Whatever it was that led to the extinction of the Mesozoic reptiles, it was probably some very far-reaching change indeed, for the life of the seas did at the same time undergo a similar catastrophic alteration. The crescendo and ending of the Reptiles on land was paralleled by the crescendo and ending of the Ammonites, a division of creatures like squids with coiled shells which swarmed in those ancient seas. All through the rocky record of this Mesozoic period there is a vast multitude and variety of these coiled shells; there are hundreds of species, and towards the end of the Mesozoic period they increased in diversity and produced exaggerated types. When the record resumes, these too have gone. So far as the reptiles are concerned, people may perhaps be inclined to argue that they were exterminated because the Mammals that replaced them competed with them, and were more fitted to survive; but nothing of the sort can be true of the Ammonites, because to this day their place has not been taken. Simply they are gone. Unknown conditions made it possible for them to live in the Mesozoic seas, and then some unknown change made life impossible for them. No genus of Ammonite survives to-day of all that vast variety, but there still exists one isolated genus very closely related to the Ammonites, the Pearly Nautilus. It is found, it is to be noted, in the warm waters of the Indian and Pacific oceans.[15] And as for the Mammals competing with and ousting the less fit reptiles, a struggle of which people talk at times, there is not a scrap of evidence of any such direct competition. To judge by the Record of the Rocks as we know it to-day, there is much more reason for believing that first the reptiles in some inexplicable way perished, and then that later on, after a very hard time for all life upon the earth, the mammals, as conditions became more genial again, developed and spread to fill the vacant world. § 5 Were there mammals in the Mesozoic period?