Chordate Classification and Vertebrate Evolution
Invertebrate Ancestors of Fish, Frogs, Lizards, Birds and Mammals
Mar 4, 2009
The Phylum Chordata contains a wide range of animals, from sea-squirts to humans, and (since some of them do not have a backbone) ‘our’ phylum includes all the vertebrates as well as a few invertebrates! Unlikely though it might seem, the Phylum Chordata is probably closely related to the Phylum Echinodermata (the starfish and sea urchins).
Chordate Biology
The chordates all have a head end and a post-anal tail at some stage in their life-cycle (maybe as a larval form or in the embryo). Their gut has slits at the front end and runs below the nervous system. There is a stiffening rod (the notochord in ‘lower’ forms or a vertebral column in ‘higher’), which is just above the gut. There are three subphyla:
- Urochordata (or ‘Tunicates’) are very different from the higher vertebrates, most being little more than small filter-feeding sacs.
- Hemichordata (or ‘Lancelets’ – previously called ‘Amphioxus’) are a bit ‘fish-like’, but they do not have a backbone and spend their lives stuck in sediment filter-feeding like the tunicates.
- Craniata all have a recognisable head, but the Hagfish and Lampreys do not possess bone (and can not therefore be called vertebrates). Many other craniates are collectively known as ‘fish’ (a word which has no strict taxonomic definition) and this includes all the sharks and the boney fish as living examples, as well as many other extinct forms. All the four-legged animals (tetrapods) also fit in here.
Tetrapods
One type of bony fish, the lobe-fins (see ‘Coelacanths’), was the ancestor of all the four-legged creatures, including us. These four classes of tetrapod (Amphibia, Reptilia, Aves, and Mammalia) are the best known of the chordates or vertebrates.
- Amphibians – (Amphibia) – were the original land tetrapods. They are ancestral to the reptiles and all that followed after. Their way of reproducing (with eggs and young that need to be in freshwater) limits their distribution to some extent, but frogs, toads, newts and salamanders still exist – as well as the rather strange caecilians which burrow and look more like worms.
- Reptiles – (Reptilia) – have scales and lay eggs. They have a long and chequered history. The most famous must be the now extinct ‘Dinosaurs’, who were wiped out when an asteroid collided with the earth around 65 million years ago. Snakes, lizards, turtles and crocodiles remain as common living forms – and it could be argued that the dinosaurs themselves live on, but now as birds.
- Birds – (Aves) – are characterised by their feathers, eggs, beaks instead of toothed jaws, and their warm-bloodedness. They have four legs - but the front ones have become wings!
- Mammals – (Mammalia) – can sweat, have hair, are warm-blooded, and for the most part grow their young inside the female (although a few still lay eggs). There are over twenty ‘orders’ including the primates – where humans belong.
 |
 |
 |
 |
 |
