Vertebrate Evolution Lecture 24

1. Vertebrate Evolution Lecture 24

3. Chordates are members of the phylum Chordata Lancelets Tunicate Fig. 21.40 Chordates • Their nearest animal relations are the echinoderms • However, chordates employ a truly internal endoskeleton • Chordates are quite diverse

4. Distinguishing features of chordates • 1. Notochord • A stiff, but flexible rod, that forms beneath the nerve cord • 2. Nerve cord • A single dorsal nerve to which other nerves are attached • 3. Pharyngeal slits • A series of slits behind the mouth into the pharynx • 4. Postanal tail • A tail that extends beyond the anus • All chordates have all four of these at some time in their life

6. With the exception of tunicates and lancelets, all chordates are vertebrates • Distinguishing features of vertebrates • 1. Backbone • A bony vertebral columnreplaces the notochord • 2. Head • Well-differentiated, with skull and brain • All vertebrates have an internal skeleton made of bone and cartilage against which the muscles work • This makes possible great size and movement

7. The first vertebrates evolved around 470 mya in the oceans • Jawless fish – • Ostracoderms – toothless, scavangers • Agnathans • Hagfish • Lampreys • Cartilaginous skeleton • Invasion of the land • First – fungi and plants (around 500 mya) • Second – arthropods (around 410 mya) • Third – vertebrates (360-280 mya) • Amphibians were the first to live on land

8. Fig. 20.08

9. An evolutionary timeline

10. Fish • Four characteristics • 1. Gills • Used to extract dissolved oxygen gas from water • 2. Vertebral column • An internal skeleton with a spine surrounding the dorsal nerve cord • 3. Single-loop blood circulation • Blood flow: Heart  Gills  Body  Heart again • 4. Nutritional deficiencies • Inability to synthesize the aromatic amino acids

11. Fig. 22.10 The Evolution of Jaws • Jaws addressed two challenges faced by predators • 1. How to grab and hold prey • 2. How to pursue prey • Jaws evolved from arch supports (about 410 mya)

12. Fish Evolution • Early forms: Spiny fishes and placoderms • Sharks: light skeleton – cartilage • Class Chondrichthyes • Skates and rays are flattened sharks that are bottom-dwellers • advanced reproduction system – internal fertilization

15. Class: Osteichthyes - Bony Fish • Stiff internal skeletons – firm support for muscle action • Swim bladder – creates neutral buoyancy • Gil cover - Operculum – allows fish to pump gills to circulate water while motionless • Lateral line system – sensory – detects changes in pressure, predators near by, presence of prey

16. Fig. 20.11

20. Red-eyed tree frog Fig. 22.14 22.5 Amphibians Invade the Land • Amphibians are direct descendants of fishes • They are the first vertebrate to walk on land • They include • Frogs • Toads • Salamanders • Caecilians

21. Amphibians • Carnivores • Developed from lobe finned fish • Leg development required skeletal modification • Terrestrial adaptation – means to acquire oxygen: • Cutaneous respiration – moist skin • Lung development – internal moist surface • Pulmonary circulation • 3 chambered heart – oxygenated blood separated from deoxygenated blood

22. Fig. 22.15 Evolution of legs Characteristics of Amphibians • 1. Legs: prob. development from lobe finned fish

23. Characteristics of Amphibians • 2. Lungs • Provide a more efficient means of respiration than gills • 3. Cutaneous respiration • Respiration directly across the skin supplements the use of lungs • 4. Pulmonary veins • Two large veins that return aerated blood to the heart for repumping • 5. Partially divided heart • Separates the blood circulation into two separate paths • Pulmonary and systemic

24. Fig. 25.13b • The heart beat in fishes has a peristaltic sequence • Starts at the rear (SV) and moves to the front • Gill respiration provides fully oxygenated blood to the body • However, circulation is sluggish • This limits rate of oxygen delivery to rest of body

25. Amphibian and ReptileCirculation • The advent of lungs resulted in two circulations • 1. Pulmonary circulation • Delivers blood to the lungs • 2. Systemic circulation • Delivers blood to the rest of the body

26. Fig. 25.14a • The amphibian heart has two structural features that reduce mixing of oxygenated & deoxygenated blood • 1. The atrium is divided into two chambers by a septum • 2. Conus arteriosus is partially separated by another septum • Amphibians in water supplement the oxygenation of blood by a process called cutaneous respiration

27. Fig. 20.14

29. Amphibian Reproduction • Dependent on water environment • Eggs laid in water • External fertilization in watery environment • Young develop in water • Some unique adaptations enable colonization of dry habitats • Example: young develop in mouth of adult • Adults become terrestrial

30. Reptiles

31. Reptiles Improved Adaptation to Terrestrial Live • Dry Skin – prevention of desiccation • Thoracic breathing – Efficient system of drawing air in and out of lungs • Improved cardio-vascular • Improved leg attachments – rapid movement

32. Fig. 25.14b • Among reptiles, additional modifications have further reduced the mixing of blood in the heart • The ventricle is partially divided into two chambers by a septum • The separation is complete in the crocodiles • They thus have completely divided pulmonary and systemic circulation

33. Fig. 20.04b

34. Fig. 20.03

35. Fig. 20.04a

36. Reptile – Reproduction • Eggs laid on and develop on land – even by aquatic species • Internal fertilization • Egg developed as self contained survival/developmental structure

37. Chorion: Allows O2 entry • Amnion: Fluid-filled cavity • Yolk sac: Provides food • Allantois: Excretes waste Fig. 22.17 Amniotic egg • Watertight - four membranes

39. Birds – Class Aves Reptile Adapted for Flight • Scales on leg – evidence for reptilian ancestry • Temperature regulation • Feathers – Did they arise for flight or insulation? • Improvements to oxygen gathering/circulation • Four chambered heart • Flight improvements • Skeletal modifications • Wing bones • Keel • Light weight

40. Fig. 20.16

43. http://home13.inet.tele.dk/palm/origbird.htm http://www.nature.com/nature/journal/v420/n6913/fig_tab/nature01196_F5.html

44. Fig. 20.05

45. Fig. 20.17

46. Mammals • Major characteristics: • Mammary Glands – modified sweat glands • Hair – Keratin filled cells • Middle ear structure • Other characteristics: • Extended care for young • Behavioral flexibility – reflection of brain development • Dentition – heterodent – 4 types of teeth • Highly developed for life on land – some returned to water

47. Fig. 20.19

48. Mammals – Major Groups • Prototherians • Shelled eggs • Few living examples: duck billed platypus Monotremes • Therians • Viviparous – young born alive • Includes: • Marsupials – young born early, develop in pouch • Examples: kangaroo, opossum • Placental – development in placenta within uterus • Example: us (humans)

49. Fig. 22.22 Other Characteristics of Modern Mammals • Placenta • Characteristic of most mammals • Brings the bloodstream of mother and fetus into close contact • The two don’t mix

50. Mammal Diversity • Empty niches became available with extinction of dinosaurs • Radial evolution to fill niches occurred – great diversity developed