Paleozoic Era: Cambrian Period

539–485 Ma

The Cambrian: an evolutionary explosion

Trilobites, crustaceans, and small shellies, oh my! The Cambrian Period is characterized by a rapid diversification of lifeforms that paleontologists call the Cambrian Explosion. Here, we begin to see a burgeoning ecosystem with some of the first representatives of modern animal groups and new behaviors. We see animals hunting each other and creating complex burrow structures for shelter and camouflage—for example, see our priapulid worms, Ottoia, poking out of the sediment and lying in wait for prey scurrying across the seafloor, such as Hallucigenia with its paired rows of defensive spikes. We see small invertebrates living amongst our first animal-dominated reef systems, like Wiwaxia, our spiny, armored friend in the foreground, hanging out amongst various sponges in front of our stromatolitic and archaeocyathid reefs). During the Cambrian, the supercontinent Pannotia was in the process of breaking up and most continental land masses were clustered in the Southern Hemisphere. The sea level was high, flooding our continents with warm, shallow seawater—perfect for the development of new niches and evolutionary expansion of marine animals. While many Cambrian fossils are preserved in extraordinary detail in deposits known as lagerstätten, there are always difficulties when it comes to the identification and classification of these early animals. No other animal perfectly encapsulates this struggle than the Anamolocaris, seen swimming with large eye stalks and curved feeding appendages. The original descriptions of this animal in the late 19th century consisted instead of separate, disarticulated portions of the animal: the front feeding appendages were found on their own and identified as strange, shrimp-like tails; the abdominal segments were identified as parts of a large unknown arthropod; and the mouth parts were identified as an awkward pineapple-ring-shaped jellyfish. It wasn’t until 1985 that a fully articulated Anamolocaris fossil was discovered, with abdominal segments connected to the front feeding appendages with a mouthpart similar to the misidentified jellyfish. The animal was then redescribed as a large swimming arthropod predator, though many new insights are still being gained today from continued research. Several paleontologists at Mizzou are actively researching the Cambrian Period, its uniquely preserved fossils, and the diversity of early animal life it holds.