This lecture will cover the early evolution of limbs starting at the late Cambrian period and moving forward until specialised pentadactyle limbs appear in the Tertiary. I will speak a lot about the skeleton and rather little about anything else - simply because we know much more about the skeleton since it fossilises so well. Muscles, nerves and blood vessels do not.
The following table is an example of the geological timescale with the periods related to approximate ages and biological events [geological time].
The earliest vertebrates are related to modern lampreys and hagfishes and are known as AGNATHANS or jawless fish. They are often parasitic; feeding on other fish by boring a hole through the skin and eating the victim from the inside out. Early examples of an agnathans are the ostracoderms (500 mya Ordovician) [for example Hemicyclaspis].
There is still no certainty about the origin of true, paired fins. However, Anaspids (500 mya) have lateral fin foldsrunning all the way down the sides of the body and it is assumed that true fins evolved by the segmental loss of parts of this fold [Anaspids].
Agnathans have only two semi-circular canals and lack jaws and proper paired fins. About 450 mya (Silurian) the first true jawed fish appeared. These were the ACANTHOIDIANS [for example Climatius].
They have three semi-circular canals and (most importantly for us today) true fins. Especially note the presence of pectoral and pelvic fins: hopefully these names are beginning to ring a bell. These changes are believed to signify a change from a bottom dwelling to a free swimming lifestyle: actively seeking prey (hence the fins) and eating it (jaws).
Paired fins are specialized flaps of the body wall that serve as steering organs in swimming (the main force coming from muscles in the trunk). There are two groups - pectoral and pelvic [pectoral fins].
The base of a fish's fin is a flat plate of bone attached to the muscles of the body wall. The fin itself is a flexible fan of small bones that stick out sideways from the plate. The bony fan is moved by two groups of small muscles: a group of elevators above (dorsally) which pull it upwards and a group of depressors below (ventrally) which pull it downwards. The nerves which supply the limb muscles divide to correspond with the muscle masses into an anterior or depressor and a posterior or elevator branch of the ventral ramus of one or more spinal nerves. These muscles adjust the angle at which the fin meets the water causing the fish to turn, climb or descend as it moves along propelled by its tail [pelvic fins].
The fossil fish mentioned so far have all had a skeleton made out of cartilage rather than bone. These are Chondrichthyes (400 mya Devonian) and modern groups include sharks and rays. The fish that were ancestral to land tetrapods are Osteichthyes (400 mya), or bony fish possessing a true bony skeleton. Most bony fish do not possess bony fins - the fins are pliant paddles attached to very simple girdles. Flexibility is more important than strength for a steering rather than load bearing function.
However, there is one group of fish that have bony fins. These are the Crossopterygii represented as a living form by the Coelacanths and various fossil forms [fin of a Eusthenopteran].
The basic form of a tetrapod limb is now present in fish. We do not know why, although we can speculate that there may have been advantages in some conditions to have bony fins. Certainly there are intermediate fish forms (lung fish and musdskippers) that use their bony fins for locomotion. Terrestrial locomotion and finalising of the tetrapod body form was acheived by the amphibians [Rhipidistan & Amphibian], but the anatomical homolgy of the components can be clearly seen when compared with Crossopterygian fish.
[Rhipidistian & Amphibian Limb]
Limb bony homology can be extended [Forelimb evolutionary sequence]
Beyond amphibians (Jurassic and Cretaceous - reptiles, birds and mammals), limbs improve for locomotor efficiency. There is a movement towards a more upright stance rather than the amphibian sprawling stance, and some tendency towards bipedalism among reptiles and birds.
As often in evolution, basic forms of structures can evolve to perform a number of tasks. Tetrapod forelimbs are a prime example of this by becoming used for grasping, climbing, flying, burrowing and swimming as well as terrestrial locomotion [Comparative Limbs].
In addition, there are a specific set of sequential modifications that have occured in a number of groups to improve locomotor efficiency. This has basically consisted of elongating and straightening the limb and reducing the peripheral weight so that moving the limb becomes more energy efficient. This weight reduction has usually resulted in the loss of bones and the elongation of distal limb elements [plantigrade etc.].
Interestingly, human locomotor adaptations are rather different reflecting the fact that we have only very recently (late Tertiary) become terrestrial.
Alexander RM. Animals. Cambridge: Cambridge University Press. 1990.
Hildebrand M. Vertebrate Structure by M. Hildebrand. New York: John Wiley & Sons. 1995, 4th ed.
Strickberger MW. Evolution. Boston: Jones and Bartlett Publishers. 1990.