Notes: This nautilus lives in waters at a depth of 600 to 800 meters. Often their habitat is at the bottom of a steep slope under coral reefs.

At night they use their natural jet propulsion to maneuver up over 325 meters to feed.

Why an “explosion”?• What were conditions like at the end of the Proterozoic?

Supercontinent was breaking up

climate warming after glacial periodIncreased oxygen in atmosphereMore coastal habitat – shelf environmentsEukaryotic cells added to diversityStage was set for larger cells, specialized cells, and multi-celled

organismsSexual reproduction allowed for shuffling of alleles increasing variation

within species

Result? Rapid diversification of life forms and numbers of families and species occurred during the early Paleozoic. No “explosion” – it took millions of years.

• Diorama of the environment and biota – of the

Phyllopod bed of the Burgess Shale, • British

Columbia, Canada

Burgess Shale

• On August 30 and 31, 1909, – Charles D. Walcott, • geologist and head of the Smithsonian Institution,

– discovered the first soft-bodied fossils – from the Burgess Shale, – a discovery of immense importance in deciphering

the early history of life– beautifully preserved on bedding planes

Burgess Shale Soft-Bodied Fossils

• a rare glimpse into a world previously almost unknown – that of the soft-bodied animals that lived some

530 million years ago• a much more complete picture – of a Middle Cambrian community – than deposits containing only fossils of the hard

parts of organisms (the bias of the fossil record favors hard parts to be preserved)

More Complete Picture of a Middle Cambrian Community

• 60% of the total Burgess Shale assemblage – of more than 100 genera is composed of soft-

bodied animals, – a percentage comparable to present-day marine

communities• What conditions led to the remarkable

preservation of the Burgess Shale fauna? • The site of deposition of the Burgess Shale – was located at the base of a steep submarine

escarpment

Sixty Percent Soft-Bodied

• The animals lived in and on mud banks – that formed along the top of this escarpment

• Periodically, this unstable area – would slump and slide down the escarpment as a

turbidity current• At the base, the mud and animals carried with it

were deposited in a deep-water anaerobic environment devoid of life, predators, and O2.

Reason for the Preservation

• In such an environment, – bacterial degradation did not destroy the buried

animals– and they were compressed by the weight of the

overlying sediments – and eventually preserved as carbonaceous

impressions

Carbonaceous Impressions

• Evolution and plate tectonics forcing:Early Paleozoic characterized by

– The opening and closing of ocean basins, (Wilson Cycles)

– transgressions and regressions of epeiric seas, • as evidenced by Cratonic Sequences

– the formation of mountain ranges, • Orogenies that formed Mobile Belts

– and the changing positions of the continents – had a profound effect on the evolution – of the marine and terrestrial communities

Study of Paleozoic Life

• appearance of skeletonized animals – near the Precambrian-Cambrian boundary

• marine invertebrates – began a period of relatively rapid evolution of body

types– Paleozoic marine invertebrate community greatly

diversified

– Actually the “explosion” had taken place over millions of years during the Early Cambrian Period

Tremendous Biologic Change

• At the beginning of the Paleozoic Era, – animals with skeletons appeared rather abruptly in

the fossil recordMicroscopic, then larger, visible shelly fauna

The Cambrian Explosion

• A conical sclerite* of Lapworthella from Australia* a piece of

the armor covering

– This specimen is several millimeters in size

Lower Cambrian Shelly Fossil

• Archaeooides, an enigmatic spherical fossil from the Mackenzie Mountains, Northwest Territories, Canada– This

specimen is several millimeters in size

Lower Cambrian Shelly Fossil

• : (1) It provides protection against ultraviolet radiation,

allowing animals to move into shallower waters; (2) it helps prevent drying out in an intertidal

environment; (3) it provides protection against predators– Recent evidence of actual fossils of predators

• and specimens of damaged prey, • as well as antipredatory adaptations in some animals, • indicates that the impact of predation during the

Cambrian was great

What are advantages of an Exoskeleton

• Reconstruction of Anamalocaris– a predator from the Early and Middle Cambrian– It was about 45 cm long and probably fed on

trilobites – Its gripping appendages presumably carried food

to its mouth

Cambrian Predator

• Wounds to the body of the trilobite Olenellus robsonensis– The wounds have healed, demonstrating that they

occurred when the animal was alive and were not inflicted on an empty shell

Wounded Trilobite

• With predators playing an important role – in the Cambrian marine ecosystem, – any mechanism or feature – that protected an animal – would certainly be advantageous – and confer an adaptive advantage to the organism

(4) A fourth advantage is that – a supporting skeleton, whether an exo- or

endoskeleton, – allows animals to increase their size – and provides attachment sites for muscles

Advantages of an Exoskeleton

• Plankton are mostly passive and go where currents carry them– Plant plankton • such as diatoms, dinoflagellates, and various algae,

– are called phytoplankton and are mostly microscopic– Many are photosynthesizers

– Animal plankton are called zooplankton and are also mostly microscopic• Examples of zooplankton include foraminifera,

radiolarians, and jellyfish

Planktonic organisms

• The nekton are swimmers – and are mainly vertebrates • such as fish

– the invertebrate nekton • include cephalopods -- chambered, coiled shellfish

Nektonic organisms

• Organisms that live – on or in the seafloor make up the benthos

• They can be characterized – as epifauna (animals) or epiflora (plants), • for those that live on the seafloor,

– or as infauna, • which are animals living in and moving through the

sediments

Benthic organisms

• The benthos can be further divided – into those organisms that stay in one place, – called sessile, – and those that move around on or in the seafloor,– called mobile

Sessile and Mobile

Marine Ecosystem

Mobile epifauna: gastropod, starfish

Infauna:

worm, bivalve

• Where and how animals and plants live in the marine ecosystem

Marine Ecosystem

Plankton:

Jelly fish

Nekton: fish

cephalopod

Benthos: d-k

Sessile epiflora:

seaweed Sessile epifauna:

bivalvecoral crinoid

• Although almost all the major invertebrate phyla – evolved during the Cambrian Period – many were represented by only a few species

• While trace fossils are common – and echinoderms diverse,

• the organisms that comprised the majority of Cambrian skeletonized life were– trilobites, – inarticulate brachiopods, – and archaeocyathids

Cambrian Skeletonized Life

Cambrian Marine Community• Floating jellyfish, swimming arthropods,

benthonic sponges, and scavenging trilobitesReconstruction

• Trilobites were – by far the most conspicuous element – of the Cambrian marine invertebrate community – and made up about half of the total fauna

• Trilobites were – benthonic – mobile – sediment-deposit feeders – that crawled or swam along the seafloor

Trilobites

• They first appeared in the Early Cambrian, – rapidly diversified, – reached their maximum diversity – in the Late Cambrian, – and then suffered mass extinctions – near the end of the Cambrian – from which they never fully recovered

• As yet no consensus exists on what caused the trilobite extinctions

Trilobites

– possible reduction of shelf space, – increased competition, – rise in predators• cooling of the seas may have played a role,

at the end of the Ordovician Period

Trilobite Extinctions

• Cambrian brachiopods – were mostly primitive types called inarticulates

• They secreted a chitinophosphate shell, – composed of the organic compound chitin – combined with calcium phosphate (like teeth)

• Inarticulate brachiopods – also lacked a tooth-and-socket-arrangement – along the hinge line of their shells

Cambrian Brachiopods

• The articulate brachiopods, – which have a tooth-and-socket arrangement, – were also present – but did not become abundant – until the Ordovician Period

Articulate Brachiopods

• The third major group of Cambrian organisms – were the archaeocyathids

• These organisms – were benthonic sessile suspension feeders – that constructed reeflike structures

Archaeocyathids

• Restoration of a Cambrian reeflike structure built by archeocyathids

Cambrian Reeflike Structure

Back to the Burgess Shale….

• The diversity of organisms preserved in the mud that is now the Burgess Shale shows– Most modern phyla are represented– Additional organisms cannot be placed in the

evolutionary tree– A primitive chordate may have existed – “Pikaia”

Pikaia, chordateancestor of vertebrates??

• Ottoia, a carnivorous worm

Rare Preservation: Burgess Shale

• Wiwaxia, a scaly armored sluglike creature whose affinities remain controversial

Rare Preservation: Burgess Shale

• Hallucigenia, a velvet worm

Rare Preservation: Burgess Shale

• Waptia, an anthropod

Rare Preservation: Burgess Shale

• Thus, the phyla of the Cambrian world – were viewed as being essentially the same in

number – as the phyla of the present-day world, – but with fewer species in each phylum

• According to this view, the history of life – has been simply a gradual increase in the diversity

of species – within each phylum through time

• The number of basic body plans – has therefore remained more or less constant – since the initial radiation of multicelled organisms

Cambrian Phyla

• The Cambrian invertebrate community – was dominated by trilobites, inarticulate brachiopods, and

archaeocyathids, • the Ordovician was characterized – by the adaptive radiation of many other animal

phyla, • such as articulate brachiopods, bryozoans, and corals

Striking Changes in Ordovician

• Recreation of a Middle Ordovician seafloor fauna with cephalopods, crinoids, colonial corals, trilobites, and brachiopods

Middle Ordovician Seafloor Fauna

• The end of the Ordovician – was a time of mass extinctions in the marine realm

• More than 100 families of marine invertebrates became extinct,

• and in North America alone, – approximately one-half of the brachiopods and

bryozoans died out• What caused such an event? – Many geologists think these extinctions – were the result of the extensive glaciation – that occurred in Gondwana – at the end of the Ordovician Period

Mass Extinctions

• Mass extinctions, – those geologically rapid events – in which an unusually high percentage – of the fauna and/or flora becomes extinct,

• have occurred throughout geologic time – for instance, at or near the end of the • Ordovician, • Devonian, • Permian, • and Cretaceous periods

– and are the focus of much research and debate

Mass Extinctions

• The Silurian and Devonian reefs – were dominated by – tabulate and colonial rugose corals and

stromatoporoids• While the fauna of these Silurian and Devonian

reefs – was somewhat different – from that of earlier reefs and reeflike structures, – the general composition and structure – are the same as in present-day reefs

Silurian and Devonian Reefs

• Reconstruction of a Middle Devonian reef from the Great Lakes area – with corals, cephalopods, trilobites, crinoids,

and brachiopods

Middle Devonian Reef

• Restoration of a Silurian brackish-marine bottom scene – near Buffalo New York– with algae, eurypterids,

gastropods, worms, and shrimp

Silurian Brackish-Marine Scene

• Ammonoids are excellent guide fossils – for the Devonian through Cretaceous periods – with their distinctive suture patterns, – short stratigraphic ranges, – and widespread distribution

Ammonoids

• A late Devonian ammonoid cephalopod

Ammonoid Cephalopod

– from Erfoud, Morocco

– The distinctive suture pattern, short stratigraphic range, and wide geographic distribution make ammonoids excellent guide fossils

• Another mass extinction – occurred near the end of the Devonian – and resulted in a worldwide near-total collapse – of the massive reef communities

• On land, however, the seedless vascular plants – were seemingly unaffected,

• Thus, extinctions at this time – were most extensive in the marine realm, – particularly in the reef and pelagic communities

Another Mass Extinction

• Paleogeography of North America during the Permian Period

Permian Period

• From Glass Mountains of West Texas– including algae, productid

brachiopods, cephalopods, sponges, and corals

Permian Patch-Reef Community

• The greatest recorded mass extinction event – to affect Earth – 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

The Permian Marine Invertebrate Extinction Event

• Diversity for marine invertebrate and vertebrate families

Phanerozoic Diversity

– 3 episodes of Paleozoic mass extinctions are visible

– with the greatest occurring at the end of the Permian Period

• What caused such a crisis – for both marine and land-dwelling organisms?

• Various hypotheses have been proposed, – but no completely satisfactory answer – has yet been found

• Some scenarios put forth to explain the extinctions include – (1) a meteorite impact such as occurred at the end

of the Cretaceous Period – (2) a widespread marine regression resulting from

glacial conditions,

Mass Extinction

– (3) a reduction in shelf space due to the formation of Pangaea,

– (4) oceanographic changes such as anoxia, salinity changes, and turnover of deep-ocean waters

• It appears that the Permian mass extinction – took place over millions of years – at the end of the Permian Period, – which would seemingly rule out a meteorite

impact

Permian Mass Extinction