Paleozoic Life History: Vertebrates and Plants

1. Chapter 13 Paleozoic Life History: Vertebrates and Plants

2. Tetrapod Footprint Discovery • Tetrapod trackway • at Valentia Island, Ireland • These fossilized footprints • which are more than 365 million years old • are evidence of one of the earliest four-legged animals on land

3. Tetrapod Footprint Discovery • The discovery in 1992 of fossilized Devonian tetrapod footprints • more than 365 million years old • has forced paleontologists to rethink • how and when animals emerged onto land • The Late Devonian trackway • has helped shed light on the early evolution of tetrapods • the name is from the Greek tetra, meaning four and podos, meaning foot • Based on the footprints, it is estimated • that the creature was longer than 3 ft • and had fairly large back legs

4. Tetrapod Wader • Furthermore, instead of walking on dry land • this animal was probably walking or wading around in a shallow, tropical stream, • filled with aquatic vegetation and predatory fish • This hypothesis is based on the fact that • the trackway showed no evidence of a tail being dragged behind it • Unfortunately, there are no bones associated with the tracks • to help in reconstructing what this primitive tetrapod looked like

5. Why Limbs? • One of the intriguing questions paleontologists ask is • Why did limbs evolve in the first place? • It probably was not for walking on land • In fact, many scientists think • aquatic limbs made it easier to move around • in streams, lakes, or swamps • that were choked with water plants or other debris

6. Unanswered Questions • Presently, there are many more unanswered questions • about the evolution of the earliest tetrapods • than there are answers • During the 1990s, only a few Devonian tetrapods were known • Today, paleontologists have a more detailed knowledge • and are able to fill the gaps between the fish and amphibians • leading to more complete fish-amphibian phylogeny

7. New Information from Fossils • As more paleoenvironmental and paleoecologic data and analyses • from a variety of sites • are made available, • A better understanding of the linkage • between morphological changes and the environment • is fast emerging. • New technologies now provide the means • to extract more and more detailed information • from the fossils

8. Vertebrates and Plants • Previously, we examined the Paleozoic history of invertebrates, • beginning with the acquisition of hard parts • and concluding with the massive Permian extinctions • that claimed about 90% of all invertebrates • and more than 65% of all amphibians and reptiles • Now we examine • the Paleozoic evolutionary history of vertebrates and plants

9. Transition from Water to Land • One of the striking parallels between plants and animals • is the fact that making the transition from water to land, • both plants and animals had to solve the same basic problems • For both groups, • the method of reproduction was the major barrier • to expansion into the various terrestrial environments • With the evolution of the seed in plants and the amniote egg in animals, • this limitation was removed, • and both groups were able to expand into all the terrestrial habitats

10. Vertebrate Evolution • A chordate (Phylum Chordata) is an animal that has, • at least during part of its life cycle, • a notochord, • a dorsal hollow nerve cord, • and gill slits • Vertebrates, which are animals with backbones, are simply a subphylum of chordates

11. Characteristics of Chordates • The structure of the lancelet Amphioxus illustrates the three characteristics of a chordate: • a notochord, a dorsal hollow nerve cord, and gill slits

12. Phylum Chordata • The ancestors and early members of the phylum Chordata • were soft-bodied organisms that left few fossils • so little is known of the early evolutionary history of the chordates or vertebrates • Surprisingly, a close relationship exists between echinoderms and chordates • They may even have shared a common ancestor, • because the development of the embryo is the same in both groups • and differs completely from other invertebrates

13. A Very Old Chordate • Yunnanozoon lividum is one of the oldest known chordates • Found in 525 MY old rocks in Yunnan province, China • 5 cm-longanimal

14. Spiral Versus Radial Cleavage • Echinoderms and chordates • have similar • embryonic development • In the arrangement of cells resulting from spiral cleavage, • cells in successive rows are nested between each other • In the arrangement of cells resulting from radial cleavage, • cells in successive rows are directly above each other • This arrangement exists in both chordates and echinoderms

15. Echinoderms and Chordates • Both echinoderms and chordates have similar • biochemistry of muscle activity • blood proteins, • and larval stages • The evolutionary pathway to vertebrates • thus appears to have taken place much earlier and more rapidly • than many scientists have long thought

16. Hypothesis for Chordate Origin • Based on fossil evidence and recent advances in molecular biology, • vertebrates may have evolved shortly after an ancestral chordate acquired a second set of genes • the ancestor probably resembled Yunnanozoon • According to this hypothesis, • a random mutation produced a duplicate set of genes • allowing the ancestral vertebrate animal to evolve entirely new body structures • that proved to be evolutionarily advantageous • Not all scientists accept this hypothesis and the evolution of vertebrates is still hotly debated

17. Fish • The most primitive vertebrates are fish • and some of the oldest fish remains are found • in Upper Cambrian Deadwood Formation, • northeastern WY • Here, phosphatic scales and plates of Anatolepis, • a primitive member of the class Agnatha • have been recovered from marine sediments.

18. Fish • All known Cambrian and Ordovician fossil fish • have been found in shallow nearshore marine deposits, • while the earliest nonmarine (freshwater) fish remains • have been found in Silurian strata • This does not prove that fish originated in the oceans, • but it does lend strong support to the idea

19. Fragment of Primitive Fish • A fragment of a plate from Anatolepis cf. A. heintzi from the Upper Cambrian marine Deadwood Formation of Wyoming • Anatolepis is one of the oldest known fish

20. Ostracoderms — “Bony Skinned” Fish • As a group, fish range from the Late Cambrian to the present • The oldest and most primitive of the class Agnatha are the ostracoderms • whose name means “bony skin” • These are armored jawless fish that first evolved during the Late Cambrian • reached their zenith during the Silurian and Devonian • and then became extinct

21. Geologic Ranges of Major Fish Groups

22. Bottom-Dwelling Ostracoderms • The majority of ostracoderms lived on the seafloor • Hemicyclaspis is a good example of a bottom-dwelling ostracoderm • Vertical scales allowed Hemicyclaspis to wiggle sideways • propelling itself along the seafloor • while the eyes on the top of its head allowed it to see predators approaching from above • such as cephalopods and jawed fish • While moving along the sea bottom, • it probably sucked up small bits of food and sediments through its jawless mouth

23. Devonian Seafloor • Recreation of a Devonian seafloor showing: an acanthodian (Parexus) a ray-finned fish (Cheirolepis) • a placoderm (Bothriolepis) an ostracoderm (Hemicyclaspis)

24. Swimming Ostracoderm • Another type of ostracoderm, • represented by Pteraspis • was more elongated and probably an active swimmer • although it also seemingly fed on small pieces of food it could suck up

25. Evolution of Jaws • The evolution of jaws • was a major evolutionary advantage • among primitive vertebrates • While their jawless ancestors • could only feed on detritus • jawed fish • could chew food and become active predators • thus opening many new ecological niches • The vertebrate jaw is an excellent example of evolutionary opportunism • The jaw probably evolved from the first two or three anterior gill arches of jawless fish

26. Evolutionary Opportunism • Because the gills are soft • they are supported by gill arches composed of bone or cartilage • The evolution of the jaw may thus have been related to respiration rather than feeding • By evolving joints in the forward gill arches, • jawless fish could open their mouths wider • Every time a fish opened and closed its mouth • it would pump more water past the gills, • thereby increasing the oxygen intake • The hinged forward gill arches enabled fish to also increase their food consumption • The evolution of the jaw for feeding followed rapidly

27. Evolution of Jaws • The evolution of the vertebrate jaw • is thought to have occurred • from the modification of the first two or three anterior gill arches • This theory is based on the comparative anatomy of living vertebrates

28. Acanthodians • The fossil remains of the first jawed fish are found in Lower Silurian rocks • and belong to the acanthodians, • a group of small, enigmatic fish • characterized by • large spines, • scales covering much of the body, • jaws, • teeth, • and reduced body armor

29. Acanthodians: Most Abundant during Devonian • Although their relationship to other fish has not been well established, • many scientists think the acanthodians • included the probable ancestors of the present-day • bony and cartilaginous fish groups • The acanthodians were most abundant during the Devonian, • declined in importance through the Carboniferous, • and became extinct during the Permian

30. Other Jawed Fish • The other jawed fish, the placoderms • whose name means “plate-skinned” • evolved during the Late Silurian • Placoderms were heavily armored, jawed fish • that lived in both freshwater and the ocean, • and like the acanthodians, • reached their peak of abundance and diversity during the Devonian

31. Placoderms • The Placoderms exhibited considerable variety, • including small bottom dwellers • as well as large major predators such as Dunkleosteus, • a Late Devonian fish • that lived in the midcontinental North American epeiric seas • It was by far the largest fish of the time • attaining a length of more than 12 m • It had a heavily armored head and shoulder region • a huge jaw lined with razor-sharp bony teeth • and a flexible tail • all features consistent with its status as a ferocious predator

32. Late Devonian Marine Scene • A Late Devonian marine scene from the midcontinent of North America featuring the giant placoderm, Dunkleosteus

33. Age of Fish • Many fish evolved during the Devonian Period including • the abundant acanthodians • placoderms, • ostracoderms, • and other fish groups, • such as the cartilaginous and bony fish • It is small wonder, then, that the Devonian is informally called the “Age of Fish” • because all major fish groups were present during this time period

34. Cartilaginous Fish • Cartilaginous fish, • class Chrondrichthyes, • represented today by • sharks, rays, and skates, • first evolved during the Early Devonian • and by the Late Devonian, • primitive marine sharks • such as Cladoselache • were quite abundant

35. Cartilaginous Fish Not Numerous • Cartilaginous fish have never been • as numerous nor as diverse • as their cousins, • the bony fish, • but they were, and still are, • important members of the marine vertebrate fauna • Along with cartilaginous fish, • the bony fish, class Osteichthyes, • also first evolved during the Devonian

36. Ray-Finned Fish • Because bony fish are the most varied and numerous of all the fishes • and because the amphibians evolved from them, • their evolutionary history is particularly important • There are two groups of bony fish • the common ray-finned fish (subclass Actinopterygii) • and the less familiar lobe-fined fish (subclass Sarcopterygii) • The term ray-finned refers to • the way the fins are supported by thin bones that spread away from the body

37. Ray-Finned and Lobe-Finned Fish • Arrangement of fin bones for (a) a typical ray-finned fish (b) a lobe-finned fish • Muscles extend into the fin • allowing greater flexibility

38. Ray-Finned Fish Rapidly Diversify • From a modest freshwater beginning during the Devonian, • ray-finned fish, • which include most of the familiar fish • such as trout, bass, perch, salmon, and tuna, • rapidly diversified to dominate the Mesozoic and Cenozoic seas

39. Lobe-Finned Fish • Present-day lobe-finned fish are characterized by muscular fins • The fins do not have radiating bones • but rather articulating bones • with the fin attached to the body by a fleshy shaft • allowing a powerful stroke and • making the fish a powerful swimmer • Three orders of lobe-finned fish are recognized: • coelacanths • lungfish • and crossopterygians

40. Coelacanths • Coelacanths are marine lobe-finned fish • that evolved during the Middle Devonian • and were thought to have gone extinct • at the end of the Cretaceous. • In 1938, a fisherman caught a coelacanth • in the deep waters off Madagascar, • and several dozen more have been caught since then • in Madagascar and in Indonesia

41. Lungfish Fish • Lungfish were fairly abundant during the Devonian, • but today only three freshwater genera exist, • one each in South America, Africa, and Australia • Their present-day distribution presumably • reflects the Mesozoic breakup of Gondwana • The “lung” is actually a modified swim bladder • that most fish use for buoyancy in swimming • In lungfish, this structure absorbs oxygen, • allowing them to breathe air • when the lakes or streams in which they live become stagnant or dry up.

42. Lungfish Respiration • When the lakes become stagnant and dry up, • the lungfish burrow into the sediment to prevent dehydration • and breathe through their swim bladder • until the stream begins flowing or the lake fills with water • When the water is well oxygenated, • however, lungfish rely upon gill respiration

43. Amphibians Evolved from Crossopterygians • The crossopterygians are an important group of lobe-finned fish • because amphibians probably evolved from them • However, the transition to amphibians • is not as simple as once portrayed • Among the crossopterygians • the rhipidistians appear to be the ancestral group • These fish, reaching over 2 m in length, • were the dominant freshwater predators • during the Late Paleozoic.

44. Amphibian Ancestor • Eusthenopteron, • a good example of a rhipidistian crossopterygian, • had an elongated body • that enabled it to move swiftly in the water, • as well as paired muscular fins that may have been used for moving on land • The structural similarity between crossopterygian fish • and the earliest amphibians is striking • and one of the most widely cited transitions • from one major group to another

45. Rhipidistian Crossopterygian and Eusthenopteron

46. Fish/Amphibian Comparison • Similarities between the crossopterygian lobe-finned fish and the labyrinthodont amphibians • Their skeletons were similar

47. ulna radius humerus Comparison of Limbs • Comparison of the limb bones • of a crossopterygian (left) and an amphibian (right) • Color identifies the bones that the two groups have in common

48. Comparison of Teeth • Comparison of tooth cross sections show • the complex and distinctive structure found in • both crossopterygians (left) and amphibians (right)

49. Paleozoic Evolutionary Events • Before discussing this transition • and the evolution of amphibians, • we should place the evolutionary history of Paleozoic fish • in the larger context of Paleozoic evolutionary events • Certainly, the evolution and diversification of jawed fish • as well as eurypterids and ammonoids • had a profound effect on the marine ecosystem

50. Defenseless Organisms • Previously defenseless organisms either • evolved defensive mechanisms • or suffered great losses, possibly even extinction • Recall that trilobites • experienced extinctions at the end of the Cambrian, • recovered slightly during the Ordovician, • then declined greatly from the end of the Ordovician • to final extinction at the end of the Permian