Some recent items on the origin of suckling in mammals. I don't know if a more formal paper will be published soon.
Brasilitherium, ancient mammal relative could probably form a tight seal between its tongue and palate and might have suckled.Â
Origin of the Mammalian Fauces
fromÂÂ5th International Palaeontological Congress
Alfred Crompton, Catherine Musinsky, Jose Bonaparte, Bhart-Anjan BhullarÂ & Tomasz Owerkowicz (2018)
Speculations on the origin of the mammalian fauces: therians and monotremes compared.
5th International Palaeontological CongressÂAbstracts Book:Â
In therian mammals, several muscles control the transport of food through the fauces: the tensor veli palatini tenses the soft palate between the pterygoid hamuli; intrinsic tongue muscles change the tongueâs shape; and the palatoglossus acts together with extrinsic tongue muscles and the mylohyoid to draw the tongue against the soft palate and form a tight seal between the oral cavity and oropharynx. To determine the evolution of the fauces region of mammals, we studied serial sections of a pouch young marsupial, CT scans of non-mammalian cynodonts, ictidosaurs, mammaliaforms, and extant therians and monotremes. The heterodont dentition (incisors, canines and postcanines) of Permian therapsids suggests some processing of food occurred in the postcanine region, which would have required a way to prevent food from passing prematurely into their wide-open pharynx. Non-mammalian cynodonts may have achieved this by pushing the tongue against the pterygopalatine bosses of the pterygoids, but that would not have provided the tight seal achieved by therian mammals. Ictidosaurs and mammaliaforms of the Late Triassic were the first to attain such a tight seal in this region. The medial portion of the reptilian posterior pterygoideus muscle shifted its origin from the posterior surface of the transverse process of the pterygoid to a soft palate between the pterygopalatine bosses and formed the tensor veli palatini. We conclude that the pterygopalatine boss-not the ectopterygoid and not the lateral portion of the transverse process of the pterygoid-was the homologue of the mammalian pterygoid hamulus, and suggest that the palatoglossus arose at the same time as the tensor veli palatini. The lateral portion of the posterior pterygoideus in mammalian ancestors originated on the posterolateral border of the transverse process. When this process was lost in the transition to mammals, the origin of the muscle shifted to the pterygoid hamulus and palatine to form the mammalian medial pterygoid muscle. Monotremes extensively modified the fauces region: the palatine extends backward under the pterygoid portion of the pterygoid hamulus, and the hamulus portion retains its contact with the soft palate behind the palatines. The tensor veli palatini and palatoglossus are lost, but an enlarged mylohyoid helps the tongue break down invertebrates between the keratinized surfaces on the ventral surface of the palatine and dorsal surface of the tongue.Â