A question for the biology folks

[wearestardust]

A probably silly question for the biology folks, following from some of my recent reading, and demonstrating my confusion.

In one book (about dinosaurs) I saw a cladogram that showed sinapsids branching off before reptilia. However, elsewhere, I have seen sinapsids and therapsids described as "reptiles" ("mammal-like reptiles" to be precise).

Now, here's the thing that's confusing me, or alternatively, that I'm confused about. My understanding is that everything upstream of a point on a cladogram is in the same family (hence birds are in the clade reptilia, assuming the evoution of birds from therapods). However, if that is so, and sinapsids are reptiles, then where does that put us? Or (perhaps) was the description of sinapsids as reptiles perhaps being used a shorthand?

[Slothhead]

Actually this is a good question and one that i have just applied 5000 words in explanation for an article being published in two parts in an upcoming mag.



http://www.geol.umd.edu/~tholtz/G104/amniota.jpg

But for the skinny version of it.

This picture is relatively good.

Modern "reptiles" are actualy diapsids (which is why turtle location has caused some problems) and synapsids are where we fit.
SO the first tetrapods were anapsids (no frenestrae) then we have a split that leads to us, being synapsids (single frenestrae).

The position of turtles in the above picture is actually incorrect, but that doesnt matter too much.

Does this answer your question?
D

[wearestardust]

Actually this is a good question and one that i have just applied 5000 words in explanation for an article being published in two parts in an upcoming mag.

...

But for the skinny version of it.

This picture is relatively good.

Modern "reptiles" are actualy diapsids (which is why turtle location has caused some problems) and synapsids are where we fit.
SO the first tetrapods were anapsids (no frenestrae) then we have a split that leads to us, being synapsids (single frenestrae).

The position of turtles in the above picture is actually incorrect, but that doesnt matter too much.

Does this answer your question?
D

If what that means is: talk of sinapsids as being reptiles is strictly speaking wrong, and perhaps shorthand for the lay reader instead of explaining what anapsids or amniotes are, then yes it does. (where I came across it was in a Scientific American collection of essays on dinosaurs, and then in Nick Lane's (http://www.nick-lane.net/)book Life Ascending: the ten great inventions of evolution, particularly in the chapter on evolution of homeothermy).

That cladogram looks pretty similar to the one I was referring to, though it's returned to the library so I can't compare.

Actually, in my reckoning one of the coolest things I have learned this year is the dimetrodons were sinapsids, not archosaurs.

[Slothhead]

Yeah thats right, dimetrodons and mammals are part of the same clade.

The problem with the word reptile is that there sort of isnt such a thing as reptiles. We often use the word for descriptive purposes. So the first tetrapods are often termed amphibian like, as that is what they would have been 'like'.

Similarly we describe a good number of animals as reptile like with reference to 'modern reptiles' which also doesnt mean much as there is no single clade of reptiles.

Also, be aware that there is a difference between ectothermy/endothermy and homeothermy/poikilothermy.


It is fair to say though that the distant ancestors to mammals were more "reptile-like" than mammal like and the later ones being more mammal like than reptile like. But there is no hard line that divides the two, if you had to draw a line on a cladogram you would have to use a thick pen and cover an area.

What most people finding surprising is that the lineage that leads to modern mammals split quite some time ago, where as most people think that it is relatively recent.

[wearestardust]

Yeah thats right..., dimetrodons and mammals are part of the same clade.

The problem with the word reptile is that there sort of isnt such a thing as reptiles. We often use the word for descriptive purposes. So the first tetrapods are often termed amphibian like, as that is what they would have been 'like'.

Similarly we describe a good number of animals as reptile like with reference to 'modern reptiles' which also doesnt mean much as there is no single clade of reptiles.

Also, be aware that there is a difference between ectothermy/endothermy and homeothermy/poikilothermy.


It is fair to say though that the distant ancestors to mammals were more "reptile-like" than mammal like and the later ones being more mammal like than reptile like. But there is no hard line that divides the two, if you had to draw a line on a cladogram you would have to use a thick pen and cover an area.

What most people finding surprising is that the lineage that leads to modern mammals split quite some time ago, where as most people think that it is relatively recent.

Thanks very much, that makes it all very clear.

Re your last comment: I was certainly in that boat, and had no idea of that point until quite recently, and was unaware of the prevelance/dominance of sinapsids/therapids in the Permian until even more recently.

Generally, I'm finding out that I don't know squat about biology and I'm quite enthused to learn more. On which point: yes I learned yesterday (in a hot bath, which somehow seems relevant) of the distinction between different terms relating to body temperature. Plus the term "inertial endothermy".

[davo]

I love learning stuff here, thanks all, extremely interesting! :)

[Slothhead]

Re: ectothermy and endothermy etc.

These are exciting parts especially when you let the mind off the leash a little, for example, imagine if rather than endotherms we were ectotherms, how would this change society. When you explore it there are many many changes that would take place, but this begs the question. Is it possible for an ectothermic animal to get to the intelligence of a human.
This is a hard question because then we have to define the term intelligence but it is an interesting thought experiment. But if we just assume the social structure and level as well as the ability to develop technology, if that is what we mean, it is a lot of fun.