Post by Deleted on Sept 20, 2013 23:32:18 GMT 5
Information about early/basal/plesiomorphic synapsid skin.
A new scenario for the evolutionary origin of hair, feather, and avian scales
"In living mammals, the main morphological difference between gland and hair embryonic morphogenesis is that only the hair bud is associated to a true dermal condensation, and linked to an increase in the presence of the beta-catenin pathway. The synapsid lineage, which separated from the amniote taxa in the Pennsylvanian about 310 million years ago, may have evolved a glandular rather than a scaled integument, with a thin alpha-keratinized layer adorned with alpha-keratinized bumps. Those bumps may have even presented some cysteine-rich alpha-keratins, precursors of the hair-type keratins. In addition, the first synapsids may have developed both a lipid barrier outside the epidermis, similar to current amphibians living in xeric habitats, and some lipid complex with the alpha-keratins of the stratum corneum as in current mammals (Lillywhite, 2006), as a means to strengthen the barrier against water loss of the integument. For approximately 100 million years, the characters inherited from the synapsid ancestor evolved to the mammaliaform node. The mammaliaform then evolved until the divergence of the first mammalian lineage, the monotremes, followed by the marsupials and the eutherians. The living representatives of these lineages possess a high diversity of cutaneous glands: sebaceous, sweat and mammary glands, which are most commonly associated to hair follicles. For example, in monotremes, even the mammary glands are associated to hair follicles, an association which is transiently retained in marsupial embryos (Long, 1969) but lost in eutherian embryos. In the latter, the mammary glands are not only isolated from hairs, but even the nipple prevents hair from forming around it by the means of the BMP pathway (Mayer et al. 2008). During evolution, this pathway may have been progressively co-opted to suppress hair follicle formation in the nipple vicinity, first in marsupials and then more efficiently in eutherians."
Mammalian skin evolution: a reevaluation
"Early Theropsid Amniotes evolved a skin structurally and functionally similar to that of modern toads, living amphibians whose reduced epidermal mucogenicity and cornification permits them to walk over dusty driveways and hide in garages!"
So non-therapsid synapsids very likely had naked, glandular, amphibian-like skin. Any thoughts?
A new scenario for the evolutionary origin of hair, feather, and avian scales
"In living mammals, the main morphological difference between gland and hair embryonic morphogenesis is that only the hair bud is associated to a true dermal condensation, and linked to an increase in the presence of the beta-catenin pathway. The synapsid lineage, which separated from the amniote taxa in the Pennsylvanian about 310 million years ago, may have evolved a glandular rather than a scaled integument, with a thin alpha-keratinized layer adorned with alpha-keratinized bumps. Those bumps may have even presented some cysteine-rich alpha-keratins, precursors of the hair-type keratins. In addition, the first synapsids may have developed both a lipid barrier outside the epidermis, similar to current amphibians living in xeric habitats, and some lipid complex with the alpha-keratins of the stratum corneum as in current mammals (Lillywhite, 2006), as a means to strengthen the barrier against water loss of the integument. For approximately 100 million years, the characters inherited from the synapsid ancestor evolved to the mammaliaform node. The mammaliaform then evolved until the divergence of the first mammalian lineage, the monotremes, followed by the marsupials and the eutherians. The living representatives of these lineages possess a high diversity of cutaneous glands: sebaceous, sweat and mammary glands, which are most commonly associated to hair follicles. For example, in monotremes, even the mammary glands are associated to hair follicles, an association which is transiently retained in marsupial embryos (Long, 1969) but lost in eutherian embryos. In the latter, the mammary glands are not only isolated from hairs, but even the nipple prevents hair from forming around it by the means of the BMP pathway (Mayer et al. 2008). During evolution, this pathway may have been progressively co-opted to suppress hair follicle formation in the nipple vicinity, first in marsupials and then more efficiently in eutherians."
Mammalian skin evolution: a reevaluation
"Early Theropsid Amniotes evolved a skin structurally and functionally similar to that of modern toads, living amphibians whose reduced epidermal mucogenicity and cornification permits them to walk over dusty driveways and hide in garages!"
So non-therapsid synapsids very likely had naked, glandular, amphibian-like skin. Any thoughts?