Anomalocaris

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Anomalocaris canadensis
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Anomalocaris
Temporal range:
Ma[1]
ROMIP 51212, a largely complete specimen of Anomalocaris canadensis.
Life restoration of Anomalocaris canadensis.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: †Dinocaridida
Order: †Radiodonta
Family: †Anomalocarididae
Genus: †Anomalocaris
Whiteaves, 1892
Species
  • A. canadensis Whiteaves, 1892
    • =A. whiteavesi Walcott, 1908
    • =A. gigantea Walcott, 1912
    • =A. cranbrookensis Resser, 1929
  • A. daleyae Paterson, García-Bellidob & Edgecombe, 2023[2]

(8 more unnamed species[3])

Anomalocaris ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of

stem-group arthropods
.

It is best known from the type species A. canadensis, found in the Stephen Formation (particularly the Burgess Shale) of British Columbia, Canada. The species A. daleyae is known from the somewhat older Emu Bay Shale of Australia.[2] Other undescribed remains are known from China and the United States.[3]

Like other radiodonts, Anomalocaris had swimming flaps running along its body, large compound eyes, and a single pair of segmented, frontal appendages, which in Anomalocaris were used to grasp prey. Measuring up to 38 cm (1.25 ft) long excluding frontal appendages and tail fan,[4] A. canadensis is one of the largest animals of the Cambrian, and thought to be one of the earliest examples of an apex predator,[5][6] though others have been found in older Cambrian lagerstätten deposits.

Since the original description in late 19th century,

fossilized parts and misidentified as the body parts of other animals.[8] Its radiodont affinity was revealed in 1980s, specifically in a 1985 journal article by Harry B. Whittington and Derek Briggs.[9] The trunk and mouth were reconstructed after another radiodont genus until the corrections done in 1996[8] and 2012.[10] It is the type genus of Anomalocarididae, a family which previously included all radiodonts but recently only Anomalocaris and a few closely-related taxa.[3]

Discovery and identification

Peytoia nathorsti
(bottom) from British Columbia. The latter was originally assigned to the former species.

From the start, Anomalocaris fossil was misidentified, followed by a series of misidentifications and taxonomic revisions.

phyllocarid crustaceans, and gave the full scientific name Anomalocaris canadensis. He describes 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 or ventral appendages of the body segments and the relative position of the caudal spine.[7]

In 1928, Danish paleontologist Kai Henriksen proposed that Tuzoia, a Burgess Shale arthropod which was known only from the carapace, represented the missing front half of Anomalocaris.[8] The artists Elie Cheverlange and Charles R. Knight followed this interpretation in their depictions of Anomalocaris.[8]

Not known 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, who mistook it for a jellyfish and placed it in the genus 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.[18] In the same publication in which he named Peytoia, Walcott named Laggania, a taxon that he interpreted as a holothurian.

In 1966, the Geological Survey of Canada began a comprehensive revision of the Burgess Shale fossil record, led by

Cambridge University paleontologist Harry B. Whittington.[8] In the process of this revision, Whittington and his students Simon Conway Morris and Derek Briggs would discover the true nature of Anomalocaris and its relatives, but not without contributing to the history of misinterpretations first.[18] 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.[19] 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".[20] 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.[18][21] 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.[9] The two genera have now been placed into the order Radiodonta[8] 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.[10]

In 2011[22] and 2020,[23] compound eyes of Anomalocaris were recovered from a paleontological dig at Emu Bay on Kangaroo Island, Australia, proving that Anomalocaris was indeed an 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.[22]

In 2021, "A." saron[24] and "A." magnabasis[25] were reassigned to the new genus Houcaris, in the family Tamisiocarididae.[26] In the same year, "A." pennsylvanica was reassigned to the genus Lenisicaris.[3] 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.[27] In 2023, "A". kunmingensis was reassigned to the new genus Guanshancaris in the family Amplectobeluidae.[28] Multiple phylogenetic analyses also suggested that "A". briggsi (tamisiocaridid) was not a species of Anomalocaris either,[29][30][31][4][32][33][34] and it was reassigned to the genus Echidnacaris in the family Tamisiocarididae in 2023.[2] In the same 2023 study, a new species of Anomalocaris, A. daleyae, was described based on remains found in the Emu Bay Shale in Australia.[2]

Description

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.[4][27] Previous estimation up to 1 m (3.3 ft)[20] 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[4]) and the size of largest frontal appendage (up to 18 centimetres (7.1 inches) in length when extended).[18][4]

Anomalocaris propelled itself through the water by

undulating the flexible flaps on the sides of its body.[35] Each flap sloped below the one more posterior to it,[36] and this overlapping allowed the lobes on each side of the body to act as a single "fin", maximizing the swimming efficiency.[35] The construction of a remote-controlled model showed this mode of swimming to be intrinsically stable,[37] 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.[8][38] It is difficult to distinguish lobes near the tail, making an accurate count difficult.[36] For the main trunk flaps, the type species A. canadensis had 13 pairs.[38]

  • Oral cones of Anomalocaris (A), Peytoia (B) and Hurdia (C), the former showing unique triradial, tuberculated and wrinkled structures.
    Oral cones of Anomalocaris (A), Peytoia (B) and Hurdia (C), the former showing unique triradial, tuberculated and wrinkled structures.
  • The mobility of radiodont trunk appendages (like Anomalocaris).
    The mobility of radiodont trunk appendages (like Anomalocaris).
  • The head sclerite structure of Anomalocaris.
    The head sclerite structure of Anomalocaris.

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 have scale-like tubercles near the mouth opening.[10][39] Such an oral cone is very different from those of a typical hurdiid radiodont like Peytoia and Hurdia, which is smooth and tetraradial.[10][33] As a shared character across radiodonts, Anomalocaris also had three sclerites on the top and side of its head.[33] The top one, known as a head shield, dorsal carapace or H-element, was shaped like an laterally-elongated[40] oval, with a distinct rim on the outer edge.[38] The remaining two lateral sclerites, known as P-elements, were also ovoid, but connected by a bar-like outgrowth.[33] The P-elements were previously misinterpreted as two huge compound eyes.[38][33]

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.[8] Each frontal appendage of Anomalocaris usually had 14 podomeres (segmental units, at least 1 for shaft and 13 for distal articulated region), with each appendage being laterally-flattened (taller than wide).[38] Most podomeres were tipped with a pair of endites (ventral spines).[38] The endites themselves were both equipped with multiple auxiliary spines, which branches off from the anterior and posterior margin of the endites.[24][39][41][38][1]

The tail was a large tail fan, composed of three

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.[38]

Based on fossilized eyes from the

opsins suggest that Anomalocaris may have had dichromatic color vision.[43]

Paleobiology

Diet

Grasping movement of the frontal appendage of A. canadensis.

The interpretation of Anomalocaris as an active

apex predators known to exist.[5]

However, the long-standing idea that Anomalocaris fed on hard-bodied animals, especially its ability to penetrate mineralized

arthropod cuticles, causing the prey's exoskeleton to rupture and allowing the predator to access its innards.[47] This behaviour was originally thought to have provided an evolutionary pressure for trilobites to roll up, to avoid being flexed until they snapped.[47]

Ecological reconstruction of Anomalocaris hunting 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.

vetulicolian; free-swimming arthropods like isoxyids and hymenocarines; Nectocaris).[5][6]

Bicknell et al. (2023) examined the frontal appendages of Anomalocaris, suggesting it was an active nektonic apex predator. Postured with the frontal appendages outstretched, Anomalocaris would have been able to swim with maximized speed, similar to modern predatory water bugs. 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 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, Cambroraster, Titanokorys, Stanleycaris) and artiopods (e.g. Sidneyia) would have been benthic predators in the Burgess Shale.[5][6]

Paleoecology

Specimens of Anomalocaris have been found worldwide spanning from

Chengjiang Biota, Hongjingshao Formation, Balang Formation and the Kaili Formation of China, as well as the Eagar Formation and Weeks Formation in the United States.[3]

Anomalocaris canadensis lived in the Burgess Shale in relatively great numbers.[1] In the Burgess Shale, Anomalocaris is more common in the older sections, notably the Mount Stephen trilobite beds. However, in the younger sections, such as the 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,[20] and is currently synonymized to A. canadensis.[38]

Other unnamed species of Anomalocaris live in vastly different environments.[3] For example, Anomalocaris cf. canadensis (JS-1880) lived in the Maotianshan Shales,[3] a shallow tropical sea or even being delta[48] in what is now modern China. Anomalocaris daleyae (Emu Bay Shale) lived in a comparable environment; the shallow, tropical waters of Cambrian Australia.[3] The Maotianshan Shale and the Emu Bay Shale are very close in proximity, being separated by a small landmass, far from the Burgess Shale.[3] These two locations also included "Anomalocaris" kunmingensis and "Anomalocaris" briggsi respectively, species that previously attributed[49][50][39][51] but taxonomically unlikely to be a member of Anomalocaris nor even Anomalocarididae.[3][52]

See also

Footnotes

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  8. ^
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  16. ^ Briggs DE (1979). "Anomalocaris, the largest known Cambrian arthropod". Palaeontology. 22 (3): 631–664.
  17. ^ Whiteaves JF (1892). "Description of a new genus and species of phyllocarid Crustacea from the Middle Cambrian of Mount Stephen, B. C.". The Canadian Record of Science. 5 (4).
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  19. JSTOR 1303799
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  20. ^ a b c Briggs DE (1979). "Anomalocaris, the largest known Cambrian arthropod". Palaeontology. 22 (3): 631–664.
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External links

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