Anomalocaris, meaning “unlike other shrimp,” was a captivating creature from the Cambrian Period (520 to 500 million years ago), a time of rapid diversification of life. It offers insights into early ecosystems and the evolution of complex life forms, making it an iconic example of Cambrian fauna.
An Enigmatic Cambrian Giant
Anomalocaris possessed several striking physical features, making it a formidable presence in the Cambrian seas. It had large, stalked compound eyes, which provided keen vision for hunting prey. Its segmented body was flanked by a series of undulating lateral fins or lobes, allowing it to propel itself efficiently through the water. A prominent fan-shaped tail further aided its propulsion and maneuverability.
The most distinctive features were its pair of large, spiny, grasping appendages located at the front of its head. These flexible, segmented limbs were equipped with multiple auxiliary spines, suggesting they were used to capture and manipulate prey. Behind these appendages was a unique circular mouth, often described as resembling a pineapple ring, composed of 32 overlapping plates that radiated inwards. Anomalocaris canadensis could reach lengths of up to 37.8 centimeters, making it one of the largest animals of its era and an early example of an apex predator.
The Shifting View of Its Diet
For a long time, based on its impressive size and powerful-looking appendages, Anomalocaris was widely considered a top predator that fed on hard-bodied prey, such as trilobites. Scientists initially believed its circular mouth with radiating plates was designed to crush the tough exoskeletons of these ancient arthropods. This view was also supported by the presence of “bite marks” on some trilobite fossils.
However, recent research has challenged this long-held hypothesis, suggesting a different primary diet. Current understanding indicates Anomalocaris likely fed on soft-bodied organisms or possibly functioned as a filter feeder, rather than crushing hard shells.
Fossil Clues to Feeding Habits
The revised understanding of Anomalocaris’s diet is supported by detailed analysis of its fossilized remains. Studies of its mouthparts indicate they lacked the strong, mineralized surfaces necessary to effectively crush hard-shelled prey like trilobites. In fact, 3D modeling of the mouth suggests it could not fully close and its plates were flexible, not rigid like teeth, making them unsuitable for breaking hard shells. The absence of wear marks on the mouthplates further supports that they did not regularly interact with hard, abrasive materials.
The structure of its grasping appendages also provides clues; while formidable, their spines were relatively fragile, making them more suitable for capturing and manipulating soft-bodied prey. Furthermore, the lack of fossilized trilobite remains directly within Anomalocaris gut contents or coprolites (fossilized feces) has contributed to the new hypothesis. Some large coprolites containing trilobite fragments, once attributed to Anomalocaris, are now believed to have been produced by other organisms. These combined fossil clues paint a picture of Anomalocaris as a swift, visual predator that likely “pincushioned” soft, swimming creatures in the water column rather than crunching armored prey on the seafloor.