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Radiodonta is an extinct order of stem-group
arthropods Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
that was successful worldwide during the
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
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, but they also included sediment sifters and filter feeders. Some of the most famous species of radiodonts are the
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
taxa '' Anomalocaris canadensis'', '' Hurdia victoria'', ''
Peytoia nathorsti ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'', '' Titanokorys gainesi, Cambroraster falcatus'' and '' Amplectobelua symbrachiata''. The later surviving members include the subfamily Aegirocassisinae from the Early Ordovician of
Morocco Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...
and the
Early Devonian The Early Devonian is the first of three Epoch (geology), epochs comprising the Devonian period, corresponding to the Lower Devonian Series (stratigraphy), series. It lasted from and began with the Lochkovian Stage , which was followed by the Pr ...
member '' Schinderhannes bartelsi'' from Germany.


Etymology

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, 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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
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, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two originally refer to the family
Anomalocarididae Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
, 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 The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
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 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 (head carapace complex), bore arthropodized frontal appendages, ventral mouthparts (oral cone), and stalked
compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...
s. The tapering trunk is composed of multiple body segments, each associated with pairs of flaps and gill-like structures (setal blades).


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 (the last term is discouraged since the homology between frontal appendages and the original, morphologically distinct megacheiran great appendages is questionable.). 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. The frontal appendage consists of two regions: the shaft ('peduncle', 'base' or 'proximal region' in some studies) and the distal articulated region (also referred to as 'claw'). A triangular region covered by soft cuticle (arthrodial membrane) 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 Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...
and the labrum of Euarthropoda (all arose from ocular
somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryogenesis, embryonic stage of somitogenesis, along the head-to-tail axis in segmentation (biology), segmented animals. ...
), while subsequent studies also suggest a deutocerebral origin and homologous with the
chelicerae The chelicerae () are the arthropod mouthparts, mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated ...
of
Chelicerata The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...
and the antennae or ' great appendages' of other arthropods (all arose from post-ocular somite 1). 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. File:20191221 Radiodonta frontal appendage Anomalocarididae Amplectobeluidae.png, Frontal appendages of
Anomalocarididae Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
, Amplectobeluidae, and possibly related species File:20191228 Radiodonta frontal appendage Tamisiocarididae Cetiocaridae.png, Frontal appendages of Tamisiocarididae File:20191229 Radiodonta frontal appendage Hurdiidae.png, Frontal appendages of Hurdiidae


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 ('jaws' in previous studies). 3 or 4 tooth plates might be enlarged, giving the oral cone a triradial (e.g. ''
Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'', '' Echidnacaris'') or tetraradial (e.g. Hurdiidae, '' Lyrarapax'') 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 (
carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...
) 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. The P-elements may connect to each other as well as the H-element by a narrow anterior extension (P-element neck or 'beak'). The head sclerites are small and ovoid in
Anomalocarididae Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
and Amplectobeluidae, but often enlarged in Hurdiidae, corresponded to their distinct body shapes (streamlined in Anomalocarididae/Amplectobeluidae but often compact in Hurdiidae). The head bore two stalked
compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...
s, 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 possess an additional median eye behind the H-element. Contrary to the original diagnosis, the division of body segments (segmental boundaries) can be visible externally and no known member of Radiodonta (except the putative radiodont '' Cucumericrus'') 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. File:20210914 Radiodonta body flaps lobes structures variations.png, Variations of radiodont body flaps File:Radiodonta body flaps movement.gif, The movement of radiodont flapping appendages File:Video animation of Cambroraster.webm, Video animation of ''Cambroraster falcatus'', showcasing the movement of the trunk flaps File:20200103 Radiodonta gnathobase‐like structures.png, 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. The flaps may have numerous vein-like structures (referred to as 'strengthening rays', 'flap rays', 'tranverse rods', 'transverse lines' or 'veins'). The flaps on the neck region (referred to as 'reduced flaps', 'neck flaps', 'head flaps', 'anterior flaps' or 'differentiated flaps') 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. Numerous elongated blade-like extensions (referred to as lanceolate blades or lamellae) arranged in a row, forming bands of
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
-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 (lobopods) of gilled
lobopodia Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as ...
ns, 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. File:20220724 Hurdiidae.png, '' Stanleycaris'', '' Hurdia'', '' Aegirocassis'', ''
Peytoia ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'' 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. File:20210626 Anomalocaris.png, ''
Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'' is a member of the
anomalocarididae Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
family, which at one point included all radiodonts, but now only includes a few genera such as '' Lenisicaris''. File:20210912 Amplectobeluidae.png, '' Amplectobelua'' and '' Lyrarapax'' are representatives of the amplectobeluidae which is a very inclusive family of mainly Chinese radiodonts. File:20191228 Radiodonta frontal appendage Tamisiocarididae Cetiocaridae.png, '' Echidnacaris'' and '' Tamisiocaris'' are examples of the family tamisiocarididae which were exclusively suspension feeding radiodonts from the Cambrian.


Internal structures

Traces of
muscles Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
,
digestive system The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
and
nervous system In biology, the nervous system is the complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...
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 divercula (digestive glands). 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 cord The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice ve ...
s 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 Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...
. 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 ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'', 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 A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...
of a
horseshoe crab Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not true crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scor ...
. 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 In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...
events may have occurred among radiodonts, a behavior which also has been reported in some other
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
arthropods such as
trilobites Trilobites (; meaning "three-lobed entities") are extinct marine arthropods that form the class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most successful of all early animals, ...
.


Diet

Radiodonts had diverse feeding strategies, which could be categorized as
raptorial In biology (specifically the anatomy of arthropods), the term ''raptorial'' implies much the same as ''predatory'' but most often refers to modifications of an arthropod leg, arthropod's foreleg that make it function for the grasping of prey whi ...
predators, sediment sifters, or suspension, filter feeders. For example, raptorial predators like ''
Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'' 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 Pincer may refer to: *Pincers (tool) *Pincer (biology), part of an animal *Pincer ligand, a terdentate, often planar molecule that tightly binds a variety of metal ions *Pincer (Go), a move in the game of Go *"Pincers!", an episode of the TV series ...
. 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 ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'' 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 Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
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 ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'' 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 ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'', '' 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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analyses suggest that radiodonts, alongside opabiniids ('' Opabinia'' and '' Utaurora''), are stem-group
arthropods Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
just basal to deuteropoda, a clade including upper stem (e.g. fuxianhuiids and bivalved arthropods) and crown Euarthropoda (e.g.
Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...
,
Chelicerata The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...
and Mandibulata). This interpretation is supported by numerous arthropod groundplan found on radiodonts and opabiniids, such as stalked
compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...
s, digestive glands, trunk appendages forming by dorsal and ventral elements (precursor of arthropod biramous appendages). Compared to opabiniids, which possess posterior mouth opening and fused frontalmost appendages (comparable to euarthropod posterior-facing labrum/hypostome complex), radiodonts on the other hand featured euarthropod-like dorsal sclerite (H-element) and arthropodization (frontal appendages) on their head regions, alongside cuticularized gut termini. 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. 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. Basal deuteropods that possess a mixture of radiodont/opabiniid characters like '' Kylinxia'' and '' Erratus'', may represent intermediate forms between radiodonts, opabiniids and other euarthropods. 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 ''Kerygmachela kierkegaardi'' is a Kerygmachelidae, kerygmachelid Lobopodia#Gilled lobopodians, gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with i ...
'' (right) File:20191028 Megacheirans Leanchoilia Haikoucaris Yohoia Fortiforceps.png, Megacheira or 'great appendage arthropods', a
class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...
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 ''Kerygmachela kierkegaardi'' is a Kerygmachelidae, kerygmachelid Lobopodia#Gilled lobopodians, gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with i ...
'', which occasionally united under the class Dinocaridida with opabibiids and radiodonts. 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 certainly bore lobopod underneath each of their flaps. Taxa even basal to 'gilled lobopodians' are siberiids like '' Megadictyon'' and '' Jianshanopodia'', a group of
lobopodia Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as ...
ns 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
lobopodian grade, alongside the other two extant panarthropod phyla
Tardigrada Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged Segmentation (biology), segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who calle ...
and
Onychophora Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...
. Previous studies may suggest radiodonts as a group other than stem-arthropods, such as a hitherto unknown
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
; cycloneuralian worms undergone convergent with arthropods (based on the cycloneuralian-like radial mouthparts); stem chelicerate euarthropods alongside megacheirans also known as great appendage arthropods (based on the similarity between radiodont frontal appendages, megacheiran great appendages and
chelicerae The chelicerae () are the arthropod mouthparts, mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated ...
); or '' Schinderhannes bartelsi,'' which resolved as a hurdiid radiodont in recent analyses, as a species more closely related to euarthropods than other radiodonts (based on some putative euarthropod-like features found on ''Schinderhannes''). 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
ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phylum (biology), phyla. The grouping of these animal phyla into a single ...
n plesimorphy, since they also have been found in panarthropods such as
tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them . In 1776, th ...
and some
lobopodia Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as ...
ns; radiodonts lacking definitive euarthropod features such as trunk tergites and multiple head appendages, and the megacheiran great appendages were considered to be deutocerebral, which could be non-homologous to the radiodont protocerebral frontal appendages; putative euarthropod characters found on the single ''Schinderhannes'' fossil is questionable and may present other radiodont-like structures.


Interrelationships

Traditionally, all radiodont species have been placed within one family,
Anomalocarididae Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
, hence the previous common name 'anomalocaridid' and it was still occasionally used to refer the whole order even after reclassification. Since the reassignment done by Vinther ''et al.'' 2014, most of the radiodont species were reclassified within three new families: Amplectobeluidae, Tamisiocarididae (formerly Cetiocaridae), and Hurdiidae. Including Anomalocarididae, the four recent radiodont families may form the clade Anomalocarida. The original description of the order Radiodonta included ''
Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'', '' Laggania'' (later known as ''
Peytoia ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
''), '' Hurdia'', '' Proboscicaris'', '' Amplectobelua'', '' Cucumericrus'', and '' Parapeytoia''. However, '' Proboscicaris'' is now regarded as a junior synonym of '' Hurdia'', and '' Parapeytoia'' is considered to be a Megacheiran. Due to the limited discovery, The position of '' Cucumericrus'' within Radiodonta is unclear, as it was either unselected by
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analysis or resolved in a polytomy with Radiodonta and Euarthropoda. File:20210207 Cucumericrus decoratus trunk appendage.png, One of the poorly-known body parts (trunk appendage) of '' Cucumericrus decoratus''; this species may not represent a true radiodont. File:20191228 Radiodonta frontal appendage Anomalocaris briggsi.png, 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, followed by a handful of subsequest studies with more or less modified results. In most analysis, '' Caryosyntrips'' is the basal-most genus, but either resolved in a polytomy with other radiodonts and Euarthropoda (alongside '' Cucumericrus'' if included) or outside of Radiodonta, casting doubt on its radiodont affinity. With the exclusion of questionable ''Caryosyntrips'' and ''Cucumericrus'', the
monophyly In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent comm ...
of Radiodonta is widely supported, with a few results suggest possible
paraphyly Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
(either the Anomalocarididae+Amplectobeluidae clade or Hurdiidae
sister A sister is a woman or a girl who shares parents or a parent with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to ref ...
to Euarthropoda). Putative
synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ...
of monophyletic Radiodonta including tripartite head sclerite complex and differentiated neck region. The genus ''
Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'' in a broader sense always found to be
polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...
, usually with '' "Anomalocaris" kunmingensis'' and ''"Anomalocaris" briggsi'' resolved as a member of Amplectobeluidae and Tamisiocarididae respectively. Interrelationship of Amplectobeluidae is uncertain, as the amplectobeluid affinities of '' Lyrarapax'' and '' Ramskoeldia'' were occasionally questioned. 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). Tamisiocarididae was often suggested to be sister group of Hurdiidae in the 2010s,Dryad Data
/ref> but this position became questionable in subsequent studies. * Radiodonta ** ?'' Cucumericrus'' (radiodont affinity questionable) ** ?'' Caryosyntrips'' (radiodont affinity questionable) ** Anomalocarida *** '' Paranomalocaris'' (placed within Anomalocarididae by some studies.) *** '' Laminacaris'' (placed within Amplectobeluidae by some studies.) *** '' Houcaris'' (either placed within Anomalocarididae, Amplectobeluidae or Tamisiocarididae.) *** '' Innovatiocaris'' ***
Anomalocarididae Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
**** ''
Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...
'' (in a broader sense, some species may placed within the other families.) **** '' Lenisicaris'' **** Shucaris (either placed within Anomalocarididae or Amplectobeluidae) *** Amplectobeluidae **** '' Lyrarapax'' (position questioned by some studies.) **** '' Amplectobelua'' **** '' Ramskoeldia'' (position questioned by some studies.) **** '' Guanshancaris'' **** Shucaris (either placed within Anomalocarididae or Amplectobeluidae) *** Tamisiocarididae **** '' Tamisiocaris'' **** '' Echidnacaris'' **** '' Houcaris'' *** Hurdiidae/ Peytoiidae **** Aegirocassisinae ***** '' Aegirocassis'' ***** '' Pseudoangustidontus'' ****''
Peytoia ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'' **** '' Schinderhannes'' (position questioned by some studies.) **** '' Stanleycaris'' **** '' Mosura'' **** Hurdiinae ***** '' Hurdia'' ***** '' Pahvantia'' ***** '' Ursulinacaris'' ***** '' Cambroraster'' ***** ?'' Zhenghecaris'' (putative hurdiid radiodont) ***** '' Cordaticaris'' ***** '' Buccaspinea'' ***** '' Titanokorys''


History

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 ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'' 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
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. Prior to their recognition as a group, radiodont specimens had been assigned to five different phyla:
Porifera Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...
,
Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
,
Echinodermata An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, ...
,
Annelida The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to variou ...
, and
Arthropoda Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated ( metameric) segments, and paired jointed appendages. ...
. The first known radiodont specimens were collected from the trilobite beds of Mount Stephen by Richard G. McConnell of the
Geological Survey of Canada The Geological Survey of Canada (GSC; , CGC) is a Canadian federal government agency responsible for performing geological surveys of the country developing Canada's natural resources and protecting the environment. A branch of the Earth Science ...
in 1886 or 1888. These specimens were named '' Anomalocaris canadensis'' in 1892 by GSC paleontologist Joseph Whiteaves. Whiteaves interpreted the specimens, now known to be isolated frontal appendages, as the abdomen of a phyllocarid
crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
. Additional radiodont specimens were described in 1911 by Charles Walcott.Walcott, C. D. 1911a
Middle Cambrian holothurians and medusae
Cambrian geoogy and paleontology II. Smithsonian Miscellaneous Collections, 57: 41–68.
He interpreted an isolated oral cone, which he named ''
Peytoia nathorsti ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...
'', as a jellyfish, and a poorly-preserved but relatively complete specimen, which he named ''Laggania cambria'', as a holothurian. In 1912 Walcott named '' Hurdia victoria'' and ''H. triangulata'' based on isolated H-elements, which he interpreted as the carapaces of crustaceans.WALCOTT, C. D. 1912
Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata
Smithsonian Miscellaneous Collections, 57: 145–228.
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. 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 The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
paleontologist Harry B. Whittington. 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. In 1979,
Derek Briggs Derek Ernest Gilmor Briggs (born 10 January 1950) is an Irish palaeontologist and taphonomist based at Yale University. Briggs is one of three palaeontologists, along with Harry Blackmore Whittington and Simon Conway Morris, who were key in ...
recognized that the fossils of ''Anomalocaris'' were appendages, not abdomens, but interpreted them as walking legs alongside "Appendage F". 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 both represent the same species and ''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''. Even after these recognitions, partial misidentifications (e.g. oral cone and frontal appendages of ''Peytoia'' had been assigned to ''Anomalocaris'' and ''Hurdia'', respectively) had been revealed by subsequent studies as well. 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 Anomalocarididae (occasionally mis-spelt Anomalocaridae) is an extinct family of Cambrian radiodonts, a group of stem-group arthropods. Around 1990s and early 2010s, Anomalocarididae included all radiodont species, hence the previous equivalen ...
, 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

{{Taxonbar, from=Q3291852 * Cambrian Series 2 first appearances Dinocaridida Early Devonian extinctions Prehistoric arthropod orders