To begin with, a dinosaur mythologist is any author or illustrator who writes, paints, sculpts, etc. about dinosaurs, and in so doing, gives its readers and viewers a stronger impression and greater idea of their existence and grandeur.
Strictly speaking, the first Christian dinosaur mythologist is Arthur Conan Doyle, author of The Lost World, which has supplied the premise for many a subsequent adventure-story. Yet in another sense, he is not a Christian dinosaur mythologist, insofar as there is no Christian mythology in The Lost World whatsoever. It is pure science fiction: an adventure-story based on a then recent scientific discovery. There is no attempt to incorporate that discovery into the myths appropriated by his forebears; indeed, he seems to have been under no pressure to do so, and few writers since have made the effort. This is wise, inasmuch as it avoids confusing the reader; but it leaves a lacuna in the readers’ body of knowledge, which any reader familiar with Christian mythology from childhood (as many then were, and some still are) would desire to fill.
Into this lacuna, then, steps the work of J.R.R. Tolkien, and in particular his posthumous collection of incipient myths, published under the title of the Silmarillion, which gives a theogony and a heroic age to the fantasy-land invented by him in his better-known stories of The Hobbit and its sequel, the Lord of the Rings trilogy.
In ch. 1 of the Silmarillion, we learn that the Valar, the gods of the story’s world, once lived “in the midmost parts” of the world (equidistant from the Poles), under the light of two towering Lamps (one at each Pole), while Melkor, the Satanas of Tolkien’s mythos, lurked in his caves in the far north; where, under his influence, the lands around became polluted or inhospitable, while “beasts became monsters of horn and ivory and dyed the earth with blood”. How long this uneasy peace continued, the text does not say; millions of years at most, thousands at least. At any rate, it came to an end when Melkor shattered the towering Lamps and spilt ‘destroying flame’ where they fell, “so that the first designs of the Valar [for the shape of the world] were never after restored”. If, despite the author’s professed ‘dislike’ of allegory, this is meant to refine on the legend of the Fall of Lucifer (perfected two centuries prior by Milton), Tolkien is one of the first Christian mythologists to acknowledge the existence and extinction of the dinosaurs.
This theme may even be said, with a further stretch of the imagination, to continue in the story of Eönwe, the divine avenger, who descends like a Younger Dryas impact and changes the climate and landscape to something not unlike the modern shape of the world, actually identified (in The Fall of Gondolin) with the Anthropocene epoch, and captures Melkor, several million years later. Melkor himself is said in the Valaquenta (the theogony of the story’s world, and prologue of the Silmarillion) to have originally been a god of ‘Darkness and Cold’, and so may be assimilated, in his arrival on Earth, to the beginning of the Ice Age; but he is also a volcano spirit in the stories of Beren & Luthien, Children of Hurin, and The Fall of Gondolin, later in the text, and may be identified, therefore, with volcanic climate-change.
Tolkien, of course, was unaware of the Permo-Triassic boundary and its cause, the Permian Extinction or ‘Great Dying’, of which in his time nothing had yet been written in England. Yet given the author’s usual style, it is likely that had he been aware of that event, he would have portrayed it in the same way and assigned it the same cause. His Valaquenta, Silmarillion, Beren & Luthien, Children of Hurin, Fall of Gondolin, and Downfall of Numenor (Atlantis), collectively known as the History of Middle-Earth are all ‘catastrophist’, or in his own phrase, ‘eucatastrophist’ stories, of the history of a world through a sequence of disasters and revolutions. The zoohistory of the Mesozoic Era also suits his usual theme of past regimes of life (such as an empire, or an entire subspecies of hominin) rising to a pinnacle of grandeur and coming to a tragic fall, which is essentially an outline of the existence of the dinosaurs as now imagined by humanity. Thus it is not incredible that he should incorporate the Mesozoic and Ice Ages into his mythology, even if only in passing.