The Warm Miocene Sea That Used to Cover the American Southeast

The Miocene epoch (23.03–5.33 million years ago) was a period of sustained global warmth. During its climatic peak — the Mid-Miocene Climatic Optimum, approximately 15–17 million years ago — global mean temperatures were roughly 3–4°C warmer than today. Polar ice sheets were significantly reduced, and global sea levels were substantially higher than present, flooding low-lying coastal plains worldwide.

For the North American Southeast, this meant that much of peninsular Florida sat beneath a warm, shallow epicontinental sea. This was not a deep-water environment — it was a productive, sunlit, reef-fringed tropical sea comparable in character to the modern Bahamas Platform or the shallow southern Gulf of Mexico. The Florida Platform of the Middle Miocene was a series of shallow banks, shoals, and marine grassbeds, with warm currents circulating nutrients from deeper offshore water.

The fauna of this Miocene Florida sea was extraordinary in its diversity. The apex predator was Otodus megalodon, but it shared the water with at least 25 other shark species: Isurus hastalis (the broadtooth mako, comparable in size to a modern mako), Hemipristis serra (snaggletooth), Galeocerdo aduncus (ancestral tiger shark), Carcharhinus species (reef and bull shark ancestors), Odontaspis ferox (sand tiger relatives), and more.

Whale diversity during the Miocene reached one of its highest points in the fossil record. Baleen whales (Mysticeti) were diversifying rapidly — the fossil record from Virginia and Maryland preserves ancestral balaenid, balaenopterid, and cetotheriid lineages. These whales were megalodon's primary prey, and their diversity and abundance directly supported megalodon populations. Early toothed whales (sperm whale relatives, dolphin ancestors) occupied lower trophic levels. Dugongs in the genus Metaxytherium grazed on seagrass beds in coastal shallows. The same seafloor that accumulated all of these fossils became the Bone Valley Formation.

The extraordinary phosphate richness of the Bone Valley and similar formations is a direct record of ancient marine productivity. Phosphate concentrations in marine sediments form when organic material (fish, plankton, marine mammals) decomposes under oxygen-poor seafloor conditions, releasing phosphorus into pore waters that precipitate as carbonate fluorapatite.

The Miocene Florida seaway experienced repeated episodes of coastal upwelling — zones where nutrient-rich water rose to the surface, fueling explosive phytoplankton growth and the food web it supported. This productivity drove high organic loading to the seafloor, which drove phosphate precipitation in the accumulating sediment. The result was a seafloor that is, geologically speaking, a natural phosphorus trap — and by extension, a trap for the calcium phosphate of every fish and shark tooth that settled into it.

The fertilizer industry recognized this chemistry and began mining the Bone Valley Formation in earnest in the early 20th century. What the mining industry calls economic phosphate rock, paleontologists call one of the richest Miocene fossil archives in the world.

The Miocene warm climate peaked around 15 million years ago and then cooled in a series of steps associated with the growth of Antarctic ice sheets. As water was locked into polar ice, global sea levels fell. The shallow epicontinental sea over Florida gradually retreated — first from the north, then progressively southward over millions of years.

The Miocene sediments — now the Bone Valley Formation — were stranded as dry land, buried beneath younger Pliocene and Pleistocene sediments. In the most recent geologic chapter, the Peace River and its predecessors incised through the overlying layers and began eroding the Bone Valley. The fossil teeth that wash up on Venice Beach today have been in transit — geologically speaking — for millions of years: from Miocene seafloor, through slow burial, through mineralogical transformation, through gradual exhumation, and finally into the swash zone where you find them.