Lesson Overview

1. Lesson Overview 26.2 Chordate Evolution and Diversity

2. The Earliest Chordates • The Cambrian fossil deposits include some early chordate fossils, such as Pikaia, which is shown in the figure. • Scientists first thought it was a worm but then determined that it had a notochord and paired muscles arranged in a series, like those of simple modern chordates.

3. Modern Chordate Diversity • Modern chordates consist of six groups: the nonvertebrate chordates (4%) and the five groups of vertebrates (96%)—fishes (largest group), amphibians, reptiles, birds, and mammals.

4. Cladogram of Chordates • The cladogram of chordates presents current hypotheses about evolutionary relationships among chordate groups. It also shows at which points important vertebrate features, such as jaws and limbs, evolved.

5. Cladogram of Chordates • The circles represent the appearance of certain adaptive features, such as jaws and limbs, during chordate evolution. Each time a new body plan adaptation evolved, a major adaptive radiation occurred.

6. Nonvertebrate Chordates • Fossil evidence suggests that the ancestors of living nonvertebrate chordates diverged from the ancestors of vertebrates more than 550 million years ago. • Two chordate groups lack backbones: tunicates and lancelets.

7. Jawless Fishes • The earliest fishes appeared in the fossil record about 510 million years ago. They had no true jaws or teeth, and their skeletons were made of cartilage.

8. Jawless Fishes • Lampreys and hagfishes both lack vertebrae and have notochords as adults. • Lampreys are filter feeders as larvae and parasites as adults. • Hagfishes have pinkish gray, wormlike bodies, secrete incredible amounts of slime, and tie themselves into knots!

9. Sharks and Their Relatives • Early fishes also evolved jaws, paired pectoral (anterior) and pelvic (posterior) fins. • Jaws make it possible to bite and chew plants and other animals, paired fins offered more control of body movement, while tail fins and powerful muscles gave greater thrust.

10. Bony Fishes - Ray & Lobe-Finned • Ray-finned fishes are aquatic vertebrates with skeletons of true bone; most have paired fins, scales, and gills. • Lobe-finned fishes are bony fishes that evolved fleshy fins supported by larger, more substantial bones. Ancestors of four-limbed vertebrates, or tetrapods.

11. Amphibians • Amphibians are vertebrates that also, with some exceptions, require water for reproduction, breathe with lungs as adults, have moist skin with mucous glands, and lack scales and claws.

12. The Unique “Fishapod” • The Tiktaalik fossil shows both fish and tetrapod features, so its discoverers informally refer to it as a “fishapod”—part fish, part tetrapod. • Early amphibians evolved ways to breathe air and protect themselves from drying out, which fueled another adaptive radiation.

13. Reptiles • Reptiles were the first vertebrates to evolve adaptations to drier conditions. • A reptile is a vertebrate with dry, scaly skin, well-developed lungs, strong limbs, and shelled eggs that do not develop in water.

14. Reptiles • Reptiles were the first vertebrates to evolve adaptations to drier conditions. • A reptile is a vertebrate with dry, scaly skin, well-developed lungs, strong limbs, and shelled eggs that do not develop in water. • Terrestrial Adaptations include: vertebrate with dry, scaly skin, well-developed lungs, strong limbs, and shelled eggs that do not develop in water.

15. Enter the Dinosaurs • The Triassic and Jurassic periods saw a great adaptive radiation of reptiles. Dinosaurs lived all over the world, and they were diverse in appearance and in habit. • The evolutionary lineage that led to modern birds came from one group of feathered dinosaurs.

16. Exit the Dinosaurs • At the end of the Cretaceous Period, a worldwide mass extinction occurred. According to current hypotheses, it was caused by a series of natural disasters: a string of volcanic eruptions, a fall in sea level, and a huge asteroid smashing into what is now the Yucatán Peninsula in Mexico. • After these events, dinosaurs, along with many other animal and plant groups, became extinct both on land and in the sea.

17. Birds • Birds are reptiles that regulate their internal body temperature (endothermy.) • They have an outer covering of feathers; strong yet lightweight bones; two legs covered with scales that are used for walking or perching; and front limbs modified into wings.

18. Bird Roots • Recent fossil discoveries support the hypothesis that birds evolved from a group of dinosaurs. • The first birdlike fossil discovered was Archaeopteryx, which shows both bird characteristics (flight feathers) and dinosaur characteristics (teeth and bony tail).

19. Mammals • Characteristics unique to mammals include mammary glands in females that produce milk to nourish young, and hair. • Mammals also breathe air, have four-chambered hearts, and regulate their internal body temperature.

20. The First Mammals • They were very small and resembled modern tree shrews. • While dinosaurs ruled, mammals remained generally small and were probably active mostly at night. • After the great dinosaur extinction, mammals diversified, increased in size, and occupied many niches.

21. Modern Mammals • By the beginning of the Cenozoic Era, three major groups of mammals had evolved—monotremes, marsupials, and placentals.