This document summarizes key characteristics of vertebrate evolution from early chordates like sea squirts to modern birds. It describes: 1) Early chordates like sea squirts that have larval forms displaying chordate traits but adult forms that differ significantly. 2) Lancelets and hagfish which retain more chordate traits as adults than sea squirts. 3) Jawless fish like lampreys that were early vertebrate lineages but lacked jaws. 4) The divergence of jawed fish into cartilaginous fish like sharks and bony fish like ray-finned and lobe-finned varieties. 5) The transition from fish to early tetrap

1. Chapter 34: Vertebrates (sea squirts through birds)

2. Chordate Phylogeny

4. Chordate Characteristics

5. Subphylum Urochordata The urochordates include the tunicates, a.k.a., sea squirts The tunicates are mostly sessile, filter feeding animals that look almost nothing like a chordate However, their larval form possesses all of the basic characteristics of a chordate

6. Tunicate: Urochordate

7. Tunicate Larva The notochord serves as a primitive internal support structure

8. Subphylum Cephalochordata Cephalochordates, the lancelets, retain a chordate body plan throughout life In addition to the four defining chordate characteristics, lancelets, as well as tunicate larvae, have somites Somites are blocks of musculature arranged in segments along the bodies of lancelets and fishes (as well as derivations of this segmentation found in tetrapods)

9. Lancelet: Cephalochordate

10. Class Myxini Class Myxini are represented in modern times by the hagfish Technically hagfish are not fish; they are more like “sophisticated” lancelets

11. Hagfish, a Craniate

12. Notochord, Nerve Cord, Etc.

14. Class Cephalaspidomorphi Class cephalaspidomorphi is represented in extant lineages by lampreys Jawless fish were the vertebrate forerunners of the jawed fish Lampreys in addition lack paired fins

15. Lamprey: Jawless Vertebrate Lamprey have vertebrae (backbones) but no jaw As vertebrates, lampreys are (just barely) members of subphylum Vertebrata

16. Lamprey Larvae are like Lancelets Lamprey larvae are suspension feeders that resemble lancelets, even burying themselves in sediment as to lancelets

17. Early (and armored) Jawless Fish

18. Superclass Gnathsomata Jawed fish are referred to as gnathostomes (superclass gnathsomata) for their defining feature: jaws Jawed fish also have skulls, vertebrae, and paired appendages (fins)

19. Early Gnathostomes Many vertebrate characteristics are adaptations to vigorous swimming Jaws, of course, aid in eating Especially carnivorous eating…

20. Class Chondrichthyes Class Chondrichthyes includes the sharks and rays Members of this class are named for their cartilaginous skeletons, i.e., unmineralized (or less mineralized/ossified) skeletons

21. Class Chondrichthyes Members

23. The Osteichthyans These are bony fish, both ray-finned and lobe-finned fish

25. Class Actinopterygii Members of class Actinopterygii are the ray-finned The lobe-finned fish are members of Sarcopterygii

26. Anatomy of a Ray-Finned Fish

27. Class Actinistia

28. A Coelacanth Lobe fins are supported by musculature and a bony skeleton; they may be employed for “walking” upon the bottom and other substrate found within bodies of water

29. Class Dipnoi Members of class Dipnoi are the lung fish The lungfish are named for the lungs they retain and use for gulping air, especially when air is less readily available within the freshwaters in which they reside

30. Various Extant Lungfish

31. Class Amphibia Amphibians are the lineage descended from the earliest tetrapods: the terrestrial vertebrates

32. Various Extant Amphibians The amphibians were the dominant terrestrial vertebrates during the Carboniferous period, i.e., the same time the seed-less, vascular plants dominated the land Most amphibians are dependent on the water, minimally for reproduction; this is because their eggs are not desiccation resistant In addition, many amphibians employ their skin for gas exchange, thus requiring that their skin remain moist Thus, amphibians tend to be not as well-adapted to long term, especially multi-generational excursions away from moist habitats

33. Tetrapods The tetrapods are the terrestrial descendants of some lobe-finned fish, starting approximately with the amphibians

34. Reconstruction of an Extinct Tetrapod

35. Class Reptilia Note how the reptiles are indicated as sister group to the mammals In fact, the reptiles and mammals together form a clade known as the amniotes The defining features of the amniotes are keratinized (waterproof) skin and the amniotic (shelled) egg

36. Class Reptilia, Examples

37. Class Reptilia is Paraphyletic Class Reptilia, if it excludes the birds, is paraphyletic

38. The Amniotes The amniotes were the first fully terrestrial vertebrates, achieving true freedom from water except, of course, for the need to drink

39. Amniotic Egg The amniotic egg is shelled, an adaptation to desiccation prevention The amniotic egg employs extraembryonic membranes to transfer stored nutrients and water, exchange gasses, and remove wastes

40. The Amniote Phylogeny

41. Giant Aquatic Reptiles Non-Archosaur Giant Reptiles (i.e., these are lizards!)

43. Feathers and Light-Weight Bones

44. Archaeopteryx More primitive extinct birds were more saurischian dinosaur like