When a continent splits apart to form a new ocean basin, the rifting process is not a tidy affair. The splitting begins with a large number of normal faults that open rift valleys. Some of these may eventually join up to form a single large basin that eventually becomes an ocean, but many of the basins are left out and become rifting backwaters. Because they’re low areas, the basins often fill with water to become long, narrow lakes that can persist for hundreds of thousands of years before eventually filling up with sediment.
This is what was happening in eastern North America during the Late Triassic and Early Jurassic, as Pangea broke apart and the Atlantic Ocean began to form. As the main Atlantic rift opened, a series of rift basins were left behind spread from Nova Scotia to Georgia, including 6 major basins in Virginia. The deposits associated with these rifts are collectively referred to as the Newark Supergroup. Lake sediments are an excellent setting for fossil preservation, and all of these basins include numerous fossils, as I’ve noted in various posts about the Solite Quarry (which is part of the Danville Basin) and Dinosaur State Park (in the Hartford Basin). As might be expected in a lake deposit, some of the most common fossils are fish.
Ptycholepis is one of the small fresh water fish found in the Newark Supergroup, as well as in several European countries. Ptycholepis is easily identified in these deposits because of its unusual scales, which are much longer than they are deep and have a notched posterior margin (Schaeffer et al.1975):
One feature of the Atlantic rifting is that it didn’t occur everywhere simultaneously. It seems that the Atlantic “unzipped”, starting in the south first. That means that the deposits in the southern basins are generally older than the ones in the north. Given the age gradient, it might be expected that the fish faunas would change from south to north, and that is in fact what Olsen et al. (1982) reported. In the Newark Supergroup Ptycholepis was first reported from Connecticut, and most of the North American specimens have come from Connecticut and New Jersey. We have so far not found Ptycholepis in the older deposits at the Solite Quarry, even though we’ve been excavating there for 20 years and have recovered large numbers of other fish. In fact, the only reported specimens of Ptycholepis from Virginia are from Prince William County (top) and Fauquier County (below) in the northern part of the state. The Culpeper Basin, northernmost of Virginia’s rift basins, is exposed in both these counties and represents the youngest Newark Supergroup sediments in the state.