Paleozoic Life
Learning Objectives
The end of the Precambrian is marked by the appearance of abundant shelled organisms.
The Burgess Shale contains a great diversity of well preserved Cambrian fossils (movie).
At the beginning of the Cambrian there was no life on land, life was still only in the oceans. There was a wide variety of habitat styles:
Epifaunal: organisms that lived on the sea floor
Infaunal: organisms that burrowed into the sediment on the sea floor
Pelagic: organisms that floated or swam in the water of the oceans
Sessile: organisms that were attached for the majority of their life cycle, did not have the ability to move from place to place
Motile: organisms that could move freely through swimming, crawling and burrowing
FEEDING STYLES:
Filter Feeders: organisms that obtain food through the filtering of sea water
Sediment Feeders: organisms that obtain their food through ingesting sediment much as an earthworm
Herbivores: organisms that feed on photosynthetic organisms and other autotrophs
Carnivores: organisms that feed on animals
Scavengers: organisms that feed on waste matter
The oceans of the Early Paleozoic contained a wide variety of organisms. Scientists know about these organisms from the fossil record and infer their behavior from modern species. A "snapshot" of Early Paleozoic ocean life would include:
I) Arthropods:
All arthropods are similar in that they have an exoskeleton, paired, jointed appendages and a segmented body.
1) Trilobites: most abundant group in the Cambrian, trilobites became extinct in the Permian. Trilobites were motile, epifaunal organisms believed to be scavengers. They occupied two ecological zones, deep ocean and shallow shelf. Trilobites are important guide fossils because their morphology changed frequently.
2) Ostracodes: very small organisms (.5 - 4mm), ostracodes lived in a shell. Ordovician - present
3) Eurypterids: scorpion like looking organisms, eurypterids were believed to be predators. Some obtained lengths of 3 meters. Ordovician - Permian
II) Brachiopods:
Brachiopods were the most abundant organisms of the Paleozoic, comprising 30% of all life. They were sessile, infaunal filter feeders. Brachiopods possess two valves (shells) and are symmetric left to right but not top to bottom. 120 species still exist today. Brachiopod morphology
III) Archaeocyathid:
A conical shaped sessile, epifaunal filter feeder. Abundant in the Early Paleozoic, extinct by Permian.
IV) Sponges (Porifera):
An organism with a sac like body supported by spicules of calcium carbonate or silica. It is the spicules that remain in the fossil record. Filter feeding, usually sessile organisms. Over 3000 species of sponge exist today that are not much different from their Paleozoic ancestors.
V) Cnidaria:
All Cnidaria possess stinging cells (nematocysts) and exist in either a sessile (polyp) or motile (medusa) stage. Modern members include jellyfish and coral. Paleozoic Cnidaria consisted of simple corals. Corals are epifaunal, sessile filter feeders.
VI) Bryzoa:
Colonial mass animals that look like twigs. Individual bodies or zooids are contained in calcium. Over 4000 modern members but even more abundant in the Paleozoic.
VII) Mollusks:
A very diverse group, all members possess a head, a visceral mass and a foot although these organs are arranged very differently in each of the three classes.
1) Bivalves: modern species include clams, oysters and mussels. Bivalves are motile filter feeders with two shells that are symmetric top to bottom but not left to right. Appear in the Cambrian but not abundant until the Late Paleozoic.
2) Gastropods: possess a shell into which they can contract like a snail. Motile and often predaceous they appear in the Cambrian but are not abundant until the Late Paleozoic.
3) Cephalopods: modern members include octopus and squid, Paleozoic forms consisted of nautiloids and ammonites, organisms that lived within a shell with non-stinging tentacles much like today's squid.
VIII) Echinoderms:
Organisms with a five part body plan and radial symmetry, modern members include starfish and sand dollars. Early Paleozoic echinoderms were sessile stalked organisms such as crinoids, blastoids and cystoids. These became extinct replaced by more motile forms.
IX) Graptolites:
Planktonic colonial organisms that live in a cup on a stem. Usually preserved as a carbon film, graptolites were thought to be extinct since Late Paleozoic but recently an organism was found that is believed to be a graptolite.
X) Conodonts:
Very small (<1mm) plate like looking fossils. Very useful as guide fossils but geologists not quite sure what conodonts were or if they were actually part of another larger organism. Most accepted idea is that they belong to a chordate, perhaps some sort of jawless fish.
XI) Vertebrates (Chordates):
A very diverse group. All chordates have a central hollow nerve chord or notochord, pharyngeal slits in the embryo and an internal skeleton. The first chordate, Pikaia, appeared in the Cambrian and resembled an eel.
There are many classes of vertebrates. Paleozoic fossils contain a record of fish evolution.
Fish: 5 classes
1) Agnatha: jawless fish, represented today by lampreys and hagfish. Agnatha first appear in the Ordovician. Fins are not well developed and they are not good swimmers. Agnatha are particle feeders or predators (they latch on and suck nutrients from a larger organism's body). During the Paleozoic some possessed bony armor.
2) Placodermi: jawed fish, some possessed bony body armor. Appear in the Silurian, extinct in the Permian, Placodermi were not efficient swimmers because of rudimentary fin arrangement.
3) Acanthodii: jawed fish with more well developed fins, no bony armor, more movement because more agile. Appear in the Silurian, extinct in the Permian.
4) Chondrichthyes: cartilaginous fish, internal skeleton is composed of cartilage rather than bone. Appearing in the Late Silurian/Early Devonian these fish had 2 sets of flexible paired fins and were much more efficient swimmers. Today this class is represented by sharks and rays.
5) Osteichthyes: bony fishes, this is a very diverse group to which modern fish belong. Fins arrangement is sophisticated and varies widely within class. Appear in the Devonian.
Throughout the Paleozoic fish evolve in 3 distinct ways:
- Fin arrangement becomes more specialized
- Fish become more streamlined, better swimmers with improved jaw structure
- Gill arrangement evolves resulting in more efficient metabolism
Plants:
1) At the beginning of the Paleozoic stromatolites are still the dominant plant form. They were widespread in the Cambrian becoming increasingly restricted after the Ordovician. Some modern members exist today.
2) Receptaculids: a lime secreting algal mat common in the Ordovician.
Land Plants:
To move ashore plants had to evolve the following adaptations:
- A support system: in the water the plant is supported by water on land the plant must provide its own support
- Water tight covering: on land a plant must prevent dehydration
- Reproductive strategy: in water a plant sheds its gametes into the water for pollination
- Structures to obtain water and nutrients: in the water these are readily available to a plant
Psilophytes: the first undisputed land plants were psilophytes appearing in the Middle Silurian.
The earliest land plants were primitive, resembled ferns and remained close to the water. Plants reproduced by shedding gametes into the water and the earliest plants did not have well developed vascular tissues.
Lycopsids: fern like trees with a scaly bark that appeared in the Late Devonian. They possessed more efficient vascular tissue, a stronger bark and support system. Leaves emerged directly from the trunk of the tree and they still reproduced by spores in the water. By Mississippian and Pennsylvanian time vast swampy lowlands were covered with these trees. These organic deposits eventually form the extensive coal beds found in the Appalachian region.
Sphenopsids: these plants reproduced by spores that were carried on the wind. First appearing in the Pennsylvanian these plants resembled horsetails or rushes.
Gymnosperms: appearing in the Late Pennsylvanian these trees were wind pollinated and possessed a seed. The seed allowed the plant to move away from the water for reproduction. Modern members include conifers and pines.
The evolutionary trend of plants in the Paleozoic was toward an independence from water.
- Reproduction improved from gametes shed into the water to wind pollination and eventually to a seed.
- Bark improved allowing plants to inhabit drier environments.
- Support structures improved leading to the evolution of tall trees.
- Root systems and vascular tissue improved to allow the plants to obtain water and nutrients more efficiently.
HOWEVER there were still NO flowering plants at the end of the Paleozoic.
Invertebrate evolution in the Paleozoic:
During the Late Paleozoic alot of deformation occurred. This increased erosion which increased sedimentation in the shallow shelf environments. Habitats started to shrink and continents started a drying trend as Pangea formed. The tendency in invertebrate evolution was toward mobility:
Echinoderms: crinoids and other stalked varieties (sessile) decline while motile forms evolve (starfish)
Brachiopods (sessile) decline while Bivalves (motile) increase
Ammonites (motile) increase
Defense mechanisms improve as predators become more efficient. Brachiopods develop spines and heavier shells as sharks develop broad, flat teeth for crushing. Slow moving Trilobites with weak shells begin to decline. Food gathering techniques improve.
Arthropods: a large increase in the Late Paleozoic. Insects appear in the middle Devonian and explode to over 400 forms by Pennsylvanian. Scorpions, cockroaches, winged insects, spiders all appear.
Vertebrate Evolution in the Paleozoic:
Fish: discussed above, evolution towards more mobility and more efficient jaw and gill structures. By Late Paleozoic Osteichthyes and Chondrichthyes are the dominant classes.
Crossopterygian: a lungfish that is believed to be the ancestor of amphibians. To move ashore vertebrates needed to develop specific adaptations:
- Watertight skin
- Eye coverings and tear glands
- Ears for hearing
- More efficient lung and circulatory system
- Stronger skeleton for support and walking
- Rigid backbone and framework
Icthyostegides: earliest amphibian fossil, Late Devonian. Amphibians still dependent on the water:
- Skin must be kept moist
- Reproduction in water
- Eggs laid in water and early development of individual in water
During the Mississippian and Pennsylvanian amphibians dominate the land. Coal deposits from the Appalachian region contain a very complete record of amphibian life during the Late Paleozoic including both adult and tadpole stages. There was great variety among amphibians during the Late Paleozoic. Amphibians from this time were slow moving with a heavy skeletal structure.
At the end of the Paleozoic Pangea forms and the climate changes. Inland seas begin to dry and the swampy lowland environments start to shrink. As cooler and drier conditions prevail many amphibian species become extinct. Frogs, toads, salamanders and newts survive.
Reptiles: reptiles possess evolutionary adaptations that allowed them to move away from the water.
- Water tight skin that did not have to stay moist
- Reproduction on land and an amniotic egg
- More efficient jaw and tooth structure for slicing food (amphibs swallowed food whole)
- Lighter skeletal framework leads to more agility and mobility
The first reptiles appeared in the Pennsylvanian and were small lizard like creatures. As the land dried reptiles, with the ability to move away from the water, were able to exploit new environments. By Permian reptiles were becoming the dominant life form on Earth.
Pelecosaurs: fin backed reptiles such as Dimetredon.
Mass Extinction:
A mass extinction occurs when 50% or more of existing species become extinct in a short geologic time. At the end of the Permian it is estimated that 90% of all species went extinct.
Causes for the Permian extinction:
1) Pangea forms: this causes a change in climate, particularly drying, that decimated the amphibians. Climate becomes more severe because ocean currents blocked. Oceans overall became deeper and shallow shelf habitats shrink leading to extinction of many marine invertebrates.
2) Flood Basalts: large scale flood basalts and volcanism change climate and increase acidity of precipitation.
