page_title: Radiodonta
section_title: Frontal appendage
hierarchical_section_title: Radiodonta / Description / Frontal appendage
caption_attribution_description: Anomalocaris canadensis Whiteaves, 1892 grasping claw (~8.5 cm long), preserved as a carbonized film in slightly metamorphosed shale from the Middle Cambrian-aged Burgess Shale of southwestern Canada (YPM 35138, Yale University’s Peabody Museum, New Haven, Connecticut, USA). The Middle Cambrian-aged Burgess Shale is the most famous fossil deposit on Earth. It is located near the town of Field in Yoho National Park, southeastern British Columbia, western Canada. The deposit is famous for its spectacular soft-bodied preservation - the organisms have had their appendages &amp; internal organs preserved. Many tens of thousands of fossils have been collected from the Burgess Shale Formation over the last century. Including known, but unnamed species, and excluding known or demonstrable junior synonyms, the Burgess Shale biota totals at least ~280 species. Many claim that Charles Walcott discovered the Burgess Shale Lagerstätte (as soft-bodied fossil deposits are called by paleontologists) in 1909. However, it was actually discovered in 1886 or 1888 by Richard McConnell, based on anomalocarid appendage material from Mt. Stephen, in the Campsite Cliff Member of the Burgess Shale Formation. The main collecting localities have been two quarries (Walcott Quarry &amp; Raymond Quarry) on the western side of the ridge connecting Mt. Field and Wapta Mountain a little north-northeast of Field. Numerous other smaller localities have been identified in the same area &amp; for many, many kilometers to the south. Collecting at the Burgess Shale was most intense in 1910-1917 (Charles Walcott), 1925-1930 (Harvard’s Museum of Comparative Zoology), 1966-1967 (Geological Survey of Canada), and 1975-2000s (Royal Ontario Museum). Some of the most celebrated problematic fossil organisms from the Burgess Shale are the anomalocarids, the namesake of which is Anomalocaris canadensis Whiteaves, 1892. The genus name “Anomalocaris” means “strange shrimp”, which is ironically appropriate, because the name was given to fossil remains identified as shrimp bodies. But, anomalocarids are anything but shrimp. These creatures had medium-sized to large bodies (extrapolated up to ~3 meters long) with a head having a pair of grasping claws &amp; short-stalked eyes &amp; a pineapple ring-shaped mouth, plus a body with two lateral rows of swimming flaps. The first anomalocarid fossils (isolated grasping claws) were discovered in the 1880s from British Columbia’s Burgess Shale Formation. They were identified as shrimp bodies lacking heads (see this photo). In general, paleontologists didn’t recognize that these Anomalocaris fossils represented parts of a much larger organism. The true nature of the complete Anomalocaris organism wasn’t realized until very rare complete specimens were excavated from Burgess Shale quarries by the Royal Ontario Museum. The "headless shrimp" fossils turned out to be grasping claws at the head end of a large animal that could not be classified with any traditional arthropod group. New high-level taxa have been created to accommodate it (see Collins, 1996, Journal of Paleontology 70: 280-293). Classification: Arthropoda, Dinocarida, Radiodonta, Anomalocaridae Stratigraphy: Walcott Quarry Member, Burgess Shale Formation, Ptychagnostus praecurrens Interval-zone, lower Marjuman Stage, middle Middle Cambrian. Locality: Walcott Quarry, western side of ridge between Mt. Field &amp; Wapta Mountain, north-northeast of the town of Field, southeastern British Columbia, southwestern Canada.
context_page_description: Radiodonta is an order of stem-group arthropods that was successful worldwide during the Cambrian period, and included the earliest large predators known. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the latter originally refer to the family Anomalocarididae, which previously included all but recently only a few species of this order. Some of the most famous species of radiodonts are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, Peytoia nathorsti, and Amplectobelua symbrachiata, the Ordovician Aegirocassis benmoulai and the Devonian Schinderhannes bartelsi.
context_section_description: The anterior structures on the head are a pair of frontal appendages. They may referred as 'claws', 'grasping appendages', 'feeding appendages', or 'great appendages' in previous studies, but recently the latter was no longer used as the frontal appendages were considered non-homologous to megacheiran great appendages. They are scleritzed and segmented (arthropodized), bearing ventral spines (endites) on most of their segmental units (podomeres), and the endites may bore additional rows of spines (auxiliary spines) on their anterior and posterior margins. The frontal appendage could divided into two regions: the shaft ('peduncle' in previous studies) and distal articulated region. Triangular region covered by soft cuticle (arthrodial membrane), may occur between the ventral side of each podomere and provide flexibility. 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 and great appendages of other arthropods, which are deutocerebral (asore from post-ocular somite 1). Since the morphology of the frontal appendages, especially those of the endites are always differ between species, they are one of the most important characters in species identification. In fact, many radiodonts are only known from a handful of fossilized frontal appendages.
caption: Frontal appendage of Anomalocaris canadensis.
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