Tooth Talk: How Teeth Evolved Into ‘Ultimate Cutting Tools’

The largest shark that ever lived – the Megalodon - meaning "big tooth" - is an extinct species of shark that lived approximately 23 to 3.6 million years ago. It is known only from its gigantic bladelike teeth, which can be more than seven inches long. These teeth are described by some scientists as the "ultimate cutting tools," and took millions of years to evolve into their final form. Megalodon's earliest ancestor - Otodus obliquus - sported three-pronged teeth that could have acted like a fork for grasping and tearing fast-moving fish. In later megatooth shark species, teeth flattened and developed serrated edges, transitioning to a knifelike shape for killing and eating fleshy animals like whales and dolphins.

The final tooth evolution in this lineage of powerful predators took 12 million years, a study shows. An analysis of teeth from megalodon and its immediate ancestor, Carcharocles chubutensis, traced the unusually slow, gradual shift from a large tooth flanked by mini-teeth - known as lateral cusplets - to teeth without these structures. "This transition was a very long, drawn-out process, eventually resulting in the perfect cutting tool - a broad, flat tooth with uniform serrations," said study lead author Victor Perez, a doctoral student in geology at the Florida Museum of Natural History. "It's not yet clear why this process took millions of years and why this feature was lost."

Teeth offer a wealth of information about an animal, including clues about its age, when it lived, its diet and whether it had certain diseases. Megalodon's teeth suggest its hunting style was likely a single-strike tactic, designed to immobilize its prey and allow it to bleed out. "It would just become scavenging after that," Perez said. "A shark wouldn't want to grab and hold onto a whale because it's going to thrash about and possibly injure the shark in the process."

The researchers carried out a "census of teeth," analyzing 359 fossils with precise location information from the Calvert Cliffs on the western shore of Maryland's Chesapeake Bay - an ocean in C. chubutensis and megalodon's day. The cliffs provide an uninterrupted rock record from about 20 to 7.6 million years ago, a period that overlaps with these megatooth sharks. The researchers noted a consistent decrease in the number of teeth with lateral cusplets over this time span. About 87 percent of teeth from 20 to 17 million years ago had cusplets, falling to about 33 percent roughly 14.5 million years ago. By 7.6 million years, no fossil teeth had cusplets.

Shark Teeth And Gum Disease

Adult C. chubutensis had cusplets while adult megalodon did not, but this feature is not a reliable identifier of which species a tooth belonged to. Juvenile megalodon could have cusplets, making it impossible to discern whether a tooth with cusplets came from C. chubutensis or a young megalodon. Some teeth analyzed for the study had tiny bumps or pronounced serrations where cusplets would be. A set of teeth from a single shark had cusplets on some, no cusp lets on others, and replacement teeth with reduced cusplets.

Paleontologists cannot pinpoint exactly when megalodon originated or when C. chubutensis went extinct, said Perez, who began the project as an intern at the Calvert Marine Museum. "As paleontologists, we can't look at DNA to tell us what is a distinct species. We have to make distinctions based off of physical characteristics. We feel it's impossible to make a clean distinction between these two species of sharks. In this study, we just focused on the evolution of this single trait over time."

Lateral cusplets may have been used to grasp prey, which could explain why they disappeared as these sharks shifted to a cutting style of feeding. Another possible function was preventing food from getting stuck between the sharks' teeth, which could lead to gum disease. But if the cusplets served a purpose, why lose them? "It's still a mystery," Perez continued. "We're wondering if something was tweaked in the genetic pathway of tooth development."

Elephant Weight Affected By Changing Teeth

Elephants usually have 26 teeth: the incisors, known as the tusks, 12 deciduous premolars, and 12 molars. Unlike most mammals, which grow baby teeth and then replace them with a single permanent set of adult teeth, elephants are polyphyodonts that have cycles of tooth rotation throughout their lives. Teeth are not replaced by new ones emerging from the jaws vertically as in most mammals. Instead, new teeth grow in at the back of the mouth and move forward to push out the old ones.

The first chewing tooth on each side of the jaw falls out when the elephant is two to three years old. The second set of chewing teeth falls out at four to six years old. The third set falls out at nine to 15 years of age, and set four lasts until 18 to 28 years of age. The fifth set of teeth falls out at the early 40s. The sixth and usually final set must last the elephant the rest of its life. Elephant teeth have loop-shaped dental ridges, which are thicker and more diamond-shaped in African elephants.

The teeth of most mammals, including humans, are only replaced once in a lifetime, when the milk teeth give way to the permanent teeth. This one change is enough to adapt to the increasing size of the jaw. But elephants increase greatly in size and weight over the course of their lives - from a starting weight of 100 kilograms to several tons in adulthood. One single change of teeth would not be enough for the enormous growth of the jaw. That's why the teeth of elephants are replaced a total of five times over their lifespan. On each side of the jaw they have only one single tooth in use at a time which is slowly pushed forwards by a new bigger tooth out of the mouth, breaking off in pieces. If you look inside an elephant's mouth you will see either only one single tooth or pieces of the old tooth behind which part of the new tooth is pushing through, a process that is called molar progression.

As a result of this process, the elephants' chewing surface gets bigger when two teeth are present on one side at the same time, and then smaller again when there is only one tooth on each side. For that reason there are times when it is easier for the animals to eat more or chew the same amount more finely and increase the intake of digestible food. Researchers at the Vetsuisse Faculty of the University of Zurich have now observed weight fluctuations in elephants living in zoos, which can be explained by these changes of teeth. "We actually wanted to find out whether zoo elephants that have offspring are lighter than those who have not reproduced," says Marcus Clauss of the Clinic for Zoo Animals, Exotic Pets and Wildlife.

Christian Schiffmann, a Ph.D. candidate, visited nearly every zoo in Europe and recorded the weight of the elephants. The researchers noticed a pattern - the animals continually gained weight from childhood to adulthood, and then their bodyweight fluctuated by 300 kilograms in long cycles of around a hundred months. "At first we thought it might have something to do with the seasons or with reproduction," says Schiffmann. "But the cycle is a lot longer than one year, and we found the same pattern in groups that were not reproducing. The only other plausible explanation was the unusual tooth change process in elephants."

As elephants reproduce all year round but do experience seasonal fluctuations in the amount of food available, animals of various ages and tooth stages have access to differing quality and quantity of food. The weight of these elephants is therefore influenced by other factors alongside the change of teeth. It is only in zoos, where food availability is comparatively stable, that the pattern can be clearly observed. This study is a great example of how research into zoo animals can provide new biological findings that would not be possible by observing animals in the wild.

Tooth Fossils Fill Six-Million-Year-Old Gap In Primate Evolution

Researchers have used fossilized teeth found near Lake Turkana in northwest Kenya to identify a new monkey species. This discovery helped fill a six-million-year gap in primate evolution. Understanding the evolution of Old World monkeys is important because, along with the great apes and humans, they belong to the anthropoid group of primates - primates that resemble humans. “The monkey fossil discovery grew out of a more extensive study of a section of sedimentary rocks in Kenya that contain a large number of different types of fossils, including several hundred mammal and reptile jaws, limbs, and teeth,” says UNLV geoscientist Terry Spell, a member of the international research team that discovered the species that lived 22 million years ago. “This adds to our understanding of the earliest evolutionary history of Old World monkeys, including changes in their diet with time to include more leaves. Monkeys originated at a time in the past when Africa and Arabia were together as an island continent. Plate tectonic motions pushed this land mass into the Eurasian land mass at 20 to 24 million years ago, and an exchange of animals and plants occurred. It is unclear if competition with newly introduced species or changing climate conditions drove changes in diet.”

Scientists named the newly discovered monkey species Alophia (“without lophs”) due to the lack of molar crests on its teeth - a phenomenon that sets them apart from geologically younger monkey fossils. Old World monkeys are the most successful living superfamily of nonhuman primates with a geographic distribution that is surpassed only by humans. The group occupies a wide spectrum of land to tree habitats and have a diverse range of diets. They evolved to develop a signature dental feature of having two molar crests which to this day allows them to process a wide range of food types found in the varying environments of Africa and Asia.

The Teeth Of Alaska Lake Seals

Hundreds of harbor seals live in Iliamna Lake, the largest body of freshwater in Alaska and one of the most productive systems for sockeye salmon in the Bristol Bay region. Although how the seals first colonized the lake remains a mystery, it is thought that sometime in the distant past, a handful of harbor seals likely migrated from the ocean more than 50 miles upriver to the lake, where they eventually grew to a consistent group of about 400.

Scientists now know these "colonizing" seals must have found the lake suitable enough to stay and raise their offspring. Generations later, the lake-bound seals appear to be a genetically distinct population from their ocean-dwelling cousins even though they are still managed as part of the larger Eastern Pacific harbor seal population. But if the lake seals are distinct and show signs of local adaptation to their unique ecological setting, this would mean that their conservation especially in the face of the rapidly changing climate of western Alaska and proposed industrial developments should differ from that of nearby marine populations.

Lifelong chemical records stored in their sequentially growing canine teeth show that the Iliamna Lake seals remain in freshwater their entire lives, relying on food sources produced in the lake to survive. In contrast, their relatives in the ocean are opportunistic feeders, moving around to the mouths of different rivers to find the most abundant food sources, which includes a diverse array of marine food items in addition to the adult salmon returning to Bristol Bay’s nine major watersheds. These findings are described in a paper published in Conservation Biology. "We clearly show these seals are in the lake year-round, throughout their entire lives," says lead author Sean Brennan, a postdoctoral researcher at the University of Washington's School of Aquatic and Fishery Sciences. "This gives us critical baseline information that can weigh in on how we understand their ecology, and we can use that information to do a better job developing a conservation strategy."

The study comes at a time when federal agencies are considering whether to permit mining activities in Bristol Bay, a region teeming with wildlife, including Alaska sockeye salmon. Iliamna Lake, and the seals and other animals that live there, is located in the heart of the proposed Pebble Mine project. The U.S. Army Corps of Engineers recently released a draft environmental impact statement that analyzes the project's proposal, presents alternative plans and gives the public a chance to comment. Ultimately, the document will help decide whether the controversial mine is approved.

Chemical Signatures In Teeth

Iliamna Lake harbor seals, because of their current conservation status, aren't assessed as a distinct and ecologically significant population in the project's draft environmental impact analysis. If the seals are determined to be a distinct population, that has important implications for how the Iliamna Lake system is managed, the study's authors said. The lake and its resident fishes would then be considered critical habitat for seals. Separately, federal regulators have considered whether the lake seals should be named a distinct population, but scientists have been unable to agree on whether the seals are both distinct, and ecologically and evolutionarily significant, mainly because little is known about their ecology including whether adult lake seals potentially migrate to the ocean to feed each year.

Brennan was a doctoral student at the University of Alaska Fairbanks when he heard about early efforts to evaluate whether the lake seals were a distinct population. Chemical tracing methods he was using to track the life patterns of salmon could also work for the seals, he realized. "The light just went off in my head," Brennan said. "What I was doing for salmon was directly applicable to this population of seals." Brennan and collaborators at the UW, University of Utah and University of Alaska Anchorage looked at the chemical signatures present in the teeth of lake seals during each year of their life to better understand where they moved and what they ate. Specifically, the scientists drilled into the growth lines of the seals' canine teeth, then measured the ratio of heavy and light isotopes of carbon, oxygen, and strontium present in each growth layer.

Because of the young bedrock geology of the Kvichak River watershed, which encompasses Iliamna Lake, strontium isotope levels in the ocean are consistently much higher than in the lake. Unlike other elements, strontium signatures in mammal teeth directly reflect what animals assimilate from their environment, in particular, what they eat. Therefore, by looking at the strontium isotope ratios over the course of a seal's life, the researchers saw that the ratios were consistent with lake signatures meaning these seals only live in Lake Iliamna, depend principally on fish produced within the lake, and do not migrate to the ocean.

They also determined that young seals eat very little adult sockeye salmon. But later in life, the seals shift to supplement their diets with the seasonally abundant sockeye salmon that return each summer to the lake. The researchers say this method could be used to better understand the life patterns of other elusive mammals around the world, such as river dolphins in the Amazon or the Mekong Basin. Broadly, marine mammals in coastal regions are among the most endangered animals on Earth. "In terms of the broader picture of aquatic mammal conservation across the globe, I think we show that strontium isotopes can be really powerful because they collapse a lot of uncertainty,” Brennan added. “This method is completely underutilized across the world."

British Teeth Research Reveals Ancient Diets

Researchers analyzing the teeth of Britons from the Iron Age to the modern day have unlocked the potential for using proteins in tooth tartar to reveal what our ancestors ate. Dental plaque accumulates on the surface of teeth during life and is mineralized by components of saliva to form tartar or "dental calculus," entombing proteins from the food we eat in the process. Identifying evidence of many foods, particularly plant crops, in diets of the past is a challenge as they often leave no trace in the archaeological record. But proteins are robust molecules that can survive in tartar for thousands of years.

Archaeological tooth tartar has previously been shown to preserve milk proteins, but the international study, led by researchers at the University of York and the Max Planck Institute for the Science of Human History, proved for the first time that it can also reveal more precise information about a wider range of food proteins, including those from plants. The discovery could provide new insights into the diets and lifestyles of our ancestors, adding to the value of dental remains in our understanding of human evolution. The team plans to use the results of this study to help refine their protein-detection methods, and to explore particular problem areas of ancient diet research.

This approach may be particularly useful in the detection of understudied vegetative crops, especially in regions where macrobotantical remains are not preserved. "It may offer a more precise way of identifying foodstuffs compared to other methods such as ancient DNA and isotope analysis as it can distinguish between different crops and indicate whether people were consuming dairy products, like milk or cheese." says Dr. Camilla Speller, senior author from the Department of Archaeology at the University of York. Analyzing 100 archaeological samples from across Britain, as well as 14 samples from living dental patients and recently deceased individuals, the research team found that potential dietary proteins could be found in about one third of the analyzed samples. "In the teeth we look at from individuals who lived around the Victorian era we identified proteins related to plant foods, including oats, peas and vegetables in the cabbage family,” Speller continued. “Occasionally, we find evidence of milk and oats in the same mouth - I like to think it's from eating porridge!"

In the modern samples, the researchers found proteins that reflected a global British diet, such as those related to potatoes, soybeans and peanuts, as well as milk proteins. "While there is still a lot we don't know, this is exciting because it shows that archaeological dental calculus harbors dietary information, including food products that ordinarily do not survive in archaeological sites," added Dr. Jessica Hendy, first author from the Department of Archaeology at the Max Planck Institute in Germany.

Ancient Dental Plaque Nicotine Extracted

A team of scientists including researchers from Washington State University has shown for the first time that nicotine residue can be extracted from plaque, also known as "dental calculus," on the teeth of ancient tobacco users. Their research provides a new method for determining who was consuming tobacco in the ancient world and could help trace the use of tobacco and other intoxicating plants further back into prehistory. "The ability to identify nicotine and other plant-based drugs in ancient dental plaque could help us answer longstanding questions about the consumption of intoxicants by ancient humans," says Shannon Tushingham, a WSU assistant professor of anthropology and co-author of a study on the research in Journal of Archaeological Science Reports. "For example, it could help us determine whether all members of society used tobacco, or only adults, or only males or females."

Tracing the ancient spread of tobacco in the Americas has traditionally relied on the presence of pipes, charred tobacco seeds and the analysis of hair and fecal matter. However, these items are rare in the archeological record and are hard to link to particular individuals. As a result, tobacco use has been difficult to document archeologically. On the other hand, dental plaque adheres to the surfaces of teeth and mineralizes over time, preserving a wide range of substances that are in the mouth. It is easy to link to particular individuals because it can be removed directly from teeth. Nevertheless, dental plaque was largely ignored by archaeologists until recently. However, using modern and highly sensitive instrumentation, scientists have found they can detect and characterize trace amounts of a wide variety of compounds, including proteins, bacterial DNA, starch grains and other plant fibers in dental plaque.

Because nicotine is detectable in the dental plaque of contemporary smokers, Tushingham and her collaborators wanted to find out if it also preserved in plaque taken from people who lived long ago. She and David Gang, a professor in the WSU Institute of Biological Chemistry, Korey Brownstein, a graduate student in the WSU Molecular Plant Sciences Program, and Jelmer Eerkens, an anthropologist at the University of California, Davis, collaborated with members from the Ohlone tribe in San Francisco Bay to extract plaque from the teeth of eight individuals, buried between 6,000 and 300 years ago, and analyze it for nicotine. Using ordinary dental picks, Eerkens and his team at UC Davis extracted the dental plaque from the ancient teeth and then sent it to Tushingham and Gang's labs at WSU for analysis. The WSU researchers used liquid chromatography-mass spectrometry to test the samples for nicotine and other plant-based drugs like caffeine and the muscle relaxant atropine.

Among the samples they analyzed, two tested positive for nicotine, demonstrating for the first time that the drug can survive in detectable amounts in ancient plaque. One of these individuals, an adult man, was also buried with a pipe. A surprise came from the molar of an older woman, which also tested positive for nicotine. "While we can't make any broad conclusions with this single case, her age, sex, and use of tobacco is intriguing," Eerkens said. "She was probably past child-bearing age, and likely a grandmother. This supports recent research suggesting that younger adult women in traditional societies avoid plant toxins like nicotine to protect infants from harmful biochemicals, but that older women can consume these intoxicants as needed or desired."

While the researchers did not detect evidence of any other plant-based drugs in this particular study, they believe dental plaque could be used help trace the use and spread of other intoxicants as well. "We think a wide variety of plant-based, intoxicating chemicals could be detected in ancient dental plaque," Brownstein said. "It really opens up a lot of interesting avenues of discovery."


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