Anomalocaris (from Ancient Greek anomalous, meaning "unusual," and karis, meaning "shrimp," with the intended meaning "unlike other shrimp") is an extinct group of radiodonts, which are early marine arthropods. The most well-known species is A. canadensis, found in the Stephen Formation, especially the Burgess Shale, in British Columbia, Canada. Another named species, A. daleyae, was discovered in the older Emu Bay Shale in Australia. Other, unnamed species of Anomalocaris have been found in China and the United States.
Like other radiodonts, Anomalocaris had swimming flaps along its body, large compound eyes with many lenses, and a single pair of segmented, front appendages used to catch prey. It measured about 34.2 to 37.8 centimeters (13.5 to 14.9 inches) long, not including its front appendages or tail. Anomalocaris was one of the largest animals of the Cambrian period and is believed to have been one of the first apex predators. However, other large predators have been found in older Cambrian rock layers.
When Anomalocaris was first described in 1892, only its front appendages were known, which were initially thought to belong to a shrimp-like arthropod. Because early discoveries included only parts of the body, Anomalocaris had a complicated history of classification and was once mistaken for the related genus Peytoia. Complete body fossils of Anomalocaris were not fully described until the late 20th century. It is the type genus of Anomalocarididae, a family that once included all radiodonts but now includes only Anomalocaris and a few closely related species.
Discovery and identification
At first, scientists made mistakes when identifying the Anomalocaris fossil, and these errors continued for many years. Stephen Jay Gould, who wrote about the Cambrian explosion in his 1989 book Wonderful Life, described the history of Anomalocaris as a story filled with confusion, challenges, and finally a clear understanding that combined parts from three different groups into one creature—the largest and most powerful animal of the Cambrian period.
Anomalocaris fossils were first found in 1886 by Richard G. McConnell, a scientist from the Geological Survey of Canada (GSC). After learning about rich fossils in the Stephen Formation in British Columbia, McConnell climbed Mount Stephen on September 13, 1886. He discovered many trilobites and two unknown fossils. In 1891, Henri-Marc Ami, a paleontologist at GSC, collected more trilobites, brachiopods, and 48 additional unknown fossils. Joseph Frederick Whiteaves, a GSC paleontologist, studied these 50 fossils in 1892. He believed they were the abdomens of a type of crustacean and named the species Anomalocaris canadensis. He explained the features: the body segments were unusually flat, and the ventral appendages were nearly as tall as the segments themselves. The name Anomalocaris means "unusual shrimp," and the species name canadensis refers to Canada.
In 1928, Danish paleontologist Kai Henriksen suggested that Tuzoia, a Burgess Shale arthropod known only from its carapace, was the missing front part of Anomalocaris. Artists Elie Cheverlange and Charles R. Knight later used this idea in their drawings of Anomalocaris.
At the time, scientists were unaware that parts of Anomalocaris relatives had already been described but not recognized. In 1911, Charles Doolittle Walcott discovered a fossil he thought was a jellyfish and named it Peytoia. He also found a frontal appendage but did not notice its similarity to Whiteaves’ discovery, instead calling it a feeding appendage or tail of another creature, Sidneyia. In the same study, Walcott named Laggania, which he believed was a type of sea cucumber.
In 1966, the Geological Survey of Canada began a major review of the Burgess Shale fossils, led by Harry B. Whittington of Cambridge University. Whittington and his students, Simon Conway Morris and Derek Briggs, eventually uncovered the true nature of Anomalocaris and its relatives. However, they first contributed to the history of misinterpretations. In 1978, Conway Morris noticed that Laggania’s mouthparts matched Peytoia’s but thought Laggania was a mix of Peytoia and a sponge. In 1979, Briggs realized that Anomalocaris fossils were appendages, not abdomens, and suggested they were the legs of a giant arthropod. He also identified a feeding appendage from Sidneyia as belonging to a similar creature, called "appendage F." Later, Whittington discovered a frontal appendage connected to Peytoia’s mouthparts, linking the two. Researchers eventually recognized that Peytoia, Laggania, and Anomalocaris’ appendages belonged to the same group of large creatures. These species are now classified in the order Radiodonta, also called radiodonts or anomalocaridids. Since Peytoia was named first, it is the accepted name for the entire animal. However, the original frontal appendage came from a larger species, now called Anomalocaris.
In 2011, compound eyes of Anomalocaris were found in Emu Bay Shale on Kangaroo Island, Australia. This discovery confirmed that Anomalocaris was an arthropod, as scientists had suspected. The eyes also showed that advanced arthropod eyes evolved early, before jointed legs or hardened exoskeletons. This specimen was later identified as a new species, A. daleyae.
Many species previously linked to Anomalocaris have been reclassified. In 2021, A. saron and A. magnabasis were moved to the new genus Houcaris, but later studies suggested H. saron belongs to a different family. A. pennsylvanica was reassigned to Lenisicaris in 2021. In 2022, a specimen previously thought to be Anomalocaris or A. saron was given a new genus, Innovatiocaris. In 2023, A. kunmingensis was moved to Guanshancaris, and A. briggsi was reassigned to Echidnacaris.
Radiodonts like Anomalocaris are early relatives of modern arthropods. They share traits such as segmented frontal appendages and compound eyes but lack features of modern arthropods, like two-branched trunk limbs. A diagram from Liu et al. (2026) shows their evolutionary relationships:
† "Lobopodia" (an ancestor to tardigrades, onychophorans, and arthropods)
† Radiodonta (e.g., Anomalocaris)
† Isoxyidae + "Great appendage bivalved forms"
† Artiopoda (including Trilobita)
Description
Anomalocaris was much larger than many animals living at the same time. A complete specimen of A. canadensis, ROMIP 51211, measured up to 20.5 cm (8.1 in) long. Without the frontal appendages and tail fan, it measured about 17.4 cm (6.9 in). The largest frontal appendage reached 18 cm (7.1 in) when fully extended. This specimen would have been up to 34.2–37.8 cm (1.12–1.24 ft) long without the frontal appendages and tail fan. Earlier estimates of up to 1 m (3.3 ft) are unlikely because the body length of A. canadensis is about twice the length of its frontal appendages. A. daleyae, found in Australia, is larger than A. canadensis. Its largest appendage measured up to 18.3 cm (7.2 in), suggesting the animal was between 34.8–51.2 cm (1.14–1.68 ft) long.
Anomalocaris moved through water by wiggling the flexible flaps on its body. These flaps overlapped, working together like a single fin to help it swim efficiently. A remote-controlled model showed this method of swimming was stable, meaning Anomalocaris likely did not need a complex brain to stay balanced. The body was widest between the third and fifth flaps and narrowed toward the tail. Three small flaps were present on the neck. Lobes near the tail are hard to count accurately. The type species, A. canadensis, had 13 pairs of main body flaps.
Anomalocaris had a unique, disk-shaped mouth called an oral cone. This mouth was made of several plates arranged in three parts. Three large plates were present, with three to four medium plates between them and smaller plates in between. Most plates had wrinkles and tiny bumps near the mouth. This structure is different from the smooth, four-part mouths of other radiodonts like Peytoia and Hurdia.
Like other radiodonts, Anomalocaris had three hard plates on its head. The top plate, called a head shield, was oval-shaped with a rim. Two side plates, called P-elements, were also oval but connected by a bar-like growth. These were once mistaken for large compound eyes.
Fossils from the Emu Bay Shale show that A. daleyae had compound eyes 30 times more powerful than those of trilobites, which were once thought to have the best eyes of their time. One eye had over 24,000 lenses (ommatidia), giving it vision as sharp as a modern dragonfly, which has 28,000 lenses per eye. Studies of proteins suggest Anomalocaris may have had the ability to see two colors.
The tail was a large fan made of three pairs of wide, fin-like lobes and one final lobe-like piece. Earlier studies thought the tail helped swim, but newer research suggests it was used more for steering. Gills, in the form of thin, hair-like structures called lanceolate blades, were arranged in rows on the top of the body. Two gills were attached to each body segment, separated by a central line.
The most notable feature of Anomalocaris was its two frontal appendages, large limbs in front of the mouth. Each appendage had 14 segments, with the shaft and distal part made of 13 segments. The appendages were flat and taller than wide. Most segments had pairs of spines called endites. These spines had smaller spines branching from their front and back edges.
Paleobiology
Scientists have long believed that Anomalocaris was an active predator, as its specialized frontal appendages and glands in the middle part of its body strongly suggest it hunted other animals. Among Burgess Shale fossils, A. canadensis was one of the largest creatures, making it one of the first known top predators in its environment.
For many years, it was thought that Anomalocaris fed on animals with hard shells, such as trilobites, because it could break through their tough exoskeletons. However, recent studies suggest it likely hunted softer prey instead. Some Cambrian trilobites have marks on their shells that match the shape of Peytoia’s mouthparts, not Anomalocaris’s. Fossilized feces, or coprolites, containing trilobite remains are large enough to suggest radiodonts, like Anomalocaris, could have produced them. However, since Anomalocaris had no hard tissues, it may not have been able to break trilobite shells. These coprolites might instead have come from other trilobites, like those in the genus Redlichia. Another idea is that Anomalocaris grabbed one end of its prey with its mouth while using its frontal appendages to rock the other end, causing the prey’s shell to break. This behavior may have led trilobites to evolve ways to protect themselves, like curling into a ball.
The lack of wear on Anomalocaris’s mouthparts suggests they rarely touched hard trilobite shells, which may have been better suited for softer prey. Some scientists think A. canadensis could have eaten hard-shelled animals because of its short, strong spines on its frontal appendages. However, this idea is based only on comparisons with other radiodonts that had fragile appendages. Most A. canadensis specimens show no damage to their appendages, suggesting they did not hunt hard-shelled prey. Unlike Peytoia, which has a rectangular mouth with short spines, A. canadensis has a smaller, irregular mouth opening, indicating it used suction to eat soft prey. Three-dimensional models of radiodont appendages also suggest A. canadensis likely hunted small, soft-bodied animals like vetulicolians, free-swimming arthropods, and Nectocaris.
In 2023, Bicknell and colleagues studied Anomalocaris’s appendages, concluding it was a fast-swimming top predator. With its appendages stretched out, it could swim quickly, like modern predatory water bugs. Its large eyes, similar to those of dragonflies, mantis shrimps, and robberflies, suggest it used a strategy called "predictive target interception," calculating prey movement to intercept them. Anomalocaris likely hunted free-swimming animals, not benthic ones like trilobites, to avoid damaging its appendages on the seafloor. Other radiodonts and artiopods, such as Hurdia and Sidneyia, were more likely to hunt on the seafloor.
Paleoecology
Fossils of Anomalocaris have been discovered in many places around the world, dating back to the early Cambrian period, from Cambrian Stage 3 to the Guzhangian. In addition to the Burgess Shale and Emu Bay Shale, fossils have been found in the Chengjiang Biota, Hongjingshao Formation, Balang Formation, and Kaili Formation in China, as well as the Eagar Formation and Weeks Formation in the United States.
Anomalocaris canadensis was common in the Burgess Shale. In the Burgess Shale, this species was more frequently found in the older layers, such as the Mount Stephen trilobite beds. In the younger layers, like the Phyllopod bed, Anomalocaris could grow much larger—about twice the size of those found in the older trilobite beds. Some scientists once thought these large specimens were a different species, Anomalocaris gigantea, but recent studies suggest they are the same species as A. canadensis.
Other Anomalocaris species lived in different environments. For example, Anomalocaris cf. canadensis (JS-1880) lived in the Maotianshan Shales, which were part of a shallow tropical sea or river delta in what is now China. Anomalocaris daleyae, found in the Emu Bay Shale, lived in a similar environment in early Cambrian Australia. The Maotianshan Shale and Emu Bay Shale are near each other, separated by a small landmass, and are far from the Burgess Shale. These two locations also contained "Anomalocaris" kunmingensis and "Anomalocaris" briggsi, but these are no longer classified as Anomalocaris species.