Temporal range: Late Triassic – Middle Jurassic 210–166 Ma
|Life restoration of a Megazostrodon model, Natural History Museum, London|
Kermack, Mussett & Rigney, 1973
Morganucodonta ("Glamorgan teeth") is an extinct order of primitive mammaliaformes similar to crown-group mammals. Their remains have been found in southern Africa, Western Europe, Arizona and China. The morganucodonts were most likely insectivorous and nocturnal, seeking prey while avoiding predators that were usually active by day. Such a trait may have been inherited by their descendants to avoid the predatory dinosaurs, though some of the latter were nocturnal as well.1
Morganucodonts 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).
Unlike Sinoconodon and the therapsids, the teeth of morganucodonts were 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 Furthermore, the primitive postcanine teeth have been replaced by true molars and premolars.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 found in morganucodonts, 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.
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.
Because morganucodonts possessed the dentary-squamosal jaw joint, systematists like G. G. Simpson (1959)2 considered the morganucodonts to be mammals, and most contemporary paleontologists continue to use this classification.3 Others, however limit Mammalia to the crown group and so exclude more basal mammaliaform orders like Morganucodonta.4
- Schmitz, Lars; Motani, Ryosuke (2011). "Nocturnality in dinosaurs inferred from scleral ring and orbit morphology". Science 332: 705–708. doi:10.1126/science.1200043.
- Simpson, George Gayord (1959). "Mesozoic mammals and the polyphyletic origen of mammals". Evolution 13 (3): 405–414.
- Kemp, T. S. (2005). The Origin and Evolution of Mammals. Oxford University Press. p. 3. ISBN 0198507607.
- Rowe, T. (1988). "Definition, diagnosis, and origin of Mammalia". Journal of Vertebrate Paleontology 8 (3): 241–264.
- Rich, PV; Fenton, MA; Fenton, CL; THV, Rich. The Fossil Book: A Record of Prehistoric Life. p. 519. ISBN 9780486293714.
- Palmer, D, ed. (2006). The Illustrated Encyclopedia of the Prehistoric World. Book Sales. p. 342. ISBN 9780785820864.
- Kielan-Jaworowska, Z; Luo, ZX; Cifelli, RL (2004). Mammals from the Age of Dinosaurs. Columbia University Press. pp. 168–183. ISBN 9780231119184.
- "Mesozoic Mammals; Basal Mammaliaformes, Morganucodontidae, Megazostrodontidae and Hadrocodium, an internet directory". Karin and Trevor Dykes. Retrieved January 2013.