The lower jaw of Morganucodon

Abstract

The genus Morganucodon is found in Yunnan, China, in normal (non-karstic) sedimentary deposits (Lufeng beds) of probable Rhaetian age; and in Wales in karstic deposits in the Carboniferous Limestone. These latter deposits cannot be younger than Sinnemurian or older than Rhaetic. A new suborder—Morganucodonta—of the Triconodonta is created for Morganucodon and its allies. Morganucodon and Eozostrodon are not synonyms. The lower jaw of Morganucodon resembles closely that of an advanced cynodont, except for the presence of a squamosal-dentary joint in the former. There was no reduction in the functional importance of the reptilian (quadrate-articular) jaw-joint in passing from the cynodont condition to that of Morganucodon. The mechanism of shearing is discussed. The action of the cheek-teeth is pure shear. Primarily, the function of the squamosal-dentary articulation was to resist couples produced by the shearing, thus primitively, the presence of efficient shearing cheek-teeth is associated with a squamosal-dentary articulation. The skull of Morganucodon will be described in a later paper.