From Jaw to Ear: Transition Fossil Reveals Ear Evolution in Action (Yanoconodon)

Scientific American ^ | 14 March 2007 | David Biello

[Stultis]

March 14, 2007
From Jaw to Ear: Transition Fossil Reveals Ear Evolution in Action
Now hear this: early mammal fossil shows how sensitive ear bones evolved
The mammal ear is a very precise system for hearing—enabling everything from human appreciation of music to the echolocation of bats. Three tiny bones known as ossicles—the hammer (malleus), anvil (incus) and stirrup (stapes)—work together to propagate sound from the outside world to the tympanic membrane, otherwise known as the eardrum. From there, the sound is transmitted to the brain and informs the listener about pitch, intensity and even location. But it has been a mystery how this delicate system evolved from the cruder listening organs of our reptilian ancestors. Paleontologists have scoured fossil records in search of signs of how the jawbones of reptiles migrated and became the middle ear of mammals. Now Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh and his colleagues have found one: Yanoconodon allini, an intermediate between modern mammals and their distant ancestors. "It helps to show a transitional structure in the long process of evolution of mammal ears," Luo says. The Luo team found the new tiny mammal—just five inches (12.7 centimeters) long—in the Yan Mountains of Hebei Province in China. Similar rocks in other formations date to the Mesozoic era 125 million years ago when dinosaurs roamed Earth and early mammals are thought to have been relegated to scurrying through the undergrowth. Yanoconodon sports three cusps on its molars for feeding on insects and worms as well as a long body compared with its stubby limbs, ideal for scrabbling in the dirt for dinner. "This particular mammal has a very long body but relatively short limbs," Luo says. "By looking at the claw structure, hand bones and foot bones, our general interpretation is that it is a mammal that lived on the ground surface or perhaps was capable of digging." More importantly, the nearly complete fossil shows a separation between the jawbones and the inner-ear bones, but one that is incomplete. Yanoconodon's stirrup, anvil and hammer bones are still connected to the jaw by another bone—gone from adult modern mammals. In fact, they display the same layout as mammal embryos do today, before the cartilage precursors of the jaw and ear bones separate during gestation. "Reptiles have [a] jaw full of ear bones from mammals and mammals have an ear full of jawbones of reptiles," Luo notes. "Proportion of the ear bones [is] already like those of modern mammals [in this animal] but the reptilian connection to the jaw is retained." This means Yanoconodon not only picked up the high frequencies associated with modern mammal hearing but also the vibrations transmitted through the ground. "It has not completely lost this ability to sensitively detect ground vibrations through the jaw but has gained some of the modern mammal ability to hear airborne sounds," Luo adds. The extinct early mammal had some other unusual features, including more vertebrae than any terrestrial mammal alive today. This means that in this feature it closely resembled monotremes (egg-laying mammals like the platypus), whereas other features brought it closer to marsupials and placental mammals. Regardless, it represents a key middle step in evolving the exquisitely sensitive modern mammal ear.

[Stultis]

Go to the source and there's a figure you can click on and enlarge that shows the comparative form and position of these bones in a mammal-like reptile (Morganucodon), the new fossil Yanoconodon and a modern mammal (the opossum).

[FreeperinRATcage]

Three tiny bones known as ossicles—the hammer (malleus), anvil (incus) and stirrup (stapes)—work together to propagate sound from the outside world to the tympanic membrane, otherwise known as the eardrum

This is all I needed to read. Doesn't ANYONE fact check anymore?

The three bones work to transmit impulses FROM the tympanic membrane to the cochlea. The Tympanic membrane receives the sound from the outside world.

[Coyoteman]

This is all I needed to read. Doesn't ANYONE fact check anymore?

The three bones work to transmit impulses FROM the tympanic membrane to the cochlea. The Tympanic membrane receives the sound from the outside world.

And do you think its the scientists, or perhaps the writer, who made this error?

[Stultis]

Good catch. I missed that.

Also there is false journalistic mystery mongering in the claim that:

But it has been a mystery how this delicate system evolved from the cruder listening organs of our reptilian ancestors.

In fact we have known for decades the basics of how this evolution occurred, first by inference (how it must have happened, to the extent that the nature of this and other fossils were predicted before they were found) and later (as the article in fact mentions) from embryological studies.

Still it's interesting to now have a fossil that shows another of the transitional stages in actuality.

[muir_redwoods]

"And do you think its the scientists, or perhaps the writer, who made this error?"

Why C-Man, I'm surprised at you, when has a professional journalist ever been known to make such an error?

[Oztrich Boy]

Still it's interesting to now have a fossil that shows another of the transitional stages in actuality.

You mean something has been found that evolutionists predicted? My world is askew. Maybe there's something to this evolution theory after all.

[Nathan Zachary]

Don't bet on it. They are comparing a fossil of an elephant to that of a possum (with a fossilized wood tick embedded in it) It will turn out to be another hoax as usual, like that fossilized salamander.

[Coyoteman]

Here is an article which has a lot of similar data on transitionals (particularly Section 4).

It effectively demolishes the entire creationist argument. Excellent reading!

[RunningWolf]

That is a demolishing?

During Darwin's time, inductive reasoning would have declared that there are no white crows, then they found white crows.

[RunningWolf]

Interesting article Stultis, thx for posting it.

"Reptiles have [a] jaw full of ear bones from mammals and mammals have an ear full of jawbones of reptiles," Luo notes. "Proportion of the ear bones [is] already like those of modern mammals [in this animal] but the reptilian connection to the jaw is retained."

Well that is one way of looking at it I guess. Kind of a circular pre-determined way IMO. I might suggest that they 'step out of the box' at least for 30 minutes every 3 months.

Of course in the halls of the castle Academia, Me, my pov,and my words might be rejected right away.

[Stultis]

Well that is one way of looking at it I guess. Kind of a circular pre-determined way IMO.

Circular how? There is absolutely no question about the homology of the ear ossicles in mammals with bones that are located in the reptilian jaw. I.e. Reptilian Stapes --> Mammalian Stapes, R. Quadrate --> M. Incus, R. Articular --> M. Malleus.

As the article noted, albeit in passing, this can be confirmed in the embryological development of mammals. (At least in kangaroos. I don't happen to know if it's been observed in other species as well.) The bones actually start in their "old" position in the jaw.

And then we can see the same thing happening progressively in the fossils. Even before the discovery of this new creature, which merely shows an additional stage, we had the following (source):

Figure 26. The jaw articulation and ear ossicles of four vertebrates. Although novelty does arise in evolution, an exceedingly common pattern is the alteration of an existing structure. Not one of the three ear bones of mammals -- stapes, incus, and mallus -- is really new. the stapes, which began as a part of the hyomandibular in the primitive fish, became the single ear bone in amphibians, reptiles, and birds. The incus was a prominent upper jawbone (pterygoquadrate) in primitive fishes and became reduced to the quadrate in amphibians, reptiles, and birds. It formed a jaw joint with the articular in the lower jaw, the later a modification of Meckel’s cartilage. In the mammal-like reptiles the dentary of the lower jaw begins to enlarge: and finally, in the mammals it forms a new jaw joint with squamosal of the upper part of the skull. Of the two bones of the old joint, the quadrate becomes the incus and the articular becomes the malleus.

NOTE: The proceeding is focusing mainly on showing the shifting of the jaw-joint from Quadrate/Articular in reptiles to Dentary/Squamosal in mammals. If you're focusing on the ear ossicles themselves you can actually see those better, in the reptilian and primitive mammal positions looking at the jaw from the inside.

BTW, note that some of the Mammal-Like Reptiles actually had a DOUBLE jaw joint, with both the reptilian and mammalian articulations in place simultaneously. This arrangement was PREDICTED by evolutionists years before the actual fossils were found as a necessary stage in the transition.

[SunkenCiv]

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