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Morganucodon (nicknamed Morgie) is an extinct early genus of mammal. It is commonly held to be a transitional form in the evolution between "mammal-like reptiles" and "true mammals". It is remarkable for having a doubly-articulated jaw, marking the transition between two bones in the jaw of reptiles becoming the "hammer" and "anvil" bones of the middle ear of mammals. The relationship between the jaw bones of reptiles and middle-ear bones of mammals was noticed in the early 19th century, but there was no known fossil evidence for such a transition before the discovery of Morganucodon in 1949.
"It was a small animal with a skull 2-3 cm in length and a presacral body length of about 10 cm [4 inches]. In general appearance it would have looked like a shrew or mouse." It has been suggested that many of the early mammals, which were generally small, had to make a living "In the Shadow of Dinosaurs",an environment where being small and furry (and therefore capable of being active in the cool of the night) represented a niche unavailable to the dinosaurs.
Fossils of Morganucodon species and their near relatives are abundant and found in many parts of the world. There are thousands of fragments found in the original location alone, and others elsewhere in Europe, as well as in China, South Africa, North America and India. There are not just a few fossils from a single location discovered at a single time by a single researcher.
Morganucodon is not the only "no-longer-missing link" between reptiles and mammals. Such a transition, of course, is not made up of a single event, and is not the straight line "upward" as many popular representations may suggest, but is part of a complex web of relationships of many different living things. Like all other living things, Morganucodon had its place in the "tree of life", part of the complex pattern of common-descent-with-modification that is so fundamental to the evolutionary relationships of life.
- Dimetrodon - an early "mammal-like reptile" which is rather widely recognized, famous for the "sail" on it back, often popularly mistaken to be a dinosaur.
- Yanoconodon - first reported in the literature in 2007
The closest relatives of the genus Morganucodon form the biological family Morganucodontidae. Some of the genera mentioned as belonging to that family include:
- Eozostrodon - this was actually discovered shortly before Morganucodon but is only represented by a few teeth
Transition from jaw to middle ear
These two small bones in the middle ear are part of the signature of mammals among the vertebrates. All mammals have them, and no reptiles, birds, or other vertebrates do. So they have the appearance of being an unbridgeable gap separating mammals from other forms of life. This structure has therefore been called the Definitive mammalian middle ear. It has been suggested (by Wesley R. Elsberry) that the structure fits one criterion for Irreducible complexity, in that the function (transmitting sound) of the chain of three bones cannot be satisfied when one of the bones is missing; but that it does not fit the other criterion, in that the definitive mammalian middle ear has evolved. The seeming conflict (with the assertion that irreducible complexity cannot evolve) is yet to be resolved.
Discovery of the transition
In the early 19th century, it was discovered that these two bones are really the equivalents of two bones in the jaws of reptiles, where they function as part of the jaw joint. The basis of this identification was not any evolutionary theory, but comparative anatomy and embryology. This has been called the "Raupert-Gaupp Theory" after the names of two anatomists who established the homology.
When the theory of evolution was developed in the mid 19th century, a homology such as this was typical of one major line of inference for evolution. On occasion, an evolution-denier could point to the seeming implausibility of a transitional form between jaw-bones and ear-bones, which would demand double duty for these bones. Thus the discovery of Morganucodon, with such a double-duty, double-articulated jaw, neatly filled in a seemingly unbridgeable gap.
The confirmation of the "Reichert-Gaupp Theory" by the discovery of transitional fossils thus tends to confirm the reliability of a methodology relying on developmental biology and comparative biology alone.
More recently, discoveries in genetics have made it possible to explore the genes that may be responsible for these structures, thereby confirming the evolutionary relationship from yet another, totally distinct, line of inference. The gene responsible has been identified as one related to the bagpipe gene in insects.
There is a plausible source of selection pressure favoring the middle ear having these bones. Early mammals were very often small, probably nocturnal, with a diet of insects or other small invertebrates. Having these bones in the middle ear has been demonstrated to give better hearing in high frequencies. Hearing in high frequencies would have an advantage for getting around in the dark, and for finding insects.
Significance of the transition
Some evolution-deniers may object that evolution is, at best, an idle speculation of no use. But this transition presents a case of the usefulness of evolutionary biology.
Certain consequences of the knowledge about the evolutionary origins of the ossicles have been suggested.
Neil Shubin has pointed out the better understanding of human anatomy, in particular of the paths of the cranial nerves and muscles, that comes from appreciating their evolutionary origins. Cranial anatomy is one of the things in biology that can be covered by the saying, "nothing in biology makes sense except in the light of evolution."
There are possible consequences for our understanding of some birth defects and genetic diseases of hearing.
The transitional forms give additional cases for developing physical models for the understanding of the functioning of hearing in humans.
- Evolution of the Ear in Mammals Based on testimony by Kevin Padian in the case of Kitzmiller v. Dover
- page 143 in T. S. Kemp, The Origin and Evolution of Mammals, Oxford University Press, 2005. ISBN 0-19-850760-7.
- N. C. Fraser and H.-D. Sues, In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods, Cambridge University Press, 1994 ISBN 0-521-45242-2.
- Peter J. Bowler, Life's Splendid Drama: Evolutionary Biology and the Reconstruction of Life's Ancestry 1860-1940 University of Chicago Press, 1996 ISBN 0-226-06921-4. Chapter 6, "The Origin of Birds and Mammals"
- Stephen Jay Gould, Eight Little Piggies: Reflections in Natural History. W. W. Norton, 1993 ISBN 0-393-03416-X. Chapter 6, "An Earful of Jaw"
- The theory of evolution is useless, without practical application. Mark Isaak: Index to Creationist Claims
- Neil Shubin, Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. Pantheon Books, 2008 ISBN 978-0-375-42447-2. Chapter 10, "Ears"
- Susan Amin, Eva Matalova, Carol SImpson, Hiroki Yoshida and Abigail S. Tucker, "Incudomalleal joint formation: the roles of apoptosis, migration and down regulation", BMC Developmental Biology volume 7 (December 2007) 134. doi:10.1186/1471-213X-7-134 http://www.biomedcentral.com/1471-213X/7/134