Marine Diversity II
October 3, 2002
Precambrian paleobiology, continued
The Ediacaran Fauna – the Vendian Period
The biological big bang – the Cambrian Period
Rate of diversification
increasing diversity
hard parts
The Burgess Shale
Phanerozoic diversity
booms and busts
patterns – evolutionary faunas
The first fossil evidence of eucaryotic cells comes from fossils called acritarchs - spherical microfossils that have thick and complex organic walls.
-They appear to be the resting spores of free-floating, aquatic, and eucaryotic algae.
-the oldest ones found are from 1,600 million year old rocks in the Ural Mountains.
The oldest metazoan (multicellular) organisms date from about 600 million years ago:
The Ediacaran fauna (examples on handout) - known first from south Australia; now known from around the world
--Vendian Period, last time period in Proterozoic
--all soft-bodied
--all simple geometries, with radial or bilateral symmetry, some with an anterior—posterior differentiation
--ancestors of jellyfish and the like or evolutionary dead ends?
Interesting as the Vendian Edicaran Fauna is, it doesn't
seem to have survived or left much if anything in the way of descendants. The next wave in the metazoan diversification
is marked by very different organisms and marked by a very significant change
in the evolution of life: the first skeletons.
Cambrian diversification: The biological big bang
The topic now is the diversification of metazoan, or multicellular forms of life. This is an interesting topic for a number of reasons, among them:
-that it happened at all;
how long did it take? When did it happen? (>900 Ma or ~540 Ma)
why did it happen / what might have prompted it
The Cambrian Event--the biological big bang
-the first shelly fossils appear in the early Cambrian and they are typified by small
cones and tubes of various sorts – the “small shellies”
-also abundant trace fossils for the first time
-skeletons both calcium phosphate and calcium carbonate
-all the major phyla appear in the Cambrian for the first time
--recalibrating the base of the Cambrian. Story from Science Sept. 1993. Uranium-dating volcanic rock fragments in a conglomerate at the base of the Cambrian.
Instead of having 65 My for the origin of the major phyla (570 - 505 Ma), as originally thought, the recalibrated time scale suggests 35 Ma (540-505) instead. New calibration affects our estimation of Cambrian evolutionary rates: fast. “The Biological Big Bang”.
A graph of the number of families of metazoans during the latest Proterozoic and Cambrian (on handout) shows the phenomenon.
1. a diversification that began about 600 million years ago, at the end of a Proterozoic period called Vendian.
2. and picked up speed at the start of the Cambrian.
-ability to get above surface (cleaner water?)
-ability to enclose water within shell for cleaner filter feeding
-protection from predators
-surfaces for attachment of muscles and thus greater mobility
-ability to grow larger (later in the Early Cambrian)
In the Early Cambrian, the diversification accelerates:
-larger animals become abundant for the first time
-trace fossils (tracks, trails and burrows) become more numerous
-representatives of almost all the major metazoan fossil groups appear for the first time: (branching earlier; where are the fossils?)
-arthropods (including trilobites)
-cnidaria (corals)
-sponges
-brachiopods
-molluscs
-echinoderms
-first evidence of predation (healed bite marks on trilobites.
Triggers for the big bang:
1. external, environmental changes:
oxygen concentration increased?
some other chemical change?
2. internal, biological trigger.
Changes in the genetic or developmental system to cause increased
variation?
Increased selection pressure from predation?
Glimpse of Cambrian world comes from one of those rare fossil deposits that contains the traces of soft-bodied organisms as well as those with hard parts. This is the
Burgess Shale Fauna
The Burgess Shale is a Middle Cambrian deposit of marine shales that is exposed in the Canadian Rockies. The animals lived at the base of a submarine cliff, and populations were often wiped out and buried by submarine landslides of soft mud.
Preserved there are not only the usual bits of the hard-shelled Cambrian fauna of trilobites, sponges and brachiopods, but the films of soft tissue of some really weird and wonderful stuff. (on handout)
2. Hallucigenia
3. Anomalocaris - two species shown, note circular mouth on one above. -whole animal the largest in the fauna - up to .6 meter (about 2 feet long). Thought to be a predator.
4. Sanctacaris
5. Pikaia - probably a chordate - note stiffened rod and muscle bands.
6. Marrella - not a trilobite, but vaguely similar - common form in the Burgess Shale.
7. Olenoides – trilobite with anterior and posterior antennae
8. Opabina – nozzle and five eyes
Lessons of the Burgess Shale:
1. Middle Cambrian faunas were quite morphologically diverse; the word is "disparity" their morphologies were very unlike each other. Even so soon after the origin of metazoans. Evolution can proceed quite rapidly.
2. While there are some descendants of these wild and wonderful Burgess beasts, most lineages died out. Evolution can be thought of as experimentation followed, most commonly, by extinction. Survival is the exception, not the rule.
Phanerozoic marine diversity (handout)
Plots number of marine families known as fossils through the Phanerozoic.
The pattern: Rapid Cambrian-Ordovician increase, mid- to late Paleozoic plateau, Permo-Triassic crash (mass extinction), Mesozoic-Cenozoic increase (with small crash at K/T). Two rapid diversifications, one long plateau. Why?
A potential sampling bias: available rock volume in each period varies through time – is the diversity curve a function of sampling?
The big five mass extinctions (end Ordovician, end Devonian, Permo-Triassic, end Triassic, end-Cretaceous.) More later when we discuss extinctions.
The three faunas: deconstructing Phanerozoic diversity (Fig 13.3 on handout): “Evolutionary faunas” -- Groups of animals that show the same pattern of diversity through time
Cambrian fauna – inarticulate brachiopods, trilobites
Paleozoic fauna – articulate brachiopods, crinoids, rugose and tabulate
corals, cephalopods
Modern fauna – bivalve and gastropod mollusks, crustaceans, fish,
sharks