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Regarding scales, this is the way I understand it. (Clarification may be

Bony scales are a primitive feature of vertebrates. Early fish scales were
preserved partly because of their bone content. Rhipidistians (primitive
choanates) had rhomboidal scales all around. 

This pattern changed with early tetrapods. Smaller scales dominated the
dorsal and lateral surfaces while larger V-shaped scales covered the belly,
probably to prevent chafing.

Tiny circular scales covered the flanks and dorsal surface of terrrestrial
pre-amniotes like _Gephyrostegus_ while V-shaped scales continued protecting
the ventral surfaces. These were all ossified and thus preserved.

The only scales that remained ossified on _Hylonomus_ a primitive amniote,
were the belly scales. The dorsal and lateral scales may have become
keratinized at this time (negative evidence).  Keratin does not (usually)
fossilize, but it does seal in moisture while bony scales do not. Keratin
allows amniotes to bask without drying out, expanding their range, etc. etc.

David Peters