The Earth Through Time
Chapter 12—Life of the Paleozoic
CHAPTER OUTLINE FOR TEACHING
I. Overview of Life’s Expansion
A. Cambrian: vast expansion of shelly marine life forms; jawless fishes
B. Ordovician: most modern phyla established by this time
C. Late Paleozoic: land plants and vertebrates (tetrapods and amniotes) are very successful in populating the continents
D. Mass Extinctions: within Ordovician and Devonian; end of Permian
II. Invertebrates of the Paleozoic: arrival of animals with shells
A. Review of Significant Early Cambrian Fossils
1. Cloudina
2. Anabarites
3. Aldanella
4. Lapworthella
B. Review of Significant Middle Cambrian Fossils
1. Aysheaia
2. Leanchoila
3. Waptia
4. Anomalocaris
5. Marrella
6. Hallucigenia
7. Opabinia
8. Pikaia
9. Cathaymyrus and Yunnanozoon
C. Extraordinary Cambrian Soft-Body Fossil Sites
1. Burgess Shale, British Columbia, Canada
a. viewed as one of the most important faunas in fossil record
b. impressions and films on bedding planes
c. limited exposure near Mt. Wapta, BC
d. discovered by C.D. Walcott in 1909
e. four groups of arthropods (trilobites, crustaceans, scorpions, insects)
f. other groups: sponges, onycophorans, crinoids, sea cucumbers, chordates, other unknown forms
2. Chengjian site, Yunnan Province, China
a. about 10 million years older than Burgess Shale
b. discovered in 1984
c. early chordates and oldest fishes
D. The Cambrian explosion of life
1. Sudden increase in the pace of evolution
2. Possible causes
a. more beneficial climatic conditions following glacial episodes and the related demise of the Ediacaran animals
b. changes in the sequences of HOX genes that control body architecture
c. combination of factors, some of which may not be known
E. The Great Ordovician Biodiversification Event (GOBE)
1. Middle Ordovician, only 40 million years after the Cambrian explosion
2. Tripling of global biodiversity in only 25 million years
3. Accompanied fragmentation of continents and proliferation of shallow, warm water habitats
4. New groups
a. trilobites
b. brachipods
c. bivalve mollusks
d. gastropods
e. coralline animals
5. Newly diverse life strategies
a. epifaunal and infaunal
b. filter and sediment feeding
c. herbivores, carnivores, and scavengers
III. Invertebrates of the Paleozoic
A. Diversification of Many Groups
1. Epifaunal animals
2. Infaunal animals (bioturbation)
3. Filter-feeding animals
4. Sediment-feeding animals
B. Unicellular Groups
1. Foraminifera (calcareous microfossils)
a. range Cambrian to present
b. more numerous and varied by Carboniferous
c. global distribution during Pennsylvanian-Permian (fusulinids)
2. Radiolarians (siliceous microfossils)
a. range Early Paleozoic to present
b. most abundant in Mesozoic rocks
C. Marine Invertebrate Groups
1. Colonial cup animals
a. archaeocyathids
b. sponges (Porifera)
(i) well-known guide fossils
· Cambrian: Protospongia
· Silurian: Astraeospongium
· Devonian: Hydnoceras
(ii) sponge-like fossils: stromatoporoids
c. corals and other cnidarians
(i) rugose or tetracorals
(ii) scleractinid or hexacorals
(iii) tabulates
2. Bryozoans
3. Brachiopods
a. articulate
b. inarticulate
4. Mollusks
a. placophorans
b. monoplacophorans
c. bivalvia or pelecypoda
(i) clams
(ii) mussels
(iii) scallops
(iv) oysters
d. gastropods
e. cephalopods
5. Arthropods
a. trilobites
b. ostracods
c. eurypterids
6. Echinoderms
a. asteroidea (starfish)
b. ophiuroidea (brittle stars)
c. echinoidea (sea urchins)
d. edioasteroidea (ancestral starfish and sea urchins)
e. crinoidea (crinoids)
f. cystoidea (cystoids)
g. blastoidea (blastoids)
h. rhombifera
i. diploporita
7. Graptolites: primitive chordates called pterobranchs
IV. Vertebrates (Chordates) of the Paleozoic
A. Basic Groups
1. Non-Amniotic vertebrates
a. fish and amphibians
b. require water to reproduce
2. Amniotic vertebrates
a. all higher vertebrates
b. have enclosed eggs
3. Chordates
a. stiff, elongate support structure with central nerve cord
b. gill slits and ventral and dorsal blood circulation
B. Fishes: five taxonomic classes
1. Agnathids (jawless fish): ostracoderms
2. Acanthodians and Placoderms (archaic jawed fish)
a. placoderms
b. arthrodires
c. antiarchs
3. Chondrichthyans (cartilaginous fish)
4. Osteichthyans (bony fish)
a. actinopterygians (ray-fined fish)
b. sarcopterygians (lobe-finned fish)
(i) dipnoans (lungfish)
(ii) crossopterygians (lobe-finned fish with limbs)
· rhipidistians
· coelacanths
C. Conodont elements (microfossil teeth)
1. Range: Late Paleozoic-Triassic
2. Fossil hard parts: tooth-like calcium phosphate fossils
3. Whole body form: eel-like, 40 mm long, a primitive jawless vertebrate with many teeth (Promissum)
D. Crossopterygians: transitional animals between fish and amphibians that had forelimbs
1. Tiktaalik
2. Eusthenopteron
E. Tetrapods
1. Amphibians (egg-laying tetrapods that reproduce in water)
a. ichthyostegids: fish-like amphibians (basal tetrapods)
b. labyrinthodonts: teeth similar to ray-fin fishes
2. Reptiles, Birds, and Mammals (Amniotic tetrapods)
a. Late Carboniferous subdivision
(i) reptiles
(ii) synapsids
b. Permian-Triassic groups
(i) therapsids (mammal-like reptiles)
(ii) cynodonts (an especially mammal-like group)
V. Plants of the Paleozoic
A. Stromatolites and stromatolitic reefs: Cambrian-Ordovician
B. Non-stromatolitic algae of Early Paleozoic
1. Chlorophytes
2. Receptaculids (lime-secreting algae sometimes called “sunflower corals”)
C. Land plants
1. Bryophytes (mosses, liverworts, hornworts)
2. Tracheophytes (ferns and trees)
a. lycopsids (scale trees)
(i) Lepidodendron
(ii) Sigillaria
b. cordaites
VI. Mass Extinctions
A. Late Ordovician: global cooling event due to Gondwanaland’s ice caps
1. Phase-one victims: planktonic and nektonic organisms (graptolites, acritarchs, and many nautloids)
a. associated with global cooling, which shifted living groups toward the equator
b. sea level was lowered and epeiric seas and shallow shelf areas lost due to regression
2. Phase-two victims: benthic organisms (surviving trilobites; great reduction in numbers of corals, conodonts, bryozoans, and brachiopods)
a. associated with global cooling
b. sea level rising due to ice-sheet melting
c. warmer conditions stressed cold-adapted groups
B. Late Devonian: global cooling due to Gondwanaland glaciation
1. Decimated Devonian reef communities including tabulate corals and stromatoporoids
2. Severely reduced: rugose corals, brachiopods, goniatites, trilobites, conodonts, and placoderms
3. Marine invertebrates in general: loss of 70% of families
4. Occurred over 20 million years, therefore an ecological crisis of some kind
a. eutrophication or anoxic conditions in the oceans
b. continental glaciation: cooling and regression of seas
C. Late Permian (“Mother of Mass Extinctions”)
1. Continental effects
a. 70% of land animal species exterminated
b. extinction of families of amphibians, primitive reptiles, mammal-like reptiles
2. Marine effects
a. most strongly affected the tropics
b. 90% of marine species exterminated
c. extinction of fusulinids, rugose corals, many crinoids, productid brachiopods, lacy bryozoans, many ammonoid groups
3. Factors
a. extreme global warming
b. greenhouse gas effects
c. massive flood basalt eruptions
d. carbon-dioxide poisoning
e. ozone layer damage or loss
Answers to Discussion Questions
1. In the Burgess Shale, Pikaia is a chordate. In the Chengjian site, China, Cathysmyrus and Yunnanozoon are chordates. Chordates have a notochord (at some stage of development) and a related nerve cord on the dorsal side of the notochord. Fossils with evidence of these features are classified as chordates.
2. Here are the age assignments of the fossils indicated:
fusulinids Pennsylvanian-Permian
archaeocyathids Early-Middle Cambrian
Archimedes Mississippian
3. The presence of stinging cells, or cnidocytes, characterizes all members of the Phylum Cnidaria, and thus the phylum is so named. The extinct forms are tabulate and rugose corals.
4. Cephalopods of the Subclass Nautiloidea have straight or gently undulating sutures, whereas the Ammonoidea have more complex sutures.
5. Conodont elements consist of cones, bars, and blades bearing tiny denticles or cusps. These elements are made of calcium phosphate (a variety of the mineral apatite), a material common in teeth of vertebrates.
6. By the end of Paleozoic, many invertebrate groups had become extinct, including fusulinids, rugose corals, many families of crinoids, productid brachiopods, trilobites, lacy bryozoans, and many groups of ammonoids.
7. Shallow shelf and epeiric sea environments are affected by episodes of continental glaciation when sea level is lowered due to ice buildup at high latitude. Cooling of water also makes calcium carbonate precipitation move difficult, so the amount of carbonate deposition is greatly decreased.
8. Echinoderms are largely sessile spiny skinned animals which generally consist of a crown, stem, and arms. Trilobites are mobile, three-lobed arthropods which were highly successful Paleozoic animals, but are extinct today. Mollusks are a diverse group which all have in common a foot and a mantle (which secrets shell, if any). The Mollusks have various strategies of life: infaunal, epifaunal, and mobile. The Brachiopods are symmetrical bivalves that are filter feeders. The Brachiopods still exist but were at their peak of diversity during Paleozoic. The Bryozoa are colonial organisms consisting of collections of zooecia; they generally form branching or encrusting shapes. The Porifera (sponges) consist mainly of an osculum, choanocytes, mesenchyme, and flagellae. The Porifera are attached forms; their fossil record is mainly as spicules of silica or calcium carbonate.
9. Evolution of the shell provided obvious physical protection for the invertebrate. Further, the shell permitted more efficient locomotion in water and sediment.
10. (a) Ostracaderms and placoderms – The former are Early Paleozoic armored, jawless fishes (Agnatha), and the latter, armored jawed fishes.
(b) Osteichthyes and chondrichthyes – The former are bony fishes, and the latter, cartilaginous fishes (e.g., sharks).
(c) Sarcopterygians and labyrinthodonts – The former are lobe-finned fish, and the latter, a lung fish with specialized teeth made of infolded enamel.
(d) Infaunal and epifaunal invertebrates – The former live within sediment or rock, and the latter, upon sediment or rock.
(e) Synapsids and diapsids – The former type of tetrapod has one temporal fenestra (opening) on each side of the skull, and the latter, two fenestra on each side.
11. Tiktaalik was a transitional animal between fish and amphibians that had forelimbs. These forelimbs had the beginnings of fingers, wrist bones, elbows, and shoulders. The upper arm (humurus) resembled both that of the fish and the amphibian.
12. Therapsid reptile characteristics indicating eventual mammalian evolution include: fewer bones in the skull, enlarged lower jaw at the expense of more posterior elements, double ball-and-socket articulation of skull and neck, differentiation of teeth (incisors, canines, cheek, teeth), limbs indirect vertical alignment beneath body, and ribs reduced in neck and lumber region (promoting flexibility).
13. Chlorophytes, or green algae, are considered ancestral to land plants because of chlorophytes’ adaptation to freshwater and presence of chlorophyll (green pigment).
14. The vascular system in plants was a key step in evolution of land-dwelling species because it allowed conduction of water and dissolved minerals and gave strength to the plant so it could withstand gravity’s force and wind. The first plant to make this transition was Cooksonia, small, leafless stems that were thin and evenly branching (with branches surmounted by sporangia). Aglaophyton and Rhynia which followed had woody, vascular tissue.
15. Synapsid reptiles have a single, lower temporal opening (fenestra) on each side of the skull.
16. a and e
17. a
18. b and c
19. d
20. d
21. e
22. b
23. a
24. d
25. e
26. a
Chapter Activities
Student activities for in-depth learning.
1. Using the resources on Anomalocaris at http://www.trilobites.info/anohome.html, write a description of this unusual looking arthropod. In your description, note what uses the various features of Anomalocaris’ body plan might have been. Make a sketch of Anomalocaris and label the parts. When did this arthropod live and where has it been found?
2. Use the web resources at the web pages on placoderms located at the University of California, Berkeley, Museum of Paleontology web site as follows: http://www.ucmp.berkeley.edu/ vertebrates/basalfish/placodermi.html. What is the nature of a placoderm? What is the geological range of this fish? Make a sketch of the bony skull of the placoderm. Why was bony armor of importance to such fish at this time in geological history?
Chapter 12—Life of the Paleozoic
CHAPTER OVERVIEW
The evolution of early soft-bodied life Proterozoic forms is compared with the diversification of harder shelled Paleozoic forms. The evolution of those Paleozoic (Cambrian through Permian) forms exhibited an exponential expansion of life. This increase in the invertebrate phyla is prominently noted during Cambrian in shell-bearing trilobites and brachiopods. Cambrian history of the Burgess Shale Fauna is discussed in terms of one of its most important members, Pikaia, the earliest known member of the phylum chordata. Several other interesting members of the Burgess Shale are also described. The development of unicellular animals (protistans) and Paleozoic metazoan invertebrates is also detailed.
The evolutionary history of jawless fish beginning during Silurian to their dominance in the seas during Devonian is traced. Devonian is known as the “Age of Fish.” With the domination of the sea by fish; the first land animals, the ichthyostegids, made their appearance only to be foreshadowed by the amniotes that become more dominant. The reptiles dominance on land during Late Paleozoic and the anatomical characteristics shared by reptiles, therapsids, and mammals is discussed. Other major advances include receptaculids (green algae), and tracheophytes (trees, ferns, and flowering plants). The earliest evidence of the invasion of the continent by plants is by tetrads during Ordovician. This chapter concludes with a discussion of worldwide extinctions during Late Ordovician, Late Devonian, and Late Permian with supportive evidence given for those extinctions.
LEARNING OBJECTIVES
By reading and completing information within this chapter, you should gain an understanding of the following concepts:
· Explain the significant changes in the fossil record in comparing Precambrian with Paleozoic.
· Explain the importance of the Burgess Shale, both morphologically as a fossil form of chordate and as a stratigraphic marker.
· Describe some morphological differences in the following Paleozoic organisms: Protista, Foraminifera, radiolarians, archaeocyathids, Porifera, Cnidaria , Bryzoa, Brachiopoda, Mollusca, Arthropoda, Echinodermata, and graptolites.
· Trace the evolutionary history of fish from an early form such as the lancelet (Branchiostoma), Ostrocoderms, Acanthodians, and Placoderms to the two major groups, Chondrichthyes (cartilaginous fish) and Osteichthys (bony skeleton forms).
· Describe the evolutionary history of amphibians, including the characteristics they share with fish and reptiles.
· Describe the significance of the amniotic egg in the evolutionary transition of amphibians to reptiles.
· Describe the three anatomical traits possessed by therapsids that suggest they are on the main line of evolution toward mammals.