By Dan May
On Our Land
Dan May
Most US states officially recognize state birds (Connecticut’s is the American robin), flowers (mountain laurel) and even trees (white oak). However, only a few recognize dinosaurs.
In 2017 Connecticut formally designated Dilophosaurus as its state dinosaur. This animal is locally known from its tracks as seen at Dinosaur State Park in Rocky Hill, but figured prominently in the first Jurassic Park movie in 1993 based on Michael Crichton’s 1990 book.
In the movie, the dinosaur was depicted as about four feet tall with a fictionalized ability to spit venom after expanding a multicolored neck frill, which terrorized one of the villains before the dinosaur stunned and presumably ate him. As a well-studied fossil, though, Dilophosaurus was a bipedal carnivore over 20 feet long and 10 feet high at the hip. It was one of the first carnivorous dinosaurs and ranged across North America, with the most complete fossils excavated in Arizona.
Dilophosaurus walked the Earth about 190 million years ago in a geologic epoch known as the Early Jurassic. Note that this was about 120 million years before its rapacious costars in the movie – the velociraptors and tyrannosaurus rex – were reigning as predators in an epoch called the Late Cretaceous. Despite their link to the name Jurassic, dinosaurs were abundant in both the preceding time period called the Triassic and also in the subsequent Cretaceous.
Collectively the Triassic, Jurassic and Cretaceous time periods are known as the Mesozoic Era, and the film franchise might have been more aptly named Mesozoic Park.
The Mesozoic spanned a period from one global mass extinction event about 245 million years ago to the extinction event that caused the demise of the dinosaurs about 65 million years ago. The absolute ages of specific dinosaurs are based on the radiometric age of volcanic rocks that are interlayered with the sedimentary rocks that contain the fossils themselves. In Connecticut, Dilophosaurus tracks are found in Jurassic sandstone and mudstone of ancient river floodplains, across which basaltic lavas like those erupting today in Iceland or the Canary Islands intermittently erupted.
Dilophosaurus likely roamed across all of New England, but early Jurassic rocks are only preserved in a narrow north-south band in central Connecticut. Beginning about 200 million years ago, the state was subject to a plate tectonic event that nearly split the state in two from north to south. This ‘rifting’ event slowly displaced bedrock in a rift-valley downward more than a mile relative to the older metamorphic rocks like those underlying Orange and Milford that had formed during earlier mountain building.
This rifted zone is about 10 miles wide near New Haven and extends northward along what is now the I-91 corridor beyond the Massachusetts border, where it is about 20 miles wide. As it was developing in the Jurassic, a rift valley formed and filled in with sediment eroded from the bordering highlands as well as volcanic rocks that were erupted during rifting. Nearly all the cliff-forming features seen along Route 15 and I-91 from West Rock State Park northward are erosion-resistant basaltic lavas erupted into and onto river floodplains in the rift valley.
As you drive north on the Wilbur Cross Parkway to enter the tunnel going through West Rock in Hamden, you are passing thorough a 190 million year old basaltic intrusion called a diabase by geologists and trap rock locally. The red rocks that are exposed sporadically along the Parkway here are part of a formation called the New Haven Arkose. The sandstone, mudstone and coarser cemented gravels of this formation are best exposed in North Haven along the Route 40 bypass connecting the parkway to I-91. Preserved Jurassic sedimentary rocks in the rift valley further north contain Dilophosaurus tracks.
Rift valleys are good fossil locales as sediment accumulates rapidly and the often-accompanying volcanic eruptions provide good age controls. Perhaps the best-known rift valleys forming today are part of the East African Rift System. Instead of dinosaurs, the fossils here provide a partial record of hominid and human evolution over the past 10 million years. These are smaller animals than those of the Mesozoic, but equally voracious predators.
Dan May is a geologist and professor of environmental science at the University of New Haven. He can be contacted at dmay@newhaven.edu.