The Early Paleozoic World:

the Cambrian explosion of life

EPSC233 Earth & Life History (Fall 2002)

Recommended reading:

STANLEY “Earth System History”Chapter 13, pp. 344-350.

Keywords: Phanerozoic eon, Paleozoic era, Cambrian period, Tommotian fauna, phylum (pl. phyla), evolutionary radiation, invertebrate animals.

The 3.5 billion years old Archean cells preserved in chert, and the low C13/C12 ratios of graphite inclusions in 3.8 billion year old BIFs suggest that life arose within 100 million years of the end of heavy meteoritic bombardment.

Biomarkers suggest that eukaryotes may have evolved from prokaryotes 2.7 billion years ago.

The Ediacaran fauna (earliest probable animals) evolved 570 million years ago.

Prior to the discovery of Ediacaran fossils, the Precambrian time was referred to as the “Azoic” eon (i.e. “without life”) .

This was most irritating to the early defenders of the hypothesis that life had evolved from simpler to more complex forms... including Darwin.

Like today, these early evolutionists eager to find primordial fossils did eventually find them!

A metamorphic rock from the Grenville province (1.1 billion years-old) found in 1858 by William Logan, the founder of the Geological Survey of Canada.

In 1864, Sir William Dawson (father of North American paleobotany) named the fossil “Eozoon canadense” and affirmed its parentage with protozans (eukaryotic unicellular organisms). To the end of his life, he defended the thesis of a biological origin for this primordial fossil. Darwin eagerly accepted Eozoon as support for his ideas.

In 1894, the detailed description of a green and white layered metamorphic rock collected near Vesuvius (Italy) convinced geologists that the layering must have formed without the influence of life.

The 1946 discovery of cm-sized fossils in the Ediacara hills of the Flinders Range, Australia, presented a new enigma to paleontologists.

Was the “Garden of Ediacara” a community of organisms without predators?

Ediacaran fossils are generally found in nearshore sandstones. It would have taken a tough skin to resist abrasion by quartz grains.

A modern sea pen (essentially a “soft coral”). Sea pens leave only scattered spicules (mineral needles) when they die.

What were the Ediacarans/Vendozoans?

• Were they animals with a quilted structure (thick walls separating compartments like air mattress)?

What were the Ediacarans/Vendozoans?

• Were they “endosymbionts”, i.e. a primitive eukaryotic animal that lived in symbiosis with photosynthetic bacteria (a bit like modern corals?)...

• Were they lichens?

• Were they preserved in sandstone because they were covered (and protected) by cyanobacterial colonies?

Was the diverse Ediacaran fauna a short-lived evolutionary phase, spanning only 15-20 million years at the end of the Neoproterozoic?

Work is ongoing, in the Mackenzie Mountains and Mexico where older impressions (pre-Varangian ice age) have been found.

Radiometric dates are rare, so “chemostratigraphy” is increasingly used.

Neoproterozoic (900-550 Ma) is marked by:

- decrease in extent of stromatolites (being grazed on by herbivores?)- increase in burrows & feeding traces (do animals evolve new feeding/hiding strategies but leave no body fossils because most are soft-bodied?)- appearance & decline of the Ediacaran fauna (under pressure from the predators and grazers who left the trace fossils?)

After 570 Ma, Neoproterozoic fine-grained rocks (shales and siltstones) show simple burrows and feeding traces, probably made by worm-like organisms.

Were they grazers, responsible for the demise of the Ediacaran fauna and the decline of stromatolites?

Very few Neoproterozoic fossils suggest that life forms (algal or animal?) were starting to armor their body with CaCO3 coverings.

In Namibia (Germs, 1972), Canada (British Columbia, Hoffman and Mountjoy, 2001), Mexico and southwestern U.S., calcareous tubes and stemmed sheaths have been found. These enigmatic fossils are known as Cloudina and Namacalathus.

Namacalathus

Very thin shells. Overall shape reconstructed by tomography (from digital photographs taken as the rock is polished, gradually exposing the specimen).

Cloudina

Is the Cambrian explosion “for real”?

Evidence of life forms becomes more obvious in strata of Cambrian age.

Many different kinds of animals started to produce hard (mineralized) parts (exoskeletons, shells, teeth, sclerites…) of calcium carbonate (CaCO3) and calcium phosphate (material similar to the mineralized part of our bones and teeth).

Fossils preserved as flat impressions are easy to overlook. They are flat and visible only on bedding surfaces.

The rock must be broken up into plates to reveal the fossils.

Hard parts are usually easier to find than impressions of soft-bodied animals because:

• they are often more resistant than the rest of the rock in which they were preserved. Weathering tends to make such fossils “stand out” from the rock.

• Hard parts are sometimes replaced by a mineral more resistant than the rest of the rock.

The fossil record suggests that animal life evolved faster during the 40 million years of the Cambrian period than during the rise of the Ediacaran fauna.

Three evolutionary steps have been recognized:

1) SSF (small shelly fossils)2) Tommotian fauna3) larger, skeletalized fauna

The lowermost Cambrian contains a “small shelly fauna” (SSF) of simple fossils: vase- and tube-shaped, or teeth-like.

(All are a few millimeters in size.)

The SSF is succeeded by the richer Tommotian fauna, first discovered in Siberia.

- all are small hard parts, most < 1 cm.- many are unlike any hard parts found in living animals, or fossils from strata younger than Cambrian age.- a few belong to groups that survive today.

Many of the smaller fragments were probably sclerites, i.e. scales and spines which covered small, armored, worm-like animals.

Aldamella, a primitive mollusk

Anabarella,ancestral to present-day mollusks

This Tommotian fauna was “short-lived”, lasting perhaps 3-4 million years.

The Tommotian fauna is followed by fossils from much larger animals, reaching a few centimeters to nearly two meters.

Most of them belonged to phyla that have survived to this day.

The most abundant type of animal remains in the Cambrian fossil record are arthropods.

horseshoe crab scorpion

The phylum Arthropoda, today, includes insects, spiders, crabs… All animals without a spine or internal skeleton, but with a light exoskeleton, a segmented body and jointed legs.

Trilobites were the earliest arthropods. They appear in Cambrian strata and diversified rapidly.

Trilobite enrolled for protection.

Planktonic form(floated or swam)

Benthic(crawler)

Trilobites may have been around earlier but became “visible” in the Cambrian when they started to reinforce their exoskeletons with CaCO3...

Traces similar to those found with Cambrian trilobites occur in Neoproterozoic strata and lowermost Cambrian strata.

Proterozoic traces are shallow, simple, often “resting traces” rather than deeper meandering feeding traces.

resting traces of jellyfish?

Trichophycus pedum is a trace fossil at the base of Cambrian strata.

Fortune Head, Newfoundland: thereference section for base of Cambrian

The trace fossil Trichophycus pedum - marks the first occurrence of fairly complex metazoan animals.- occurs nearly worldwide- sometimes with the last Ediacaran fossils, but usually in strata above them. The first shelly fossils clearly appear later.

Trichophycus pedum was officially chosen in 1991 as the most useful fossil to mark the boundary between the Proterozoic and the Cambrian.

Other Cambrian phyla include:

benthic animals (bottom-dwellers)

Edrioasteroid (primitive echinoderm, ancestral to starfish) on a brachiopod shell.

Some of the middle Cambrian fossils were so unlike anything known today that the first reconstructions mistakenly included parts from different animals... If Tuzoia is a bivalved shrimp: is this its shrimp-like body?

Tuzoia-Anomalocaris hybrid

Tuzoia