The Cambrian Explosion. Sept. 20, 2005 The biological big bang the Cambrian Period Rate of diversification Increase in diversity Appearance of hard parts The Burgess Shale as a window into Cambrian life Notes on the video The last of the Proterozoic 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? Dating the event The base of the Cambrian is defined by the first appearance of fossils with hard parts (the small shelly fossils described below. Because the fossils themselves can t be dated directly, the best estimate of the age of the base of the Cambrian comes from radiometric dates (Uranium- Lead) on zircons extracted from volcanic fragments in breccias that occur just below the first fossils. See the diagram below. Because these are dates on the rock fragments in the sedimentary strata, the strata themselves could be younger than the fragments.
Dating the event Bowring et al., 1993. Science 261: 1293-1298. 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 -molecular evidence (based on molecular similarities among living representatives) suggest that the different phyla diverged from each other perhaps 1 Ga years ago. But the groups don t appear in the fossil record for another 500 Ma! Why?
Small shellies from the Early Cambrian. Benton and Harper, 1997 A graph of the number of families of metazoans during the latest Proterozoic and Cambrian (Benton and Harper, 1997) 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. Hard parts appear for the first time at the base of the Cambrian Why skeletons? Possibilities: all or none may be true: -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: (molecular studies suggest branching earlier, but where are the fossils?)
-arthropods (including trilobites) -cnidaria (corals) -sponges -brachiopods -molluscs -echinoderms -first evidence of predation (healed bite marks on trilobites. http://www.fossilnews.com/2000/fftrilo/ffmay00.html Possible triggers for the big bang: 1. external, environmental changes: oxygen concentration increased? Continental break-up Climate change global warming after Snowball Earth 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. (see below) 1. Aysheaia - intermediate between arthropod and annelid worm; interpreted to be a predator on sponges 2. Hallucigenia - spiky side up! 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. Olenoides trilobite with anterior and posterior antennae 7. Opabina nozzle and five eyes Briggs, Erwin and Collier, 1994 Some 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.
Notes on the video: Explosion of Life from the series The Shape of Life Features 1. The Burgess Shale fauna as an example of the Cambrian diversification of life. 2. The diversity of annelid worms and their roles in nutrient cycling Note: while the Burgess Shale is Cambrian, it s Middle Cambrian, not Early Cambrian, so it s the late stages of a biological explosion that began 30 million years before. Interpreting scrappy remains the road kill, or flattened fauna of the Burgess shale Anomalocaris - major predator Opabinia another predator Pikaia earliest chordate; ancestor to vertebrates Aysheaia marine worm: living representative is terrestrial Predation as a selective force in evolution - origin of spines, bristles and other anti-predatory features. Possible causes of the Cambrian explosion: 1. Genetic revolution: The importance of sponges the first metazoans. Cnidarians motion. Basic body plans basic architectures or blueprints of the major phyla appear at about the same time, or in a flash as the video states (that is, within 20 or so million years of each other). 2. Increase in oxygen levels allows size increase. 3. Predators and the beginning of an evolutionary arms race. Annelid worms Diversity of morphology Diversity of habitats Diversity of feeding types Note effect of worm tubes of Diopatra on sediment movement in the intertidal zone Impact of life on the Earth (think stromatolites and oxygen here it s worm burrowing and nutrient cycling). The innovation of the annelid design and the ability to burrow. How might worms have saved the planet from a world of ice? Key scientists featured in the program: Desmond Morris studies the Burgess Shale fauna Rudy Raff works on the biology of the Cambrian explosion Damhnait McHugh studies living annelid worms