The Cambrian Explosion

Introduction

The Cambrian explosion refers to a relatively short period in evolutionary time when a wide diversity of different phyla began to apparently suddenly appear in the fossil record.  The Cambrian explosion, otherwise known as the Cambrian radiation, began around 541 ± 1 Ma and lasted for the next 20-25 million years (Erwin et al. 2011).  This episode in Earth’s history is evidence for the appearance of the first representatives to all the major groups of organisms including the forerunners to ourselves in the phylum Chordata (Chernicoff et al. 2002).

In 1909 while out riding with his family near Mount Wapta in the Canadian Rockies of British Columbia the palaeontologist Charles Walcott made a startling discovery in the rocks he was traversing.  This discovery would offer a new insight into the dramatic acceleration of evolutionary diversity.  Wolcott an expert in the Cambrian period working at the Smithsonian Institution knew that some of the fossils he was looking at were very unlike anything that had been seen before.  He immediately recognised the importance of his find.  The fossils had been preserved in rocks known as the Burgess shale.  Over the course of the proceeding years over 60,000 specimens were discovered, including 170 new species and became known as the Burgess shale fauna (Chernicoff et al. 2002).

What was so exciting to science was that these animals were incredibly well preserved in the fossil record.  The reason they were so well preserved was that before fossilisation it was thought they were living at the bottom of a steep cliff in warm, shallow, well oxygenated water (Chernicoff et al. 2002).  They were swept into deeper water by an underwater landslide where they were buried beneath sediment.  This low-oxygen environment protected the specimens from scavengers (Chernicoff et al. 2002).   These new animals which appeared had diverse body plans and what made them unique was their ability to bio-mineralize hard body parts such as shells and exoskeletons (Crowley 2015).   Prior to this, Ediacaran fauna exhibited the first record of radiation of multicellular animals which had evolved from simpler forms in the Proterozoic eon (Crowley 2015).   Most fauna from this period were soft bodied, evidence of which were first found in the Ediacaran hills in  Australia in the 1940s (Crowley 2015).  Some of these may have been diploblasts, consisting of two different cell layers, an outer ectodermal layer and an internal endodermal layer (Marshall 2006). In the late Ediacaran, the first skeletonised simple animal forms appear such as the tube shaped Cloudina (Grant 1990) and the slightly more complex Namacalathus (Amthor et al. 1997).  These ani
mals do not have any known extant relatives and it has been suggested that they constitute a completely different phylum known as Vendoza (Crowley 2015).

Many of the animals which are seen in later times had evolved from the early Cambrian but also, many forms which are not seen in later groups are believed to be evolutionary experiments by some palaeontologists. It is believed that as many as 20 different types of animals fit into this category (Chernicoff et al. 2002).  These animals are thought to have gone extinct due to natural selection.  Simple bilateral body forms were evident in the Ediacaran but it was during the Cambrian explosion animals with a triploblastic bilateral body plan appeared for the first time (Marshall 2006). Triploblasts grow with three cell layers, the outer ectoderm, the intermediate mesoderm and the internal endoderm (Marshall 2006).  The Cambrian explosion was highly significant in the development of life on Earth not only due to the diversity of the animals seen but also the disparity.  Diversity refers to the number of species present while disparity refers to the morphological or distinct differences between species (Marshall 2006).   The most common animal present in Cambrian seas are the trilobites.  From the phylum Arthropoda, which includes today’s insects, crustaceans, myriapods and arachnids, trilobites were a diverse class with over 600 genera in the Cambrian alone (Chernicoff et al. 2002). The body plan was segmented and consisted of an exoskeleton which was discarded during moulting when the animal was growing.  They had adapted to swimming, crawling and planktonic modes of transport (Chernicoff et al. 2002).  Predators were also evident in the Cambrian. A giant arthropod predator known as Anomalocaris grew to 60cm and had a pair of sharp appendages used for capturing and cutting up their prey (Chernicoff et al. 2002).  Other groups of animals which shared ocean habitats with trilobites were brachiopods.  These animals which resemble molluscs had a pair of coiled tentacles covered in cilia which allowed the animal to filter the water for food and breathe oxygen.  Their shells consisted of two valves which mirrored each other made by biomineralising calcium carbonate or calcium phosphate from the surrounding seas (Chernicoff et al. 2002).  Another important group to appear in the Cambrian explosion are in the phylum Mollusca.  Modern day relatives include, snails, clams and squid.  These early branchiopods and molluscs became known as the small shellies (Marshall 2006).  A specimen with a very unusual body plan, a row of tentacles along the underside of the body coupled with a row of sharp spikes along the top was eventually placed in the class of arthropods known as onycophorans or velvet worms.  (Chernicoff et al. 2002).  The Pikaia is thought to be the earliest precursor to backboned animals or Chordates. (Chernicoff et al. 2002).

Similar fossils to those found at the Burgess shale were found at 30 other locations around the world which would suggest that these animals were abundant in the waters of the Cambrian oceans.  The most important discoveries after the Burgess shale discovery is in the Yunnan province in south eastern China (Chernicoff et al. 2002).

Causes of the Cambrian Explosion

The explanations as to the main drivers of this seemingly sudden emergence in diversity and disparity in life has puzzled researchers for decades.  Charles Darwin was quoted as saying “To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer” (Darwin 1876). Some of the reasons have centred on environmental changes such as the rapid oxygenation of the oceans, changes in ocean chemistry, release of methane into the atmosphere warming the planet leading to conditions where rapid diversification was possible (Zhang et al. 2014).  A mass extinction event at the boundary of the Ediacaran-Cambrian period, eliminating existing Ediacaran fauna making way for the explosion in Cambrian life forms (Laflamme et al. 2013) and ecological and developmental causes have also been suggested (Zhang et al. 2014).

The evidence for a mass extinction event at the Ediacaran-Cambrian boundary is problematic, some clades such as Erniettomorphs disappearing at or near the boundary and others such as Dickinsoniomorphs dying out long before this while other groups such as Porifera survive well into the Phanerozoic (Laflamme et al. 2013).  The rise of predation at this time may have driven evolution in some clades through an arms race together with more complex multi-layered ecosystem into creating more diverse biota (Laflamme et al. 2013).  The Cambrian saw the first animals with the ability to crush shells, many specimens of trilobites exhibit evidence of scarring (Laflamme et al. 2013).  In conjunction with an evolutionary arms race, bioengineering of the oceans is said to have played a role in altering the dynamics of ecosystems (Laflamme et al. 2013).  Efficient filter feeders such as sponges would have limited the role of microbial mats and thus increased the levels of O₂ in the oceans. Bioturbation and burrowing by newly evolved bilaterans could have altered the habitats of existing Ediacaran animals.  This would have had a knock-on effect, presenting greater opportunity for growth in the taxa to become bigger and more complex (Gaidos et al. 2007).

Notwithstanding the changes in biotic interactions and ecological conditions there must have been major underlying abiotic changes taking place in the Earth’s dynamic systems to provide the building blocks for these new animals to develop.  A lot of research has concentrated on correlating the fossil record of the Neoproterozoic-Cambrian divide with changes in the physical and geo-chemical environment (Peters & Gaines 2012). Analysis of stratigraphic sediments from 540-480 Ma show an increase in ocean alkalinity and an expansion in the shallow continental shelves surrounding land masses (Peters & Gaines 2012).  This would have a profound effect on organisms living there.  Geochemical weathering of the continents would have mobilised large amounts of organic and inorganic compounds following the break-up of Rodinia and successive glaciation events during the Neoproterozoic era.  The physical reworking of soil and underlying sediment laid down a stratigraphic layer separating the continental crystalline basement rock and the younger Cambrian marine sediment.  This is known as the Great Unconformity (Peters & Gaines 2012).    It has been suggested that continental denudation of the land and the altering of ocean chemistry can be linked to the adaptation of bio-mineralising organisms (Peters & Gaines 2012).  The Great Unconformity can be seen in the stratigraphic record right around the world.  The oceans chemistry would have gone through some very important changes in composition.  CO₂ would have been consumed from the atmosphere while vital ions used in the building blocks of metazoans such as HCO₃^–, Ca²⁺, H₃SiO₄^–, SO₄^2-, Cl^–,Na⁺, Mg²⁺, K⁺ and Fe³⁺ would have been released into the oceans (Peters & Gaines 2012).  Evidence in the rock record shows a lot of carbonates were precipitated from sea water, deep sea carbon sinks were not evident until later in the Mesozoic so a combined source of shallow seas and weathered continental rocks provided a rich source of carbonates available for biomineralisation (Peters & Gaines 2012). Cambrian sediments show a relatively higher amount of Glauconite to sediments today.  This material rich in H₃SiO₄^–, K⁺,Fe³⁺ is further evidence for an input of continental weathering during the Great Unconformity (Peters & Gaines 2012).  Furthermore evidence of a three-fold increase in Ca²⁺ from the Neoproterozoic to the Cambrian are seen in evaporate fluid inclusions (Peters & Gaines 2012).  Calcium carbonate is the key ingredient in the shells and skeletons of all sea animals but would also have been toxic to other sea creatures such as Ediacaran phyla (Peters & Gaines 2012).

Conclusion

The Cambrian explosion was one of the most important periods of time for the development of biological life on planet earth.  The Burgess shale in Canada along with 30 other sites globally is evidence that these new animals were abundant and widespread.  Up to this point animals had a simple body plan, were mostly soft bodied and occupied limited ecological niches. Then at approximately 541 ±1 Ma more complex, triploblastic bilateral organisms with the ability to biomineralise skeletons and exoskeletons began to appear.  These metazoans had a greater diversity and greater disparity never seen before.  They gave rise to all of the animal phyla seen on earth today.  It was also a time for evolutionary experimentation.  Many of the animals that thrived in this environment have no extant relatives.  Palaeontologists have debated the causes for the Cambrian explosion.  Some have pointed to an extinction event during the Ediacaran-Cambrian boundary however, the fossil evidence is far from conclusive as some of the Ediacaran animals such as Porifera survived into the Cambrian era.  There is a substantial amount of evidence to suggest a combination of environmental factors contributed to the right conditions for the rapid diversification of different life forms.  An increase in the O₂ concentration of the atmosphere along with large scale releases of inorganic compounds into the oceans especially HCO₃^–, Ca²⁺, H₃SiO₄^–, SO₄^2-, Cl^–,Na⁺, Mg²⁺, K⁺,Fe³⁺  due to continental weathering is evidenced in the stratigraphic record of the Great Unconformity.  Shallow, warm, well oxygenated seas with a prolonged influx of key ingredients allowed for more complex organisms to appear which was then accelerated by trophic and ecological interactions such as predation allowed Cambrian metazoans to replace Ediacaran forms and to diversify rapidly over a relatively short space in evolutionary time.

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