Transcript Chapter 12
Paleozoic Life: Invertebrates Burgess Shale Paleozoic Life • Geology drove biology – evolution and plate tectonics • Evolution affected by – the opening and closing of ocean basins – transgressions and regressions of epeiric seas – formation of mountains – changing positions of the continents Tremendous Biologic Change • Dramatic biologic change in Paleozoic – appearance of skeletonized animals – marine invertebrates adaptive radiation and evolution • diversification and extinction • culminating at the end of the Paleozoic Era in the greatest mass extinction in Earth history Archaeooides, an enigmatic spherical Cambrian fossil from the Mackenzie Mountains, Northwest Territories, Canada – specimen is several millimeters in size Cambrian Explosion • Beginning of the Paleozoic Era – animals with skeletons appeared abruptly in the fossil record • Sudden and rapid appearance of new animals in the fossil record – rapid, however, only in the context of geologic time – millions of years during the Early Cambrian Period • How many million? Not a Recent Discovery • Early geologists observed sudden appearance of skeletonized animals • Charles Darwin addressed this problem in On the Origin of Species – such an event was difficult to reconcile with his newly expounded evolutionary theory http://ethomas.web.wesleyan.edu/scie639/EXP L.gif Sharp Contrast • The sudden appearance of shelled animals – contrasts sharply with the biota living during the preceding Proterozoic Eon • Up until the evolution of the Ediacaran fauna – Earth was populated primarily by single-celled organisms • The Ediacaran fauna • consists primarily of multicelled softbodied organisms Restoration of the Ediacaran Environment Time Between • Long time period assumed to have existed – between the extinction of the Ediacaran fauna – and the evolution of the first Cambrian fossils • That gap has narrowed in recent years – with the discovery of more fossils • known Proterozoic fossil assemblages continue right to the base of the Cambrian – recent work from Namibia indicates that Ediacaranlike fossils are even present above the first occurrence of Cambrian index fossils Triggering Mechanism • Hypotheses about triggers: – critical threshhold • both biological and geological • Environmental trigger (Snowball Earth): – Earth was glaciated one or more times during the Proterozoic – followed by global warming during the Cambrian • Evolutionary trigger: – appearance of Hox genes • (how do we know this?) Snowball Earth • Shallow seas formed as supercontinents broke up • organic matter could be produced rapidly and- decomposition was restricted • atmospheric O2 goes up; CO2 and CH4 go down; • Usually when glaciers form, their formation stops CO2 consumption by rock weathering • thwarted by equatorial continents • eventually volcanoes outgassed enough CO2 to melt the Snowball Hox genes • all metazoans (animals except sponges + algae) have the same genetic controls on body organization • Hox genes: master switch in body pattern creation (‘put a leg here and an eye there’) • these genes first appeared in the Cambrian When Hox genes mutate... Skeletons (1) protection against ultraviolet radiation, allowing animals to move into shallower waters (2) prevent drying out in an intertidal environment (3) protection against predators – recent evidence of actual fossils of predators • and specimens of damaged prey • indicates that the impact of predation during the Cambrian was great Cambrian Ecology • Reconstruction of Anamalocaris – a predator from the Early and Middle Cambrian – about 45 cm long and probably fed on trilobites – gripping appendages presumably carried food to its mouth Wounded Trilobite • Wounds to the body of the trilobite Olenellus robsonensis – wounds have healed suggesting predation Cambrian Marine Community • Body plans • Most documented evolutionary experiments • Almost all the major invertebrate phyla evolved during the Cambrian Period – many were represented by only a few species • Majority of Cambrian skeletonized life: – trilobites – brachiopods – archaeocyathids Cambrian Marine Community • Floating jellyfish, swimming arthropods, benthonic sponges, and scavenging trilobites Trilobites • Most conspicuous element of the Cambrian marine invertebrate community – about half of the total fauna – benthonic – mobile – sediment-deposit feeders • Appeared in the Early Cambrian, and rapidly diversified, – reached their maximum diversity in the Late Cambrian, – and then suffered mass extinctions near the end of the Cambrian, never fully recovered Trilobite Extinctions • No consensus on what caused the trilobite extinctions – plate tectonics -> changed ecological niches – increased competition – rise in predators • A cooling of the seas may have played a role – particularly for the extinctions that took place at the end of the Ordovician Period http://www.nmsu.edu/~geology/zuhl/trilobite.jpg Cambrian Brachiopods • Cambrian brachiopods – not abundant until the Ordovician Period http://www.palaeontology.geo.uu.se/Mainp ages/Brachiopoda/brach Ordovician Brachiopod Archaeocyathids • Archaeocyathids, an extinct group of sponges – benthonic sessile suspension feeders – constructed reeflike structures • The rest of the Cambrian fauna consisted of representatives of the other major phyla, – including many organisms that were short-lived evolutionary experiments http://www.carleton.ca/~tpatters/teaching/intro/ cambrian/cambrianex16.html Cambrian Reeflike Structure • Restoration of a Cambrian reeflike structure built by archeocyathids Burgess Shale Soft-Bodied Fossils • 1909, Charles D. Walcott • Smithsonian Institution, – first soft-bodied fossils from the Burgess Shale – a discovery of immense importance in deciphering the early history of life • Walcott and his collecting party split open blocks of shale – yielding the impressions of a number of soft-bodied organisms – beautifully preserved on bedding planes Burgess Shale • Presented a much more complete picture of a Middle Cambrian community • What conditions led to the remarkable preservation of the Burgess Shale fauna? Reason for the Preservation • Animals preserved in the Burgess Shale – lived in and on mud banks – that formed along the top of a steep submarine escarpment • Periodically, this unstable area would slump – slide down the escarpment as a turbidity current – mud and animals carried with it were deposited in a deep-water anaerobic environment Rare Preservation: Burgess Shale • Ottoia, a carnivorous worm Rare Preservation: Burgess Shale • Wiwaxia, a scaly armored sluglike animal Rare Preservation: Burgess Shale • Hallucigenia, a velvet worm Rare Preservation: Burgess Shale • Waptia, an anthropod How Many Phyla arose during the Cambrian? • At the center of that debate are the Burgess Shale fossils • Early hypothesis: most Burgess Shale organisms are part of existing phyla • Competing hypothesis: Cambrian had many more phyla Burgess Body Plans • Highly diverse – Gould: more diverse than ever before – Evolutionary experimentation Ordovician Marine Community • A major transgression that began during the Middle Ordovician (Tippecanoe sequence) – resulted in the most widespread inundation of the N. A. craton • This vast epeiric sea, which experienced a uniformly warm climate during this time – opened numerous new marine habitats – soon filled by a variety of organisms Striking Changes in Ordovician • Both sedimentation patterns and fauna underwent striking changes • Ordovician was characterized by the adaptive radiation of many animal phyla • brachiopods • Bryozoans • corals – with a consequent dramatic increase in the diversity of the total shelly fauna Middle Ordovician Seafloor Fauna • Cephalopods, crinoids, colonial corals, trilobites, and brachiopods Brachiopods •Brachiopods —present since the Cambrian —began a period of major diversification —in the shallow-water marine environment during the Ordovician Graptolites • Excellent guide fossils – especially abundant – most graptolites were planktonic – most individual species existed for less than a million years • Due to the fragile nature of their organic skeleton – most commonly found in black shales Conodonts – well-known small toothlike fossils – composed of the mineral apatite • (calcium phosphate) – the same mineral that composes bone Mass Extinctions • End of the Ordovician a time of mass extinctions in the marine realm – > 100 families of invertebrates became extinct • What caused such an event? – another massive glaciation? Silurian and Devonian Marine Communities • Ordovician extinction -> rediversification, recovery of: – brachiopods, bryozoans, gastropods, bivalves, corals, crinoids, and graptolites Massive Reef Builders • Silurian and Devonian were times of major reef building • While most of the Silurian radiations of invertebrates represented repopulating of niches • Organic reef builders diversified in new ways – building massive reefs – larger than any produced during the Cambrian or Ordovician Middle Devonian Reef • corals, cephalopods, trilobites, crinoids, and brachiopods Ammonoids • Excellent guide fossils ! – for the Devonian through Cretaceous periods – with their distinctive suture patterns – short stratigraphic ranges – widespread distribution Another Mass Extinction • Another mass extinction occurred near the end of the Devonian – worldwide near-total collapse of the massive reef communities – were most extensive in the marine realm Another Mass Extinction • The tropical groups were most severely affected – in contrast, the polar communities were seemingly little affected • Apparently, an episode of global cooling – was largely responsible for the extinctions near the end of the Devonian – WHY? Actors in Extinctions • During such a cooling, the disappearance of tropical conditions – would have had a severe effect on reef and other warm-water organisms • Cool-water species, on the other hand, could have simply migrated toward the equator • The closing of the Iapetus Ocean and the orogenic events of the Late Devonian – undoubtedly also played a role in these extinctions – by reducing the area of shallow shelf environments where many marine invertebrates lived Remember the connection between the tectonic and faunal changes! Carboniferous and Permian Marine Communities • The Carboniferous invertebrate marine community (Mississipian and Pennsylvanian) – responded to the Late Devonian extinctions – in much the same way as the Silurian invertebrate marine community responded to the Late Ordovician extinctions – that is, by renewed adaptive radiation and rediversification Mississippian Marine Life – blastoids – crinoids – lacy bryozoans – brachiopods – small corals Restricted Permian Marine Faunas • The Permian invertebrate marine faunas resembled Carboniferous faunas – not as widely distributed – restricted size of the shallow seas on the cratons – reduced shelf space along the continental margins Permian Period • Paleogeography of North America during the Permian Period Permian Patch-Reef Community • From Glass Mountains of West Texas – algae, productid brachiopods, cephalopods, sponges, and corals The Permian Marine Invertebrate Extinction Event • Greatest recorded mass-extinction event – occurred at the end of the Permian Period • Before the Permian ended – roughly 50% of all marine invertebrate families – and about 90% of all marine invertebrate species became extinct Phanerozoic Diversity • Diversity of marine invertebrate and vertebrate families – 3 episodes of Paleozoic mass extinction are visible – with the greatest occurring at the end of the Permian Period Permian Mass Extinction • Potential causes: – (1) a meteorite impact (like the end of the Cretaceous Period) – (2) a widespread marine regression resulting from glacial conditions – (3) volcanic eruptions -> CO2 -> warming -> collapsed ocean circulation -> anoxia -> euxinia Permian Mass Extinction • It appears that the Permian mass extinction – took place over an 8-million-year interval at the end of the Permian Period – which would seemingly rule out a meteorite impact: see reading for essay three for more discussion of this Biota Dramatically Changed • Regardless of the ultimate cause of the Permian mass extinctions, – the fact is that Earth's biota was dramatically changed • Triassic marine faunas were of low diversity – but the surviving species tended to be abundant – and widely distributed around the world