Paleozoic Life History: Invertebrates

Chapter 12 Paleozoic Life History: Invertebrates

Burgess Shale • Diorama of the environment and biota • of the Phyllopod bed of the Burgess Shale, • British Columbia, Canada

Burgess Shale Soft-Bodied Fossils • 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 • Walcott and his collecting party split open numerous blocks of shale, • many of which yielded the impressions • of a number of soft-bodied organisms • beautifully preserved on bedding planes

Thousands of Fossil Specimens • Walcott returned to the site the following summer • and located the shale stratum • that was the source of his fossil-bearing rocks • in the steep slope above the trail • He quarried the site • and shipped back • thousands of fossil specimens • to the United States National Museum of Natural History, • where he later catalogued and studied them

More Complete Picture of a Middle Cambrian Community • The importance of Walcott's discovery • is that it allowed geologists a rare glimpse into a world previously almost unknown • that of the soft-bodied animals that lived some 530 million years ago • The beautifully preserved fossils • from the Burgess Shale • present a much more complete picture • of a Middle Cambrian community • than deposits containing only fossils of the hard parts of organisms

Sixty Percent Soft-Bodied • In fact, 60% of the total fossil 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

Reason for the Preservation • The animals • whose exquisitely preserved fossil remains • are found in the Burgess Shale • 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

Carbonaceous Impressions • 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

Study of Paleozoic Life • We will examine the history of Paleozoic life • as a system of interconnected biologic and geologic events • Evolution and plate tectonics • are the forces that drove this system • The opening and closing of ocean basins, • transgressions and regressions of epeiric seas, • the formation of mountain ranges, • and the changing positions of the continents • had a profound effect on the evolution • of the marine and terrestrial communities

Tremendous Biologic Change • A time of tremendous biologic change • began with the appearance of skeletonized animals • near the Precambrian-Cambrian boundary • Following this event, marine invertebrates • began a period of adaptive radiation and evolution • during which the Paleozoic marine invertebrate community greatly diversified • Indeed, the history of the Paleozoic marine invertebrate community • was one of diversification and extinction, • culminating at the end of the Paleozoic Era • in the greatest mass extinction in Earth history

The Cambrian Explosion • At the beginning of the Paleozoic Era, • animals with skeletons • appeared rather abruptly in the fossil record • In fact, their appearance is described • as an explosive development • of new types of animals • and is referred to as • the "Cambrian explosion" by most scientists

The Cambrian Explosion • This sudden and rapid appearance • of new animals in the fossil record • is rapid, however, only in the context of geologic time, • having taken place over millions of years • during the Early Cambrian Period

Not a Recent Discovery • Early geologists observed • that the remains of skeletonized animals • appeared rather abruptly in the fossil record • Charles Darwin addressed this problem • in On the Origin of Species • and observed that, • without a convincing explanation, • such an event was difficult to reconcile • with his newly expounded evolutionary theory

Sharp Contrast • The sudden appearance of shelled animals • during the Early Cambrian • 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, • which is found on all continents except Antarctica, • consists primarily of multicelled soft-bodied organisms

Soft-Bodied Organisms • Microscopic calcareous tubes, • presumably housing worm-like suspension feeding organisms, • have also been found at some localities • In addition, trails and burrows, • which represent the activities of worms • and other sluglike animals • are also found associated • with Ediacaran faunas throughout the world • The trails and burrows • are similar to those made by present-day soft-bodied organisms

Time Between • Until recently, it appeared that • a fairly long time period existed • between the extinction of the Ediacaran fauna • and the evolution of the first Cambrian fossils • That gap has been considerably narrowed • in recent years with the discovery • of new Proterozoic fossiliferous localities • that continue right to the base of the Cambrian

Hotly Debated Topic • Nonetheless, the cause of the sudden appearance • of so many different animal phyla • during the Early Cambrian • is still a hotly debated topic • Newly developed molecular techniques • that allow evolutionary biologists • to compare the similarity of molecular sequences • of the same gene from different species • is being applied to the phylogeny of many organisms

Early Invertebrate History • In addition, new fossil sites • and detailed stratigraphic studies • are shedding light • on the early history and ancestry • of the various invertebrate phyla

Triggering Mechanism • It appears likely that the Cambrian explosion • probably had its roots firmly planted in the Proterozoic • However, the mechanism • that triggered this event is still unknown and • was likely a combination of factors, • both biological and geological • For example, geologic evidence • indicates Earth was glaciated • one or more times during the Proterozoic, • followed by global warming during the Cambrian

Hox Genes • These global environmental changes • may have stimulated evolution • and contributed to the Cambrian explosion • Recent work on Hox genes, which are • sequences of genes that control the development of individual regions of the body, • shows that the basic body plans for all animals • was apparently established • by the end of the Cambrian explosion, • and was only slightly modified since then

Major Event in Earth's History • Whatever the ultimate cause of the Cambrian explosion, • the appearance of a skeletonized fauna • and the rapid diversification of that fauna • during the Early Cambrian • was a major event in Earth's history

The Emergence of a Shelly Fauna • The earliest organisms with hard parts • are Proterozoic calcareous tubes • found associated with Ediacaran faunas • from several locations throughout the world • These are followed by other microscopic skeletonized fossils • from the Early Cambrian • and the appearance of large skeletonized animals • during the Cambrian explosion

Lower Cambrian Shelly Fossil • 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 • The tube of an anabaritid from the Mackenzie Mountains, Northwest Territories, Canada • This specimen is several millimeters in size

Why Skeletons • Along with the question of • why did animals appear so suddenly in the fossil record • is the equally intriguing one of • why they initially acquired skeletons • and what selective advantage this provided • A variety of explanations • about why marine organisms evolved skeletons • have been proposed, • but none is completely satisfactory or universally accepted

Advantages of an Exoskeleton • The formation of an exoskeleton • confers many advantages on an organism: (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

Cambrian Predator • 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

Wounded Trilobite • 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

Advantages of an Exoskeleton • 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

It Is Unknown Why Organisms Evolved Mineralized Skeletons • There currently is no clear answer about • why marine organisms evolved mineralized skeletons • during the Cambrian explosion and shortly thereafter • They undoubtedly evolved • because of a variety of biologic and environmental factors

Mineralized Skeletons Were Successful • Whatever the reason, • the acquisition of a mineralized skeleton • was a major evolutionary innovation • allowing invertebrates to successfully occupy • a wide variety of marine habitats

Paleozoic Invertebrate Marine Life • Having considered the origin, differentiation, and evolution • of the Precambrian-Cambrian marine biota, • we now examine the changes • that occurred in the marine invertebrate community • during the Paleozoic Era

Marine Invertebrate Communities • Rather than focusing on • the history of each invertebrate phylum, • we will survey the evolution • of the marine invertebrate communities through time, • concentrating on the major features and changes that took place • To do that, we need to briefly examine • the nature and structure • of living marine communities so that • we can make a reasonable interpretation • of the fossil record

The Present Marine Ecosystem • In analyzing the present-day marine ecosystem, • we must look at where organisms live, • how they get around, • as well as how they feed • Organisms that live in the water column • above the seafloor • are called pelagic • They can be divided into two main groups: • the floaters, or plankton, • and the swimmers, or nekton

Plankton • 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 • Animal plankton are called zooplankton and are also mostly microscopic • Examples of zooplankton include foraminifera, radiolarians, and jellyfish

Nekton • The nekton are swimmers • and are mainly vertebrates • such as fish; • the invertebrate nekton • include cephalopods

Benthos • 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

Sessile and Mobile • 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

Marine Ecosystem • Where and how animals and plants live in the marine ecosystem Plankton: Jelly fish Sessile epiflora: seaweed Nekton: fish cephalopod Sessile epifauna: bivalve Benthos: d-k crinoid coral

Marine Ecosystem Infauna: worm, bivalve Mobile epifauna: gastropod, starfish

Feeding Strategies • The feeding strategies of organisms • are also important in terms of their relationships • with other organisms in the marine ecosystem • There are basically four feeding groups: • suspension-feeding animals remove or consume microscopic plants and animals as well as dissolved nutrients from the water; • herbivores are plant eaters; • carnivore-scavengers are meat eaters; • and sediment-deposit feeders ingest sediment and extract the nutrients from it

Marine Ecosystem coral crinoid bivalve Suspension feeders:

Marine Ecosystem worm sediment-deposit feeder Herbivores: gastropod Carnivores-scavengers: starfish

Organism's Place • We can define an organism's place • in the marine ecosystem • by where it lives • and how it eats • For example, an articulate brachiopod • is a benthonic, • epifaunal suspension feeder, • whereas a cephalopod • is a nektonic carnivore

Trophic Levels • An ecosystem includes several trophic levels, • which are tiers of food production and consumption • within a feeding hierarchy • The feeding hierarchy • and hence energy flow • in an ecosystem comprise • a food web of complex interrelationships among • the producers, • consumers, • and decomposers

Primary Producers • The primary producers, or autotrophs, • are those organisms that manufacture their own food • Virtually all marine primary producers are phytoplankton • Feeding on the primary producers • are the primary consumers, which are mostly suspension feeders

Other Consumers • Secondary consumers feed on • the primary consumers, • and thus are predators, while tertiary consumers, which are also predators, feed on the secondary consumers • Besides the producers and consumers, • there are also transformers and decomposers • These are bacteria that break down the dead organisms • that have not been consumed • into organic compounds that are then recycled

Marine Food Web • Showing the relationships • among the • producers, • consumers, • and decomposers

When the System Changes • When we look at the marine realm today, • we see a complex organization of organisms • interrelated by trophic interactions • and affected by changes in the physical environment • When one part of the system changes, • the whole structure changes, • sometimes almost insignificantly, • other times catastrophically

Paleozoic Life History: Invertebrates