Fossils of the Coastal Plain
The Coastal Plain is underlain by a wedge of flat-lying Cretaceous and Cenozoic unconsolidated sediments. These sediments preserve striking evidence of how much sea life has changed from the Paleozoic to the Cenozoic.
Figure 4.20: Scleractinian coral, Tertiary (6 cm).
Fossils from the Coastal Plain are very different from the Paleozoic marine fossils found in the Inland Basin and Appalachian/Piedmont regions. The Coastal Plain sediments are especially rich in mollusks (bivalves and gastropods, and, in Cretaceous sediments, cephalopods). Shark teeth, scleractinian corals (Figure 4.20), barnacles, and sand dollars are also found in the Coastal Plain but are rare or absent in Paleozoic rocks. Brachiopods, so common in Paleozoic sedimentary rocks, are all but absent in the Coastal Plain.
Cretaceous
Figure 4.21: Oyster, Gryphaea, Cretaceous (5 cm).
Figure 4.23: Belemnite cephalopod, Cretaceous (7 cm long).
Good outcrops of marine and non-marine Cretaceous fossils are found in New Jersey and famous outcrops along the Chesapeake and Delaware canals in Delaware. Outcrops with clams are common, especially including oysters such as Gryphaea (Figure 4.21), in some cases creating small oyster "reefs." Snails, ammonoids, belemnites (Figure 4.23), and claws of the shrimp Calianassa are common.
Figure 4.22: Hadrosaurus, Cretaceous.
Rare remains of dinosaurs and other terrestrial vertebrate organisms have also been found in the Cretaceous sediments of the Coastal Plain. Parts of marine reptiles, such as mosasaurs, giant crocodiles, plesiosaurs, bony fishes, turtles, lizards, snakes, and even a wing and neck bone from a pterosaur have been found in sediments in the Chesapeake and Delaware canals. Perhaps the most celebrated discovery was that of a duck-billed dinosaur known as Hadrosaurus in Haddonfield, New Jersey (Figure 4.22). Discovered by John Hopkins in 1835, the bones did not receive much attention until pieces were sent to Joseph Leidy in Philadelphia, who determined that they were from a dinosaur that stood on its hind legs. This was America’s first dinosaur skeleton find.
Cephalopods
Cephalopods are swimming predators with tentacles and a beak-shaped mouth that move using a jet of water. The group includes belemnites, nautiloids, ammonoids, squid and octopi. A mass extinction between the Cretaceous and Tertiary eliminated many varieties of cephalopods. The shells of cephalopods range from long straight cones to spirals, but some have internal shells or no significant shell at all, like the octopus. Early to mid-Paleozoic rocks of the Inland Basin preserve cephalopods, mostly nautiloids, but they are generally neither abundant nor well preserved. The Nautilus is the only living member of this group. Belemnites, bullet-shaped fossils related to squids, were common in the Cretaceous seas. Though belemnites are commonly found in Cretaceous sediments of the Coastal Plain, they did not survive past the Cretaceous. Ammonoids, with somewhat more complex chambering than nautiloids, first appear in Devonian-age rocks. They were especially successful in the Mesozoic, and went extinct at the end of the Cretaceous with the dinosaurs.
Tertiary
The best-known fossil-rich sections of the Tertiary Coastal Plain deposits in the Northeast are at Calvert Cliffs in Maryland. Barnacles (Figures 4.24), crab claws, sand dollars and bryozoans are present in relatively small amounts, in addition to the mollusks that characterize deposits in the Coastal Plain region (Figures 4.25 and 4.26). Among the vertebrates found at Calvert Cliffs have been inner ear bones, teeth, and vertebrae of many species of whales and porpoises, shark teeth (Figures 4.27), ribs, jaws, crocodile teeth, turtle remains, stingray spines and teeth, and skeletal elements of bony fish. There is only one species of brachiopod at Calvert Cliffs, an organism that had dominated the Paleozoic sea.
Figure 4.24: Barnacles, Balanus, Tertiary (5 cm wide).
Barnacles
Barnacles are filter-feeding crustaceans, something like shrimps living upside down in a box. They are surrounded by calcare- ous plates and live attached to shells, such as the large Chesapecten shells found in Tertiary sediments of the Coastal Plain. It is sometimes possible to find them whole, often still attached to other shells.
Gastropods
Gastropods are known popularly as snails. Unlike bivalves, gastropods have only one shell. The soft parts of gastropods are anatomically similar to bivalves, but the foot is made to crawl along the surface and the shell is usually coiled. Gastropods are only sometimes found in early and mid-Paleozoic sedimentary rocks of the Inland Basin, but are extremely common in the Cenozoic rocks of the Coastal Plain, and in Late Pleistocene glacial lake deposits and marine deposits in the Northeast.
Figure 4.25: Gastropods. Left: Ecphora, Tertiary (6 cm long); Right: Turritella, Tertiary (3.5 cm long).
Bivalves
Bivalves are often called clams, but they also include scallops, mussels, cockles and oys- ters. Bivalves have a protective pair of hinged valves and feed off particles in the water by collecting them with their gills. Bivalves are known as filter feeders. During the Paleozoic, bivalves lived mostly on the surface of the ocean floor. In the Mesozoic, bivalves became extremely diverse and some evolved the ability to burrow into ocean floor sediments. Bivalves are the dominant fossils in the Cretaceous and Cenozoic Coastal Plain sediments. They are also common in late Pleistocene marine sediments, for example along the St. Lawrence Seaway and and submerged the coast of Maine and in smaller numbers in Paleozoic rocks of the Inland Basin and freshwater deposits of the Triassic rift basins.
Figure 4.26 : Bivalves of the Coastal Plain. Left: Chesapecten, Tertiary (9.5 cm wide); Right: Mercenaria, Tertiary (7 cm wide).
Shark Teeth
Shark teeth are one of the more common and prized fossils to come from Coastal Plain deposits. A good knowledge of how shark teeth vary both among sharks and in position within the mouth can often aid in identification. Among the shark teeth found in Tertiary Coastal Plain sediments in Mary- land are teeth of the requiem, tiger, mako, angel and great white sharks. The fossil great white shark, or Carcharocles megalodon, is most famous for its enormous size, having teeth over 15 cm long. Since shark skeletons are cartilaginous, except for teeth and vertebrae, there has been consider- able debate over the actual body size of Carcharocles. Even conservative estimates, though, suggest a body size well over 17 meters long, nearly twice the size of the living great white sharks.
Figure 4.27: Shark teeth from Tertiary sediments at Calvert Cliffs. Left: Tiger and sand shark teeth (~3 cm long); Right: Carcharodocles (7 cm long).
