Michael D. Coe, a Yale anthropologist who devoted his career to proving that the ancient Maya incubated an elaborate written language that had previously been undervalued by many scholars, died on Sept. 25 in a hospital in New Haven. He was 90 .

The cause was a stroke, his son Andrew said.

Dr. Coe was instrumental in deciphering the Maya script and in translating and validating the authenticity of what became known as the Grolier Codex — a document found in a Mexican cave that was believed to have been written around the 13th century on fig bark. It is now considered the earliest existing manuscript in the Americas.

First exhibited at the Grolier Club in Manhattan in 1971, it is one of only four written Maya works known to have survived marauding Spanish conquistadors and purges by Roman Catholic priests.

Dr. Coe defied contemporary critics who believed that the Maya hieroglyphics had been randomly inscribed and did not represent a recorded language. Instead, he embraced a discovery by the Soviet scholar Yuri Knorozov, who deciphered phonetic syllables in the Maya writing system, which allowed the texts to be read and spoken in their original language as well as translated .