This section covers mainly the
Mesozoic mammals, together with the monotremes. One of the major
problems in this area is Dr. Thomas Rich. That is, everyone else
seems fairly happy with the current arrangement, save that Dr.
Rich keeps finding jaw bones that look for all the world like
placental mammal (eutherian) bones, but are (a) older than any
placental mammal ought to be and (b) located in Australia -- a
placenta-free zone. Rich et
al. (2001). The (now) conventional wisdom is that the
characterstic "tribosphenic" molars of mammals evolved
twice and that Dr. Rich's specimens are the results of convergent
evolution in some monotreme ancestors, Australosphenida,
and not placentals misplaced in time and space, Ausktribosphenidae.
Luo et al. (2001a);
Luo et al. (2002).
With the unyeilding commitment to principled choice which
characterizes our commentary, we have used both terms.
This reflects the fact that, while we officially think Dr. Rich
is wrong, we also have a sneaking suspiscion that he may be right.
In either event, we are now completely prepared to claim that we
supported the winner all along, no matter who may prevail. (ATW
The following is a condensed reminder
of some of the many changes from basal synapsids to
basal mammals. Beyond this level, there appears to be an
unreasonable amount of disagreement on the phylogenetic branching
order. I have recently seen two completely different published
phylogenies of basal mammals, both in standard texts, and both of
which purported to rely on exactly the same reference!
- Metabolic rate: transition to more or
less full homeothermy inferred from geographic range,
nocturnal habit, etc.
- Temporal fossa: increase in size;
confluence with orbit.
- Zygomatic Arch: development; replacement
of jaw adductor by masseter as principal jaw muscle;
greatly increased capacity for oral processing of food.
jaw: dentary becomes only significant element of
mandible; development of coronoid process; reduction of
post-dentary elements; reflected lamina of angular;
dentary-squamosal jaw articulation. See infra, ear.
- Dentition: development of heterodont
dentition with incisors, canines, pre-molars and molars;
"permanent" (diphyodont) teeth with prismatic
enamel; increasingly fixed pattern of occlusion; definite
- Palate: full secondary palate.
- Ear: reflected lamina of the angular (tympanic);
retroarticular process of articular; conversion of post-dentary
bones to sensory use in middle ear. Reflected lamina may
have been resonating chamber, followed by development of
tympanic membrane framed by increasingly small and
gracile reflected lamina and/or retroarticular process
- Pineal foramen: pineal foramen initially
becomes more conspicuous, then recedes and is lost.
- Skull table: development of parietal
crest with muscular attachment on outside of dermal bones.
- Braincase: parietal and squamosal spread
downward to cover braincase, gradually replacing (in
advanced mammals) neurocranium while providing muscle
attachment on lateral (formerly dorsal) surface;
epipterygoid changes from pillar supporting parietal and
braincase to alisphenoid element of skull, continuous
with parietal, squamosal, & petrosal (fused otic
capsule). Brain size does not increase relative
- Skull fusions: fusion of parietals and
frontals, otic capsule, occipitals, numerous other
elements in therians; loss of dermal bones, e.g. post-orbitals.
- Skull attachment: double occipital
condyle, condyles move ventrally, development of
mammalian circular form.
- Spine: loss of lumbar ribs (reversed in
& lost again in Mammalia); increase in number of
sacral vertebrae (??); reduction of tail; vertebral
articulations changed to accommodate dorsoventral
undulation; vertebrae amphiplatyan (flat on both ends),
implying loss of notochord remnants (?).
- Limb girdles: reduction (e.g. pubes,
coracoids) or loss of ventral elements; more vertical
orientation of limbs.
- Limbs: more vertical orientation;
elongation of humerus & femur; digits shorter;
- Integument: fur?; mammalian muzzle
- Habit: primitively large carnivores;
great reduction in size; development of omnivorous and
herbivorous adaptations. --ATW 001202