The asteroid that struck the Earth 66 million years ago changed far more than life on land. Ocean ecosystems also collapsed in the aftermath of the strike, and, like the non-avian dinosaurs on land, almost all the impressive marine reptiles that had swum the ancient seas disappeared. Then, as life recovered, the oceans offered possibilities that had not been open for over 180 million years—waters virtually free of large predators. There, early whales could take the evolutionary plunge.
Among the most impressive of these new beasts were the basilosaurids. Named after Basilosaurus, one of the first fossil whales ever discovered, these oceanic carnivores were seagoing success stories. And a pair of recent fossil discoveries underscores how these early whales opened up a variety of niches long before the earliest dolphins or baleen whales would evolve.
Basilosaurus and its close relatives have long stood as epitomes of aquatic adaptation in the epic story of whale evolution. The ancestors of the first whales lived on land, but about 55 million years ago early whales were spending more time in the water. They gained adaptations such as denser bones that acted as a kind of ballast. Over millions of years, a variety of early whales lived between the shore and the open ocean, with some lineages becoming ever more adapted to a life spent entirely in the water. By 41 million years ago, whales included the basilosaurids—sinuous forms with sharp teeth, long bodies and only the vestiges of their hind legs sticking out from their sides.
Against the background of aquatic adaptation, Basilosaurus has long stood out as an iconic whale no longer tied to the shore. The beast became a scientific darling because of a rich fossil record. “Much of the emphasis on Basilosaurus happens to be because we’ve known about this particular fossil species for over 175 years, and from relatively complete skeletons,” says Smithsonian National Museum of Natural History whale researcher Nicholas Pyenson.
Decades of research has affirmed that Basilosaurus was an impressive predator, using piercing and slicing teeth to feed on large fish, sharks and even other whales. But Basilosaurus only represents one part of what paleontologists are beginning to recognize as a much more significant moment in whale evolution, in which Basilosaurids included some of the largest and smallest fossil whales known.
In August of this year, paleontologists described Perucetus, an absolutely massive basilosaurid. The whale may have been the largest animal of all time, a giant that rivals or even exceeds the proportions of today’s blue whale. Paleontologists never expected that basilosaurids, still early in the whale evolutionary story, would have evolved such stupendous size. A week later, a different team of paleontologists described Tutcetus, another basilosaurid that was wholly different. Tutcetus was dolphin-sized, the smallest basilosaurid yet known.
Up until recently, University of Liège paleontologist Rebecca Bennion says, basilosaurids were thought to be large, unspecialized predators. “Their skulls were certainly very similar, all suggesting predation at higher trophic levels,” she says, meaning that basilosaurids were hunting large prey that were likely formidable carnivores themselves. Her own research into Basilosaurus and other fossil whales found that these carnivores were similar to giant seagoing reptiles like Mosasaurus, sharing long and low skulls full of gripping teeth, a case of convergent evolution between apex predators over 25 million years apart. But now Tutcetus and Perucetus change that perception. Because of their difference in sizes and anatomy, the whales indicate that basilosaurids were not all rapacious hunters of large prey.
Even though the size of Perucetus is currently debated, the whale was certainly enormous and might have required a life almost crawling over the ocean bottom to munch on clams and other mollusks. Tutcetus, by comparison, was a smaller hunter that was feeding on fish and trying to avoid becoming lunch for a larger whale.
The new whales aren’t part of a linear progression of early whales from sea to land, but part of a varied and many-branching family. “There is a misconception in some of the ways that we tell the story of whale evolution,” Pyenson says. Rather than increasing adaptation to the water in a linear fashion, early whales are best looked at as an array of evolutionary “experiments.” And basilosaurids hold an important place in this story of evolutionary transformations.
Basilosaurids thrived between about 33 million and 43 million years ago. And even though earlier whales were capable of crossing oceans, basilosaurids were among the first whales to live their entire lives in the water without ever coming back to shore. “The common ancestor of all aquatic whales probably looked a lot like the basilosaurids Zygorhiza or Dorudon,” Pyenson says, but about the size of the smaller Tutcetus. These whales looked like littler, shorter versions of Basilosaurus, down to their wedge-shaped skulls fitted with gripping and slicing teeth. The animals resembled the last common ancestor of all today’s whales in a group called the pelagiceti, or the fully aquatic whales.
In the seas, the basilosaurids began to take on a variety of different forms over millions of years. While Pyenson cautions that ancient whale species diversity wasn’t like what we see today, he notes that “basilosaurids explored a vast range of ecomorphs and sizes.”
The broad array of differences raises questions about basilosaurid evolution. And the discoveries of new forms such as Tutcetus and Perucetus lead to queries about how such whales evolved in the first place, as well as what eventually led basilosaurids to become extinct as more modern whales began to spread through the oceans.
Further investigations into the evolution and natural history of basilosaurids will rely on future finds, an endeavor that has put greater emphasis on undoing the colonialist practices of the past. Some of the best places to find fossil whales are countries such as Egypt and Peru, but scientists from those regions have often been excluded as scientists from Western nations visit to collect fossils and take them far from where they were uncovered.
“For fossil whales, I think there is a strong colonial imprint that constrains the path of science in Latin America, especially in Peru,” Pyenson says. Paleontologists in Peru need more funding and support to develop their research and find new fossil sites—rather than having the nation’s fossils described and named by researchers practicing “parachute science,” when scientists from institutions with more funding and support visit other countries to extract fossils from another, often in the global south, to build their collections and reputations.
Paleontologists in Egypt have faced similar challenges. Tutcetus, Pyenson notes, is the first fossil whale from Egypt named by Egyptian paleontologists—even though researchers have been visiting the country’s fossiliferous rocks for generations. The whale’s description is a strong reminder that paleontology is a science carried out by people carrying their own biases and has struggled with a legacy of colonialism that is only just beginning to be undone.
To understand how life on Earth changed through time, paleontology needs perspectives from all over the world—a global effort to piece together what truly happened so long ago. After all, fossil whales like Basilosaurus and its kin crossed entire oceans and are found on multiple continents. If experts are going to fully envision the lives of these vanished whales, the effort requires the insight of researchers from all over the planet.