The Origin and Evolution of Mammals - Moodle

118 THE ORIGIN AND EVOLUTION OF MAMMALS
interpreted as the sites of attachments of sheets of
turbinals that were cartilaginous rather than bony,
and which presumably enlarged the area of olfactory
epithelium available several-fold.
Jacobson’s, or the vomero-nasal organ, is a characteristic
tetrapod olfactory organ situated in the
floor of the nasal cavity, utilised primarily in social
signalling by detecting pheromones (Duvall 1986).
Undoubtedly the organ was present in all the
mammal-like reptiles since it is still present and often
well developed in modern mammals, but there has
been much debate and confusion about its position
and possible relationship with the prominent
foramen and adjacent canal in the septomaxillary
bone of the snout region of synapsids (Fig. 4.10(f)).
In a detailed comparison with living tetrapods,
Hillenius (2000) concluded that in non-mammalian
synapsids, Jacobson’s organ was a paired tubular
structure lying in the floor of the front part of the
nasal cavity, and that it had an association with the
naso-lacrimal, or tear duct. He proposed that this
duct ran from the glands in the orbit, particularly a
Harderian gland, through the snout and opened
at the septomaxillary foramen (Fig. 4.10(g)). He
assumed that, as in many living tetrapods, the exudate
of the Harderian gland is a serous fluid that
moistens the nasal area, so that it collects odour
molecules, which then pass backwards to Jacobson’s
organ for detection. In living mammals, the situation
has changed (Fig. 4.10(h)). Jacobson’s organ
receives molecules directly from the oral cavity,
usually via a naso-palatine duct. The naso-lacrimal
duct no longer has a direct association with Jacobson’s
organ, but discharges directly to the external nostril
and fleshy rhinarium of the snout. The nature of the
fluid derived from the Harderian gland has also
altered, and is high in lipids. It serves two functions,
one is to deliver pheromones to the body surface,
and the other is to provide waterproofing material
to be spread over the fur. This changed function in
mammals is associated with reduction of the septomaxilla
bone and loss of its foramen and canal,
and therefore this osteological modification may be
an indication of the presence of insulating fur, and
of more complex social behaviour. Related to this,
Duvall (1986) speculated that social signalling by
pheromones was an essential precursor of the evolution
of lactation and mammalian levels of maternal
care. Cynodonts still possessed the primitive form
of septomaxilla, foramen, and canal, whereas mammals
from Morganucodon onwards have the reduced
version lacking the foramen. Perhaps this is evident
that cynodonts were uninsulated and lacked
complex behaviour, though it is hard to be convinced
by such tenuous reasoning.
Brain
The external features of the brains of mammals and
birds are indicated very well by the impressions on
the inner surfaces of the braincase because the brain
is almost entirely enclosed within bone. The relatively
much smaller brains of other amniotes are
not so enclosed and therefore it is impossible to
reconstruct completely the size, or the relative sizes
of the different parts from cranial material alone.
Typically the hindbrain, medulla oblongata, and
cerebellum are determinable, and also the form of
the dorsal surface. But the sides and floor of the
midbrain and forebrain, including the cerebral
hemispheres are not. Therefore, there is scope for
considerable disagreement about the anatomical
evolution of brains in synapsids.
An endocast of the brain of a specimen of the
pelycosaur Dimetrodon was described long ago by
Case (1897; Hopson 1979), and indicates a primitive,
tubular structure lacking evidence for large
expansions of any of its regions.
For the basal therapsids, Kemp (1969b) reconstructed
the brain of gorgonopsians on the basis of
a number of acetic acid-prepared braincases of
large specimens (Fig. 4.11(a)). One of these has
sheets of a crystalline material that may have
replaced cartilaginous sheets present in life, and if
this interpretation is correct, then a fairly complete
representation of the external form of its brain is
possible. Compared to the pelycosaur endocast, the
cerebellum was significantly larger and there is an
impression of a relatively large optic lobe. There is
no room for the cerebrum to have been greatly
enlarged, but it is possible that it was significantly
larger than the pelycosaur tubular form.
Several authors have attempted to reconstruct
the brain of cynodonts, with inconsistent results.
According to Hopson’s (1979) review of endocranial
casts, all cynodonts possessed a tubular brain
at the upper end of the size range of those of