The abundance of plant life in the Colorado area during the Cretaceous period is indicated by the extent of coal deposits of this age. About one-fourth the area of the state is underlain by coal seams varying in thickness from a few inches to fifty feet or more, most of it being Cretaceous. In the northern Colorado district the coal-bearing formation is the Laramie. Near Denver there is some coal in the Arapahoe formation which overlies the Laramie and is of later age.
Coal mines often produce excellent plant fossils, and occasionally other evidence of prehistoric life. In a mine near Canon City, Colorado, a series of natural casts of dinosaur feet was taken from the overlying rock after the coal had been removed. One of these, in the Denver Museum of Natural History, is seen to consist of sandstone inside a very thin layer of dark clay. Flattened against the lower surface is the carbonized stem of a Cretaceous plant which grew in the swamp where the coal deposit was formed.
Since the shape of dinosaur feet is unmistakable we can only assume that a large reptile of this type walked over the surface of swampy ground in which a great thickness of decaying vegetation had accumulated. A layer of mud settled over the top and became sufficiently firm to retain the mold of the feet as the animal moved along. Any plant material either on the mud or included in it was pushed to the bottom of the impressions and flattened out by the weight of the huge creature. Then sand was washed into the footprints from some nearby source during a heavy rainstorm.
Following these events there was probably a subsidence of the area, and a great thickness of rock-making sediments was built over the ancient swamp. The buried vegetation gradually became converted into coal, the sand consolidated into a firm sandstone, and the mud produced the shales forming the roof of the present mine, which is now at an elevation of a mile above sea level as a consequence of the general uplifting of the Rocky Mountain region during late Cretaceous and subsequent time.
When the coal was removed, the hard sandstone casts separated readily from the softer shales surrounding them. A small amount of the shale adheres to the sandstone, and some of the flattened vegetation, now in the condition of coal, still remains attached.
MESOZOIC INVERTEBRATES
As in other eras, the invertebrates fluctuate with the periods. Characteristic forms appear, become more or less prominent, then in many cases decline or disappear. Variations among the mollusks are particularly helpful in the identification of rocks which originated in the Cretaceous seas. Clam-like bivalves of the genus Inoceramus, the straight-shelled ammonids known as Baculites, and oysters, are locally common in some of the formations exposed a few miles west of Denver.
The ammonids, or “ammonites,” were extremely abundant throughout the world during Mesozoic time. Their shells were chambered like those of the pearly nautilus, a related cephalopod mollusk inhabiting tropical seas at the present time. While only four species of the Nautilus tribe are living today, thousands of species of ammonids swarmed the prehistoric seas. Many new forms came into existence in Triassic time but few survived the period. A pronounced revival occurred in the Jurassic, only to be followed by a decline and eventual extinction at the close of the Cretaceous. Ammonites measuring three or four inches across the diameter of the coiled shell were about average size, but diameters up to three or four feet were not uncommon. Externally the shells were ornamented with ribs, knobs, and spines; inside was a pearly lining. The partitions were thin and composed of the same pearl-like substance as the lining. Each partition becomes wavy as it approaches the shell, and the line of union has a distinct pattern which is seen in specimens which have lost the outer shell layer. This wavy suture line becomes more complicated in the later members of the race, and the peculiar markings developed by the repeated partitions provide a convenient method of identification.
The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes now living, comprise another group of carnivorous mollusks. These, however, had lost the external shell, and the usual fossil is part of an internal shell or “skeleton,” known as the guard. This limy structure has the form of a pointed cigar, and is seldom over a foot long although the total body-length of the larger animals was commonly about six to eight feet. Several hundred species have been described, the majority being of Jurassic age. They declined rapidly toward the close of the era.
The invertebrate life of the Mesozoic was strongly dominated by mollusks, with cephalopods in first place, the bivalve pelecypods and the single-valve gastropods or snail-like forms sharing subordinate positions. The dominating trilobites, sea-scorpions, and tetracorals of the Paleozoic had disappeared, while the brachiopods and crinoids were greatly modified and more like the forms which live today.