Chapter 34

1. Chapter 34 Vertebrates

2. Overview: Half a Billion Years of Backbones • Early in the Cambrian period, about 530 million years ago, an astonishing variety of animals inhabited Earth’s oceans • One type of animal gave rise to vertebrates, one of the most successful groups of animals

3. Fig. 34-1

4. The animals called vertebrates get their name from vertebrae, the series of bones that make up the backbone • There are about 52,000 species of vertebrates, including the largest organisms ever to live on the Earth • Vertebrates have great disparity, a wide range of differences within the group

5. Concept 34.1: Chordates have a notochord and a dorsal, hollow nerve cord • Vertebrates are a subphylum within the phylum Chordata • Chordates are: • Bilaterian animals • They belong to the clade of animals known as Deuterostomia • Two groups of invertebrate deuterostomes are more closely related to vertebrates than to other invertebrates: • The urochordates, and • cephalochordates

6. Derived Characters of Chordates • All chordates share a set of derived characters • Some species have some of these traits only during embryonic development • Four key characters of chordates: • Notochord • Dorsal, hollow nerve cord • Pharyngeal slits or clefts • Muscular, post-anal tail

7. Fig. 34-3 Dorsal, hollow nerve cord Muscle segments Notochord Mouth Anus Pharyngeal slits or clefts Muscular, post-anal tail

8. Notochord • The notochord: • A longitudinal, flexible rod between the digestive tube and nerve cord • It provides skeletal support throughout most of the length of a chordate • In most vertebrates: • A more complex, jointed skeleton develops • The adult retains only remnants of the embryonic notochord

9. Dorsal, Hollow Nerve Cord • The nerve cord of a chordate embryo: • Develops from a plate of ectoderm • The ectoderm rolls into a tube dorsal to the notochord • It develops into the central nervous system: • The nervous include: • The brain, and • the spinal cord

10. Pharyngeal Slits or Clefts • Pharyngeal clefts: • Grooves in the pharynx of most chordates • They develop into slits that open to the outside of the body • Functions of pharyngeal slits: • Suspension-feeding structures in many invertebrate chordates • Gas exchange in vertebrates (except tetrapods, vertebrates with limbs) • Develop into parts of the ear, head, and neck in tetrapods

11. Muscular, Post-Anal Tail • Chordates have a tail posterior to the anus • In many species, the tail is greatly reduced during embryonic development • The tail contains skeletal elements and muscles • It provides propelling force in many aquatic species

12. Lancelets • Lancelets (Cephalochordata): • Are named for their bladelike shape • Are marine suspension feeders • They retain characteristics of the chordate body plan as adults

13. Fig. 34-4 Cirri 2 cm Mouth Pharyngeal slits Atrium Notochord Digestive tract Atriopore Dorsal, hollow nerve cord Segmental muscles Anus Tail

14. Tunicates • Tunicates (Urochordata): • Are more closely related to other chordates than are lancelets • Are marine suspension feeders commonly called sea squirts • As an adult, a tunicate draws in water through an incurrent siphon, filtering food particles • They most resemble chordates during their larval stage, (may last only a few minutes)

15. Fig. 34-UN2 Cephalochordata Urochordata Myxini Petromyzontida Chondrichthyes Actinopterygii Actinistia Dipnoi Amphibia Reptilia Mammalia

16. Fig. 34-5 Incurrent siphon to mouth Water flow Notochord Dorsal, hollow nerve cord Excurrent siphon Tail Excurrent siphon Excurrent siphon Atrium Muscle segments Incurrent siphon Pharynx with slits Intestine Anus Stomach Intestine Tunic Atrium Esophagus Pharynx with slits Stomach An adult tunicate A tunicate larva

17. Early Chordate Evolution • Ancestral chordates may have resembled lancelets • Tunicate genome sequencing has identified genes shared by tunicates and vertebrates • Gene expression in lancelets holds clues to the evolution of the vertebrate form

18. Fig. 34-6 BF1 Otx Hox3 Nerve cord of lancelet embryo BF1 Hox3 Otx Brain of vertebrate embryo (shown straightened) Midbrain Forebrain Hindbrain

19. Concept 34.2: Craniates are chordates that have a head • The origin of a head opened up a completely new way of feeding for chordates: active predation • Craniates share some characteristics: a skull, brain, eyes, and other sensory organs

20. Derived Characters of Craniates • Craniates have two clusters of Hox genes; lancelets and tunicates have only one cluster • One feature unique to craniates is the neural crest, a collection of cells near the dorsal margins of the closing neural tube in an embryo • Neural crest cells give rise to a variety of structures, including some of the bones and cartilage of the skull

21. Fig. 34-7 Neural crest Neural tube Dorsal edges of neural plate Migrating neural crest cells Notochord

22. In aquatic craniates the pharyngeal clefts evolved into gill slits • Craniates have a higher metabolism and are more muscular than tunicates and lancelets • Craniates have a heart with at least two chambers, red blood cells with hemoglobin, and kidneys

23. The Origin of Craniates • Fossils from the Cambrian explosion 530 million years ago document the transition to craniates • The most primitive of the fossils are those of the 3-cm-long Haikouella • Haikouella had a well-formed brain, eyes, and muscular segments, but not a skull

24. Fig. 34-8 5 mm Segmented muscles Pharyngeal slits

25. Fig. 34-8a 5 mm

26. Fig. 34-8b Segmented muscles Pharyngeal slits

27. In other Cambrian rocks, paleontologists have found fossils of even more advanced chordates, such as Myllokunmingia • Myllokunmingia had a skull and was a true craniate

28. Hagfishes • The least derived surviving craniate lineage is Myxini, the hagfishes • Hagfishes have a cartilaginous skull and axial rod of cartilage derived from the notochord, but lack jaws and vertebrae

29. Fig. 34-UN3 Cephalochordata Urochordata Myxini Petromyzontida Chondrichthyes Actinopterygii Actinistia Dipnoi Amphibia Reptilia Mammalia

30. Fig. 34-9 Slime glands

31. Concept 34.3: Vertebrates are craniates that have a backbone • During the Cambrian period, a lineage of craniates evolved into vertebrates • Vertebrates became more efficient at capturing food and avoiding being eaten

32. Derived Characters of Vertebrates • Vertebrates underwent a second gene duplication involving the Dlx family of transcription factors • Vertebrates have the following derived characters: • Vertebrae enclosing a spinal cord • An elaborate skull • Fin rays, in the aquatic forms

33. Lampreys • Lampreys (Petromyzontida) represent the oldest living lineage of vertebrates • They are jawless vertebrates inhabiting various marine and freshwater habitats • They have cartilaginous segments surrounding the notochord and arching partly over the nerve cord

34. Fig. 34-UN4 Cephalochordata Urochordata Myxini Petromyzontida Chondrichthyes Actinopterygii Actinistia Dipnoi Amphibia Reptilia Mammalia

35. Fig. 34-10

36. Fig. 34-10a

37. Fig. 34-10b

38. Fossils of Early Vertebrates • Conodonts were the first vertebrates with mineralized skeletal elements in their mouth and pharynx

39. Fig. 34-11 Dental elements

40. Other armored, jawless vertebrates had defensive plates of bone on their skin

41. Fig. 34-12 Pteraspis Pharyngolepis

42. Origins of Bone and Teeth • Mineralization appears to have originated with vertebrate mouthparts • The vertebrate endoskeleton became fully mineralized much later

43. Concept 34.4: Gnathostomes are vertebrates that have jaws • Today, jawed vertebrates, or gnathostomes, outnumber jawless vertebrates

44. Derived Characters of Gnathostomes • Gnathostomes have jaws that might have evolved from skeletal supports of the pharyngeal slits

45. Fig. 34-13-1 Gill slits Cranium Mouth Skeletal rods

46. Fig. 34-13-2 Gill slits Cranium Mouth Skeletal rods

47. Fig. 34-13-3 Gill slits Cranium Mouth Skeletal rods

48. Other characters common to gnathostomes: • An additional duplication of Hox genes • An enlarged forebrain associated with enhanced smell and vision • In aquatic gnathostomes, the lateral line system, which is sensitive to vibrations

49. Fossil Gnathostomes • The earliest gnathostomes in the fossil record are an extinct lineage of armored vertebrates called placoderms

50. Fig. 34-14 0.5 m