A tiny gland, invisible to the naked eye, kept dinosaurs from the sea.
For 150 million years, dinosaurs commanded the land with unrivaled authority, yet the ocean — teeming with reptilian giants — remained forever beyond their reach. A study published in Historical Biology now suggests the barrier was not one of limb or lung, but of chemistry: most dinosaurs appear to have lacked the salt-excreting glands above the eyes that allow modern seabirds to drink from the sea and survive. Only the spinosaurs, those elongated, fish-hunting wanderers of rivers and deltas, show fossil evidence of developing this quiet physiological key — a door they approached but never fully opened before extinction closed it for them.
- The ocean was not empty during the age of dinosaurs — ichthyosaurs, plesiosaurs, and mosasaurs ruled it — yet not one of those marine giants was a true dinosaur, a gap that has puzzled paleontologists for generations.
- New fossil analysis by Andrea Cau's team at the University of Southampton identifies bone structures in spinosaur skulls consistent with salt-excreting glands, the first such evidence found in non-avian dinosaurs.
- The finding reframes the question: the obstacle to ocean colonization may not have been how dinosaurs moved through water, but whether their bodies could chemically tolerate it.
- Most dinosaurs, the study implies, lacked the anatomical space in their skulls to house these glands — an invisible physiological ceiling that kept an entire lineage of dominant creatures from claiming the sea.
- Spinosaurs appear to have been evolutionary experimenters, their elongated snouts and repositioned nostrils accidentally creating room for a partial solution — but extinction arrived before the experiment could conclude.
For 150 million years, dinosaurs ruled the land absolutely — yet the oceans remained the domain of other creatures entirely: ichthyosaurs shaped like dolphins, plesiosaurs with improbable necks, mosasaurs of terrifying ferocity. Not one of those ocean giants was a dinosaur. The question of why has long haunted paleontology.
A study published in Historical Biology now offers an answer both unexpected and elegant. The barrier, researchers suggest, may have resided not in limb anatomy or locomotion, but in the skull itself — specifically in tiny glands capable of expelling salt. Seawater carries roughly 35 grams of salt per liter, and for a land-adapted animal, drinking it triggers a physiological crisis. Modern seabirds solve this through supraorbital salt glands positioned above the eyes, which filter salt from the bloodstream and expel it through the nostrils. Without such structures, sustained ocean life is nearly impossible.
Andrea Cau of the University of Southampton led a team that found bone features in spinosaur fossils consistent with precisely these glands — anatomical markers for both the gland itself and the canal through which salt would have been expelled. It is the first relatively solid evidence of such an adaptation in non-avian dinosaurs. The spinosaurs' extraordinarily elongated snouts and repositioned nostrils appear to have created sufficient skull space to house the glands — an accidental architecture that Cau's team describes as an exaptation, a structure that evolved for one purpose and proved useful for another.
Yet the finding carries a sobering implication: if spinosaurs had to evolve these glands independently, most other dinosaurs almost certainly lacked them entirely. For decades, explanations for the absence of marine dinosaurs focused on mechanical constraints — how they moved, how they reproduced, how they competed. This research points instead toward something more fundamental: the invisible chemistry of survival. The living descendants of dinosaurs — penguins, albatrosses, gulls — did eventually conquer the oceans, but only after specific skull modifications created room for efficient salt excretion. Most non-avian dinosaurs, it seems, never had that room to begin with. Spinosaurs came closest, haunting the rivers and coastal shallows, experimenting with an adaptation that might have changed everything — but the door, for them and all their kin, remained just barely closed.
For 150 million years, dinosaurs ruled the land with absolute dominion. Creatures weighing dozens of tons thundered across continents. In the skies above them, pterosaurs spread wings wider than small aircraft. And in the oceans below, marine reptiles of staggering power hunted and thrived—ichthyosaurs shaped like dolphins, plesosaurs with necks that seemed to have no end, mosasaurs that could match any modern shark in ferocity. Yet here lies a peculiar gap in the prehistoric world: not a single one of those ocean giants was actually a dinosaur.
The puzzle deepens when you consider that some dinosaurs appeared to be moving toward the water. Spinosaurus stands as the most famous example—a predator with an elongated snout and conical teeth that almost certainly spent much of its time hunting fish in rivers, lagoons, and coastal shallows. Yet even Spinosaurus never made the leap to becoming a true marine dinosaur, a creature that could claim the open ocean as its domain the way its cousins claimed the land.
Why? A study published in Historical Biology offers an answer that is both unexpected and elegant. The barrier may not have been in the legs, or the lungs, or even in how these animals swam. It may have been in the skull itself—specifically, in tiny glands capable of expelling salt. To understand this requires first grasping a fundamental problem of ocean life: seawater contains roughly 35 grams of salt per liter. For an animal accustomed to fresh water or land, drinking salt water becomes a physiological crisis. The excess salt disrupts the delicate osmotic balance within cells, forcing the organism to expend enormous energy simply to purge it.
Modern marine mammals like whales and seals have solved part of this problem through kidneys of extraordinary efficiency. But many seabirds employ a different strategy entirely. They possess specialized glands positioned directly above their eyes that filter salt from the bloodstream and expel it through the nostrils. Watch a gull, an albatross, or a penguin long enough, and you will see them shed tiny droplets of concentrated salt from their beaks. These structures are called supraorbital salt glands, and they represent a physiological innovation that proved crucial for ocean colonization.
Here is where the research led by Andrea Cau of the University of Southampton becomes striking. His team identified in fossils of spinosaurs a series of bone features consistent with the presence of these salt glands. They found anatomical markers related both to the gland itself and to the canal through which accumulated salt would have been expelled. This would represent, according to Cau, the first relatively solid evidence of such an adaptation in non-avian dinosaurs. Yet the finding creates a paradox: the only dinosaur group showing clear signs of developing a physiological tool useful for salty environments is precisely the group that came closest to the water. The deeper implication is more troubling still. If spinosaurs had to evolve these glands, does that mean most other dinosaurs lacked them entirely? The study suggests yes—and that absence may have functioned as an invisible evolutionary barrier.
For decades, explanations for the absence of marine dinosaurs focused on mechanical constraints: limb anatomy, locomotion, reproduction, competition with established marine reptiles. But this new research points toward something more fundamental: the regulation of salt and water within the body itself. The hypothesis is that many dinosaurs simply lacked the anatomical flexibility to develop efficient salt glands. Modern birds, the living descendants of dinosaurs, did manage to colonize the oceans—penguins, petrels, albatrosses, cormorants, gulls all evolved extraordinarily effective salt-excretion systems. But this was possible, the study argues, only because of specific anatomical changes in the skull region that created space to house these glands above the eye sockets.
Spinosaurs appear to have arrived at a partial solution through an independent evolutionary path. Their extremely elongated snout, the position of their nostrils, and certain modifications to the skull would have created sufficient space to position the glands in a location similar to that seen in modern seabirds. Cau's team describes this as an "exaptation"—a structure that evolved originally for one purpose and later became useful for another. This does not make Spinosaurus a true ocean animal. Most evidence still suggests that spinosaurs exploited rivers, deltas, and coastal zones rather than open seas comparable to those dominated by ichthyosaurs and mosasaurs. But it does suggest they were experimenting with an adaptation that few other dinosaurs seemed to possess—a small physiological innovation that might have opened a door that remained closed for all the rest.
Citas Notables
The problem may not have been in the legs, lungs, or swimming ability, but in the skull itself—specifically in tiny glands capable of expelling salt.— Andrea Cau, University of Southampton, leading the study published in Historical Biology
La Conversación del Hearth Otra perspectiva de la historia
So dinosaurs ruled the land for over a hundred million years, yet never made it into the ocean. That seems almost impossible given how successful they were everywhere else.
It does seem strange at first. But the ocean isn't just a bigger version of land. It's a fundamentally different chemical environment. Saltwater creates a problem that land animals never had to solve.
And that problem is salt itself?
Exactly. Your body wants to maintain a certain balance of salt and water inside your cells. When you're surrounded by saltwater, that balance gets disrupted. You lose water, gain salt, and your cells start to fail. It's a slow poison.
But other animals figured it out. Whales, seals, fish—they all live in the ocean.
They did, but each group solved it differently. Whales and seals have incredibly efficient kidneys. Seabirds developed these specialized glands above their eyes that filter salt directly from the blood and spit it out through the nose. It's elegant.
And dinosaurs couldn't do that?
Most couldn't. Their skulls apparently didn't have the right shape or flexibility to accommodate those glands. Spinosaurus came close—its long snout and modified skull gave it space to develop them. But even that wasn't enough to push it fully into the ocean.
So it's not about swimming ability or strength. It's about plumbing.
In a way, yes. It's about basic physiology. The ocean demanded a specific anatomical solution, and most dinosaurs simply weren't built to provide it.