1. The Vertebrate Story
Part 1
Introduction

2. Introduction
The story of vertebrate history unfolds over more than half a billion years.
This period has seen some of the largest, most complex organisms to ever exist.
Vertebrate diversity can be attributed to opportunity and to their novel features.
Vertebrates occupy marine, freshwater, terrestrial, and aerial environments.
Two features, the vertebral column and the cranium give the group its name.

3. All vertebrates are also members of the chordate family; and possess, at some time, each of the five chordate features.
Notochord
Dorsal Hollow Nerve Cord
Pharyngeal Slits
Sub-Pharyngeal Organ
Post Anal Tail

4. Vertebral Column
Composed of serially repeating vertebrae and form the backbone that defines the major body axis.
Made of either bone or cartilage.
Squeezed between successive vertebrae are thin compression pads, called intervertebral disks.
A single vertebrae consists of a solid cylindrical body, often enclosing the notochord, a dorsal neural arch, that encloses the spinal cord, and a ventral hemal arch enclosing blood vessels.

6. Cranium
Composed of a composite of bone and/or cartilage and supports the sensory organs of the head.
The term cephalization refers to the movement of the sensory structures into a single anterior position.
Vertebrate evolution has been characterized by a wide array of cranial structures that collectively form the head.
Currently the defining characters are the presence of neural crest cells and epidermal placodes.
These embryonic structures give rise to the sensory structures, bones, and teeth within the head.

7. Origin The origin of modern vertebrates took place in marine waters.
Early evolution was characterized by an increasingly active lifestyle hypothesized to proceed in three steps.
Suspension-feeding pre-vertebrates
Agnathans
Gnathostomes

8. Pre-Vertebrates
These species arose within a sister group, the protochordates.
Similar in form to amphioxus, but free swimming and better adapted to estuarine environments.
The shift from this pre-vertebrate to a more vertebrate condition involved two changes in the pharynx:
The development of a circular band of muscles.
Strong, spiny cartilage replaced the collagen in he pharyngeal bars.

9. This new muscular pump would have initially assisted in feeding, by increasing the amount of water circulation .
This removed size limits that imposed by a ciliary pump .
Although gill were not present until much later, these changes also helped push water across the pharyngeal bars and increased the amount respiratory exchange in this area.

10. Agnathan
The appearance of the pharyngeal pump brings us to the agnathan condition.
Early agnathans were likely suspension, or deposit feeders.
They still employed cilia and mucus in the pharyngeal basket to assist in food collection, however, the movement was now due to the muscular pumping of water through the pharynx.

11. Gnathostome
The transition from agnathan to gnathostome involved a switch in feeding strategy.
Transition species would have been raptorial feeders that plucked individual food particles from the water column.
With the advent of jaws suction feeders would have become active, aggressive feeders that were no longer limited to feeding on small particles that fit easily into the mouth or pharynx.

12. Classification Traditional taxonomy divides vertebrates into classes.
Amphibians, reptiles, birds and mammals are collectively termed tetrapods.
All other vertebrates are fishes.
Vertebrates with jaws are gnathostomes, those without are agnathan.
Vertebrates whose embryos develop in a self-contained fluid sac (amnion) are called amniotes.
All of these will be observed in lab.

13. Chordate Morphology

14. Tonight's Activities
Locate the 5 chordate features in the amphioxus and amocetes larva.
You will probably not find the sub-pharyngeal organ in either specimen.
Be able to compare the two organisms.
Amphioxus is a chordate, protocertebrate
Amocetes is the larval form of the hagfish, a true vertebrate.

15. Amphioxus I Ammocetes