Editing Synapsida (section)

==== Mammary glands ====
Early synapsids, as far back as their known evolutionary debut in the Late Carboniferous period,<ref name="Oftedal-2002a"/> may have laid parchment-shelled (leathery) eggs,<ref name="Oftedal-2012"/> which lacked a calcified layer, as most modern reptiles and [[monotreme]]s do. This may also explain why there is no fossil evidence for synapsid eggs to date.<ref name="Oftedal-2002b">{{cite journal |last=Oftedal |first=Olav T. |date=2002-07-01 |title=The origin of lactation as a water source for parchment-shelled eggs |journal=Journal of Mammary Gland Biology and Neoplasia |volume=7 |issue=3 |pages=253–266 |issn=1083-3021 |pmid=12751890|doi=10.1023/A:1022848632125|s2cid=8319185}}</ref> Because they were vulnerable to desiccation, secretions from [[Apocrine sweat gland|apocrine]]-like glands may have helped keep the eggs moist.<ref name="Oftedal-2002a"/>

According to Oftedal, early synapsids may have buried the eggs into moisture laden soil, hydrating them with contact with the moist skin, or may have carried them in a moist pouch, similar to that of monotremes ([[echidna]]s carry their eggs and offspring via a temporary pouch<ref>{{Cite web |url=http://www.life.umd.edu/classroom/bsci338m/Lectures/Monotremes.html |title=Monotremes and marsupials |website=www.life.umd.edu |access-date=2018-08-23}}</ref><ref>{{cite web |title=Life History and Ecology of the Monotremata |website=www.ucmp.berkeley.edu |url=http://www.ucmp.berkeley.edu/mammal/monotremelh.html |access-date=2018-08-23}}</ref>), though this would limit the mobility of the parent. The latter may have been the primitive form of egg care in synapsids rather than simply burying the eggs, and the constraint on the parent's mobility would have been solved by having the eggs "parked" in nests during foraging or other activities and periodically be hydrated, allowing higher clutch sizes than could fit inside a pouch (or pouches) at once, and large eggs, which would be cumbersome to carry in a pouch, would be easier to care for. The basis of Oftedal's speculation is the fact that many species of [[Anura (frog)|anura]]ns can carry eggs or tadpoles attached to the skin, or embedded within cutaneous "pouches" and how most [[salamander]]s curl around their eggs to keep them moist, both groups also having glandular skin.<ref name="Oftedal-2002b"/>

The glands involved in this mechanism would later evolve into true mammary glands with multiple modes of secretion in association with hair follicles. Comparative analyses of the evolutionary origin of milk constituents support a scenario in which the secretions from these glands evolved into a complex, nutrient-rich milk long before true mammals arose (with some of the constituents possibly predating the split between the synapsid and [[sauropsid]] lines). [[Cynodont]]s were almost certainly able to produce this, which allowed a progressive decline of yolk mass and thus egg size, resulting in increasingly [[altricial]] hatchlings as milk became the primary source of nutrition, which is all evidenced by the small body size, the presence of [[epipubic bone]]s, and limited tooth replacement in advanced cynodonts, as well as in [[mammaliaforms]].<ref name="Oftedal-2002a">{{cite journal |last=Oftedal |first=Olav T. |date=2002-07-01 |title=The mammary gland and its origin during synapsid evolution |journal=Journal of Mammary Gland Biology and Neoplasia |volume=7 |issue=3 |pages=225–252 |issn=1083-3021 |pmid=12751889 |doi=10.1023/a:1022896515287 |s2cid=25806501}}</ref><ref name="Oftedal-2012">{{Cite journal |last=Oftedal |first=O.T. |date=2012-03-01 |title=The evolution of milk secretion and its ancient origins |journal=Animal |volume=6 |issue=3 |pages=355–368 |doi=10.1017/S1751731111001935 |doi-access=free |issn=1751-732X |pmid=22436214|bibcode=2012Anim....6..355O }}</ref>