Quick Bite Field Guide: Weird Whales and Swimming Sloths
Marine mammals are fascinating beasts and the subject of our latest Quick Bite episode! Whales, manatees, seals, otters…they’ve all gone back to the water and evolved all kinds of spectacular adaptations to making a living in a soggy setting. Toothed whales evolved an ability to “see” the underwater world around them using echolocation – basically sonar – to track prey with high-pitched sounds and echoes. A 28 million year-old fossil from South Carolina called Cotylocara shows toothed whales could echolocate early in their evolutionary history. A more surprising adaptation to life in the water was preserved with another new whale fossil called Semirostrum. The new whale has a long chin which was probably used as a sensitive probe to track down buried prey on the seafloor. Our final study examines how a SLOTH adapted to life in the ocean. Thalassocnus was a relative of giant ground sloths, a land-dwelling group of animals. A recent study showed how Thalassocnus gradually acquired thickened bones, a trait that has been observed in nearly every mammal that has gone back to the water. Even if it’s a weird animal to imagine clawing through the water, it adapted to that lifestyle in exactly the way paleontologists expected!
Click below for images of these incredible creatures and for more discussion of the science behind these discoveries!
Cotylacara macei – Meaning “Cavity head”, Cotylacara is the oldest known whale – about 28 million years old – with the muscle attachment sites necessary for echolocation.
Semirostrum ceruttii – Meaning “Half nose”, Semirostrum was a porpoise that lived off the coast of California from 5-2 million years ago. It probably used its long chin filled with large, sensitive nerves to explore the ocean floor.
Thalassocnus – Meaning “Lazy one of the Sea”. The giant sloth genus Thalassocnus was recognized as a marine mammal decades ago, but a recent study showed how quickly the animal adapted to the water. Over 4 million years, successive species thickened their dense bones to act as ballast, an adaptation shared with more familiar marine mammals like manatees and whales.
Sounding the Fathoms with Cotylacara
It’s tough being a land animal trying to make it in the ocean. For one thing, land-based eyes are adapted for seeing through the air. Underwater, the density of the fluid messes with our focus and it’s harder to see clearly. Whales’ eyes are adapted to seeing clearly under the waves, but water is also very good at absorbing light, making it harder for anything to see far down in the ocean. Toothed whales, technically called Odontocentes, share an ingenious adaptation for “seeing” clearly underwater – no matter the light conditions – using sound.
A lot like a submarine using sonar to explore the depths of the ocean, toothed whales produce high pitched sounds that bounce off objects around them, sending echoes back to the whale. The whale then assembles these sound waves in their brains so that they can “see” the sea lion frolicking ahead or the rugged reef below. Baleen whales, called mysticetes, are the giant krill-swallowing animals like blue whales and humpback whales and they do not have the ability to echolocate.
In order to echolocate, a toothed whale produces a high pitched sound at the phonic lips. Not be be confused with the lips around your mouth, the phonic lips of a whale actually sit below the blowhole. The muscles used to make the echolocating sound are the same muscles used to wiggle your nose! In baleen whales, these muscles are small, but in toothed whales they are well developed with large attachment sites on the skull. If paleontologists wanted to understand how long ago echolocation evolved, they needed to find an early skull with the attachment sites for phonic lip muscles. Drum roll, please…
Enter Cotylacra, the oldest echolocating whale! The 28 million year old fossil whale is from Berkeley County, South Carolina, where it was discovered in a drainage ditch in a subdivision. The paleontologists working on the discovery, a team from New York, Texas, and South Carolina, recognized the new species as an early relative of toothed whales based on its teeth and other details of its skull. They also noticed the faint attachments for the phonic lips near where the blowhole would have been. They observed the skull had space for the melon, an organ mostly made of fat that focuses the sounds made at the phonic lips.
Cotylacara, meaning “open head” in reference to special divots on its forehead, does not have the well-developed echolocation anatomy sported by modern whales but its skull suggests that it could have produced and received the noises that made its watery home easier to navigate. This new fossil bridges the gap between ancient whales without echolocation and modern toothed whales with a sophisticated ability to see with sound. While paleontologists did not know exactly where they would find the first echolocator or how old it would be, they did predict that some of the oldest toothed whales would have evidence for echolocation ability in their skulls.
But sometimes the fossil record throws up something we never could have seen coming…even with sonar…
Probing the Depths with Semirostrum
Near San Diego, California, paleontologists discovered a strange porpoise with a long, jutting lower jaw. Its chin projected out from below its upper jaw, nearly doubling the length of the whale’s face! That’s unlike anything seen in modern whales, fossil whales…or any other mammal! There are plenty of whales with long, narrow beaks but why would Semirostrum, meaning “half nose”, have only this giant chin?
Using the same digital scanning technology doctors use to get an idea of what’s going on inside the human body, the paleontologists scanned the skull of the Semirostrum and found the jaw had large canals for branches of the mental nerve, the same nerve that emerges on your chin and provides feeling along your jaw. Semirostrum’’s mental nerve had lots of places to leave the mandible, making the long chin a highly sensitive structure. Today there are birds that have long lower beaks like the skimmer. Skimmers use their beaks to explore the water and sediment for food. Maybe Semirostrum used its sensitive chin the same way, slowly moving above the sediment, probing for a dinner of clams, oysters, and other invertebrates? That is the hypothesis the paleontologist team presents for Semirostrum.
The paleontologists who studied Semirostrum found more support for this slow-moving feeding behavior, called benthic feeding, in the skeleton of the new whale. The shoulder and chest bones of Semirostrum had large muscle attachments, showing this was a beefy porpoise. River dolphins are living relatives of Semirostrum, and some species of river dolphin have very similar large muscle attachments for big flipper muscles. These big muscles allow river dolphins to move precisely, if not quickly, through their murky habitat. Semirostrum was beautifully suited to cruising the California shoreline 4 million years ago, equipped with echolocation inherited from Cotylocara-like ancestors, strong, precise flipper muscles like a river dolphin, and a special, sensitive mandible perfect for searching for buried prey.
A little further south there was something even weirder slowly moving along the Pacific shoreline…
Lazy Jones’ Locker
Paleontologists knew there had to be a toothed whale ancestor that had early adaptations for echolocation, because all living toothed whales share this adaptation. Paleontologists did not expect a porpoise with a long lower jaw, but it seems like a reasonable adaptation that other animals have converged on. But it’s hard to think of a description paleontologists were less prepared for than: SWIMMING SLOTH.
Sure modern tree sloths have occasionally been fished out of rivers, swimming from bank to bank, but this is not normal behavior for the bemused South American mammals. The giant relatives of the small tree sloths were lumbering, shaggy animals with weirdly shaped feet, massive claws, and skulls highly adapted to chewing a lot of plants. Some of these ancient ground sloths were the size of cats, whereas others were the size of elephants! If there was a group of animals paleontologists never expected to find cavorting in the ocean, it was sloths.
But the fossil record doesn’t know (nor care) what we expect. Instead, it preserves life as it was in all of its unexpected diversity, including the remains of Thalassocnus, meaning the “lazy one of the sea”. This genus was first found in the Pisco Formation of Peru in 1995. Based on the rocks that surrounded the fossils, paleontologists knew the animal must have been far out to sea. But there weren’t a lot of obvious indicators that this animal had its sea-legs: No flippers, no blowhole, no flukes.
But Thalassocus did have thick bones.
A big problem for any animal with lungs trying to make a living in the ocean is it is constantly fighting the air that keeps it alive. Air is far less dense than water, so an air-filled structure has a strong tendency to float. Fish, sharks, crabs: none of these animals need to carry around bags of air to help them breath. But whales, manatees, plesiosaurs, and penguins have all evolved thickened bones to help add a little heft to balance their lungs. And sloths did it, too. This adaptation and the discovery of skeletons in marine deposits is convincing evidence that there were once human-sized sloths moving below the waves.
Bones can be roughly divided into two sections: the thick “cortical” bone around the rim which takes on the support duties, and the trabecular bone at the core where fatty marrow and thin struts reside. If you eat chicken or ribs, remember how the bone looks thick and pale on the outside and brown and mushy inside? The outer portion is the cortical bone, whereas the inner portion is the trabecular bone. Animals adapting to a marine lifestyle evolve thick cortical bone relative to their internal trabecular bone so they have ballast to counteract their buoyant lungs. This bone thickening and swelling, called “osteosclerosis” and “pachyostosis”, has been observed in the early relatives of whales and sea cows. These lineages still had four limbs, but their ticketed bones show they are getting ready for a life at sea. Thickened bone proceeds other, more elaborate adaptations to the water.
Because the Pisco Formation preserves 8 million years of geological history, a recent study of Thalassocnus was able to show the gradual thickening of the animal’s bone through these 8 million years. This record of gradually thicker, stouter bone is the most detailed record of this critical adaptation to life in the water ever recorded for any marine animal. The paleontologists working on the project also discovered sloths and anteaters (close relatives of sloths) generally have thicker bone than other mammals, so giant sloths may have been had a slight evolutionary advantage to making the transition to the water; their bones might have been thick to being with! Maybe sloths aren’t that unexpected in the water after all!
Just because paleontologists have solid evidence that sloths were in the water, that doesn’t mean we know exactly what they were doing there. But there are good hints. Through the course of Thalassocnus’s evolution, it’s skull became narrower, and it’s jaw more spoon-like. The sea sloth seems to be adapting to a manatee-like lifestyle, fueling itself with sea grass and other marine plants. The giant sea sloth would not have been the most agile creature off the Peruvian coast, but agility isn’t a prerequisite for a happy life at sea. Manatees are fully adapted to life in the ocean, but they aren’t exactly jumping through hoops.
This study demonstrates that Thalasocnus, whose existence may have been unexpected, adapted to its marine habitat in expected ways. In fact, Thalassocnus may provide the best record known of the key evolutionary transition from land-based bone architecture to ocean-based bone architecture of ANY animal.
The first echolocating whale, Cotylacara. A giant-chinned porpoise, Semirostrum. The evolution of a swimming sloth, Thalassocnus. Each discovery is a mix of the expected and the unexpected, but all can be understood with the tools available to paleontologists from physiology, anatomy, radiology, geology, and wildlife biology. Each discovery tests old ideas and helps form new questions.
That’s why it’s a paleontologists life for me!
At the Water’s Edge by Carl Zimmer 1998. If you want to learn more about whale evolution and the elaborate evolutionary transitions required to get back into the water, spend some time with this accessible book. Zimmer, a spectacular science writer, takes a hard look at how animals got out of the water and started walking, then turned around and lost their legs to start swimming again.
How Whales Walked into the Sea by Faith McNulty Illustrated by Ted Rand. 1999. This book is aimed at people between 7 and 10. It may be a little out of date (we know hippos are the close relatives of whales, not mesonychids), but it still provides a solid introduction to the strange story of whale evolution.
Return to the Sea: The Life and Evolutionary Times of Marine Mammals by Annalisa Berta Illustrated by James L. Sumich and Carl Buell. 2012. When paleontologists grapple with the adaptations required to get mammals back into the water, we usually focus on whales, but plenty of other mammals have made the transition and have great evolutionary stories to tell to prove it. Berta tackles them all including seals and sea cows. In the end, Berta puts marine mammals into a modern context, exploring how humans are affecting the diversity and habitats of these fascinating, endangered creatures.
Geisler, Jonathan H., Matthew W. Colbert, and James L. Carew. 2014. A new fossil species supports an early origin for toothed whale echolocation. Nature 508: 383-386 doi:10.1038/nature13086
Amson, Eli, Christian de Muizon, Michel Laurin, Christine Argot, and Vivian de Buffrènil. 2014. Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru. Proceedings of the Royal Society: Biological Sciences 281: 20140192. http://dx.doi.org/10.1098/rspb.2014.0192
Racicot, Rachel A., Thomas A. Demere, Brian L. Beatty, and Robert W. Boessenecker. 2014. Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California. Current Biology 24: 774-779. http://dx.doi.org/10.1016/j.cub.2014.02.031
Filed under: Cenozoic, Cetacea, Convergence, Eocene, Field Guide, Fossils, Functional Morphology, Marine, North America, Oligocene, Paleontology, Pliocene, Podcast, South America, Xenarthra, dolphin, echolocation, marine biology, ocean, porpoise, sloth, whale