Geol 006 Dinosaurs and their Mesozoic World

J Bret Bennington

The Triassic-Jurassic Transition

The Triassic to Jurassic Transition in Eastern North America

The breakup of Pangaea began in the Late Triassic with the rifting of North America from North Africa. Numerous rift valleys developed along the suture between the two continents. As the continents separated, the continental crust was stretched and thinned. Large blocks of the crust rotated and sank along faults to create elongate rift valleys boardered by highlands. These rift valleys hosted river systems, floodplains, and large, perennial lakes. Magma from the Earth's mantle periodically intruded its way into the sedimentary layers accumulating in the basins, sometimes solidifying within the layers to form sills and other times erupting out onto the surface to form extensive lava flows.

Basins in Pennsylvania, Maryland, Virginia and the Carolinas contain mostly Late Triassic Age Rocks, whereas basins in New Jersey, Connecticut, Massachussetts, and Nova Scotia contain both Late Triassic and Early Jurassic age strata.

Newark Supergroup

The sequence of rock layers deposited in the developing rift basins is named the Newark Supergroup by geologists, in deference to the Newark Basin in central New Jersey that preserves the thickest sequence of Triassic and Jurassic rock layers on the east coast (as much as 5 km). Within each basin the Newark Supergroup is divided into a series of formations that are defined locally. Basalt lava flows found in the sequence are also given names because they are distinctive and correlatable within a basin.

Newark Basin

This is the largest east coast basin, stretching for 225 miles from southeastern New York, through New Jersey and into Pennsylvania. The formations of the Newark Basin are:

Boonton Fm

Hook Mountain basalt

Towaco Fm Jurassic

Preakness basalt

Feltville Fm

Orange Mountain basalt

Triassic - Jurassic Boundary

Passaic Fm

Lockatong Fm Triassic

Stockton Fm

The strata of the Newark Basin are tilted to the west, a result of downward movement of the western edge of the basin along the Ramapo Fault. Because of the tilt, the edges of the layers are exposed along the surface, with younger strata to the west and older strata to the east. The igneous layers are erosion resistant and form prominent ridges on the land surface. The three lava flows form the Watchung Mountains while an igneous sill that is exposed at the eastern edge of the basin forms the Palisades Cliffs along the western shore of the Hudson River.

The Hartford (Connecticut Valley) Basin

This is a north-south trending basin lying along the course of the Connecticut River in the center of the state, extending northward into Massachussetts. This basin has subsided along its eastern border fault to produce tilted strata as are present in the Newark Basin.

Portland Fm

Hampden Basalt

East Berlin Fm

Holyoke Basalt Jurassic

Shuttle Meadow Fm

Talcott Basalt

Triassic - Jurassic Boundary

New Haven Arkose

Sedimentary layers within the formations of the Newark Supergroup tend to show cycles of deposition. These cycles have been related to changes in climate occurring over thousands of years that caused the expansion and contraction of large rift lakes. During wetter parts of the climate cycle the lakes grew extensive and deep, depositing thinly laminated black shales in the center of the basins. As the lakes shallowed during drier parts of the climate cycle they accumulated beige silt and sand layers. Frequent drying episodes are shown by the abundant mudcracks seen in many of the sediment layers. During times when the lakes dried up completely red sandstones, conglomerates, and arkoses were deposited by rivers and flash floods moving sediments from the uplands into the basin.

Late Triassic Dinosaurs of the East Coast

Late Triassic dinosaurs are known only from a few teeth and the abundant footprints and trackways they left behind. Much more common in Late Triassic sediments are the bones and teeth of aquatic animals such as phytosaurs and metoposaurs (labyrinthodont amphibians) that lived in the rift lakes.

Ornithischians

• Tiny teeth of small ornithischian dinosaurs have been found in North Carolina and Pennsylvania.
• Small, three toed footprints and smaller, four toed handprints are common in Late Triassic rocks. Called Atreipus and Gregaripus, these footprints were probably made by small fabrosaur-like dinosaurs. Gregaripus trackways are usually found in groups with larger individuals positioned to the outside of smaller individuals.

Theropods

• No bones or teeth are known from Theropod dinosaurs on the East Coast.
• Trackways called Grallator and Anchisauripus of a small theropod dinosaur. Larger footprints called Kayentapus are also known from Virginia.

Prosauropods

• Badly preserved bones of prosauropods are known from Nova Scotia.
• Ducklike, hind footprints called Agrestipus are known from Virginia.

Early Jurassic Dinosaurs of the East Coast

• Early Jurassic dinosaur bones are known from Nova Scotia and Connecticut. To date the only animals that have been found are two types of prosauropod - a small species called Anchisaurus (about 8 feet long) and a larger species called Ammosaurus (about 13 feet long) - and the incomplete skeleton of a theropod carnivore called Podokesaurus, which is probably a ceratosaur similar to Coelophysis, but it is difficult to say for certain as the skeleton was incomplete and was destroyed in a fire many years ago.
• It is known that a relatively large theropod was running around on the East Coast in the Early Jurassic because of the abundance of large, clawed, three-toed trackways called Eubrontes (the name given to the footprint itself, not the dinosaur that made the footprint). Eubrontes trackways are preserved in abundance at Dinosaur State Park in Rocky Hill, Connecticut. One possible candidate for the maker of Eubrontes is a large carnosaur called Dilophosaurus found in the Lower Jurassic Kayenta Formation in Arizona.
• A smaller, clawed, three-toed footprint called Grallator is common in both Late Triassic and Early Jurassic strata in North America, and is attributed to smaller ceratosaurs like Coelophysis.
• Anomoepus - footprints similar to Atreipus and Gregaripus - are from a small ornithischian dinosaur.

Dinosaur faunas of the Early Jurassic of the East Coast appear to be similar to those of the Late Triassic, perhaps with a greater abundance of large predators. The main groups represented are:

Ceratosaurian Theropods

Small ornithischians

Prosauropods