5.4 An Age of Hardship and Death¶
This great period of Mesozoic life, this second volume of the book of life, is indeed an amazing story of reptilian life proliferating and developing. But the most striking of all the story remains to be told. Right up to the latest Mesozoic Rocks we find all these reptilian orders we have enumerated still flourishing unchallenged. There is no hint of an enemy or competitor to them in the relics we find of their world. Then the record is broken. We do not know how long a time the break represents; many pages may be missing here, pages that may represent some great cataclysmal climatic change. When next we find abundant traces of the land plants and the land animals of the earth, this great multitude of reptile species had gone. For the most part they have left no descendants. They have been “wiped out”. The pterodactyls have gone absolutely, of the plesiosaurs and ichthyosaurs none is alive; the mosasaurs have gone; of the lizards a few remain, the monitors of the Dutch East Indies are the largest; all the multitude and diversity of the dinosaurs have vanished. Only the crocodiles and the turtles and tortoises carry on in any quantity into Cainozoic times. The place of all these types in the picture that the Cainozoic fossils presently unfold to us is taken by other animals not closely related to the Mesozoic reptiles and certainly not descended from any of their ruling types. A new kind of life is in possession of the world.
This apparently abrupt ending up of the reptiles is, beyond all question, the most striking revolution in the whole history of the earth before the coming of mankind. It is probably connected with the close of a vast period of equable warm conditions and the onset of a new austerer age, in which the winters were bitterer and the summers brief but hot. The Mesozoic life, animal and vegetable alike, was adapted to warm conditions and capable of little resistance to cold. The new life, on the other hand, was before all things capable of resisting great changes of temperature.
Whatever it was that led to the extinction of the Mesozoic reptile, 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 though 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 today 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.
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 today, 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.