Radiodonta is a clade of stem-group arthropods that was successful worldwide during the Cambrian period, and included the earliest large predators known. Some of the most famous species included in Radiodonta are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, and Peytoia nathorsti, the Ordovician Aegirocassis benmoulai and the Devonian Schinderhannes bartelsi.
The name Radiodonta (Latin] for radius "spoke of a wheel" and Greek for odoús "tooth") refers to the radial arrangement of tooth plates surrounding the mouth.
Traditionally, all taxa currently included within Radiodonta have been placed within one family, Anomalocarididae. The original description of the order Radiodonta included Anomalocaris, Laggania, Hurdia, Proboscicaris, Amplectobelua, Cucumericrus, and Parapeytoia. Proboscicaris is now regarded as a junior synonym of Hurdia, and Parapeytoia is considered to be a megacheiran. The phylogenetic position of Cucumericrus is unclear, as it is in a polytomy with Radiodonta and Euarthropoda and thus may belong to either or neither.
- ?Caryosyntrips serratus
- ?Cucumericrus decoratus
- Paranomalocaris multisegmentalis
- Anomalocaris pennsylvanica
The first in-depth phylogenetic analysis of Radiodonta was conducted by Vinther et al. in 2014, and it was expanded by Cong et al. later that year by the addition of Lyrarapax unguispinus. The analysis was further modified in 2015 by Van Roy et al. with modified characters and the inclusion of Cucumericrus decoratus and Aegirocassis benmoulae.
The original diagnosis of order Radiodonta is as follows:
Radiodontids are bilaterally symmetrical, elongate arthropods with a nonmineralized cuticle typically most robust in the jaws and claws. The body is subdivided into two tagmata, much like the prosoma and opisthosoma of chelicerate arthropods.
Typically, the front part shows no external segmentation, bears one pair of preoral claws, a pair of prominent eyes, and ventral jaws with radiating teeth. Some forms have additional rows of teeth and three or four postoral gnathobasic limb pairs. The trunk is metameric, typically with about 13 segments laterally developing imbricating lobes for swimming and gills for respiration, and may end in a prominent three-part tail. Some forms have gnathobasic trunk limbs.
The most anterior structures on the head are the frontal appendages. They are homologous with the antennae of Onychophora and the labrum of Arthropoda, and not homologous with the chelicerae of Chelicerata or the antennae of other arthropods. The mouth is on the ventral side of the head and is surrounded by a ring of tooth plates known as the oral cone.
Contrary to the original diagnosis, no known member of Radiodonta is known to have legs. Rather, the appendages were modified into fin-like lateral flaps. These flaps are homologous to the endopod of the biramous limbs of many arthropods, and dorsal to them were a second set of flaps which bore gills and are homologous to the exopod.
- Collins, Desmond (1996). "The "evolution" of Anomalocaris and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.)". Journal of Paleontology. 70 (2): 280–293.
- Vinther, Jakob; Stein, Martin; Longrich, Nicholas R.; Harper, David A. T. (2014). "A suspension-feeding anomalocarid from the Early Cambrian". Nature. 507: 496–499. doi:10.1038/nature13010. PMID 24670770.
- Daley, Allison C.; Budd, Graham E.; Caron, Jean-Bernard; Edgecombe, Gregory D.; Collins, Desmond (2009). "The Burgess Shale anomalocaridid Hurdia and its significance for early euarthropod evolution". Science. 323 (5921): 1597–1600. doi:10.1126/science.1169514. PMID 19299617.
- Van Roy, Peter; Daley, Allison C.; Briggs, Derek E. G. (2015). "Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps". Nature. 522: 77–80. doi:10.1038/nature14256. PMID 25762145.
- Cong, Peiyun; Ma, Xiaoya; Hou, Xianguang; Edgecombe, Gregory D.; Strausfeld, Nicholas J. (2014). "Brain structure resolves the segmental affinity of anomalocaridid appendages". Nature. 513: 538–42. doi:10.1038/nature13486. PMID 25043032.
- Daley, Allison C.; Edgecombe, Gregory D. (2014). "Morphology of Anomalocaris canadensis from the Burgess Shale". Journal of Paleontology. 88 (1): 68–91. doi:10.1666/13-067.
- Van Roy, Peter; Daley, Allison C.; Briggs, Derek E. G. (2013). Anomalocaridids had two sets of lateral flaps. 57th Annual Meeting of The Paleontological Association. Zurich, Switzerland.