Anomalocaris is an extinct genus of arthropod that lived during the Cambrian in North America, Australia and China.
History[]
Many invertebrate remains have been mistaken for Anomalocaris: frontal appendages found by Richard McConnell in 1886 or 1888 and described Joseph Whiteaves as phyllocarid crustacean abdomens, appendages from related groups and separate genera lumped into Anomalocaris, Tuzoia as the front half of the animal (now seen as false, but was adopted by Charles R. Knight and Elie Cheverlange in their artworks) and mouth parts now referred to Peytoia. Whiteaves gave it the name "unlike other shrimps" (or also sometimes translated to "abnormal" or "odd shrimp"), but it was not yet known that these parts were of what we traditionally known as Anomalocaris.
Collection wall full of Anomalocaris specimens at the Royal Ontario Museum.
Credit: Clumsystiggy on DeviantArt.
Charles Walcott discovered the first Anomalocaris and referred it to Peytoia as a jellyfish. He also found an appendage that (unbeknownst to him) was similar to Whiteaves' genus and instead thought it belonged to Sidneyia as a feeding arm or tail. The Geological Survey of Canada in 1966 began to revise the Burgess Shale fossil record, led by Harry Whittington, who found that (alongside Simon Conway Morris and Derek Briggs) Anomalocaris was different than thought before: that the mouth was of Peytoia (Conway, 1978), that the appendage of Sidneyia was of a similar taxon and that the "abdomens" were actually appendages (Briggs, 1979) and that elements spread between Anomalocaris, Peytoia and Laggania were all mixed specimens.
Collection wall with a Anomalocaris specimens and model at the Royal Ontario Museum.
Credit: Clumsystiggy on DeviantArt.
Stephan Jay Gould (1989) explains how he thought Anomalocaris is a fossil taxon he believed to be of a more diverse set of phyla in the Cambrian in his book Wonderful Life, but others have disputed this. Historically, Anomalocaris has been used to house various anomalocaridid remains that are not part of the genus, and sometimes not even anomalocaridid in nature.
A. daleyae was described by Paterson et al. (2023) based on oral cones and a pair of associated frontal appendages from the Emu Bay Shale.
Description[]
Anomalocaris was a large animal, once estimated to have been up to a meter long based on the ratio of body parts but now known to be only 28 centimeters long. By undulating the flexible flaps of armour that lined it's body and overhung at the side, creating a rythmic wave, Anomalocaris was able to propel through water by using each flap effectively like a fin. Models of Anomalocaris show that this type of swimming would be very stable, and that it would have needed a complex brain to keep balance during movement. Towards the 3rd and 5th lobes, the body was widest, and then narrowed towards the tail. 3 smaller lobe pairs were located on the neck. Lobes nearer to the tail are harder to differentiate, which makes an accurate count difficult A. canadensis has 13 pairs of main trunk flaps and ELRC 20001 has at least 11 pairs.
BPM 1034b, a new species from the Weeks Formation.
An oral cone surrounded the mouth, comprised of 3 large, 3-4 medium and several small plates between them, triradially-oriented. Scale-like tubercles near to the mouth opening are present on the plates and the tops are wrinkled. This is very different to hurdiids. Two larfe frontal appendages sat near to the mouth at the front of the head. 3 sclerites on top of the head are shared with all radiodonts. The top one is known as a head shield, dorsal carapace or H-element; it was laterally-elongate, ovular and has a distinct rim on the outer edge. The other two (the P-elements) were ovoid and connected by a bar-shaped overgrowth. These were once misinterpreted as massive compound eyes.
Credit: Junnn11 on the Wikimedia COmmons.
14 podomeres are found on the appendages, and they are laterally flattened. Each is tipped by a pair of endites that bear multiple auxiliary spines that branch off from the anterior and posterior of the endites. A tail fan with 2 or 3 large, lateral, fin-shaped lobes and a terminal lobe-like piece was attached to the tail, similarly to a telson is present in A. canadensis. Older studies claim these were more like fins but newer studies find they helpted to steer. The gills were long, thin and hair-like structures arranged in setal blades that attached by the margin to the top of the animal, with two setal blades attached per segment. Down the middle, the gills were separated in two. It may have had dichromatic colour vision, with 16,000 lenses in each eye and resolution in a 3-centimeter-wide eye only rivaled by modern dragonflies.
Credit: Junnn11 on the Wikimedia Commons.
Anomalocaris was likely an active predator based on the raptorial appendages, mid-gut glands and its great size. However, it is unsure if it could penetrate the exoskeleton of trilobites. W-shaped bites on trilobites referred to Peytoia and trilobite parts in coprolites larger than any contemporaneous fauna (so they are likely belonging to Anomalocaris) are evidence for this, but Anomalocaris did not bear mineralized parts to strike into exoskeletons and the coprolites may instead be Redlichia. It has been suggested that Anomalocaris grabbed one end of a trilobite with their mouths and used their appendages to rock the animal back and forth, rupturing their exoskeletons by the cuticle and allowing for access to their internal anatomy. This would explain why trilobites evolved to roll up in balls to avoid this weakness. Anomalocaris may have also fed on newly-molted trilobites. However, little wear on radiodont oral cones suggests they did not often contact mineralized shells, and were better feeding on smaller, soft-bodied animals by suction, since their structure would fail against exoskeletons. Short, robust spines on the appendages may have allowed feeding on animals with exoskeletons, however. Later found out, this was based on species later found to be tamisiocaridids. Still, Anomalocaris canadensis bears a suction-adapted oral cone. Of the same species, 3D modelling reveals feeding on soft prey with 2-5 centimeter diameters, such as Nectocaris.
Paleoecology[]
The Mural Formation anomalocaridid.
A. canadensis is known from the Burgess Shale and is more common in older sections, but much larger (~2x the size) in younger sections. These larger individuals were once called A. gigantea, but they have been merged with A. canadensis. A. daleyae is known from the Emu Bay Shale. These areas were separated by a small landmass but were close in the Cambrian, but far from the Burgess Shale as it was in the Cambrian. The Kaili Biota, Hongjingshao, Balang and Weeks Formations all bear an unnamed Anomalocaris species, making about 6 total unnamed species. A small, undescribed, fragmented anomalocaridid frontal appendage is from the "waterfall quarry" of the Mural Formation, which was similar to the Burgess Shale, and is similar in appearance to A. canadensis.
Species[]
A. caandensis model at the Royal Ontario Museum.
Credit: Clumsystiggy on DeviantArt.
- A. canadensis
- A. daleyae
- A. n. sp. Kaili Biota
- A. n. sp. Hongjingshao Formation
- A. n. sp. Balang Formation
- A. n. sp. Weeks Formation (BPM 1034b)
Reassigned Species[]
- "A." saron
- "A." magnabasis
- "A." pennsylvanica
- "A." briggsi
- "A." nathorsti
- "A." kunmingensis
Synonyms[]
- A. emmonsi
- A. kokomoensis
- A. lineata
- A. whiteavesi
- A. gigantea
- A. cranbrookensis
Gallery[]
Reconstructions[]
