Literature

We are witnessing a transformation of thinking about the early evolution of mammals. There are now over 310 Mesozoic mammaliaform genera, but two-thirds of these have been named very recently (in the last 25 years). Prior to this period, based on the limited fossil data, morphological and ecological diversification was assigned to the period after the dinosaurs became extinct.
The fossil record was interpreted with the "broad generalisation that Mesozoic mammals failed to develop any ecomorphological specializations. They were viewed as small animals with a generalised feeding and unspecialized limb structure for terrestrial habits, and without the widely divergent ecological specializations of Cenozoic descendants".
This picture has had to be totally revised in the light of evidence to the contrary. A great variety of specialised animals have been documented, some large and others small. The assumption that the Mesozoic mammal groups lacked ecological specializations "is now falsified by discoveries of several new Mesozoic mammals with convergences to highly specialised extant mammals." Consequently, these "prevailing patterns" in the newly discovered fossils confront "out-of-date views that a few long branches of Cenozoic or extant mammals would extend deep into the Mesozoic".
The revised approach provides no resolution to the widely documented mismatch between the fossil record and molecular time estimates for the origins of the mammalian lineages.

"The older molecular dates would predict an abundance of long branches, and a long delay of diversification within each long branch after a branch's origin. However, studies using morphological data of both fossil and extant taxa demonstrate that there are few or no such lineages with a long evolutionary lag time. This discrepancy is so systemic and widespread that it cannot be explained by the difference between minimum age constraint (represented by actual fossils) and the timing of origin that can be hypothetically estimated by molecules in marsupial and placental evolution."

A further benefit in looking at the new data collectively is that it raises significant questions about the stepwise assembly of complex mammal structures advanced in "the traditional paradigm of early mammal evolution".

"Some best-documented 'textbook' scenarios are acquisitions of key characters along a transformation series: transformations of the mammalian middle ear and the jaw hinge, and evolution of the tribosphenic molars."

It was recognised that these innovations were "so intricate and unique" that they can have happened only once and it was thought that the transformation process could be documented. However, with a richer database and a greater number of cases to analyse, the picture has become confusing.

"Character conflicts are inevitable when more characters become available from better 'transitional' fossils. For the several key mammaliaform structures known to have evolved by incremental or step-wise assembly, their precursory conditions have shown character conflicts in the recently found fossils."

There are two ways of approaching this conundrum. The first recognises that there are design issues needing our attention (but this is typically rejected from further consideration). The second suggests that these structures must have evolved many times along many different pathways, and that we have overstated the complexity issues needing to be addressed. This is the option taken in this paper:

"This suggests labile evolutionary experiments before the accomplishment of the complex structure. [. . .] Perhaps, most interestingly, successive waves of Mesozoic mammal diversification multiplied the chances for many short-lived lineages to iteratively experiment with developmental patterning and ecological diversification that were previously known only for Cenozoic mammals, but are now shown to be widespread among Mesozoic mammals."

Suffice to comment here that this suggestion is a hypothesis that depends on labile developmental patterning and the simplification of complexity. Design inferences have a significant role here in testing the validity of such approaches. For more on Yanoconodon and the alleged jaw to ear transition, go here.

Transformation and diversification in early mammal evolution
Zhe-Xi Luo
Nature 450, 1011-1019 (13 December 2007) | doi:10.1038/nature06277

Abstract: Evolution of the earliest mammals shows successive episodes of diversification. Lineage-splitting in Mesozoic mammals is coupled with many independent evolutionary experiments and ecological specializations. Classic scenarios of mammalian morphological evolution tend to posit an orderly acquisition of key evolutionary innovations leading to adaptive diversification, but newly discovered fossils show that evolution of such key characters as the middle ear and the tribosphenic teeth is far more labile among Mesozoic mammals. Successive diversifications of Mesozoic mammal groups multiplied the opportunities for many dead-end lineages to iteratively evolve developmental homoplasies and convergent ecological specializations, parallel to those in modern mammal groups.