Plates of life

Early life had a tough time to sustain itself on earth. One of the detrimental conditions which constantly bombards the planet comes from space in the form of radiation. In the early earth history the surface of the planet had unprotected exposure to incoming radiation which did not encourage growth. For macro life, the conditions were inclement.

Eventually the world became covered with water, which insulates from and dilutes radiation.

It is believed that most  of life on earth started, more than once, in water and much later life forms transferred and evolved onto the land which still was bombarded by radiation. So being safe from harmful radiation was an primary environment from which life forms could sustain their presence until a more protective atmosphere and environment was established.

Being on land meant more exposure to radiation, so living organisms had to adapt

for that, resulting for the most part in protective, thicker, outer pigment and even more so for plates. The radiation was such a hazard that thick plates were developed to allow the organism to adapt.  many species have spent a long life lineage due to armor plate protection

one may note that the thicker plates on the animals are found on the upside of the 

body whence radiation approaches