Part IV: The History of Life

The Cambrian Explosion and beyond...

The earliest animal remains

These date to 580-600 million years ago (Ma) [later Precambrian]

Often very abundant, these are thought to represent burrows and tubes of worm-like creatures

[if remains of worms, these ancient animals must have been Bilateria]

Trace fossils

The Ediacaran Period

• 560-580 Ma

• named for Ediacaran hills in SE Australia, where the first deposits were found

• flattened, non-mobile, non-predatory animals with uncertain affinities to modern forms

Dickensonia, a common fossil of unknown affinity

Ediacaran fossils

probably an ancestor of sea pens (Pennatulacea)

Kimberella, the only known bilaterally symmetrical Precambrian animal

Ediacaran animals

• disappeared by 560 Ma

• ancestors of Cambrian animals, or…

• a failed experiment in animal evolution?

tick marks are 12 my

Paleozoic:

From start

of Cambrian

through

major (end-

Permian)

extinction

tick marks are 12 my

Paleozoic

Cambrian explosion!

540-525 Ma

The Burgess Shale

• 520-515 my old Cambrian deposits

• in Yoho Provincial Park, British Columbia

The Burgess Shale• perhaps the most spectacular fossils ever

found

• exquisitely preserved remains of invertebrates from most phyla (and several chordates)

• many predatory and highly complex

The Burgess Shale “Problematica”

HallucigeniaAnomalocaris

Opabinia Wiwaxia

From Smithsonian Inst., NMNH

From AAAS

Opabinia regalis was probably an arthropod

the “nozzle” was a claw

Most Burgess Shale animals are clearly from modern phyla

Olenoides, a trilobite

a priapulid worm (with muscles

showing)

Pikia, a cephalochordate

Vauxia, a sponge

dinner, 520 my ago

Molecular phylogenies of the animal phyla allow us to “order” major events in animal evolution that occurred in the Cambrian.

Cambrian: a revolution in animal evolution

Chordate synapomorphies

Origin of bilateral symmetry (?)

Protostomes

Major patterns of embryonic development (e.g. gastrulation)

Precambrian diversification of bilateral animals (~600 my)

Cambrian explosion of fossilizable forms (~20 my)

Some studies of protein evolution push origins of phyla back to 1000-1200 mya

tick marks are 12 my

Paleozoic

Cambrian explosion!

540-525 Ma

480 Ma

440 Ma

425 Ma

365 Ma

360 Ma

tick marks are 7.5 my

Mesozoic190 Ma

150 Ma

110 Ma

Cenozoictick marks are 2.7 my 30 Ma

5-6 Ma??

Macroevolution, according to...

Darwin (1859)Eldredge and Gould (1972)

Gradual morphological change occurs continuously

Morphological evolution is not associated with speciation

Phyletic gradualism

Darwin (1859)

Morphological change occurs in bursts

Most change occurs at speciation

“Stasis” otherwise

Punctuated equilibrium

Eldredge and Gould (1972)

Tests for punctuated equilibrium vs gradualism• must avoid “circular reasoning”

(species are recognized by breaks in morphology)

• valid tests require– a good phylogeny– coexistence of species after

speciation

Punctuated equilibrium in fossil Bryozoa

Jackson and Cheetham 1994

From Futuyma 2005

From Futuyma 2005

From Futuyma 2005

Why stasis…a lack of genetic variation?

No.

Morphologically conservative horseshoe crabs show as much (or more) genetic divergence as between king crabs and hermit crabs.

from Avise et al. (1994)

Stasis may occur due to “zigzag” evolution

24 different shell characters in 3 Pliocene bivalve lineages change, but fluctuate around a mean value...

from Stanley and Yang (1987)

Extinction

The marine fossil record shows that diversity has increased, more or less steadily, to the present.

This has been punctuated by 5 major “mass extinctions.”

from Primack, 3rd ed., Sinauer

The end-Permian extinction eliminated ~ 95% of the species, and >50% of the families on Earth

Mass Extinction (% of Families): The Big Five

Based on Stanley (1979), from Freeman and Herron (1998)

Lyellian curves are used to estimate extinction rates

• find the X-value (in My) at which 50% of the species are extinct

• double this value

– this yields the time (in My) for 100% turnover

– this is also the average duration of a single species in the fossil record (its “survivorship”)

• the “lifespan” of mammal species (~1.5 My) is much less than that of Pacific bivalve molluscs (~15 My)

• this suggests great variation in extinction rates (and speciation rates?) across lineages

• still, species durations in most groups range from 1-10 My

From Freeman and Herron (1998)

Other analyses of Lyellian curves confirm that extinction rates are highly variable

For example, Tertiary extinction rates for tetrapods are much greater than those of insects or bivalves

Survivorship of species with range >2500 km is 10 times that of species with range <1000 km. From Jablonski (1986)

Marine species extinction rates depend on larval dispersal

Fossils of species with plankton-feeding larvae (planktotrophs) persist 3 times as long as nonplanktotrophs. From Jablonski (1986)

Extinction rates depend on larval

dispersal powers

Why? Planktotrophs have longer periods of development, allowing greater dispersal and broader geographic range. This “buffers” against extinction.

Recent bird and mammal extinction rates

• the best data on recent extinction rates• a dramatic rise in extinction rate after 1850• this is followed by a drop 1950-2000

Predicting future extinctions: The Species Report Card (NHDC and The Nature Conservancy 1997)

Aquatic inverts, FW fishes, flowering plants are most vulnerable to future extinctions