Part IV The History of Life The Cambrian

Part IV: The History of Life The Cambrian Explosion and beyond. . .

Trace fossils 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]

The Ediacaran Period • 560 -580 Ma • named for Ediacaran hills in SE Australia, where the first deposits were found • flattened, non-mobile, nonpredatory animals with uncertain affinities to modern forms

Ediacaran fossils Dickensonia, a common fossil of unknown affinity 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?

Paleozoic: tick marks are 12 my From start of Cambrian through major (end. Permian) extinction

Paleozoic 540525 Ma Cambrian explosion! tick marks are 12 my

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” Hallucigenia Anomalocaris Opabinia Wiwaxia

Opabinia regalis was probably an arthropod From Smithsonian Inst. , NMNH the “nozzle” was a claw From AAAS

Most Burgess Shale animals are clearly from modern phyla Pikia, a cephalochordate Olenoides, a trilobite dinner, 520 my ago Vauxia, a sponge a priapulid worm (with muscles showing)

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

Protostomes Major patterns of embryonic development (e. g. gastrulation) Origin of bilateral symmetry (? ) Chordate synapomorphies

Some studies of protein evolution push origins of phyla back to 1000 -1200 mya Precambrian diversification of Cambrian explosion of fossilizable forms (~20 my) bilateral animals (~600 my)

440 Ma Paleozoic 540525 Ma Cambrian explosion! 425 Ma 480 Ma 365 Ma 360 Ma tick marks are 12 my

150 Ma Mesozoic 190 Ma tick marks are 7. 5 my 110 Ma

Cenozoic tick marks are 2. 7 my 5 -6 Ma? ? 30 Ma

Macroevolution, according to. . . Eldredge and Gould (1972) Darwin (1859)

Phyletic gradualism Darwin (1859) Gradual morphological change occurs continuously Morphological evolution is not associated with speciation

Punctuated equilibrium Eldredge and Gould (1972) Morphological change occurs in bursts Most change occurs at speciation “Stasis” otherwise

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

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 from Stanley and Yang (1987) 24 different shell characters in 3 Pliocene bivalve lineages change, but fluctuate around a mean value. . .

Extinction

The marine fossil record shows that diversity has increased, more or less steadily, to the present. from Primack, 3 rd ed. , Sinauer This has been punctuated by 5 major “mass extinctions. ”

Mass Extinction (% of Families): The Big Five The end-Permian extinction eliminated ~ 95% of the species, and >50% of the families on Earth

Lyellian curves are used to estimate extinction rates Based on Stanley (1979), from Freeman and Herron (1998)

• 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

From Freeman and Herron (1998) • still, species durations in most groups range from 1 -10 My

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