The Burgess Shale

Anomalocaris canadensis

The largest Burgess Shale predator, first described in 1892

3D animation of Anomalocaris canadensis.

Animation by Phlesch Bubble © Royal Ontario Museum

Taxonomy:

Kingdom: Animalia
Phylum: Arthropoda
Higher Taxonomic assignment: Order Radiodonta, Family Anomalocarididae
Species name: Anomalocaris canadensis
Remarks:

Anomalocaris is the most iconic member of Radiodonta, the extinct group of arthropods characterized by a circular tooth-lined mouth and a single pair of jointed frontal appendages (Collins 1996). Within this group, Anomalocaris belongs to the eponymous family Anomalocarididae, which possess long, multisegmented grasping appendages with numerous trident-like spines (Vinther et al. 2014).

Described by: Whiteaves
Description date: 1892
Etymology:

Anomalocaris – from the Greek anomoios, “unlike,” and the Latin caris, “crab” or “shrimp,” thus, “unlike other shrimp.”

canadensis – from Canada, the country where the Burgess Shale is located.

Type Specimens: Lectotype – GSC3418 in the Geological Survey of Canada, Ottawa, Canada.
Other species:

Burgess Shale and vicinity: none.

Other deposits:

Other deposits: Several species have been described from widely distributed deposits, including Anomalocaris pennsylvanica from the Kinzers Formation, USA (Pates and Daley 2018), Anomalocaris magnabasis from the Pioche and Carrara Formations (Pates et al. 2019), A. cf. canadensis from the Emu Bay Shale, Australia (Daley et al. 2013), A. cf. canadensis. from the Eager Formation (Briggs 1979), British Columbia, Canada, and possibly A. sp. from the Balang Formation of China (Liu 2013). As a consequence of the complex history of research on Anomalocaris (see below), several species previously assigned to this genus likely belong to other genera (e.g., Daley et al., 2013; Wang et al., 2013; Wu et al., 2021), but not all have yet been reassigned.

Age & Localities:

Age:
Middle Cambrian, Wuliuan stage, Burgess Shale Formation (approximately 505 million years ago).
Principal localities:

The Collins, Raymond and Walcott Quarries on Fossil Ridge. The Trilobite Beds, Tulip Beds (S7) and the Collins Quarry on Mount Stephen. Additional localities on Mount Field, Mount Stephen, and near Stanley Glacier.

History of Research:

Brief history of research:

Anomalocaris famously has a complex history of description because parts of its body were described in isolation before it was realized they all belonged to the same animal. The frontal appendage of Anomalocaris was described by Whiteaves (1892) as the body of a shrimp. The mouth parts were described by Walcott (1911) as a jellyfish called Peytoia nathorsti. A full body radiodontan specimen was originally described as the sea cucumber Laggania cambria (Walcott 1911), and re-examined by Conway Morris (1978) who concluded it was a superimposition of the “jellyfish” Peytoia nathorsti on top of a sponge. Henriksen (1928) attached Anomalocaris to the carapace of Tuzoia, but Briggs (1979) suggested instead that it was the appendage of an unknown arthropod, an idea that turned out to be correct. In the early 1980s, Harry Whittington was preparing an unidentified Burgess Shale fossil from the Geological Survey of Canada by chipping away layers of rock to reveal underlying structures, when he solved the mystery of Anomalocaris‘s identity. Much to his surprise, Whittington uncovered two Anomalocaris “shrimp” attached to the head region of a large body, which also had the “jellyfish” Peytoia as the mouth apparatus. Similar preparations of other fossils from the Smithsonian Institution in Washington DC revealed the same general morphology, including the Laggania cambria specimen studied by Conway Morris (1978), which was reinterpreted as a second species of Anomalocaris. Thus, Whittington and Briggs (1985) were able to describe two species: Anomalocaris canadensis, which had a pair of Anomalocaris appendages, and Anomalocaris nathorsti, which had a different type of frontal appendage and includes the original specimen of Laggania cambria and Peytoia nathorsti. Bergström (1986) re-examined the morphology and affinity of Anomalocaris and suggested it had similarities to the arthropods. Collecting at the Burgess Shale by the Royal Ontario Museum in the early 1990s led to the discovery of several complete specimens, which Collins (1996) used to reconstruct Anomalocaris canadensis with greater accuracy. This also led to a name change of Anomalocaris nathorsti to Laggania cambria, although it was later argued that the name Peytoia nathorsti had priority (Daley and Bergström 2012). Daley and Bergström (2012) re-examined the material and were the first to recognize that the mouthpart of A. canadensis was triradially organized, distinct from prior interpretations of a tetraradial organization as in Peytoia. Daley and Edgecombe (2014) conducted the most recent comprehensive revision of A. canadensis based on all available material. Anomalocaris has been the subject of many studies discussing its morphology (e.g. Moysiuk and Caron 2019; Paterson et al. 2020; Zeng et al. 2022), affinity (e.g., Chen et al., 2004; Daley et al., 2009; Hou et al., 1995; Vinther et al., 2014), ecology (e.g., Nedin, 1999; Rudkin, 1979; Vannier et al., 2014) and functional morphology (e.g., de Vivo et al., 2021; Sheppard et al., 2018; Usami, 2006).

Description:

Morphology:

Anomalocaris is a dorsoventrally flattened animal with a relatively flexible exoskeleton. It has a segmented body, with sixteen lateral swimming flaps bearing gills, and a prominent tail fan, which consists of three pairs of prominent fins that extend upward from the body (Daley and Edgecombe 2014). Repeated paired gut glands are associated with the body segments in some specimens. The head region bears one pair of frontal jointed appendages, two large dorsal eyes on stalks, three small rounded plates, and a ventrally oriented circular mouth apparatus (Whittington and Briggs 1985; Daley and Edgecombe 2014; Moysiuk and Caron 2019). The mouthparts are composed of three large, tubercle-covered plates separated by a series of smaller plates, all with orally-directed teeth (Daley and Bergström 2012). The frontal appendages are elongated and have fourteen segments, each with a pair of trident-like spikes projecting from the ventral surface (Briggs 1979). The most complete Anomalocaris specimen is 25 cm in length, although isolated fragments suggest individuals could reach a larger size, perhaps up to 100 cm.

Abundance:

The Anomalocaris frontal appendage is extremely common at the Mount Stephen Trilobite Beds, and several hundred specimens of isolated frontal appendages and mouth parts have been collected from Mount Stephen and the Raymond Quarry on Fossil Ridge (O’Brien and Caron 2016; Nanglu et al. 2020). These parts are relatively rare at Walcott Quarry, where fewer than 50 specimens are known (Caron and Jackson 2008). About ten complete body specimens are known from the Raymond Quarry.

Maximum Size:
About 100 cm.

Ecology:

Life habits: Nektobenthic, Mobile, Nektonic
Feeding strategies: Carnivorous
Ecological Interpretations:

The streamlined body would have been ideal for swimming. Undulatory movements of the lateral flaps propelled the animal through the water column and might have also served in gill ventilation (Whittington and Briggs 1985; Usami 2006). The tail fan would have enabled rapid turning, allowing Anomalocaris to chase fast-moving prey (Sheppard et al. 2018). A predatory lifestyle is suggested by the large eyes, frontal appendages with spines, gut glands, and spiny mouth apparatus (Whittington and Briggs 1985; Vannier et al. 2014). The frontal appendages are highly flexible, suggesting an ability to precisely manipulate prey items (de Vivo et al. 2021). The circular ring of plates around the mouth, shared with other radiodontans, is unique in the animal kingdom. Although the precise functioning of these mouthparts remains unclear, it seems likely that the plates could be pivoted to bring the teeth into contact with prey or to create suction to draw prey into the mouth (Daley and Bergström 2012). It has been suggested that Anomalocaris may have preyed on trilobites because some Cambrian trilobites have round or W-shaped healed wounds, interpreted as bite marks (Rudkin 1979), and large fecal pellets composed of trilobite parts have been found in the Cambrian rock record (Nedin 1999). It was proposed that Anomalocaris could have fed by grasping one end of the trilobite in the mouth apparatus and rocking the other end back and forth with the frontal appendages until the exoskeleton cracked (Nedin 1999). However, the non-biomineralized mouth apparatus of Anomalocaris was arguably too weak to penetrate the calcified shell of trilobites and it never shows any sign of breakage or wear, rendering this hypothesis less plausible (Whittington and Briggs 1985; Daley and Bergström 2012). Other Cambrian predators, such as larger trilobites, have been proposed as alternative candidates responsible for coprolites and trilobite injuries (Bicknell et al. 2021, 2022). It remains conceivable that Anomalocaris could have fed on freshly moulted “soft-shell” trilobites as well as other soft bodied organisms (Rudkin 2009).

References:

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