Chapter 34

1. Chapter 34 Vertebrates

2. Half a Billion Years of Backbones • By the end of the Cambrian period, some 540 million years ago • An astonishing variety of animals inhabited Earth’s oceans • One of these types of animals • Gave rise to vertebrates, one of the most successful groups of animals

3. Figure 34.1 • The animals called vertebrates • Get their name from vertebrae, the series of bones that make up the backbone

4. There are approximately 52,000 species of vertebrates • Which include the largest organisms ever to live on the Earth

6. The Origin of Chordates • The phylum Chordata is defined by the presence of four morphological features • pharyngeal gill slits • a notochord • a dorsal hollow nervecord • a muscular tail that extends past the anus

7. Dorsal,hollownerve cord Brain Notochord Musclesegments Mouth Anus Pharyngealslits or clefts Muscular,post-anal tail Figure 34.3 Derived Characters of Chordates • All chordates share a set of derived characters • Although some species possess some of these traits only during embryonic development

12. Notochord • The notochord • Is a longitudinal, flexible rod located between the digestive tube and the nerve cord • Provides skeletal support throughout most of the length of a chordate • In most vertebrates, a more complex, jointed skeleton develops • And the adult retains only remnants of the embryonic notochord

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

14. Pharyngeal Slits or Clefts • In most chordates, grooves in the pharynx called pharyngeal clefts • Develop into slits that open to the outside of the body • These pharyngeal slits • Function as suspension-feeding structures in many invertebrate chordates • Are modified for gas exchange in aquatic vertebrates • Develop into parts of the ear, head, and neck in terrestrial vertebrates

15. Muscular, Post-Anal Tail • Chordates have a tail extending posterior to the anus • Although in many species it is lost during embryonic development • The chordate tail contains skeletal elements and muscles • And it provides much of the propelling force in many aquatic species

16. Tunicates • Tunicates, subphylum Urochordata • Belong to the deepest-branching lineage of chordates • Are marine suspension feeders commonly called sea squirts

17. Notochord Dorsal, hollownerve cord Tail Excurrent siphon Musclesegments Incurrentsiphon Intestine Stomach Atrium Pharynx with slits (c) A tunicate larva is a free-swimming butnonfeeding “tadpole” in which all fourchief characters of chordates are evident. • Tunicates most resemble chordates during their larval stage • Which may be as brief as a few minutes Figure 34.4c

18. Incurrentsiphonto mouth Excurrentsiphon Excurrent siphon Atrium Pharynxwith numerousslits Anus Intestine Tunic Esophagus Stomach (b) In the adult, prominent pharyngeal slits function in suspension feeding, but other chordate characters are not obvious. (a) An adult tunicate, or sea squirt, is a sessile animal (photo is approximately life-sized). Figure 34.4a, b • As an adult • A tunicate draws in water through an incurrent siphon, filtering food particles

19. Tentacle 2 cm Mouth Pharyngeal slits Atrium Notochord Digestive tract Atriopore Dorsal, hollownerve cord Segmentalmuscles Anus Tail Figure 34.5 Lancelets • Lancelets, subphylum Cephalochordata • Are named for their bladelike shape

20. Lancelets are marine suspension feeders • That retain the characteristics of the chordate body plan as adults

21. BF1 Otx Hox3 Nerve cord of lancelet embryo BF1 Hox3 Otx Brain of vertebrate embryo(shown straightened) Midbrain Forebrain Hindbrain Figure 34.6 • Gene expression in lancelets • Holds clues to the evolution of the vertebrate form

22. 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 common characteristics • A skull, brain, eyes, and other sensory organs

23. Neuraltube Neuralcrest Dorsal edgesof neural plate Ectoderm Ectoderm (a) The neural crest consists of bilateral bands of cells near the margins of the embryonic folds that form the neural tube. (b) Neural crest cells migrate todistant sites in the embryo. Migrating neuralcrest cells Notochord Figure 34.7a, b Derived Characters of Craniates • One feature unique to craniates • Is the neural crest, a collection of cells that appears near the dorsal margins of the closing neural tube in an embryo

24. (c) The cells give rise to some of the anatomical structuresunique to vertebrates, including some of the bones and cartilage of the skull. Figure 34.7c • Neural crest cells • Give rise to a variety of structures, including some of the bones and cartilage of the skull

25. The Origin of Craniates • Craniates evolved at least 530 million years ago • During the Cambrian explosion

26. (a)Haikouella. Discovered in 1999 in southern China, Haikouella had eyes and a brain but lacked a skull, a derived trait of craniates. Figure 34.8a • The most primitive of the fossils • Are those of the 3-cm-long Haikouella

27. 5 mm (b) Haikouichthys.Haikouichthys had a skull and thus is considered a true craniate. Figure 34.8b • In other Cambrian rocks • Paleontologists have found fossils of even more advanced chordates, such as Haikouichthys

28. Slime glands Figure 34.9 Hagfishes • The least derived craniate lineage that still survives • Is class Myxini, the hagfishes

29. Hagfishes are jawless marine craniates • That have a cartilaginous skull and axial rod of cartilage derived from the notochord • That lack vertebrae

30. Vertebrates • craniates that have a backbone • During the Cambrian period • A lineage of craniates evolved into vertebrates • A series of innovations occurred as the vertebrate lineage diversified

32. Fossil vertebrates from 480 million years ago were the first fossils to contain bone in the form of an exoskeleton enveloping the body. • The first vertebrates with jaws appear 430 million years ago. • The appearance of tetrapods (animals with four limbs) and the transition to land are dated at about 357 million years ago • The first amniotes appeared 20 million years after the emergence of tetrapods.

33. Derived Characters of Vertebrates • Vertebrates have • Vertebrae enclosing a spinal cord • An elaborate skull • Fin rays, in aquatic forms

34. Lampreys • Lampreys, class Cephalaspidomorphi • Represent the oldest living lineage of vertebrates • Have cartilaginous segments surrounding the notochord and arching partly over the nerve cord

35. Figure 34.10 • Lampreys are jawless vertebrates • Inhabiting various marine and freshwater habitats

37. Dorsal viewof head Dentalelements Figure 34.11 Fossils of Early Vertebrates • Conodonts were the first vertebrates • With mineralized skeletal elements in their mouth and pharynx

38. Pteraspis Pharyngolepis Figure 34.12 • Armored, jawless vertebrates called ostracoderms • Had defensive plates of bone on their skin

39. Origins of Bone and Teeth • Mineralization • Appears to have originated with vertebrate mouthparts • The vertebrate endoskeleton • Became fully mineralized much later

40. Gnathostomes • vertebrates that have jaws • Today, jawless vertebrates • Are far outnumbered by those with jaws

41. The Vertebrate Jaw • Vertebrates could not harvest food by biting until jaws evolved. The leading hypothesis for the origin of the jaw is that mutation and natural selection increased the size and modified the orientation of the gill arches. (Figure 33.11).

45. Lines of Evidence • Support the gill-arch hypothesis: • Both gill arches and jaws consist of flattened bars of bony or cartilaginous tissue that hinges and bends forward. • Both jaws and gill arches are derived from neural crest cells. • The muscles that move jaws and gill arches are derived from the same population of embryonic cells.

46. Other Charactersistics of Gnathostomes • Enhanced sensory systems, including the lateral line system • An extensively mineralized endoskeleton • Paired appendages

47. (a) Coccosteus, a placoderm Figure 34.14a Fossil Gnathostomes • The earliest gnathostomes in the fossil record • Are an extinct lineage of armored vertebrates called placoderms

48. (b) Climatius, an acanthodian Figure 34.14b • Another group of jawed vertebrates called acanthodians • Radiated during the Devonian period • Were closely related to the ancestors of osteichthyans

49. ClassChondrichthys • Sharks, Rays, and Their Relatives) Have a skeleton that is composed primarily of cartilage • The cartilaginous skeleton • Evolved secondarily from an ancestral mineralized skeleton

50. (a) Blacktip reef shark (Carcharhinus melanopterus).Fast swimmers with acute senses, sharks have paired pectoral and pelvic fins. Pelvic fins Pectoral fins (b) Southern stingray (Dasyatis americana).Most rays are flattened bottom-dwellers thatcrush molluscs and crustaceans for food. Some rays cruise in open water and scoop food into their gaping mouth. Figure 34.15a, b Sharks and Rays • The largest and most diverse subclass of Chondrichthyes