Fossils of the Exotic Terrane

Rocks of the Exotic Terrane region in general do not have preserved fossil communities because either they are metamorphosed sedimentary rocks or they are igneous rocks without fossil communities. Interesting exceptions occur in Paleozoic rocks, which include both fossils from the Ordovician onward, fossils from the accreted terranes, and fossils of organisms that lived in the Iapetus Ocean. Fossils after accretion of the terranes to the Northeast contain organisms that actually lived in the region and were not transported on the terrane.

Paleozoic

Fossils as old as the Cambrian are known from the eastern Massachusetts Boston Basin and are preserved in rocks of the exotic terrane Avalonia. These rocks are believed to have originally been part of northern Africa, sutured to the side of North America during the Acadian mountain-building event. The fossil assemblages are like those in northern Africa but different from equivalent-aged fossils in North America. These fossil assemblages were among the earliest evidence for continental plate movement and exotic terranes.

Figure 4.28: Maine state fossil, Pertica quadrifaria, Devonian. 

Graptolites have been found in Ordovician rocks in Maine, possibly existing within the Iapetus Ocean. Silurian and Devonian fossils (which also existed in the Paleozoic inland ocean) are found in limestones from Maine and New Hampshire, including corals, crinoids, brachiopods, trilobites, bryozoans, bivalves, and tentaculids. Devonian fossils in New Hampshire, though badly preserved in schist, include brachiopods. The Devonian plant fossil, Pertica quadrifaria, (Figure 4.28) is preserved in parts of northern Maine, an area of the Exotic Terrane that experienced only weak metamorphism.

Pennsylvanian basins found in Rhode Island and Massachusetts contain a rich record of plant fossils, and rarer amphibian tracks, insects, and arachnids. These fossils are similar to those preserved in Pennsylvanian-age rocks in the Inland Basin, though in less extensive deposits.

Triassic to Jurassic

Figure 4.29: Triassic-Jurassic rift basin in the Exotic Terrane.

The western Massachusetts Connecticut River Valley, part of a Triassic rift basin (Figure 4.29), is the site of Pliny Moody’s 1800 description of three-toed dinosaur footprints, which he thought were made by a raven from Noah’s Ark (Figure 4.30). This was also the area of Edward Hitchcock’s early to mid-1800’s interpretation of hundreds of dinosaur tracks as bird footprints. At one locality in Connecticut, the footprints are so numerous that a museum has been built over them. In some localities there have also been discoveries of dinosaur bones, fish (Figure 4.31), crocodiles, and other vertebrates. One well-known discovery by a teenage fossil collector was of an unusual gliding reptile. However, as in the Newark and Gettysburg rift basins in the Appalachian/Piedmont, vertebrate bone fossils are rare compared to the numerous footprints.

Figure 4.30: Three-toed dinosaur track from Holyoke, Massachusetts (13 cm long).

Figure 4.31: Fish, Semionotus, early Jurassic, Massachusetts (5.5 cm long).