Millions of years ago, an early relative of modern mammals had a tiny, saddle-shaped bony structure connected to the jaw. Today, that set of bones, called the hyoid, help all mammals to chew and swallow. Now scientists say the mammal hyoid may be one secret to our eventual success. It enabled mammals to spread into all the different ecological niches they occupy today.
Microdocodon gracilis was a shrew-sized animal. It lived about 165 million years ago in east Asia. Chang-Fu Zhou led the team that examined an M. gracilis fossil. He’s a vertebrate paleontologist. He works at the Paleontological Museum of Liaoning in Shenyang, China. The team discovered that the fossil included a beautifully preserved hyoid. That structure bears a striking resemblance to the hyoids of modern mammals. The researchers reported their findings July 19 in Science.
When it comes to food, mammals have found ways to survive in many types of environments. Modern species have teeth specially adapted for their differing diets. Large carnivores like lions and tigers have sharp, cutting blades. Some small mammals have high cusps on their teeth to help crunch insects. Other species have ridge-packed teeth to help grind down plants.
But one thing all mammals have in common is that we chew. That breaks food into tiny pieces before swallowing. That’s unlike, say, reptiles. Those animals swallow food whole, says Zhe-Xi Luo. He’s a vertebrate paleontologist at the University of Chicago in Illinois. Also, mammals’ mouths, throats and tongues are designed to be flexible and strong enough to suckle milk. This is a defining trait of mammals.
“The food transport and the swallowing of the chewed-up food is all controlled by muscle related to this highly mobile bone” called the hyoid, Luo says.
All jawed vertebrates, from fishes to felines, have a hyoid. But in mammals the hyoid is uniquely mobile. Luo likens it to a child’s backyard swing, with a curved seat suspended by two chains. “[The hyoid] can bend and is flexible, like a kid swinging back and forth,” he says.
Scientists didn’t know if the evolution of the flexible hyoid pre-dated the rise of mammals or came later. The Jurassic Period dates to between 201 million years and 145 million years ago. In that time, early mammalian relatives were already diversifying in terms of food niches. And they had a variety of different kinds of teeth. That suggested the animals likely had some sort of mobile hyoid, Luo says. But what the bones really looked like remained unclear.
Enter M. gracilis. It represents a key stage in mammal evolution. Early mammal relatives that lived in the late Permian and Triassic periods, before the Jurassic, had a rigid hyoid. Mammals that arose during the Cenozoic Era had a mobile hyoid. The Cenozoic stretches from 65 million years ago to modern times. It is sometimes called the Age of Mammals.
M. gracilis is the first Jurassic mammal relative found with a well-preserved hyoid. This enabled researchers to see the distinctive shape. “Once we knew what to look for, we started to search for … similar structures in other extinct mammal lineages,” Luo says. “And we found them left and right.”
That suggests this structure was common among mammal relatives from the Jurassic. And it may have been an important part of mammals’ ultimate ability to diversify, the researchers say. “Mammals just chewed and swallowed themselves into evolutionary success,” Luo says.
The scientists realized something else as well. The presence of a mobile hyoid pre-dates the evolution of another defining feature of mammals. This is the three middle ear bones, and their separation from the jaw. Those ear bones are called the malleus, incus and stapes. They are the secret to mammals’ sharp hearing.
Some thought that separation might have also led to development of the mobile hyoid in mammals. Instead, this new fossil evidence suggests these two adaptations were not directly related.
That was a particularly interesting aspect of the discovery, says vertebrate paleontologist Simone Hoffmann. He’s at the New York Institute of Technology in Old Westbury. He coauthored a commentary in the same issue of Science.
“The hyoid is used for chewing, but also for suckling milk, which is very closely tied to mammals,” says Hoffmann. “And [this fossil] suggests the hyoid bones may have developed way before mammals themselves. It might change our understanding of what attributes we typically think of as mammal attributes.”
adaptation (in biology) A process by which an organism or species becomes better suited to its environment. When a community of organisms does this over time, scientists refer to the change as evolution.
carnivore An animal that either exclusively or primarily eats other animals.
Cenozoic Era The most recent of the three major time period defining animal history. The Cenozoic spans roughly 65 million years, from the extinction of the non-avian dinosaurs.
commentary (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.
development (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.
diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).
evolutionary An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.
extinct An adjective that describes a species for which there are no living members.
feline Adjective for something having to do with cats (wild or domestic) or their behaviors.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
incus One of the bones of the middle ear. The word means “anvil” in Latin. It transfers sound vibrations from another bone, called the malleus, to a third, known as the stapes.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
Jurassic Lasting from about 200 million to 145.5 million years ago, it’s the middle period of the Mesozoic Era. This was a time when dinosaurs were the dominant form of life on land.
malleus One of the bones of the middle ear. The word means “hammer” in Latin. It transmits the vibrations of the eardrum to another bone called the incus.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
muscle A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.
niche A small or narrow pocket that sets something apart, or perhaps offers a region of protection. (In ecology) The term for the role that an organism plays in its community.
paleontologist A scientist who specializes in studying fossils, the remains of ancient organisms.
Permian A time in the distant geologic past, about 250 million to 300 million years ago. Many reptiles rose to prominence on land; these were not yet dinosaurs. Many large invertebrates ruled the oceans during this period. But most would die off at the end of the Permian, as it gave way to a new geologic period known as the Triassic.
reptile Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.
shrew A mouse-sized, insect-eating mammal. Related to moles, it’s chiefly active at night. Shrews have a long, pointed snout and tiny eyes. Despite looking somewhat mouse-like, a shrew is not a rodent (which a mouse is).
species A group of similar organisms capable of producing offspring that can survive and reproduce.
stapes One of the bones of the middle ear. The word means “stirrup” in Latin. It transfers sound vibrations from another bone, called the incus, to the inner ear.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
trait A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.
Triassic Period A time in the distant geologic past, about 200 million to 250 million years ago. It’s best known as the period during which dinosaurs first emerged.
variety (in agriculture) The term that plant scientists give to a distinct breed (subspecies) of plant with desirable traits. If the plants were bred intentionally, they are referred to as cultivated varieties, or cultivars.
vertebrate The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.
Journal: C.-F. Zhou et al. New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science. Vol. 365, July 19, 2019, p. 276. doi: 10.1126/science.aau9345.
Journal: S. Hoffmann and D.W. Krause. Tongues untied. Science. Vol. 365, July 19, 2019, p. 222. doi: 10.1126/science.aay2061.