Benton M.J. Introduction to paleobiology and the fossil record

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228 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

(a)

5 μm

2 μm

5 μm

2 μm

5 μm

2 μm

(c)

(b)

Figure 9.18 Some coccolith morphotypes:

(a) coccospheres of the living Emiliana huxleyi,

currently the most common coccolithophore

(×6500), and (b) Late Jurassic coccolith

limestone (×2000). (c) Coccolith plate styles:

1 and 2, Coccolithus pelagus; 4 and 5, Oolithus

fragilis; 5 and 6, Helicosphera carteri. In

C. pelagus and H. carteri growth was upwards

and outwards with the addition of layer upon

layer of calcite; in O. fragilis growth was

different with curved elements, in non-parallel to

crystal cleavage directions. (a, b, courtesy of

Jeremy Young; c, courtesy of Karen Henriksen.)

liths, but these are mostly in taxa with a

limited fossil record. More interesting prob-

lems are posed by the alternation of holococ-

colith-bearing and heterococcolith-bearing

phases in the life cycle of a single species. In

modern coccolithophores the taxonomy is

being adjusted to reﬂ ect this as data become

available.

Together with diatoms, dinoﬂ agellates and

picoplankton (tiny, single-celled plankton

0.2–2.0 μm in size), coccolithophores are the

most abundant phytoplankton in modern

oceans. The greatest diversity is developed in

the tropics. Dependence on sunlight for pho-

tosynthesis restricts the group to the photic

zone, with a depth range of 0 m to about

150 m. Within wave-mixed surface waters

there is normally only a slight vertical strati-

ﬁ cation of assemblages, but a quite different

assemblage is often developed beneath the

thermocline.

Evolution and geological history

Rare coccoliths ﬁ rst appeared in the Late Tri-

assic and increased in numbers during the

Jurassic and Cretaceous; the group peaked in

the Late Cretaceous, and chalk from that

interval is almost entirely composed of these

nannofossils. Only a few species survived the

end-Cretaceous extinction event but they

radiated again during the Cenozoic, recover-

ing their numbers and abundance. However,

in the last 4–5 myr there has been a marked

decline in the abundance of larger coccoliths

and, as a result, they have become less abun-

dant in oceanic sediments, typically forming

only 10–30% of modern calcareous oozes.

Biostratigraphic zonal schemes using cocco-

lithophores have been established from the

Jurassic to the present day, and these are

widely applied because they are reliable and

operate over great distances. Moreover, basic

biostratigraphic analyses of coccolithophore

samples can be carried out rapidly, typically

requiring less than an hour per sample. This

is because nannofossils are abundant enough

to be studied in simple strew mount prepara-

tions and can be reliably identiﬁ ed in cross-

polarized light. Nannofossils only occur in

low-energy marine sediments and are easily

destroyed by diagenesis, but when they are

present they provide an ideal means of rapidly

dating sediments.

ing modern and fossil taxonomies. Some

modern coccolithophores are polymorphic,

producing several different types of cocco-

PROTISTS 229

Diatoms

Diatoms are unicellular autotrophs that are

included among the chrysophyte algae; they

are characterized by large green-brown chlo-

roplasts. Both individuals and loosely inte-

grated colonies of diatoms occur in a range of

aquatic environments from saline to freshwa-

ter and across a range of temperatures, being

particularly common in the Antarctic plank-

ton. Both benthic and planktonic life modes

occur, although within the plankton one group

– the Centrales – prefer marine environments;

the Pennales, on the other hand, are more

common in freshwater lakes (Box 9.11).

Morphology and classiﬁ cation

The diatom cell is contained within a siliceous

skeleton or frustule comprising two unequally-

sized valves or thecae (Fig. 9.19). The smaller

hypotheca ﬁ ts into the larger epitheca; the

valve plates and congula of both valves

interface with the congulum of the epitheca

covering that of the hypotheca to form a

connective seal.

During reproductive ﬁ ssion, both the parent

valves are used as the epitheca by the off-

spring, which then constructs its own

hypotheca. This process occurs a number of

times each day, progressively reducing the size

of the fustule. A stage of sexual reproduction

kicks in to restore the growth momentum of

the individual.

Classiﬁ cation of the group is based on shell

morphology (Box 9.11).

Evolution and geological history

Both diatom frustules and, more commonly,

endospores are preserved in the fossil record.

A Late Jurassic assemblage from western

Siberia that includes Stephanopyxis may be

the oldest known diatom ﬂ ora. The ﬁ rst

diverse ﬂ oras appeared during the mid-Creta-

ceous with almost 10 families recorded from

Aptian rocks; the group further diversiﬁ ed

after the Turonian. Nearly 100 genera of

centric diatoms are recorded from the Upper

Cretaceous. Some of the ﬁ rst pennate diatoms

appeared during the Paleogene, colonizing

freshwater environments for the ﬁ rst time; the

group reached an acme during the Miocene.

Remarkably, diatom frustules can accumu-

late as thick deposits of diatomite (sometimes

up to 500 m thick), which is a very porous

sediment, often with 80% as spaces, and per-

meable with a density of about 0.5 g cm

−1

These diatomites, also termed kieselguhrs and

tripolis, are widely used as puriﬁ ers for ﬁ lter-

ing drinks, medicines and water. Over 2

million tons are extracted each year for com-

mercial use. Modern sedimentation rates

suggest that 4–5 mm of diatomaceous ooze is

deposited over 1000 years; such an ooze cur-

rently occupies over 10% of the ocean ﬂ oor

today. Major commercial deposits occur in

the Miocene of the Ardèche, France and in the

Pliocene and Pleistocene of Cantal, France are

some of the main suppliers, although other

deposits occur in Spain, Germany and Russia.

The Miocene Monterey Formation in Califor-

nia is particularly widespread, occurring in

both onshore and offshore basins; this diato-

maceous mudstone is also the source and

reservoir rock for most of California’s

petroleum.

Chitinozoans

Chitinozoans are most common in ﬁ ne-

grained sediments, usually those deposited in

anoxic environments, and are associated with

pelagic macrofauna such as graptolites and

nautiloids together with acritarchs. In some

lithologies, such as black slates, chitinozoans

are the only fossils preserved. These associa-

tions, together with their widespread geo-

graphic range, suggest that chitinozoans were

at least pelagic. The group has proved

extremely useful for both regional and global

epivalve + epicingulum = epitheca

epitheca + hypotheca = frustule

epivalve

girdle

hypovalve

hypovalve + hypocingulum = hypotheca

hypocingulum

epicingulum

Figure 9.19 Descriptive morphology of the

diatoms.

230 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 9.11 Classiﬁ cation of siliceous-walled groups: diatom classiﬁ cation

Two main divisions are recognized based on their shell morphologies: the Centrales, as the name

suggests, have round valves with pores radiating in concentric rows from the valve center; the Pen-

nales have more elliptical valves with the pores arranged in pairs (Fig. 9.20). The latter are usually

characterized by a median gash or raphe.

Order CENTRALES

Suborder COSCINODISCINEAE

• Valves with a ring of marginal processes

Suborder RHIZOSOLENIINEAE

• Valves are unipolar

Suborder BIDDULPHIINEAE

• Valves are bipolar

Order PENNALES

Suborder ARAPHIDINEAE

• Valves without a raphe

Suborder RAPHIDINEAE

• Valves with a raphe

CENTRALES

Coscinoconus

Achnanthes

Surirella

Asterolampra

Cocconeis

Eunotia

PENNALES

Figure 9.20 Some diatom morphotypes: Coscinoconus (×250), Asterolampra (×400), Cocconeis

(×360), Achnanthes (×150), Surirella (×200) and Eunotia (×400).

PROTISTS 231

correlations and is a key part of global bio-

stratigraphic schemes for the Ordovician and

Silurian systems.

Morphology and classiﬁ cation

Chitinozoans are small (between 50 and

2000 μm), ﬂ ask- to vase-shaped, hollow vesi-

cles with smooth or ornamented surfaces (Fig.

9.21). The vesicles were thought to have con-

sisted of a protein called pseudochitin similar

in composition to the graptolite rhabdosome,

but recent research suggests that they are

actually composed of networks of kerogen,

and chitin is in fact absent from the pyroly-

sates vaporized from the vesicle (Jacob et al.

2007). The vesicle encloses a chamber that

ranges in shape from spherical through ovoid

to cylindrical and conical forms. The chamber

opens through an aperture at the oral end,

either directly or at the end of a neck with a

collar. The aperture is closed by an operculum

that may be supported by the prosome. The

base of the vesicle may be ﬂ at or extended as

a variety of structures, for example a copula

(long hollow tube), mucron (short hollow

tube), siphon (bulb-like process) or peduncle

(solid process). There are nearly 60 genera of

chitinozoans.

The precise afﬁ nity of the group remains

uncertain (Box 9.12). Chitinozoan vesicles

were probably tightly sealed, and they occur

as chains and clusters that suggest they may

have been eggs or egg capsules or even

dormant cysts. Chitinozoans have, in fact,

been interpreted in the past as egg cases of a

huge range of invertebrates, such as annelids,

echinoderms, gastropods and graptolites, but

they were probably the products of some soft-

bodied, worm-like animal during a pelagic

life stage.

Two main groups of chitinozoans have

been established based on the way the vesicle

is sealed, and are further subdivided accord-

ing to the outline or silhouette of the vesicle

together with modiﬁ cations of the neck. The

Operculatifera have a relatively simple oper-

culum and they lack a neck (including the

Desmochitinidae with small subspherical ves-

icles), whereas the Prosomatifera have a more

complex opercula with a prosoma and a well-

developed neck (including the Conochitinida

and the Lagenochitinidae, the second with a

recessed operculum) (Fig. 9.23).

Evolution and distribution

Possible chitinozoans, in the form of Desmo-

chitina-like sacs, have been reported from the

Upper Proterozoic of Arizona, but the ﬁ rst

true chitinozoans appeared during the

Tremadocian (Early Ordovician) and subse-

quently diversiﬁ ed rapidly during the Early

Ordovician, evolving hundreds of different

species spread across at least 50 genera. This

diversity continued through the Silurian with

all the three main groups represented. They

declined during the Devonian, disappearing

ﬁ nally at the top of the Famennian, when the

last remaining lagenochitinid went extinct.

Through time the group developed smaller,

self-contained chambers with an increased

complexity of ornament and a greater degree

of apparent coloniality (Fig. 9.23).

(a)

recessed operculum

aperture

lip

neck

body

chamber

base

(b)

aperture

outer

wall

operculum

flange

neck

inner wall

spines

prosome

spine filled with

spongy matter

Figure 9.21 Descriptive morphology of the chitinozoans: (a) Operculatifera (simplexoperculate),

Lagenochitina, and (b) Prosomatifera (complexoperculate), Ancyrochitina.

232 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Review questions

1 The move from prokaryote to eukaryote

cell types was a major evolutionary jump.

How was this transition achieved and

what sorts of implications did it have for

life on Earth?

2 Foraminifera have been widely used by oil

and gas companies in exploration. Why

are they so useful?

3 Radiolarians have proved very useful in

sorting out the stratigraphy of mountain

belts. Why are they superior to other

micro- and macrofossil groups in these

types of studies?

4 Chromistan groups such as the cocco-

lithophores and diatoms have a funda-

mental effect on the stability of atmospheric

and oceanic systems on our planet.

But such nannoplankton appeared rela-

Box 9.12 The chitinozoan Rosetta Stone

But what really were chitinozoans? Material from the Ordovician of Estonia, described as the chi-

tinozoan “Rosetta Stone”, may have partially solved the problem. Individual vesicles are linked

together in a coiled, chain-like structure; each vesicle belongs to the same species, Desmochitina

nodosa Eisenack (Fig. 9.22). It is unlikely that these were eggs of a metazoan, because larvae would

be unable to escape from the tightly sealed and connected chambers. Therefore Paris and Nolvak

(1999) postulated that the coiled, chain-like structure represents an intermediate, immature stage,

perhaps an intra-oviduct phase, prior to the ﬁ nal egg-laying event. Unfortunately, distribution in

time and space of chitinozoans does not match any skeletonized metazoan group. So we are back

to speculation. Possibly chitinozoans are related to a soft-bodied “chitinozoan animal” and the

search is on to ﬁ nd this animal in one or more of the Paleozoic Lagerstätten.

Figure 9.22 Chitinozoan apparatus: a large cluster of Desmochitina nodus interpreted as an egg

clutch of the chitinozoan animal; the opercula are not present suggesting that the animals had

already hatched (×70). (Courtesy of Florentin Paris.)

Sphaerochitina Urnochitina Conochitina Ancyrochitina

Colonial

arrangements

Figure 9.23 Some chitinozoan morphotypes: Sphaerochitina (×160), Urnochitina (×160), Conochitina

(×80), Ancyrochitina (×240) and colonial arrangements (×40).

PROTISTS 233

tively late in the geological record. Is

there any evidence for a Paleozoic

nannoplankton?

5 The identity of the chitinozoans may have

been solved but how should these fossils

be classiﬁ ed?

Further reading

Armstrong, H.A. & Brasier, M.D. 2005. Microfossils,

2nd edn. Blackwell Publishing, Oxford, UK.

Bignot, G. 1985. Elements of Micropalaeontology.

Graham and Trotman, London. (Useful overview of

all the main microfossil groups.)

De Wever, P., Dumitrica, P., Caulet, J.P., Nigrini, C. &

Caridroit, M. 2001. Radiolarians in the Sedimentary

Record. Gordon and Breach Science Publishers, the

Netherlands. (Key reference on radiolarian

paleontology.)

Haeckel, E. 1862. Die Radiolarien (Rhizopoda Radi-

aria). Eine Monographie. Reimer, Berlin. (Classic

reference on Radiolaria, beautifully illustrated.)

Jenkins, D.G. & Murray, J.W. 1989. Stratigraphical

Atlas of Fossil Foraminifera, 2nd edn. British

Micropaleontology Association and Ellis Horwood

Ltd, London. (Well-illustrated account of the

foraminiferans.)

Lipps, J.H. (ed.) 1993. Fossil Prokaryotes and Protists.

Blackwell Scientiﬁ c Publications, Oxford, UK. (Mul-

tiauthor compilation of the prokaryote and protist

microfossil groups.)

References

Armstrong, H.A. & Brasier, M.D. 2005. Microfossils,

2nd edn. Blackwell Publishing, Oxford, UK.

Cavalier-Smith, T. 2002. The phagotrophic origin of

eukaryotes and phylogenetic classiﬁ cation of proto-

zoa. International Journal of Systematic and Evolu-

tionary Microbiology 52, 297–354.

Corsetti, F.A., Olcott, A.N. & Bakermans, C. 2006. The

biotic response to Neoproterozoic snowball Earth.

Palaeogeography, Palaeoclimatology, Palaeoecology

232, 114–30.

De Wever, P., Dumitrica, P., Caulet, J.P., Nigrini, C. &

Caridroit, M. 2001. Radiolarians in the Sedimentary

Record. Gordon and Breach Science Publishers, the

Netherlands.

Haeckel, E. 1862. Die Radiolarien (Rhizopoda Radi-

aria). Eine Monographie. Reimer, Berlin.

Haeckel, E. 1904. Kunstformen der Natur. Verlag des

Bibliographischen Institut, Leipzig.

Henriksen, K., Young, J.R., Bown, P.R. & Stipp, S.L.S.

2004. Coccolith biomineralisation studied with

atomic force microscopy. Palaeontology 47, 725–

43.

Jacob, J., Paris, F., Monod, O., Miller, M.A., Tang, P.,

George, S.C. & Bény, J.-M. 2007. New insights into

the chemical composition of chitinozoans. Organic

Geochemistry 38, 1782–8.

Keeling, P.J., Burger, G., Durnford, D.G. et al. 2005.

The tree of eukaryotes. Trends in Ecology and Evo-

lution 20, 670–6.

Paris, F. & Nolvak, J. 1999. Biological interpretation

and palaeobiodiversity of a cryptic fossil group: the

“chitinozoan animal”. Geobios 32, 315–24.

Servais, T., Lehnert, O., Li, J., Mullins, G.L., Munnecke,

A., Nützel, A. & Vecoli, M. 2008. The Ordovician

biodiversiﬁ cation: revolution in the oceanic trophic

realm. Lethaia 41, 99–110.

Tyszka, J. 2006. Morphospace of foraminiferal shells:

results from the moving reference model. Lethaia 39,

1–12.

Vecoli, M., Lehnert, O. & Servais, T. 2005. The role of

marine microphytoplankton in the Ordovician bio-

diversiﬁ cation event. Notebooks on Geology, Memoir

2005/2, 69–70.

Chapter 10

Origin of the metazoans

Key points

• Relatively few basic body plans have appeared in the fossil record; most animals have

a triploblastic architecture, with three fundamental body layers.

• Molecular data show there are three main groupings of animals: the deuterostomes

(echinoderm–hemichordate–chordate group), the spiralians (mollusk–annelid–

brachiopod–bryozoans–most ﬂ atworms–rotifers (platyzoans) group) and the ecdysozo-

ans (arthropod–nematode–priapulid plus other taxa group). Together, the spiralians and

ecdysozoans are usually called the protostomes.

• Five lines of evidence (body fossils, trace fossils, fossil embryos, the molecular clock and

biomarkers) suggest that the metazoans had originated prior to the Ediacaran,

600 Ma.

• Snowball Earth by coincidence or design was a pivotal event in metazoan history; bila-

terians evolved after the Marinoan glaciation.

• The ﬁ rst metazoans were probably similar to the demosponges, occurring ﬁ rst before

the Ediacaran.

• The Ediacaran biota was a soft-bodied assemblage of organisms largely of uncertain

afﬁ nities, reaching its acme during the Late Proterozoic, which may represent the earliest

ecosystem dominated by large, multicellular organisms.

• The Tommotian or small shelly fauna was the ﬁ rst skeletalized assemblage of metazoans;

this association of Early Cambrian microfossils contains a variety of phyla with shells

or sclerites mainly composed of phosphatic material.

• The Cambrian explosion generated a range of new body plans during a relatively short

time interval.

• The Ordovician radiation was marked by accelerations in diversiﬁ cation at the family,

genus and species levels together with increased complexity in marine communities.

Consequently, if my theory be true, it is indisputable that before the lowest Silurian

[Cambrian of modern usage] stratum was deposited, long periods elapsed, as long as,

or probably far longer than, the whole interval from the Silurian age to the present

day; and that during these vast, yet quite unknown, periods of time, the world

swarmed with living creatures.

Charles Darwin (1859) On the Origin of Species

ORIGIN OF THE METAZOANS 235

ORIGINS AND CLASSIFICATION

When did the ﬁ rst complex animals, the meta-

zoans, appear on Earth and what did they

look like? How could complex, multicelled

animals evolve from the undifferentiated

single-celled organisms of most of the Pre-

cambrian? Why did they take almost 4

billion years to appear? These questions have

puzzled scientists, including Charles Darwin,

for over two centuries. In the last few decades

a range of multidisciplinary techniques, from

molecular biology to X-ray tomography, has

helped generate new testable hypotheses

regarding the origins of our early ancestors.

Apart from the fossil evidence of metazoan

body and trace fossils, the investigation of

minute fossil embryos, carefully calibrated

molecular clocks and more recently biomark-

ers have placed the investigation of Precam-

brian life at the top of many scientiﬁ c

agendas.

The ﬁ rst metazoans: when and what?

Life on our planet has been evolving for nearly

4 billion years. Molecular data suggest meta-

zoans have probably been around for at least

600 myr (Fig. 10.1), during which time,

according to some biologists, as many as 35

separate phyla have evolved. Five lines of evi-

dence have ﬁ gured prominently in the search

for the earliest metazoans: body fossils, trace

fossils, fossil embryos, the molecular clock

and biomarkers.

Much controversy still surrounds the timing

of their origin. Was there a long cryptic inter-

val of metazoan evolution prior to the Edia-

caran – a time when we do not ﬁ nd fossils

preserved, either because the animals lacked

preservable bodies, or they were small, or

perhaps a combination of both? Or, as the

recalibrated molecular clocks suggest, can

animal origins be tracked back only to the

Ediacaran, when there was also a sudden rise

493

494

543

546

549

545

535

561

579

604

634

664

Cryogenian Ediacaran

Early

EarlyMMLate Late

N-D

B/T

Cambrian Ordovician

700 635 600 575 555 542 513 501 488 472 461 443 Ma

200

160

120

Number of classes/orders

Crinozoa

Eleutherozoa

Hemichordata

Pteriomorpha

Nuculoida

Vetigastropoda

Sorbeochoncha

Phyllodicida

Spionidea + Serpulidea

Hoplonemertea

Heteronemertea

Arthropoda

Priapulida

Anthozoa

Hydrozoa

Calcispongia

Demospongia

“Porifera”

Cnidaria

Ecdysozoa Spiralia Deuterostomia

604

metazoan orders

metazoan classes

molecular clock estimates

molecular clock calibration

feeding larva

non-feeding larva

Oman biomarker record

Figure 10.1 Time scale and tempo of early animal evolution: the key metazoan groups are shown with

the putative age of their last common ancestor, together with an estimate of the respective numbers of

classes and orders indicated against a stratigraphy indicating key biological and chemical events. N–D,

Nemakit-Daldynian; T, Tommotian; A, Atdabanian; B/T, Botomian. (Courtesy of Kevin Peterson.)

236 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

in oxygen levels in the deep ocean (Canﬁ eld

et al. 2007).

Body fossil evidence

Body fossils of basal metazoans in the Edia-

caran Period are few and far between. The

morphology of an early metazoan fossil must

be clearly described and convincingly illus-

trated, different organs and tissues identiﬁ ed,

and comparisons drawn with other extant

and fossil organisms. Many Upper Precam-

brian successions have been subjected to

intense metamorphism and tectonism (see p.

48) and are now located in some of the Earth’s

mountain belts. The chances of ﬁ nding ade-

quately preserved fossils are slight. Neverthe-

less, the earliest undoubted metazoans occur

within the widespread Ediacara biota (see p.

242) dated at approximately 600–550 Ma.

Moreover the fact that a relatively advanced

metazoan, the mollusk Kimberella, possibly

equipped with a foot and radula (see p. 330),

occurs within the Ediacara biota from south-

ern Australia and Russia could suggest a

history of metazoan evolution prior to the

Ediacaran. But although a strong case can be

made for a signiﬁ cant Proterozoic record for

the cnidarians and sponges and perhaps some

other metazoans, the Cambrian explosion still

marks the arrival, center stage, of the bilateri-

ans (Budd 2008).

Trace fossil evidence

Trace fossils are the behavior of organisms

recorded in the sediment (see p. 510). By their

very nature they occur in place and thus

cannot be transported or reworked by cur-

rents. Nevertheless these too must be convinc-

ingly demonstrated as biogenic and the age of

their enclosing sediments accurately deter-

mined. If and when metazoans developed

locomotory organs, such as the molluskan

foot, and digestive systems, we might expect

to ﬁ nd burrows and trails together with fecal

pellets. Records of trace fossils from rocks

older than 1 Ga in India (Seilacher et al. 1998)

and over 1.2 Ga in the Stirling biota of Aus-

tralia (Rasmussen et al. 2002) generated

considerable excitement (Fig. 10.2). Both

suggested metazoan life older than 1 Ga but

both are now considered questionable (Jensen

2003). The oldest undoubted locomotory

trace fossils are from about 550 Ma (Droser

et al. 2002) from northwest Russia, whereas

fecal strings have been reported from rocks

some 600 Ma (Brasier & McIlroy 1998) sug-

gesting the existence of an ancient digestive

system. In fact no convincing trace fossils are

known from successions older than the Mari-

noan glaciation (635 Ma), the second main

icehouse event associated with snowball Earth

(see p. 112).

Embryo fossil evidence

Fossil Neoproterozoic embryos are now

known from a number of localities, although

claims that they represent sulfur-oxidizing

bacteria or that they are not embryos at all

have their advocates. Some of the best studied

examples are from the Doushantuo Forma-

tion, South China. The part of the formation

yielding the embryos was ﬁ rst dated at approx-

imately 580 Ma, predating much of the Edia-

caran but postdating the Marinoan glaciation.

Revised dates seem to suggest that the faunas

are younger and that they overlap with the

older Ediacaran assemblages. Cell division

and cleavage patterns are obvious although it

is difﬁ cult to assign the material to distinct

metazoan groups in the absence of juvenile

and adult forms. There are, however, a lack

of epithelia even in clusters of over 1000 cells

Figure 10.2 Putative trace fossils from the

Precambrian of Australia, showing

Myxomitodes, a presumed trail of a mucus-

producing multicellular organism about 1.8–2

billion years old from Stirling Range, Western

Australia. (Photo is approximately 65 mm wide.)

(Courtesy of Stefan Bengtson.)

ORIGIN OF THE METAZOANS 237

suggesting that the embryos examined are

those, at best, of stem-group metazoans

(Hagadorn et al. 2006); they could equally

well be fungi or rangeomorphs (enigmatic

frond-like fossils). Nonetheless the Doushan-

tuo embryos, although unplaced taxonomi-

cally, provide our earliest body fossil evidence

for probable metazoan life, albeit very basal,

and a fascinating insight into embryologic

processes in deep time (Donoghue 2007)

(Box 10.1).

Molecular evidence

Not only have the morphologies of organisms

evolved with time, but so too have their mol-

ecules. This forms the basis of the concept of

the molecular clock (see p. 133). The molecu-

lar clock has opened up tremendous possibili-

ties to date, independently of direct fossil

evidence, the times of divergence of say the

mammals from the reptiles or the brachio-

pods from the mollusks. Nevertheless,

attempts to date the divergences of the various

groups of metazoans have proved controver-

sial. For example, the last common ancestor

of the bilaterians, the metazoan clade exclud-

ing the sponges and cnidarians, has been vari-

ously placed at anywhere between 900 and

570 Ma. Why is there such a spread of ages

in a seemingly exact science? The rates of

molecular evolution in various groups are

unfortunately not constant. The vertebrates

appear to have reduced their rates of molecu-

lar change through time. So, using the slow

vertebrate rates of molecular evolution to

calibrate the date of origin of Bilateria gives

dates that are too ancient (900 Ma). On the

other hand, using mean bilaterian rates of

molecular evolution gives a date (570 Ma)

that is more in keeping with evidence from

the fossil record (e.g. Budd & Jensen 2000)

and thus makes the Cambrian explosion much

more of an explosion of animals rather than

fossils (Peterson et al. 2004). Nevertheless the

most recent molecular clock data (Peterson

et al. 2008) suggest a major phase of meta-

zoan radiation within the Ediacaran, prior to

that in the Cambrian. This radiation probably

set the agenda for metazoan macroevolution

for the rest of geological time.

Box 10.1 Synchrotron-radiation X-ray tomographic microscopy

Fossil embryos from the Upper Neoproterozoic and Cambrian are providing some important clues

about the origin and early evolution of the metazoans. They are, however, tiny and notoriously hard

to study. Nevertheless Phil Donoghue and his colleagues (2006) are beginning to accumulate a large

amount of new information on the composition, structure and cell division within these minute

organisms together with their modes of preservation. Synchrotron-radiation X-ray tomographic

microscopy (SRXTM) has provided a whole new way of scanning embryos without actually destroy-

ing them (Fig. 10.3). The embryos, most of them 1 mm across or smaller, are held steady in a high-

energy beam of photons, and multiple “slices” are produced, spaced a few microns apart. Using

imaging software, these slices can be combined to create a detailed three-dimensional model of the

internal structure of the fossil. Embryos assigned to the bilaterian worm, Markuelia, together with

Pseudooides, variously show the process of cell cleavage and development of possible blastomeres,

clusters of cells produced by cell division after fertilization, rather than yolk pyramids, which are

more typical of the arthropods. This high-tech methodology has already demonstrated a real prospect

for identifying the animals themselves and charting their early stages of development, some 600 Ma.

It also can reject the claims that such fossils were the planula larvae of cnidarians, minute bilaterians

or the early stages of gastrulation (see p. 240) of hydrozoans or bilaterians. It has, however, been

recently suggested that many of these embryonic structures were created by bacteria (see p. 190).

But not all.

Read more about this topic at http://www.blackwellpublishing.com/paleobiology/.

Continued