Download Outline 14: Paleozoic Life

Outline 14:
Paleozoic Life
Radiation of the Animal
Phyla
Cambrian Life
• The first animals evolved about 100 my
before the start of the Cambrian. These
are the Ediacaran fossils of the latest
Proterozoic.
• None of these animals had hard parts.
• Base of the Cambrian defined by first
animals with hard parts.
Life at the end
of the
Proterozoic
Cambrian Life
• Early Cambrian fossils consist mostly of
trilobites, brachiopods, archaeocyathids,
and small little shells.
Cambrian trilobites cruising on Saturday night
Typical
Cambrian
trilobites
Modern horseshoe crabs
look similar to trilobites,
but they are not closely
related. Example of a
“living fossil.” Trilobites
are extinct.
A living Inarticulate Brachiopod. Very
common fossils in the Cambrian.
Modern Inarticulate Brachiopods in their burrows
Modern Inarticulate Brachiopods for dinner in
southeastern Asia.
Cambrian Archaeocyathids:
Reef-Forming Animals
Examples of small shelly fossils from the
Early Cambrian. Scale bars are 0.1 mm.
Cambrian Life
• The Middle Cambrian Burgess Shale
records the “Explosion of Life.” All
known phyla had appeared by then.
• A phylum is a major body plan.
Examples: Mollusca, Annelida,
Arthropoda, Chordata, etc.
Animals got their start in the Ediacaran,
followed by the Cambrian “Explosion of Life.”
Sponges
Kevin Peterson, Dartmouth
The Cambrian
Explosion
made the
cover of
TIME.
Burgess Shale Fossils
• Most are soft-bodied fossils, a very rare
kind of fossilization.
• Of today’s 32 living phyla, 15 are found
in the Burgess Shale. The other 17 are
microscopic or too delicate to be
preserved.
• Another 10 extinct phyla are also found
in the Burgess Shale.
Burgess Shale Fossils
• Assume that all 32 living phyla were
alive in the Middle Cambrian.
• Add the 10 extinct phyla for a total of
42 phyla. That’s more phyla than
today!
• Thus, Cambrian phyla were more
diverse than today.
A Paradox
• There were more body plans (phyla) near
the start of animal life than today.
• However, there were many fewer
species.
• This doesn’t match the expectation of
slow evolutionary diversification of life.
The Pattern of Animal
Evolution
•
•
•
•
Initial radiation of phyla.
Reduction by natural selection.
No new phyla since the Cambrian.
Diversification within remaining phyla.
A Hypothesis
• The genome of early animals was less
rigid, not as “hardwired” as later animals.
Adaptive mutations were more possible.
• A wide variety of body plans were
produced by mutations.
• Natural selection eliminated some of
these body plans.
A Hypothesis
• Body plans that survived became the
modern phyla.
• 500 m.y. of evolution has made genomes
more rigid and more species rich.
• Mutations required to make a new body
plan would be lethal. Phyla were locked
in.
The Burgess Shale of British Columbia,
Canada: record of the Cambrian Explosion
Mt. Stephen in Yoho National Park, Canada
Geologists at the Burgess Shale quarry
Trilobites!
Paleontologist collecting a slab of fossils
Trilobites with
preserved legs and
antennae
The strange
animals of the
Middle Cambrian
Burgess Shale
Opabinia and Amwiskia, representatives of
two extinct phyla
Opabinia
The first sea scorpion on the attack!
Marella, extinct class of arthropods
Marella, extinct class of arthropods
Marella as
Cambrian road kill
(or a squished bug?)
Yohoia, an extinct class of arthropods
Specimens of
lobopods
Living and fossil lobopods
Burgess Shale worm Ottoia
A spiny “worm,”
Wiwaxia
Hallucigenia, a spiny lobopod
Which way is up?
Hallucigenia
Correct Interpretation
Original
Interpretation.
Anomalocaris, the
largest predator of
the Cambrian and an
extinct phylum.
Trilobite with
a bite mark,
possibly from
Anomalocaris
Pikaia
Anomalocaris in hot
pursuit of Marella
Pikaia, an early chordate
Pikaia, a chordate from the Burgess Shale
Yunnanozoan, a chordate from the early
Cambrian of China
Primitive chordates: Tunicates or Sea Squirts. Adults have
a pharynx with gill slits. Larval forms are free-swimming
and have a notochord. Fish are thought to have evolved
from the larval form by precocious sexual maturation.
Chordate evolution
Branchiostoma, the lancelet; a
primitive living chordate
Invertebrates after the Cambrian
Phylum Cnidaria: colonial corals
Phylum Cnidaria: horn coral
Skeleton of a modern coral
A living sea anemone, relative of corals
Living coral reefs
Living coral reefs
Phylum Bryozoa - fossils
Phylum Bryozoa – living animals
Phylum Brachiopoda
BIVALVIA
Phylum
Mollusca
Mollusca: Class
Bivalvia
Fossil marine bivalve, Kansas
Phylum Mollusca: Class Gastropoda
Phylum Mollusca:
Class Cephalopoda
Nautilus
Nautilus
A Paleozoic Cephaplopod
Phylum Arthropoda
An Ordovician Trilobite
A Silurian Trilobite
The Devonian
Trilobite
Phacops rana
The compound eye
of Phacops rana
A death assemblage of Phacops rana
Eurypterid or
“Sea Scorpian”,
Silurian of New
York
A Cenozoic crab
Phylum Echinodermata
Crinoid
Blastoid
A living crinoid at a depth of 692 m, Bahamas
Slab of Mississippian crinoids – note the long stems
for feeding high above the substrate
Asteroid
Ophiuroid
Starfish feeding on bivalves
Devonian starfish
Echinoids:
sand dollar (left)
sea biscuit (below)
Holothurian: sea cucumber