou, Edgecombe, & Strausfield	2014[9]	Amplectobeluidae	Cambrian Stage 3	China
Lyrarapax trilobus	Cong, Daley, Edgecombe, Hou, & Chen	2016[5]	Amplectobeluidae	Cambrian Stage 3	China
Amplectobelua symbrachiata	Hou, Bergström, & Ahlberg	1995[23]	Amplectobeluidae	Cambrian Stage 3	China
Amplectobelua stephenensis	Daley & Budd	2010[20]	Amplectobeluidae	Wuliuan	United States		Unknown
Guanshancaris kunmingensis	Zhang et al.	2023[67]	Amplectobeluidae	Cambrian Stage 4	China		Unknown
Tamisiocaris borealis	Daley & Peel	2010	Tamisiocarididae	Cambrian Stage 3	Greenland		Incomplete[7]
Ursulinacaris grallae	Pates, Daley & Butterfield	2019	Hurdiidae	Wuliuan	Canada		Unknown
Schinderhannes bartelsi	Kühl, Briggs, & Rust	2009[32]	Hurdiidae	Emsian	Germany	Incomplete[3]	Incomplete[3]
Stanleycaris hirpex	Pates, Daley, & Ortega-Hernández	2018[69]	Hurdiidae	Wuliuan	Canada		P-element is unknown, possibly absent[74]
Peytoia nathorsti	Walcott	1911[75]	Hurdiidae	Wuliuan–Drumian	Canada  United States		Incomplete[3]
Peytoia infercambriensis (formerly Cassubia infercambriensis)[76]	Lendzion	1975	Hurdiidae	Cambrian Stage 3	Poland		Unknown
Aegirocassis benmoulai	Van Roy, Daley, & Briggs	2015[10]	Hurdiidae (Aegirocassisinae)	Tremadocian	Morocco
Hurdia victoria	Walcott	1912[77]	Hurdiidae	Wuliuan–Drumian	Canada  Czechia
Hurdia triangulata	Walcott	1912[77]	Hurdiidae	Wuliuan	Canada
Cambroraster falcatus	Moysiuk & Caron	2019[3]	Hurdiidae	Wuliuan	Canada
Pahvantia hastata	Robison & Richards	1981	Hurdiidae	Drumian	United States
Cordaticaris striatus	Sun, Zeng, & Zhao	2020[70]	Hurdiidae	Drumian	China	Incomplete[70]
Zhenghecaris shankouensis	Vanner, Chen, Huang, Charbonnier, & Wang	2006	Hurdiidae	Cambrian Stage 3	China	Unknown
Buccaspinea cooperi	Pates, Lerosey-Aubril, Daley, Kier, Bonino & Ortega-Hernández	2021[71]	Hurdiidae	Drumian	United States		Unknown
Titanokorys gainesi	Caron & Moysiuk	2021[37]	Hurdiidae	Wuliuan	Canada
Pseudoangustidontus duplospineus	Van Roy & Tetlie	2006	Hurdiidae (Aegirocassisinae)	Tremadocian	Morocco		Unknown
Pseudoangustidontus izdigua	Potin, Gueriau & Daley	2023	Hurdiidae (Aegirocassisinae)	Tremadocian	Morocco		Incomplete[68]