- Explain why most famous museum dinosaurs come from western North America even though the East Coast has an important dinosaur record.
- Describe how Pangaea, continental rifting, and the Western Interior Seaway affected dinosaur movement and isolation.
- Distinguish body fossils from trace fossils and explain what East Coast footprints reveal.
- Explain how a mega-bias or unconformity produced a major gap in the eastern fossil record.
- Describe how Appalachia preserved relict dinosaur lineages as a refugium.
A Hidden Dinosaur History in the East
Famous western fossils can overshadow the East Coast's important record of dinosaur discoveries.
The American Museum of Natural History in New York displays many iconic dinosaurs, but most of those specimens came from the American West. Their distant origins do not mean that eastern North America lacks dinosaur fossils.
Important Evidence That Was Overshadowed
North America's earliest dinosaur discoveries included fossils from New Jersey.
Some of the first dinosaurs discovered in North America came from New Jersey. One wasHadrosaurus foulkii, described in 1858. Eastern rocks may also hold the continent's geologically oldest dinosaur evidence, although discoveries in the West have received far more attention. This leads to a central puzzle: where is the rest of the eastern record, and why are dinosaurs such asTriceratopsnot found in Central Park?
How Evolution and Geology Shaped the East
Changing continents and uneven fossil preservation created an eastern dinosaur world with its own history.
Evolutionary change and geological events worked together to produce a distinctive, isolated dinosaur world along the East Coast.
Pangaea and the Triassic Footprint Record
Dinosaurs appeared during the Middle Triassic, perhaps by 245 million years ago. North America then belonged to the connected supercontinentPangaea. Some of the earliest undisputed dinosaur evidence is from present-day South America, and the joined continents allowed early dinosaurs to walk between landmasses. At the time, North America lay just north of the equator, while the Appalachian Mountains dominated its eastern side.
Early dinosaur bones are uncommon in this region. However, the Late Triassic to Early Jurassic rocks of theNewark Supergroupcontain many fossil footprints. These footprints aretrace fossils—evidence of an organism's activity rather than its body.
- Varied tracks show that several kinds of dinosaurs already inhabited the East Coast during the early Mesozoic Era.
- Most tracks came from small, three-toed carnivores as long as 3 meters, probably resembling western dinosaurs such asCoelophysis. Larger prints indicate carnivores about 3 to 4 meters long, perhaps animals likeDilophosaurus.
- Tracks combined with limited fossil remains point to early relatives of giant sauropods, including the still comparatively small, 6-meter-longAnchisaurus.
- Other footprints document small ornithischians, members of the broader lineage that later included large plant-eaters such asTriceratopsandAnkylosaurus.
What can footprints reveal that body fossils may not, and what can footprints leave uncertain?
Footprints demonstrate that dinosaurs were present and provide clues about features such as size and foot shape. Unlike bones, they often cannot identify exactly what the trackmaker looked like or how it was related to other dinosaurs.
Rifting During the Jurassic
Near the Triassic–Jurassic transition about 200 million years ago, North America began pulling away from Africa and Europe. The Atlantic Ocean formed during this separation, isolating dinosaurs in the East. Early Jurassic evidence there remains dominated by footprints, which confirm that dinosaurs persisted but had not yet reached very large sizes.
The Great Gap in the Record
A regional preservation failure erased much of the East Coast's dinosaur history.
From the Middle Jurassic through the Early Cretaceous, the eastern record loses roughly 50 to 70 million years of dinosaur fossils. Meanwhile, the western Morrison Formation preserves celebrated dinosaurs such asApatosaurus,Allosaurus, andStegosaurus. Reconstructions show no great ocean or mountain barrier between eastern and western North America then, so the contrast can misleadingly suggest that eastern dinosaurs disappeared while western ones prospered.
Why So Much Time Is Missing
Dinosaurs probably remained in the East during the Jurassic and Early Cretaceous, but local geology failed to preserve their remains. Without rock from an interval, fossils from that interval cannot survive in the record.
Paleontologists continue to discuss what happened to the missing rocks. Two explanations are considered:
- Previously formed rocks may have been worn away across millions of years.
- There may have been no suitable space where sediment could collect and become new rock. As the Appalachians eroded, rivers may have transported sediment toward the Atlantic instead of depositing it in basins capable of burying dinosaur bodies.
Consequently, millions of years in the evolution of eastern dinosaurs remain almost entirely undocumented.
Appalachia in the Cretaceous
The fossil record returns with recognizable dinosaurs, followed by isolation and distinctive eastern lineages.
Eastern dinosaur fossils reappear around 115 million years ago in the Cretaceous, and the animals now seem more familiar.
- Near present-day Washington, D.C., Early Cretaceous fossils include a huge sauropod, predatory dromaeosaurs, armored ankylosaurs, and a ceratopsian.Acrocanthosaurus, a giant predator first known from the West, also occurs in Maryland. Its presence on both sides of the continent supports movement between East and West.
- About 90 million years ago, the Western Interior Seaway flooded land between the Arctic Ocean and the Gulf of Mexico, dividing North America into two landmasses.
- Laramidia lay to the west and preserves most of the continent's dinosaur fossils. A land bridge connected it with Asia at intervals.
- Appalachia formed in the east as a completely isolated island continent.
Appalachian dinosaur finds become somewhat more frequent after this split, though they remain scarce beside Laramidia's record. The eastern animals finally attain sizes typical of Late Cretaceous dinosaurs, but their forms differ from dinosaurs living at the same time in the West.
Older Lineages Surviving in Isolation
Appalachia sheltered relict dinosaur lineages after related groups had vanished elsewhere. Two stages of isolation helped turn it into a refugium:
- Jurassic rifting separated eastern North America from Europe and Africa as Pangaea broke apart.
- The Cretaceous Western Interior Seaway later divided Appalachia from Laramidia.
Several Appalachian dinosaur groups illustrate this pattern:
- The tyrannosaursAppalachiosaurusfrom Alabama andDryptosaurusfrom New Jersey were relatives ofT. rex. Although they lived alongside the era of Laramidia's better-known tyrannosaurs, they occupied basal positions on the family tree and retained combinations of traits associated with much older members of the group. Scientists had once believed tyrannosaurs with those traits were gone, replaced by larger and more specialized relatives.
- Duck-billed dinosaurs includingHadrosaurus,Eotrachodon, andLophorhothonalso survived on the eastern island. They lie near the base of the hadrosaur family tree even though more derived relatives had evolved elsewhere. The 85-million-year-oldEotrachodonfrom Alabama may belong to a lineage that ultimately produced Laramidia's Late Cretaceous duck-billed dinosaurs. Basal members nevertheless remained in Appalachia, kept older features, and followed separate evolutionary paths.
- Horned dinosaurs, or ceratopsians, show a comparable pattern of older lineages continuing in Appalachia.
In this way, the island acted as a haven for ancient branches that survived and kept evolving apart from their western relatives, producing the East Coast's lost dinosaur world.
An Evolutionary Route of Its Own
Isolation allowed eastern dinosaurs to persist and develop separately from their western contemporaries.
Even with a vast gap in preservation, available fossils show that Appalachia supported diverse dinosaurs. Many belonged to older branches that had disappeared from Laramidia but endured and evolved on the isolated eastern landmass. The Western Interior Seaway kept later western dinosaurs such asT. rexfrom reaching the East. Appalachia therefore preserved a distinctive community that escaped extinction locally and continued evolving through the later Mesozoic. Places that appear overlooked can be exactly where unusual species endure.
Check Your Understanding
Use these questions to review the evidence, geological events, and evolutionary patterns.
What is a trace fossil, and what do eastern trace fossils chiefly reveal about early dinosaurs?
A trace fossil records an organism's activity rather than its body. Eastern footprints show that dinosaurs were present, varied in type, and included small three-toed carnivores, larger carnivores, early sauropod relatives, and small ornithischians.
How did Pangaea affect the spread of the earliest dinosaurs?
Pangaea joined North America to other continents, so early dinosaurs could move across connected land rather than being stopped by oceans between those landmasses.
What is the Newark Supergroup, and which kind of dinosaur evidence is abundant in it?
It is a group of eastern rocks dating from the Late Triassic into the Early Jurassic. It preserves numerous fossil footprints, or trace fossils.
Which geological event divided North America into Laramidia and Appalachia?
Around 90 million years ago, the Western Interior Seaway flooded a corridor from the Arctic Ocean to the Gulf of Mexico, leaving Laramidia in the west and isolated Appalachia in the east.
What does mega-bias or unconformity mean here, and how does it limit knowledge of eastern dinosaurs between the Middle Jurassic and Early Cretaceous?
It is a regional failure to preserve rock from a long interval. Because roughly 50 to 70 million years of relevant eastern rock and fossils are missing, the dinosaurs that probably lived there during that span are largely unknown.
In Appalachia's dinosaur history, what are a relict lineage and a refugium?
A relict lineage is a surviving remnant of an older group that vanished elsewhere. A refugium is a region where isolated populations from formerly widespread groups remain alive; Appalachia served that role for several dinosaur lineages.
How did Appalachiosaurus and Dryptosaurus contrast with tyrannosaurs living at the same time in Laramidia?
The Appalachian species were basal tyrannosaurs that retained mixtures of traits typical of much older members of the family. Contemporary Laramidian tyrannosaurs belonged to the larger, more specialized forms that scientists once thought had replaced dinosaurs with those older traits.
Common misconceptions
Watch out: The famous dinosaurs displayed in New York were mostly discovered near the museum.
Instead: Most were transported from the American West, although the East Coast has its own significant dinosaur record.
Watch out: The long gap in eastern fossils proves that dinosaurs vanished from the East.
Instead: Dinosaurs probably continued living there, but the region did not preserve the rocks and fossils needed to document them.
Watch out: Appalachia's distinctive Late Cretaceous dinosaurs must have appeared there suddenly.
Instead: They most likely descended from lineages that occupied the region during the missing interval and survived in isolation as relict groups.
Lesson recap
- The Newark Supergroup's footprints record a varied early eastern dinosaur community even though body fossils are scarce.
- A regional mega-bias removed roughly 50 to 70 million years from the East Coast fossil record.
- The Western Interior Seaway separated Appalachia from Laramidia during the Late Cretaceous.
- As an isolated refugium, Appalachia preserved older tyrannosaur, hadrosaur, and ceratopsian lineages that evolved along distinct paths.