Cambrian Explosion

By | August 11, 2016

About 530 million years ago, a huge range of new animals appeared in the seas, including the first creatures with clear legs, heads, sense organs, skeletons, and shells. All the main categories of invertebrates (animals without backbones) known today seem to have evolved almost at once, as well as some weird creatures quite unlike anything else. Scientists call this mysterious burst of life the Cambrian explosion.

Wiwaxia (we-WAX-ee-a)


This Wiwaxia fossil from the Burgess Shale fossil bed in Canada is about 500 million years old. The armorlike plates on its back are called sclerites.

Wiwaxia looked like a tiny, armored porcupine, since its body was covered with protective spines and rows of overlapping armor plates. Its flat lower surface, where the mouth was located, did not have any protection. The mouth had two or three rows of sharp, conical teeth that may have been used to scrape algae from the seabed. Wiwaxia had no distinct head or tail and was probably blind, relying on touch and smell to find the way.

■ When 505 million years ago (Middle Cambrian)

■ Fossil location Canada

■ Habitat Seafloor

■ Length 1–2 in (3–5 cm)

Anomalocaris a-NOM-a-low-CAR-iss


Fossils of Anomalocaris often show only a small part of the body. This is one of its feeding claws.

A little like a giant shrimp, Anomalocaris was the largest animal found in the Burgess Shale fossil bed of Canada. Experts suspect it was the top predator in Cambrian seas and used a pair of spiked claws attached to its head to grasp prey such as trilobites. It had no legs but could swim by flexing its segmented body and waving the flaps on its sides. Large compound eyes indicate it had good vision and hunted by sight.

■ When 505 million years ago (Middle Cambrian)

■ Fossil location Canada, S. China

■ Habitat Oceans

■ Length Up to 3 ft (1 m)

Echmatocrinus ECK-mat-oh-crine-us



Echmatocrinus lived attached to the seafloor, its cone-shaped body topped by a ring of 7–9 tentacles, each bearing small side-branches. The surface of the main cone was covered with a jigsaw of hard, protective plates. When it was first discovered, scientists thought Echmatocrinus might be related to starfish, but it lacks the five-sided symmetry of the starfish family. Some experts think it might instead be a kind of coral.

■ When 505 million years ago (Middle Cambrian)

■ Fossil location Canada

■ Habitat Oceans

■ Length 1 in (3 cm) wide, below the tentacles

Ottoia ot-OY-ah


Ottoia fossil

Ottoia was a kind of worm that lived in U-shaped burrows, which is why its fossils are usually curved. Its mouth was covered with tiny hooks and could be turned inside out like a sock to capture small animals from the muddy seafloor. Fossilized food remains inside Ottoia’s gut reveal that it was a cannibal, preying on its own kind as well as devouring small shelled animals. Ottoia is one of the most common early Cambrian fossils, with around 1,500 known specimens.

■ When 505 million years ago (Middle Cambrian) to now

■ Fossil location Canada

■ Habitat Oceans

■ Length 1.–3. in (4–8 cm)

Hallucigenia ha-lucy-JEAN-ee-a



■ When 505 million years ago (Middle Cambrian)

■ Fossil location Canada, China

■ Habitat Oceans

■ Length Up to 1 in (2.5 cm)

Hallucigenia is one of the strangest animals from the Cambrian Period. At one end is a large blob that may be a head, but with no mouth or eyes. It may simply be a stain on the fossil and not a part of the animal. Running along the wormlike body were rows of sharp spines and rows of fleshy tentacles. Orginally the spines were thought to be legs, but scientists now think the fleshy tentacles were the legs, despite not being arranged in pairs.



Burgess Shale Formation – Rocky Mountains, Canada

Most of these two pages come from the Burgess Shale Formation in the Rocky Mountains in Canada. Littering the ground at this famous mountaintop site are hundreds of beautifully preserved animal fossils dating back almost to the very dawn of animal life. The Burgess Shale contains imprints of soft body parts that normally don’t fossilize and reveals that invertebrate life was already amazingly varied half a billion years ago.

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