Embryos collected from beached whales hold the key to a little-understood slice of whale evolution [Image credit: wellcome images]
Jars of pale whale embryos line the shelves in a corner of the Large Mammal Warehouse in Vernon, Calif., used by the Natural History Museum of Los Angeles County. Some of these embryos are an inch or two long, smaller than your little finger. Others are nearly two foot in size, and have already begun to resemble large-headed whales with minute flippers and closed eyes. They are long dead (the oldest specimens may be over a hundred years old), retrieved one by one from beached whales that rescuers could not send back to sea. The embryos were then preserved in formalin, a solution of formaldehyde in water, and tucked away in the warehouse for future scientists to study.
Now, a few of those embryos have helped illuminate a little-understood sliver in the evolution of cetaceans, the order of marine mammals to which whales and dolphins belong (dolphins, like orcas and sperm whales, fall into the suborder of odontoceti, or toothed whales). In a study published on Feb.19 in the journal PeerJ, researchers scrutinized both fossils and embryos to investigate how modern whales evolved teeth that are very different in shape and number from those of most mammals.
In examining the embryos, researchers found a gene called Bmp4, which is responsible for forming incisors, teeth used for biting into food. It is typically found at the front of mammalian jaws, including ours, but Armfield’s team found that it was also expressed at the back of the jaw in a few embryos belonging to the pantropical spotted dolphin, Stenella attenuata. This difference, they believe, allows more incisor-like teeth to develop in both the back and front of the jaw.
Around 34 million years ago, the ancestors of today’s whales experienced a sea change in the arrangement of their teeth. They began to grow more teeth than most mammals, but these teeth lacked the variety and complex cusps of those belonging to other species, such as pigs and people. The result is that most whales today have scores of simple teeth that largely resemble each other. The broad expression of Bmp4 in the embryos the researchers examined could explain how this happened.
“We’d never seen that in another mammal,” says Brooke Armfield, the study’s lead author, a researcher at the University of Florida in Gainesville who studies the genetics underlying the evolution of mammalian teeth.
Teeth, whether they are fossilized or developing within an embryo, allow scientists a glimpse into a mammal’s life and history. Their cusps and points are riddled with clues to matters ranging from what an animal ate to how the path of its evolution unspooled over millions of years. And, in the case of cetaceans, their changing contours trace a history that spans land and sea.
Georges Cuvier, the renowned French paleontologist and anatomist from the seventeenth and eighteenth centuries, is said to have once proclaimed, “Show me your teeth and I will tell you who you are.”
And, in an evolutionary sense, they can reveal where you have been.
Teeth are formed from an outer layer of shiny enamel and inner layer of dense tissue called dentine, but these organic materials actually make up only a small percentage of the substance found in teeth. The enamel of cetacean teeth is roughly 97 to 98 percent minerals (mostly calcium and phosphate, along with traces of sodium, strontium, and fluoride), while the dentine is roughly 70 percent minerals. By comparison, bones are about 50 to 60 percent mineral. While soft tissues decompose and are not preserved, highly mineralized structures like teeth are tougher and more likely to stick around as fossils.
This is fortunate for scientists, as there is a lot of information to be gleaned from fossilized teeth. They can tell us how old an animal was based on how many layers they have, like the rings in trees. Fossilized teeth can additionally reveal information about the diet of an animal, based on their shape and how worn they are.
Most mammals have several different kinds of teeth in their mouths that can be grouped into incisors, canines, pre-molars, or molars. All of them serve a different purpose when a mammal begins to chew, such as tearing food up (canines) or grinding it down (molars).
They also fit neatly together towards the back of the jaw, top teeth to bottom, like the two halves of a friendship necklace. This property is called “precise occlusion,” and it does not exist in modern whales. Nor do whales chew their food, as they lack the complex teeth with many raised cusps that other mammals — including us — have.
“They just swallow and eat [their] prey whole,” says Carolina Loch, a PhD candidate at University of Otago in Dunedin, New Zealand who studies fossilized dolphin teeth. “If you look at their stomachs you can find completely whole fish. You could basically eat them because they’re just completely intact in there.”
It was not always this way. At one point in time, the ancestors of modern whales chewed their food properly and had a more varied set of teeth. They lived on land, walked on four legs, and were likely herbivorous. But during the Eocene period, about 52 million years ago, they began to spend more and more time in the water, trading limbs for flippers and tail flukes as they became fully aquatic.
They began “taking their jaws and snapping them to catch prey in the water, similar to what you would see in an alligator,” says Armfield. They no longer needed to be able to chew their food. Eventually, when the Eocene gave way to the Oligocene, about 34 million years ago, the ancestors of today’s whales began to sprout more, simpler teeth. This trend in cetaceans is a prime example of how evolution helps organisms adapt to their changing environments by altering how they express genes such as Bmp4.
Today, dolphins like the ones Armfield and her colleagues studied come the closest to what scientists call true homodonty, the condition of having teeth that are all identical, the extreme end of whales’ evolution away from varied teeth. However, even the pointed pegs that fill dolphins’ mouths are not quite identical.
Instead, “the differences between one tooth and the next adjacent tooth are so small that on the whole it’s more of a continuum,” says Brian Beatty, a paleontologist at the New York Institute of Technology (NYIT) College of Osteopathic Medicine in Old Westbury, who has specialized in marine mammals. Other species are less strongly committed to homodonty, such as the Amazon and Ganges river dolphins, which sport conical teeth in front and molar-like teeth in back, or the narwhal with its enormous protruding incisor, often likened to a unicorn’s horn.
Other whales, such as the gray whale, the humpback whale, and the enormous blue whale, do no have any teeth at all, preferring to strain their food through thick baleen plates. However, at the beginning of the Oligocene, the ancestors of these whales had not yet lost their teeth. They followed the trend of developing more and simpler teeth along with the other predecessors of modern whales.
Typically, studies that examine developing animals are done on mice, which are cheap, easy to house and breed quickly in laboratory conditions. Mice, however, aren’t ideal models for studying teeth; they have a specialized arrangement of teeth that differs from that of most mammals, including whales. Armfield and colleagues decided to compare the dolphin embryos to embryos belonging to both mice and pigs, which are more closely related to cetaceans and possess teeth that are more typical of mammals.
As an organism nears the time of its birth, it takes on more and more characteristics unique to its species. But early in their development, many animal embryos look very similar, and deciphering which genes and proteins an animal uses to put itself together can give scientists a unique window to peek into its evolutionary history. The shape an animal will take depends upon where these genes are expressed in its growing body and when during development they become active. The protein coded by Bmp4, one of the genes Armfield and her team examined, is for instance involved in both limb and tooth development in mammals.
In Armfield’s study, scientists examined two genes, Bmp4 and Fgf8, responsible for setting in motion the growth of incisors and molars, respectively. The protein coded by Bmp4 appeared at the front of the pig, mouse, and dolphin mouths. Additionally, in all three species, the protein associated with the other gene, Fgf8, was found only at the back of the jaw, where molars grow. However, in the dolphins, Bmp4-encoded proteins were also expressed at the back of the jaw.
In other words, the protein responsible for overseeing the development of incisor-like teeth was found throughout the range where dolphins grow teeth. Armfield and her team concluded that this departure from typical mammalian protein arrangement could be the basis for the increased numbers of simple, cone-like teeth seen in so many cetaceans.
Because the back of the jaw now expresses the incisor-inducing gene, says Armfield, “we can interpret that as producing the incisor-like morphology we see in the dolphins,” says Armfield.
So far, the study has met a warm reception.
“I think they demonstrated pretty well that changes in gene expression in the teeth have at least influenced the development of the teeth we see in modern toothed whales compared to the fossils,” says Mark Uhen, an expert on cetacean evolution at George Mason University in Fairfax, Va.
“This is a significant contribution, given that very little is known of the molecular changes and evolutionary patterns involved in tooth differentiation and number in whales,” adds Annalisa Berta, who researches cetacean evolution at San Diego State University, in an email.
The trend towards many teeth, all the same, is well represented in the fossil record, as teeth are more often preserved than other body parts. And, importantly for Armfield and her team, fossilized teeth can reveal more about an animal than its age and diet; they also contain clues to the arc of an organism’s evolution.
In addition to examining embryos in their study, the researchers combed over fossilized teeth and their modern-day descendents, focusing on the Eocene and Oligocene. During the Eocene, the fossils took on a range of forms, or morphologies.
“That’s when they’re really starting to become more and more aquatic,” says Armfield. “They kind of play around with their cusp morphologies in a sense so you see them lose cusps, you see them gain cusps.”
However, by the beginning of the Oligocene this interlude of experimentation with different morphologies had waned, and the trend towards more plentiful and less complex teeth emerged. Armfield and her team suggest that these characteristics are tied together, with the teeth they examined becoming more uniform over time as they increased in number.
What makes this study unique is that the researchers investigated both how cetacean teeth physically changed over millions of years and how it might have happened on a mechanistic level.
“Maybe it is not a definitive answer, but it is a really interesting explanation for this,” says Loch. “I think that was a great exercise of understanding an evolutionary problem by using a broad toolset.”
NYIT’s Beatty praises the researchers for tapping knowledge of the connection between the genes Bmp4 and Fgf8 and tooth development and applying it to the dolphin teeth. “I think they’ve done an extraordinary job of aiming at answerable questions,” he says.
Yet many more questions remain, such as how the results compare to gene expression in other cetaceans or what other genes might be involved. And those questions may not be answerable. The collection of dolphin embryos Armfield and her colleagues scrutinized is a rare stroke of fortune, the jar-filled warehouse corner an anomaly. It is, not surprisingly, very rare for pregnant whales to become irrevocably stranded within reach of people who happen to be versed in the art of tissue preservation.
But collecting more specimens is not feasible. Many whales are hard to reach as well as being endangered or on their way to extinction. Besides, no one wants to go kill Flipper to study some proteins.
However, on the rare occasions when a pregnant whale does become stranded and cannot be saved, there is a wealth of information to be mined.