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

Was the evolution of early cells from simple organic compounds in Precambrian oceans a slow, improbably process? Or did life simply need... - liquid water - lack of free oxygen - simple amino acids - shelter from heavy meteorite showers

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. Ediacaran fauna, the arliest probably animals, evolved 570 million years ago.

Ediacaran remains are generally found in nearshore sandstones. In finer-grained sedimentary rocks of Neoproterozoic age (570-545 million years ago), the presence of crawling traces and burrows suggest that other kinds of animals were also evolving. Are they the traces of predators and grazing herbivores? Did they put an end to the Ediacaran lifestyle, and restricted the extent of stromatolites?

Is the Cambrian explosion “for real”? Evidence of life forms becomes more obvious in strata of Cambrian age. 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 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 intervals have been recognized: 1) SSF 2) Tommotian fauna 3) 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 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.

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 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: the reference 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.

Cambrian trilobites, lacking claws or specialized mouth parts for chewing, were probably not predators… There were larger animals around. Scattered remains of a diverse Cambrian soft-bodied fauna were found first in the Rocky Mountains of B.C., in the Burgess Shale…

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

Paleontologists were groping in the dark until complete specimens were found in Early Cambrian strata from Chengjiang, China. - soft bodied fauna, like the Ediacarans - but 30 million years younger - evidence of predation and burrowing - most phyla surviving today are represented - but also complex fossils of uncertain affinities

Some re-assembly was required... jaws interpreted as jellyfish claws = Anomalocaris Up to 2 meters long