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.
- Lower 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 dental formula.
- 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 to diapsids.
- 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 advanced cynodonts
& 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; calcaneal heel
- Integument: fur?; mammalian muzzle
- Habit: primitively large carnivores; great reduction in size; development of omnivorous and herbivorous adaptations. --ATW 001202