Fossils of the Exotic Terrane

Overview

The Exotic Terrane region preserves some fossils from Paleozoic rocks from the Cambrian onward. The earliest fossils either come from accreted terranes or are fossils of organisms that lived in the Iapetus Ocean; thus, these fossils do not represent organisms that lived on the ancient North American landmass. Nevertheless, some younger Paleozoic (Carboniferous), Mesozoic, and Cenozoic fossils were deposited after accretion of the terranes to the Northeast and are remains of organisms that actually lived in the region and that were not transported from elsewhere.

Paleozoic fossils

Cambrian fossils

Boston and the surrounding area lies on a crustal fragment, which was probably at one time connected to what is now northwestern Africa. Fossils as old as the Cambrian are known from the eastern Massachusetts Boston Basin and are preserved in rocks of the exotic terrane called Avalonia. These rocks are believed to have originally been part of northern Africa; they were sutured to the side of North America during the Acadian mountain-building event. The Avalonia fragment was left attached to North America when the Atlantic Ocean began to form as Pangea began to come apart.

The fossil assemblages are like those in northern Africa but different from equivalent-aged fossils in North America. This “paradoxical” distribution gave the name to a group of Cambrian trilobites–Paradoxides–which today can be found in northern Africa and in the rocks around Boston, especially around the suburb of Braintree. The paradoxidid trilobites of the Braintree Slate are among the largest known trilobites. These fossil assemblages were among the earliest evidence for continental plate movement and exotic terranes.

Cambrian fossils from deepwater habitats are found in Maine. Like the fossils from the Boston Basin, these fossils did not originally form on the North American continent, but, rather, are from a fragment that was accreted later.

Ordovician to Devonian fossils

Graptolites have been found in Ordovician rocks in Maine; these possibly existed 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 trimerophyte (a type of early vascular plant) Pertica quadrifaria is preserved in parts of northern Maine, an area of the Exotic Terrane that experienced only weak metamorphism.

Carboniferous fossils

The Alleghanian Orogeny caused folding and uplift in the rocks of New England during the Carboniferous. In Rhode Island, the Narragansett Basin formed and eventually filled with sediments eroding off of surrounding highlands. Swamp forests grew in the basin, resulting in the formation of small coal deposits. These deposits include plant and animal fossils.

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, although the deposits in the Exotic Terrane are less extensive. Pennsylvanian rocks of the Rhode Island Formation contain fossil insects, including some of the largest known cockroaches.

Mesozoic fossils

The Connecticut River Valley, which runs from Long Island Sound, through the middle of Connecticut and Massachusetts, and between Vermont and New Hampshire, is part of a failed rift basin, associated with the earliest phase of the breakup of Pangea in the Late Triassic and Early Jurassic. Between around 255 and 245 million years ago, this rift valley was filled with lakes similar to the Great Lakes of the East African rift valley today. In these lakes swam a great diversity of fishes, and along their shores walked a great diversity of terrestrial reptiles, including dinosaurs. These animals left abundant fossils in the shales and sandstones that accumulated in and around the lakes, which are called the Newark Series or Newark Supergroup. Fossils of the Newark Supergroup rift lakes are also found in Virginia and North Carolina in the southeastern U.S.

Dinosaur trackways in this area are historically important. Pliny Moody (1790-1868) is the first person known to have found three-toed dinosaur footprints from the region around 1800. Edward Hitchcock (1793-1864), a geologist and professor at Amherst College in Massachusetts, later interpreted the abundant dinosaur tracks in the region as bird footprints.

In the New England area, several areas with trackways have been preserved for the public. Dinosaur Footprints in Holyoke, Massachusetts, is a park that conserves footprints discovered in the 1920s during highway construction. Hundreds of footprints discovered in the 1960s during another construction project are preserved under a dome at Dinosaur State Park in Connecticut. Eubrontes, one of the types of tracks found in the Connecticut Valley, is the state fossil of Connecticut.

While dinosaur footprints are common in the Connecticut River Valley, bones are rare. One of the few fossil dinosaur skeletons that has been found is that of Podokesaurus holyokensis, a housecat-sized Triassic dinosaur that walked on two legs. The skeleton of Podokesaurus was discovered by Mignon Talbot (1869-1950), a professor at Mount Holyoke College in Massachusetts, in 1910 while she was out walking with her sister. Talbot described Podokesaurus in 1911. Unfortunately, the fossil was destroyed in a fire in 1916, and no other fossils of Podokesaurus have been discovered. Podokesaurus was designated the state dinosaur of Massachusetts in 2021.

The Early Jurassic prosauropod dinosaur Anchisaurus polyzelus has also been found in the Newark Supergroup of Massachusetts and Connecticut. Anchisaurus may be the dinosaur that made Eubrontes tracks.

Cenozoic fossils

While woolly mammoths (Mammuthus primigenius) and other Pleistocene mammals roamed New England during interglacial intervals when the region was not covered by large sheets of ice, relatively few Pleistocene fossils are preserved. The scarcity of Pleistocene fossils in the region is probably a result of the destruction of fossils during periods of glaciation and the acidity of the soil, which dissolves bone over time.

Plant fossils have been recorded from lake sediments of Glacial Lake Hitchcock in eastern Vermont, as well as other places in the region.