Radiodonta
Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. Radiodonts are distinguished by their distinctive frontal appendages, which are morphologically diverse and were used for a variety of functions. Radiodonts were among the earliest large predators, and they also included sediment sifters and filter feeders. Some of the most famous species of radiodonts are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, Peytoia nathorsti, Titanokorys gainesi, Cambroraster falcatus and Amplectobelua symbrachiata. The later surviving members include the subfamily Aegirocassisinae from the Early Ordovician of Morocco and the Early Devonian member Schinderhannes bartelsi from Germany.
Etymology
The name Radiodonta refers to the radial arrangement of tooth plates surrounding the mouth, although this feature is suggested to be absent in some radiodont species.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 and lateral elements; outgrowths from frontal appendages bearing auxiliary spines; and reduced anterior flaps or bands of lamellae and strong tapering of body from anterior to posterior.
Members of Radiodonta are known as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, 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. A nearly complete specimen of a juvenile Lyrarapax unguispinus measured only, making it among the smallest radiodont specimens known, though adults reached a length of 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. The largest known Cambrian radiodont was Amplectobelua, reaching lengths of up to based on an incomplete specimen. Anomalocaris canadensis was also relatively large, estimated up to long, and the Cambrian hurdiid Titanokorys approached around long.The body of a radiodont could be divided into two regions: head and trunk. The head is composed of only one body segment known as the ocular somite, covered by sclerites, bore arthropodized frontal appendages, ventral mouthparts, and stalked compound eyes. The tapering trunk is composed of multiple body segments, each associated with pairs of flaps and gill-like structures.
Frontal appendage
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. They are sclerotized and arthropodized, bearing ventral endites on most of their podomeres, and the endites may bear additional rows of auxiliary spines on their anterior and posterior margins. The frontal appendage consists of two regions: the shaft and the distal articulated region. A triangular region covered by soft cuticle may occur on the ventral side between podomeres and provide flexibility. Their purported pre-ocular and protocerebral origin suggest they are homologous to the primary antennae of Onychophora and the labrum of Euarthropoda, while subsequent studies also suggest a deutocerebral origin and homologous with the chelicerae of Chelicerata and the antennae or 'great appendages' of other arthropods. 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. In fact, many radiodonts are only known from a handful of fossilized frontal appendages.Oral cone
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. Three or four tooth plates might be enlarged, giving the oral cone a triradial or tetraradial appearance. 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. Detail reconstruction of some amplectobeluid oral cones are speculative, but they possibly did not present a typical radial arrangement.Head sclerites, eyes and trunk
Three head sclerite complex formed by a central H-element and a pair of P-elements cover the dorsal and laterovental surface of the animal's head. The P-elements may connect to each other as well as the H-element by a narrow anterior extension. The head sclerites are small and ovoid in Anomalocarididae and Amplectobeluidae, but often enlarged in Hurdiidae, corresponded to their distinct body shapes. The head bore two stalked compound eyes, which may have had mobility, and are located between the gaps formed by the posterior regions of the H-element and P-elements. The compound eyes of Echidnacaris are exceptionally unstalked. Some species of Hurdiid possess an additional median eye behind the H-element.Contrary to the original diagnosis, the division of body segments can be visible externally and no known member of Radiodonta is known to have pediform trunk appendages. The trunk has numerous body segments, tapering from anterior to posterior, with the anterior three or four segments significantly constricted into a neck region.
The trunk appendages were fin-like body flaps, 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. The flaps may have numerous vein-like structures. The flaps on the neck region are significantly reduced. In some species, jaw-like feeding appendages called gnathobase-like structures arose from each of the bases of their reduced neck flaps. Numerous elongated blade-like extensions arranged in a row, forming bands of gill-like structures known as setal blades, covered the dorsal surface of each body segment. At least in Aegirocassis, each of the lanceolate blades are covered in wrinkles. The ventral flaps may be homologous to the endopod of the biramous limbs of euarthropods and lobopodous limbs 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. The trunk may end either with a tail fan compose of 1 to 3 pairs of blades, a pair of long furcae, an elongated terminal structure, or a featureless blunt tip.
Internal structures
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. 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 diverticula.The brain of radiodonts was simpler than the three-segmented 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. In Anomalocaris, morphology of the tail fan even suggests it could rapidly change its swimming direction efficiently. 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. Bands of setal blades with wrinkling lanceolate blades may have increased the surface area, suggesting they were gills, providing the animal's respiratory function. 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, a behavior which also has been reported in some other Cambrian arthropods such as trilobites.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. With the smaller head carapace complex and large surface of arthrodial membranes, frontal appendages of these taxa had greater flexibility. 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. 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. 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.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.