Tetrapod Footprint Discovery in Paleozoic Era

1. Chapter 13 Paleozoic Life History— Vertebrates and Plants

2. Tetrapod Footprint Discovery • Tetrapod trackway • at Valentia Island Ireland • These fossilized fooprints • which are more than 365 million years old • are evidence of one of the earliest four-legged animals on land • Photo courtesy of Ken Higgs, U. College Cork, Ireland

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 newly discovered 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 wasn't 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 • The scant fossil evidence also seems to support this hypothesis

6. Unable to Walk on Land • Fossils of Acanthostega, • a tetrapod found in 360 million year old rocks from Greenland, • reveals an animal with limbs, • but one clearly unable to walk on land • Paleontologist Jenny Clack, • who recovered hundreds of specimens of Acanthostega, • points out that Acanthostega's limbs were not strong enough to support its weight on land, • and its ribcage was too small for the necessary muscles needed to hold its body off the ground

7. Acanthostega Had Gills and Lungs • In addition, Acanthostega had gills and lungs, • meaning it could survive on land, but was more suited for the water • Clack believes that Acanthostega • used its limbs to maneuver around • in swampy, plant-filled waters, • where swimming would be difficult • and limbs would be an advantage

8. Unanswered Questions • Fragmentary fossils • from other tetrapods living at about the same time as Acanthostega • suggest that some of these early tetrapods • may have spent more time on dry land than in the water • At this time, there are many more unanswered questions • about the evolution of the earliest tetrapods • than there are answers • However, this is what makes the study of prehistoric life so interesting and exciting

9. 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 • In this section, we examine • the Paleozoic evolutionary history of vertebrates and plants

10. Transition from Water to Land • One of the striking parallels between plants and animals • is the fact that in passing 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

11. 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

12. 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

13. 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

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

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

16. 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

17. 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

18. Fish • The most primitive vertebrates are fish • and some of the oldest fish remains are found in Upper Cambrian rocks • All known Cambrian and Ordovician fossil fish • have been found in shallow nearshore marine deposits, • while the earliest nonmarine 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 • a primitive member of the class Agnatha (jawless 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 • 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 three 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 • 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 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 • that evolved during the Late Silurian were the placoderms, • whose name means “plate-skinned” • 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 mid-continental 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

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 Middle 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 • and the less familiar lobe-fined fish • 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 Diversified • 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 • Two major groups of lobe-finned fish are recognized: • lungfish • and crossopterygians

40. 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 • Studies of present-day lung fish indicate that lungs evolved • from saclike bodies on the ventral side of the esophagus

41. Lungfish Respiration • These saclike bodies enlarged • and improved their capacity for oxygen extraction, • eventually evolving into lungs • When the lakes or streams in which lungfish live • become stagnant and dry up, • they breathe at the surface • or burrow into the sediment to prevent dehydration • When the water is well oxygenated, • however, lungfish rely upon gill respiration

42. Amphibians Evolved from Crossopterygians • The crossopterygians are an important group of lobe-finned fish • because amphibians evolved from them • During the Devonian, two separate branches of crossopterygians evolved • One led to the amphibians, • while the other invaded the sea

43. Coelacanths • The crossopterygians that invaded the sea, • called the coelacanths, • were thought to have become extinct at the end of the Cretaceous • In 1938, however, • fisherman caught a coelacanth in the deep waters of Madagascar, • and since then several dozen more have been caught, • both there and in Indonesia

44. Rhipidistians — Ancestors of Amphibians • The group of crossopterygians • that is ancestral to amphibians • are rhipidistians • These fish, attaining lengths of over 2 m, • were the dominant freshwater predators • during the Late Paleozoic

45. Amphibian Ancestor • Eusthenopteron, • a good example of a rhipidistian crossopterygian, • had an elongate body • that enabled it to move swiftly in the water, • as well as paired muscular fins that could be used for locomotion on land • The structural similarity between crossopterygian fish • and the earliest amphibians is striking • and one of the better documented transitions • from one major group to another

46. Eusthenopteron • Eusthenopteron, • a member of the rhipidistian crossopterygians • had an elongate body • and paired fins • that it could use to move about on land • The crossopterygians are thought to be amphibian ancestors

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

48. 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

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

50. 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