The Origin and Evolution of Mammals - Moodle

Although there is no mammal-like contact between
the dentary and the squamosal bone, eucynodonts
do have a secondary contact between one of the
postdentary bones, the surangular, and the
squamosal. It lies immediately laterally to the articular–quadrate
hinge and presumably functioned to
stabilise the jaw articulation. The front ends of the
paired dentaries have fused to form a strong,
immobile symphysis. The eucynodont postcranial
skeleton (Jenkins 1971b) shows a number of evolutionary
modifications towards the mammalian condition,
such as the appearance of a definite acromion
process on the scapular for attachment of the clavicle.
The ilium is more expanded and the pubis more
reduced compared to Thrinaxodon. The head of the
femur is well turned in and the major and minor
trochanters are enlarged and very mammalian in
form. In the early eucynodonts from the Cynognathus
Assemblage Zone of South Africa, such as
Diademodon and Cynognathus, the costal plates on the
ribs are still well developed, but by the Middle
(a)
Cynognathus
EVOLUTION OF MAMMAL-LIKE REPTILES 65
Triassic they are either greatly reduced, as in certain
traversodontids, or completely absent, assumed to
have been secondarily lost, as in probainognathians.
While there are several non-mammalian eucynodont
taxa that are well categorised and more or less
universally accepted, the interrelationships among
them remains a matter of controversy, and at least
three recent cladistic analyses have produced results
at variance with one another. Profoundly embedded
in this taxonomic disagreement is disagreement
about exactly which of the non-mammalian cynodonts
is the most closely related to Mammalia, an
issue taken up later in the section. The more or less
unchallenged monophyletic subgroups of eucynodonts
are these.
● Cynognathidae: a family of relatively primitive
carnivores
● Diademodontoidea: two families of herbivorous
eucynodonts, Diademodontidae and Traversodontidae
(b)
cor.pr.
cor.pr.
ang.pr.
ART
refl.lam.
ART
refl.lam.
Figure 3.21 (left) (a) Skull of Cynognathus in four views. Approx. skull length 29 cm (Broili and Schröder 1934). (b) Medial and lateral views of
lower jaw (Kermack et al. 1973). (right) Probainognathian cynodonts. (b) Skull of Lumkuia fuzzi in four views. Skull length approx. 6 cm (Hopson
and Kitching 2001) (c) Skull of Probainognathus jenseni in four views (Carroll 1988, from Romer 1970). (d) Chiniquodon (Probelesodon). Skull length
approx. 11 cm ( Romer 1969). (e) Lateral view of skull of Ecteninion lunensis. Skull length approx. 12 cm (Martinez et al. 1996). ART, articular.
ang.pr, angular process. cor.pr, coronoid process. refl.lam, reflected lamina of the angular. Continued overleaf