Across the wider region the answer is yes, and more than once: rock laid down in the sea brackets 100 and 250 million years ago. Around 66 and 100 million years ago the fossils dug up near the city include both sea animals and land animals. Counting which kind turns up more often would not settle what the place was like — some animals fossilise far more readily than others, and a river can carry a land bone out to sea. So this page shows you both witnesses and what each one can and cannot tell you.
Five ages in the past of this place, each one a window rather than a day. Each says which witness spoke — and where our evidence cannot answer yet, it says that too.
“Ma” means millions of years ago. The dashed mark means our sources have nothing for that age — which is a finding, not a fault.
Around 100 million years ago: sea rock, and land animals in the digs
Witness one — the rock of the region
Across the wider region, the rock from 100 million years ago — the Mowry Shale — was laid down in the sea. That is a statement about roughly 28,600 square kilometres of region, centred about 68 km from the city — not about the ground under one street.
Witness two — the fossils dug up nearby
24 fossils dug up near Denver have an age range covering 100 million years ago: 11 from the sea, 13 from land. The closest of them was found about 18 km away — near the city, never under it.
The regional column spanning 100 Ma is marine; the fossils dug up near this place whose age range contains 100 Ma are 11 marine, 13 terrestrial and 0 unclassified. These are different claims about different things — a region tens of thousands of km² wide, and individual digs kilometres away — and both are shown. The nearby fossil record is mixed — it holds both sea animals and land animals whose age ranges overlap this age.
This is not a mistake in the science, and it is not something to tidy away. A rock layer describes a whole region over a long stretch of time; a fossil occurrence is local evidence from one find in one place — but its age can span a wide stretch of time too. Both can be true, and working out how is exactly what geologists do.
You might think you could just count the fossils and see which side wins. You cannot, and the reason is worth knowing. Some animals turn into fossils far more easily than others — a shell or a tooth survives where a soft body does not — and some places have simply been dug and studied more than others. A river can wash a land animal’s bone out into the sea, so a bone found in sea rock does not always mean the animal lived in the sea. Counting the finds tells you what has been found and recorded. It does not tell you what the place was like.
Travel back in time
The ground Denver sits on has not always been here. A plate model can wind the continents backwards and say roughly where it was — roughly being the important word.
250
million years agojump to the evidence250 million years ago, the plate model puts this ground almost on the equator. This is one of the ages the models argue about most: published reconstructions put it as much as 5.2° of latitude apart, which is several hundred kilometres. Treat it as one model's answer, not a photograph.
200
million years agojump to the evidence200 million years ago, the plate model puts this ground about 15° north of the equator. Published models agree more closely here, within about 2.7° of latitude, but it is still one model's answer rather than a photograph.
150
million years agojump to the evidence150 million years ago, the plate model puts this ground about 24° north of the equator. Published models disagree by about 4.4° of latitude here — a few hundred kilometres — so it is one model's answer, not a photograph.
100
million years agojump to the evidence100 million years ago, the plate model puts this ground about 37° north of the equator. Published models disagree by about 3.4° of latitude here — a few hundred kilometres — so it is one model's answer, not a photograph.
66
million years agojump to the evidence66 million years ago, the plate model puts this ground about 43° north of the equator. Published models agree more closely here, within about 1.4° of latitude, but it is still one model's answer rather than a photograph.
What was the water doing?
Five ages, oldest first. Each one has two kinds of witness — the rock of the wider region, and the fossils dug up near the city — and they are kept apart on purpose, because they are answers to different questions.
Each age here is a marker on the timeline, not a single day. The evidence that belongs to it comes with an age range of its own that overlaps the marker — so it can tell you what this stretch of time was like, and never what was happening on one exact date.
The rock of the region says sea
Around 250 million years ago
250 million years ago, the plate model puts this ground almost on the equator. This is one of the ages the models argue about most: published reconstructions put it as much as 5.2° of latitude apart, which is several hundred kilometres. Treat it as one model's answer, not a photograph.
The rock of the region
Across the wider region, the rock from 250 million years ago — the Lykins Fm — was laid down in the sea. That is a statement about roughly 28,600 square kilometres of region, centred about 68 km from the city — not about the ground under one street.
The fossils dug up nearby
No fossil dug up near here has an age range that covers 250 million years ago, so there is no local evidence for this exact age.
Our evidence has no witness here
Around 200 million years ago
200 million years ago, the plate model puts this ground about 15° north of the equator. Published models agree more closely here, within about 2.7° of latitude, but it is still one model's answer rather than a photograph.
An open question
Neither of the evidence sources we use can answer this one: our regional rock record holds no unit for 200 million years ago at this point, and no fossil dug up nearby covers that age either. That is a real gap in the evidence, not a gap in the past — and saying so is more useful than guessing.
The rock of the region says land
Around 150 million years ago
150 million years ago, the plate model puts this ground about 24° north of the equator. Published models disagree by about 4.4° of latitude here — a few hundred kilometres — so it is one model's answer, not a photograph.
The rock of the region
Across the wider region, the rock from 150 million years ago — the Morrison Fm — was laid down on land, not under the sea. That is a statement about roughly 28,600 square kilometres of region, centred about 68 km from the city — not about the ground under one street.
The fossils dug up nearby
28 fossils dug up near Denver have an age range covering 150 million years ago: 0 from the sea, 28 from land. The closest of them was found about 19 km away — near the city, never under it.
The rock of the region says sea
Around 100 million years ago
100 million years ago, the plate model puts this ground about 37° north of the equator. Published models disagree by about 3.4° of latitude here — a few hundred kilometres — so it is one model's answer, not a photograph.
The rock of the region
Across the wider region, the rock from 100 million years ago — the Mowry Shale — was laid down in the sea. That is a statement about roughly 28,600 square kilometres of region, centred about 68 km from the city — not about the ground under one street.
The fossils dug up nearby
24 fossils dug up near Denver have an age range covering 100 million years ago: 11 from the sea, 13 from land. The closest of them was found about 18 km away — near the city, never under it.
The regional column spanning 100 Ma is marine; the fossils dug up near this place whose age range contains 100 Ma are 11 marine, 13 terrestrial and 0 unclassified. These are different claims about different things — a region tens of thousands of km² wide, and individual digs kilometres away — and both are shown. The nearby fossil record is mixed — it holds both sea animals and land animals whose age ranges overlap this age.
The rock of the region says land
Around 66 million years ago
66 million years ago, the plate model puts this ground about 43° north of the equator. Published models agree more closely here, within about 1.4° of latitude, but it is still one model's answer rather than a photograph.
The rock of the region
Across the wider region, the rock from 66 million years ago — the Dawson Arkose Fm and Denver Fm and Arapahoe Fm — was laid down on land, not under the sea. That is a statement about roughly 28,600 square kilometres of region, centred about 68 km from the city — not about the ground under one street.
The fossils dug up nearby
113 fossils dug up near Denver have an age range covering 66 million years ago: 8 from the sea, 105 from land. The closest of them was found about 0.5 km away — near the city, never under it.
The regional column spanning 66 Ma is non-marine; the fossils dug up near this place whose age range contains 66 Ma are 8 marine, 105 terrestrial and 0 unclassified. These are different claims about different things — a region tens of thousands of km² wide, and individual digs kilometres away — and both are shown. The nearby fossil record is mixed — it holds both sea animals and land animals whose age ranges overlap this age.
What was dug up nearby?
Every fossil below was found somewhere near Denver, never under it — so each one comes with the distance to where it was actually dug up.
Around 66 million years ago
A Tyrannosaurus rex from 66 million years ago was dug up about 12 km from here, in the Denver, Arapahoe County.
A Triceratops from 66 million years ago was dug up about 8.2 km from here, in the Denver, Jefferson County.
A fossil the database can only call “Dinosauria” from 66 million years ago was dug up about 0.5 km from here, in the Denver, Jefferson County.
A Ceratopsidae from 66 million years ago was dug up about 4.5 km from here, in the Denver, Jefferson County.
A Compsemys victa from 66 million years ago was dug up about 5.1 km from here, in the Laramie, Jefferson County.
Around 100 million years ago
A Baculites from around this time (its own age range is 83.6–85.7 million years) was dug up about 14 km from here, in the Niobrara (Smoky Hill Chalk), Arapahoe County.
A Stramentum haworthi from around this time (its own age range is 83.6–85.7 million years) was dug up about 14 km from here, in the Niobrara (Smoky Hill Chalk), Arapahoe County.
A Hadrosauropodus leonardii from around this time (its own age range is 100.5–113.2 million years) was dug up about 16 km from here, in the Muddy Sandstone, Jefferson County.
Around 150 million years ago
An Apatosaurus ajax from 150 million years ago was dug up about 19 km from here, in the Morrison (Brushy Basin), Jefferson County.
A fossil the database can only call “Dinosauria” from 150 million years ago was dug up about 19 km from here, in the Morrison, Jefferson County.
A fossil the database can only call “Dinosauria” from 150 million years ago was dug up about 19 km from here, in the Morrison, Jefferson County.
A fossil the database can only call “Dinosauria” from 150 million years ago was dug up about 19 km from here, in the Morrison, Jefferson County.
We show these places to the nearest few kilometres rather than pinning them exactly. The scientists who recorded them know the spot far better than that — the full record is one click away in the Paleobiology Database, and it is free for anyone to read. We round them here because a map made for children is not a map for digging things up, not because anything is being hidden.
What changed?
The ground under Denver has not stayed still. The plate model puts it near the equator 250 million years ago and at the latitude it sits at today, and the rock of the region flips between sea and land more than once along the way.
The rock at the surface today is younger than all of it. The geological map of this exact point returns Mesozoic sedimentary rocks (Mesozoic), Paleogene sedimentary (Cenozoic), Gravels and alluviums (Pleistocene), Broadway Alluvium (Pleistocene).
This page says nothing about how high Denver sits, or how it got that way — we have no source for that yet, so we do not guess.
Try it yourself
A dinosaur was dug up a few kilometres from here, and it drank water. So does everyone in Denver today — and it is the same water, going round again. Work out how much of your next glass has been through a dinosaur.
Around 100 million years ago, two kinds of evidence describe this place. The rock of the wider region was laid down in the sea. The fossils dug up near the city include both sea animals and land animals. Both are good evidence, and both are answers to slightly different questions.
Would counting the fossils settle it? Why not — and what would you weigh instead?
How sure are we?
Column facts describe an area of thousands to tens of thousands of km², centred tens of kilometres from the town coordinate. They are claims about a region, never about the rock beneath the town.
Fossil facts are a bounding-box sample around the town, each with its own distance from it; a fossil is never "here". The absence of fossils is not evidence of the absence of life — only of the absence of a recorded, published dig in this box.
Positions are model outputs, not observations. Independent plate models disagree by hundreds of kilometres at these ages, and that spread is the honest size of the uncertainty.
The land/sea question is answered only by the regional rock and the local fossils, separately, and never by the reconstruction — which says where this ground was and cannot say whether it was wet. There is deliberately no single field holding a one-word answer, because answering in one word would require merging a regional claim with a local one (CLAUDE.md rule 5). The nearby fossils are reported as a census of what has been found — which kinds are present, and how many of each — and never counted into a verdict: occurrence records are not statistically equivalent observations, so a plurality of them cannot establish an environment (science review, 2026-08-30).
Fossils are binned to their nearest slice age by age-range midpoint, with boundaries at the inter-slice midpoints 83, 125, 175 and 225 Ma. Those boundaries are an editorial choice, not a published stratigraphic scheme; a reviewer may prefer stage boundaries. Every occurrence keeps its own age range so the choice can be re-examined.
Sources
Macrostrat — the rock of the regionRead from version 2.3.9 of its data service on 27 August 2026.Peters, S.E. et al. Macrostrat: a platform for geological data integration and deep-time research. https://macrostrat.org, accessed on (date)Palaeogeography: Wright, N., Zahirovic, S., Müller, R. D., and Seton, M. Towards community-driven paleogeographic reconstructions: integrating open-access paleogeographic and paleobiology data with plate tectonics. Biogeosciences 10:1529-1541
The Paleobiology Database — the fossils dug up nearbyPBDB publishes no dataset version string; the data-service path (data1.2) and the datainfo access_time bound this claim.Fossil occurrence data from the Paleobiology Database (https://paleobiodb.org), released under CC0 1.0.Palaeo-coordinate cross-check (Paleobiology Database `pgm=gplates`): Wright et. al., 2013; Towards community-driven paleogeographic reconstructions.Palaeo-coordinate cross-check (Paleobiology Database `pgm=scotese`): Scotese, C.R., 2021. An Atlas of Paleogeographic Maps: The Seas Come In and the Seas Go Out, Annual Reviews of Earth and Planetary Sciences, v. 49, p. 669-718.Palaeo-coordinate cross-check (Paleobiology Database `pgm=seton`): Seton et. al., 2012; Global continental and ocean basin reconstructions since 200 Ma.
The EarthByte plate model — where this ground used to beMULLER2019 version 3.0, run with pyGPlates 1.0.0, licensed CC BY 4.0.GPlates: Müller, R. D., Cannon, J., Qin, X., Watson, R. J., Gurnis, M., Williams, S., Pfaffelmoser, T., Seton, M., Russell, S. H. J., Zahirovic S. (2018). GPlates: Building a virtual Earth through deep time. Geochemistry, Geophysics, Geosystems, 19, 2243-2261. https://doi.org/10.1029/2018GC007584Reconstructions computed with the GPlates Web Service (EarthByte Group, The University of Sydney).Plate model: Müller, R. D., Zahirovic, S., Williams, S. E., Cannon, J., Seton, M., Bower, D. J., Tetley, M. G., Heine, C., Le Breton, E., Liu, S., Russell, S. H. J., Yang, T., Leonard, J., and Gurnis, M. (2019), A global plate model including lithospheric deformation along major rifts and orogens since the Triassic. Tectonics, vol. 38, https://doi.org/10.1029/2018TC005462. Licensed CC BY 4.0.Plate model: Zahirovic, S., Eleish, A., Doss, S., Pall, J., Cannon, J., Pistone, M., Tetley, M. G., Young, A., & Fox, P. (2022). Subduction kinematics and carbonate platform interactions. Geoscience Data Journal, 9(2), p.371-383. Licensed CC BY 4.0.Coastline and static-polygon data by the EarthByte Group, licensed CC BY 3.0 Unported.
This record was assembled on from the datasets above. The ancient positions are reconstructions from a published plate model, not observations.
