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

2. Derived Characters of Chordates
All chordates share a set of derived characters
Although some species possess some of these traits only during embryonic development
Muscle segments
Brain
Mouth
Anus
Dorsal, hollow nerve cord
Notochord
Muscular, post-anal tail
Pharyngeal slits or clefts
Figure 34.3

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. Tunicates most resemble chordates during their larval stage
Which may be as brief as a few minutes
Pharynx with slits
Notochord
Tail
Dorsal, hollow nerve cord
Atrium
Stomach
Intestine
Excurrent siphon
Incurrent siphon
Muscle segments
(c) A tunicate larva is a free-swimming but nonfeeding “tadpole” in which all four chief characters of chordates are evident.
Figure 34.4c

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

10. Dorsal, hollow nerve cord
Lancelets
Lancelets, subphylum Cephalochordata
Are named for their bladelike shape
Tentacle
Mouth
Pharyngeal slits
Atrium
Digestive tract
Atriopore
Segmental muscles
Anus
Notochord
Dorsal, hollow nerve cord
Tail
2 cm
Figure 34.5

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

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. 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
Notochord
(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 to distant sites in the embryo.
Migrating neural crest cells
Ectoderm
Dorsal edges of neural plate
Neural crest
Neural tube
Figure 34.7a, b

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

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

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

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

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

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

26. Fossils of Early Vertebrates
Conodonts were the first vertebrates
With mineralized skeletal elements in their mouth and pharynx
Dorsal view of head
Dental elements
Figure 34.11

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

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

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

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

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

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

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

41. Ray-Finned Fishes Class Actinopterygii, the ray-finned fishes
Includes nearly all the familiar aquatic osteichthyans
(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

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

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

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