Ancient peoples developed a number of explanations for fossils that they found.  Some thought fossils were the remains of mythical giants and animals.  In Greece, in the 6th century B.C., Xenophanes claimed that fossils of marine animals found on land indicated the sea once covered the land. Aristotle thought that sea animals had been stranded after swimming through cracks in rocks to the sites where their fossils were found.  Although Aristotle was mistaken on this point, the great contribution of the early Greeks was that they attributed the phenomena observed in the natural world to natural processes which could be observed and studied.  

Hence it is clear that in the investigation of Nature, or Natural science, as in every other, there must first of all be certain defined rules by which the acceptability of the method of exposition may be tested

--Aristotle, Parts of Animals, Book I.I, p.53


Aristotle appreciated, as modern scientists do, that an understanding of the natural world often results from the study of seemingly insignificant organisms.

.it now remains to speak of animals and their Nature.  So far as in us lies, we will not leave out any one of them, be it never so mean; for though there are animals which have no attractiveness for the senses, yet for the eye of science, for the student who is naturally of a philosophic spirit and can discern the causes of things, Nature which fashioned them, and provides joys which cannot be measured.

--Aristotle, Parts of Animals, Book I.v, p. 99


     Aristotles extensive observations allowed him to appreciate the similarities between the anatomical structures of diverse organisms.  Comparative anatomy is essential to the classification of living and extinct animals today.

I have now described the arrangement of the parts which are on the visible surface; and, as I said, they mostly have their own proper names and are well known through their familiarity.  With the inner parts the reverse is true.  They are for the most part unknownat least, those of man are, and hence we have to refer to those of other animals, the natural structure of whose parts those of man resemble

Aristotle, Historia Animalium I, p 57


Thus, if our description proceeds by taking the attributes for every species, we shall be obliged to describe the same ones many times over, namely, those which although they occur in different species of animals are themselves identical and present no difference whatever.

--Aristotle, Parts of Animals, Book I.I, p. 55

Unfortunately, after the fall of Rome, the spirit of scientific investigation was all but extinguished in Western civilization, although many Islamic scholars continued in the tradition of the early Greeks.  More than 1,500 years passed before the investigative philosophy of the early Greeks spurred what we refer to as modern science.  The lack of a scientific tradition is evident in the predominant explanations for fossils from the Dark and Middle Ages.  Rather than studying fossils to learn about the natural world, fossils were viewed only in terms of their relevance to religious beliefs.  Some interpreted fossils of large animals as remains of giant humans.

But the large size of the primitive human is often proved to the incredulous by the exposure of sepulchres, either through the wear of time or the violence of torrents or some accident, and in which bones of incredible size have been found or have rolled out.  I myself, along with some others, saw on the shore of Utica a mans molar tooth of such size, that if it were cut down into teeth such as we have, a hundred, I fancy, could have been made out of it.  But that, I believe, belonged to some giant.  For though the bodies of ordinary men were larger then than ours, the giants surpassed all in stature.

--St. Augustine, City of God, Book XV. 9


Concerning the days before the Flood, the Glorious Historian has told us, There were giants on the earth, in those days, Could any undoubted ruins and remains of those giants be found under the earth, among the other subterraneous curiosities in our days, it would be an illustrious confirmation of the Mosaic History, and an admirable obturation on the mouth of Atheism!

Cotton Mather, from Semonin, p. 29


Johann Scheuchzer described a giant salamander fossil as Homo diluvii testis, Man, a witness of the Flood.  

A giant fossil salamander which occurs abundantly in the Upper Miocene of Switzerlandnamed Homo diluvii testis [1726].  The motto attached to the figure reads:
Oh sad remains of bone, frame of poor Man of Sin

Soften the heart and mind of recent sinful kin.

Hogben, 1930, p. 141


Some felt that fossils were the remains of dragons and other animals mentioned in the Bible.

In 1443, when foundations were being dug for St. Stephens Cathedral in Vienna, the thighbone of a mammoth turned up.  It was hung on the gates of the city, which henceforth bore the name “Giant’s Gate.â€