Which of the fontalle is the last to close after birth
Anterior fontanelle
Posterior fontanelle
Sphenoidal fontanelle
Mastoid fontanelle
Ref. The Short Textbook of Pediatrics. Pg 550
 

Anterior fontanelle closes  around 10-18 months
Posterior fontanelle closes around 8 weeks
Persistent posterior fontanelles is earliest sign of hypothyroidism