| Anomalocaris | |
|---|---|
%2c_Royal_Tyrrell_Museum%2c_Drumheller%2c_Alberta%2c_2025-07-13.jpg) | |
| Cast of TMP 2023.003.0003, a complete specimen ofAnomalocaris canadensis | |
 | |
| Life restoration ofAnomalocaris canadensis | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | †Dinocaridida |
| Order: | †Radiodonta |
| Family: | †Anomalocarididae |
| Genus: | †Anomalocaris Whiteaves, 1892 |
| Species | |
(8 more unnamed species[3]) | |
Anomalocaris (fromAncient Greekανώμαλος, meaning "unlike", andκαρίς, meaning "shrimp", with the intended meaning "unlike other shrimp") is anextinctgenus ofradiodont, anorder of early-divergingstem-group marinearthropods. It is best known from the type speciesA. canadensis, found in theStephen Formation (particularly theBurgess Shale) ofBritish Columbia,Canada. The other named speciesA. daleyae is known from the somewhat olderEmu Bay Shale ofAustralia.[2] Other unnamedAnomalocaris species are known fromChina and theUnited States.[3]
Like otherradiodonts,Anomalocaris had swimming flaps running along its body, largecompound eyes, and a single pair of segmented,frontal appendages, which inAnomalocaris were used to grasp prey. Estimated to reach 34.2–37.8 cm (13.5–14.9 in) long excluding the frontal appendages and tail fan,[4]Anomalocaris is one of the largest animals of the Cambrian, and thought to be one of the earliest examples of anapex predator,[5][6] though others have been found in older Cambrianlagerstätten deposits.
Anomalocaris was only known from its frontal appendages when it was first described in 1892, which were originally considered to be the body of a shrimp-like arthropod. Due to initially only being known from isolated body parts,Anomalocaris has a convoluted taxonomic history, being historically confused with the related radiodont genusPeytoia, with complete body specimens ofAnomalocaris only being described in the late 20th century. It is the type genus ofAnomalocarididae, a family which previously included allradiodonts but recently onlyAnomalocaris and a few closely relatedtaxa.[3]
From the start,Anomalocaris fossil was misidentified, followed by a series of misidentifications and taxonomic revisions.[7][8] AsStephen Jay Gould, who popularised theCambrian explosion in his 1989 bookWonderful Life, appropriately described:
[The story ofAnomalocaris 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 single reconstructed creature, the largest and fiercest of Cambrian organisms.[9]
Anomalocaris fossils were first collected in 1886[8] by Richard G. McConnell of theGeological Survey of Canada (GSC). Having been informed of rich fossils at theStephen Formation in British Columbia, McConnell climbedMount Stephen on 13 September 1886.[10][11] He found abundant trilobites, along with two unknown specimens.[12] In August 1891,Henri-Marc Ami, Assistant Palaeontologist at GSC, collected many trilobites andbrachiopod fossils,[13] along with 48 more of the unknown specimens.[14] The fifty specimens were examined and described in 1892 by GSC paleontologistJoseph Frederick Whiteaves.[15][16] Whiteaves interpreted them as the abdomens ofphyllocaridcrustaceans, and gave the full scientific nameAnomalocaris 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 nameAnomalocaris (fromανώμαλος, unlike,—καρίς, a shrimp,i.e., unlike other shrimps) [the species name referring to Canada] is suggested by the unusual shape of theuropods or ventral appendages of the body segments and the relative position of the caudal spine.[12]
In 1928, Danish paleontologist Kai Henriksen proposed thatTuzoia, a Burgess Shale arthropod which was known only from the carapace, represented the missing front half ofAnomalocaris.[8] The artists Elie Cheverlange andCharles R. Knight followed this interpretation in their depictions ofAnomalocaris.[8]
Not known to scientists at the time, the body parts of relatives ofAnomalocaris had already been described but not recognized as such. The first fossilized mouth of such a kind of animal was discovered byCharles Doolittle Walcott, who mistook it for ajellyfish and placed it in the genusPeytoia. 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 coexistedSidneyia.[17] In the same publication in which he namedPeytoia, Walcott namedLaggania, a taxon that he interpreted as aholothurian.
In 1966, the Geological Survey of Canada began a comprehensive revision of the Burgess Shale fossil record, led byCambridge University paleontologistHarry B. Whittington.[8] In the process of this revision, Whittington and his studentsSimon Conway Morris andDerek Briggs would discover the true nature ofAnomalocaris and its relatives, but not without contributing to the history of misinterpretations first.[17] In 1978, Conway Morris recognized that the mouthparts ofLaggania were identical toPeytoia, but concluded thatLaggania was a composite fossil made up ofPeytoia and the spongeCorralio undulata.[18] In 1979, Briggs recognized that the fossils ofAnomalocaris were appendages, not abdomens, and proposed that they were the walking legs of a giant arthropod, and that the feeding appendage Walcott had assigned toSidneyia was the feeding appendage of similar animal, referred to as "appendage F".[15] 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 toAnomalocaris and mouthpart similar toPeytoia.[17][19] Whittington linked the two species, but it took several more years for researchers to realize that the continuously juxtaposedPeytoia,Laggania and frontal appendages (Anomalocaris and "appendage F") actually represented a single group of enormous creatures.[7] The two genera have now been placed into the order Radiodonta[8] and are commonly known as radiodonts or anomalocaridids. SincePeytoia was named first, it is the accepted correct name for the entire animal. However, the original frontal appendage was from a larger species distinct fromPeytoia and "Laggania" and therefore retains the nameAnomalocaris.[20]
In 2011,compound eyes ofAnomalocaris were recovered from a paleontological dig atEmu Bay Shale onKangaroo Island, Australia, proving thatAnomalocaris was indeed anarthropod 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.[21] This specimen was later identified as that of a new species ofAnomalocaris,A. daleyae.[2]
Numerous species have been previously referred toAnomalocaris, but subsequent analyses have doubted this generic assignment,[22][23][24] and reclassified them within different genera. In 2021, "A."saron[25] and "A."magnabasis[26] were reassigned to the new genusHoucaris in the familyTamisiocarididae,[27] but subsequent analysis suggests thatH. saron is a member of the familyAmplectobeluidae instead and thatH?magnabasis (recovered as a sister taxon of Amplectobeluidae) does not form a monophyletic clade with other species ofHoucaris.[28] In the same year, "A."pennsylvanica was reassigned to the genusLenisicaris.[3] In 2022, specimen ELRC 20001 that was treated as an unnamed species ofAnomalocaris or whole-body specimen ofA. saron got a new genus,Innovatiocaris.[29] In 2023,"A". kunmingensis was reassigned to the new genusGuanshancaris in the familyAmplectobeluidae.[30] Multiple phylogenetic analyses also suggested that"A". briggsi (tamisiocaridid) was not a species ofAnomalocaris either,[4][31][32][33] and it was reassigned to the genusEchidnacaris in the familyTamisiocarididae in 2023.[2]
Anomalocaris was gigantic in comparison to contemporary animals. A complete specimen ofA. canadensis, ROMIP 51211, is measured up to 20.5 cm (8.1 in) long[8][34] (17.4 cm (6.9 in) long when excluding the frontal appendages and tail fan[4]). The largest frontal appendage is measured up to 18 cm (7.1 in) long when extended,[35] and this specimen ofA. canadensis would have reached up to 34.2–37.8 cm (1.12–1.24 ft) in body length excluding the frontal appendages and tail fan.[4]Dryad data 04[29] Previous body length estimation up to 1 m (3.3 ft)[15] is unlikely based on the ratio of body parts[35] (body length measured only about 2 times the length of frontal appendage inA. canadensis[4]) and the size of largest frontal appendage.[35]A. daleyae (formerlyA. cf.canadensis orA. aff.canadensis) from theEmu Bay Shale of Australia is larger thanA. canadensis, with the largest known appendage measuring up to 18.3 cm (7.2 in) long, which would have belonged to an individual between 34.8–51.2 cm (1.14–1.68 ft) long.[2][4]
Anomalocaris propelled itself through the water byundulating the flexible flaps on the sides of its body.[36] Each flap sloped below the one more posterior to it,[37] and this overlapping allowed the lobes on each side of the body to act as a single "fin", maximizing the swimming efficiency.[36] The construction of a remote-controlled model showed this mode of swimming to be intrinsically stable,[38] implying thatAnomalocaris 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 three pairs of small flaps on the constricted neck region.[8][35] It is difficult to distinguish lobes near the tail, making an accurate count difficult.[37] For the main trunk flaps, the type speciesA. canadensis had 13 pairs.[35]
Anomalocaris had an unusual disk-like mouth known as an 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.[20][39] Such an oral cone is very different from those of a typicalhurdiid radiodont likePeytoia andHurdia, which is smooth and tetraradial.[20][32]
As a shared character across radiodonts,Anomalocaris also had three sclerites on the top and side of its head.[32] The top one, known as a head shield, dorsal carapace or H-element, was shaped like a laterally-elongated[40] oval, with a distinct rim on the outer edge.[35] The remaining two lateral sclerites, known as P-elements, were also ovoid, but connected by a bar-like outgrowth.[32] The P-elements were previously misinterpreted as two huge compound eyes.[35][32]
Based on fossilized eyes from theEmu Bay Shale, which belong to the speciesAnomalocarisdaleyae,[2] the stalked eyes ofAnomalocaris 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 moderndragonfly, which has 28,000 lenses in each eye.[21] Additionally, estimation ofecdysozoanopsins suggest thatAnomalocaris may have haddichromaticcolor vision.[41]
The tail was a large tail fan, composed of three[8][35] pairs of large, lateral fin-shaped lobes and one terminal lobe-like tailpiece.[35] Previous studies suggest the tail fan was used to propel it through Cambrian waters,[19][36] while furtherhydrodynamic study rather suggest it was more adapted to providesteering function.[34] The gills of the animal, in the form of long, thin, hair-like structures known aslanceolate 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.[35]
Perhaps the most notable part ofAnomalocaris are the two frontal appendages, large limbs positioned in front of the mouth, at the front of the head.[8] Each frontal appendage ofAnomalocaris usually had 14podomeres (segmental units, at least 1 for shaft and 13 for distal articulated region), with each appendage being laterally flattened (taller than wide).[35] Most podomeres were tipped with a pair of endites (ventral spines).[35] The endites themselves were both equipped with multiple auxiliary spines, which branches off from the anterior and posterior margin of the endites.[25][39][42][35][1]
The interpretation ofAnomalocaris as an activepredator is widely accepted throughout the history of research,[7][8][20] as its raptorial frontal appendages and mid-gut glands strongly suggest a predatory lifestyle.[43][44][5] In the case ofA. canadensis, its outstanding size amongstBurgess Shale fauna also make it one of the firstapex predators known to exist.[5]
However, the long-standing idea thatAnomalocaris fed on hard-bodied animals, especially its ability to penetrate mineralizedexoskeleton oftrilobites, has been questioned, with many recent studies considering it more likely thatAnomalocaris exclusively hunted soft-bodied prey.[45][20][5][6] Some Cambriantrilobites have been found with round or W-shaped "bite" marks, which were identified as being the same shape as the mouthparts ofPeytoia (previously misidentified as those ofAnomalocaris[46][20]). Stronger evidence thatAnomalocaris atetrilobites comes fromcoprolites, which containtrilobite parts and are so large that the radiodonts are the only known organism from that period large enough to have produced them.[46] However, sinceAnomalocaris lacks any mineralized tissue, it seemed unlikely that it would be able to penetrate the hard, calcified exoskeleton of trilobites.[46] Rather, thecoprolites may have been produced by different organisms, such as thetrilobites of the genusRedlichia.[39] Another suggested possibility was thatAnomalocaris 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 ofarthropod cuticles, causing the prey'sexoskeleton to rupture and allowing the predator to access its innards.[46] This behaviour was originally thought to have provided anevolutionary pressure fortrilobites to roll up, to avoid being flexed until they snapped.[46]
The lack of wear onradiodont mouthparts suggests they did not come into regular contact with mineralizedtrilobite 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 oftrilobites.[45][39]A. canadensis was suggested to have been capable of feeding on organisms with hardexoskeletons due to the short, robust spines on its frontal appendages.[39][26] However, this conclusion is solely based on the comparison with the fragile frontal appendages ofsuspension feedingradiodonts (e.g.Echidnacaris andHoucaris spp.).[27] The typical lack of damage to the endites on the frontal appendages ofA. canadensis (with damage only present on a single specimen) suggests that they were not used to grasp hard-shelled prey.[6] As opposed toPeytoia whose oral cone is more rectangular with short protruding spines, the oral cone ofA. canadensis has a smaller and more irregular opening, not permitting strong biting motions, and indicating a suction-feeding behavior to suck in softer organisms.[20] Three-dimensional modelling of variousradiodont frontal appendages also suggest thatA. canadensis is more capable to prey on smaller (2–5 cm or 0.8–2.0 inches in diameter), active, soft-bodied animals (e.g.vetulicolian; free-swimming arthropods likeisoxyids andhymenocarines;Nectocaris).[5][6]
Bicknellet al. (2023) examined the frontal appendages ofAnomalocaris, suggesting it was an activenektonic apex predator. Postured with the frontal appendages outstretched,Anomalocaris would have been able to swim with maximized speed, similar to modern predatorywater 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 ofnektonic andpelagic soft-bodied animals. It probably would have not huntedbenthic animals liketrilobites, 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 otherradiodonts (e.g.Hurdia,Cambroraster,Titanokorys,Stanleycaris) andartiopods (e.g.Sidneyia) would have beenbenthic predators in theBurgess Shale.[5][6]
Specimens ofAnomalocaris have been found worldwide spanning fromCambrian Stage 3 to theGuzhangian. Aside from the Burgess Shale and Emu Bay Shale, fossils have been found in theChengjiang Biota,Hongjingshao Formation,Balang Formation and theKaili Formation of China, as well as theEagar Formation andWeeks 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 theMount Stephen trilobite beds. However, in the younger sections, such as thePhyllopod 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,[15] and is currentlysynonymized toA. canadensis.[35]
Other unnamed species ofAnomalocaris live in vastly different environments.[3] For example,Anomalocaris cf.canadensis (JS-1880) lived in theMaotianshan Shales,[3] a shallow tropical sea orriver delta[47] 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,[48][49][39][50] which are no longer considered anomalocaridids.[30][2][3][51]
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