Radiodonta

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, ==Definition==

The original diagnosis of order Radiodonta in 1996 is as follows: In 2014, the clade Radiodonta was defined phylogenetically as a clade including any taxa closer to Anomalocaris canadensis than Paralithodes camtschaticus. 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. Members of Radiodonta are known as radiodonts, radiodontids, although the last two originally refer to the family Anomalocarididae, which previously included all species of this order but is now restricted to only a few species. ==Description==

Most radiodonts were significantly larger than the other Cambrian fauna, with typical body lengths of large taxa varying from . The largest described radiodont is the Early Ordovician species Aegirocassis benmoulai, which may have grown up to long. An isolated frontal appendage of a hurdiid from the Ordovician with a length less than half that of the juvenile Lyrarapax is known, but it is not known whether this specimen pertains to an adult. Anomalocaris canadensis was also relatively large, estimated up to long,). 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. or 'proximal region' while subsequent studies also suggest a deutocerebral origin and homologous with the chelicerae of Chelicerata and the antennae or 'great appendages' of other arthropods (all arose from post-ocular somite 1). and are located between the gaps formed by the posterior regions of the H-element and P-elements.) is known to have pediform trunk appendages (legs). 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. or 'veins'). The flaps on the neck region (referred to as 'reduced flaps', or 'differentiated flaps' The brain of radiodonts was simpler than the three-segmented (compose of pro-, deuto- and tritocerebrum) brains of euarthropods, but further interpretations differ between studies. Based on Cong et al. 2014, the brain 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. Based on Moysiuk & Caron 2022, the frontal appendage nerves arose from the ventral deutocerebrum, the second brain segment. The previous "frontal appendage nerves" actually represent median eye nerve. In both interpretations, posterior to the brain was a pair of apparently unfused ventral nerve cords which ran through the animal's neck region. == Paleoecology ==

Physiology 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. Pairs of dorsal flaps, which make up a tail fan in some species, may have helped steering and/or stabilizing the animal during locomotion. 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. Diet Radiodonts had diverse feeding strategies, which could be categorized as raptorial predators, sediment sifters, or suspension, filter feeders. 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. Oral cones of radiodonts may have been used for suction and/or biting. 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. 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, 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. ==Classification==

Taxonomic affinities }} File:20220213 Opabiniidae Opabiniids.png|The opabiniids Opabinia (top) and Utaurora (bottom), were close relatives of the radiodonts File:20210310 Kylinxia zhangi.png|The presumed radiodont/opabiniid-euarthropod intermediate Kylinxia, shares many of the characteristics found in both dinocaridids and euarthropods — later studies have considered this relationship far less direct File:Erratus.png|A life reconstruction of the basal deuteropod Erratus, which helped reveal the evolution of arthropod trunk appendages File:20221119 Mieridduryn bonniae diagrammatic reconstruction.png|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 a clade including upper stem (e.g. fuxianhuiids and bivalved arthropods) and crown Euarthropoda (e.g. Artiopoda, Chelicerata and Mandibulata). This interpretation is supported by numerous arthropod groundplan found on radiodonts and opabiniids, such as stalked compound eyes, 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. File:20191217 Siberiida Siberion Megadictyon Jianshanopodia.png|The siberiid lobopodians Siberion (upper left), Megadictyon (bottom center) and Jianshanopodia (upper right) File:20210730 Gilled lobopodians Pambdelurion Kerygmachela.png|The 'gilled lobopodians' Pambdelurion (left) and Kerygmachela (right) File:20191028 Megacheirans Leanchoilia Haikoucaris Yohoia Fortiforceps.png|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 are occasionally united under the class Dinocaridida with opabibiids and radiodonts. or Schinderhannes bartelsi, which resolved as a hurdiid radiodont in recent analyses, 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; radiodonts lacking definitive euarthropod features such as trunk tergites and multiple head appendages, which could be non-homologous to the radiodont protocerebral frontal appendages; With the exclusion of questionable Caryosyntrips and Cucumericrus, the monophyly of Radiodonta is widely supported, (placed within Anomalocarididae by some studies. (placed within Amplectobeluidae by some studies. • Anomalocarididae • Anomalocaris (in a broader sense, some species may placed within the other families. • Shucaris (either placed within Anomalocarididae or Amplectobeluidae • Aegirocassis) • Stanleycaris • Mosura • Falciscaris • Hurdiinae • Hurdia • Pahvantia • Buccaspinea • Titanokorys • Liantuoia (may be a synonym of Hurdia) ==History==

, the original "Laggania cambria''" File:Anomalocaris canadensis grasping claw, Burgess Shale.jpg|Frontal appendage of Anomalocaris canadensis File:Peytoia nathorsti Laggania cambria oral cone, Burgess Shale.jpg|Oral cone of Peytoia nathorsti File:Hurdia victoria USNM PAL 57718.jpg|H-element of Hurdia victoria File:USNM PAL 57490.jpg|Paired frontal appendages from an unnamed hurdiid radiodont He interpreted an isolated oral cone, which he named Peytoia nathorsti, as a jellyfish, and a poorly preserved but relatively complete specimen, which he named Laggania cambria, as a holothurian. Isolated frontal appendages of Peytoia and Hurdia, collectively known as "Appendage F" in Briggs 1979, were all identified as those of Sidneyia at that time. The Geological Survey of Canada initiated a revision of Burgess Shale fossils in 1966, overseen by Cambridge University paleontologist Harry B. Whittington. The taxon Radiodonta itself was coined in 1996 by Desmond Collins, after it was established that Anomalocaris and its kin represented a distinctive lineage with arthropod affinities rather than a hitherto unknown phylum. Collins also established the class Dinocarida to contain the order Radiodonta as well as the Opabiniidae, which he recognized as distinct due to its lacking the distinctive oral cone structure of radiodonts. Radiodonta was first given a phylogenetic definition in 2014. Radiodonta was originally viewed as containing a single family, Anomalocarididae, but it was divided into four families in 2014: Amplectobeluidae, Anomalocarididae, Cetiocaridae, and Hurdiidae. The name Cetiocaridae did not conform to the International Code of Zoological Nomenclature and so was renamed Tamisiocarididae in 2019. Until the 2010s, radiodonts were typically considered to be uniformly large apex predators, but discoveries of new species over the course of that decade led to a considerable increase in the known ecological and morphological diversity of the group. ==References==