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Updated: May 22, 2025
Even among the 'Reptilia' the class which exhibits the largest proportion of entirely extinct forms of any one type, that of the 'Crocodilia', has persisted from at least the commencement of the Mesozoic epoch up to the present time with so much constancy, that the amount of change which it exhibits may fairly, in relation to the time which has elapsed, be called insignificant.
Our existing 'Nautilus' has its representative species in every great formation, from the oldest to the newest; and 'Loligo', the squid of modern seas, appears in the lias, or at the bottom of the mesozoic series, in a form, at most, specifically different from its living congeners.
Although the Mesozoic era is shorter than the Paleozoic, as measured by the thickness of their strata, yet its duration must be reckoned in millions of years. Its predominant life features are the culmination and the beginning of the decline of reptiles, amphibians, cephalopod mollusks, and cycads, and the advent of marsupial mammals, birds, teleost fishes, and angiospermous plants.
The remains of these animals occur throughout the series of Mesozoic formations, from the Trias to the Chalk, and there are indications of their existence even in the later Palæozoic strata. Most of these reptiles, at present known, are of great size, some having attained a length of forty feet or perhaps more.
The Arctogaeal province is at present enormous, while the Australian is relatively small. Why should not these proportions have been different during the Mesozoic epoch?
A very similar lesson is to be learned from the fossils found lying exposed on the deep ocean bottom; for they are about as likely to be Palæozoic or Mesozoic as Tertiary.
There were rude, half-defined fishes in the Silurian, and probably the beginning of amphibians in the Devonian, and some small mammalian forms in the Mesozoic time, and doubtless rude studies of the genus Homo in Tertiary times. Nature works up her higher forms Jike a human inventor from rude beginnings. Her first models barely suggest her later achievements.
The two animals, however, belong to very different branches of the reptile world, and are by no means the most formidable of the Mesozoic reptiles. Many orders of the land reptiles sent a branch into the waters in an age which, we saw, was predominantly one of water-surface.
Along the Atlantic coast the Mesozoic peneplain may be traced shoreward to where it disappears from view beneath an unconformable cover of early Tertiary marine strata. The beginning of the Tertiary was therefore marked by a subsidence.
With the close of the Permian the world of animal and vegetable life had so changed that the line is drawn here which marks the end of the old order and the beginning of the new and separates the Paleozoic from the succeeding era, the Mesozoic, the Middle Age of geological history.
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