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.
Kevin Peterson, Dartmouth
Animals got their start in the Ediacaran, followed by the Cambrian “Explosion of Life.”
Sponges
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
Original Interpretation.
Correct Interpretation
Anomalocaris, the largest predator of the Cambrian and an extinct phylum.
Trilobite with a bite mark, possibly from Anomalocaris
Anomalocaris in hot pursuit of Marella
Pikaia
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
Phylum Mollusca
BIVALVIA
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
AsteroidOphiuroid
Starfish feeding on bivalves
Devonian starfish
Echinoids:
sand dollar (left)
sea biscuit (below)
Holothurian: sea cucumber
