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eehiram
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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)
3. Neurology: Limbic system versus cortex regions in synapsids (reptiles from whom therapsids diverged; see above quote ) brain → therapsids brains → therians (mammalia) brains
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:
[...]More than 200 million years ago, a divergence from small repitiles (synapsids) gave rise to the therapsids, the ancestors of mammals. By the Jurassic, plant-eating and meat-eating mammals--therians--had evolved. Most were mouse-sized.
Therians and dinosaurs coexisted in the Cretaceous. Then the last of the dinosaurs vanished at the K-T boundary (Section 19.7). With their extinction, new adaptive zones opened up for mammals.
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.
[Therians] had hair and modifications in jaws, teeth, and body form. For instance, their four limbs were positioned upright under their trunk. This skeletal arrangement made it easier to walk erect, but a trunk higher from the ground was not as stable. At that time the cerebellum, a brain region dealing with the body's balance and spatial positioning, started expanding.
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?Primates evolved from mammals about 60 million years ago, in tropical Paleocene forests. Like the small rodents and tree shrews they resembled, [...]
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?