Mol Biol Evol. 1999 Mar;16(3):410-8 doi:10.1093/oxfordjournals.molbev.a026122

Boore JL, Daehler LL, Brown WM.

Complete sequence, gene arrangement, and genetic code of mitochondrial DNA of the cephalochordate Branchiostoma floridae (Amphioxus)

AGA encodes serine in amphioxus mitochondrial DNA. AGG is absent. AGA and AGG are reduced in frequencey compared to the AGY serine codons, which suggests a that the common ancestor of chordates had a precondition for AGR reassignment.