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 organisms Middle Cambrian age (shortly after Cambrian Explosion of Early Cambrian) About 505 ma A snapshot of life assemblages (in general, fossil record only preserves hard parts of organisms) Surprisingly diverse assemblage with very unusual forms

Vauxia -spongeVauxia “forest” with Leanchoilia

Wiwaxia A worm, a mollusc, or something completely different ?

Marella An early arthropod (presumably related to trilobites) Marella with squished-out guts Marella caught in the act of molting

Anomalocaris An unusual shrimp-like arthropod ? Peytoia A jellyfish ? Anomalocaris: A Case of Mistaken Identity

Anomalocaris: A composite of components previously Believed to be separate organisms (Peytoia) (Original Anomalocaris) (Peytoia)

Anomalocaris – oblique view A “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

Pikaia earliest 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) Carnivores Suspension 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 number of phyla through time Gould’s View: Sudden appearance of phyla, removal of many by mass extinction

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