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