I thought that was a rock! Upon closer inspection it is an awesome little lizard, with brilliant camo capabilities!There's actually a cute little lizard at his feet but it may be hard to see on the pictures.
So I'm just looking at my newly finished models and their awesome skull helmets. Then the thought struck me, is there any lore reason at all why they are wearing them?I've read all of the Lizardmen's army books and all their lore, but I do not recall reading about a reason behind these skulls.
Do any of you know of a reason?

From the 8th Edition Lizardmen army book, page 55, Chakax's special rules:
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Even though he is a special character with special rules, the idea seems to be inline with what you are discussing. It can be assumed that it is applicable to regular temple guard as well.
Hmm... I thought that it were skulls of little stegadons back in fifth edition... Or were that the skink priests' masks?... Maybe "belongs to" means it was his helmet... Not his skull...
But the idea of the dead warriors still supporting their brothers after giving their lives for the great plan sounds nicely fitting for lizardmen...