[wet]

Originally Posted by naiad

The baleen plates of whales are modified hairs, made from keratin. They are not teeth. You are probably right that they started off as sensory bristles, and eventually became food filters.

The teeth and scales of sharks have an identical structure (denticles). Both are replaced constantly, as they do not grow as the fish grows and cannot repair themselves if damaged. Lampreys and hagfish have only teeth and no scales, so the teeth must have migrated from the mouth to the rest of the body when they evolved into sharks and rays.

Bony fish scales are very different. They are not like teeth, and are not replaced unless they are lost in an accident. They grow as the fish grows, and normally last for its lifetime. The teeth still have to be replaced, as they are similar to those of sharks and rays. The bony fishes must have lost the tooth-like scales on the body at some point, and re-evolved scales of a very different type.

Seems that bony fishes ancestors had mutations which selected for calcium/phosphorus accumulation in the internal cartilage (bones) rather than calcium/phosphorus in the external scales, leaving the scales more flexible but the bones firmer. (I had thought this was due to living in freshwater, but now I think it was more likely a chance mutation.)

They started off as plates in the skin, which fitted together edge to edge and did not overlap, like floor tiles. Gars, sturgeon and other primitive fish have these scales (ganoid).

These sound like the difference between thin incisor teeth and wide molars, the base of the scales widened becoming plate-like. Perhaps a freshwater adaptation.

Almost all the advanced bony fish species have scales which overlap, like roof tiles (cycloid and ctenoid).

I think hydrodynamics is strongly influential with these types.

A few have naked skin, or plates made of bone.

When the first fishes left the water, they lost their scales, which were probably heavy and easily damaged on land. Some primitive fish species (Australian Lungfish, Arowana) have these large heavy scales, so it is easy to understand why they lost them. This left them with soft moist skin, like modern amphibians. This is light and flexible, but has the disadvantage of losing water by evaporation and being delicate. At some point they improved on this by re-evolving scales, becoming the reptiles. These scales are different once again, being made of keratin (not living tissue, except at the root). Feathers are thought to have evolved from reptile scales. The beak is also made of keratin, like feathers. Hair is also thought to have evolved from reptile scales.

I'm wondering where the reptiles got their keratin...I think there is a frog which during the larval stage has a keratinous snout and/or keratinous pseudo-teeth, this too could have spread from the mouth out around the body as scales.

Tadpoles with keratinous beaks, at tadpole photos:
Tetrapod Zoology : Frogs and toads: sheer, untold awesomeness

Ancient amphibian imprints: 3 naked salamanders
Experts discover rare amphibian imprints - Yahoo! News

Feathers, hair, beaks, horn, claws and fingernails are keratin protein.
Both meat and keratin are protein, but keratin has no nutritional value AFAIK.

Platypus males have poison ankle spurs, are there any fish with poison spines or glands on the rear non-caudal fins? (I forgot the proper name for the rear lateral fins) Catfish have poison spines on the front fins and dorsal fin but not on the rear fins AFAIK.