At first glance, its skeleton appears almost familiar.
It has the solid, barrel-shaped build of a hippopotamus, the heavy bones of a marine herbivore, the forward-projecting teeth of an animal with very strong opinions about vegetation, and four stout limbs that look as though they might have carried it across the seafloor—or at least helped it negotiate the shallows with considerable determination.
But this is neither a hippopotamus nor a manatee.
It is a desmostylian: a member of an entirely extinct order of aquatic and semi-aquatic mammals that once lived around the northern Pacific Rim.
The museum specimen has often been identified in photographs and older references as Paleoparadoxia.
More precisely, it is now classified as Neoparadoxia cecilialina, a close relative within the paleoparadoxiid family. Taxonomy is a living science, and fossil animals occasionally receive new names as researchers examine their skeletons in greater detail. The animal has not changed, of course. It remains magnificently deceased and entirely unconcerned about the paperwork.
Neoparadoxia cecilialina lived approximately 10.5 million years ago during the Late Miocene Epoch, when the coastline, climate and marine communities of California were very different from those we know today.
This remarkably complete skeleton was recovered from rocks of the Monterey Formation in Orange County, California. The individual was approximately 2.4 metres—or about eight feet—long when it died. Researchers determined that it was not yet fully grown. An adult may have reached approximately 2.7 metres in length, making it roughly the length of a large sofa, although considerably more difficult to arrange around a coffee table.
It may have been about eleven years old at the time of its death, though estimating the age of an extinct mammal with no perfect modern equivalent requires careful comparison and a certain amount of scientific humility.
And desmostylians demand humility.
They were such peculiar animals that scientists have debated almost every aspect of them: how they moved, how much time they spent on land, what they ate, how they were related to other mammals and whether their heavy bodies were better suited to swimming, wading or walking underwater.
Imagine a hippo, a manatee and a small elephant submitting design suggestions for the same animal.
Then imagine the committee approving all three.
Desmostylians appeared during the Oligocene and survived into the Miocene, living along the margins of the North Pacific. Their fossils have been found on both sides of the ocean, including western North America and Japan. Unlike seals, whales and sea cows, however, they left no living descendants. The entire order vanished.
That makes Desmostylia especially intriguing. We are not merely looking at an unusual extinct species; we are looking at the remains of a completely lost experiment in marine mammal evolution.
The name Desmostylia comes from Greek words referring to “bundled pillars,” a wonderfully architectural description of the animals’ extraordinary back teeth.
Their molars were composed of clustered columns of enamel, giving them an appearance rather like bundles of tightly packed tubes.
These were not delicate little teeth designed for nibbling parsley.
They formed a powerful crushing surface, suited to processing tough vegetation. Desmostylians are generally interpreted as herbivores that fed in or near coastal waters, possibly consuming aquatic plants, sea grasses, algae or shoreline vegetation.
Exactly what appeared on the menu remains debated because stomach contents and polite Miocene restaurant receipts have not survived.
The front of the mouth was equally impressive. Neoparadoxia had forward-projecting incisors and enlarged canine-like teeth. Combined with the deep lower jaw, they gave the skull a wonderfully strange profile.
Some reconstructions make the mouth look rather like the scoop of a small excavator.
I do not wish to suggest that Neoparadoxia was the backhoe of the Miocene, but if it had arrived at a construction site wearing a reflective vest, few of us would have questioned its credentials.
Those teeth and jaws may have helped the animal grasp, uproot or gather vegetation from the seabed. Its long snout could have been used to crop plants while its heavy body remained submerged in shallow coastal water.
The skeleton also reveals an animal built very differently from a whale or dolphin. Its limbs were still substantial and weight-bearing. Rather than being transformed into sleek flippers, they retained strong bones and distinct digits. This suggests that Neoparadoxia could support itself on the bottom and perhaps travel on land, although it would hardly have been graceful.
There is still scientific disagreement about precisely how desmostylians moved. Some studies have interpreted paleoparadoxiids as slow swimmers or bottom walkers inhabiting relatively shallow water. Their dense bones could have acted as ballast, helping them remain submerged without bobbing helplessly back to the surface.
Think of a diver’s weight belt, except the weight belt is the skeleton and removing it is not an option.
Dense bones are found in several aquatic or semi-aquatic mammals. They can help counteract the buoyancy produced by lungs and body fat, allowing an animal to remain at a preferred depth while feeding.
Research into desmostylian bone structure suggests that different members of the group developed different degrees of aquatic adaptation.
Paleoparadoxia and its relatives possessed particularly dense bones and may have moved slowly through shallow coastal environments, hovering, paddling or walking along the bottom as they searched for food.
This was not a creature built for chasing tuna across the open ocean.
It was built to approach a bed of aquatic vegetation with patience, mass and purpose.
Its broad body, powerful shoulders and large pelvis suggest an animal capable of pushing itself through water and over uneven substrates. On land, it may have moved with an awkward, rolling gait. In the water, the same body could have been far better supported.
The result was likely an animal that looked somewhat cumbersome from our perspective but was well adapted to its own coastal world. Evolution does not work toward elegance as judged by humans. It works toward surviving long enough to reproduce.
Sometimes the result is a gazelle.
Sometimes the result is an eight-foot marine mammal that resembles a hippopotamus wearing an elephant costume over a manatee foundation garment.
For many years, desmostylians were commonly placed near proboscideans—the group containing elephants—and sirenians, which include manatees and dugongs. More recent analyses have complicated that picture, and their precise position within the mammalian family tree remains debated.
This uncertainty is not a failure of palaeontology. It is one of the reasons the science is so compelling.
Fossils preserve evidence, but rarely all of it. Researchers must compare the shapes of bones, patterns of tooth development, microscopic bone structure, geological age and evolutionary relationships. New discoveries can strengthen an old interpretation or turn it neatly on its head.
Neoparadoxia is a beautiful reminder that classification is not merely the act of attaching a label to a display. It is an ongoing investigation into ancestry, adaptation and deep time.
The museum skeleton is particularly valuable because it is so complete. Many fossil mammals are known from isolated teeth, fragments of jaw or scattered bones. A nearly complete skeleton allows palaeontologists to examine how the skull, spine, ribs, limbs and pelvis worked together.
One tooth can tell us that an animal existed.
A skeleton can begin to tell us how it lived.
The individual displayed in Los Angeles is also scientifically important because it represents one of the youngest known desmostylians. It lived near the end of the group’s long evolutionary history, shortly before these peculiar marine mammals disappeared from the fossil record.
Why did they vanish?
We do not yet have a simple answer. Changes in sea level, climate, coastal habitats, available vegetation and competition with other marine herbivores may all have contributed. As desmostylians disappeared, sirenians—manatees and dugongs—became the surviving large marine herbivores.
That does not necessarily mean that sea cows marched into the North Pacific and evicted them with tiny legal notices. Evolutionary replacement is rarely so tidy. Environmental change can alter habitats and food sources, favouring one group while placing another under increasing pressure.
Whatever happened, the desmostylian experiment eventually ended.
Today, their bones survive in Miocene marine rocks around the Pacific and in museum collections where we continue to ask questions of them.
This is why museum displays matter.
A mounted skeleton is not simply an attractive object placed in a hall for paleontological tourism. It is the visible result of discovery, excavation, preparation, anatomical study, comparison and scientific revision. It allows schoolchildren, researchers, travellers and gloriously enthusiastic fossil people to stand face-to-face with an organism unlike anything alive today.
We are accustomed to imagining marine mammals as whales, dolphins, seals, walruses, manatees and dugongs. Neoparadoxia challenges that tidy modern picture. The ancient oceans supported entire forms of mammalian life that have since disappeared.
Ten and a half million years ago, something large and heavy moved through California’s coastal waters. It lowered its peculiar head toward the bottom, used its formidable jaws to gather vegetation and carried a skeleton dense enough to help anchor it beneath the waves.
It had no way of knowing that one day its bones would stand in a museum in Los Angeles, surrounded by visitors attempting to decide whether it looked more like a hippo, a manatee, an elephant or an ambitious piece of earth-moving equipment.
The scientifically correct answer is that it was none of them.
It was a desmostylian. It was Neoparadoxia cecilialina. And it was gloriously, marvellously strange.
Sources: Natural History Museum of Los Angeles County—Vertebrate Paleontology, Smithsonian Ocean—Desmostylian locomotion and ecology, PLOS ONE—Bone structure and aquatic adaptation in Desmostylia, and Royal Society Open Science—Desmostylian fossil history.
Lead Image: Asset id: 2710687265. Los Angeles, USA 9.04.2025: Preserved skeleton of a prehistoric Paleoparadoxia fossil mammal displayed at the Natural History Museum, Los Angeles
