The Cambrian Explosion
The Cambrian explosion established virtually all the major animal body forms that would exist thereafter, including many that became extinct. ~ English anthropologist Roger Lewin
Pannotia breaking up led to the abrupt disappearance of Ediacaran biota. The radiative revolution of the Cambrian (542–485 MYA) followed. It was facilitated by a surge in oxygen which incited animal evolution in shallow marine waters. Extreme fluctuations in oxygen during the Cambrian period alternately provoked dramatic diversifications and mass extinctions (during anoxic times).
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Discoveries pertinent to evolutionary biology have at first been typically interpreted within the conventional mindset of the day. Decades or generations may pass before reinterpretation, as Cambrian fossils exemplify.
Burgess Shale
Observe always that everything is the result of a change and get used to thinking that there is nothing Nature loves so well as to change existing forms and make new ones like them. ~ Roman Emperor Marcus Aurelius in the 2nd century
The fossil beds that became known as the Burgess Shale were discovered by Canadian geologist Richard McConnell in 1866. McConnell noticed unusual black shale outcroppings in the Canadian Rockies of British Columbia, studded with tiny fossils.
McConnell’s find came to the attention of American paleontologist Charles Doolittle Walcott, administrator of the Smithsonian Institution. Walcott made his 1st expedition there in 1907. He made several more over the next 17 years.
All told, Walcott accumulated 65,000 specimens of Cambrian life, which he proceeded to catalog into the existing taxa of modern times. Walcott saw no meaningful evidence of evolution in the curiosities he had carved out.
From 1924 into the 1930s, American paleontologist Percy Raymond collected further fossils from the Burgess Shale formation, to no great advance in understanding.
After that, interest dwindled until the early 1960s, when English paleontologist Harry Whittington initiated his own survey. Whittington was prodded into action by a fellow trilobite enthusiast while the two were rummaging in Raymond’s collection.
Whittington had a team to examine his and Walcott’s fossils, which had languished in a Smithsonian storeroom since Walcott’s death in 1927. Papers poured out of the group in the early 1970s, diagnosing many of the specimens as hitherto unknown animal forms; whence the legend of the Cambrian explosion.
Discoveries in the early 21st century pushed back into the Ediacaran period evolutionary developments that were previously thought to have occurred during the Cambrian. For example, complex nervous systems and brains developed during the Ediacaran.
Whatever the jumping off point from the Ediacaran, a relatively sparse number of species encountered conducive conditions at the onset of the Cambrian that put evolution into hyperdrive. Genetic codes changed 5.5 times faster than they do now. New traits arose at 4 times today’s pace.
The fossil record of oceanic animal forms exploded during the early Cambrian. 100 different phyla (body plans) seem to have come into being, including many unique and bizarre creatures. Today there are 30 or so phyla.
Geological events on land and under the seabed created the conditions for the Cambrian. The warm seas became spiced with species. Seawater became more alkaline, affording a profound expansion of shallow marine habitats.
Oceanic oxygen levels rose during the Cambrian. Photosynthetic plankton played a role, but so did the animals that ate them.
Dead organisms sink, decomposing as they go. Because decomposition consumes oxygen, this would have kept ocean waters anoxic; but filter-feeding sponges started a clearing process. Over time, this helped oxygenate the oceans.
The sponges basically march from shallower to deeper water, oxygenating as they go. ~ American evolutionary biologist Douglas Erwin
While overall oceanic oxygenation occurred during the Cambrian, there were pulses when the global marine oxygen level dropped. These short-lived catastrophes may have emptied ecological niches and spurred evolutionary developments. Whereas rising oxygen permitted animals to prosper, it may have been intermittent drops that doled out diversity.
Rising sea levels during the Cambrian increased erosion: spilling nutrients, such as calcium, phosphate, and potassium, into the oceans. Less acidic water meant greater viability and longevity of these marine nutrients.
Calcium was the key ingredient that afforded the building of hard shells and skeletons. These durable structures spelled protection.
With predation came an intense set of evolutionary pressures: spurring the development of better burrowers, faster-moving creatures, stronger predators with sharpened weapons, increasingly sophisticated senses, and so on. The proliferation of life created an evolutionary cascade, spawning predators in prodigious variety. Prey adapted to evade these threats. The predator-prey gyre began in earnest during the Cambrian.
Hallucigenia
505 MYA lived a slight (0.5–3.5 cm) wormy creature with 7–8 pairs of legs that ended in claws, and long spikes the rose from its body. It was hard to tell the head from the tail. Hallucigenia clearly had features for defense against predators.
Caterpillars would later take this form, albeit distinctly. The descendants of Hallucigenia include arthropods and modern velvet worms.
Fish Appear
The first fish were glorified aquatic worms. They arose 530 MYA, with a distinct head and tail, gills for breathing, camera-like eyes, and a notochord for support. The term notochord is broadly used as a flexible rod-shaped body, but more specifically refers to cartilage back support, which evolved into a backbone.
The earliest fish were jawless and boneless. Vertebrate backbones appeared ~500 MYA, while jawed fish emerged over 420 MYA.
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Free-floating fauna in the ocean were not as prolific during the Cambrian as they would be later. While micro-bial mats were no longer ubiquitous, most Cambrian animal life that didn’t swim lived on or near the sea floor.
Whereas the Ediacaran witnessed the rise of the life’s lasting biomechanisms, the Cambrian’s great creation was carnivory: animals were eating other animals with zest. This set the evolutionary clock racing, sparking an incredible burst of speciation. While the Ediacaran was of evolution within animals, the Cambrian was of interaction between animals.
Crustaceans became the dominant invertebrate during the Cambrian. They occupied the shallow waters near seashores, which flooded inland during the early Cambrian.
The animal cardiovascular system was well-developed by 520 BYA. The vascular system of some ancient arthropods was more complex than what is found in many modern crustaceans: with simplification came efficiency.
Trilobites
One crustacean came to dominate the Cambrian seas: the trilobite. From 526 MYA these armored arthropods diversified to 17,000 species.
Trilobites were the cockroaches of their age. These shallow-benthic marine arthropods were common in a variety of environments, including tidal flats, with an amazing diversity of shapes and sizes.
By 520 MYA, trilobites were 2/3rds of all marine species. Most trilobites were less than 10 cm, but one, Paradoxides, at some 60 centimeters, was a prodigious paradox.
In typical radiation, trilobites accommodated themselves to many lifestyles: plankton-feeding swimmers, seabed scavengers, filter feeders, and predators. Olenidae, a family of trilobite, evolved a symbiotic relationship with sulfur-eating bacteria, and were thereby able to tolerate the oxygen-poor, sulfur-rich seabed at the time. Trilobites were one of the 1st animals to evolve a complex digestive system.
475 MYA, somewhat mysteriously, many trilobites abruptly vanished: declining to 1/3rd of the sea species, as mollusks, corals, and other stationary filter feeders rose. Stagnation – brought on by a strong thermocline – has been suggested, as has a sea level drop followed by a rise, though how this might have provoked trilobite decimation is unclear. Anyway, something begat a drawn-out decline of trilobites.
Glaciation at the end of the Ordovician wiped out the dominant trilobite group, the Ibex Fauna, while another group, the Whiterock Fauna, swam through the extinction event unscathed. Adaptive tolerances to variable conditions paid off.
The coming of jawed fish put a crunch on trilobites. Still they hung on. A long run on life’s stage – 340 million years – came to a close for trilobites in the excruciating mass extinction event at the end of the Permian, 250 MYA.
