During most of the Late Cretaceous (100.5 to 66 million years ago) the eastern half of North America formed Appalachia (named for the Appalachian Mountains), an island land mass separated from Laramidia to the west by the Western Interior Seaway, and from Hudsonia by the Hudson Seaway. This seaway had split North America into two massive landmasses due to a multitude of factors such as tectonism and sea-level fluctuations for nearly 40 million years. The seaway eventually expanded, divided across the Dakotas, and by the end of the Cretaceous, it retreated towards the Gulf of Mexico and the Hudson Bay.
This left the island masses joined in the continent of North America as the Rocky Mountains rose. From the Cenomanian to the end of the Campanian ages of the Late Cretaceous, Appalachia was separated from the rest of North America. As the Western Interior Seaway retreated in the Maastrichtian, Laramidia and Appalachia eventually connected. Until this connection, its fauna was isolated, and developed very differently from the tyrannosaur, ceratopsian, hadrosaurid, pachycephalosaur and ankylosaurid dominated fauna of the western part of North America, known as "Laramidia".
Due to high sea levels, subsequent erosion, and the lack of orogenic input of sediment into the Western Interior Seaway unlike the east coast of Laramidia, no terrestrially formed deposits have survived, with most dinosaur remains originating from seaborne carcasses that were transported into marine environments. This same lack of terrestrial sediments is also present on the western coast of Laramidia. Some sediments in the northern part of the range have been removed by glacial erosion of the Laurentide Ice Sheet during Quaternary glaciations, but it is difficult to ascertain how much sediment has been removed, or whether these sediments would have been any more productive than those that remain. Thus relatively little is known about Appalachia in comparison to Laramidia, with the exception of plant life, marine life and the insects trapped in amber from New Jersey.
In addition, due to a lack of interest in Appalachia, many fossils that have been found in Appalachia lie unstudied and remain in the inaccurate genera to which they were assigned in the days of E. D. Cope and O. C. Marsh. Only a few fossils of the terrestrial creatures that were found in this region have given us a brief glimpse into what life was like here during the Cretaceous period. However, the area has seen a bit of a resurgence of interest due to several discoveries made in the past few years. As mentioned earlier, not much is known about Appalachia, but some fossil sites, such as the Woodbine Formation, Navesink Formation, Ellisdale Fossil Site, Mooreville Chalk Formation, Demopolis Chalk Formation, Black Creek Group and the Niobrara Formation, together with ongoing research in the area, have given us a better look into this forgotten world of paleontology.
Geography
Appalachia stretched from Quebec and Newfoundland and Labrador all the way down to the Eastern United States and west to the Midwestern United States. Fossils found in these regions indicate that the area was covered in coastal plains and coastal lowlands during the Cretaceous period. Some scientists have proposed the idea that an archipelago of islands had formed during the time that the Western Interior Seaway had divided Laramidia and Appalachia until the end of the Cretaceous. This archipelago would have allowed dinosaurs to migrate into the Gulf Coast and possibly explain why there are some noticeable distinctions between the fauna in the two assemblages of Appalachia.
For instance, the southeastern assemblage (which consists of the Carolinas and the Gulf Coast sites) has some tyrannosauroids such as Appalachiosaurus, some hadrosauroids such as Eotrachodon and Lophorhothon, nodosaurs, dromaeosaurs, and new leptoceratopsian while the northern assemblage (which consists of New Jersey, Delaware, and Maryland) has some tyrannosauroids such as Dryptosaurus, hadrosauroids such as Hadrosaurus, smaller theropods, and a possible lambeosaur in the area.
Range
Vertebrate fossils have been found along the Atlantic Seaboard as well as other states like Alabama, Georgia, North Carolina, South Carolina, Mississippi, Missouri, Kentucky, Tennessee, Kansas, Nebraska, Iowa, and Minnesota. Parts of Canada that were a part of Appalachia during the Cretaceous include, Manitoba, Ontario, Quebec, Nunavut, New Brunswick, Newfoundland and Labrador, and Nova Scotia.
Fauna
Dinosaurs
From the Cenomanian to the Maastrichtian, Appalachia was inhabited by various groups of dinosaurs, including hadrosauroids, hadrosaurs, nodosaurs, leptoceratopsians, indeterminate ornithopods, tyrannosauroids, dromaeosaurs, ornithomimids, and indeterminate maniraptors. There is also plausible fossil evidence of chasmosaurs, lambeosaurs, sauropods, carcharodontosaurs, caenagnathids, troodontids, and coelurosaurs that might have inhabited the area. In 2022, fossils unearthed from the Woodbine Formation in Texas confirmed that carcharodontosaurs, troodontids, and coelurosaurs did indeed inhabit Appalachia.
Tyrannosaurs In Late Cretaceous North America, the dominant predators were the Tyrannosauroidea, huge predatory theropods built for ripping flesh from their prey. In Appalachia, dryptosaurs were the top predators in this clade. Rather than developing the huge heads and massive bodies of their western relatives such as Gorgosaurus, Albertosaurus and Lythronax, dryptosaurs had more in common with the basal tyrannosaurs like Dilong and Eotyrannus. Dryptosaurs were characterized by long arms with three fingers; while they were not as large as the largest tyrannosaurids, fossils from the Potomac Formation in New Jersey show that some of them did evolve some of the large-bodied features that can be found on other tyrannosaurs. Three genera of valid Appalachian tyrannosaurs are known, Dryptosaurus, Appalachiosaurus, and the recently discovered Teihivenator while other indeterminate fossils lie scattered throughout most of the southern United States like Georgia, North Carolina, and South Carolina. Fossil foot bones from Appalachian deposits indicate another, unnamed tyrannosauroid measuring , indicating that diversity in these basal tyrannosauroids remained high during the Late Cretaceous.
These fossilized teeth possibly belong to a species of Appalachiosaurus or an undescribed species of a new tyrannosaur. There is also the possibility of a fourth tyrannosaur known from Applachia known as Diplotomodon, but the genus is considered dubious. Fossils from New Jersey and Delaware, most notably in the Mt. Laurel Formation and Merchantville Formations, have revealed that the primitive tyrannosauroids were much more diverse than expected, and some of them grew to lengths of 8 to 9 meters long, making them around the same size as some of the more advanced tyrannosaurs found in Laramidia. Fossils in Cenomanian deposits further indicate tyrannosauroids had been a constant presence in Appalachia since the Middle Cretaceous.
Dromaeosaurs The dryptosaurs were not the only predatory dinosaurs in Appalachia. Indeterminate dromaeosaur fossils, possibly belonging to Velociraptorinae and Saurornitholestinae, and teeth, most closely matching those of Saurornitholestes, have also been unearthed in Appalachia as well; mostly in the southern states like Missouri, North Carolina, South Carolina, Alabama, Mississippi and Georgia. Finds from the Campanian Tar Heel Formation in North Carolina indicate that there may have been dromaeosaurids of considerable size; intermediate between genera such as Saurornitholestes and Dakotaraptor. Though known only from teeth, the discovery indicates large dromaeosaurids were part of Appalachia's fauna. Along with the dromaeosaurid remains, tyrannosauroid and possible ornithomimid remains have been unearthed in Missouri as well. It should also be noted that dromaeosaur remains are more commonly found in the southern region of Appalachia when compared to the northern region, with their sizes ranging from smaller forms to gigantic forms. Recent fossils unearthed in New Jersey show that maniraptorans were present in Appalachia as well.
Ornithomimids Various ornithomimid bones, such as Coelosaurus, have also been reported from Appalachia from Missouri, Mississippi, Alabama, Georgia and as far north in states like Maryland, New Jersey, and Delaware, but it is now believed that some of these are the bones of juvenile dryptosaurs while others belong to various undescribed species of ornithomimids. As of 2019, no distinct species of ornithomimosaurs have been identified yet, mostly due to the fact that no complete skeleton has been unearthed yet. However, it can be assumed that most of them were around the same size as their Laramidian relatives, though there is one specimen that could have reached a large size similar to Gallimimus or Beishanlong. In 2022, fossils from the Eutaw Formation from Mississippi revealed that large ornithomimids did indeed roam Appalachia.
Other theropods Fossils from the Woodbine Formation in Texas, one of the few fossil sites that is one of Appalachia's more well preserved fossils, reveal that other theropods might have roamed Appalachia around the time when the Western Interior Seaway first formed, they include possible specimens of allosauroids, troodontids, caenagnathids, dromaeosaurs, and tyrannosaurs. The most of these being the carcharodontosaurid Acrocanthosaurus.
Hadrosaurs Another common group, arguably the most widespread species in the area, of Appalachian dinosaurs were the hadrosaurs which were represented by three groups including Hadrosauromorpha, Hadrosauroidea, and the Hadrosauridae which is now considered to be their "ancestral homeland"; eventually making their way to Laramidia, Asia,Bolotsky, Y.L. & Kurzanov, S.K. 1991. [The hadrosaurs of the Amur Region.] In: [Geology of the Pacific Ocean Border]. Blagoveschensk: Amur KNII. 94-103. [In Russian] Europe, South America and Antarctica where they diversified into the lambeosaurine and saurolophine dinosaurs, though some of the primitive hadrosaurs were still present until the end of the Mesozoic. While the fossil record shows a staggering variety of hadrosaur forms in Laramidia, hadrosaur remains for Appalachia show less diversity due to the relative uncommon number of fossil beds.
However, a decent number of hadrosaurs are known from Appalachia with Protohadros, Claosaurus, and Lophorhothon representing Hadrosauromorpha, Hypsibema crassicauda and Hypsibema missouriensis representing Hadrosauroidea, and Hadrosaurus and Eotrachodon representing Hadrosauridae. These hadrosaurs from Appalachia seem to be closely related to the crestless hadrosaurs of Laramidia like Gryposaurus and Edmontosaurus, despite the fact that they are not considered to be saurolophines. Claosaurus is known from a specimen which floated into the Interior Seaway and was found in Kansas, might also be from Appalachia, since it was found closer to the Appalachia side of the seaway and is unknown from Western North America. Hadrosaur remains have even been found in Iowa, though in fragmentary remains, Tennessee, most notably from the Coon Creek Formation.