Chapter 12 Life of the Paleozoic
Paleozoic Invertebrates Representatives of most major invertebrate phyla were present during Paleozoic, including sponges, corals, bryozoans, brachiopods, mollusks, arthropods, and echinoderms. Almost all of the common invertebrate phyla in existence today had appeared by Ordovician.
Paleozoic Vertebrates Vertebrates evolved during Paleozoic, including: Fishes Amphibians Reptiles Synapsids ("mammal-like reptiles") The first vertebrates were jawless fishes, which are found in rocks as old as Cambrian in China.
Paleozoic Vertebrates An advanced lineage of fishes with primitive lungs and stout fins gave rise to the fourlegged animals or tetrapods. The transition from water-dwelling vertebrates to land-dwelling vertebrates depended on the evolution of the amniotic egg.
Paleozoic Plants The first primitive land plants appeared near the end of Ordovician. Vascular plants expanded across the land, forming great forests during Devonian. The plants progressed from seedless, sporebearing plants to plants with seeds but no flowers (gymnosperms).
Paleozoic Extinctions Several mass extinctions occurred during Paleozoic, including the largest extinction of all at the end of Permian. Other mass extinctions occurred at the end of Ordovician and Devonian.
Diversity during Paleozoic Red arrows mark extinction events
Paleozoic Life Summary of invertebrate phyla
The first animals with shells are called small shelly fossils. Small shelly fossils are found at the base of Cambrian, and during Late Neoproterozoic. Most disappeared by at the top of the first stage within Lower Cambrian.
Small Shelly Fossils Many had phosphatic shells, few mm in size. Shells and skeletal remains of primitive molluscs, sponges, and animals of uncertain classification, such as Cloudina, that secreted a calcareous tube.
Cambrian Diversification The initial Paleozoic diversification is known as "the Cambrian explosion." Abrupt appearance of many types of animals about 535 million years ago, followed by rapid evolution. During that episode of explosive evolution, all major invertebrate phyla appeared in the fossil record (except Bryozoa).
Soft-Bodied Fossils in the Burgess Shale The extraordinarily well-preserved Middle Cambrian Burgess Shale fauna of Canada provides a window into the past to view the spectacular diversity of Middle Cambrian. Many soft-bodied organisms are preserved in black shale, along with the soft parts of animals with shells, such as legs and gills of trilobites.
Animals in the Burgess Shale 1. Several groups of arthropods, including trilobites and crustaceans 2. Sponges 3. Onycophorans (velvet worms) 4. Crinoids 5. Molluscs 6. Corals 7. Three phyla of worms 8. Chordates (Pikaia) 9. Many others
Pikaia - One of the Oldest Chordates Pikaia is a fish-like lower chordate from the Burgess Shale. Modern representatives are called lancelets, such as the genus Amphioxus.
Chordates Chordates have a notochord or dorsal stiffening rod associated with a nerve chord, at some stage in their development. In vertebrates, the notochord is surrounded by and usually replaced by a vertebral column during embryonic development. Vertebrates are chordates, but Pikaia pre-dates the evolution of vertebrae. It is thought that vertebrates evolved from organisms similar to Pikaia.
Predators in the Cambrian Seas The giant predator of the Cambrian seas, Anomalocaris, up to 60 cm long. Predators would have caused selective pressures on prey. The need to avoid being eaten probably encouraged the evolution of hard protective shells. Predation probably also caused an increase in diversity of prey, as they evolved to better survive predation.
The Chengjiang fauna In 1984, the Lower Cambrian Chengjiang fossil site was discovered in Yunnan Province, China. More than 100 species of invertebrates have been found, with extraordinary preservation, including many soft bodied forms.
The Chengjiang fauna Jelly fish Annelid worms Cnidaria Porifera (sponges) Brachiopods Arthropods Early chordates similar to Pikaia The world's oldest known fish (Myllokunmingia) Other species of unknown phyla
Oldest Known Fish The world's oldest known fish, Myllokunmingia, from the Maotianshan Shale near Chengjiang, in the Yunnan Province of China. 535 million years old.
Ordovician Diversity Following a slight dip in diversity at the end of Cambrian, Ordovician seas experienced renewed diversification. Global diversity tripled. The number of genera increased rapidly, and the number of families increased from about 160 to 530. The increase was particularly notable among trilobites, brachiopods, bivalve molluscs, gastropods, and corals.
Late Ordovician Extinction An extinction event at the end of Ordovician led to an abrupt decline in diversity. This extinction event was apparently related to the growth of glaciers in Gondwana, coupled with a reduction in shallow water habitat associated with the lowering of sea level.
Diversity and extinction during Paleozoic Red arrows mark extinction events
Silurian Diversity Diversification of marine animals occurred again at the beginning of Silurian. The period ended with only a slight drop in diversity.
Devonian Diversity During Devonian, there was continued diversification, but this ended with another fairly large extinction event, which extended over about 20 million years. Roughly 70% of marine invertebrates disappeared. Because of the long duration, the extinction is unlikely to have been caused by a sudden, catastrophic event.
Carboniferous-Permian Diversity During Early Carboniferous, diversity once again increased. Diversity of marine animals remained fairly constant throughout Carboniferous and Permian. Late Permian is marked by a catastrophic extinction event which resulted in the total disappearance of many animal groups.
Unicellular Organisms in the Paleozoic Seas The principal groups of Paleozoic unicellular animals with a significant fossils record are the foraminifera and the radiolaria, which belong to Phylum Sarcodina. These organisms are unicellular eukaryotic organisms, and belong to Kingdom Protista.
Foraminifera Name: Foraminifera means "hole bearer." Chief characteristics: Unicellular. Related to the amoeba, with pseudopods. Foraminifera build tiny shells (called tests) which grow by adding chambers. Some species (called agglutinated foraminifera) construct tests of tiny particles of sediement. This is the most primitive test. Other forams construct tests of calcium carbonate.
Foraminifera Geologic range: Cambrian to Holocene. Modes of life: Benthic or benthonic (bottom dwellers) Planktic or planktonic (floaters).
Fusulinid foraminifera (fusulinids) Fusulinids were abundant during Late Paleozoic (primarily Pennsylvanian and Permian). Their tests were similar in size and shape to a grain of rice. Their internal structure is complex and used to distinguish different species. Important guide fossils during Pennsylvanian and Permian because they evolved rapidly, were abundant, and widespread geographically.
Radiolaria Chief characteristics: Unicellular. Test or shell composed of opaline silica Ornate lattice-like skeleton Often spherical or radially symmetrical with spines Geologic range: Precambrian or Cambrian to Holocene. Rare during Early Paleozoic. More abundant during Mesozoic and Cenozoic. Mode of life: Planktonic. Marine only.