Athena Review Image Archive ™
Morganucodon oehleri skull
Skull of Morganucodon oehleri (after Rowe et al. 2011).
Morganucodon
was a small, early stem mammal dating from the the Late Triassic to
Middle Jurassic periods (~ 205-180 mya). Three main genera are known, Morganucodon, Eozostrodon, and Haldanodon,
with widespread distribution in the northern hemisphere (Laurasia), as
well as Africa and India in the southern, Gondwanan region
(Kielan-Jaworoska et al. 2004).
Morganucodon means "Glamorgan tooth." The Welsh variant Morganucodon watsoni
is represented by abundant fossils. It was discovered and named in
1949-1958 by Walter Georg Kuhne from Duchy Quarry in Glanmorgan, Wales.
Fossils include teeth, jaw fragments, and post-cranial bones. The
deposits at Glanmorgan are dated from the Sinemurian stage of the Early
Jurassic, at about 200-190 mya (Kermack et al. 1981).
The Chinese species Morganucodon oehleri (whose
skull is pictured here) was discovered in the Early Jurassic,
Hettangian-Sinomurian Formation in the Lufeng Basin of Yunnan province.
The species is named for a complete skull found in 1941, first
identified by E.T. Oehler in 1948, and later described by W. H. Rigney.
Additional skulls of M. oehleri were found in Yunnan in the 1970s and
80s by the Beijing Institute of Vertebrate Paleontology and
Paleoanthropology (IVPP) (Crompton and Luo 1994).
Morganucodon,
with a skull 27-38 mm long, a body length of about 10 cm (4 in), and an
estimated body weight from 27-80 grams, probably resembled a shrew or
mouse. Morganucodon was
likely nocturnal and spent the day in a burrow. The diet appears to
have been insects and other small animals (Luo et al 2001; Kemp 2005).
Various
skull features representing mammalian traits first appear in
morganocodonts. They have a mammalian jaw joint plus a tiny,
retained reptilian joint, a condition which prevents their being
classed as fully mammalian. The orbit around the eye is fully enclosed,
as in all mammals. The development of the mammallian middle ear is also
advanced in morganuconodonts, with both Meckel's groove and the angular
bone more reduced than in Sinoconodon or nonmamalian cynodonts.
M. oehleri
shows an increased brain size over contemporarty non-mammalian land
animals, with enlarged olfactory bulbs, neocortex, pyriform cortex, and
cerebellum.
According to Rowe et al. (2011), its relative brain size
had expanded in two evolutionary pulses to mammalian levels. The first
related to increased resolution in olfaction and improvements in
tactile sensitivity. The second phase of olfactory enhancement then
enlarged the brain to mammalian levels.
The
origin of crown Mammalia (the direct ancestors of present mammals) saw
a third pulse of olfactory enhancement, with ossified ethmoid turbinals
supporting an expansive olfactory epithelium in the nasal
cavity.
References:
Kemp T.S. 2005. The origin and evolution of mammals. Oxford University Press.
Kielan-Jaworowska, Zofia, Cifelli, Richard L, and Luo, Zhe-Xi 2004. Mammals from the age of dinosaurs: origins, evolution, and structure. New York, Columbia University Press.
Luo, Crompton, and Sun 2001
Rowe,T., Macrini, T.E., and Luo, Z.-X. 2011. Fossil evidence on origin of the mammalian brain. Science 332, 955-957.
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