Evolution and the Fossil Record
Fossil Record
rocks dated from 542 to about 488 Ma include fossils representative of most animal phyla living today, such as...
crustaceans and other arthopods
onychophorans
sipunculid worms
segmented worms
mollusks
chordates
Confirms periods of rapid diversification, such as the Cambrian Explosion
demonstrates evidence of five episodes of mass extinctions
Methods of Formation:
- Amber and Freezing
least altered remains
provide the most information
rare
- Permineralization and Replacement
when structures are buried in sediments
original shape is preserved but composition is altered
comon
- Natural Molds and Casts
originate when remains decay after being buried in sediment
preserve information about surface shape but no internal details
- Trace fossils
record behavior instead of form
examples: dinosaur tracks, copralites
difficult to determine which species made it
Life on an Evolving Earth
life evolved on a world that was itself changing, driven primarily by plate tectonics constantly rearranging continents and oceans
Phanerozoic
Paleozoic: ancient life
Mesozoic: middle life
Cenozoic: recent life
Evolution
morphology is a critical source of info
DNA data is mostly unavailable
fossil species are recognized on the basis of their anatomy (morphospecies concept)
convergent or parallel evolution can be difficult to recognize
juveniles and adults can sometimes be confused for separate species
fossil record provides data on aspects of evolution that can never be studied or replicated in the lab
Ediacaran Biota
precambrian
first unequivocal evidence for macroscopic life
565-544 Ma; end of the Proterozoic era
difficult to classify
most preserved as impressoin
include forms of life no longer represented on E today
includes sponges, jellyfish, and comb jelly relatives
small in size (a few cm) and relatively simple morphology
fossilized embryos support the hypothesis that bilaterians evolved before Cambrian
Cambrian
Burgess Shale Fauna
is a Lagerstatte
records an astonishing variety of large and complex bilaterally symmetric animals
located in British Columbia, Canada
preserved even soft-bodied Cambrian fossils
provided an unparalleled view of the early diversification of life in the Late Cambrian ocean
most animal phyla alive today make their first appearance in the fossil record during the Cambrian
fauna includes:
wide array of complex and unusual arthropods, including trilobites
segmented worms
wormlike priapulids and sipunculids
diversity of mollusks
several chordates, including jawless vertebrates
Problematica
some species in the Burgess Shale Fauna are so unusual that they have been hard to classify
now grouped with living phyla, or at least with early Phanerozoic fossils of known affinity
Cambrian Explosion: Rapid appearance of many large and complex animals
an astonishing variety of body plans, cell types, and developmental patterns evolved
What caused this?
Sudden increase in seawater during Proterozoic and increase in atmospheric oxygen in mid-Cambrian
more oxygen -> bigger bodies are possible
larger size is a prerequisite for the evolution of tissues
higher metabolic rates are required for active movement
Mass extinction eliminated much of Ediacaran fauna at end of Proterozoic, making opportunity for the tiny deuterostomes and protostomes to evolve in response to the changed conditions
Major Transitions
Fish-Tetrapod Transition
leading up to it...
by Ordovician, terrestrial plants had evolved
insects began exploiting resources on land from the Silurian or earliest Devonian
Devonian is often referred to as the "age of fishes", with astonishing diversity displayed by aquatic vertebrates
past few decades have shed light on the sequence of character evolution associated with the first land living, limbed vertebrates: the tetrapods
includes...
fin turned into limb
structural changes to withstand the increased effects of gravity
Tiktaalik is a transition form, a "fishapod"
all sarcopterygians
Ichthyostega
Acanthostega had well-developed joints between consecutive vertebrae
Eusthenopteron, an aquatic taxa
enlarged rib attachments
Why?
classic theory is largely based on ecology of Australian lungfish, which moves between ponds that shrink during the dry season
late Devonian was substantially warmer than today
untapped foods available on land such as insects
Dinosaur-Bird Transition
Archaeopteryx has been famous for its position as the oldest definitive fossil bird
theropods
mix of avian and reptilian features
plesiomorphies include
teeth
long tail
three clawbearing fingers on the hand
synapomorphy is well-developed feathers
clade of bipedal, carnivorous dinosaurs that includes birds as its living descendants
early members have many features commonly associated with birds and flight
Paleontologists have found feathers from animals that clearly could not fly
Sinosauropteryx
bears fuzzy, filamentous integumentary structures
provided the first evidonce of a down-like covering in theropods
Why did feathers first evolve?
display: as sexual signaling devices
thermoregulation: for insulation or to aid in brooding eggs
Origin of Mammals
fossil record of synapsid lineage captures transition from reptile-like forms in the Carboniferous to the earliest mammals by the Early Jurassic
Cynodonts
include Procynosuchus ad Thrinaxodon
clade the includes mammals as its living representatives
evolutionary transition concerns middle ear
in early synapsids, modern reptiles, and birds, only a single element (the stapes) is present
in mammals this structure includes an air-filled cavity with three bones (malleus, incus, and stapes)
Extinction
Background Extinctions
affects limited number of species
Mass Extinction
96% of phanerozoic extinctions
result of a normal evolutionary process
ongoing extinction of individual species due to environmental or ecological factors
large scale
sudden extinction that is geographically and taxonomically widespread
ultimate fate of all species
about 4% of phanerozoic extinctions
A decline in rate of both origination and extinction across the Phanerozoic
seen in several studies of marine organisms
hypotheses:
optimization of fitness
energy input to E has been decreasing
has happened five times
Sixth mass extinction?
majority of recently extinct species inhabited islands, whose extinctions have usually resulted from human hunting or the introduction of nonnative predators or competitors
current concern is focused on habitat loss due to expanding human populations
current human pop is 7 billion, growing at 1.2% a year, resulting in 12+ billion by 2050
unless human population growth declines rapidly, threats to natural habitats will grow in intensity over the next several decades
Extinctions are now occuring at 100-1000x the background rate. If this continues and all rare species go extinct, it would take less than 100 years for 60% of living species to be wiped out
Three types of approaches to predict how continued habitat destruction will affect extinction rates:
- Multiply # of species found per hectare in different environments by rates of habitat loss measured from satellite photos
- Quantify the rate that well-known species are moving from threatened to endangered to extinct status in the lists maintained by conservation groups
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