The fossil record is very important. It forms the basis of the knowledge we have today, about extinct species. It also demonstrates how animals have evolved. By looking at fossils it is possible to group particular animals together based on their shared anatomical, physiological and behavioural characteristics. There are many gaps in the fossil history and questions that have not been answered, as well as varying opinions in regards to many fossils origins and monophyletic groupings, however, it is clear that the present day vertebrates are extremely diverse.

The pie-chart below demonstrates this diversity by indicating the number of species within the major groups of  vertebrates.

The diversity of the vertebrates is tabulated below. (Note: different figures from the pie-chart above, why? New species are being discovered all the time and different organisations have different ideas and views).

The timeline below shows when the major groups of vertebrates (as indicated by the chart and table) first appeared in the fossil history.

SO HOW DID IT ALL START?

To discuss the origin of the vertebrates we look to the protochordates; their anatomy and development provides evidence of possible vertebrate origins. Protochordates, as well as vertebrates belong to the Phylum Chordata. This is because they share the following characteristics that are shown at some point in each members development but are not necessarily possessed throughout the animals entire life.

The shared characteristics of chordates are:

1. Notochord

2. Dorsal Hollow Nerve Cord

3. Muscular Postanal Tail

4. Endostyle : a ciliated groove on the floor of the pharynx that secretes mucus for trapping food particles during filter feeding. The endostyle present in adult tunicates, cephalochordates and larval lampreys are homologous with the thyroid gland of vertebrates.  

5. Gill Slits

In addition the vertebrates have:

6. Brain

7. Skull

8. Ventral Heart

9. Stomach

SO IN THE BEGINNING...........

Within the phylum Chordata are several sub-phyla: Hemichordata, Urochordata and Cephalocordata; these are all non-vertebrate chordates.

Sub-Phylum Hemichordata (“half-chordates”).

• Sedentary worm-like animals with pharyngeal slits; indication of a dorsal hollow nerve cord and notochord.
• Ciliated larvae and a body built around three regions (prosoma, mesosoma, metasoma), like that of developing echinoderms.
• Thus form a link group between echinoderms one side and protochordates the other.

Class Pterobranchia.

• Sessile
• Sometimes tube-dwelling
• A crown of arms and tentacles on the back of the head that form the filter-feeding apparatus
• U-shaped gut

Class Enteropneusta. (Acorn worms)

• Live in a mud-burrow
• Use the proboscis to gather food particles.
• Larva has a post-anal tail (like vertebrates) but this is shed at maturity, leaving the anus terminal
• Straight through gut

  Sub-phylum Urochordata (“tail-chordates”).

• Are called 'tail-chordates' because the notochord is restricted to the tail.
• Sea squirts; sessile marine filter-feeders, and their planktonic relatives.

  Class Ascidiacea.

• Sea squirts
• Enlarged filter-feeding pharynges with an endostyle secreting a mucus filter on the inside, and a dorsal lappet (rather than a groove) collecting up the food entrapped in the mucus dorsally.
• Don’t look much like vertebrates, but have a tadpole larva, with a tail, notochord and muscle blocks, which looks more like one.
• Larva does not feed, and when it settles to metamorphose into an adult it loses its tail.  

    Class Thaliacea.

• Planktonic, barrel-like, tunicates
• Feed, with a filter-feeding mechanism like that of the adult sea squirt.

Class Larvacea.

• Planktonic tadpole-larva-like tunicates
• Filter-feed through a complex house
• Molecular evidence suggests are the most primitive urochordates.

A table to show how the anatomy and development of the protochordates provides evidence for the origins of the vertebrates

The subphylum Cephalochordata is a very important link in the fossil record because they share some characteristics with vertebrates that are absent in tunicates such as myomeres, a circulatory system, specialised excretory cells and a vertebrate-like tail fin. Furthermore, amphioxus and vertebrates share some similar embryonic features such as mesoderm and neural tissues.

These features form the basis of the discussion for the next lecture.    

Further reading:

Carroll, R.L. (1988) Vertebrate Palaeontology and Evolution. Chapter 2: pages 16 – 21  

Pough, F.H., Janis, C.M. & Heiser J.B. (2002) Vertebrate Life.  Chapters 1 –2: pages 1 – 46