The Origin and Evolution of Mammals - Moodle

116 THE ORIGIN AND EVOLUTION OF MAMMALS
at the jaw articulation that was borne by the quadrate
and articular, so allowing them even greater
mobility. The great reduction in size of the reflected
lamina of the angular of cynodonts, but not its
actual loss was always a mystery, but can now be
seen to be a reduction to a suitably delicate bone
supporting the tympanum without adding excessively
to its mass. In the course of a detailed review
of the evolution of the quadrate in cynodonts and
early mammals, Luo and Crompton (1994) have
provided very detailed evidence for the supposed
transition. From a primitive condition represented
by Thrinaxodon, through a sequence of increasingly
advanced forms to the early mammal Morganucodon,
there is a reduction in size of the quadrate. This was
coupled with a shift in the orientation of its contact
with the squamosal, creating an increasingly effective
axis for transmitting vibrations between the
presumed tympanic membrane and the stapes.
Indeed, a direct comparison of their reconstructions
of the morphology of this region in Thrinaxodon
and Morganucodon shows a remarkable similarity
(Fig. 4.9(h)). The final step occurred in later mammals,
where the tympanic bone and ear ossicles lost
all contact with the dentary bone in the adult, being
now supported only by the crista parotica, which is
a small process of the periotic bone. They are protected
within a partial bony housing formed from
the surrounding bones, alisphenoid or periotic.
The implication of Allin’s theory for pre-cynodont
stages of mammal-like reptiles is that they did not
possess a tympanic membrane for sound reception,
and therefore had very poor impedance matching
for converting air-borne sound waves to waterborne
sound waves in the inner ear. In pelycosaurs,
the massiveness of the stapes indicates that it still
served its primitive function of mechanical support
between the braincase and palate. Nevertheless,
even at this stage the stapes was associated with an
open fenestra ovalis so must have been involved in
sound reception. There is no difficulty postulating
detection of low-frequency, high-amplitude sound,
received via the ground or the side of the skull. It
would have been transmitted by intermolecular
vibrations within the bony tissue, rather than by
vibration of a light, low-inertia stapes. In the gorgonopsians
the stapes is lightened by the presence
of a very large stapedial foramen, suggesting that
a mechanism involving vibration of the bone as a
whole had evolved, a mechanism potentially capable
of detecting higher-frequency sound. Even so,
there is no indication of the presence of a specialised
tympanic membrane, so the lower jaw as a
whole was probably still the initial sound receptor.
Therocephalians do not possess a stapedial foramen,
an unexpected and unexplained fact, but certainly
not one that suggests acute high-frequency
hearing ability in this group. The reflected lamina
of the angular of primitive therapsids is large and
primarily a site of muscle attachment so any role it
may have had as an incipient eardrum would have
been at best highly inefficient. Thus the anatomical
evidence points to the cynodonts, with their
reduced reflected lamina, as the first to develop a
dedicated tympanic membrane on the lower jaw.
The cochlea of mammals is considerably longer
than that of cynodonts, indicating enhanced sensitivity
to a range of frequencies (Kielan-Jaworowska
et al. 2004). It has become housed within a swollen
promontorium on the ventral side of the skull,
formed entirely from the periotic bone, a feature
which, Luo et al. (1995) suggest, increased the
sound insulation of the organ from the surrounding
cranial structures.
Olfaction
Most modern mammals have a very acute sense of
smell in terms of range of discrimination and acuity
of reception. Little can be inferred about the sense
of smell of non-mammalian synapsids, but what
signs there are strongly suggest early elaboration of
the faculty. In modern mammals, the nasal cavity is
large and the area of the olfactory epithelium is
greatly increased by thin, bony turbinals extending
into it from the roof and sides (Fig. 4.10(a)). The
naso-turbinals in the roof, and the ethmo-turbinals
in the postero-dorsal region of the cavity are the
main olfactory areas; the maxillo-turbinals lying
laterally along the course of the airflow through the
nasal cavity are associated with mucous epithelium
related to the respired air and endothermy, as discussed
elsewhere. No turbinal bones as such have
been discovered in any non-mammalian synapsid,
but fine ridges on the internal surfaces of the nasal
and frontal bones are universally present from
pelycosaurs onwards (Fig. 4.10(b)–(e)). These are