This is in no way intended to disparage Mayor’s admirable book, but after reviewing nineteenth century books on geology, this simply isn’t true, and I think I may know what really happened.
This is not to say that everyone believed it, of course. Many believed down to modern times that fossil giants were (a) the giants of the Bible, (b) medieval forgeries, or (c) the work of the Devil. I didn’t think anyone really believed fossils were the devil’s work, but I discovered that George Hakewill (c. 1578-1649), the English clergyman, wrote that “the wit and art of Man goes farre, but, being assisted by the Devill's help, it produces effects almost incredible,” suggesting an origin point for the ruder belief that the Devil made the fossils himself. As far back as Holinshed’s Chronicles (1577-1587), the author scoffed at the notion that giant’s bones were those of elephants (fossils being then unknown), indicating that such identifications were already common enough to need refutation. In 1633, a writer named Stow in the Survey of London reviewed such bones and declared they “might bee of an elephant.”
At any rate, the question is how we moved from a period at the end of the nineteenth century when scientists and scholars knew that fossil elephants and other mammals were mistaken for the skeletons of giants to the modern era when paleontologist Peter Dodson of the University of Pennsylvania could write will all honesty in the forward to The First Fossil Hunters that the entirety of the Classical evidence for fossils was news to him.
I think the answer lies in the decline of Classical education. Every educated Victorian (which was admittedly a small number, mostly of the elite) would have been well-versed in the Classics, at least the Latin writers—even in Victorian times Greek was dying out. The broad general education of Victorian times meant that even scientific specialists in paleontology would have a broad liberal arts background and would have been able to drawn on Classical texts and myths when investigating fossil finds at various Mediterranean locations. For example, every educated person would have the knowledge of Vergil’s Aeneid to immediately know that Sicily was the reputed home of Polyphemus the Cyclops. Similarly, the understanding of paleontology would allow them to read into Classical texts, like Pausanias’ description of the patella of Ajax (1.35.4-5), fossil finds.
But as the twentieth century wore on, the liberal arts declined, specialization increased, and scholars no longer had any particularly deep knowledge of areas outside their increasingly narrow fields. Few Classical scholars have any particularly strong knowledge of paleontology, and few paleontologists are familiar with the fine details of Classical literature. Probably more than a few have never read Homer, much less Pausanias. This isn’t to fault them—it is impossible given the sheer volume of human knowledge today to have detailed knowledge of more than a small slice of the whole. But it does point to the need for an interdisciplinary approach to knowledge to avoid having subjects that cross disciplinary boundaries fall into the cracks.