The Origin and Evolution of Mammals - Moodle

60 THE ORIGIN AND EVOLUTION OF MAMMALS
Cynodontia
The cynodonts were the last major group of the Late
Permian radiation of therapsids to make an appearance,
being first recorded just before the close of the
period as rare elements of the Dicynodon Assemblage
Zone fauna of South Africa and equivalent aged
beds in Zambia, Russia, and also Western Europe
(Sues and Munk 1996). Despite this modest beginning,
the cynodont taxon survived the end-Permian
and underwent a broad radiation in the Triassic, one
branch of which culminated in the mammals. Thus
they are fundamental to understanding the transition
from basal amniote to mammalian morphology.
Cynodonts share a number of characters with the
Therocephalia and the two are usually included in
a single group Eutheriodontia. The main similarities
are:
● narrowing of the intertemporal skull roof as the
temporal fenestrae expanded inwards, creating an
elongated, narrow sagittal crest
● reduction of the postorbital and postfrontal bones
● broadening of the epipterygoid
● discrete process of the prootic bone that contacts
the quadrate ramus of the pterygoid
● differentiation of the vertebral column into distinct
thoracic and lumbar regions, with the lumbar
ribs short, horizontal, and immovably attached
● tendency to elongate the ilium and reduce the
pubis in the pelvic girdle.
At one time it was thought that among the therocephalians,
the Whaitsiidae were closest to the cynodonts,
on the basis of a number of shared similarities
such as the extremely broad epipterygoid, reduction
of the suborbital fenestra, and closure of the
interpterygoid vacuity (Kemp 1972b). Cladistic
analysis, however, indicates that these features are
better interpreted as convergences in the two taxa
(Hopson and Barghusen 1986), perhaps associated
with a similar mode of strengthening of the skull
against powerful jaw action.
Cynodonts are a very well categorised group;
Hopson and Kitching (2001) noted no less than
27 synapomorphies, mostly cranial, of which the
more prominent ones are the following.
● Deepening and lateral flaring of the zygomatic
arch, and development of an adductor fossa on the
lateral face of the dentary bone, both characters
associated with the invasion of the lateral surface of
the jaw by adductor musculature.
● Sagittal crest between the paired temporal fenestrae
with deep, laterally facing surfaces for origin of
the temporalis musculature.
● Reflected lamina of the angular reduced.
● Quadrate and articular bones that constitute the
jaw articulation reduced in size.
● Dentition differentiated into unserrated incisors
and canines, followed by a series of simple anterior
postcanines, and more complex posterior postcanines
bearing accessory cuspules.
● Secondary palate formed from crests along the
lateral sides of the choanal vault.
● Epipterygoid greatly expanded as a major structural
component of the sidewall of the braincase
region.
● Occipital condyle double.
● Marked differentiation of thoracic from lumbar
regions of the vertebral column.
● Scapula deeply concave and coracoid reduced.
● Ilium expanded well forwards and pubis
reduced.
● Femur with inturned head, and strongly developed
major and minor trochanters.
Procynosuchia—primitive cynodonts
Procynosuchus
The Late Permian primitive cynodont Procynosuchus
is very well-known from several South African
skulls, and a virtually complete, acid-prepared
skeleton from the contemporaneous Madumabisa
Mudstone of Zambia described in detail by Kemp
(1979, 1980c). It has been recorded in Germany
(Sues and Boy 1988). By therapsid standards it was
a relatively small animal with a skull length of up
to 14 cm and a presacral length of about 40 cm.
As a primitive-grade cynodont, Procynosuchus
(Fig. 3.19(a)) illustrates an incipient stage in the
evolution of several advanced cynodont and mammalian
features. The first important one concerns
the dentition. The postcanine dentition has become
differentiated into five anterior premolariform
teeth, each of which consists of a slightly recurved
cusp with a small basal swelling at the back of the
tooth, followed by eight molariform teeth which