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The Cambrian Explosion and Beyond

Chapter 17

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Fig. 2.18 The geological time scale

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Limitations of the fossil record

• Hard parts – shells, bones, teeth – most likely to be fossilized because they decay slowly and are more durable

• In order to be fossilized, a specimen generally needs to be covered quickly by water-borne or wind-borne sediments (sand, mud, ash)

• Lack of oxygen is favorable for fossilization• The fossil record consists primarily of hard parts

left in depositional environments such as river deltas, beaches, flood plains, marshes, lakeshores, and the sea floor — the fossil record is biased

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Early animals – Ediacaran fauna(565 – 544 mya)

• First multicellular animals appear about 565 million years ago (mya)

• Simple, small, asymmetric or radially symmetric – sponges, jellyfish

• Few bilaterally symmetric forms, such as Kimberella, which has uncertain affinities, but appears to be mollusc-like

• Dickinsonia considered by some to be an annelid worm, and by others to be cnidarian (jellyfish) polyp

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Kimberellahttp://www.ucmp.berkeley.edu/vendian/kimberella2.html

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Dickinsoniahttp://www.ucmp.berkeley.edu/vendian/dickinsonia.jpg

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The Cambrian explosion

• Cambrian period 543 – 495 mya• “Explosive” appearance of large, complex,

bilaterally symmetric animals, segmented animals with limbs, antennae, shells, external skeletons, and notochords – including arthropods, molluscs, annelids, and chordates within the period 543 – 506 mya

• Almost all living animal phyla are present• Burgess shale fauna (Simon Conway Morris;

Stephen Jay Gould, Wonderful Life, 1989)

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The first animals: phylogeny and fossils (Fig. 17.12)

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Macroevolutionary Patterns

• Adaptive radiation• Punctuated equilibrium versus gradualism• Extinction• Taxon survivorship curves• Mass extinctions• The Cretaceous – Tertiary (K-T) impact extinction• Anthropogenic extinction

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Adaptive Radiation

• An adaptive radiation occurs when a single or a small group of ancestral species rapidly diversifies into a large number of descendant species that occupy a wide variety of ecological niches

• Adaptive radiations can occur when:– A species colonizes a new region where there are no or few

competitors (i.e., lots of empty niches) – Galápagos finches, Hawaiian Drosophila and silver swords

– A taxon acquires an important adaptation – evolution of flight in birds

– A taxon is released from competition after extinction of a dominant group – radiation of mammals after extinction of the dinosaurs (?)

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Darwin’s Finches (Fig. 3.4)

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Adaptive radiation (Fig. 17.13)

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Punctuated Equilibrium

• Niles Eldredge and Stephen Jay Gould, 1972• The fossil record for some groups reveals that

morphological evolution consists of long periods of stasis with little or no change and very short periods during which morphological change occurs in association with speciation

• Presented as a challenge to the “conventional” picture of morphological evolution described by the modern synthesis

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The controversy• Phyletic gradualism

– New species arise by transformation of large ancestral groups (often without splitting – anagenesis)

– Transformation occurs over all or a large part of the ancestral species geographic range

– Transformation is even and slow– Evolution occurs more or less at the same rate during and between

speciation events

• Punctuated equilibrium– A small subgroup of the ancestral form gives rise to a new group

by splitting – cladogenesis– New species originates in a small part of the ancestral species

geographic range – peripheral isolates model– New species develop rapidly, then may replace ancestral species– Between speciation events there is stasis

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Patterns of morphological change: punctuated equilibrium and gradualism (Fig. 17.15)

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Explaining the fossil record

• Darwin was aware of this “problem” and he explained the apparent discontinuities and sudden transitions in the fossil record as being due to the incompleteness of the fossil record

• Eldredge and Gould argued that sudden transformations are not artifacts – speciation occurs rapidly and in small populations and is, therefore, unlikely to leave a fossil record

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Testing punctuated equilibrium

• Strong tests of punctuated equilibrium vs. phyletic gradualism are difficult

• Need a complete stratigraphic sequence• Are morphospecies biological species?• Cryptic species?• Need multiple specimens and populations of each species

in order to determine the range of variation within species

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Punctuated change in cheilostome Bryozoa (Jackson and Cheetham 1994) (Fig. 17.16)

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Who wins?

• Erwin and Antsey (1995) – review of 58 tests of punctuated equilibrium

• “Paleontological evidence overwhelmingly supports a view that speciation is sometimes gradual and sometimes punctuated, and that no one mode characterizes this very complicated process in the history of life.”

• About 1/3 of the studies support a combination of gradualism and stasis

• Time-scale effects – when the resolution of the fossil record is on the order of 10’s of thousands of years, morphological change may appear “sudden”, but if we had been present during a 10,000 year period to witness it, it might have seemed gradual

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Genetic and morphological change in two arthropod clades (Fig. 17.18)

a. Horseshoe crabs today are almost identical to those that lived 150 million years agob. Hermit crabs and alliesGenetic distances based on 16s rRNA gene sequences

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Extinction

• Extinction is the ultimate fate of all species• Mass extinctions

– The big five– Global in extent– Involve a broad range of organisms– Rapid

• “Background” extinction– Accounts for the vast majority of extinctions

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Distribution of species extinction intensities(Raup 1994) (Fig. 17.20)

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Patterns of extinctions of families through time (Benton 1995) (Fig. 17.21)

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Survivorship curves for taxa

• Leigh Van Valen (1973) showed that the probability of extinction of a taxon was independent of its age – a taxon does not become more, or less, likely to go extinct as it gets older

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Survivorship curves for genera and families(Van Valen 1973) (Fig. 17.22)

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How long does a species of marine bivalve exist? (Jablonski 1986) (Fig. 17.23)

Species with planktonic larvae have longer durations than do species with direct development

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Geographic range affects the survivorship of bivalve and gastropod species (Jablonski 1986) (Fig. 17.24)

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The Cretaceous – Tertiary (K-T) extinction

• World-wide iridium anomaly at the K-T boundary dated to 65 mya

• Iridium is rare in the Earth’s crust but more abundant in meteorites

• Based on the amount of iridium required to produce the deposits seen at the K-T boundary, Alvarez et al. (1980) estimated that a 10 km (6.2 mi) diameter asteroid hit the earth

• The asteroid theory is supported by the presence of shocked quartz crystals, glass microtektites, and a 180 km (112 mi) diameter crater in the ocean near the Yucatán Peninsula

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Iridium anomaly at

the K-T boundary

(Fig. 17.25 b)

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Location and shape of the Chicxulub crater (Schultz and D’Hondt 1996) (Fig. 17.26)

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Effects of a 10 km asteroid impact

• Widespread wild fires

• Acid rain (from release of SO2)

• Darkness and intense cooling from blockage of sunlight – reduced photosynthesis

• Tsunami (up to 4 km high)• Severe earthquakes ?• Increased volcanism ?

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Probability of extinction at the K-T boundary and geographic range of marine bivalves (Jablonski and Raup 1995)

(Fig. 17.28)

Provinces are graphic regions that share similar flora and fauna

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Anthropogenic extinctions - 1

• Between 1600 and 1993, humans observed extinction of 486 animal species and 600 plant species (Smith et al. 1993)

• Most of these extinctions occurred in North America, the Caribbean, Australasia, and Pacific Islands

• Currently, the taxa with the highest proportion of endangered species include: Palms – 925 of 2,820 species (33%) Gymnosperms – 242 of 758 species (32%) Birds – 1,029 of 9,500 species (11%) Mammals – 505 of 4,500 species (11%)

• Widespread habitat destruction is probably the greatest anthropogenic cause of extinction at present

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Anthropogenic extinctions - 2

• Pacific island birds– Steadman (1995) estimates 2,000 species have become

extinct over last 2,000 years – almost 20% of all bird species

– 60 endemic Hawaiian species extinct in last 1,500 yr

– 44 New Zealand species extinct since human colonization – including 8 species of moas, the largest known birds

– On the island of ‘Eua only 6 of 27 land birds present before human occupation are still living

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Exinction of forest birds on the island of ‘Eua (Tonga) (Jablonski and

Steadman 1995)

(Fig. 17.29)

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Deforestation in the Brazilian Amazon (Skole and Tucker 1993) (Fig. 17.30)

a. 1978b. 1988

During this period of time the annual loss of forested area was about 15,000 km2, an area approximately equivalent to the state of Connecticut per year