Abnormal Shrimp: An Apex Predator or Barely Chewing?


By Allison Kubo Hutchison

The same animal was once described by paleontologists as a shrimp, jellyfish, sea cucumber, and a sponge at different times during its study. Anomalocaris, Latin for “abnormal shrimp”, is a creature of exceeding strangeness to modern hominids; it is related to modern-day shrimp with a flat segmented body, faceted eyes on stalks, and two grabbing appendages which more closely resemble tentacles.

Artistic Rendering of Anomalocaris magnabasis
Species could grow up to a meter long, significantly larger than most other beasts in the Cambrian. It swam by undulating the segmented flaps along the sides of its body and its large faceted eyes offset from the body mass could peer around for prey. Despite its strange appearance, the scientist has often thought the ecological niche of the Anomalocaris can be likened to that of the majestic lion or humans ourselves. Nothing could take it on and all feared it. It was long thought that Anomalocaris was an apex predator of its time and fed upon trilobites and other hard-bodied animals (Read here about what trilobites fed on).
Fossilized mouth plate of Anomalocaris canadensis Whiteaves was first identified as a jellyfish fossil.

However, these claims have been questioned by further analysis of the Anomalocaris mouthparts. Its unusual mouthparts stumped paleontologists leading to decades of misidentification. Anomalocaris has a disk-shaped, or perhaps donut-shaped, mouth composed of plates pointing inward and forming a ring. Computer modeling of the mouth found it difficult to produce pressures to break the hard shells of the trilobite. In addition, the lack of chipped and broken mouthparts suggests they weren’t used for the hard work of crushing trilobites.

However, Anomalocaris have the characters one would expect to see of a predator in that time: large eyes, body size, mobility, and appendages posed to bring food to its maw. The morphology, other than the weakness of the jaws, suggests that it was a predator. We know that something was preying on trilobites as they have found coprolites (paleontology speak for fossilized poop) with trilobite shells and even fossils of trilobites with damaged shells from failed attacks. Some suggest that Anomalocaris fed on freshly moulted arthropods when the shell was relatively weak. Modern-day aquatic predators are even able to detect molting hormones and target the soft new shells.

There is a large range in Anomalocaris species frontal appendages which suggests that like the trilobite they were specialized feeders. Indeed, some may not have fed on hard-bodied trilobites and preferred soft worms or even fed primarily as cannibals. However, they remain an important part of the Cambrian ecosystem and the story of how life evolved from the “Abnormal Shrimp” to us.

Rendering of the varied shape and size of the anomalocaris frontal appendages.

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