Temporal evolution: is early mammal evolution best characterized by major long branches reaching deep into the Mesozoic and by the long evolutionary fuse that delayed diversification within long branches? short-lived branches with a short evolutionary fuse before diversification?

.Ecological diversification: is lineage splitting of early mammals decoupled from, or correlated with ecological diversification?

.Morphological transformation: are originations of key mammalian characters singular evolutionary events, or iterative convergences despite their complexity?

Temporal pattern of early mammal evolution
The evolution of early mammals occurred in successive diversifications or episodes of quick splitting of relatively short-lived clades.
Diversification of premammalian mammaliaforms—the extinct relatives outside mammals—during the Late Triassic and Early Jurassic, the Middle Jurassic diversification of docodonts, theriiform mammals, and the australosphenidan mammals that are basal to monotremes, the Late Jurassic diversification within the extinct theriiform groups that are closer to marsupials and placentals than to monotremes, and the Early Cretaceous divergence of the marsupial lineage and the placental lineage.
Cenozoic placentals and marsupials represent a new episode of diversification in succession to the Cretaceous stem eutherians and metatherians
Successive diversification of Mesozoic mammaliaforms as a whole.
It is uncommon for any Mesozoic group to maintain a long history with little diversity.
Broad discrepancies between the dating by: - fossils and the estimate by moleculesthat lineages could phylogenetically diverge long before their morphological diversification - splits of early mammal lineages were not accompanied by morphological differences and were ‘silent’ with regard to their ecological diversification.

Ecological diversification on Mesozoic mammals
There is no question that this spectacular ecomorphological diversification accelerated in an Early Cenozoic adaptive radiation of mammals into the niches vacated on the extinction of non-avian dinosaurs.

The hypothesis of the decoupling of phyletic divergence from ecological diversifications rests on the assumption that the major Mesozoic mammal groups lacked ecological specializations, other than generalized habits. This assumption is now falsified by discoveries of several new Mesozoic mammals with convergences to highly specialized extant mammals.

Fossorial behaviour was documented by taphonomic evidence or some premammaliaform cynodonts, but only recently did the fossorial skeletal specializations (such as scratch digging) become known for mammaliaform lineages.

The hypertrophied digging-limb features and the unique columnar and enamel-less teeth were developed in the Late Jurassic Fruitafossor 150 million years (Myr) ago, 100 Myr before a similar character complex evolved convergently in armadillos and aardvarks, among placentals.

New skeletal fossils suggest that some (although not all) Mesozoic eutherians and metatherians and their near kin also developed such adaptations(climb on uneven terrain) , as shown by the elongate intermediate phalanges and convex profiles of manual and pedal distal phalanges, and in the tarsus, among other skeletal features.

They unveiled a new picture in which ecological diversification is not unique to the Early Cenozoic mammalian radiation, and that many dead-end Mesozoic mammal clades developed similar ecomorphotypes long before the analogous modern mammals.

The decoupling hypothesis can certainly be rejected as a rationale for the gap between molecular time estimates and the first appearance in fossil data of the major placental and marsupial lineages. Correlation of ecomorphological specializations with phylogenetic splitting is a basic feature of Mesozoic mammal evolution. Cenozoic placental carnivores are an independent case for correlated ecomorphological and phyletic diversifications.

Transformation of key evolutionary apomorphies
In more derived premammalian mammaliaforms, the dentary is so enlarged as to have a condyle articulating with the squamosal glenoid, forming the true mammalian jaw hinge, known as the temporomandibular joint. Further along the evolution of living mammals, the middle ear became detached from the mandible to form the ‘cranial middle ear’, or the definitive mammalian middle ear. The detachment of the middle ear from the mandible in adults and the mobile suspension of the middle ear via the incus to the cranium are crucial for sensitivity of the mammal middle ear.
The stapedial process is present in most mammaliaforms, fulfilling the crucial function of the out-lever of the middle-ear lever system for the impedance-match and amplification, but is absent in the incus of most premammaliaform cynodonts. has a discontinuous distribution: it is present in tritylodontids, Brasilitherium and Morganucodon, but absent in the tritheledontid. The loss of the quadratojugal and the development of the stapedial process are not only homoplastic in their overall distributions, but are also in conflict with each other. It is abundantly evident that separate evolutionary experiments occurred repetitively during the transformation of the incus structure for better impedance-match and hearing sensitivity.

In premammalian outgroups, the middle ear is attached to the dentary. In extant mammals the connection between the dentary and Meckel’s element is conserved in embryonic and fetal stages, but lost in the adult owing to the re-absorption of embryonic Meckel’s cartilage, the homologue to part of the prearticular. Because reabsorption of
Meckel’s cartilage is crucial for extant mammals to complete the ontogeny of their middle ear, an early ossification of Meckel’s cartilage influenced the retention of the ectotympanic–dentary connection in some major Mesozoic mammal groups. This provides a common ontogenetic heterochrony as a main mechanism for the homoplastic evolution of a critical component of the mammalian middle ear.

Dental evolution was far more labile in Mesozoic mammals than can be inferred from Cenozoic mammals and is consistent with the functional analysis that there was more than one pathway to combine slicing and crushing functions, as exemplied by tribosphenic and pseudotribosphenic molars for more effective faunivory and omnivory, in early mammalian history.

Concluding remarks
Two classic examples of this paradigm are the sensitive hearing by the sophisticated middle ear in the earliest mammaliaforms, leading to exploitation of the nocturnal niches, and the versatile functions of the tribosphenic molar in northern tribosphenic mammals, leading to the great diversification of marsupials and placentals.

No evidence to the contrary, it was granted that processes of evolutionary innovation leading to ecological diversification were singular events—these evolutionary innovations of mammals are so intricate and unique that it would be unlikely for these sophisticated structures to be homoplastic.
Homoplasies in the definitive mammalian middle ear by the ossified Meckel’s cartilage in eutriconodonts are a case of developmental heterochrony.

lineage splits are accompanied by significant ecological diversification and by more labile developmental patterning in early mammal evolution.