The Origin and Evolution of Mammals - Moodle

140 THE ORIGIN AND EVOLUTION OF MAMMALS
It is tantalising that so little is yet known of this
form, since it is likely that it will prove to be of considerable
importance in finally understanding the
relationships between tritylodontids, tritheledontans,
and the earliest groups of mammals. Further specimens
are awaited with interest.
Haramiyida
The next earliest known mammals are the haramiyidans,
which first appear in the Upper Norian part
of the Upper Triassic of Germany. This gives a date
for their appearance in the fossil record of about
212 Ma (Lucas and Hunt 1994), and they survived
until at least the Upper Jurassic of Tanzania (Heinrich
1999). Like Adelobasileus, the haramiyidans are tantalisingly
poorly understood, although in their case it
is for want of cranial material. Until recently they
were known only as isolated teeth, incisors, premolars,
and relatively large multicusped, doublerooted
molars (Sigogneau-Russell 1989a). Each
individual molar tooth consists of a broad crown
carrying a row of three large cusps along one edge
and a row of five smaller cusps along the opposite
one (Fig. 5.1(b) and (c)). The rows are connected at
one end by a transverse crest, and between them
lies a concave basin. For a long time it was assumed
that all the molariform teeth, both uppers and lowers,
were identical, and that the uppers must have
been simply reversed versions of the lowers. In this
arrangement, the three cusps of an upper tooth
would have occluded in the basin of a lower.
Simultaneously, the three cusps of the lower tooth
would have occluded in the basin of the same upper
tooth (Parrington 1947).
Most specimens were put into one of two genera,
either Haramiya or Thomasia, on the basis of small
differences in the teeth, although Sigogneau-
Russell (1989a) suggested that the two genera actually
represented upper and lower teeth, respectively.
Subsequently, Jenkins et al. (1997) described the first
known skeletal remains of a haramiyidan.
Haramiyavia comes from a Late Triassic locality in
East Greenland, and includes of a pair of dentaries
and a maxilla, all bearing teeth (Fig. 5.1(d)), along
with some of the postcranial bones. It shows that
Sigogneau-Russell’s (1989) interpretation was correct:
teeth of the Haramiya kind are the lowers, teeth
of the Thomasia kind the uppers. Haramiyavia also
demonstrates that the group has a fully mammalian
dentition. The dentary is not completely preserved
so it is uncertain whether it had a dentary condyle,
although the shape of the hind region of the bone is
very similar to that of other early mammals so that
it seems likely that a fully developed condyle was
indeed present in life.
The new material allows the tooth action to be
interpreted. There are four incisors, which are rather
specialised, for the first three lowers are strongly
procumbent, producing a near-horizontal surface
against which the four short, vertical upper incisors
could work. The incisors are followed by a diastema
and then a small canine tooth. The four increasingly
complex premolariform teeth are followed by three
molariforms, which made up the main chewing surfaces.
The wear facets on the opposing teeth indicate
that the action was completely orthal, with virtually
no horizontal movement of the lower teeth against
the uppers, either side-to-side or to-and-fro.
Haramiyavia (Fig. 5.1(d)) was a little larger than
most other mammals of the time, with a skull length
estimated to be about 4 cm. Its skeleton was lightly
built and agile, to judge by the handful of postcranial
bones preserved. It seems that the haramiyidans were
already specialised as small herbivores, or at least
omnivores, with a simple, crushing action of the dentition
that might have dealt with seeds, for example.
The relationships of the Haramiyida have always
been a mystery, because they are so specialised yet
occur so early in mammalian history. There had
even been doubts expressed about whether they
were true mammals at all, although the discovery
of Haramiyavia certainly dispels them. Comparing
them with the other mammal groups, the commonest
concept is that haramiyidans are basal relatives
of the multituberculates, as argued by Butler and
McIntyre (1994; Butler 2000). The most important
evidence for this is the form of the molariform
teeth, with their two multiple rows of cusps, and
the evidence for an element of antero-posterior
occlusal action in some genera, particularly Thomasia
(Haramiya). However, the similarities in molar structure
is not close, and the force of the occlusal character
is considerably weakened by the absence of
horizontal movement of the teeth in Haramiyavia
(Jenkins et al. 1997). There is really no good