# Anomalocaris

By [QuantumBladeck](https://paragraph.com/@oncrypted) · 2023-08-31

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**_Anomalocaris_** ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of [radiodont](https://www.wikiwand.com/en/Radiodonta), an [order](https://www.wikiwand.com/en/Order_\(biology\)) of early-diverging [stem-group](https://www.wikiwand.com/en/Stem-group) [arthropods](https://www.wikiwand.com/en/Arthropod).

It is best known from the type species _A. canadensis,_ found in the [Stephen Formation](https://www.wikiwand.com/en/Stephen_Formation) (particularly the [Burgess Shale](https://www.wikiwand.com/en/Burgess_Shale)) of British Columbia, Canada. The species _A. daleyae_ is known from the somewhat older [Emu Bay Shale](https://www.wikiwand.com/en/Emu_Bay_Shale) of Australia. Other remains are known from China and the United States. Originally, several [fossilized](https://www.wikiwand.com/en/Fossilized) parts discovered separately (the mouth, frontal appendages and trunk) were thought to be three separate creatures, a misapprehension corrected by [Harry B. Whittington](https://www.wikiwand.com/en/Harry_B._Whittington) and [Derek Briggs](https://www.wikiwand.com/en/Derek_Briggs) in a 1985 journal article.

Like other radiodonts, _Anomalocaris_ had swimming flaps running along its body, large [compound eyes](https://www.wikiwand.com/en/Compound_eye), and a single pair of segmented, "[frontal appendages](https://www.wikiwand.com/en/Radiodonta#Frontal_appendage)", which in _Anomalocaris_ were used to grasp prey. Measuring up to 38 cm (1.25 ft) long excluding frontal appendages and tail fan, _A. canadensis_ is one of the largest animals of the Cambrian, and thought to be one of the earliest examples of an [apex predator](https://www.wikiwand.com/en/Apex_predator), though others have been found in older Cambrian [lagerstätten](https://www.wikiwand.com/en/Lagerst%C3%A4tte) deposits.

**Discovery and identification**
--------------------------------

From the start, _Anomalocaris_ fossil was misidentified, followed by a series of misidentifications and taxonomic revisions. As [Stephen Jay Gould](https://www.wikiwand.com/en/Stephen_Jay_Gould), who popularised the [Cambrian explosion](https://www.wikiwand.com/en/Cambrian_explosion) in his 1989 book [_Wonderful Life_](https://www.wikiwand.com/en/Wonderful_Life_\(book\)), appropriately described:

> _\[The story of Anomalocaris is\] a tale of humor, error, struggle, frustration, and more error, culminating in an extraordinary resolution that brought together bits and pieces of three "phyla" in a singe reconstructed creature, the largest and fiercest of Cambrian organisms._

_Anomalocaris_ fossils were first collected in 1886 by Richard G. McConnell of the [Geological Survey of Canada](https://www.wikiwand.com/en/Geological_Survey_of_Canada) (GSC). Having been informed of rich fossils at the [Stephen Formation](https://www.wikiwand.com/en/Stephen_Formation) in British Columbia, McConnell climbed [Mount Stephen](https://www.wikiwand.com/en/Mount_Stephen) on 13 September 1886. He found abundant trilobites, along with two unknown specimens. In August 1891, [Henri-Marc Ami](https://www.wikiwand.com/en/Henri-Marc_Ami), Assistant Palaeontologist at GSC, collected many trilobites and [brachiopod](https://www.wikiwand.com/en/Brachiopod) fossils, along with 48 more of the unknown specimens. The fifty specimens were examined and described in 1892 by GSC paleontologist [Joseph Frederick Whiteaves](https://www.wikiwand.com/en/Joseph_Frederick_Whiteaves). Whiteaves interpreted them as the abdomens of [phyllocarid](https://www.wikiwand.com/en/Phyllocarid) [crustaceans](https://www.wikiwand.com/en/Crustacean), and gave the full scientific name _Anomalocaris canadensis_. He describe the crustacean characters:

> _Body or abdominal segments, which, in all the specimens collected, are abnormally flattened laterally, a little higher or deeper than long, broader above than below, the pair of ventral appendages proceeding from each, nearly equal in height or depth to the segment itself... The generic name Anomalocaris (from ανώμαλος, unlike,—καρίς, a shrimp, i.e., unlike other other shrimps) \[the species name referring to Canada\] is suggested by the unusual shape of the_ [_uropods_](https://www.wikiwand.com/en/Uropod) _or ventral appendages of the body segments and the relative position of the caudal spine._

In 1928, Danish paleontologist Kai Henriksen proposed that [_Tuzoia_](https://www.wikiwand.com/en/Tuzoia), a Burgess Shale arthropod which was known only from the carapace, represented the missing front half of _Anomalocaris_. The artists Elie Cheverlange and [Charles R. Knight](https://www.wikiwand.com/en/Charles_R._Knight) followed this interpretation in their depictions of _Anomalocaris_.

Unbeknownst to scientists at the time, the body parts of relatives of _Anomalocaris_ had already been described but not recognized as such. The first fossilized mouth of such a kind of animal was discovered by [Charles Doolittle Walcott](https://www.wikiwand.com/en/Charles_Doolittle_Walcott), who mistook it for a [jellyfish](https://www.wikiwand.com/en/Jellyfish) and placed it in the genus [_Peytoia_](https://www.wikiwand.com/en/Peytoia). Walcott also discovered a frontal appendage but failed to realize the similarities to Whiteaves' discovery and instead identified it as feeding appendage or tail of the coexisted [_Sidneyia_](https://www.wikiwand.com/en/Sidneyia). In the same publication in which he named _Peytoia_, Walcott named _Laggania_, a taxon that he interpreted as a [holothurian](https://www.wikiwand.com/en/Sea_cucumber).

In 1966, the Geological Survey of Canada began a comprehensive revision of the Burgess Shale fossil record, led by [Cambridge University](https://www.wikiwand.com/en/Cambridge_University) paleontologist [Harry B. Whittington](https://www.wikiwand.com/en/Harry_B._Whittington). In the process of this revision, Whittington and his students [Simon Conway Morris](https://www.wikiwand.com/en/Simon_Conway_Morris) and [Derek Briggs](https://www.wikiwand.com/en/Derek_Briggs) would discover the true nature of _Anomalocaris_ and its relatives, but not without contributing to the history of misinterpretations first. In 1978, Conway Morris recognized that the mouthparts of _Laggania_ were identical to _Peytoia_, but concluded that _Laggania_ was a composite fossil made up of _Peytoia_ and the sponge [_Corralio undulata_](https://www.wikiwand.com/en/Capsospongia). In 1979, Briggs recognized that the fossils of _Anomalocaris_ were appendages, not abdomens, and proposed that they were the walking legs of a giant arthropod, and that the feeding appendage Walcott had assigned to _Sidneyia_ was the feeding appendage of similar animal, referred to as "appendage F". Later, while clearing what he thought was an unrelated specimen, Harry B. Whittington removed a layer of covering stone to discover the unequivocally connected frontal appendage identical to _Anomalocaris_ and mouthpart similar to _Peytoia_. Whittington linked the two species, but it took several more years for researchers to realize that the continuously juxtaposed _Peytoia_, _Laggania_ and frontal appendages (_Anomalocaris_ and "appendage F") actually represented a single group of enormous creatures. The two genera have now been placed into the order Radiodonta and are commonly known as radiodonts or anomalocaridids. Since _Peytoia_ was named first, it is the accepted correct name for the entire animal. However, the original frontal appendage was from a larger species distinct from _Peytoia_ and "_Laggania_" and therefore retains the name _Anomalocaris_.

In 2011 and 2020, [compound eyes](https://www.wikiwand.com/en/Compound_eye) of _Anomalocaris_ were recovered from a paleontological dig at [Emu Bay](https://www.wikiwand.com/en/Emu_Bay,_South_Australia) on [Kangaroo Island](https://www.wikiwand.com/en/Kangaroo_Island), Australia, proving that _Anomalocaris_ was indeed an [arthropod](https://www.wikiwand.com/en/Arthropod) as had been suspected. The find also indicated that advanced arthropod eyes had evolved very early, before the evolution of jointed legs or hardened exoskeletons.

In 2021, "_A._" _saron_ and "_A._" _magnabasis_ were reassigned to the new genus [_Houcaris_](https://www.wikiwand.com/en/Houcaris), in the family [Tamisiocarididae](https://www.wikiwand.com/en/Tamisiocarididae). In the same year, "_A._" _pennsylvanica_ was reassigned to the genus _Lenisicaris_. In 2022, specimen ELRC 20001 that was treated as an unnamed species of _Anomalocaris_ or whole-body specimen of _A. saron_ got a new genus, [_Innovatiocaris_](https://www.wikiwand.com/en/Innovatiocaris). In 2023, _"A". kunmingensis_ was reassigned to the new genus [_Guanshancaris_](https://www.wikiwand.com/en/Guanshancaris) in the family [Amplectobeluidae](https://www.wikiwand.com/en/Amplectobeluidae). Multiple phylogenetic analyses also suggested that _"A". briggsi_ (tamisiocaridid) was not a species of _Anomalocaris_ either, and it was reassigned to the genus [_Echidnacaris_](https://www.wikiwand.com/en/Echidnacaris) in the family [Tamisiocarididae](https://www.wikiwand.com/en/Tamisiocarididae) in 2023. In the same 2023 study, a new species of _Anomalocaris, A. daleyae,_ was described based on remains found in the [Emu Bay Shale](https://www.wikiwand.com/en/Emu_Bay_Shale) in Australia.

**Description**
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Size estimation of _Anomalocaris_.

For the time in which it lived, _Anomalocaris_ was gigantic, _A. canadensis_ is estimated to be up to 37.8 cm (1.24 ft) long excluding frontal appendages and tail fans. Previous estimation up to 1 m (3.3 ft) is unlikely based on the ratio of body parts (body length measured only about 2 times the length of frontal appendage in _A. canadensis_, respectively) and the size of largest frontal appendage (up to 18 centimetres (7.1 inches) in length when extended).

_Anomalocaris_ propelled itself through the water by [undulating](https://www.wikiwand.com/en/Lateral_undulation) the flexible flaps on the sides of its body. Each flap sloped below the one more posterior to it, and this overlapping allowed the lobes on each side of the body to act as a single "fin", maximizing the swimming efficiency. The construction of a remote-controlled model showed this mode of swimming to be intrinsically stable, implying that _Anomalocaris_ would not have needed a complex brain to manage balance while swimming. The body was widest between the third and fifth lobe and narrowed towards the tail, with additional 3 pairs of small flaps on the constricted neck region. It is difficult to distinguish lobes near the tail, making an accurate count difficult. For the main trunk flaps, the type species _A. canadensis_ had 13 pairs.

![](https://storage.googleapis.com/papyrus_images/0ca01b0912753fd579cb99c3a71336f6e7a2a45790c390acc0066c86bb68edac.png)

_Anomalocaris_ had an unusual disk-like mouth known as oral cone. The oral cone was composed of several plates organized triradially. Three of the plates were quite large. Three to four medium sized plates could be found between each of the large plates, and several small plates between them. Most of the plates wrinkled and possess scale-like tubercles near the mouth opening. Such an oral cone is very different from those of a typical [hurdiid](https://www.wikiwand.com/en/Hurdiidae) radiodont like _Peytoia_ and [_Hurdia_](https://www.wikiwand.com/en/Hurdia), which is smooth and tetraradial. As a shared character across radiodonts, _Anomalocaris_ also possessed three sclerites on the top and side of its head. The top one, known as a head shield, dorsal carapace or H-element, was shaped like an laterally-elongated oval, with a distinct rim on the outer edge. The remaining two lateral sclerites, known as P-elements, were also ovoid, but connected by a bar-like outgrowth. The P-elements were previously misinterpreted as two huge compound eyes.

![](https://storage.googleapis.com/papyrus_images/f306c10a097cd99d8d1a87ae9cd1ba2d0da750fb6c25dc627c1b95a9747273eb.png)

Frontal appendages of _Anomalocaris_, with examples from multiple species.

Two large frontal appendage were positioned in front of the mouth, at the front of the head. Each frontal appendage of _Anomalocaris_ usually possessed 14 podomeres (segmental units, at least 1 for shaft and 13 for distal articulated region), with each appendage being laterally-flattened (taller than wide). Most podomeres were tipped with a pair of endites (ventral spines). The endites themselves were both equipped with multiple auxiliary spines, which branches off from the anterior and posterior margin of the endites.

The tail was a large tail fan, composed of three pairs of large, lateral fin-shaped lobes and one terminal lobe-like tailpiece. Previous studies suggest the tail fan was used to propel it through Cambrian waters, while further [hydrodynamic](https://www.wikiwand.com/en/Hydrodynamic) study rather suggest it was more adapted to provide [steering](https://www.wikiwand.com/en/Steering) function. The gills of the animal, in the form of long, thin, hair-like structures known as [lanceolate](https://www.wikiwand.com/en/Lanceolate) blades, were arranged in rows forming setal blades. The setal blades were attached by their margin to the top side of the animal, two setal blades per body segment. A divide ran down the middle, separating the gills.

Based on fossilized eyes from the [Emu Bay Shale](https://www.wikiwand.com/en/Emu_Bay_Shale), which belong to the species _Anomalocaris_ _daleyae,_ the stalked eyes of _Anomalocaris_ were 30 times more powerful than those of trilobites, long thought to have had the most advanced eyes of any contemporary species. With one specimen having over 24,000 lenses in one eye, the resolution of the 3-centimetre-wide (1.2 in) eyes would have been rivalled only by that of the modern [dragonfly](https://www.wikiwand.com/en/Dragonfly), which has 28,000 lenses in each eye. Additionally, estimation of [ecdysozoan](https://www.wikiwand.com/en/Ecdysozoan) [opsins](https://www.wikiwand.com/en/Opsins) suggest that _Anomalocaris_ may have had [dichromatic](https://www.wikiwand.com/en/Dichromacy) [color vision](https://www.wikiwand.com/en/Color_vision).

**Paleobiology**
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### **Diet**

Grasping movement of the frontal appendage of _A. canadensis_.

The interpretation of _Anomalocaris_ as an active [predator](https://www.wikiwand.com/en/Predation) is widely accepted throughout the history of research, as its raptorial frontal appendages and mid-gut glands strongly suggest a predatory lifestyle. In the case of _A. canadensis_, its outstanding size amongst Burgess Shale fauna also make it one of the first [apex predators](https://www.wikiwand.com/en/Apex_predators) known to exist.

However, the long-standing idea that _Anomalocaris_ fed on hard-bodied animals, especially its ability to penetrate mineralized [exoskeleton](https://www.wikiwand.com/en/Exoskeleton) of trilobites, has been questioned, with many recent studies considering it more likely that _Anomalocaris_ exclusively hunted soft-bodied prey. Some Cambrian trilobites have been found with round or W-shaped "bite" marks, which were identified as being the same shape as the mouthparts of _Peytoia_ (previously misidentified as those of _Anomalocaris_). Stronger evidence that _Anomalocaris_ ate trilobites comes from [coprolite](https://www.wikiwand.com/en/Coprolite), which contain trilobite parts and are so large that the anomalocarids are the only known organism from that period large enough to have produced them. However, since _Anomalocaris_ lacks any mineralized tissue, it seemed unlikely that it would be able to penetrate the hard, calcified shell of trilobites. Rather, the coprolites may have been produced by a different organisms, such as the trilobites of the genus [_Redlichia_](https://www.wikiwand.com/en/Redlichia). Another suggested possibility was that _Anomalocaris_ fed by grabbing one end of their prey in its oral cone while using its frontal appendages to quickly rock the other end of the animal back and forth. This produced stresses that exploited the weaknesses of [arthropod cuticles](https://www.wikiwand.com/en/Arthropod_cuticle#Mechanical_properties), causing the prey's exoskeleton to rupture and allowing the predator to access its innards. This behaviour was originally thought to have provided an evolutionary pressure for trilobites to roll up, to avoid being flexed until they snapped.

![](https://storage.googleapis.com/papyrus_images/781ea14ab325aebfd9e83b1d4102e844b451a85d173fc039b08b720295b61f56.png)

Ecological reconstruction of _Anomalocaris_ hunting [_Isoxys_](https://www.wikiwand.com/en/Isoxys), after posture estimated in Bicknell _et al._ (2023)

The lack of wear on radiodont mouthparts suggests they did not come into regular contact with mineralized trilobite shells, and were possibly better suited to feeding on smaller, soft-bodied organisms by suction, since they would have experienced structural failure if they were used against the armour of trilobites. _A. canadensis_ was suggested to have been capable of feeding on organisms with hard exoskeletons due to the short, robust spines on its frontal appendages. However, this conclusion is solely based on the comparison with the fragile frontal appendages of [suspension feeding](https://www.wikiwand.com/en/Suspension_feeding) radiodonts (e.g. _"A". briggsi_ and _Houcaris_ spp.). The typical lack of damage to the endites on the frontal appendages of _A. canadensis_ (with damage only present on a single specimen) suggests that they were not used to grasp hard-shelled prey. As opposed to _Peytoia_ whose oral cone is more rectangular with short protruding spines, the oral cone of _A. canadensis_ has a smaller and more irregular opening, not permitting strong biting motions, and indicating a suction-feeding behavior to suck in softer organisms. Three-dimensional modelling of various radiodont frontal appendages also suggest that _A. canadensis_ is more capable to prey on smaller (2–5 cm in diameter), active, soft-bodied animals (e.g. [vetulicolian](https://www.wikiwand.com/en/Vetulicolian); free-swimming arthropods like [isoxyids](https://www.wikiwand.com/en/Isoxyida) and [hymenocarines](https://www.wikiwand.com/en/Hymenocarina); [_Nectocaris_](https://www.wikiwand.com/en/Nectocaris)).

Bicknell _et al._ (2023) examined the frontal appendages of _Anomalocaris_, suggesting it was an active [nektonic](https://www.wikiwand.com/en/Nekton) apex predator. Postured with the frontal appendages outstretched, _Anomalocaris_ would have been able to swim with maximized speed, similar to modern predatory [water bugs](https://www.wikiwand.com/en/Nepomorpha). Its eyes would be suitable to hunt prey in well-lit waters. _Anomalocaris_ would have hunted various free-swimming animals since there are a large diversity of nektonic and pelagic soft-bodied animals. It probably would have not hunted [benthic](https://www.wikiwand.com/en/Benthos) animals like trilobites, considering the possibility of damaging the frontal appendages on the substrate while trying to grab prey from seafloor at speed. Instead, other animals such as other radiodonts (e.g. [_Hurdia_](https://www.wikiwand.com/en/Hurdia), [_Cambroraster_](https://www.wikiwand.com/en/Cambroraster), [_Titanokorys_](https://www.wikiwand.com/en/Titanokorys), [_Stanleycaris_](https://www.wikiwand.com/en/Stanleycaris)) and [artiopods](https://www.wikiwand.com/en/Artiopoda) (e.g. [_Sidneyia_](https://www.wikiwand.com/en/Sidneyia)) would have been benthic predators in the Burgess Shale.

**Paleoecology**
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Specimens of _Anomalocaris_ have been found worldwide spanning from [Cambrian Stage 3](https://www.wikiwand.com/en/Cambrian_Stage_3) to the [Guzhangian](https://www.wikiwand.com/en/Guzhangian). Aside from the Burgess Shale and Emu Bay Shale, fossils have been found in the [Chengjiang Biota](https://www.wikiwand.com/en/Chengjiang_Biota), Hongjingshao Formation, [Balang Formation](https://www.wikiwand.com/en/Balang_Formation) and the [Kaili Formation](https://www.wikiwand.com/en/Kaili_Formation) of China, as well as the Eagar Formation and [Weeks Formation](https://www.wikiwand.com/en/Weeks_Formation) in the United States.

_Anomalocaris canadensis_ lived in the Burgess Shale in relatively great numbers. In the Burgess Shale, _Anomalocaris_ is more common in the older sections, notably the [Mount Stephen trilobite beds](https://www.wikiwand.com/en/Mount_Stephen_trilobite_beds). However, in the younger sections, such as the [Phyllopod bed](https://www.wikiwand.com/en/Phyllopod_bed), _Anomalocaris_ could reach much greater sizes; roughly twice the size of its older, trilobite bed relatives. These rare giant specimens have previously been referred to a separate species, _Anomalocaris gigantea_; however, the validity of this species has been called into question, and is currently [synonymized](https://www.wikiwand.com/en/Synonym_\(taxonomy\)) to _A. canadensis_.

Other unnamed species of _Anomalocaris_ live in vastly different environments. For example, _Anomalocaris_ cf. _canadensis_ (JS-1880) lived in the [Maotianshan Shales](https://www.wikiwand.com/en/Maotianshan_Shales), a shallow tropical sea or even being [delta](https://www.wikiwand.com/en/River_delta) in what is now modern China. _Anomalocaris daleyae_ (Emu Bay Shale) lived in a comparable environment; the shallow, tropical waters of Cambrian Australia. The Maotianshan Shale and the Emu Bay Shale are very close in proximity, being separated by a small landmass, far from the Burgess Shale. These two locations also included _"Anomalocaris" kunmingensis_ and _"Anomalocaris" briggsi_ respectively, species that previously attributed but taxonomically unlikely to be a member of _Anomalocaris_ nor even Anomalocarididae.

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*Originally published on [QuantumBladeck](https://paragraph.com/@oncrypted/anomalocaris)*