Robert Van Voorst explains the history of the reference to Thallus in George Syncellus' Chronography.
Robert E. Van Voorst, Jesus outside the New Testament: An Introduction to the Ancient Evidence (Grand Rapids: Eerdmans, 2000) 20-23
Thallos: The Eclipse at Jesus' Death
The earliest possible reference to Jesus comes from the middle of the first century. Around 55 C.E., a historian named Thallos wrote in Greek a three-volume chronicle of the easter Mediterranean area from teh fall of Troy to about 50 C.E. Most of his book, like the vast majority of ancient literature, perished, but not before it was quoted by Sextus julius Africanus (ca. 160- ca. 240), a Christian writer, in his History of the World (ca. 220). This book likewise was lost, but one of its citations of Thallos was taken up by the Byzantine historian Georgius Syncellus in his Chronicle (ca. 800).