The Cambrian Period

This is a summarising look at the earliest period of the Phanerozoic eon and the Palaeozoic era. Lasting from 541 to 490 Ma, the Cambrian was the starting point for our own ancestry as well as the ancestry for most animal phyla that we see today. The Cambrian Period is named after the Latin name for Wales. It was named this way because of Adam Sedgwick, a geology pioneer studied strata. In fact Charles Darwin was one of Sedgwick's students, Sedgwick later disagreed with Darwin's theory of Evolution.

The climate of the the Cambrian period began relatively cool, with temperatures steadily increasing to peak towards the middle of the period. We know this

Above: The Burgess Shale. Image Credit: burgess-shale.rom.on.ca

because there is evidence of retreating glaciers at the turn of the eon 541 Ma. This was caused by increasing temperatures. 

At this time, the continents of the Earth are all joined together, forming one supercontinent called Rodinia. During the Cambrian, Rodinia began to break up, this left two, still giant, land masses. The first, Gondwana, lay over the where our South Pole is today. Gondwana would give rise to Africa, Australia, South America, Antarctica and parts of Asia almost 500 million years later. The other, Laurentia, was on the equator and would later become North America and Europe. 

Due to melting of such a massive volume of ice, the sea levels of the Cambrian Earth rose dramatically. This created new habitats in shallow sea environments. Although the oceans were teeming with life, the land was a desolate wasteland with no life at all due to the inhospitable environment. 

Beneath the waves, there is a different story. Fossils have revealed a genus of microorganisms that formed a layer above the then anoxic (containing no oxygen) mud on the sea floor. This layer of organisms is what the first multicellular organisms 'grazed' on. To combat this the microorganisms evolved to burrow in the mud, and through bioturbation the mud became oxygenated. This added oxygen allowed the for the building of coral reefs and animals with calcium carbonate exoskeletons. This is because these organisms 'breathe' by absorbing oxygen through their exoskeletons, an abundance in oxygen meant that this gas transfer was more efficient fuelling growth. Respiration of cyanobacteria has also been theorised to be the cause for a sudden surge in oxygen in the oceans.

Above: Holochroal eye of a trilobite. Image credit:
trilobites.info

In the Cambrian, we find the famous trilobites evolve. It was in the trilobite that we see a complex evolution; eyes. These eyes are separated into two categories; holochroal and schizochroal. Holochroal is a compound eye with more than 15,000 lenses tightly packed together to form an image, schizochroal on the other hand are compound eyes with larger lenses which are separated by sclera.  

Above: Schizochroal eye of a trilobite. Image credit:
trilobite.info

The Cambrian also saw the first predator. At one metre long, Anomalocaris was the largest animal in the ocean. With an armoured exoskeleton and crushing jaws, this monster was truly formidable. The only thing to threaten an Anomalocaris would be another Anomalocaris.

During the middle Cambrian there was a mass extinction. Here many of the reef builders and primitive trilobites met their end. The cause is unknown for certain, one hypothesis suggests that a depletion in oxygen was the cause. This was the result of upwelling of cooler waters from the ocean depths. Although this pushed various species to extinction, it created more opportunities throught the creation of more habitats, a possible cause of the Cambrian Explosion, a sudden radiation of species.

Possibly the most famous fossil bed of Cambrian age is the Burgess Shale in British Columbia, Canada. Here there are 12 species of late Cambrian trilobites, as opposed to the two species exhibited by the Sirius Passet of Greenland and China, which is of similar age. Amazingly, the Burgess Shale has ancestral representatives of all modern day animal phyla, with the exception of the Bryozoa. This is where Anomalocaris

Above: Artists impression of Anomalocaris sp. image
credit: Quade Paul

was found. Alongside was the earliest chordate, a small animal called Pikaia. At only four centimetres long, it was tiny compared to it's predators. It is evident to see from the fossils that a nerve chord ran along a ridge from behind the head to almost the end of it's body. It also had a segmented muscle structure that is surprisingly similar to the musclature of modern day animals. The earliest jawless fish, Haikoichthyes, is also found in the shale.

The Cambrian period was closed by another mass extinction. James Miller of the South West Missouri State University, suggests that because we find glacial deposits at the open of the Ordovician in South America, that is was glaciation and a colder climate was the cause of this extinction. Glaciers lock up vast amounts of water, including ocean water, on the land as ice. This reduces the oxygen and destroys the shallow sea habitats. Therefore, the organisms that are adapted to this environment cannot adapt quick enough to the change and die out.