Mammalian Middle Ear Evolution

Hello AlphaOmegakid, What the theory of evolution says is that mechanisms of evolution we see in the world today are sufficient to explain patterns of life in the past, and thus it predicts that we will find patterns consistent with evolutionary processes in the fossil record (and in the genetic record). This prediction is what is tested by the fossil and genetic record.The theory predicts that populations will change over time, that there will be a succession of hereditary traits in the populations from generation to generation, as new mutations are added to the existing mix, and old ones are eliminated (natural selection or genetic drift).The theory predicts that isolated populations will necessarily have different successions of hereditary traits because (a) they will have different sets of new mutations in each population and (b) they will be living in different ecologies, so natural selection will favor traits matched to the different ecologies rather than for conformity.Thus the theory of evolution predicts that in any hereditary lineage there will be a transition from a set of hereditary traits at point {A}, to a different, though related, set of hereditary traits at point {C}, and that at any point {B} intermediate in time between {A} and {C} the organisms will have intermediate sets of traits: some will be shared with {A} and not {C} and some will be shared with {C} and not {A}, and that there will still be many (if not most) traits that are shared by {A}, {B} and {C}. Thus fossil {B} is a transitional fossil because it is an intermediate form along the hereditary path from {A} to {C}.There is only a relative relationship to time (from generation to generation), there is no strict number of fixed mutations and selected new traits per year, decade, century. There is no metric for how much change has to happen either. Well yes, but that is because the fossil record is the record of what actually occurred, and because, so far, the theory of evolution is capable of explaining it by everyday evolutionary processes. This demonstrates the validity of the theory: each fossil is a test of the theory, and no fossil in the billions found, has yet invalidated the theory. Yes. Finding a development that is not in earlier organisms and which does not have any possible morphological development from homologous features with ancestors. No. It is found in the earliest mammals because it is a feature that is used to define when the first mammals evolved (hair an mammary glands not fossilizing that well), one of the diagnostic features. The hereditary lineage of the transition from reptile jaw and ear to mammal jaw and ear goes through many intermediate stages, several including a double jointed jaw as part of the transition.http://www.geocities.com/...naveral/Hangar/2437/therapsd.htm
Palaeos: Page not foundEven if you found a similar development in an earlier synapsid, that would not invalidate the branch of evolution leading through therapsid to mammaliforms to mammals: it would just be another branch, likely starting from a common ancestor with similar start along the lineage of transition from reptile to mammal. Actually the last in the synapsid - therapsid - cynodont lineage of transition from reptile ear to mammal ear was roughly 240 Million to 200 Million years ago, to mammaliforms (note you can move up and down, and in and out in detail, on this cladogram by clicking on it):

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The Late Triassic Epoch of the Triassic Period: 228 to 200 million years ago

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The following is a condensed reminder of some of the many changes from basal synapsids to basal mammals. ...

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1. Metabolic rate: transition to more or less full homeothermy inferred from geographic range, nocturnal habit, etc.
2. Temporal fossa: increase in size; confluence with orbit.
3. Zygomatic Arch: development; replacement of jaw adductor by masseter as principal jaw muscle; greatly increased capacity for oral processing of food.
4. Lower jaw: dentary becomes only significant element of mandible; development of coronoid process; reduction of post-dentary elements; reflected lamina of angular; dentary-squamosal jaw articulation. See infra, ear.
5. Dentition: development of heterodont dentition with incisors, canines, pre-molars and molars; "permanent" (diphyodont) teeth with prismatic enamel; increasingly fixed pattern of occlusion; definite dental formula.
6. Palate: full secondary palate.
7. Ear: reflected lamina of the angular (tympanic); retroarticular process of articular; conversion of post-dentary bones to sensory use in middle ear. Reflected lamina may have been resonating chamber, followed by development of tympanic membrane framed by increasingly small and gracile reflected lamina and/or retroarticular process
8. Pineal foramen: pineal foramen initially becomes more conspicuous, then recedes and is lost.
9. Skull table: development of parietal crest with muscular attachment on outside of dermal bones.
10. Braincase: parietal and squamosal spread downward to cover braincase, gradually replacing (in advanced mammals) neurocranium while providing muscle attachment on lateral (formerly dorsal) surface; epipterygoid changes from pillar supporting parietal and braincase to alisphenoid element of skull, continuous with parietal, squamosal, & petrosal (fused otic capsule). Brain size does not increase relative to diapsids.
11. Skull fusions: fusion of parietals and frontals, otic capsule, occipitals, numerous other elements in therians; loss of dermal bones, e.g. post-orbitals.
12. Skull attachment: double occipital condyle, condyles move ventrally, development of mammalian circular form.
13. Spine: loss of lumbar ribs (reversed in advanced cynodonts & lost again in Mammalia); increase in number of sacral vertebrae (??); reduction of tail; vertebral articulations changed to accommodate dorsoventral undulation; vertebrae amphiplatyan (flat on both ends), implying loss of notochord remnants (?).
14. Limb girdles: reduction (e.g. pubes, coracoids) or loss of ventral elements; more vertical orientation of limbs.
15. Limbs: more vertical orientation; elongation of humerus & femur; digits shorter; calcaneal heel
16. Integument: fur?; mammalian muzzle
17. Habit: primitively large carnivores; great reduction in size; development of omnivorous and herbivorous adaptations. --ATW 001202

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Those are the "diagnostic traits" of mammals, and it includes the ear.After that we find fossils of small shrew-like mammals that take us up to 70kyrs ago and the great meteor catastrophe\opportunity (its a matter of POV eh?). That fossils found would be intermediate in form between ancestors and descendants. Think of your sister getting her wisdom teeth in much sooner than you do, even though you are older -- does this out of sequence formation invalidate your existence or heredity from your parents?Enjoy.Edited by RAZD, : million not thousand, added map

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LOL - posting late and tired. 240Myr.

Thanks Wounded King, So we are looking for something between this guy ...(1) 75 million year old shrewlike mammal Bad News for Dinos Was Good News for Mammals

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The fossil of a shrewlike animal uncovered a decade ago in Mongolia's Gobi Desert set off one of the most extensive probes ever into the origins of placental mammals, the vast majority of all living mammals (which excludes marsupials and egg-layers, like the platypus).

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THE SKULL THAT STARTED IT ALL: This skull, belonging to a shrewlike animal, that dates back more than 70 million years, kicked off a comprehensive fossil analysis that led to a determination that placental mammals arose roughly 65 million years ago.
ILLUSTRATION: PAUL BOWDEN/CMNH

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To properly age and classify the Mongolian fossil Maelestes gobiensis, estimated to be between 71 million and 75 million years old, Wible and his team compared it with 409 features culled from the skulls, teeth and skeletal remains of other animals ranging in age from present-day mammals to those estimated to have lived over 100 million years ago. In an attempt to determine whether it was a placental mammal, the scientists constructed a tree charting the evolution of placental mammals beginning well in the Cretaceous. "We wanted to find out what our fossil was," Wible says, "and we wanted to test whether any of [the other] Cretaceous fossils could be placentals."

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Wible and his colleagues report in Nature that when they finished analyzing and classifying the specimens, they discovered that none dating back to the Cretaceous appeared to be placental mammals; it seemed such mammals more likely evolved some 65 million years ago, which would support the long held "explosive model'' theory that a dino die-off made way for them to spring up.

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That talks about another diagnostic characteristic of modern mammals, but not about the ears. The skull however looks like mammal ear structure from what I can see with the jaw and the arch.... and this guy?(2) 195 million year old shrew-like mammal(iform?) Oldest mammal is found: Origin of mammals is pushed back to 195 million years

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Discovery of the skull of a shrewlike animal the size of a paper clip pushes back the origin of mammals, including humans, to 195 million years ago. Found in China, the tiny skull shows evidence that the first mammals evolved from reptiles 45 million years earlier than widely believed.

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A model of a shrewlike creature that may have been the first true mammal, the most distant ancestor of the class of animals that includes present-day elephants and humans. The reconstruction was made from a 195 million-year-old fossil skull (above right). It was no bigger than a paper clip and weighed less than an ounce.

"Previously, the remains of the first true mammals were only found in deposits 150 million years old," significantly younger than the newly discovered skull, notes Alfred Crompton, Fisher Research Professor of Natural History at Harvard University.

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New jaws, ears, teeth

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The little almost-mammal weighed a scant 2 grams, considerably less than an ounce. Crompton suspects that it was warm-blooded and possessed hair, although that cannot be proven. H. wui probably was too small to eat anything but insects. It was, in turn, eaten by lizards and other reptiles. According to the accepted definition, mammals have three linked bones in the middle ear, part of the apparatus that translates vibrations of the air into nerve signals that register as sound in the brain. H. wui has these bones, which are believed to have once been part of the jaws of reptiles.

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Mammals are also defined by a lower jaw consisting of a single bone that both holds teeth and forms a joint with the upper jaw. In contrast, reptiles have many bones in their lower jaws, some of which make up the jaw joint. A series of fossils, dating back 200 years, show these latter bones becoming smaller and smaller. Paleontologists believe these bones eventually migrated to the middle ear, but until now, no good evidence of this existed. H. wui fills this gap and shows that we hear with bones that once formed part of the jaws of reptiles.

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However, he and his colleagues are cautious about making claims for H. wui's position on the evolutionary ladder. "Not everyone will agree with our interpretation that the skull represents the oldest relative of mammals, a species that lived as long ago as 195 million years," he admits. There will be controversy until more fossils like it are found in rocks as old.

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I note that they don't mention your cartilage, and this picture is even harder to see any detail on.On the other hand, AlphaOmegaKid's Yanoconodon seems a little younger but not necessarily more advanced than H. wui above.

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Yanoconodon is a monotypic genus of extinct early mammal whose representative species Yanoconodon allini lived 125 million years ago during the Mesozoic in what is now China. It is considered to be a transitional fossil due to the formation of its middle ear, which is a cross between those of modern mammals and their nearest relatives, the mammaliaformes. Yanoconodon was a Eutriconodont, a group synonymous to the Triconodonts which lived during the time of the dinosaurs. Indeed, some such as Repenomamus grew so large that they were able to eat small dinosaurs in some cases.[1] In particular, Yanoconodon is considered to be closely related to Jeholodens.[2]

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One begins to wonder how many (very?) different organisms can be described as "shrew-like" ...Enjoy

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we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
... to learn ... to think ... to live ... to laugh ...
to share.

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