Harry Potter and the Chamber of Secrets may well offer up an explanation for why Voldemort's Dark Mark symbol is of a snake protruding from a skull.
The Dark Mark, as you'll remember, is left by Voldemort and his followers as a signature after a kill, as well as being inked on their skin. Here it both represents their allegiance and is used by Voldemort to summon his followers when desired.
But why the skull with a snake protruding from its mouth? Voldemort is a parseltongue and uses his pet serpent Nagini as a Horcrux to protect part of himself, so his connection to snakes isn't in question. However, it seems that, as the heir of Slytherin, his trademark stamp is much more directly linked to Salazar Slytherin, with fans spotting a link in the Chamber of Secrets – namely when the chamber is opened.
"As much as I would love to picture Lord Voldemort sitting at a desk doodling different designs for his dark mark, I do think he had some influence on it when he was sixteen," Reddit user 'whiteliesnmotivation' jokes, noting the passage in CoS which describes Harry Potter seeing the basilisk coming from the statue of Salazar Slytherin’s mouth:
“Slytherin’s gigantic stone face was moving. Horrorstruck, Harry saw his mouth opening, wider and wider, to make a huge black hole. And something was stirring inside the statue’s mouth. Something was slithering up from its depths.”
“Something huge hit the stone floor of the Chamber. Harry felt it shudder – he knew what was happening, he could sense it, could almost see the giant serpent uncoiling itself from Slytherin’s mouth.”
"I think that when young Riddle opened the Chamber for the first time he used that image to make his dark mark which is a snake coming out of a skull," the Reddit user concludes.
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As many reply, it's a 'nice catch'. And spotting these connections gives us even more reason to keep re-reading. Over and over and over again.