now i would like to discuss some transitional fossils namely transitions between fish and tetrapods between theropod reptiles and birds between synapsis and mammals in between land mammals and whales various groups of fishes were abundant in the devonian period and before namely the jawless fishes of which the lampreys and hagfishes are still with us the cartilaginous fishes the sharks and the rays and the ray-finned fishes and most of the fishes and in fact most of the vertebrate species that we have are in this group and then that would include everything trout goldfish eels sea horses salmon tuna most of the fish that you can think of would be included in the ray finn fishes the loeb finned fishes were a separate group that was uh fairly abundant in the devonian and is represented today only by two groups of just a few species and those are the lung fishes and the coelacanths but the love finn fishes were quite a bit more abundant in the devonian and the geologists tell us that the devonian was a period of frequent droughts there were adaptive four organisms to be able to survive on land and so that was the origin of the tetrapods which are commonly called the land vertebrates although i'm sure you can think of a few examples of these that have gone back into the water now the lobe fin fishes as you can see are more closely related to the tetrapods than they are to other uh fishes and the lobe fin fishes as the name suggests had their fins at the end of a fleshy bony lobe which was pre-adapted for developing into the vertebrate limb now this figure shows some of the uh transitional stages between the lobe fin fishes specifically the rippidistian fishes represented here by the genus eustanopteron and this is a little bit small so i'm going to move in a little bit closer here uh and the tetrapods represented here by ichthyostego which is an early tetrapod which we would think of as an amphibian and there are several intermediate stages uh in this transition and this shows the limb structure of these organisms as well as their skulls so if we look first of all at houston opteron which is a loeb finned fish we can see there's they have a pectoral girdle and a pelvic girdle and then a bony limb structure that the uh fin is attached to and you'll see that the fin structure includes some very familiar bones you'll see a femur and a fibula and a tibia and some other bones that are homologous with bones that are found in tetrapods now here is titalic which is halfway in between and again you'll see this is in this case this is the front limb so this is the humerus or the upper arm the radius and the ulna which is the uh forearm the ulnary and the intermedium which are bones that we have in our wrist and then other bones such are which correspond to the phalanges and now going back here this is houston opteron this is the rippidistian fish the lobe-finned fish and this is ichthyostega the uh tetrapod and you can see again the homologous bones that are found in ichthyostega which developed from the bones in the lobe fin fish ancestor now moving on to the bird transition this is a phylogenetic tree of the amniotes and the amniotes remember include the reptiles birds and mammals and these are the major groups of amniotes the mammals which develop from the synapsids and early branch of the amniotes and the major groups of what we refer to as a reptile so there are several different lineages there's a little bit of conflicting evidence of where exactly the turtles fit in but snakes and lizards the tuataras you may not be familiar with but they are lizard-like reptiles found on small islands uh in new zealand and the crocodilians which are close relatives of the dinosaurs and the birds now the dinosaurs you may have heard have gone extinct the that is not true actually the dinosaurs did not go extinct the dinosaurs that did not go extinct as you can see are still with us and these are not only reptiles but they are actually dinosaurs really by any reasonable biological definition cladistically these organisms are descendants of dinosaurs they come from within the dinosaur clade and it really makes no sense to refer to these in a in any biological meaningful biological sense as anything but dinosaurs so birds really are the dinosaurs that did not go extinct so in spite of the fact that they have a number of derived characters obviously feathers and endothermy and in many other characteristics they share a common ancestor they're part of a monophyletic group with other reptiles and in fact are part of a monophyletic group with the dinosaurs and so you may keep a dinosaur as a pet or you may have had dinosaur meat for lunch now if we look at the fossil record showing the transition between birds and other dinosaurs we can see a number of similarities archaeopteryx is an organism that lived during the jurassic period and it has a number of bird-like characteristics most notably feathers it was mostly the feather imprints that led us to classify this organism as a bird because for the most part it looks like a theropod dinosaur all of the bone structure the long bony tail looks very much like a theropod dinosaur but the feathers don't fossilize very well as a rule but the earliest found fossils of archaeopteryx did have some impressions of feathers and that's how we knew that it was related to a bird there's a number of characteristics of modern birds that we don't find in archaeopteryx such as the heeled sternum and the modifications of the pelvis and of the hand bones and so on but it does have a number of bird-like characteristics as well as typical characteristics of dinosaurs and in addition there are a number of other dinosaurs since then that have been found with uh feather impressions this is fairly rare but we do find a number of dinosaurs where the feathers have been preserved and in fact in some cases these organisms have feathers on all four limbs four winged dinosaurs and a number of groups of dinosaurs apparently did have feathers so feathers is not something that is unique to what we think of as modern bird