Part IV: The History of Life The Cambrian Explosion and beyond...
Jan 04, 2016
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