Phylogenetic trends

Each taxon of vertebrate animals seems to have its own unique skin features. As you might expect, there are some observable trends that coincide with the currently accepted arrangement of chordate phylogeny.

The layering of the skin progresses from a single layer of columnar cells in amphioxus to a layered arrangement of cells with special cornified (keratinized) structures in higher vertebrates.

The glands of the skin in amphioxus and lower fish are single celled glands and those of higher vertebrates are elaborate multicellular glands.

The dermis in lower chordates is extremely thin and dense. The dermis of mammals is composed of multiple layers and can be quite thick.

Bony armor and teeth are seen in extinct fish groups and modifications of these structures can be observed to follow divergent phylogenetic lines to form the teeth and dermal scales of modern vertebrate groups.