Limb Evolution Notes

Lecture 3 - The evolution of limbs Limbs may restrict our options for movement. 5 fingers may not be ideal but it is what we have got as from the restricted route it has come from. Geological time scale (e g Cambrian, Devonian etc) are used by name not numbers as data change as age is reassessed; whereas the names relate to particular bits of rock all over the world with particular fossils in them. The tertiary is no more - it is now called the Paleogene. The boundaries of the Pliocene have also changed. These early agnathans do not have true fins either. Hemicyclaspis was a typical armoured fish - has flippers and tail but don't necessarily think that these early appendages related to fins of mdern fish. A dorsal view of the fish very much resembles living fish today, but on the ventral side they lack articulated jaws and they have just a small opening on the front with a ridge on the bottom and it is thought they scraped food from surfaces. Interestingly in terms of locomotion is that these things probably didn't live in the water column - benthic creatures as a) they were very heavy and couldn't float and b) the details of their head anatomy shows that they only had 2 semicircular canals instead of 3, indicating that they only moved in 2 dimensions. Best explanation is that on the bottom of the sea The Cambrian is where the origin of the limb was. Silurian was the invasion of land. Triassic, Jurassic, Cretaceous was the time of dinosaurs and aquatic and flying reptiles - significant limb specialization. Early origins Earliest vertebrates were agnathans - the jawless fish. A lot less material - mostly just teeth. Agnathans are related to modern lampreys and hagfish as they lack true jaws. Early examples are ostracoderms like hemicyclaspis which was around 500 million years ago. Hemicyclaspis Origin of appendages Can see clear overlap between fish fins and tetrapod limbs. Don't really know exactly how this came about -lineage not 100% clear. What happens is that there are fish like the anaspids (500Mya) that have lateral fin folds that were not segmentally specialized, and it is suggested that limbs appeared as there were segmental losses in parts of the fold. Modern electric fish sometimes have these fins. At some point this continuous fin became specialized as a pectoral and pelvic fin but there is not an evolutionary story. All we have are fish with a continuous fin (probably more primitive) and fish with specialized shoulder areas. Anaspids were still jawless fish but they look like modern fish as they show streamlining - the fusiform shape which is particularly good at moving through water. Again they only have 2 semi-circular canals - probably didn't move up and down very much and stayed towards bottom. As having jaws means you probably aren't an active Climatius predator and evolving efficient ways of locomotion probably isn't terribly important. However, 50 million years on, during the Silurian period, in the first jawed fish, proper fins coincide, for example in the acanthoidians (eg Climatius). It has teeth, pectoral and pelvic fins and it no longer has continuous lateral strip. These fish are very modern in terms of aspect. 

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