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Tags gaps , theory , evolution

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Old 16th March 2007, 04:34 PM   #1
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More gaps for the theory of evolution.

Evolution has been in the news quite a lot recently. Check out the new cover of Newsweek:



I'll not quibble with a few of the sloppy wording and metaphor used, but it is overall a pretty insightful article. You can read it here: http://www.msnbc.msn.com/id/17542627/site/newsweek/

But the coolest recently surrounds the discovery of a new transitional fossil. Discovered in China's Hebei Province, he lived 125 million years ago during the Cretaceous period. That means he was crawling around while the 20 ton dinosuars had their hayday. He's named Yanoconodon

The most important thing about this little guy is that he provides some much needed insight into the evolution of the mammalian middle ear.



Quote:
This early mammalian ear from China is a rosetta-stone type of discovery which reinforces the idea that development of complex body parts can be explained by evolution, using exquisitely preserved fossils.
- H. Richard Lane, program director in NSF's Division of Earth Sciences

PZ Myers weighs in on the little dude (he's only about 5" long):

Quote:
The latest Nature reveals a new primitive mammal fossil collected in the Mesozoic strata of the Yan mountains of China. It's small and unprepossessing, but it has at least two noteworthy novelties, and first among them is that it represents another step in the transition from the reptilian to the mammalian jaw and ear.



Here's the beautiful little beast; as you can see, it's very small, and we need to look very closely at some details of its morphology to see what's special about it.



The first significant feature to examine is the jaw. This animal is from the Mesozoic, and a time when evolution was generating some radical changes in the feeding and sensory structures of the mammalian lineage. So first, a little background.
The primitive tetrapod jaw is a compound structure built up from multiple bones. In embryonic development, a rod-like structure called Meckel's cartilage is first to form; in modern mammals, it is resorbed later in development, and really only forms a temporary scaffold. The dentary, as you might guess from the name, is the tooth-bearing portion. Farther back are several bones, including the angular and the articular, which contribute to the body of the structure and its articulation with the skull. One of the roles of these bones is to connect to the auditory apparatus of the cranium—the jaw conducts vibrations to these bones, which then transmits them to the organs of hearing. This is not a particularly sensitive way to sense sound, since it means sound waves traveling through the air (or the ground) are going to have to be picked up by a high impedance element, the bulky jaw, before being transmitted to the ear.
In the early mammalian lineage, there is a pattern of progressive reduction of the various secondary jaw elements and an expansion of the dentary bone to take over the whole job of the jaw. We have an excellent record of the transformation of the jaw and skull elements in mammalian evolution— in short, what we see is that everything but the dentary gets smaller and smaller, and gets pushed farther and farther back towards the skull. We have transitional forms that have double articulations of the jaw with the skull—one between the old articular bone and the quadrate bone of the skull, and another between the dentary and the squamosal (the current jaw joint in modern mammals)—and then forms where the old hodge-podge of bones have been cast free of the jaw altogether.
In us, the old articular and quadrate bones have completely lost their role in supporting the jaw as a joint and instead have become imbedded in the middle ear of mammals, suspended with the stapes between two delicate membranes to specialize in conducting sound vibrations to the inner ear. What does the hearing apparatus look like in Yanoconodon?
Start by looking at a, b, c, and d in this diagram. Highlighted in blues and purples at the back of the jaw are these small bones in Morganucodon (a) and Yanoconodon (b). In d is the jaw of Repenomamus, a large Cretaceous mammal. Don't miss c—that small object is the collection of middle ear bones from Ornithorhyncus, better known as the platypus.



What we see here is that the three Mesozoic mammals all retain Meckel's cartilage as a slender, ossified splint clinging to the inner side of the jaw. In b and c, we can see that middle ear bones of both Yanoconodon and the platypus are remarkably similar, but there is one significant difference. In the platypus, those bones are not connected to the jaw at all—they have the standard mammalian middle ear, with the bones suspended remotely from other bones of the jaw and skull. In Yanoconodon, we are almost at that point. The middle ear bones are clearly delicate and specialized for function in hearing, but they retain one last tentative, delicate connection with the jaw through a contact with Meckel's cartilage. In this animal, we've caught the mammals just before they've taken that last step of fully separating the middle ear bones from the jaw.
This animal is from that time just before the hearing apparatus has let loose of its last bony mooring and said bon voyage to the jaw. It's a significant moment in history, I think.
Also look at g. This is a ventral view of the left jaw of Yanoconodon, and again you can see the middle ear bones connected by that tiny strut to the jaw. h is a drawing of the middle ear bones of the platypus in the same orientation, and i is especially neat: that's the jaw of a platypus embryo, before Meckel's cartilage is resorbed, and you can see that the middle ear bones are connected in the same way, transiently. It's one module in development that flaunts a lovely example of embryonic recapitulation of evolutionary history.
I said there were two novelties in this specimen. One is the beautiful connection between the middle ear bones and the jaw; the other is a curiosity in the number of vertebrae. Compare yourself to Yanoconodon, for instance:

# vertebrae
Region |||||| You ||||||| Yanaconodon
Cervical |||||| 7, no ribs ||||||| 7, no ribs
Thoracic |||||| 12, with ribs |||||| 18, with ribs
Lumbar |||||| 5, no ribs ||||||| 8, with floating ribs

Not only is the total number of vertebrae very much on the high end of what we see in any modern mammals, making for a rather sinuous and flexible body, but there's that odd business of the lumbar (our lower back) vertebrae having riblike bones attached to them. The authors make the point, too, that the boundary between thoracic and lumbar in Yanoconodon is somewhat arbitrary—there's a continuous gradation rather than a sharp delineation. Gain and loss of lumbar ribs seems to be a fairly common event in these early mammalian clades, as illustrated below.



The inset diagram illustrates an experiment in mouse embryos: knocking out the three Hox10 genes in mice produces a transformation just like that seen in Yanoconodon, with all the lumbar vertebrae also producing small ribs. That's very cool, in that it suggests a molecular mechanism in that the evolution of the Hox10 genes was probably responsible for the morphological variation we see in Mesozoic fossils.

--------------------------------------------------------------------------------
Luo Z-X, Chen P, Li G, Chen M (2007) A new eutriconodont mammal and evolutionary development in early mammals. Nature 446:288-293
As we all know, Yanoconodon creates "more gaps for the theory of evolution" by virtue of there now being two gaps on either side of him.
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Old 16th March 2007, 04:43 PM   #2
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Originally Posted by Level View Post
The most important thing about this little guy is that he provides some much needed insight into the evolution of the mammalian middle ear. . . . . .

As we all know, Yanoconodon creates "more gaps for the theory of evolution" by virtue of there now being two gaps on either side of him.
You know, don't you, that actual evidence will either:
a) be ignored,
b) be misinterpreted,
c) or just piss them off.

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Old 23rd March 2007, 11:16 PM   #3
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Isn't almost every fossil a "transitional fossil" to a true evolutionist? I've seen claims for there being around 30-50.

I read that the cladogram (not shown here) had six mammalian middle ear homoplasies. Isn't that amazing?
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Old 24th March 2007, 03:00 AM   #4
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Originally Posted by Level View Post
As we all know, Yanoconodon creates "more gaps for the theory of evolution" by virtue of there now being two gaps on either side of him.
Good Poe on the thread title. I was holding my breath almost until this last bit waiting for the Zeno's punch line.
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Old 24th March 2007, 03:02 AM   #5
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That two gaps for one thing is hilarious. I love it.
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Old 24th March 2007, 03:31 AM   #6
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Originally Posted by teratogenome View Post
Isn't almost every fossil a "transitional fossil" to a true evolutionist? I've seen claims for there being around 30-50.
One of my pet peeves is the casual use of laymans terms by evolution advocates in ways that confuses often terminologically illiterate Creationists. "Humans are apes" is another one that makes me crazy. It only takes a few extra seconds to reply "humans are Catahhrines just like our fellow apes," which should clue the reader into there being more to the response than gorilla = human, which is what most Creationists see when they read "humans are apes."

The problem with every fossil being a transitional fossil is that's bass ackwards from the point they're trying to make. Every fossil is a fully formed individual representative of their species which exhibits characteristics of two higher taxa. That's not that much more typing than "every fossil is transitional" and more completely explains what they're trying to say.

Tiktaalik roseae is a fully formed example of species Tiktaalik roseae, but species Tiktaalik roseae is an example of a transition between aquatic fish and terrestrial tetrapods. The actual number of these "true" transitionals (meaning in the context of the Creation/evolution debate) is in the hundreds.

And welcome to the forum btw.
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Old 24th March 2007, 05:02 AM   #7
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All creatures are transitional, including us. Fossils ain't transitional. Fossils don't reproduce.

Very interesting post, Level. No copyright problems, I trust?
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Old 24th March 2007, 07:34 AM   #8
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Originally Posted by Soapy Sam View Post
All creatures are transitional, including us. Fossils ain't transitional. Fossils don't reproduce.
See. This is exactly what I'm talking about.
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Old 24th March 2007, 10:53 AM   #9
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Originally Posted by Soapy Sam View Post
Very interesting post, Level. No copyright problems, I trust?
I probably should have linked back to PZ's post at his blog, instead of just quoting and attributing the material to him. Is there anyway I can edit that post to include the URL? http://scienceblogs.com/pharyngula/2...tional_fos.php
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Old 24th March 2007, 11:11 AM   #10
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Mr Smith and Mrs Smith (ne: Jones) have their only son, Fred.

Fred grows up to manhood and looks a little like Mr Smith and a little like Mrs Smith.

Where's the transitional person?

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Old 24th March 2007, 11:39 AM   #11
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Originally Posted by UnrepentantSinner View Post
One of my pet peeves is the casual use of laymans terms by evolution advocates in ways that confuses often terminologically illiterate Creationists. "Humans are apes" is another one that makes me crazy. It only takes a few extra seconds to reply "humans are Catahhrines just like our fellow apes," which should clue the reader into there being more to the response than gorilla = human, which is what most Creationists see when they read "humans are apes."

Forgive my ignorance but having never heard of the term before, Google can't give me a single instance of "Catahhrines" or " Catahhrine" - as a layman, am I missing something here?

-
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Old 24th March 2007, 11:42 AM   #12
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Originally Posted by Level View Post
I probably should have linked back to PZ's post at his blog, instead of just quoting and attributing the material to him. Is there anyway I can edit that post to include the URL? http://scienceblogs.com/pharyngula/2...tional_fos.php
There's a two-hour limit on editing posts, so if you really want to change it, you'll have to ask a mod now. And as a friendly tip, it's generally a good idea to only C&P stuff from other websites you -know- that you're allowed to use. And make sure to notify that you do have a permission, so that it's clear from the get-go.

Or failing to get that permission, only copy a small exerpt or two (which will fall under the "fair use" clause").

I know, it can be a bother. But that's copyright laws for you, I'm afraid. Can't do much about it. At least I can tell you had only good intentions.
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Old 24th March 2007, 12:07 PM   #13
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Originally Posted by Rrose Selavy View Post
Forgive my ignorance but having never heard of the term before, Google can't give me a single instance of "Catahhrines" or " Catahhrine" - as a layman, am I missing something here?

-
Catarrhine. Broad classification of primates that includes us and the monkeys.
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Old 24th March 2007, 12:18 PM   #14
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Originally Posted by RecoveringYuppy View Post
Catarrhine. Broad classification of primates that includes us and the monkeys.
Ah thanks it was the typo that caused the prob - - Google only offerred Catherine as an alternative - I did have a quick look at wiki under "human" and "primate" classifications without success.
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Old 24th March 2007, 04:49 PM   #15
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Originally Posted by Level View Post
As we all know, Yanoconodon creates "more gaps for the theory of evolution" by virtue of there now being two gaps on either side of him.

Ah, the Xeno's Paradox anti-evolution argument:


In order for one form to evolve into another, it must first pass through the intermediary form midway between those two. For it to get to that midway point, it must first pass through a midway point between the first form and the first midway point. Since there are an infinity of midway points, evolution can't possibly happen!

What's scary is that is very nearly the argument they make when they talk about irreducible function or whatever it's called.
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Old 24th March 2007, 07:27 PM   #16
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Originally Posted by Rrose Selavy View Post
Forgive my ignorance but having never heard of the term before, Google can't give me a single instance of "Catahhrines" or " Catahhrine" - as a layman, am I missing something here?
Originally Posted by RecoveringYuppy View Post
Catarrhine. Broad classification of primates that includes us and the monkeys.
Originally Posted by Rrose Selavy View Post
Ah thanks it was the typo that caused the prob - - Google only offerred Catherine as an alternative - I did have a quick look at wiki under "human" and "primate" classifications without success.
Terribly sorry about the typo and thanks RY for getting my back. RS, if you check out the Tree of Life (Google "tree of life web") it will have most of the taxa/clades humans fit into listed and most of the pages have some sort of definition why, or links to papers/sites that explain why.

Just for the record, "human+ape+taxonomy" might have given you something to work with from Google.
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