The Elements of Geology eBook

This eBook from the Gutenberg Project consists of approximately 328 pages of information about The Elements of Geology.

Cephalopods are now to have their day.  The archaic Orthoceras lingers on into the Triassic and becomes extinct, but a remarkable development is now at hand for the more highly organized descendants of this ancient line.  We have noticed that in the Devonian the sutures of some of the chambered shells become angled, evolving the Goniatite type.  The sutures now become lobed and corrugated in Ceratites.  The process was carried still farther, and the sutures were elaborately frilled in the great order of the Ammonites.  It was in the Jurassic that the Ammonites reached their height.  No fossils are more abundant or characteristic of their age.  Great banks of their shells formed beds of limestone in warm seas the world over.

The ammonite stem branched into a most luxuriant variety of forms.  The typical form was closely coiled like a nautilus.  In others the coil was more or less open, or even erected into a spiral.  Some were hook-shaped, and there were members of the order in which the shell was straight, and yet retained all the internal structures of its kind.  At the end of the Mesozoic the entire tribe of ammonites became extinct.

The Belemnite (Greek, belemnon, a dart) is a distinctly higher type of cephalopod which appeared in the Triassic, became numerous and varied in the Jurassic and Cretaceous, and died out early in the Tertiary.  Like the squids and cuttlefish, of which it was the prototype, it had an internal calcareous shell.  This consisted of a chambered and siphuncled cone, whose point was sheathed in a long solid guard somewhat like a dart.  The animal carried an ink sac, and no doubt used it as that of the modern cuttlefish is used,—­to darken the water and make easy an escape from foes.  Belemnites have sometimes been sketched with fossil sepia, or india ink, from their own ink sacs.  In the belemnites and their descendants, the squids and cuttlefish, the cephalopods made the radical change from external to the internal shell.  They abandoned the defensive system of warfare and boldly took up the offensive.  No doubt, like their descendants, the belemnites were exceedingly active and voracious creatures.

Fishes and amphibians.  In the Triassic and Jurassic, little progress was made among the fishes, and the ganoid was still the leading type.  In the Cretaceous the teleosts, or bony fishes, made their appearance, while ganoids declined toward their present subordinate place.

The amphibians culminated in the Triassic, some being formidable creatures as large as alligators.  They were still of the primitive Paleozoic types.  Their pygmy descendants of more modern types are not found until later, salamanders appearing first in the Cretaceous, and frogs at the beginning of the Cenozoic.

No remains of amphibians have been discovered in the Jurassic.  Do you infer from this that there were none in existence at that time?

REPTILES OF THE MESOZOIC

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The Elements of Geology from Project Gutenberg. Public domain.
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