Marcellus is still not identified, and his work is lost. According to a footnote in a 2006 Cambridge edition of Proclus' Commentary on Plato's Timaeus,
Festugière [an expert on the works of Proclus] points out that the author is
unknown, and suggests confusion with Marcianus of Heraclea, who is
known to have written, shortly before Proclus' time, a Periplous of
the Outside Sea. But Marcellus may well have existed. The title given
by Proclus reveals only that it would have been mainly about
A 1960's work by Soviet Atlantologist Zhirov said:
Nothing is known about when Marcellus lived. Most historians believe
that he lived in the 1st century B.C., but A. Bescherelle and G.
Devars state in their Dictionary (69/163) that they believe
Marcellus wrote about Atlantis long before Plato.
And the cited Dictionary by Bescherelle indeed says, but without citing any higher authority, that "by the way, the Ethiopic historian Marcellus spoke about Atlantis before the Greek philosopher."