Significance of the Cambrian Explosion

Base of Cambrian/Paleozoic/Phanerozoic

The base of the Phanerozoic Eon is defined by the first appearance of burrows (presumably made by worm-like organisms). This indicates the evolution of a coelom that could be manipulated for locomotion and feeding.

Burrowing also added an important tier to community structure (no organisms burrowed prior to this time).

Tommotian Fauna (small shelly fossils): first skeletonized organisms

Early Cambrian(scale bar: 1 mm)

Typically phosphatic (but calcified forms now known) !

Oldest Skeletons

Why Do So Many Metazoans Have Skeletons ?:

1. Receptacles for excess mineral matter - note Kidney Stones and Gallstones in humans- warm seawater commonly saturated with calcium carbonate

2. Storehouses for scarce minerals (or means of slow release)-bones and teeth, some shells composed of calcium phosphate

- phosphate rather scarce in nature, but essential for metabolism (e.g. Adenosine triphosphate- ATP)

and is also key component of genetic material-calcium essential for heart, nerve, muscle functions,enzyme activation

3. Support and muscle attachment areas for locomotory organs

4. Serves as protective cage for soft internal organs

5. In shelled organisms, serves as a box to ensure controlled environment for metabolic functions

6. Protection from predators

The Pitfalls of Preservation

It is obvious that a major change occurred in the Earth’s metazoan biota by Cambrian time (particularly in the development of skeletal tissue).

But while skeletal remains give us some indication of the magnitude of change that occurred in the earliest Phanerozoic, the perils of fossil preservation prevent us from seeing the entire biota (both skeletonized and soft-bodied).

But… we do have a window shortly after this (Middle Cambrian). This window is the Burgess Shale.

Trilobites from Middle Cambrian Burgess Shale showing soft part preservation

Charles Walcott

Discovered Middle Cambrian Burgess Shale

Burgess Shale, Yoho National Park, B.C.

Exceptionally preserved soft-bodied organismsMiddle Cambrian age (shortly after Cambrian Explosion of Early Cambrian)About 505 maA snapshot of life assemblages (in general, fossil recordonly preserves hard parts of organisms) Surprisingly diverse assemblage with very unusual forms

Vauxia -sponge Vauxia “forest” with Leanchoilia

Wiwaxia

A worm, a mollusc, or something completely different ?

Marella

An early arthropod (presumably related to trilobites)

Marella with squished-out gutsMarella caught in the

act of molting

AnomalocarisAn unusual shrimp-like arthropod ?

PeytoiaA jellyfish ?

Anomalocaris: A Case of Mistaken Identity

Anomalocaris: A composite of components previously

Believed to be separate organisms

(Peytoia)

(Original Anomalocaris)

(Peytoia)

Anomalocaris – oblique viewA “Lobopod”

Anomalocaris - reconstruction

Opabinia(Lobopod ?)

The great Hallucigenia flip-out

Hallucigenia Presently classified as onychophoran

Hallucigenia

Modern “velvet worms” (onychophorans): in tropical rainforests

An aside: are arthropods just onychophorans with exoskeletons ?

millipede

onychophoran

Pikaiaearliest known Chordate

Sanctacaris(Santa Claws)

An undoubted arthropod(A Chelicerate without chelicerae ?)

Subphylum Chelicerata includes: spiders, mites, ticks scorpions,horseshoe crabs all having feeding appendages called chelicerae

Makes one wonder what really defines this group

A similar Burgess Shale- type biota has since been found in the Chengjiang County, Yunnan Province, China.

Yu'anshan Member of the Heilinpu Formation.

These fossils are about 525 ma (Early Cambrian), and therefore slightly older than the Burgess Shale fossils.

Chengjiang Fauna

Haikouella lanceolata

Of major importance in the Chengjiang biota is the occurrence of a chordate named Haikouella (this is older than Pikaia, which was previously declared to be the oldest chordate).

It is decidedly lamprey-like, indicating the appearance of jawless, fish-like chordates by the early Cambrian (even though the oldest definite remains of jawless fish date to the Early Ordovician)

Cambrian marine community

Note differences in community structure

Ediacaran marine community

All suspension feeders (or at least passive food gatherers)

CarnivoresSuspension feeders

Deposit feeders

The first arms race !

Catastrophic Burial

Burgess Shale organisms living on foot of escarpment (and possibly on edge and top of escarpment as well) smothered by due to slumping,Killed and buried instantly

Rapid burial + low oxygen (+ possible mineralization in vicinity of cold seeps ?)

= exceptional preservation

Stephen Jay Gould – Wonderful Life

Could some Burgess Shale organisms belong to extinct phyla ?Is it possible that a phylum could be represented by few or single species ?

If so, suggests that the Cambrian Explosion produced more phyla than are present today. This view has been “softened” a bit since Gould’s publication of Wonderful Life (perhaps more Classes than today)

extinction

Conventional view:Gradual increase in numberof phyla through time

Gould’s View:Sudden appearance of phyla,removal of many by mass extinction

END OF LECTURE