How Mammal Jaw Bones Evolved into Ears

TL;DR

Mammalian jaw bones may have shifted toward the ears because moving them out of the way improved chewing, as the fossil Jeholbaatar kielanae suggests. This mouse-sized multituberculate lived about 120 million years ago and preserves a bone seemingly transitioning from jaw to ear. Its anatomy offers a diet-related hypothesis for the evolution of mammals’ three middle ear bones. Read on to see how chewing and hearing may have shaped that transition.

Transcript

[♪ INTRO] Last week in the journal Nature, researchers announced that they’ve found a new fossil that might help us understand one of the most unique features of mammals: our ears. Also, the little guy is, like, really cute... so there' that. Called Jeholbaatar kielanae, this animal was found by paleontologists in Northeast China. It lived in the C... Read More

Key Insights

• 😂 Jeholbaatar kielanae is a fossil species from the Cretaceous period in China that offers insights into the evolution of mammalian ears and jaw bones.
• 🦻 Mammalian ears have three middle ear bones, enabling better hearing and the detection of higher frequencies.
• 👂 The movement of jaw bones towards the ears in Jeholbaatar improved its grinding power and made it easier to hear over the sound of chewing.
• 😂 The findings suggest a possible connection between diet and the evolution of mammalian middle ears.
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Questions & Answers

Q: How did mammal jaw bones evolve into ear bones?

The malleus and incus evolved from bones that were once in mammalian jaws, but scientists do not fully understand why they moved toward the ears. Jeholbaatar kielanae suggests that moving a bone out of the way of chewing may have enabled a forwards-and-backwards jaw motion, improved grinding power, and made hearing easier during chewing.

Q: What does Jeholbaatar kielanae reveal about ear evolution?

Its fossil preserves an ear-region bone seemingly caught between being part of the jaw and being part of the ear. The bone’s shape and placement led researchers to propose that its movement was associated with chewing rather than changes in brain size reshaping the skull.

Q: When and where did Jeholbaatar kielanae live?

Paleontologists found Jeholbaatar kielanae in Northeast China. It lived during the Cretaceous period about 120 million years ago.

Q: What kind of animal was Jeholbaatar kielanae?

Jeholbaatar was a small mammal not much bigger than a mouse, but it was not a rodent. It belonged to the multituberculates, a successful mammal group that died out about 34 million years ago.

Q: How might Jeholbaatar’s diet have influenced its ear bones?

Jeholbaatar likely ate bugs and plants, and shifting a bone toward the ears got it out of the way of chewing. That change allowed a forwards-and-backwards jaw motion that increased grinding power and helped the animal hear over the sound of its own eating.

Q: What are the three bones in the mammalian middle ear?

The three middle ear bones are the malleus, incus, and stapes. They transfer sound from the eardrum to the cochlea and enable mammals to detect much higher frequencies than other land animals.

Q: How do mammalian middle ears differ from reptile ears?

Mammals have three complex middle ear bones: the malleus, incus, and stapes. Reptiles typically have only one ear bone, the stapes.

Q: Does Jeholbaatar explain how human middle ears evolved?

Not necessarily, because Jeholbaatar’s group was a cousin of our mammal group rather than an ancestor, and it chewed differently. Its fossil instead provides a new hypothesis for how jaw bones can become ear bones and may help explain the long success of multituberculates.

Summary & Key Takeaways

• Paleontologists have discovered a new fossil species, Jeholbaatar kielanae, from the Cretaceous period in China.

• The fossil's preserved ear bones offer insights into the evolution of mammalian ears, which have unique, complex middle ear bones.

• Research suggests that the movement of jaw bones towards the ears in Jeholbaatar may have been a result of its diet and improved grinding power.