- Morganucodonta
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Morganucodonta
Temporal range: Late Triassic to Middle Jurassic
This diagram shows the Morganucodont double jaw joint. Scientific classification Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata (unranked): Amniota Class: Synapsida (unranked): Mammaliaformes Order: †Morganucodonta
Kermack, Mussett, and Rigney, 1973The morganucodonts ("Glamorgan teeth") are an extinct group of important animals that seem to represent the earliest of mammals. These were shrew-sized, likely warm-blooded and possibly possessed mammary glands. The morganucodonts were most likely insectivorous and nocturnal, seeking prey while avoiding predators that were usually active by day. Such a trait would be inherited by their descendants to avoid the predatory dinosaurs.
Contents
Anatomy
They were equipped with an unusual structure , the "double-joint", which meant that the jaw articulation would be made up of the dentary-squamosal joint as well as a quadrate-articular one. Both the articular and quadrate would eventually become the malleus and incus. There is a trough at the back of the jaw that houses postdentary bones, such bones are absent today in mammals (all living mammals today have a jaw that is composed of a single bone, one of the defining features of Mammalia).
- Dentition
Unlike Sinoconodon and the therapsids, the teeth are diphyodont (meaning that they possessed two sets of teeth as in all living mammals) and not polyphydont (meaning that the teeth are constantly replaced, as in reptiles). Furthermore, the primitive postcanine teeth found in more archaic proto-mammals have been replaced by true molars and premolars. The teeth are structured in such a way that a one-to-one occlusion and wear facets are present.
The septomaxilla, a primitive feature also found in Sinoconodon, is present as well as a fully ossified orbitosphenoid. The anterior lamina is enlarged. The cranial moiety of the squamosal is a narrow bone that is superficially placed to the petrosal and parietal. Unlike its predecessors, the morganucodonts have a larger cerebral capacity and a longer cochlea.
- Post-cranial skeleton
The atlas elements are unfused, there is a suture between the dens and axis. The cervical ribs are not fused to the centra. The coracoid and procoracoid, which are absent in therians, are present. The head of the humerus spherical as in mammals, but the spiral ulnar condyle is cynodont-like. In the pelvic girdle, the pubis, ilium and ischium are unfused.
True mammals?
Scientists still debate if the morganucodonts should be classified as true mammals or classified as a clade outside of mammalia. An argument that is often used to classify them as non-mammals is the fact that they did not possess the three middle ear bones, they were equipped with a double jaw-joint instead (see above). Morganucodonts may be tagged as "prototherians."
Geographical Distribution
Their remains have been found in southern Africa, Western Europe, Arizona and China.
Classification
- Order Morganucodonta
- Family Morganucodontidae
- Genus Morganucodon
- Genus Eozostrodon
- genus ?Gondwanadon
- Genus Helvetiodon
- Genus Erythrotherium
- Genus Wareolestes
- Family Megazostrodontidae
- Genus Megazostrodon
- Genus Brachyzostrodon
- Genus Dinnetherium
- Genus ?Indozostrodon
- Family Morganucodontidae
Sources and Further Reading
The Fossil Book: A Record of Prehistoric Life, authored by Patricia Vickers Rich, Thomas Hewitt Rich, Mildred Adams Fenton and Carroll Lane Fenton. Page 519.
The Illustrated Encyclopedia of the Prehistoric World, consultant editor Dr. Douglas Palmer. Page 342.
Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 168-183.
External links
See also
Categories:- Prehistoric mammals
- Mesozoic mammals
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