What oceans existed during the dinosaurs?
Two that this site can quote a source naming — Panthalassa and the Tethys — and, on top of the continents rather than between them, shallow seas that came and went. One of those flooded the middle of North America and left its rock under Denver.
Start with what a source actually says
“Much of Earth’s surface was covered by a large ocean called Panthalassa.”
U.S. National Park Service, “Permian Period—298.9 to 251.9 MYA”. Courtesy of the National Park Service.
Now read the title of the page it came from. That sentence is about the Permian, and the Permian finished before the first dinosaur. It is the right sentence for the ocean and the wrong one for the dinosaurs, and putting it under this heading without saying so is exactly how a reader ends up believing something nobody wrote.
What carries across is the arrangement. One continent, one ocean around it, one sea in its crook — and the whole of the dinosaurs’ time is that arrangement coming apart. The Triassic, the Jurassic and the Cretaceous each carry their own dates, from a source, on their own pages.
How much of the world was under water
This is the part you will not find on the pages below, because it only exists when you put every map this site holds in a row. Each one is a reconstruction of the whole planet at one moment — an estimate built from evidence, not a photograph — and for each one you can count how many of its squares sit below sea level.
250 million years ago
Permo-Triassic Boundary
68.6% below sea level
200 million years ago
Late Triassic
61.4% below sea level
150 million years ago
Late Jurassic
66.6% below sea level
145 million years ago
Jurassic-Cretaceous Boundary
66% below sea level
100 million years ago
Early Cretaceous
66.2% below sea level
90 million years ago
Mid-Cretaceous
68.7% below sea level
70 million years ago
Late Cretaceous
63.4% below sea level
65 million years ago
KT Boundary
65.2% below sea level
35 million years ago
Late Eocene
63.9% below sea level
The most sea is on the 90 million year map at 68.7%, and the least on the 200 million year map at 61.4% — a spread of 7.3 points across the whole set. It goes down and it comes back up. It is not a slide from wet to dry.
Why it moves is a different question, and this site does not answer it. Counting squares on nine maps is arithmetic. Saying what made the number change would be a story, and no source here tells it.
Three seas, and what each one rests on
The pages below are not the same kind of page, and the difference is the evidence underneath them rather than the writing on top.
The Western Interior Seaway
The only one of the three you can stand on top of. Its evidence is rock: six named layers under Denver, in the order they were laid down, recording a sea arriving, getting deep, and draining away over roughly twenty-four million years.
Panthalassa
No rock, because this site holds no column that reaches it. Its evidence is a sentence — a national park service describing one ocean around one continent, and naming the animals living in it.
The Tethys Ocean
A sentence again — but the only sea here with a written ending. India ran into Asia, the water between them ran out of room, and the Himalayas went up instead.
Is this all of them?
No, and this page will not pretend otherwise. These are the seas this site can quote a source naming. A complete list of the oceans of the dinosaur age is not something it holds, and a page that produced one anyway would be inventing the parts it could not look up.
The maps above do not help with that either. They show where sea was; they never say which sea it is. That silence is the reason two of these three pages took until September to become possible at all.
Where this comes from
- The names and the maps
- Names: U.S. National Park Service, “Permian Period—298.9 to 251.9 MYA”, last updated April 28, 2023, read ; public domain. Sea shares: counted from PALEOMAP PaleoDEMs (Scotese & Wright 2018), Zenodo, doi 10.5281/zenodo.5460860, licensed CC BY 4.0 — 9 maps, each counted the same way.
- How sure is this?
- Every map is one model’s reconstruction rather than a photograph, and the share below sea level is a count of grid squares on an equirectangular grid — so it is a share of the map, and squares near the poles cover less ground than squares near the equator. It is comparable between these maps because they are all counted identically. It is not an area in square kilometres and must not be read as one.