The following summary is derived from my 2003 biology textbook: Biology: concepts and applications, 5th edition, by Cecie Starr, published by Brooks/Cole, Thomson Learning. (I took biology 101 in fall 2004. This is not homework, but rather for my self-study and interest.) Chapter 24, section 9 (a.k.a. Section 24.9): The Rise of Mammals: [paraphrased] Mammalia (mammals) take their name from their mammary glands. Females feed their young with milk. [Insert typical description of mammalia here... etcetera.] Now: [...] The description of therapsids and therians is brief, so I have to infer a lot. Perhaps someone in this forum will be kind enough to help correct me or fill out the holes in my profiles on these ancestors to mammalia, and mammalia. My questions are as follows: 1. What can briefly be posted about divergence of therapids (ancestors to mammalia) from synapsids (small reptiles)? Exempli gratia: Did changes in allele frequency occur via (section 16.11: Genetic Drift) genetic drift, as opposed to bottleneck-and-founder effect? Did the speciation occur by (section 17.4 Patterns of Speciation) cladogenesis (id est: branched split, "with populations becoming genetically isolated and then diverging [...]") versus anagenesis (id est: "changes in allele frequencies and in morphology accumulate within an unbranched line of descent")? 2. Did therians coexist with dinosaurs in an evolutionary near-plateau, videlicet, were the dinosaurs blocking further evolutionary adaptation of mammalia until K-T boundary, 65 million years ago? (Section 24.10: From Early Primates to Hominids) Could prosimians (earliest primates) have emerged earlier than 60 million years ago, but not for K-T boundary's timing to have brought the mass extinction, hence the eradication of the dinosaurs as an obstacle to mammalian evolution toward prosimians? 3. Neurology: Limbic system versus cortex regions in synapsids (reptiles from whom therapsids diverged; see above quote ) brain → therapsids brains → therians (mammalia) brains a) When did the layers of paleo-cortex develop? b) When did the additional individual layers of neo-cortex emerge?