THE LATE TRIASSIC WORLD OF THE NEWARK GROUP

   During this time period, life forms and environment (Fig. 5 and Table 2) can be considered similar to the well-known environments of the Connecticut Valley. The environment in Maryland was close to the popular image of "dinosaur times." The land was a low, forested alluvial plain dotted with lakes and volcanic fissures. The word "jungle" has been deliberately avoided, because it conjures images in people's minds of vines, palms and broad-leafed plants, none of which existed for most of dinosaur time. The kind of plants then abundant were mostly conifers, but included monkey puzzles, ferns, treeferns, cycads, cycadeoids, bennettitales, ginkgoes and horsetails (Fig. 5).

Living side by side with the dinosaurs (Coelophysis? prosauropods? Dilophosaurus?) were many strange creatures, hangers-on from earlier periods. There may have been some mammal-like reptiles and a strange reptilian "tank" known as Doswellia. Actual finds in Maryland are scarce, but they do include a phytosaur and a gavial-like archosaur, found south of Poolesville in Montgomery County (Fig. 5). Much is yet to be learned about Early Mesozoic life in Maryland.

Table 2. Some representative life forms of Late Triassic through Early Jurassic time.

THE EARLY CRETACEOUS WORLD OF THE POTOMAC GROUP

    Here one finds an interesting split. The Potomac Group beds begin with the Patuxent Formation in a time before the origin of angiosperms (flowering plants) and end with the Patapsco Formation when angiosperms were beginning to become well established.

    In the middle of the Potomac Group lies the Arundel Formation, to which nearly all of the dinosaurs of the Potomac Group can be attributed (Astrodon, "Dryptosaurus," Ornithomimus, Priconodon, Coelurus and Tenontosaurus). The Potomac Group, therefore, is far more important for its plant fossils than its limited dinosaur fauna (Fig. 7 and Table 3). This stems from the fact that almost all our food crops and most of the vegetation of today, which includes broad-leafed trees and grasses, are angiosperms.

Exactly what the vegetation was like during this time is difficult to say because it was undergoing these major changes. The land was a low-lying, delta-like region, not unlike southern Louisiana today, but the vegetation was quite different. There were still coniferous trees like Araucaryoxylon, a monkey puzzle, as well as ferns, treeferns, cycads, cycadeoides, bennettitales, ginkgoes, horsetails, and clubmosses (Figs. 10 and 11). In addition, the early ancestors of broadleaf trees, like magnolias, cottonwoods, and sycamores were growing in the area. Grasses, which did not become abundant until about 20 million years after dinosaur times, were, of course, not important. There were also some early water plants like water lilies appearing at this time. Prior to the Late Cretaceous, there was little or no evidence of floating vegetation in fresh water. Thus, many of the illustrations showing earlier Jurassic behemoths like Apatosaurus wading in swamps munching soft water-plants are erroneous.

Figure 10. Evolution of Cretaceous flora

Figure 11. Examples of Early Cretaceous flora found in the Arundel Clay. A, Sequoia cones (shown natural size); B, Cycad trunk (shown about 1/3-size).

THE LATE CRETACEOUS WORLD OF THE SEVERN AND RELATED FORMATIONS

By the Late Cretaceous, angiosperms (flowering plants) had become well established. Unfortunately, Maryland has no known terrestrial deposits from this time. Therefore, knowledge of the land vegetation is basically from washed-in fragments found in marine deposits. The same is true of the animal land life. Other than the scant dinosaur remains (Ornithomimus and a hadrosaur), little is known of the land animals. One can assume that the fauna was similar to that of other areas where the land life is better known, and that it probably contained a variety of dinosaurs, reptiles, amphibians, birds, some small mammals, and insects (Fig. 9 and Table 4). The land probably also was host to vestiges of the previously described floras, which were in decline while the angiosperms were ascendant. Cottonwoods, sycamores, willows, magnolias and other deciduous trees were becoming dominant, as were shrubs, vines, and herbaceous plants. True grasses were still not common.

In these Late Cretaceous sediments there is an abundance of marine fossils, including a large variety of invertebrates such as bivalves, snails, and ammonites (a squid-like creature with a shell) (Fig. 12). Ammonites are noteworthy not only because they were so abundant throughout dinosaur times, but also because they became extinct along with the dinosaurs. In contrast, the Late Triassic and Early Cretaceous sediments in Maryland are of continental origin and, therefore, do not contain any information about ocean life during those times.

In all, the fossil record of Maryland's dinosaur world spans a time period of about 155 million years, reaching almost from the time of the coal swamps of the Paleozoic to the jungles of the Cenozoic. Maryland has as diverse or more diverse a history during this time of great life changes as can be found in many larger states and countries.

Exactly what the vegetation was like during this time is difficult to say because it was undergoing these major changes. The land was a low-lying, delta-like region, not unlike southern Louisiana today, but the vegetation was quite different. There were still coniferous trees like Araucaryoxylon, a monkey puzzle, as well as ferns, treeferns, cycads, cycadeoides, bennettitales, ginkgoes, horsetails, and clubmosses (Figs. 10 and 11). In addition, the early ancestors of broadleaf trees, like magnolias, cottonwoods, and sycamores were growing in the area. Grasses, which did not become abundant until about 20 million years after dinosaur times, were, of course, not important. There were also some early water plants like water lilies appearing at this time. Prior to the Late Cretaceous, there was little or no evidence of floating vegetation in fresh water. Thus, many of the illustrations showing earlier Jurassic behemoths like Apatosaurus wading in swamps munching soft water-plants are erroneous

Table 3. Some representative life forms of Late Cretaceous time.

Figure 12. Marine life of the Severn Formation.