The pterygoid and ectopterygoid in mammals.

A dorsal pterygoid element and a ventral ectopterygoid element can be recognised during the development of monotremes, marsupials and eutherian mammals. Their homology with the elements so named in fossils ancestral to mammals can be established by positional evidence. In monotremes the elements remain distinct and show specialised features, including a hamular cartilage in the ectopterygoid of one specimen of Ornithorhynchus. In most higher mammals the pterygoid element is much reduced and is replaced anteriorly by the perpendicular plate of the palatine. Posteriorly the pterygoid element fuses with the ectopterygoid, in many cases before the onset of ossification. The hamular cartilage arises by chondrification within the ectopterygoid element and shows no sign of being a separate morphological entity, but must be regarded as a specialised feature associated with the architecture of the palatal musculature. There is a strong case for the value of recognising that the 'pterygoid process' of higher mammals includes both a pterygoid and an ectopterygoid moiety.