Abstract

Throughout
the Neoproterozoic to early Palaeozoic, environmental changes led to the
development and expansion of ecological niches that, alongside genetic
advancements, induced the radiation of life in the Cambrian. There are multiple
hypothesised causes for the ‘Cambrian Explosion’ including, for instance, continental
break-up. This proposed idea suggests the break-up of Pannotia ~550Ma led to
the creation of habitable shelf environments and later enabled the increased
abundance of life. Studies of Anglo-Welsh geology have proposed both
stratigraphic and fossiliferous evidence to support that the diversification of
metazoan life, such as the Arthropoda and Brachiopoda phyla, was sudden and
derived from a common ancestor. However, modern research into Hox genes and discoveries
from molecular clocks support that this change was instead a gradual occurrence
that began prior to the Cambrian. There is no singular, entirely accepted
explanation to this rapid diversification event over such a short period of
geological time, instead the Cambrian Explosion was the result of a collation
of contributing environmental and biological factors.

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1.0 Introduction

The Cambrian
period lasted from 542Ma until the base of the Ordovician at 488Ma (Bowring, et al., 2006; Landing, et al., 2000).
This is supported in Wales and England (Fig. 1) where Cambrian age strata with
ash beds have been radiometrically dated using U-Pb methods on zircons. These
dates are correlated against lithological and fossiliferous evidence from other
Cambrian rocks globally to identify the base of the Cambrian. The Cambrian
Explosion occurred 542Ma at the beginning of the Palaeozoic Era and refers to
the seemingly abrupt appearance of increasingly abundant and diverse animals
with mineralised skeleton remains found in the fossil record (Butterfield, 2007).

 

Both
extrinsic and intrinsic factors are hypothesised to have changed around this
time period to produce increasingly favourable conditions, these led to the
accelerated evolution of more complex bodies with bilateral symmetry. Extrinsic
factors refer to physical alterations in the Cambrian and Precambrian environment,
such as the Neoproterozoic glaciation events and oxygenation of oceans, that created
the environments necessary to onset evolutionary changes among flora and fauna.
Alternatively, intrinsic factors are the internal biotic adaptations to the
genes within individual organisms that initiated the diversification events
(von Bloh, 2003). As represented in British Cambrian rocks, there were distinct
variations in climate, geology and consequently life forms throughout this
period. Although these hypotheses exist as independent factors, many are indeed
naturally interconnected and therefore this report will discuss the evolutionary
triggers of the explosion and to what extent they are responsible.

 

 

2.0 Data from the Fossil Record

Data obtained
from the fossil record portrays how the diversification and abundance of life occurred
in the Cambrian period. By recording the frequency, distribution
and type of fossils upon their discovery a pattern of growing complexity in
organisms can be observed over time. 

 

In Southern
Britain, some of the most commonly occurring Cambrian fossils include
brachiopods (specifically lingulates) and trilobites (Rushton, et
al., 2000).
The high abundance of preserved trilobites occurs as a result of their
strengthened calcite exoskeleton and can be found at multiple sites throughout
Britain including Comley Quarry, Shropshire and St. Davids, Pembrokeshire
(Harvey, et al., 2011). The Lower to Middle Cambrian Comley Limestone
succession at Comley Quarry yields a wide variety of trilobites that portray
the differing stages of growth as diversification occurred. This collection includes
Britain’s earliest fossilised arthropods with soft body parts that allow the
comparison of this British succession to other global Cambrian stratotypes (Siveter, et
al., 2003).
Additionally, the Charlton Hill Acrothele
prima Shales contain fauna including common small, shelly fossils, the
lingulate brachiopods Acrothele prima (of which the shales were named after) and arthropods such as Bradoria
and later discovered trilobites (Rushton, et al., 2007).

 

Large
quantities of evidence exist outside British geology, however, the
microcontinent Avalonia where England and Wales were located in the Cambrian
was also comprised of Eastern North America, Atlantic Canada and Western Europe
(Fig. 2). Thus, evidence formed in identical environmental conditions exists here.
Few locations on Earth preserve soft-bodied Cambrian fossils, these include the
lagerstätte sedimentary deposit sites where the exceptional preservation of un-mineralised
body parts can occur (Allison & Briggs, 1993). This includes the Burgess Shale in British Columbia, Canada (Butterfield, 1995) and the Chengjiang biota in Yunnan, Southern China (Xian-Guang, et al.,
2017; Gabbott, et al., 2004) where almost
all known, meticulous information regarding the Cambrian explosion and its
radiated diversification occurs in the fossil record.

 

The Burgess Shale exhibits similar organisms to those
that would have been inhabiting oceans around Great Britain during the Middle
Cambrian. However, these are rarely preserved and for that reason the Burgess
Shale is a very unique example of Cambrian Life and its diversity. Fossilised fauna included the armoured, soft-bodied
Wiwaxia (Smith, 2014), the long, tube-like Hallucigenia with multiple legs and
spines (Smith & Caron,
2015),
the soft-bodied Opabinia regalis with
had five eyes (Budd, 1996) and the predator Anomalocaris with compound eyes and
flexible lobes either side of the body to increase mobility (Paterson, et al., 2011). All of these
represent visual confirmation of the structural diversity and evolution of
animals that occurred in the Cambrian to allow increased movement and armour as
ecological interactions within environments became altered.

 

3.0 Causes

 

3.1 Extrinsic Factors

3.1.1 Supercontinent
Breakup

The rifting of the supercontinent Pannotia occurred 555Ma
ago (Bond, et al., 1984) as a result of the
Iapetus Ocean opening. Pannotia broke up into Laurentia and a larger continent
Gondwana as well as other smaller fragments including Baltica and Siberia (McKerrow, et
al., 1992).
The rifting of this Neoproterozoic supercontinent resulted in the production of
highly habitable continental margins and shelves where there were warm, shallow
environments with high levels of nutrients and oxygen required for large biotic
productivity. Consequently; metazoan life could thrive, the diversity of fauna increased
and thus new ecological interactions and behaviours including predatism ensued in
the later Cambrian.

 

Laurentia existed nearer to
the equator and contained much of modern North America and parts of Europe. Gondwana on the other hand was towards the
South Pole and was composed of landmasses that later formed many present-day continents
including Africa, Australia, South America and parts of Asia. Evidence from palaeomagnetic
studies suggests that in the Cambrian 520Ma Britain was split between the two
landmasses Laurentia and Gondwana, northern Scotland was connected to North
America and present in Laurentia whilst the remaining Scotland, England and
Wales existed on passive margins in Avalonia, a microcontinent off of Gondwana (Scotese, 2009).

Shallow seas, created from rifting, covered much of modern Britain and organisms
such as trilobites first appeared 520Ma in these locations. As metazoan life developed,
natural selection favoured stronger, bigger animals causing the growth in
numbers of the first, larger organisms with exoskeletons (McClain &
Boyer, 2009).
Mid-Cambrian trilobites such as Eccaparadoxides acadicus and Ctenocephalus matthewi (Fig.
3) have been discovered in Shropshire, England (Rushton, et
al., 2007),
upon the reinvestigation of the Acrothele
prima Shales of Charlton Hill. Here, fragments of trilobites were found
fossilised within decalcified, medium-grained sandstone interbedded with thin
layers of mudstone.

 

3.1.2 Neoproterozoic
Glaciation

Glaciation events can create an oceanic upwelling and
alterations to ocean chemistry (Zhuravlev, 2001) through the
injection of lower density fresh water from glaciers into the higher density
sea water. This reduces down-welling as well as deep-water formation and is
significant because it induces a feedback response of cooling. This in turn can
cause carbon burial, increase oxygen levels and act as a cause for the explosion
of life (Nursall, 1959). Glacial melting
produced an increased nutrient flow and led to the influx of planktonic
organisms (Canfield, et al., 2007), upon death these
organisms buried carbon into marine sediments and consequently oxygenation
occurred. Additionally, throughout the Cambrian, the Earth gradually warmed and
caused the late Proterozoic glaciers to recede and sea levels to rise. This
glacial retreat would respectively increase flooding and produce a greater
number of shallow marine environments as discussed below.

 

During the Neoproterozoic, Snowball Earth occurred as the
Earth experienced two major glaciations in the Sturtian and Marinoan 715-635Ma (Maruyama & Santosh, 2008) however a later
glaciation in the Proterozoic known as the ‘Gaskiers’ also occurred 580Ma (Eyles & Eyles, 1989). The movement of ice
in the Precambrian resulted in the collection of tillites in the rock record of
the Avalon Peninsula, Newfoundland and supports the occurrence of glacial
activity in this time period. At this location, the post-Gaskiers sedimentary
sequence contains fossilised Ediacaran biota, these imply oxygenation occurred
after the glaciation event to maintain life (Canfield, et al., 2007). Furthermore, a
sustained oxic environment would later allow the evolution of bilateria in the
Cambrian.

 

3.1.3 Rising Sea Level

Post-glacial warming and increased continental margin lengths
led to rising sea levels during the Cambrian, this caused greater areas of
continents to become flooded with warm, shallow water and created new niches
optimal for marine lifeforms to radiate (Rushton, et al., 2000). The presence of turbidites
in Southern Britain represent how the depositional environment progressed from
deltaic to marine as lowland areas became flooded due to sea level rise. The Cambrian
rocks found in this region are of aquatic origin, varying from beds of deep continental margin slopes to shallow
shelf sandstones and limestones. Rising sea levels caused significant
alterations to depositional, sedimentary environments, this includes the
deposition of sandstone turbidites in the Harlech Grits Group of North-Western
Wales in the Early Cambrian (Young, et al., 1994).

 

3.1.4 Marine Oxygenation

Preceding the
Cambrian, the Ediacaran ocean was dysoxic with a low oxygen content and life-forms
were entirely aquatic (Zhang & Cui, 2016), by the early Cambrian the sea
floor consisted of a microbial mat positioned on top thick anoxic muds (Seilacher
& Pflüger, 1994). The Cambrian Substrate Revolution
represents how the first multicellular organisms had developed to feed on this
microbial layer and from 542-530Ma burrowers developed to dig vertically into
the sediment, due to the presence of cyanobacteria this process mixed as well
as oxygenated the muds and therefore increased the dissolved oxygen content in
marine environments (Maloof, et al., 2006). This is argued to
have contributed to the Cambrian explosion as the multicellular life present displayed
the earliest evidence of bilateral symmetry, additionally other calcifying
animals began to abruptly increase in abundance as they developed carbonate
exoskeletons that were preserved by phosphatic fossilisation (Butterfield, 2003; Bengtson, et al., 1990).

 

Currently,
the majority of data concerning the
Early Cambrian is collected from small, shelly skeletal remains and trace
fossils (Crimes & Fedonkin, 1994), this includes the fossiliferous
evidence of burrowing organisms preserved within the quartzite ‘Pipe Rock’ in North-West
Scotland (Fig. 4). At this locality, Skolithos
pipe trace fossils exist in the marine deposited sandstones where they once
burrowed (Hallam & Swett, 1966).

 

The radiation
of complex life occurred accordingly to the critical level of oxygen being
achieved, the accumulation of oxygen in the atmosphere and marine settings supported
animals in the construction of larger bodies (Budd, 2013) and mineralised
skeletons without the limitation of insufficient oxygen diffusion (Tostevin, et
al., 2016).

3.2 Intrinsic Factors

3.2.1 Hox Gene
Development

Large evolutionary change in the Cambrian has the potential
to have been caused by advancements and mutations to the similar genetic link,
known as Hox genes (Carroll, 1995) between all metazoan organisms. Hox
genes are a form of homeotic genes that control the body architecture of an
embryo throughout development and produce proteins with the ability to stimulate
genes accordingly in order to determine anatomical structure (Pearson, et
al., 2005).
Arguably, the biotic radiation could only occur if a degree of these intricate
genetics had already been achieved (Brooke, et al., 1998), this is in order to
provide the essential physical arrangement systems needed to evolve.

 

Alterations to Hox genes initiated the presence of a
through-gut as well as central nervous systems and muscles needed to cause the
development of fauna suited to directed movement such as burrowing (Seilacher, et
al., 2005).
More diverse, elaborate burrows show both horizontal and vertical orientations
in the Early Cambrian, examples include the appearance of the mud burrowing Treptichnus pedum trace fossil in
Newfoundland that marks the base of the Cambrian ~540Ma (Dzik, 2005)
and is iconic due to its behavioural patterns of controlled locomotion that
enable burrowing in shallow marine conditions (Seilacher, et al., 2005).

 

Recent evidence from developmental genetics suggests that
Hox gene duplications resulted in the creation of Hox gene clusters in many
animals (McGinnis & Krumlauf, 1992), of which a typical
cluster existed in the common ancestor of all bilaterian organisms. Alongside
fossil evidence, this suggests that there is an essential connection between the
genetics of all metazoans and that the Cambrian explosion was an act of
diversification of bilaterian animals from a last common ancestor pre-dating the
Lower Cambrian.

 

4.0 Discussion

Upon the
consideration of all provoking factors of the Cambrian Explosion, extrinsic
factors are to a great extent responsible for the Cambrian Explosion yet are inconsistent
as a solitary cause. For example; supercontinent rifting of Rodinia occurred
750Ma and therefore prior to the breaking up of Pannotia (Li, et al., 2008), in addition glaciation events such as
the Sturtian Glaciation 725Ma (Kennedy, et al., 1998) occurred before the
Gaskiers and yet neither of these factors resulted in a previous rapid
appearance of more diverse life forms. To produce a radiation of life these environmental
and ecological factors must therefore act upon and alongside existing biotic developments.
Nonetheless, intrinsic causes can also be disputed considering it is difficult
to distinguish the sources of bilaterian body structure against the sources of
the diversification to their morphology. Alternatively to conclusions produced
from recent studies, the Hox genes suggested to have caused animal evolution could
have existed prior to the Cambrian yet they were fulfilling different functions
to structural determination within organisms (Valentine, et al., 1999).

 

The fossil
record exclusively represents the Earth’s obtainable fossils, therefore organic
relics

were only
available for discovery if the remains had developed hard, fossilizable body
parts that had been preserved in a non-destructive environment. Few known pre-Cambrian
fossils exist due to the degree of preservation that previously existed in sediments,
for instance the habitats in which
organisms resided in can break down and recycle their fossiliferous remains. Additionally, the lack of prior
Ediacaran fauna fossils may have occurred as a result of the divergence of
burrowers in the Cambrian (Crimes, 1992). Sediments in which organisms were
buried were displaced and disrupted as a result of bioturbation, therefore
eliminating them from the known fossil record. As proposed by Darwin (1859), this implies ancestral, multicellular
organisms existed before their first fossil embodiments were discovered in
Cambrian strata, therefore suggesting the Cambrian explosion instead represents
a radiation in the mineralisation of organisms
(Butterfield, 2007; Runnegar, 1982).

 

The Precambrian fossil
record displays increasingly more evidence over time for metazoan life existing
prior to the Palaeozoic, more so the dating from molecular clocks also suggests
that the last common ancestor of bilaterian life existed earlier than the
Cambrian 800-1200Ma (Blair & Hedges, 2005). Recent studies of
the time between the divergence of two bilateral lineages have taken into
account more genes when comparing the degree of genetic evolution (Levinton, et
al., 2004)
to improve reliability. Such reasoning from this analysis is well refined yet the
lack of understanding regarding what pressures determine the rate of mutation
and adaptation of genes has led to a vast variation in the dating of the
evolution of metazoans. Despite the differentiation in precision, all predictions
have persistently been dated to the Precambrian however these conclusions
unfortunately lack sufficient evidence to test the existing fossil record.

 

5.0 Conclusion

The Cambrian was a period of
considerable evolutionary change, as shown by trace fossils in Britain, where
multiple major phyla groups diversified. Extrinsic factors are responsible for
the apparent Cambrian Explosion to a great extent, however without intrinsic advancements
the radiation would not have occurred despite plentiful environmental changes
such as continental rifting and sea level change. Large speculation arises as
to whether this event resembles an act of accelerated diversification or purely
the fossilisation of existing entities, this report determines that with recent
discoveries more evidence exists to support that metazoan life began before the
Cambrian (Fortey, et al., 1997) but preservation was
constrained due to the lack of hard parts in organisms and the Earths
conditions. Therefore, fauna continued to diversify within the Cambrian and
appears increasingly in the fossil record due to greater biomineralisation
increasing available fossils. It can be concluded that the exploitation of newly
created niches would have first required the genetic ability in Precambrian
organisms to dictate and assemble complex bodies. Thus supporting that extrinsic
factors are highly responsible for the explosion yet the discussed hypothesised
causes are interdependent.

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