Fishy evolution

11 October, 2009

Fossilised fish embedded in rock from Koonwarra.
Fossilised fish embedded in rock from Koonwarra.
Image: Frank Coffa
Source: Museum Victoria

Question: Why do fish have complex skeletal structures such as hands and pelvis's which seem to serve limited functions for them but are clearly necessary for us – is this not a case of evolution in reverse?
 
Answer: The bones of the paired fins (pectoral and pelvic fins) of fishes are very primitive forms of what we see in higher vertebrates. The fins of fishes and the bones of their pelvic and pectoral girdles are as essential to fishes are our limbs, hands and feet to us – fishes use their fins to move (often in very complex ways), just as we use our arms, legs, hands and feet to move. Not all fishes use their fins for swimming – for example, most anglerfishes are very poor swimmers and use their pectoral and pelvic fins to crawl or amble over the sea floor.

Although fishes have many more bones associated with their paired fins than those found in higher vertebrates, the bones of the pelvis and shoulder girdle in higher vertebrates such as humans can be traced back through time to similar structures found in fishes. The transition from the fins of fishes to the limbs of land vertebrates has involved a reduction in the number bones and modification of remaining bones.
 
There are two groups of bony fishes – the familiar ray-finned fishes (Actinopterygii – almost all the bony fishes found today) and the lobe-finned fishes (Sarcopterygii). Although only 8 species of lobe-finned fishes survive today – two species of Coelacanth, and 6 species of lungfishes (including the Australian Lungfish, Neoceratodus forsteri  - a “living fossil”), back in the Devonian and Carboniferous periods however, lobe-finned fishes were much more common than ray-finned fishes.
 
The pectoral and pelvic fins of ray-fin fishes have several rod-like bones that articulate with (join to) the pectoral and pelvic girdles. Lobe-fins, however, have fleshy pelvic and pectoral fins with well-developed bones and muscles, and both fins are joined to the body via a single bone – which allows the fin to swivel. Most of the bones of lobe-fins match directly to the limb bones of land vertebrates. The humerus joins the pectoral fin to the shoulder (or pectoral girdle), and the femur joins the pelvic fin to the pelvis. These well-developed bones and muscles enabled lobe-fins to prop up on their fins in shallow water, and eventually allowed them to move from an aquatic environment onto the land.
 
One group of lobe-fins gave rise to four-legged animals or tetrapods (meaning ‘four feet’). Amphibians (frogs, toads, salamanders, etc.), reptiles (dinosaurs, lizards, snakes, crocodiles, turtles, etc), birds and mammals (including whales, dolphins, echidnas, dogs and humans) are all tetrapods. Fossils with structures which are transitional between swimming fishes and four-legged vertebrates have been found. Tiktaalik, found in 375 million year old rocks in Canada in 2006, not only had scales, gills and thin fin-ray bones like most fish, but also had features of a four-legged vertebrate, such as sturdy wrist, neck (which was mobile) and shoulder bones, thick ribs and strong internal fin bones which would have allowed the fish to support and prop itself up in shallow water – Tiktaalik may have been one of the first vertebrates to crawl or drag itself out of the water onto the land.

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