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

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108 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

precipitation and temperature in the recent

past may have inﬂ uenced the course of human

events and almost certainly impacted on the

direction of hominid evolution during the

Late Pliocene and Pleistocene. Many short-

term climatic ﬂ uctuations have been related

to Milankovitch cycles (see p. 36), patterns of

change in climates and sedimentation patterns

that are driven by changes in the eccentricity,

obliquity and precession of the Earth’s orbit

and generally on scales from 20 to 400 kyr.

These short-term trends are associated with

evolutionary changes at the speciation level

and more local regional changes in the com-

position and structure of ecosystems (Box

4.8).

Long-term trends

As noted above, the Earth has oscillated

between greenhouse and icehouse conditions

(Box 4.9) at least ﬁ ve times in the past 900 myr

(Frakes et al. 1992). These megacycles have

been compared with patterns of change in

extinctions, sea level and volcanicity (Fig.

4.26). Moreover, there may be a correlation

between these variables and the assembly and

breakup of the supercontinents. In marine

environments two extreme states occurred:

1 The icehouse state involved unstratiﬁ ed,

unstable oceans, cool surface waters

between 2 and 25°C and bottom waters

ranging from 1 to 2°C together with

rapidly moving bottom waters, rich in

oxygen and with high productivity in

areas of upwelling.

Average global temperature (°C)

To d a y

Pleistocene

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Precambrian

CoolCoolCool

Cool

Cool War mWar mWar mWarm

Figure 4.23 Climate change through time,

showing alternations between icehouse and

greenhouse worlds. (Courtesy of Christopher

Scotese.)

Calcrete

Glendonite

Dropstone

Evaporite

Tillite

“Paratropical” = high-latitude bauxites

Arid Cold

Warm temperate

Kaolinite(and coal and evaporite)

Crocodiles

Palms and mangroves

Tropical

Cool temperate

Coal

Bauxite

Laterite

Coal

and Tillites

War m Cool

We t

Dry

Figure 4.24 Some key indicators of climate and

temperature. (Courtesy of Christopher Scotese.)

PALEOECOLOGY AND PALEOCLIMATES 109

Box 4.8 Climate change and fossil size

There is strong evidence that climate and environmental changes have controlled extinctions and

speciations, but do they have a direct inﬂ uence on the size of organisms? Daniela Schmidt and her

colleagues (2004) have investigated size changes in planktic foraminiferans during the last 70 myr

from well-dated cores furnished by various ocean drilling programs. There was a sharp decrease in

size at the Cretaceous–Paleogene boundary with the disappearance of many large taxa, and after

this extinction event high-latitude taxa remained consistently small. Fluctuations in size, however,

occurred in low-latitude assemblages (Fig. 4.25). A ﬁ rst phase (65–42 Ma) is characterized by dwarfs,

a second (42–12 Ma) contains moderate size ﬂ uctuations, whereas the third (12 Ma to present) has

the relatively large-sized taxa that typify Modern assemblages. Size increases are correlated

with intervals of global cooling (Eocene and Neogene), when there were marked latitudinal and

temperature gradients and high diversity. More minor size changes in the Paleocene and Oligocene

may have been associated with changes in productivity. Cenozoic planktic foraminiferans thus

provide strong support for a stationary model of evolutionary change, with size changes being

strongly correlated with extrinsic factors such as ﬂ uctuations in latitudinal and surface-water tem-

perature gradients.

012345

O[‰]

Bottom-water temperature [°C]

Ice-free ocean

Paleocene

600500400300

MioceneOligoceneEocene Plio.

Plt.

Age

[Ma]

Size

assemblages

[μm]

Cenozoic average

small size in

low and high

latitudes

first separation

of high- and

low-latitude

fauna

strong

size increase

in low-latitude

fauna

bottom water temperature [°C]

Mg/Ca

14 10 6 4 28012

Figure 4.25 Size changes in planktic foraminiferans from high and low latitudes during the last

70 Ma, compared to temperature proﬁ les generated from oxygen isotope data and Mg : Ca ratios.

Three phases are recognized, a ﬁ rst (65–42 Ma) with dwarf taxa, a second (42–12 Ma) with

moderate-sized taxa, and a third (12 Ma to present) with large-sized taxa. Size increases are

correlated with intervals of global cooling. (Courtesy of Daniela Schmidt.)

110 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

2 On the other hand, greenhouse oceans

were more stable, and better stratiﬁ ed

with surface waters ranging in tempera-

ture from 12 to 25°C with deep-water

temperatures between 10 and 15°C. Slow

bottom currents carried little oxygen and

productivity was generally low.

Extinctions were associated with the transi-

tions between these oceanic states.

In addition to these major climatic ﬂ uctua-

tions a series of major extinction events, some

associated with extraterrestrial causes, clearly

prompted major climate change over several

million years. Such events caused major taxo-

nomic extinctions together with major restruc-

turing of the marine and terrestrial ecosystems.

Generally, greenhouse biotas were most sus-

ceptible to extinction; their species were more

specialized and thus more exposed to environ-

mental change.

Consequences for evolution

Microevolution is obvious in many fossil lin-

eages (Benton & Pearson 2001) although the

link between speciation events and climatic

change is more controversial. Generally,

marine plankton show gradual evolution

whereas marine invertebrates and vertebrates

display a pattern of punctuated equilibria (see

p. 121). Moreover, it is probable that nar-

rowly-ﬂ uctuating, changing environments

host persistent gradualistic evolution whereas

widely-ﬂ uctuating environments host mor-

phological stasis (Sheldon 1996). This resis-

tance to morphological change is clear in a

number of lineages such as Ordovician trilo-

bites and Pliocene mollusks (see p. 123).

Short-term climatic ﬂ uctuations, for

example those associated with Milankovitch

cycles, can clearly disrupt and promote the

reassembly of both marine and terrestrial

communities. In some cases they can drive

local extinctions and radiations, for example

in the conodont and graptolite faunas of the

Silurian (see p. 434).

Climate surely drives larger-scale aspects of

evolution. For example, the Cambrian explo-

sion (see p. 249) – marked by the diversiﬁ ca-

tion of skeletal organisms and the appearance

of reef-building organisms and the ﬁ rst preda-

tors – is associated with increasingly warm

climates and higher sea levels. On land the

radiation of early terrestrial tetrapods in the

Early Carboniferous and the diversiﬁ cation

of large ﬂ ying insects in the ﬁ rst extensive

forests, in cooler climates and more exposed

land areas, have been correlated with high

levels of atmospheric oxygen (Berner et al.

2000).

Some of the largest events of all, such as

the appearance of entire new biotas and grades

of organization, for example the origin of life

itself, the development of photosynthesis and

the appearance of the metazoans, may also be

associated with climate change. The ﬁ rst two

events have been associated with a stable

Archaean crust and relatively cooler climates,

which are favorable for carbon-based life to

evolve. Metazoans appeared and diversiﬁ ed

after the decay of the near global ice sheets of

“snowball Earth” (Box 4.10), whereas skele-

tal organisms radiated during the greenhouse

climates and higher oxygen levels of the Early

Cambrian.

Biological feedbacks

If climate drives evolution, could life itself

drive climate change? Few people doubt that

humans can affect the climate, and everyone

is aware of how the industrialized nations are

Icehouse Greenhouse Ice-house Greenhouse Ice

Extinctions

Climate

Volcanism

Sea level

PC Cam Ord Sil Dev Carb Perm Tr Jur Cret Cen

Figure 4.26 Climate change through time

illustrated together with changes in sea level and

ﬂ uctuations in the intensity of volcanicity. (Based

on various sources.)

PALEOECOLOGY AND PALEOCLIMATES 111

burning fossil fuels and pumping greenhouse

gases into the atmosphere. Global climate

warming will affect the plants and animals of

the cold temperate and polar regions as climate

zones move about 100 km per century towards

the poles (Wilson 1992). Nevertheless, a

number of models for long-term climatic

change have also involved the role of feed-

backs from biological organisms. For example,

the Gaia hypothesis is an attractive model

that treats the Earth as a living system. The

constant interaction between the Earth’s living

organisms, the atmosphere and the oceans

helps keep the planet in check. The idea is

certainly not new. James Hutton (1726–1797),

the father of geology, once described the Earth

as a kind of superorganism. But there were

times in the Earth’s history, the Day After

Tomorrow ice age of snowball Earth (see p.

112) or the sustained hot climates of the Cre-

taceous world, when the Earth’s climate

seemed to be out of control. Nevertheless

some climate change can be modeled by Gaia

– some of the most marked during the Pre-

cambrian (Fig. 4.28). The diversiﬁ cation of

photosynthesizers together with consumers

from the Early Proterozoic onwards, hiked

oxygen levels concomitant with declines in

greenhouse gases. Such models promote the

vital effects of life as a stabilizing inﬂ uence on

the planet’s climate, reducing the otherwise

steady rise in the Earth’s surface temperatures.

In the same way the extensive coal swamps

and forests of the later Paleozoic may also

have contributed to an interval of cooler

climate as diversifying land plants mediated

atmospheric oxygen levels, predicting the

importance of modern rain forests as a

climatic buffer.

There is no doubt that life on planet Earth

is resilient and despite the extremes of climate

change through deep time may have, through

biological feedbacks, been able to conserve

and control its own environment.

Box 4.9 Paleotemperature: isotopes to the rescue?

Is it possible to ﬁ nd out how hot or cold the Earth really was in the past? Stable oxygen isotopes

can be extremely useful as paleothermometers but also in assessing the salinity of ancient oceans

and the extent of ancient ice caps. Oxygen has three stable isotopes, the lightest being

O, then

and the heaviest

O. The ratio of

O :

O is used in most geological investigations. When calcite is

precipitated from seawater the ratio of

O :

O increases with temperature. This ratio is also stan-

dardized with respect to standard mean ocean water (SMOW) or the Peedee belemnite standard

(PDB), Belemnitella americana from the Cretaceous Peedee Formation in South Carolina. A shift of

1‰ in Δ

O values represents a change in temperature of about 4–5°C. Unfortunately, not all shells

are precipitated in equilibrium with surrounding seawater; the vital effects of some organisms inter-

fere with the process. Moreover diagenesis can also affect isotope data. For these reasons corals,

calcareous algae and echinoderms do not give good results; on the other hand brachiopods, bivalves

and foraminiferans have yielded useful data. In addition, the lightest isotope is generally preferen-

tially found in water vapor and thus rainfall. During glacial episodes, snow and ice can act as reser-

voirs for

O, thus depleting the world’s oceans of that isotope. Thus during ice ages the oceans are

characterized by higher amounts of

O. This simple model has formed the basis for our understand-

ing of climate change over the last 1 myr and the relationships of such changes to Milankovitch

cycles (see p. 36).

A dataset of oxygen isotopes is available for time series analysis at http://www.

blackwellpublishing.com/paleobiology/.

112 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 4.10 Snowball Earth

Strong evidence suggests that a number of Late Neoproterozoic ice ages were of global extent

(Hoffman et al. 1998). The occurrence of tillites in close association with carbonates in near-equato-

rial positions has suggested to Paul Hoffman and his colleagues that during these intervals the Earth

was virtually covered by ice. These data supported a model ﬁ rst developed by Brian Harland in the

1960s, subsequently christened “snowball Earth” by Joe Kirschvink in the 1980s. But paleomagnetic

data for low-latitude ice is not the only line of evidence for a global snowball. The majority of these

glacial deposits are overlain by so-called cap carbonates. These rocks suggest deposition in extreme

greenhouse conditions, under an atmosphere of high concentrations of carbon dioxide and seawater

supersaturated with calcium carbonate. Such conditions were promoted by the high temperatures

required to kick the Earth out of its “snowball” state (Fig. 4.27). The incredible buildup of the

greenhouse gas, carbon dioxide, in the atmosphere was a direct consequence of a lack of liquid water

and the cessation of weathering processes; this buildup essentially saved the surface of the planet

from an eternal frozen state. The glacial deposits and the cap carbonates, however, are also strongly

depleted in the

C isotope; this suggests very little biological productivity was in progress that could

have removed the lighter

C isotope, causing preferential enrichment of the heavier

C stable

isotope. And, ﬁ nally, banded ironstone formations (BIFs) are a feature of the snowball interval sug-

gesting the existence of an anoxic, stratiﬁ ed ocean system. Some BIFs are even associated with ice-

rafted dropstones. Not everyone, of course, agrees with this hypothesis; some have suggested a milder

“slushball Earth” and some even deny the possibility of global ice sheets altogether. But, surely these

“freeze–fry” episodes had an important inﬂ uence on the mode of organic evolution. Biological evolu-

tion would certainly have continued, not least associated with active volcanic vents deep under the

ice and in other extreme environments. However evidence for metazoan life seems to appear directly

after snowball Earth.

(a) (b)

Figure 4.27 Snowball Earth scenario. (a) Continents are near the equator, increasing precipitation

removes CO

from the atmosphere, and with falling temperatures ice begins to spread from the

poles. (b) Ice continues to spread with temperatures further reduced by the albedo (reﬂ ection of

solar energy) effect. (c) Atmospheric CO

increases due to volcanic activity, prompting a reversal

in temperatures. (d) Greenhouse conditions return and the ice sheets recede. (Courtesy of Jørgen

Christiansen and Svend Stouge.)

PALEOECOLOGY AND PALEOCLIMATES 113

Review questions

1 Living communities contain a very wide

variety of lifestyles. Although dominated

by members of the Modern fauna, ele-

ments of the Paleozoic (suspension-

feeding) fauna are still present. Is it

possible to predict the sorts of habitats

that they prefer?

2 Modern populations can show a variety

of different size distributions. Fossil assem-

blages can also show size–frequency pat-

terns. What sorts of processes can modify

the original population polygons of a

once-living species?

3 Paleocommunities through time increased

their diversity by the expansion of eco-

space. Why then did diversity reach a

plateau during the Paleozoic but appears to

still be increasing in the Modern fauna?

4 Large-scale ecological changes seem to be

decoupled from major changes in taxo-

nomic diversity. Is this a valid observation

or are our data too crude to actually test

this?

5 The Earth has suffered huge extremes in

climate through time. Can the Gaia

hypothesis help explain these climate

swings? Will the planet ever experience a

real snowball Earth?

Further reading

Bennett, K.D. 1997. Evolution and Ecology: The Pace

of Life. Cambridge University Press, Cambridge, UK.

(Relationship between ecology and evolution from a

Quaternary perspective.)

Brenchley, P.J. & Harper, D.A.T. 1998. Palaeoecology:

Ecosystems, Environments and Evolution. Rout-

ledge. (Readable textbook on most aspects of current

paleoecology.)

Briggs, D.E.G. & Crowther, P.R. (eds) 1990. Palaeobiol-

ogy – A Synthesis. Blackwell Scientiﬁ c Publications,

Oxford. (Modern synthesis of many aspects of con-

temporary paleontology.)

Copper, P. 1988. Paleoecology: paleoecosystems, paleo-

communities. Geoscience Canada 15, 199–208.

(Concise but informative integration of the main

concepts of paleocommunity ecology.)

Cronin, T.M. 1999. Principles of Paleoclimatology. Per-

spectives in Paleobiology and Earth History. Colum-

bia University Press, New York.

Frakes, L.A., Francis, J.E. & Syktus, J.I. 1992. Climate

Modes of the Phanerozoic. Cambridge University

Press, Cambridge, UK. (Overview of ancient climates

through time.)

Lovelock, J. 1998. The Ages of Gaia. Bantam Books,

New York. (Stimulating discussion of the Gaia

hypothesis.)

Vermeij, G.J. 1987. Evolution and Escalation. An Eco-

logical History of Life. Princeton University Press,

Princeton, NJ. (Fundamental text on the inﬂ uence of

predation on the history of life.)

Vrba, E.S. 1996. Climate, heterochrony, and human

evolution. Journal of Anthropological Research 52,

Time before present (Ma)

100

1000

0.2

0.4

0.6

0.8

2.3 2.1 1.9 1.7 1.5

live

dead

methane

oxygen

methanogens

consumers

photosynthesizers

Temperature (˚C)Population Gases (ppm)

(a)

(b)

(c)

Figure 4.28 Precambrian Gaia and evolution of

the biosphere. (a) Changes in climate in a live

and lifeless world; there is a sharp fall in

temperature when oxygen appears. (b) The

changing abundance in atmospheric gases. (c)

Changes in the composition of ecosystems: both

the photosynthesizers and methanogens increase

initially when oxygen appears but the

methanogens eventually decline to a much lower

level of abundance. (Population is the proportion

of the total population in tenths.) (From

Lovelock 1998.)

114 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

1–28. (Important paper relating hominid evolution

to climate change.)

Wilson, E.O. 1992. The Diversity of Life. Belknap

Press, University of Harvard, Cambridge, MA.

(Excellent discussion of modern and past biodiver-

sity and the problems that the Earth’s ecosystems

now face.)

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Chapter 5

Macroevolution and the tree of life

Key points

• Evolution by natural selection is a core scientiﬁ c model that was set out by Darwin,

and has been conﬁ rmed again and again in every branch of biology.

• Creationist attempts to promote their religious beliefs, such as “intelligent design” or

belief in a ﬂ at Earth, are not testable and therefore are not science.

• Speciation often occurs by the establishment of a barrier, and the isolation of part of a

previously interbreeding population.

• Evolution takes place both within species lineages (phyletic gradualism) and at the time

of speciation (punctuated equilibrium); the ﬁ rst model is commonest among asexual

microorganisms that live in the open oceans, and the latter in sexual organisms that are

subject to environmental and geographic barriers.

• There may be a process of species selection, acting independently of natural selection,

but examples have been hard to ﬁ nd.

• The evolution of life may be represented by a single branching phylogenetic tree.

• Cladistics is a method of reconstructing phylogeny based on the identiﬁ cation of shared

derived characters (homologies).

• Molecular sequencing provides additional evidence for reconstructing and dating the

tree of life.

• DNA has been extracted from fossils such as woolly mammoths, but not from truly

ancient fossils.

Probably all organic beings which have ever lived on this earth have descended from

some one primordial form, into which life was ﬁ rst breathed . . . There is grandeur in

this view of life . . . that, whilst this planet has gone cycling on according to the ﬁ xed

law of gravity, from so simple a beginning endless forms most beautiful and most

wonderful have been, and are being evolved.

Charles Darwin (1859) On the Origin of Species

MACROEVOLUTION AND THE TREE OF LIFE 117

Darwin laid the framework for evolutionary

biology 150 years ago. Despite millions of

essays, books and web sites discussing evolu-

tion, no one has yet falsiﬁ ed Darwin’s theory

of evolution by natural selection, and so it

stands as the core of modern biology and

paleontology, just as Isaac Newton’s laws

stand at the heart of much of modern physics.

And yet, surprisingly, Darwin is quoted, and

misquoted, by many special-interest groups

who want to use him in support of, or against,

their views of politics, sociology and religion.

So it is important to understand what Darwin

said, how his insights affect science today, and

how paleontology relies on modern evolution

as its basis.

Charles Darwin’s On the Origin of Species

(1859) is usually remembered as the book

that made the case for natural selection as the

mechanism of evolution, sometimes called

“survival of the ﬁ ttest” (see pp. 118–19).

Since the time of Darwin, evolution has been

seen in action in the laboratory and in the

ﬁ eld, and paleontologists use the principles of

evolution to understand how species origi-

nate. The origin of species is also core to a

second theme in Darwin’s writings, namely

phylogeny, or the pattern of evolution that is

often represented as a branching tree diagram.

Darwin’s idea was that life had diversiﬁ ed to

millions of species by the continued splitting

of species from a common stem (Fig. 5.1).

Indeed, he proposed that all of life, modern

and ancient, could be followed back down the

phylogenetic tree to a single point of origin:

modern evidence conﬁ rms this remarkable

insight.

Darwin’s branching diagram also explained

for the ﬁ rst time the meaning of the natural

hierarchy of life that Linnaeus had discovered

100 years earlier (Box 5.1). This natural inclu-

sive branching hierarchy is the basis of modern

approaches to discovering the tree of life, the

single great evolutionary tree that links all

species living and extinct, from the modern

biodiversity of over 10 million species, back

to a single hypothetical species 3500 million

years ago in the Precambrian. Paleontological

aspects of evolution, such as the tree of life

and studies of processes over thousands and

millions of years, are sometimes called mac-

roevolution (“big evolution”) to distinguish

them from microevolution (“small evolu-

tion”), all the smaller-scale and shorter-term

processes studied by biologists and geneticists

in the laboratory or in the ﬁ eld.

(a) (b)

ABCD E F GH IKL

III

VII

VIII

XII

XIII

XIV

Figure 5.1 (a) Charles Darwin. (b) Branching diagram of phylogeny, the only illustration in On the

Origin of Species (1859). It shows how two species, A and I, branch and radiate through time. The

units I–XIV are time intervals of variable length, and the lower case letters (a, b, c) represent new

species.