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Paper Number: 90
Paleoneurology
of Therapsida (Synapsida) and the Evolution of soft tissue traits in the
mammalian ancestry
Benoit, J. 1,2, Manger, P.R.2, Rubidge,
B.S.1,3
1Evolutionary
Studies Institute (ESI), University of the Witwatersrand, Braamfontein,
2050, Johannesburg, South Africa
2School
of Anatomical Sciences, University of the Witwatersrand, 7 York Road,
Parktown, 2193, Johannesburg, South Africa
3School
for Geosciences, University of the Witwatersrand, Braamfontein, 2050,
Johannesburg, South Africa
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The evolution of the physiology and defining soft tissue characters
of mammals, such as mammary glands and hairs, in mammalian ancestors
(Therapsida) is difficult to assess because they do not readily
fossilize. Virtual 3D palaeoneurology using CT-scan offers opportunities
to find bony correlates of these biological and physiologcal features.
Here we show that the parietal foramen for the pineal (or third) eye
tends to be smaller and absent more frequently in eutherocephalians and
eucynodonts in a convergent manner. Among cynodonts, the parietal
foramen is completely lost in Probainognathia, the lineage leading to
mammaliaforms. Given the role played by the pineal eye to achieve fine
-tuned thermoregulation in extant ectotherms (i.e. 'cold-blooded'
vertebrates) [1], this increasingly frequent loss of the pineal eye may
be correlated with the evolution of a high metabolic rate (endothermy)
in these derived therapsids. The zone of character variability may
correspond to the transition through a mesothermic metabolism, and the
definitive loss of the parietal foramen in Probainognathia would
correspond to the acquisition of complete mammalian endothermy. Also,
the loss of the parietal foramen in all Probainognathia may be
pleiotropically linked to the appearance of mammary glands and mammalian
body hair coverage since these traits and the complete ossification of
the parietal fontanelle are all controlled by the same homeogene, Msx2,
in mice [2]. This indicates that body insulation may have evolved in
relation to the loss of the parietal foramen in Probainognathia, which
supports our hypothesis. The presence of a mammalian infraorbital canal
in the more derived probainognathians suggests that they displayed
tactile vibrissae, which strengthens the possibility of the presence of
hairs elsewhere on the body. To conclude, our data indicate that
maxillary vibrissae, fur, mammary glands, an enlarged cerebellum, and
endothermy first evolved in Probainognathia, some 246 mya, and that in
silico paleoneurology is a very powerfull tool to reconstruct the
evolution of soft tissue character and physiology.
Evolution of the frequency of presence of the parietal foramen
(Fq), average size of the parietal foramen (Size), and number of
specimens examined (n) in Therapsida.
References:
Ralph CL et al. (1979) Biol Rev 54:41-72
Satokata I (2000) Nat Genet 24(4): 391-395
