Anomalocaris

Anomalocaris ("abnormal shrimp") is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed Burgess Shale. Anomalocaris has two known species: A. canadensis and A. daleyae^[1].

The recently discovered species A. daleyae is known from the somewhat older Emu Bay Shale of Australia.^[1] Since the original description in the late 19th century,^[2] the frontal appendages were the only known fossilized parts and misidentified as the body parts of other creatures.^[3]

Anomalocaris lived in the Cambrian period, which was a long time before dinosaurs. This makes Anomalocaris very old, so their body parts haven't evolved to be as detailed as modern animals.

Anomalocaris still had compound eyes which were very big and complex compared to other animals in the earlier parts of the Cambrian, and also had segmented (has many sections) front appendages which were used to grasp its prey like arms. So while Anomalocaris was around the size of a small lobster, it still was a very good apex predator for its time.

Anomalocaris was misidentified many times, as the mouth, front appendages and trunk (the rest of its body) were discovered separately multiple times, leading scientists to think for a while that these parts of Anomalocaris were, in fact, of three different animals, with the mouth thought to be a jellyfish, the front appendages thought to be a shrimp and the trunk thought to be a sea cucumber or sponge.

Anomalocaris was huge in comparison to other animals. A complete specimen of A. canadensis, called ROMIP 51211, measures up to 20.5 cm (8.1 in) long^[3] (17.4 cm (6.9 in) long when excluding the frontal appendages and tail fan^[4]). The newly discovered A. daleyae from the Emu Bay Shale is larger than A. canadensis, with the biggest appendage measuring up to 18.3 cm (7.2 in) long, which belongs to an individual measuring between 34.8–51.2 cm (1.14–1.68 ft) long.^[1][4]

Anomalocaris was misidentified after its discovery, followed by a series of misidentifications and taxonomic revisions.^[3]

An excerpt from Stephen Jay Gould's 1989 book Wonderful Life describing Anomalocaris:

"[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 single reconstructed creature, the largest and fiercest of Cambrian organisms."^[5]

Anomalocaris fossils were first collected in 1886^[3] by Richard G. McConnell of the GSC )Geological Survey of Canada). McConnell climbed Mount Stephen on 13 September 1886.^[6] He found two unknown specimens.^[2]

In August 1891, Henri-Marc Ami collected 48 more unknown specimens.^[7] The fifty specimens were examined and described in 1892 by GSC paleontologist Joseph Frederick Whiteaves.^[2] Whiteaves thought they were the abdomens of phyllocarids, and made the name Anomalocaris canadensis. He describes the things:

"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 shrimps) is suggested by the unusual shape of the uropods or ventral appendages of the body segments and the relative position of the caudal spine."^[2]

In 2011, compound eyes of Anomalocaris were found from a dig at Emu Bay Shale on Kangaroo Island, Australia, proving that Anomalocaris was really an arthropod. This was later identified in 2023 as a newly discovered species of the genus, A. daleyae.^[1]

Anomalocaris lived in the sea, like most of the plants (which were really small algae in the Cambrian) and animals back then. Recently, people now think that Anomalocaris didn't hunt trilobites and other benthic animals that lived on the sea floor, but instead swam after nekton like basal chordates (animals that can control how they move underwater, unlike plankton).

• Amplectobelua
• Opabinia—a related genus to Anomalocaris
• Cambroraster
• Peytoia
• Shucaris

1. ↑ ^{1.0 1.1 1.2 1.3} Paterson, John R.; García-Bellidob, Diego C.; Edgecombe, Gregory D. (10 July 2023). "The early Cambrian Emu Bay Shale radiodonts revisited: morphology and systematics". Journal of Systematic Palaeontology. 21 (1). Bibcode:2023JSPal..2125066P. doi:10.1080/14772019.2023.2225066.
2. ↑ ^{2.0 2.1 2.2 2.3} Whiteaves, J. F. (1892). "Description of a new genus and species of phyllocarid Crustacea from the Middle Cambrian of Mount Stephen, B.C." Canadian Record of Science.
3. ↑ ^{3.0 3.1 3.2 3.3} "Collins D: The "evolution" of Anomalocaris and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.)".
4. ↑ ^{4.0 4.1} "New suspension-feeding radiodont suggests evolution of microplanktivory in Cambrian macronekton".
5. ↑ Gould SJ (1989). Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton. p. 194. ISBN 0-393-02705-8. OCLC 18983518.
6. ↑ "The Burgess Shale: First Discoveries".
7. ↑ "I.—The Canadian Rockies. Part I: On a Collection of Middle Cambrian Fossils obtained by Edward Whymper, Esq., F.R.G.S., from Mount Stephen, British Columbia".

Media related to Anomalocaris at Wikimedia Commons

• The anomalocaris homepage featuring swimming animation
• https://undsci.berkeley.edu/this-is-not-the-shrimp-youre-looking-for/