Australia: The Land Where Time Began |
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Cambrian Explosion Triggers – Hypotheses and
Problems Abrupt appearance of major bilaterian clades in
the fossil record during the first three stages of the Cambrian Period
has puzzled the scientific world since the 1830s. Many proposed causes
including environmental, developmental, and ecological hypotheses, are
reviewed in this paper. Nutrient availability, oxygenation, and change
of seawater composition are commonly supposed to be environmental
triggers. The nutrient input, e.g. the enrichment of phosphorus in an
environment, would cause excess primary production, but it is neither
directly linked with diversity nor disparity. Fluctuating abiotic
conditions during the Snowball Earth and the associated oxygenation
event may have stimulated the diversification of complex multicellular
organisms including diversification of macroscopic and morphologically
differentiated algae in the early Ediacaran, but did not lead to the
ecological success of metazoan or bilaterian lineages. Further increase
of oxygen level and change of seawater composition just before and
during the Early Cambrian are suggested by the high weathering rate of
the trans-Gondwana Mountains, Great Unconformity, and decline of oceanic
salinity. These are potential candidates for environmental triggers for
the Cambrian Explosion but require future, more detailed geochemical
studies to confirm. The molecular phylogeny calibrated with the
molecular clock data suggested that the developmental system of
bilaterians was established before their divergence. This, in turn,
suggests that the Cambrian Explosion requires environmental triggers.
However, there still exists the contention between deep or shallow
divergence of bilaterian clades, which remains to be solved in the
future. The deep divergence model is supported by a majority of
molecular clock studies, but is challenged by the paucity of bilaterian
fossils before and during the Ediacaran Period. The shallow model is
generally consistent with the fossil record, but has to explain the
rapidity of increase in diversity, disparity, morphological complexity,
acquisition of biomineralised shells, etc. Regardless of deep or shallow
model, the conservation of lineage-specific kernels within the gene
regulatory networks (GRNs) provides an explanation for the long-term
stability of body plans after the Cambrian Explosion, and continuous
addition of microRNAs into the GRNs seems to correspond well to the
increase in morphological complexity. As for ecological causes, some
hypotheses (e.g. adaptive radiation after mass extinction, cropping, and
geosphere–biosphere feedbacks) cannot explain the uniqueness of the
event, some others (such as Cambrian substrate revolution, predator–prey
pressure, evolution of zooplankton, and roughening of fitness
landscapes) fall into the trap of the chicken-and-egg problem because of
considering the consequence as a cause. Expansion of ecosystem
engineering in the early Cambrian might also be caused by the Cambrian
Explosion. However, ecosystem engineering associated with Ediacaran
ecosystems is likely a pivotal ecological prerequisite for the later
ecological success of bilaterian clades, particularly the engineering
effect by Ediacaran sponges that ventilated seawater by sponge pumping
and removing organic material from the water column. However, the
ecological abundance of Ediacaran sponges needs to be further
investigated. Finally, a working plan is proposed for future research.
For palaeontologists, searching for ancestors of Early Cambrian faunas
is crucial to testify the earlier divergence of bilaterian lineages.
Environmentally, precise values on the oxygen level and seawater
composition are required during the Ediacaran–Cambrian transition. A long standing issue in macroevolution in the
Cambrian Explosion has been puzzling palaeontologists and evolutionary
biologists since 1830, “William Buckland knew about it, Charles Darwin
characteristically agonised over it, and still we do not understand it”
(Conway Morris, 2000, p. 4426). Essentially all of the body plans, about
half of the extant animal phyla, is generally accepted to have made
their first appearance in the fossil record within a few 10s of millions
of years during the Early Cambrian (Valentine, 2002; Erwin et
al., 2011). The Cambrian
Explosion has, therefore, been widely realised as the most significant
evolutionary event in the history of life on Earth (Conway Morris, 2006;
Marshall, 2006; Shu, 2008; Maruyama et
al., 2013; Santosh et
al., 2013). The first appearances of animal fossils are
obviously diachronous, though relatively abrupt in a short period of
time during the Late Ediacaran to Early Cambrian (Shu et
al., 2014 in this issue). The
fossil record of basal animals, such as sponges and cnidarians, is
extended to a period of time that is significantly earlier than the
opening of the Cambrian. Therefore, it seems the Cambrian Explosion
itself appears to represent the arrival of bilaterians (Budd, 2008). It
was revealed by early analysis that the divergence of bilaterian clades
was episodic: Lophotrochozoans were diversified during the
Terrenenuvian Epic, mainly in the Fortunian Age; Ecdysozoans diversified at the beginning of
Cambrian stage 3, in spite of the fact that their fossils can be traced
back to Terreneneuvian Epoch, e.g. traces of arthropods and grasping
spines of chaetognaths; Deuterostome divergence began from the middle of
the Cambrian stage 3 (Shu, 2008; Shu et
al., 2009; Maloof et
al., 2010; Kouchinsky et
al., 2012), though the
appearance of the remains and traces of bilaterian animals has remained
abrupt (Erwin et al., 2011;
Shu et al., 2014-in this
issue). It appears that the faunas from the Cambrian
indicate that diverse animal phyla were already established in the Early
Cambrian and provided a large amount of information concerning the
diversity, disparity and morphological complexity of animal life at that
time (Nielsen, 2001). Unfortunately, not much is preserved in the fossil
record about the origin of animal phyla because the early ancestors
might have been small and/or soft-bodied, for which there was little or
no preservation and/or recognisable potential. According to Zhang et al.
it is likely, therefore, that a period of animal evolution occurred
prior to the Cambrian Explosion that is observed in the Early Cambrian.
Studies of molecular diversification and comparative developmental data
also indicate this (Wray et al.,
1996; Bromham et al., 1998;
Aris-Brosou & Yang, 2003; Peterson et
al., 2004, 2008; Blair, 2009,
evolution of transport proteins for oxygen (Decker & van 2011), and
phylogenetic analyses of Cambrian and biogeography (Fortey et
al., 1996; Lieberman, 2002),
which suggests the major animal clades diverged many 10s of millions of
years prior to their first appearance in the fossil record. The paucity
of bilaterian clade fossils during the Ediacaran challenges unavoidably
any inference on earlier divergence of bilaterian phyla.
Provisionally, the conundrum of the Cambrian Explosion appears to have
been solved by the combination of palaeontological and molecular data,
with at least the stem lineages of the major animal clades diverging
during the late Cryogenian to Early Ediacaran, and becoming ecologically
important in the Late Ediacaran to Early Cambrian (Erwin et
al., 2011). A long history of
cryptic evolution is implied that is not present in the fossil record.
The Cambrian Explosion is, therefore, largely an ecological phenomenon,
including the increasing body size and morphological complexity, as well
as widespread biomineralisation among animals.
The exact causality has yet to be established,
though many hypotheses have been proposed as to what triggered the
Cambrian Explosion, that range from environmental, developmental to
ecological causes. A joint project by Chinese and Japanese scientists
have worked on palaeontological and environmental issues since 2005, and
their preliminary progress has been published in a special issue of
Gondwana Research 14 (1-2) 2008 and this volume. This cooperative
project was planned to be continued in the next 5 years. Therefore, in
this paper Zhang et al. give
a review on the available triggering mechanisms of the Cambrian
Explosion in order to identify the shortcomings of each hypothesis, in
particular with palaeontological data, and finally explicate their
future research on this topic.
2. Hypothetical triggers According to Zhang et
al. it seems obvious that
there must have been a change in something, or a critical level that was
favourable for building large, complex bodies and the construction of
hard skeletal material. The hypotheses of such evolutionary triggers is
able to be split into extrinsic or external environmental factors, such
as tectonic settings, climatic conditions, availability of nutrients,
level of oxygen content, oceanic conditions, nutrient variability and
biotic factors that are intrinsic or internal, genetical and
developmental innovations, and ecological causes.
Environmental triggers According to Zhang et
al. of the many triggers that
have been proposed the change in oxygenation and the composition change
of seawater are the most relevant factors. A number of others, such the
tectonic background and deglaciation of Snowball Earth, merely provide
mechanisms of the increase in the level of oxygen and the fluctuation of
seawater chemistry. There are, of course, other environmental factors,
e.g. the level of atmospheric CO2, pH of seawater, and trace
metals, that are of potential importance for the evolution of animals,
though the lack of data that are specifically relevant to the Cambrian
Explosion does not allow a further discussion of these issues.
Tectonic backdrop The assembly of Gondwanaland is viewed more or
less as a polyphase accretion of what were previously disparate blocks,
which was accomplished along 3 major orogenic belts, the East African
Orogen (750-620 Ma), the Brasilina-Damara Orogen (630-520 Ma), and the
Kuungan Orogen (570-530 Ma) (Li & Powell, 2001; Meert, 2003; Rogers &
Santosh, 2004; Campbell & Squire, 2010; Meert, 2011). The
Brasilina-Damara Orogen (630-520 Ma), and the Kuungan Orogen (570-530
Ma) took place just prior to and during the Cambrian Explosion. It is
suggested by more recent palaeomagnetic and geochronological that the
closure of the Clymene Ocean occurred in the Early Cambrian (Tohver et
al., 2010, 2012). The Iapetus
Ocean opened at around the same time (Grunow et
al., 1996), which was
followed by the Mawson and Brasiliano Oceans (Meert & Lieberman, 2008).
Pannotia, the hypothetical short-lived supercontinent, formed at the
latest Neoproterozoic, just prior to the Cambrian radiations (Dalziel,
1997), was comprised of Gondwana and Laurentia adjacent to Amazonia (Li
& Powell, 2001). Regardless of the exact model of Gondwana assembly and
the debate on the existence on Pannotia, the formations of the
supercontinents was just prior to, or nearly synchronous with, the
Cambrian Explosion. According to Zhang et
al. it was reasonable,
therefore, to link biological changes that occurred at this time with
tectonic events that resulted in the assembly of supercontinents and
oceans opening. Within Gondwana, each collisional orogen extended for
several kilometres (Dalziel, 1997; Campbell & Squire, 2020). When the
huge trans-Gondwana Mountains were uplifted and eroded it would have had
a significant influence on the composition of seawater and the global
climate, which provided nutrients, such as phosphorus, to the oceanic
realm (Brasier & Lindsay, 2001; Squire et
al., 2006) and increased the
rate of burial of organic carbon that is believed to have triggered a
critical rise in the availability of atmospheric oxygen (Knoll & Walter,
1992; Campbell & Allen, 2008; Campbell & Squire, 2010). Both oxygen and a nutrient like phosphorus are,
of course, necessary for animal life. An increase in primary production
may result from the enrichment of phosphorus or other nutrients that are
limiting (Howarth, 1988; Tyrrell, 1999), though neither of these linked
directly with an increase in biodiversity (number of species) nor with
widespread disparity (the number of phyla) as was embodied by the
Cambrian Explosion. Contrary to this, there may be a decrease in
biodiversity when the continents are assembling, according to the model
of Valentine & Morris (1970). Also, the worldwide deposition of
phosphate during the Ediacaran-Cambrian might aid in the preservation of
fossils at this time, such as the phosphatised biotas from the Ediacaran
of South China and the Terreneuvian small shelly fossils around the
world (Meert & Lieberman, 2008). It the case of oxygen it is more
complicated (see below). Amalgamation and fragmentation are not unique
to Precambrian-Cambrian transition (Dalziel, 1997), and there is no
evidence that the oceanic concentration of phosphorus in the
Neoproterozoic is insufficient for the diversification of animals. The
assembly of Gondwana, therefore, provides a tectonic backup (Meert &
Lieberman, 2008), and it alone cannot be a driving force in the Cambrian
Explosion. A study of the dissolved marine concentrations of phosphate
suggested, on the contrary, the concentrations of phosphate were
significantly higher than the levels in the Cambrian (Planavsky et
al., 2010).
Snowball Earth – the aftermath It is indicated by increasing evidence that the
climate of the Earth was extremely cold during the Neoproterozoic with
the result that there were 2 or 3 episodes of global glaciations with
ice sheets extending to equatorial latitudes, i.e. Snowball Earth (Kirschvink,
1992; Hoffman et al., 1998;
Hoffman & Schrag, 2002). The duration of Snowball Earth has been
constrained to between780 Ma and 635 Ma, during which there were 2
warmer periods of interglacials. For the biological world the Snowball
Earth was catastrophic, causing biological productivity in the ocean
surface waters to collapse for millions of years (Hoffman et
al., 1998), though it is
suggested by Zhang et al. to
possibly have had a role in triggering the evolutionary bursts that
followed (Fedonkin, 2003). Many researchers have argued, however, that
ultimately it resulted in significant bursts of biological evolution,
such as the Ediacaran radiation and Cambrian Explosion (Kirschvink,
1992; Hoffman et al., 1998;
Maruyama & Santosh, 2008; Planavsky et
al., 2020; Papineau, 2010).
Again, Snowball Earth is tied to biological evolution by the nutrient
phosphorus and oxygen. Erosion and high rates of chemical weathering
during the retreat of the ice sheets has been argued to have led to
higher delivery of riverine phosphorus to seawater, and this would have
increased rates of primary productivity and burial of carbon that
produced significant quantities of atmospheric oxygen. This resulted in
the accumulation of atmospheric oxygen that paved the way to macroscopic
air-breathing organisms and led to the emergence of animals in the
Neoproterozoic-Cambrian interval (Planavsky et
al., 2020; Papineau, 2010). The harsh and fluctuating abiotic conditions, on
the other hand, such as the Snowball Earth, may have temporarily
increased the degree of altruism that was adaptive, which promoted an
increasing degree of terminal (irreversible) differentiation. It is
believed that this is of particular relevance to multicellularity of
animals as a result of irreversible differentiation, (which is highly
altruistic as it imposes a high fitness cost of the individual cell) is
more prevalent than in other multicellular eukaryotes (Boyler et
al., 2007). Also, Snowball
Earth may increase genetic diversity expressed by animals as a result of
the effect of extreme climatic stress on heat shock protein 90 (HSP90),
which results in mutant signal transduction proteins, that were
pre-existing, and developmental pathways were expressed in animals
(Baker, 2006). It seems that up to now the aftermath of the Snowball
Earth on biological radiation become increasingly evident, though it
remains hard to see how fundamentally new levels of developmental and
morphological organisation could result from a cold catastrophe
(Marshall, 2006). The Snowball Earth ended at 635 Ma, however,
about 100 Myr before the beginning of the Cambrian Explosion. During
this 100 Myr what happened with the surface environment of the Earth may
be a crucial point. From the Ediacaran to Early Cambrian the steep rise
in seawater 87Sr/86Sr is attributed to elevated
weathering rates, which could have led to increased availability of
nutrients, organic burial and to the further oxygenation of the surface
environments (e.g. Shields, 2007). Following the end of Snowball Earth
the evolution of complex macroscopic multicellular organisms, as
revealed by the Early Ediacaran Lantian biota (Yuan et
al., 2011) that is resent
immediately above the Marinoan glaciation of age, and many more Late
Ediacaran macrobiotas following the Gaskiers (McCall, 2006; Fedonkin et
al., 2007). In terms of the
Cambrian Explosion it is, possibly, the continued post-glacial rise of
atmospheric oxygen concentration that paved the way for the later animal
diversification during the Early Cambrian.
True polar wander Palaeomagnetic data that imply anomalously high
rates of apparent plate motion that characterise the Ediacaran-Cambrian
interval, the rotation or translation rates for each palaeocontinents
equivalent to or exceeding 10-20 cm/year (See Mitchell et
al., 2011, and references),
which is in marked contrast to continental movement rates of a few
centimetres/year that are more typical (Meert et
al., 1993). The true polar
wander (TPW) was, therefore, invoked in order to explain the rapid
motion of the palaeocontinents during the transition from the Ediacaran
to the Cambrian (Kirschvink et al.,
1997; Evans, 1998, 2003). An episode of true polar wander which occurred
during the Cambrian Explosion of animal life has been suggested
(Mitchell et al., 2010). It
has been argued (Kirschvink et al.,
1997) that wholesale rotation of the lithosphere was likely to have
brought about major changes in the circulation of the ocean and,
therefore, regional climates, and repeated reorganisations of the global
climatic patterns during a TPW event which would have fragmented any
large scale ecosystems that had been established, and so generating
smaller populations that were more isolated, and this would lead to a
higher rates of evolutionary branching among existing groups. A more
explicit relationship between a succession of TPW events and the
Cambrian Explosion was proposed (Kirschvink & Raub, 2003). According to
the argument:
1)
Tropical continental margins and shelf slopes which form as a result of
the fragmentation of Rodinia accumulated huge quantities of organic
carbon during the Late Neoproterozoic ;
2)
An initial phase of Ediacaran TPW moved these deposits to high
latitudes, where these, as well as other additional added organic
material were trapped as methane hydrate or permafrost and stabilised
until the geothermal gradient moved them out of the clathrate stability
field;
3)
A burst of TPW brought these deposits back to the tropics, where they
gradually warmed and released to the atmosphere, and therefore induced
pulses of global warming. As the surface temperature of the Earth
correlates powerfully with biodiversity, it was suggested (Kirschvink &
Raub, 2003) that global warming could have been the driver of the
Cambrian explosion that was forced by the decomposition of methane
clathrate, which had been induced by an inertial interchange true polar
wander event. This hypothesis, as had been noted (Marshall, 2006), did
not offer an explanation as to why an increase in biodiversity, per se,
should have led to new levels of disparity. Also, the global warming
that occurred during the Early Cambrian is not currently supported by
geological or geomechanical data, regardless of the likelihood that the
methane hydrates were brought from high latitudes to the tropical
latitudes without collapse. Also, diversification resulting from
physical stress might be suppressed by rapid fluctuations of climate.
The finding that the TPW hypothesis contradicts phylogenetic analyses of
biogeography from the Early Cambrian is the other problem. Well
established endemism is shown by faunas of the Early Cambrian
(Lieberman, 2002), which is not likely to have arisen in the rapidly
changing geography that is required by the TPW hypothesis (Meert &
Lieberman, 2004, 2008).
Seawater composition Salinity, an environmental factor, is of
considerable importance for organisms, as it controls and limits the
nature of biological activity. The solubility of atmospheric gases,
especially oxygen, is strongly affected by salinity. Therefore, the
history of the salinity of seawater is crucial for the evolution of
life. It has been estimated that the initial salinity of the oceans was
1.5-2.0 times (52.5-70‰) that of the value of the modern ocean, which is
~35‰, or possibly higher, the salinity declined significantly only after
the latest Neoproterozoic when giant evaporite basins sequestered huge
amounts of salt and brine (Knauth, 1998, 2005). Salinity of 50‰ is the
limit above which most forms of macroscopic life, which includes
metazoans, cannot tolerate salinities as a result of the high osmolarity
in hypersaline conditions that can be deleterious to cells as water is
lost to the external medium until the achievement of osmotic
equilibrium. It has been speculated (Knauth, 1998, 2005) that the
delayed decline in salinity of the ocean may have been a factor in the
Cambrian explosion of life. The modelling of ocean salinity, which was
developed by using the maximal and minimal estimates of the volumes of
existing evaporite deposits, and the use of constant and declining
volumes of ocean water through the Phanerozoic, is also suggestive of a
major decline in salinity from the Neoproterozoic to the Cambrian (Hay
et al., 2006). The salt
deposits of the Hormuz region cannot, however, be dated precisely, and
the decline that is apparent prior to the Cambrian may be an artefact of
the lack of better information (Hay et
al., 2006). Also, Zhang et
al. suggest that the high
salinity hypothesis is challenged by the abundant, diverse eukaryotic
life in the Neoproterozoic. It is indicated, on the other hand, by analyses
of primary fluid inclusions from marine halites that date to the
terminal Proterozoic (about 544 Ma) and Early Cambrian (about 515 Ma)
that CA2+ concentrations in seawater increased ~3-fold during
the Early Cambrian (Brennan et al.,
2004; Berner, 2004). Also, it was indicated by an experimental study of
sponge cells that the concentration of CA2+ could increase
binding forces between their calcium-dependant cell adhesion molecules.
Zhang et al. suggest that
this, as well as the advent of self-/non-self-recognition systems, that
are assumed to have evolved gradually prior to the Cambrian, could have
made possible the rise of the animals. It was proposed, therefore, that
the coincidence in time of primitive animals that were endowed with
self-/non-self-recognition and a surge in seawater calcium during the
Early Cambrian triggered the Cambrian explosion (Fernàndez-Busquets et
al., 2009). Rapid increase in
Ca2+ in the seas of the Early Cambrian was accompanied by a
decrease in concentrations of sulphate by more than 2-fold (Brennan et
al., 2004). This conflicts
with the palaeontological data that the intensity of bioturbation
increased significantly (McIlroy & Logan, 1999) and matgrounds evolved
into mixgrounds (Seilacher, 2007) during the Early Cambrian as a result
of sediment mixing by bioturbating organisms resulting in a severalfold
increase in sulphate concentration in seawater (Canfield & Farquhar,
2009). According to Zhang et
al. as they were writing this
paper a seemingly new hypothesis links the Great Unconformity, which
separates continental crystalline basement rock dating to the
Precambrian from the much younger Cambrian shallow marine sedimentary
deposits in several palaeocontinents, to the Cambrian explosion (Peters
& Gaines, 2012). According to this hypothesis, the formation of the
Great Unconformity involved the widespread denudation of continents
which was followed by extensive reworking of soil, regolith and basement
rock, and the enhanced chemical weathering of continental crust during
the first marine transgression on a continental scale of the
Phanerozoic; these processes would have affected the chemistry of the
seawater, such as increases in the concentrations of Ca2+ in
seawater, and thus triggered biomineralisation in multiple clades, which
thereby promoted the Cambrian explosion of marine animals (Peters &
Gaines, 2012). A possible mechanism for the increase in concentrations
of Ca2+ was provided by this hypothesis. Neither the new data
of the concentrations of Ca2+ is offered, nor is a new
explanation of why an increase in the concentration of Ca2+
in seawater, per se, should have led to such a major diversification or
biomineralisation among metazoans.
Rise of Oxygen levels It has been proposed for a long time that
atmospheric oxygen was a prerequisite for the appearance of animal life
in the fossil record (Nursall, 1959; Canfield & Teske, 1996). Increasing
levels of atmospheric oxygen have, therefore, been implicated in the
marine faunas in the late Precambrian-Cambrian (Cloud, 1976; Runnegar,
1982a, b; McMenamin & McMenamin, 1990: Gilbert, 1996; Canfield et
al., 2007). Oxygen is
essential for maintaining many metabolic and physiological processes in
metazoans, which allows for the effective aerobic respiration, mobility,
increase in body size, skeletonisation, and collagen synthesis (in
metazoans, holding cells together) and Cholesterin, which makes the cell
membranes more rigid. Accumulative levels of toxic reactive species of
oxygen (ROS), however, can be dangerous for all organisms. Animals have
evolved sophisticated mechanisms in order to cope with ROS which they
used for defence (Decker & van Holde, 2011). Of the defensive strategies
used 2 are mentioned here.
1)
Cell aggregation, with surface cells protecting interior members from
ROS. It is likely, therefore, that the protection against ROS is one of
the driving forces in evolution that drove cells towards
multicellularity in which oxygen plays a part that is an even larger
role in evolution than has been previously been suspected.
2)
Melanisation, which Zhang et al.
have suggested may have occurred on the dermal cells of primitive
metazoans. It could offer protection and could possibly have formed a
defensive cuticle against predation. The way was open for almost infinite pathways of
development of special organs, functions and diversity of animals
(Decker & van Holde, 2011), once the principle of specialisation of
cells was established. Also, it was proposed that the prevailing
concentrations atmospheric oxygen may have had a role in the evolution
of mechanisms for sensing oxygen (Tayler & McElwain, 2010), which is
mediated by hypoxia inducible factor (HIF) and acted by prolyl
hydroxylases (PHDs). It has been found that the HIF/PHD system was not
present in choanoflagellates or other protists, which suggests that it
is unique to metazoans (Leonarz et
al., 2011; Rytkönen & Storz,
2011). It is also possible that increasing levels of oxygen in the
atmosphere and oceans could have had an indirect effect on biological
innovation by increasing the availability of trace elements that are
bioessential (Ambar & Knoll, 2002).
The major problem with the suggestion of rising levels being an
environmental trigger for the Cambrian explosion is the lack of precise
values of oxygen levels prior to and during the diversification of
animal phyla. Over the last 25 years geochemical evidence of the rising
levels of oxygen during the Late Neoproterozoic has been accumulating,
though it remains difficult to distinguish atmospheric from oceanic and
global from local redox changes (Shields-Zhou & Och, 2011). It was
argued (Canfield & Teske, 1996) that according to sulphur isotope
discrimination values the level of oxygen rose above 5% of the present
atmospheric level (PAL) during the Precambrian-Cambrian transition.
Though it is still difficult to determine if the increase in
Δ34S
is related to higher oxygen levels or instead to intensive bioturbation
(Canfield & Farquhar, 2009). It is shown by the iron content of
sediments that shallow waters were typically well oxygenated throughout
the later Neoproterozoic (<742 ± 6 Ma) (Canfield et
al., 2008), much earlier than
the Cambrian explosion, whereas the deep ocean was anoxic and
ferruginous prior to and during the Gaskiers Glaciation 580 Ma and that
after this glaciation, it became oxic (Canfield et
al., 2007; Liet et
al., 2010). A very late
oxygenation of deep seas was suggested by a recent study, since Cambrian
Stage 3 (Wang et al., 2012).
Oxygen levels higher than 15% of PAL are required to oxygenate deep sea
(Canfield et al., 2007),
though persistent anoxia in the oceans in the Neoproterozoic is argued
to require levels of oxygen below 40% of PAL (Canfield et
al., 2005; Kump, 2008). The
atmospheric oxygen level was, therefore, constrained between 15% and 40%
of PAL by the Late Ediacaran. Such an estimation that is based on
uniformitarian principles was, however, questioned (Butterfield, 2009).
He argued that the oceans of the Precambrian, which were stratified, had
a turbid anoxic water column dominated by cyanobacteria, differed from
their counterparts of the Phanerozoic, that were well-mixed, with a
clear-water system that was dominated by eukaryotic algae), and
therefore deep sea geochemical signatures were not able to be used as a
direct proxy for atmospheric oxygen levels under such no
nonuniformitarian conditions (Butterfield, 2009). Erosion of the supermountains of Gondwana was
suggested to have formed more free oxygen than earlier supercontinent
amalgamations as a result of the maximum height of the Gondwanan
supermountains being greater (Campbell & Squire, 2010). In the Early
Cambrian the advent of large, mobile and skeletal animals may have been
contemporaneous with the increased oxygenation of the surface
environment (Shields-Zhou & Och, 2011), though there is a geochemical
study that shows that the oxygen content declined significantly during
the Early Cambrian (Komiya et al.,
2008). The minimum oxygen requirements for metazoans to support their
aerobic metabolism, on the other hand, are constrained loosely. It has
been suggested (Catling et al.,
2005) that there was quite a broad range from 5% to 50% PAL. The
absolute minimum oxygen content that can support metazoans varies
between species, and is probably impossible to know for the first
animals of diverse metazoan phyla. Extant marine infaunal and epifaunal
metazoans are capable of accommodating dissolved oxygen concentration
decreasing to the 10s of micromolar. They correspond to ~25% PAL, if
these concentrations are extrapolated to equilibrium of the atmosphere
with surface water that is cold and well-mixed. However, it is not known
when oxygen rose above this level, and geochemical evidence is not known
of for a stepwise rise of oxygen levels that correspond to the episodic
appearances of bilaterian clades during the Early Cambrian. Also, it
needs to be known when a given level of oxygen might select for a given
type of physiology or morphology and whether such effects might affect
morphological origins, ancestral form, etc. It is difficult, therefore,
to determine whether oxygen is a key threshold that must be crossed for
the Cambrian explosion to occur. According to Zhang et
al. they are sure that oxygen
has an important role in the development of complex, multicellular
organisms. Though the widespread distribution of anaerobic chemistry and
anaerobic mitochondria, mitochondria that are not dependant on the final
mediator of a transfer of an electron to O2, in most
eukaryotic supergroups, which includes the Metazoa, combined with the
presence of enzymes that have clear evolutionary links to an anaerobic
past, suggests that the ancestors of eukaryotes evolved in a past that
was more anoxic (See van der Giezen and Lenton, 2012 and references
therein). It has been found that many extant animals are facultative
anaerobes, having the ability to live in hypoxic or anoxic conditions
(Budd, 2008). Some loriciferans have been found living in conditions
that are permanently anoxic, such as sediments (Danovaro et
al., 2010). The claim that
increasing levels of oxygen were a prerequisite for the evolution of
animals is undermined somewhat by the widespread anaerobic pathways
across metazoan phylogeny (Mentel & Martin, 2010).
Developmental genes and molecular clock Large gene regulatory networks (GRNs) control the
development of the animal body plan, and evolution of the body plan
depends on the architecture of developmental GRNs over deep time
(Davidson & Erwin, 2006); Davidson, 2010). The composition and structural organisation of
the GRNs and, therefore, their functions and evolutionary changes are
fundamental genetic issues to understand their development and
evolutionary and evolution of animals (Carroll et
al., 2001; Davidson & Erwin,
2006, 2009; Davidson, 2010). In this context, with regard to the
Cambrian explosion, the key questions are which developmental genes
define ancestral bilaterians and whether the genes responsible for
building morphological complexity of bilaterian clades were in place
prior to and during the Cambrian explosion. What accounts for the great
differences between major metazoan linages and the long prod of stasis
in phylum- and class-level body plans since the Early Cambrians, if the
development systems of bilaterians were established prior to their
divergence in the Early Cambrian?
Deep or shallow divergence It has been found that the developmental genes
were established prior to the divergence of bilaterian clades. The
finding that many developmental bilaterian genes are present in simpler
organisms that are morphologically simple reinforces the suggestion that
the original role of these genes and regulatory networks was in the
formation of specialised cell types in specific body regions, though not
necessarily in the production of complex morphology (Erwin & Davidson,
2002). The advent of developmental genes was, therefore, a component
that was a necessary, though not sufficient, component of the Cambrian
radiation of bilaterian lineages (Erwin et
al., 2011). What then is the
genetic basis that underlies the morphological diversification of
bilaterian forms? According to Zhang et
al. the key issue is in the
timing and level of gene expression. It is suggested by the earliest use of
haemoglobins to arrive at molecular estimates that the initial radiation
of the animal phyla occurred at least 900 Ma – 1.0 Ga (Runnegar, 1982c).
Subsequent estimates of the times of metazoan divergence that were based
on molecular clock vary as new studies have been reported, ranging from
several hundred million years prior to the Cambrian explosion to close
to the base of the Cambrian (see Blair, 2009 for a complete review).
According to Zhang et al. it
was believed that some of the younger dates had been affected by severe
methodical biases (Blair, 2009). It was suggested by the most recent
dating study (Erwin et al.,
2011) that the common ancestor of all metazoans arose about 800 Ma,, the
split between the deuterostomes and protostomes occurred at around 680
Ma, and stem lineages leading to most extant bilaterian clades had
evolved by the close of the Ediacaran. Most crown group bilaterian phyla
were estimated, however, to have diverged during the later Ediacaran
through to the Cambrian, which is largely consistent with the fossil
record (Erwin et al., 2011).
It is also indicated by the phylogenetic distribution of different
oxygen transport proteins (OTPs) (haemocyanin, globin and haemerythrin),
which is a prerequisite for the development of shells and hard carapace,
also indicates that there was an early divergence of bilaterian clades,
with OPTs evolving before the appearance of shelly fossils, though
following the divergence of phyla (Decker & van Holde, 2011). The
molluscan haemocyanin evolved 700 Ma, or possibly earlier, and it was
estimated that the origin of arthropod haemocyanin was between 600 and
700 Ma. The deep divergence of bilaterian phyla, at least
in stem lineages, has been supported by a majority of molecular clock
studies, though it is suffering from a couple of challenges:
1)
The cold climate during the Late Cryogenian was not favourable for the
diversification of animals,
2)
It remains unclear whether oxygen content was sufficiently high for the
bilaterians by the Cryogenian. The paucity of fossils of bilaterians in this
period is the greatest challenge, with bilaterians being completely
absent prior to the Gaskiers Glaciation. Possible animal embryo fossils
from the Doushantou Formation, which dated to the Ediacaran, have been
reinterpreted recently as non-metazoan Holozoa (Butterfield, 2011;
Huldtgren et al., 2011).
Zhang et al. suggest that
most putative metazoans from biotas of Ediacaran type are very likely to
be within the scope of cnidarian-grade organisms below the split of the
protostomes and the deuterostomes (Erwin, 2009), with the exception of
Kimberellomorpha, which could possibly be molluscans commonly associated
with radiating trace fossils (Fedonkin, 2003). It has been accepted
widely that trace fossils are strong evidence for the presence of
bilaterian animals in the Ediacaran, whereas the bilaterian nature of
early trace fossils has been questioned by the finding that large
amoeboid protists leave macroscopic traces at the bottom of the deep
ocean (Matz et al., 2008).
The earliest putative evidence of bioturbation by bilaterians that is
known of dates to ~555 Ma. The Kimberellomorpha is the only body fossil
that is relatively convincing, which is coeval with the earliest-known
bioturbation. However, according to Zhang et
al. a small group makes
neither diversity nor disparity. That early bilaterians were small,
soft-bodied animals, and therefore were not often fossilised, is the
explanation that has been most commonly attempted, though microscopic
fossils of protists and prokaryotes have been recognised as early as the
Archaean and Lagerstätten of macroscopic soft-bodied fossils have been
known of that dated to the Ediacaran. It is also possible that the early
bilaterians were not significant ecologically, and were therefore quite
rare. The deep divergence of bilaterians can only be testified to by the
discovery of diverse body fossils prior to and during the Ediacaran. Many of the genes and subcircuits of the GRNs
underlying the building of body plans may, alternatively, remained
untapped over millions of years. Phyla of basal metazoans diverged
morphologically during the Late Ediacaran and the phyla of bilaterians
diverged in the Early Cambrian, as indicated in the fossil record (Shu
et al., 2014-in this issue).
The genetic makeup of Urbilateria is complex, though they were small and
morphologically simple. A minority of molecular studies do, however,
support the shallow divergence model. Possibly, oxygen levels as well as
other environmental factors, such as the concentration of Ca2+
in seawater, until they crossed a critical threshold in the Early
Cambrian and favourable ecosystems were well established, which
therefore caused a widespread biomineralisation and increase in
morphological complexity, body size, diversity, and disparity among
bilaterian lineages. This model is, approximately, reconciled by
palaeontological data, though it is still rare to find unequivocal body
fossils of metazoans in deposits from the Ediacaran. This view is also
challenged by animals from the Early Cambrian that were considerably
diverse and had morphology that was complex. According to Zhang et
al. it is hard to imagine all
of these arising from a single ancestral form in the brief period from
about 550-520 Ma.
The bilaterian developmental GRNs were
established prior to the diversification of body plans. Once
Kimberella was accepted
as a true bilaterian, then the establishment of developmental systems
predated the Cambrian explosion by at least 10 Ma. Though the
developmental system, which enabled the divergence of the bilaterian
body plans, is a prerequisite, it is not sufficient for the Cambrian
explosion (Marshall, 2006; Erwin et
al., 2011). If the invention
of toolkit genes per se was not the trigger for the Cambrian explosion,
then what was? According to Zhang et
al. it is being increasingly
appreciated that the Cambrian explosion was an ecological phenomenon and
environmental triggers indispensable.
Ecological causes The case for ecological explanations is now
increasing, since environmental changes at the close of the Proterozoic
and opening of the Phanerozoic form the essential backdrop of the
Cambrian explosion, and it is very likely the genetic complexity was
established long before the construction of an ecosystem that was
dominated by metazoans (see discussion above), (Carroll, 2005; Conway
Morris, 2006; Budd, 2008; Erwin et
al., 2011; Erwin & Tweedt,
2012). The Cambrian explosion is seen by many as an ecological
phenomenon, which consisted to a large extent on a cascade of knock-on
events that emerged from multicellularity and mobility (E.g. Budd,
2008). Following the appearance of the first animals the interactions
between different types of organisms as well as with environments would
lead to ecologies that more were complex. The complex ecologies were
subject to continuing expansion and feedback, and this resulted in the
evolution of metazoan-dominated ecosystems since the Early Cambrian
(Conway Morris, 2006). Of the many ecological hypotheses that have been
proposed as drivers, none of them is able to satisfy geological,
palaeontological and molecular records. While some theories are
well-suited to explaining why there was a rapid increase in diversity
and disparity, they fail to explain why the “explosion” happened when it
did (Marshall, 2006). Some have even considered consequences as causes,
e.g. increase of bioturbation and the advent of macrophageous predators,
thereby falling into the trap chicken-egg problem.
Extinction of Ediacaran biotas The most significant episodes of adaptive
radiation in the history of metazoan evolution have often been preceded
by mass extinction events. Ecologically dominant species can be removed
by extinction events and thereby makes the resources the removed species
had previously utilised making them available to surviving species.
Following mass extinction events many clades diversified rapidly, the
most famous case being the diversification of many mammal and bird
clades following the end Cretaceous extinction event (Losos, 2010).
Therefore extinction of Ediacaran macrobiotas (Narbonne, 2005) and
Cloudina-Namacalathus
assemblage (Amthor, 2003) could open up new possibilities for metazoans,
as was the case with the dinosaurs, which opened the way for mammal and
bird clades (Knoll & Carroll, 1999; Amthor et
al., 2003). However, as has
been noted (Marshall et al., 2006; Erwin et al., 2011), standard models
of radiation involved diversification for clades that were pre-existing,
e.g. the evolution of land vertebrates, and is not able to explain the
polyphyletic nature, morphological and ecological breadth, or the
extended duration of the Cambrian explosion.
Cropping This ecological theory was developed by Stanley
(1973, 1976). It argued that in terms of diversification, pre-Cambrian
systems were self-limiting, because systems in the absence of cropping
by herbivores; when advanced heterotrophy arose it would initiate
systems of diversification that were self-propagating; therefore, the
almost simultaneous origin and explosive radiations of heterotrophic
protists, metaphytes and metazoans were inevitable. The increases in
diversity under the influence of cropping could be explained by this
theory, though it was not directed against the uniqueness of the
Cambrian explosion in terms of timing and nature. The Cambrian explosion
is far more than the increase in biodiversity. Also, the evolution of
heterotrophic protists began much earlier (Porter, 2011), and
herbivorous metazoan fossils, such as
Kimberella were present
in biotas of the Ediacaran about 555 Ma. The question is what delayed
the occurrence of bilaterian clades.
Phytoplankton-zooplankton diversification In modern marine ecosystems the vast majority of
production is comprised of phytoplankton. Very few large animals are
capable of grazing on even relatively large net phytoplankton (2-200
μm). The microscopic world of marine primary productivity is linked to
the macroscopic world of large animals by zooplankton which repackage
unicellular phytoplankton as particles that are rich in nutrients some
10-100 times as large. Repackaging by herbivorous mezozooplankton
(0.2-20 mm) represents a key link in marine metazoan food chains
(Butterfield, 1997, 2001). In the Early Cambrian the phytoplankton (acritach)
underwent a major diversification, which is in marked parallel with the
Cambrian radiation of metazoans. Oceans would have been dominated by
picoplankton (0.2-μm) prior to the evolution of planktic metazoans
(Butterfield, 2009). The diversification of zooplankton might have been
stimulated by this sudden diversification, which in turn might stimulate
the evolution of large metazoans through food supply. The ecological
significance of zooplankton during the Early Cambrian is not clear,
however, because of the sparseness of zooplankton fossils of this age
(Butterfield, 1997, 2001; Vannier et
al., 2007). In addition, the
diversification of acritachs, as a component of the Cambrian explosion,
may also require a trigger. It has been suggested that the expansion of
animals into the water column in the Early Cambrian might have been
responsible for driving the evolution of large net-phytoplankton
(Butterfield, 2009). So a chicken-egg problem arises. Was the evolution
of zooplankton caused by the diversification of phytoplankton, which in
turn stimulated the radiation of animals? Alternatively, was the
increase of net-phytoplankton in size and diversity caused by the
ecological expansion of planktic metazoans? According to Zhang et
al., at this point they adopt
a simpler solution, considering the diversification of phytoplankton as
a consequence, rather than the cause of the Cambrian explosion.
Bioturbation The Cambrian explosion is manifested by explosive
radiation of bilaterian phyla in the Early Cambrian, as well as
substrate revolution during the Ediacaran-Cambrian transition, which was
the result of advents of burrowing bioturbation into the sediment depths
(Seilacher & Pflüger, 1994; Bottjer et
al., 2000). Therefore it was
assumed that bioturbation had a key role in the evolution of early
metazoan life (Meysman et al.,
2006). Microbial mats that were well-developed with poorly developed
vertically oriented bioturbation were characteristic of the seafloor of
the Ediacaran. The substrates that were mat-bound (aka matgrounds)
produced a stable, sharp water-sediment interface that had little
interaction with bottom water. The seafloor sediments of the Early
Cambrian, referred to as mixgrounds, were bioturbated heavily by
burrowing organisms, which resulted in sediment ventilation, and changes
in redox gradients. It was suggested (Bottjer et
al., 2000) that substrate
revolution had significant evolutionary and ecological impact. It was
assumed, accordingly, that benthic faunas needed to adapt to the
bioturbated mixgrounds that were newly emerging, thereby triggering the
Cambrian explosion (Thayer, 1979; Bottjer et
al., 2000; Meysman et
al., 2006). In contrast to
this, it was also suggested that the infaunal habits were a strategy to
escape the menace of predation that had evolved (Seilacher, 2007). Zhang
et al. suggest that the
effect of bioturbation would have been profound (Thayer, 1979; Meysman
et al., 2006; Erwin & Tweedt,
2012). However, infaunal bioturbators were required by the substrate
revolution to destruct the matgrounds, with the result that the chicken
and egg problem arises again (Levinto, 2001). The question then arises
was the Cambrian explosion caused by substrate revolution or did the
substrate revolution drive diversification of bilaterian clades,
including the bioturbators? Zhang et
al. suggest it is evident
that the advent of bilaterian bioturbators is a consequence rather than
a cause.
Advent of macrophageous predation The most commonly proposed ecological trigger of
the Cambrian explosion is the predator-prey pressure (Evans, 1912;
Hutchinson, 1961; Vermeij, 1990; Bengtson, 2002; Peterson et
al., 2005: Erwin et
al., 2011. A rapid increase
in body size by natural selection would result from the rise of
predation, and novel defensive mechanism such as biomineralisation to
form shells, or developing new structures or capabilities that allowed
movement into new habitats (Levinton, 2001; Erwin et
al., 2011). In the Middle
Neoproterozoic the rise of macrophageous single-celled organisms might
have led to the appearance of biomineralisation of skeletons in protists
about 750 Ma (Porter, 2011), though according to Zhang et
al. it did not trigger the
widespread biomineralisation in metazoans, most likely because animals
had not evolved by this time. It appears reasonable to assume the advent
of macrophagous predators near the end of the Neoproterozoic, therefore,
stimulated the spectacular radiation of animals (Peterson et
al., 2005), while the fossil
evidence for the presence of macrophagous predators is not certain.
Cloudina, a skeletonised
tubular fossil,
from
the Ediacaran, preserved boring holes which were considered to be
predatory traces (Bengtson & Yue, 1992; Bengtson, 2002; Hua et
al., 2003), though they are
also likely to have been nonpredatory in origin, as there are many
microbial borers that are capable of drilling holes. Body fossils that
are thought likely to be macrophagous predators first appeared in the
fossil record in the Early Cambrian. The first appearance of
protoconodonts in the Fortunian Stage (Kouchinsky et
al., 2012), were interpreted
as teeth and grasping spines of chaetognaths (Szaniawski, 2002), and
therefore represented the earliest known candidates for bilaterian
predators. The first appearance of definite large predators, such as
anomalocarids, was in Cambrian Stage 3, about 520 Ma, which postdates
significantly the biomineralisation of the metazoans. There is currently
no known evidence for macrophagous predators earlier than the Early
Cambrian. Again, the advent of macrophagous predators still requires a
trigger. It is reasonable to assume that their invasion lagged, as
predators stand high above the primary producers in trophic level. It is
apparent that predator-prey relationships are an essential component of
the Cambrian explosion, though not an ecological trigger.
Roughening of fitness landscapes The concept of fitness landscapes from genetics
to the morphogenesis is applied by this hypothesis (Marshall, 2003,
2006). It has been demonstrated by computer simulations that the
roughening of fitness landscapes could lead to increases in diversity
and disparity of land plants, and not necessarily requiring new
developmental genes (Niklas, 2004). The number of needs the organism
must satisfy controls the roughness (Niklas, 1994, 1997, 2004), rather
than the degree of interaction between the genes, as is the case of
genetic fitness landscapes (Kauffman, 1993). The needs of the organisms
are, however, often frustrated, which leads to conflicting solutions, so
that the overall design for an organism is not often achieved (the
principle of frustration) (Marshall, 2006). It is the degree of
frustration, therefore, that determines the roughness of the fitness
landscape (corresponding to diversity and disparity), i.e., as the
number of frustrated needs is increasing the fitness of the landscapes
are roughening. There were very few bilaterians in the biotas of the
Ediacaran, which shows fitness landscapes that are rather smooth
(Marshall, 2006). The faunas of the Early Cambrian, on the contrary,
contained about 20 bilaterian body plans, which show very rough
landscapes (Marshall, 2006). It was accordingly suggested (Marshall,
2006) that roughening was the primary driver of the Cambrian explosion.
This theory depicted nicely the Ediacaran-Cambrian evolutionary
transition, as well as providing a good explanation of the
diversification of body plans without invoking the invention of toolkit
genes. However, it invoked the ecological interactions between
organisms, especially the arms race (predation pressure) as a roughening
mechanism, which stated ”There were myriad predators to contend with,
and a myriad number of ways to avoid them, which in turn led to more
specialised ways of predation as different species developed different
avoidance strategies, etc.” This hypothesis therefore suffers from the
same problem as the predator-prey hypothesis (see above). Simply, in the
Early Cambrian the roughening of fitness landscapes is a consequence of
evolution.
Geosphere-biosphere feedbacks According to this hypothesis, this is based on an
Earth system model for the long-term carbon cycle by introducing 3
different types of biosphere: prokaryotes, eukaryotes, and complex
multicellular life, each of which is characterised by different global
temperature tolerance windows. The nonlinear feedback between
productivity that is temperature dependent of multicellular life and
their biotic enhancement of silicate weathering could lead to the drops
in temperature to the optimum value of 15oC for complex
multicellular life productivity by about 540 Ma. It was assumed,
therefore, that an earlier appearance of complex life could have been
initiated by the Neoproterozoic Snowball Earth events and that the
Cambrian explosion might have been mainly driven by extrinsic
environmental causes, i.e., cooling of the Earth that was gradual (von
Bloh et al., 2003). It
appears this hypothesis did not touch the essence of the Cambrian
explosion. Also, no geological evidence has been found of this cooling
event. There is at least 1 study which suggested that the surface
temperature of the Earth declined dramatically in the Paleoproterozoic
reaching values similar to those of the Phanerozoic by 1.2 Ga (Knauth,
2005).
Ecosystem engineering
This refers to the modification of the abiotic
environment by aspects that affect strongly other organisms by forming,
modifying or destroying the niches of other species (Jones et
al., 1994, 1997; Wright &
Jones, 2006; Erwin & Tweedt, 2012). Organisms that engineer ecosystems
interact nonlinearly and generate positive feedbacks that enhance
diversity (Altieri et al.,
2010). The role of positive ecological feedbacks during the
Ediacaran-Cambrian transition based on the first appearance of genetic
diversity patterns of various metazoan clades, was analysed (Erwin &
Tweedt, 2012) and the results of this analysis revealed that during the
Early Cambrian the feedback increased substantially, principally through
bioturbation (ventilating the sediment), pumping by sponges (modifying
the chemical condition of the water column) and the appearance of a
number of structural engineers, such as archaeocyathid reefs (enhancing
the complexity of the habitat). They suggested, therefore, that the
ecological expansion during the Early Cambrian was driven, in part, at
least, by ecosystem engineering (Erwin et
al., 2011; Erwin & Tweedt,
2012). As they noted, at present it is difficult to determine whether
the expansion of ecosystem engineering was merely a component of the
establishment of marine ecosystems dominated by metazoans or it was a
significant driver of this event (Erwin & Tweedt, 2012). Also, in the
Early Cambrian infaunas as well as epifaunas were involved in the
Cambrian explosion. The increase of bioturbation and the appearance of
archaeocyathid structure engineers are better understood as consequences
of the Cambrian explosion. Zhang et
al. agree with Erwin & Tweedt
(2012) that during the Ediacaran ecosystem engineering, in spite of the
small degree, might be crucial for the Cambrian Explosion, in
particular, engineering effects of sponges because they definitely
appeared in the Ediacaran (Gehling & Rigby, 1996; Clites et
al., 2012). Sponge fossils
from the Ediacaran are, however, actually rare. Therefore, their
engineering effects are somewhat speculative as the ecological feedbacks
require abundance.
Conclusions It is suggested by the fossil record and
molecular results that the basic developmental system of bilaterian
animals was in place in the Urbilateria at least 555 Ma, regardless of
whether the Urbilateria had a simple or complex morphology. The GRNs
that are responsible for controlling the development of an animal are
shared widely among metazoan linages. Zhang et
al. suggest the assembly,
reassembly, and redeployment of subcircuits of the GRNs are responsible
for the morphological diversification of bilaterian clades. The Cambrian
explosion still requires environmental triggers as the developmental
prerequisites were already present in the ancestral taxon (Gould, 2002).
The most common environmental triggers that have been proposed are the
oxygenation and change in the composition of seawater, particularly the
oxygenation, as it allows the most components of the Cambrian explosion,
such as body size increase, widespread biomineralisation, and ecological
strategies that are metabolically expensive, such as burrowing and
predation. The explosive diversification of complex multicellular
organisms as exhibited by the macrobiotas of the Early Ediacaran and the
subsequent stepwise increase of atmospheric oxygen levels that linked to
the weathering of the trans-Gondwana Mountains that finally caused the
Cambrian explosion. The seawater composition change, e.g. the increase
in concentration of Ca2+, may also contribute to widespread
biomineralisation in lineages of the metazoans. The Cambrian explosion
itself, at least partially, is an expansion of lineages of bilaterians;
therefore, the ecological events that were initiated by this event
cannot be invoked as ecological triggers. The positive feedbacks of
ecosystems of the Ediacaran, particularly by the sponge engineering, may
have contributed to the formation of the well-mixed, clear water system
that was typical of the oceans in the Phanerozoic, and thereby paved the
way for the Cambrian explosion. But the potential feedback of sponges
has been challenged by the rarity of sponge fossils in the Ediacaran.
According to Zhang et al. the
Cambrian explosion may have been triggered by environmental perturbation
near the boundary between the Ediacaran and Cambrian, and subsequent
amplification by ecological interactions within the reorganised
ecosystems (Knoll & Carroll, 1999).
Outlook for future research Zhang et
al. suggest that if the Cambrian explosion was ecological expansion
of bilaterian clades, some of the bilaterian ancestral groups, including
Urbilateria, may have already been in existence at least in the Late
Ediacaran, regardless of deep or shallow divergence. Analogy with the
evolution of vertebrates can make this inference: birds and mammals
became ecologically significant since the Cainozoic, though their
ancestors appeared in the Triassic and Jurassic. Ancestral forms of the
faunas from the Early Cambrian are most likely to be found in
exceptional preservations in the Late Ediacaran, even if they were small
or soft-bodied. The Lagerstätten of the Burgess Shale type and
phosphorous chert deposits are well known for exquisitely preserved
soft-bodied fossils and microorganisms they contained. Searching for
ancestral forms of faunas in exceptionally preserved fossil beds from
the Early Cambrian is a major task of the future palaeontological work
of Zhang and his team. They suggest that environmentally, oxygenation and seawater composition change appear to be more closely relevant to the Cambrian explosion. Precise data is still lacking, however. The increase during the Ediacaran-Cambrian transition was suggested by the tectonic events that were occurring at this period, though it is yet to be confirmed by geochemical studies. It is not clear when the oxygen level crossed the threshold that minimally meets the demand of large animals. Changes in the salinity and concentrations of many important ions (such as Ca, Mg and trace metals) of the oceans during the critical time interval are also working targets for their chemical colleagues. Sources & References Zhang, X., et
al. (2014). "Triggers for the
Cambrian Explosion: Hypotheses and problems." Gondwana Research
25(3): 896-909. |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |