Courtesy of Pratt Museum, Amherst College

FIRST IMPRESSIONS

For inquiries contact Linda L. Thomas , Curator, Pratt Museum of Natural History, Amherst College, Amherst, MA 01002

Early Mesozoic Paleontology in the Connecticut River Valley

The earth's geologic activity and the history of life, share an inseparable and interconnected path. The Connecticut Valley in western Massachusetts is a wonderful area filled with examples of both - the geological landscape here is as rich and diverse as its ancient fossil inhabitants. Movement of continental plates, and erosion by wind, water (including the Connecticut River today) and glaciers, have sculpted ever-changing landscapes and created habitable environments for a vast array of organisms for millions of years: from the Precambrian (~ 613 million years ago, or "mya") basement rocks of the Pelham Hills, to the 12,000 year old sand and clay beds of glacial Lake Hitchcock; from Early Devonian (~ 400 mya) conodont fossils in the Valley's northern reaches, to Recent plant remains in Lake Hitchcock beds.

One of the best understood time periods represented in the Connecticut Valley is the ~50+ million year span bridging the Late Triassic to Early Jurassic periods (~ 230-180 mya). It was a time of large-scale geologic activity, beginning with the break up of the super-continent Pangaea. In the middle of this geologically active period, during the several million years on either side of the Triassic-Jurassic extinction event (~ 202 mya), the earth's crust in this area continued stretching in an east-west direction. After several false starts, including here in the Connecticut Valley, a fault developed just east of Boston that eventually separated Massachusetts and Morocco, and initiated the formation of the Atlantic Ocean.

EARTH 200 MILLION YEARS AGO

What was to become the Connecticut Valley 225 million years later was in the middle of this gargantuan landmass at about 4° N latitude (vs 42° N now). The rifting also created a series of 13 large basins comprising the large basalt-sandstone rock sequence known as the Newark Supergroup that runs from Nova Scotia to South Carolina.

Two of these basins compose today's Connecticut Valley – the Deerfield basin in the northern Valley, and the Hartford basin in the remainder.

These Early Mesozoic basins were filled, over millions of years, with sediments captured from higher elevations and the pooling of basalt extruded through the thinning, stretched crust (the Holyoke and Talcott Ranges). Abundant fossils are found embedded in the exposed remains of these deposits.

The Early Mesozoic Connecticut Valley had a climate which was greatly influenced by two primary factors: the area's deeply inland position and its near-equatorial latitude (only 8° N by the Early Jurassic). Although the elevation at that time is unknown because no coastal deposits are present, today, the cyclical lakebed deposits show a tropical climate with long, dry seasons, interrupted by short, monsoonal rainy periods.

Courtesy of William Sillin

Millions of years of this seasonal climate supported numerous temporary lakes (called "playa" lakes). These lakes, in turn, provided a rich environment for tracks and traces to be made on their shores, and lake-dwelling organisms to be preserved in their bottom beds. In fact, the richest assemblage of trace fossils in the world can be found at sites in the Massachusetts Connecticut Valley. Hundreds of fish fossils have been recovered from the gray shales in Sunderland and Turner's Falls, Massachusetts. Ironically however, only a few body fossils of the land-dwelling trackmakers have been found (e.g., several prosauropod dinosaurs in Connecticut and southern Massachusetts).

Edward Hitchcock, professor at Amherst College from 1825-1864, was the person most responsible for developing the field of ichnology (the study of tracks and traces) through his exhaustive research of the Connecticut Valley fossil tracks.

Courtesy of Pratt Museum, Amherst College

In addition to his work at Amherst, he was also the first state geologist for Massachusetts, and the first President and a founding member of what was to become the American Association for the Advancement of Science. Collecting, studying and interpreting the fossil tracks became his primary scientific passion for the last 30 years of his life. Today, his collection of dinosaur, reptile, amphibian and invertebrate tracks and traces remains the largest fossil track collection in the world.

Courtesy of Pratt Museum, Amherst College

It is housed at the Pratt Museum of Natural History and is studied frequently by scientists from around the world as they work to decipher the behavior of these ancient animals and the ecological context of the Early Mesozoic. As evidence of ancient life, this collection forms an ecological "snapshot", a unique relic of ancient behavior - whether we are looking at the footprint pathways of 20 foot high dinosaurs or tiny insects, or the twisting trails of invertebrates as they moved across ancient lake shores and bottoms.

Courtesy of Pratt Museum, Amherst College

The Hitchcock Ichnology Collection includes "Noah's Raven" – the first evidence for dinosaurs that was found in North America. It is an unassuming slab of red sandstone with 3 1/2 footprints on it. When it was discovered in 1802 by Pliny Moody in South Hadley, MA, dinosaurs, and even evolution, were unknown. Forty years later, in 1842, Sir Richard Owen coined the word "dinosaur" to describe the unfamiliar and huge fossil bones that were being increasingly discovered in Europe. The New Englanders who uncovered these fossil tracks, while quarrying sandstone for building or paving material, understood the world best through the Bible. They thought the bird-like tracks were from the world at the time of the biblical flood. Hitchcock, too, thought that many of the tracks were birds (or even marsupials!). After over a century, during which Hitchcock was considered mistaken and out-dated, we now know that birds arose from the theropod group of dinosaurs. It is a mixture of irony, serendipity and astute observation that Hitchcock's interpretation of the dinosaur tracks as bird tracks has come virtually full circle!

Courtesy of William Sillin

Most of the Connecticut Valley trackmakers were small- to medium-sized, two-footed dinosaurs with three toes (e.g., Grallator), though some were probably 20 ft high and others walked at least some of the time on all four feet. Interestingly, many of them are identified as carnivores. (Did the plant-eating dinosaurs generally keep to the higher elevations where conditions for fossilization were harsher?) They left their footprints along the lake shorelines and lived among cycads, horsetails, ferns and conifers, and a scattering of smaller, scampering reptiles and lake-bound amphibians. Beetles and, by then, even flies flew around them while roaches lived in organic ditritus and waterbugs hunted in the lakes.

While fossil tracks uniquely tell us about behavior, they cannot tell us with any certainty (unless the trackmaker died in its track!) who exactly walked there. In other words, we can tell the difference on a large scale, between types of dinosaur or insect groups for example, but we cannot tell with certainty what species it was. As an example: some of the trackmakers are known to have been small, ornithischian dinosaurs, two-footed relatives of the later (and larger!) herbivores like Apatosaurus. While the print looks as if it might have been made by a dinosaur known as Hypsilophodon, no one knows with certainty if they truly are one and the same. Not only can many factors at the time the prints were made affect the appearance of a footprint, but its subsequent preservation and discovery will also affect what we eventually see.

Examples of these fossils and the story of the New England Early Mesozoic can be found at Amherst College's Pratt Museum of Natural History.

Courtesy of Pratt Museum, Amherst College

and Dinosaur State Park, 400 West Street, Rocky Hill, Connecticut