A hypothetical phylogeny of chordates

1. Chordates Craniates Vertebrates Gnathostomes Osteichthyans Lobe-fins Tetrapods Amniotes Echinodermata(sister group to chordates) Cephalaspidomorphi(lampreys) Chondrichthyes(sharks, rays, chimaeras) Amphibia(frogs, salamanders) Cephalochordata(lancelets) Actinopterygii(ray-finned fishes) Reptilia(turtles, snakes,crocodiles, birds) Actinistia(coelacanths) Urochordata(tunicates) Myxini(hagfishes) Dipnoi(lungfishes) Mammalia(mammals) Milk Amniotic egg Legs Lobed fins Lungs or lung derivatives Jaws, mineralized skeleton Vertebral column Head Brain Notochord Ancestral deuterostome Figure 34.2 • A hypothetical phylogeny of chordates

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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

10. 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

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

12. Early Chordate Evolution • The current life history of tunicates • Probably does not reflect that of the ancestral chordate

13. 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

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

15. 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

16. (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

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

18. (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

19. 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

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

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

22. Concept 34.3: Vertebrates are craniates that have a backbone • During the Cambrian period • A lineage of craniates evolved into vertebrates

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

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

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

26. 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

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

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

29. Concept 34.4: Gnathostomes are vertebrates that have jaws • Today, jawless vertebrates • Are far outnumbered by those with jaws

30. Gill slits Cranium Mouth Skeletal rods Figure 34.13 Derived Characters of Gnathostomes • Gnathostomes have jaws • That evolved from skeletal supports of the pharyngeal slits

31. Other characters common to gnathostomes include • Enhanced sensory systems, including the lateral line system • An extensively mineralized endoskeleton • Paired appendages

32. (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

33. (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

34. Chondrichthyans (Sharks, Rays, and Their Relatives) • Members of class Chondrichthyes • Have a skeleton that is composed primarily of cartilage • The cartilaginous skeleton • Evolved secondarily from an ancestral mineralized skeleton

35. (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 • The largest and most diverse subclass of Chondrichthyes • Includes the sharks and rays

36. (c) Spotted ratfish(Hydrolagus colliei). Ratfishes, or chimaeras, typically live at depths greaterthan 80 m and feed on shrimps, molluscs, and sea urchins. Some species have a poisonous spine at the front of their dorsal fin. Figure 34.15c • A second subclass • Is composed of a few dozen species of ratfishes

37. Most sharks • Have a streamlined body and are swift swimmers • Have acute senses

38. Ray-Finned Fishes and Lobe-Fins • The vast majority of vertebrates • Belong to a clade of gnathostomes called Osteichthyes

39. Nearly all living osteichthyans • Have a bony endoskeleton • Aquatic osteichthyans • Are the vertebrates we informally call fishes • Control their buoyancy with an air sac known as a swim bladder

40. Adipose fin(characteristic oftrout) Dorsal fin Caudal fin Swim bladder Spinal cord Brain Nostril Cut edge of operculum Anal fin Gills Anus Gonad Liver Heart Lateral line Stomach Urinary bladder Kidney Intestine Pelvic fin Figure 34.16 • Fishes breathe by drawing water over four or five pairs of gills • Located in chambers covered by a protective bony flap called the operculum

41. (a) Yellowfin tuna (Thunnus albacares), a fast-swimming, schooling fish that is an important commercial fish worldwide (b) Clownfish (Amphiprion ocellaris), a mutualistic symbiont of sea anemones (c) Sea horse (Hippocampus ramulosus), unusual in the animal kingdom in that the male carries the young during their embryonic development (d) Fine-spotted moray eel (Gymnothorax dovii), a predator that ambushes prey from crevices in its coral reef habitat Figure 34.17a–d Ray-Finned Fishes • Class Actinopterygii, the ray-finned fishes • Includes nearly all the familiar aquatic osteichthyans

42. The fins, supported mainly by long, flexible rays • Are modified for maneuvering, defense, and other functions

43. Figure 34.18 Lobe-Fins • The lobe-fins, class Sarcopterygii • Have muscular and pectoral fins • Include coelacanths, lungfishes, and tetrapods

44. Concept 34.5: Tetrapods are gnathostomes that have limbs and feet • One of the most significant events in vertebrate history • Was when the fins of some lobe-fins evolved into the limbs and feet of tetrapods