The Origin and Evolution of Mammals - Moodle

142 THE ORIGIN AND EVOLUTION OF MAMMALS
provided by the dentary condyle and glenoid fossa
of the squamosal complete with a postglenoid
ridge, the complete ossification of the internal wall
of the orbit, and the expansion of the brain case. Set
against these is a range of characters presumed to
be primitive since they do not occur in contemporary
mammals, but are found in tritheledontans
and/or tritylodontids. The most important is the
dentition. There are only five, multicusped postcanine
teeth, which are fairly simple and lack anything
more than a trace of a cingulum. Furthermore,
there is no evidence of true occlusion between the
upper and lower teeth. Comparison of a series of
specimens of differing sizes shows that the mode of
tooth replacement was cynodont-like rather than
mammal-like (Zhang et al. 1998; Kielan-Jaworowska
et al. 2004). The incisors and canines were replaced
several times in an alternating pattern, while anterior
postcanines were shed but not replaced. New
postcanines were added on sequentially at the back
of the tooth row.
Sinoconodon had a relatively larger body size than
contemporary mammals, with a maximum skull
length of about 6 cm. Interestingly, a wide range of
skull sizes has been found, indicating that the
growth from juvenile to adult was gradual, as in the
primitive amniote type, rather than the brief, rapid
pattern of juvenile growth that is typical of mammals
(Luo 1994). No postcranial skeleton has yet
been described, which might confirm this interpretation,
but in any event, the Sinoconodon growth
pattern is presumably correlated with its multiple
tooth replacement.
Phylogenetically, Sinoconodon was undoubtedly
close to the base of the mammalian radiation, as indicated
by its retained primitive characters. Indeed,
leaving aside the poorly known Adelobasileus and
the poorly understood haramiyids, it is more or less
universally accepted as the sister-group of the rest of
the Mammalia (Crompton and Luo 1993; Luo 1994;
Kielan-Jaworowska et al. 2004).
Morganucodonta
Parrington (1941) described two isolated teeth, one
premolariform and the other molariform, that had
been discovered by Walter Kühne in a Late Triassic,
probably Norian fissure-fill deposit from a limestone
quarry in Somerset, south-west England. They
were undoubtedly mammalian because of their multiple
cusps with a cingulum at the base, and their
double roots, and he referred them to a new genus
Eozostrodon. The main characteristic of the molariform
tooth was a linear row of three main cusps on
the crown, in which respect it resembled both carnivorous
cynodonts such as Thrinaxodon, and certain
Middle and Upper Jurassic mammals known
as triconodonts. This, plus the fact that they were
by some way the earliest mammalian remains discovered,
apart from haramiyidan teeth, pointed to
their considerable significance in the early history
of mammals. Unfortunately, little more could be said
about such extremely limited material. However,
Kühne went on to announce the discovery of further,
similar material in a quarry in South Wales
(Kühne 1949). This proved to be extraordinarily
abundant, yielding many thousands of teeth, jaws,
and other skeletal fragments, which in due course
formed the basis of the first serious appreciation of
the nature of a very early mammal. The exact date
of the South Wales fissure material is not certain
and may be latest Triassic or earliest Jurassic. There
was some early dispute about the nomenclature.
Mills (1971) described the teeth, and Kermack et al.
(1973) the lower jaw, and dentition under Kühne’s
name of Morganucodon. Parrington (1971) also
described the dentition, but under the name
Eozostrodon, in the belief that the new material was of
the same genus as his original Somerset teeth.
Reviews at time were divided. Jenkins and
Crompton (1979) and Kemp (1982) referred to the
material as Eozostrodon, but Kermack and Kielan-
Jaworowska (1971) as Morganucodon. Eventually, consensus
was reached that the two teeth of Eozostrodon
are an inadequate basis upon which to make a satisfactory
diagnosis of a genus, and Morganucodon is
now universally used for the Welsh form. Fossils
referred to various genera and species of morganucodontans
have now been found in China, South
Africa, North America, and India, as well as other
parts of Europe, indicating a virtually worldwide distribution
(Lucas and Hunt 1994).
Thanks primarily to the work of Mills (1971) on
the dentition, Parrington (1971) on the jaws and
dentition, Kermack et al. (1973, 1981) on the jaws
and skull, Jenkins and Parrington (1976) on the