C.200 mya - K-T 65 mya: emergence of mammals, coexistence w/ dinosaurs

[eehiram]

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:

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)

Primates evolved from mammals about 60 million years ago, in tropical Paleocene forests. Like the small rodents and tree shrews they resembled, [...]

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?​

 

[Simon Bridge]

You are actually asking for quite a lot of information - what efforts have you made to get it for yourself?
What education level do you need this at?

GCSE "blog" with links to New Sceintist and a pop-science book on evolution covering the ideas:
http://www.truthinscience.org.uk/tis2/index.php/evidence-for-evolution-mainmenu-65/270-synapsids-and-the-evolution-of-mammals.html

College level lecture:
http://www.webpages.uidaho.edu/biol483/lectures/Lecture2.pdf

Somewhat in between:
http://faculty.plattsburgh.edu/thomas.wolosz/pre-dino.htm

But you are asking for a great deal of fine detail which simply may not be available, or may be based on speculation. i.e. we do not have genetic information so far back.

Similarly - could prosimians have emerged earlier?
Nobody knows - that is pure speculation. If things were different - then anything could happen.
This is not a useful inquiry.

The evidence does not support an emergence of prosimians before the KT boundary.
But it is likely that the ancestors of mammals existed before the KT boundary.

Brain evolution is ongoing research - you seem to be asking about the "triune brain" model, which is not afaik mainstream, though it is still discussed as a basis for other comparative neuroanatomies. Although the models now are not as straight forward as your questions suggest, there is a tendency to put the development of the basial ganglia at ab out 500my ago and the neo-cortex is thought to have been present with the earliest mammals we know of.

Anyway - I think the links above will give you a stronger basis for asking this sort of question than the book you are using.

 

[DiracPool]

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?​

The mammalian neocortex evolved out of the dorsal aspect of a pre-mammalian brain structure called the pallium. It initially evolved primarily to handle hierarchically more complex sensory discriminations that the dorsal thalamus and related limbic regions were getting overloaded with. That is why you see the 3 outer layers of the neocortex devoted t the reception of sensory afferents, while the deeper, more primitive layers are primarily output layers.

It's important to note, though, that there were two main phases in neocortical evolution. The first is with what you can call pre-eutherian or pre-placental mammals, appeared about 200mya. These are the very primitive mammals, with only a few surviving species today such as the marsupials like the kangaroos, and monotremes like the platypus. What distinguishes these mammals is there conspicuous lack of a dedicated motor cortex. The primary sensory motor loop runs straight from the sensory dominated neocortex straight into the basal ganglia and then out to the motor effectors.

The second main phase of neocortical evolution didn't happen until 100my later (about 100mya) with the appearance of eutherian, or placental, mammals. At this stage a dedicated motor cortex is distinguishable, and the progression of mammalian brain evolution from there to the primates to the apes to humans is basically one of increasing the relative size of that frontal motor (more specifically, pre-frontal), cortex relative to the more caudal and lateral sensory cortices.

If you're interested in further reading, I would suggest Streider's book, "Principles of brain evolution", and Swanson's book, "Brain architecture."

 

[eehiram]

limited-in-scope thread on emergence of mammalia, and no K-T speculati

Simon Bridge:

1a) I merely want to start off a small, limited scope thread on the topic of emergence of mammalia, beginning with the transitional therapsids, and then continuing with the therians.

My initial post should be considered flexible, and I do not need large amounts of information about this topic. I only have a few pages in my textbook on the evolution of mammalia before the early hominidae section is arrived at.

1b) Thus, fine details are not necessarily asked for; I just wanted to start the thread off. Any posted discussion that follows on the emergence of mammalia will be of interest to me, at least.

2. I will try to avoid any further speculation about pro-simians and the K-T boundary. (I would like to passingly mention, with caution, the time frames as being charted on an exponential chart, or conversely, a logarithmic chart.)

3. You are correct to detect that part of my reading is derived from "triune brain" model. I don't understand

which is not afaik mainstream

? Thanks for information on basal ganglia and cortex / neo-cortex.

I will check the links soon.
Thanks for the prompt reply.

 

[eehiram]

Cerebral cortex structures and mammalia brain evolution to human brain

DiracPool:
Thanks for the helpful information about brain evolution and pre-mammalian pallium; mammalia from ~200mya, lack of dedicated motor cortex, basal ganglia; mammalia from ~100mya, dedicated motor cortex, frontal motor cortex, pre-frontal cortex; book recommendations; et alii.

I would like to ask if we can clarify the use of these terms, although to do so might be a quibble in light of the point of view of the human brain's current neocortex attainment:

cerebral cortex
archicortex (pallium)
paleocortex
neocortex

Unless I am mistaken, the cerebral cortex can serve as the general term for all three structures of the brain's cortex, as part of mammalian brain evolution up to the human brain.

Perhaps it might be elucidating to refer to the 3 sub-structures of the cerebral cortex in terms of their development in the course of brain evolution, specifically, the evolution of the cerebral cortex.

Please correct me if I have posted in error, by all means.

 

[DiracPool]

DiracPool:

I would like to ask if we can clarify the use of these terms, although to do so might be a quibble in light of the point of view of the human brain's current neocortex attainment:

cerebral cortex
archicortex (pallium)
paleocortex
neocortex

Unless I am mistaken, the cerebral cortex can serve as the general term for all three structures of the brain's cortex, as part of mammalian brain evolution up to the human brain.

Perhaps it might be elucidating to refer to the 3 sub-structures of the cerebral cortex in terms of their development in the course of brain evolution, specifically, the evolution of the cerebral cortex.

Hi eehiram. It's great to see that you have an interest in the evolution of mammals and their nervous systems. I encourage you to continue your pursuit of that study in that it really is a fascinating subject. So fascinating, in fact, that there continues to be heated debate among researchers as to how the vertebrate brain evolved, and especially as to how to name and classify the various brain structures and systems. For example, for a long time there was a debate as to to whether the neocortex derived from the ventral portion of the pallium homologous with the reptilian dorsal ventricular ridge (see http://www.pnas.org/content/94/7/2800.full.pdf), or whether it derived from the dorsal aspect more closely associated with the cortical olfactory system. It is only recently that a confluence of genetic, embryologic, and other techniques have soundly supported the dorsal model (see Streidter). But the jury is still out on many other related brain systems in terms of their ancestral homologous roots.

So, when you ask for a clarification on "the use of terms" such as cerebral cortex, archicortex, etc., it's not so cut and dry. Especially since I don't know what your goal is here. Is it for a test? Or a class? If so, then you can just parrot the staid textbook definitions that have been around for decades. Here's a link to get you started: http://en.wikipedia.org/wiki/Allocortex

As far as contemporary scholarly publications outside of university textbooks, most often the usage of the word "cortex" alone or in anyone of it's typical assemblages, cerebral cortex, neocortex, isocortex, refers specifically to the 6-layered variety nested up against the pial surface of the skull in placental mammals. You don't see the usage of paleocortex or archicortex, say, used very often. Typically, authors will simply refer to specific brain regions these terms encapsulate, such as the parahippocampal gyrus, piriform cortex, olfactory cortex, etc.

My advice to you is to try to resist the temptation to "modularize" the function and evolution of the mammalian/human brain into these kinds of subcategories. While looking at vertebrate brain/behavior from a "Triune" perspective can be insightful as a first approximation, taking it seriously at a detailed level is oversimplifying a complex interweave of brain systems, and is likely to lead to confusion. Again, if this is for a class, just parrot the standard definitions. However, if your intent is to really understand how the brain evolved, I would suggest picking up Larry Swanson's book or some of his more recent publications, e.g.,http://www.ncbi.nlm.nih.gov/pubmed/17267046

I like his approach because he uses a flat map cartography in relating the positions, connections, and evolutionary transitions between vertebrate genera, which is useful. Good luck!

 

[eehiram]

Thanks for corrections on brain structure terms!

DiracPool:
Thank you, for correcting me.

I am reviewing my fall 2004 community college / junior college biology 101 (Intro to Biology) class out of my interest in:
evolution in general, and The Descent of Man, as Charles Robert Darwin, FRS, entitled his now-classic 1871 book.
(This and other books of Charles Robert Darwin, FRS, as predecessors in 1800s century, were not published in 2000s century so as to encompass 1990-February 2001 completed Human Genome Project results).

I am attempting to emphasize and bring forward the less popular aspects of evolutionary biology in regard to the the ancestors to humans:
hence, my interest in > 200mya synapsids → therapsids, Jurassic therians, and 60mya prosimians.

I would like to take more biology classes, or take anthropology (biological and / or cultural) classes, in the next few years at the level of community college / junior college.

Thanks for the links! I'll check them after I check the first set of links from Simon Bridge...

 

[eehiram]

Thread is intended to be flexible, small, limited scope; interesting

Please remember this post to Simon Bridge:

1a) I merely want to start off a small, limited scope thread on the topic of emergence of mammalia, beginning with the transitional therapsids, and then continuing with the therians.

My initial post should be considered flexible, and I do not need large amounts of information about this topic. I only have a few pages in my textbook on the evolution of mammalia before the early hominidae section is arrived at.

1b) Thus, fine details are not necessarily asked for; I just wanted to start the thread off. Any posted discussion that follows on the emergence of mammalia will be of interest to me, at least.

 

[eehiram]

I've reviewed 3 links; I request manual transmission for posts/links?

Simon Bridge:
Thank you for the 3 links, which I have reviewed:

1. GCSE "blog":
This level was useful for me for an overview is called for;

2. College level lecture:
This level has the details that I would like when needed,
but not for every post, please;

3. Somewhat in between:
This level might also be useful: when intermediate level of reading material / posts are desired;

In the "control" sense, I humbly request a manual transmission for posts / links:
please skip the level 2 details until needed, then use them as if from encyclopedia or reference books;
using level 3 for majority of the posts;
but downshifting to level 1 when overview / brevity is preferable.

Thank you for your assistance in this biology / anthropology review!

 

[Simon Bridge]

Simon Bridge:
...
In the "control" sense, I humbly request a manual transmission for posts / links:
please skip the level 2 details until needed, then use them as if from encyclopedia or reference books;
using level 3 for majority of the posts;
but downshifting to level 1 when overview / brevity is preferable.

Well OK - please be aware that I, and others here, field roughly 100+ questions a week so I cannot possibly remember the detail requirements for all those who have special needs.
Therefore, the onus is on the person asking the question to specify the level the answer needs to be at. This is, after all, a free of charge service, so you can expect answers at our convenience rather than yours. ;)

I'm guessing your question has been answered well-enough for you?

 

[eehiram]

"3. Somewhat in between" seems like the safest bet for future referenc

Certainly, my question has been posted on sufficiently; I'd like to move on to another thread,
(perhaps on chemistry since the inception of Copenhagen school, or my 1 chapter on organic chemistry, both from the chemistry textbook in my possession).

I do appreciate the help and elucidation that is provided free of charge by the kind and generous people here at this forum. I apologize for not being sensitive to your time constraints in advance in my earlier post.

To give a simpler answer, I therefore pick:
"3. Somewhat in between", as the safest bet.

(I may eventually return to the general topic of mammalia in lesser discussed forms, such as prosimians; as this helps me to review the lineage to hominidae. Such a topic would appear in a new thread, later, and would bring the level of education "3. Somewhat in between" into play.)

Thank you both for posting!