I have heard that Mark’s Gospel may actually not be the earliest written. True?

Among the four gospels in the New Testament, Mark is widely accepted as the earliest gospel account. It is dated to approximately 70 A.D, while Luke, Matthew and John were composed in the following 20 years.

There were other gospel accounts written that were not canonized, and these are lumped together under the category of “apocryphal” gospels. The majority of these were almost certainly written decades or even centuries after the four canonical gospels with the exception of the Gospel of Thomas, a list of sayings attributed to Jesus. Some scholars believe Thomas was already in circulation at the time that Mark, Matthew, Luke and John were written and that they used it as a source in their own accounts. Others believe it was contemporaneous with them.
The only other tantalizing hint we have of the existence of other gospels is a comment by the evangelist Luke who noted that “many have undertaken to set down an orderly account of the events that have been fulfilled among us” (Luke 1:1). If indeed there were other gospels written before the four famous gospels we commonly read now, they have been lost in the long stretch of history.