The Origin and Evolution of Mammals - Moodle

154 THE ORIGIN AND EVOLUTION OF MAMMALS
blade-like lower premolar tooth, it became clear
that these are not multituberculates at all, but a
quite separate group, Gondwanatheria. They have
since been described from Madagascar and India as
well (Krause et al. 1997), and there is a possible, but
virtually indeterminate jaw fragment from Tanzania
(Krause et al. 2003). Thus the group is restricted to
the Gondwanan continents. The molar teeth of gondwanatheres
are hypsodont, and have transverse
enamel ridges that develop on the occlusal surface,
once the softer dentine and enamel has worn
down. They were presumably yet another group of
Mesozoic mammals adopting the rodent-like habitat
of small herbivore, feeding on tough, high-energy
vegetable matter. In the absence of more material
their relationships remain totally obscure.
Diversity and evolution
The skull of multituberculates (Fig. 5.9) is broad
with a short, blunt snout, and laterally facing orbits.
The jugal, long thought to be absent, is now known
to consist of a very thin plate attached to the inner
face of the otherwise robust zygomatic arch (Hopson
et al. 1989). Other cranial characters in which they
differ from other mammals are the absence of a
component of the palatine bone in the orbital wall,
although Hurum (1994) noted some exceptions,
and the nature of the specialised jaw joint permitting
antero-posterior movements of the lower jaw.
Perhaps to free the jaw for this propalinal movement,
multituberculates possessed ear ossicles free
of any connection to the dentary bone (Fig. 5.12(b)),
and which were therefore more or less identical to
those of living mammals (Miao and Lillegraven
1986; Hurum et al. 1996). The dentition is the most
strikingly unique character of the group. The upper
incisors are reduced in number to a maximum of
three, the second of which is enlarged, and worked
against the single, large, procumbent lower incisor.
The lower premolars are equally distinctive, with
serrated, blade-like crowns that had a shearing or
chopping action against the multicusped upper
premolars. The molars are reduced to two in number.
Both the uppers and the lowers have two rows
(three in the upper molars of more advanced forms)
of up to eight blunt cusps arranged longitudinally,
and with no connecting ridges between them.
When in occlusion, the rows of cusps of the upper
molars fit into the valleys between cusp rows of the
lowers, and vice versa, and the posterior shift of the
lower jaw then caused a very effective grinding
action, as discussed shortly.
The traditional classification of the approximately
70 genera of multituberculates was into
three superorders, the Plagiaulacoidea for the more
primitive ones of the Upper Jurassic and European
Early Cretaceous; the Ptilodontoidea for the
advanced, predominantly North American forms;
and the Taeniolabidoidea for the advanced, predominantly
Asian forms (e.g. Clemens and Kielan-
Jaworowska 1979; Hahn and Hahn 1983). However,
subsequent attempts at formal cladistic analyses
have amply confirmed the suspicion that this classification
is a very poor reflection of the phylogeny
of the group (Simmons 1993; Rougier et al. 1997;
Kielan-Jaworowska and Hurum 1997). On the other
hand, there is far from a consensus amongst these
authors on what the true interrelationships are. In
the most recent review, Kielan-Jaworowska and
Hurum (2001) analysed 62 characters amongst 32 taxa
and obtained 17,783 equally parsimonious trees.
This extremely low resolution results from a combination
of conservatism in many characters, extensive
convergence and mosaic evolution particularly in
the dentition, and the inevitable problem of missing
characters in many of the taxa that are known from
very incomplete material. At any event, they were
unable to derive from the cladogram a satisfactory
cladistic classification, and instead proposed to retain
the admittedly paraphyletic group ‘Plagiaulacida’ for
the generally primitive forms such as Plagiaulax
itself, and a possible but by no means very strongly
supported group Cimolodonta for many but not
all the more advanced forms (Kielan-Jaworowska
et al. 2004).
Paulchoffia is the most plesiomorphic ‘plagiaulacid’
(Fig. 5.9(a)). It comes from the Upper Jurassic
Guimarota Lignite Mine in Portugal and has retained
what is presumed to be the primitive multituberculate
dentition of I3/1: C0/0: PM5–4/4–3: M2/2. The
lower premolars differ from those of all other
multituberculates in their rectangular shape, and
horizontal wear facets on the crowns that indicate a
crushing rather than shearing function. The molar
teeth have a relatively small number of cusps, there
being only three or four in each row. The North