By Allison Kubo Hutchison

The same animal was once described by paleontologists at different times during their investigation as a shrimp, jellyfish, sea cucumber, and sponge. Anomalocaris, Latin for “abnormal shrimp,” is a creature that is extremely strange to modern hominids. It is related to modern shrimp with a flat, segmented body, faceted eyes on stems, and two grasping appendages that are more like tentacles.

Artist’s impression of Anomalocaris magnabasis

The species could be up to a meter long and significantly larger than most other animals in the Cambrian. It swam, billowing the segmented flaps along the sides of its body, and its large, faceted eyes, offset from the body mass, were able to search for prey. Despite its strange appearance, the scientist has often thought that the ecological niche of the Anomalocaris can be compared to that of the majestic lion or humans themselves. Nothing could accept it and everyone feared it. Anomalocaris has long been believed to be an apex predator of its time, feeding on trilobites and other hard-nosed animals (read what trilobites fed here).

The fossilized mouth plate of Anomalocaris canadensis Whiteaves was first identified as a jellyfish fossil.

However, these claims have been challenged by further analysis of the Anomalocaris mouth parts. Its unusual mouthpieces baffled paleontologists, leading to decades of misidentification. Anomalocaris has a disc-shaped, or perhaps donut-shaped, mouth made up of plates that point inward to form a ring. Computer modeling of the mouth found it difficult to generate pressure to break the trilobite’s hard shells. In addition, the lack of chipped and broken mouth parts suggests that they were not used for the hard work of crushing trilobites.

Anomalocaris, however, have the characters one would expect from a predator at this time: large eyes, height, agility, and limbs designed to bring food to its throat. The morphology, aside from the weakness of the jaws, suggests it was a predator. We know that some fell victim to trilobites as they found coprolites (paleontology suggests fossilized feces) with trilobite shells and even fossils of trilobites with damaged shells from failed attacks. Some suggest that anomalocaris fed on freshly molted arthropods when the shell was relatively weak. Modern aquatic predators are even able to recognize molting hormones and target the soft new clams.

There is a wide variety of frontal appendages in species of Anomalocaris, suggesting that, like the trilobite, they were specialized forage. In fact, 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.

Representation of the different shapes and sizes of the anomalocaris frontal appendages.



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