The skull was the framework of the head in most animals, including humanoids.
On Earth, and later human colonies, a bare skull was a commonly understood symbol for death. Death took this form as an emoji. (TV: Smile [+]Frank Cottrell-Boyce, Doctor Who series 10 (BBC One, 2017).)
Female Rills found a large skull sexually attractive. (PROSE: Galaxy Four)
The skulls of the headless monks were kept in the Seventh Transept. They were carnivorous and ate anything that came in, from rats to intruders. (TV: The Wedding of River Song [+]Steven Moffat, Doctor Who series 6 (BBC One, 2011).)
While possessed by the Mara, Tegan showed a fortune teller the image of a snake skull in her crystal ball and frightened her. (TV: Snakedance)
Faction Paradox wore the skulls of dead Time Lords from another universe. (PROSE: Alien Bodies)
Victorian showmen used to draw the crowds by taking the skull of a cat, gluing it to a fish and calling it a mermaid. The Ninth Doctor compared the Slitheen's genetically modified pig with this trick while explaining the pig's true nature to Toshiko Sato. (TV: Aliens of London [+]Russell T Davies, Doctor Who series 1 (BBC One, 2005).)
The Marshal of Atrios communicated with the Shadow through a skull-shaped device located behind a mirror. (TV: The Armageddon Factor)
Broke of the Cult of the Dark Flame forced Victor Farrison to search for the skull of Vilus Krull's original body so its energies could be used to summon the Dark Flame. (AUDIO: The Dark Flame)
Inside his confession dial, the Twelfth Doctor was killed and reborn many times by the Veil, but every time, he discovered a skull that had once been his own. (TV: Heaven Sent [+]Steven Moffat, Doctor Who series 9 (BBC One, 2015).)
The Toymaker stated that the year 5,000,000,000 was the year when the very last human killed his last enemy, after which they picked up their enemy's skull and remarked, "That is the final ball of all." (TV: The Giggle [+]Russell T Davies, Doctor Who 2023 specials (BBC One, 2023).)