Morganucodonta

Anatomy and biology

Morganucodontans had a double jaw articulation made up of the dentary-squamosal joint as well as a quadrate-articular one. This implies that they also retained one of their postdentary bones: the articular. There is a trough at the back of the jaw on the inside (lingual) where the postdentary bones sat. The double jaw articulation and retention of the postdentary bones are characteristic of many of the earliest Mammaliaformes, but are absent today in mammals: All crown-group mammals have a lower jaw that is composed of a single bone, and the articular has been incorporated into the middle ear, having become the malleus (while the quadrate has become the incus).

Unlike more basal therapsids such as Sinoconodon, morganucodontan teeth were diphyodont (meaning that they replaced their teeth once, having a 'milk' set and 'adult' set as seen in today's mammals including humans and not polyphyodont (meaning that the teeth are constantly replaced, and the animal and its teeth get larger throughout its lifetime, as in reptiles). Evidence of lactation is present in Wareolestes, via tooth replacement patterns.

Morganucodontans also had specialised teeth—incisors, canines, molars and premolars—for food processing, rather than having similarly shaped teeth along the tooth row as seen in their predecessors. Morganucodontans are thought to have been insectivorous and carnivorous, their teeth adapted for shearing. Niche partitioning is known among various morganucodontan species, different types specialised for different prey. Paceyodon davidi was the largest species of morganucodontans. It is known by a right lower molariform 3.3 mm (0.13 in) in length, which is bigger than molariforms of all other morganucodontans.

The teeth of some morganucodonts are structured in such a way that occlusion happens by the lower cusp "a" fitting anteriorly to upper cusp "A", between "A" and "B". This occlusion pattern is by no means universal among morganucodontans, however; Dinnetherium, for example, has an occlusion mechanism closer to that of the eutriconodonts, in which molars basically alternate. Wear facets are present.

The septomaxilla, a primitive feature also found in Sinoconodon, is found in morganucodontans, as well as a fully ossified orbitosphenoid. The anterior lamina is enlarged. The cranial moiety of the squamosal is a narrow bone that is superficially placed to the petrosal and parietal. Unlike its predecessors, the morganucodontans have a larger cerebral capacity and a longer cochlea.

The atlas elements are unfused; there is a suture between the dens and axis. The cervical ribs are not fused to the centra. The coracoid and procoracoid, which are absent in therians, are present. The head of the humerus is spherical as in mammals, but the spiral ulnar condyle is cynodont-like. In the pelvic girdle, the pubis, ilium and ischium are unfused. At least Megazostrodon and Erythrotherium are unique among basal mammaliaforms for lacking epipubic bones, suggesting that they did not have the same reproductive constraints.

Morganucodon at least appears to have had a long lifespan and a lower metabolism than modern mammals.