Why did the synapsids and the sauropsids evolve so differently, given the relatively small initial change?

[Gelatinous squid]

One hole in the skull rather than two, next thing you know it's fur and feathers, beaks and molars. I know evolution is quirky but that seems like slim thread to hang a lineage on. 

[Tidgy's Dad]

Well, that's exactly how evoluton works. 

A small change or changes within two separate communities of a species leads to speciation. 

You then have two different species. 

Multiply the slow variation in those two species and their descendants over millions of years and you are likely to end up with groups that are very far apart from each other in form and  function. 

Slow divergence over millions of years is what creates the diversity of life.  

[siteseer]

The extra hole in the skull isn't the only difference.  It's just the easiest one to see.  There are more subtle differences elsewhere in the skeleton too.  Someone like @jdp would be able to provide a list of some of them.

 

I remember learning about this in general as a kid.  Reptiles evolved from amphibians and mammals evolved from reptiles.  Today, paleonologists understand that amphibians, reptiles, and synapsids evolved separately from perhaps the same early tetrapod group.

Edited June 25, 2023 by siteseer

[Gelatinous squid]

9 hours ago, Tidgy's Dad said:

Well, that's exactly how evoluton works. 

A small change or changes within two separate communities of a species leads to speciation. 

You then have two different species. 

Multiply the slow variation in those two species and their descendants over millions of years and you are likely to end up with groups that are very far apart from each other in form and  function. 

Slow divergence over millions of years is what creates the diversity of life.  

True but I mean, rattlesnakes and chameleons are far more different from each other than basal synapsids and sauropsids, but are still part of the smae clade. 

[Tidgy's Dad]

13 hours ago, Gelatinous squid said:

True but I mean, rattlesnakes and chameleons are far more different from each other than basal synapsids and sauropsids, but are still part of the smae clade. 

Well, that's cheating. 

You limit the comparison to only basal synapsids and sauropsids which have had little time from when they separated from their earliest common ancestor, so they are likely to look very similar. If you look at a crocodile and a giraffe, they become pretty different. 

Yes, a rattlesnake and a chameleon are very different, but as snakes descended from lizards the comparison is not fair and a basal snake would have been a lizard and later forms very like them for millions of years.

Edited June 26, 2023 by Tidgy's Dad

[erose]

On 6/25/2023 at 5:23 PM, Gelatinous squid said:

True but I mean, rattlesnakes and chameleons are far more different from each other than basal synapsids and sauropsids, but are still part of the smae clade. 

Not to mention we only  can see changes that manifest as physical changes. Not to mention the scale of time involved that is kind of mind-boggling.

[NickG]

On 6/25/2023 at 6:45 PM, Tidgy's Dad said:

Well, that's cheating. 

You limit the comparison to only basal synapsids and sauropsids which have had little time from when they separated from their earliest common ancestor, so they are likely to look very similar. If you look at a crocodile and a giraffe, they become pretty different. 

Yes, a rattlesnake and a chameleon are very different, but as snakes descended from lizards the comparison is not fair and a basal snake would have been a lizard and later forms very like them for millions of years.

 

Please don't use the Pterosaur Heresies as evidence or for information in general (even his artwork is based in his own delusionss). Dave Peters is not a reputable or credible source for anything paleontology or anything even remotely rooted in reality. He is delusional.

 

https://tetzoo.com/blog/2020/7/23/the-david-peters-problem

 

https://en.wikipedia.org/wiki/David_Peters_(paleoartist)

[NickG]

On 6/25/2023 at 8:20 AM, Gelatinous squid said:

One hole in the skull rather than two, next thing you know it's fur and feathers, beaks and molars. I know evolution is quirky but that seems like slim thread to hang a lineage on. 

This is misleading. If the conventional topology is correct, both reptiles and synapsids evolved from "anapsid" ancestors. The earliest reptiles (like captorhinids) did not have any temporal fenestra, nor did the closest stem-amniotes to the crown (look at diadectomorphs and seymouriamorphs, for instance). However, it looks like some stem-synapsids might be stem-reptiles or stem-diapsids (Ford and Benson, 2020; Laurin and Pineiro, 2018). 

 

There are good reasons to wonder if the amniote TOL may be more complicated than we've thought, and that possibly some basal synapsids and basal reptiles might be stem-amniotes and not part of either clade, but we need a lot more descriptive work of important Paleozoic (an)amniote fossils, recognition of more morphological characters, etc.

Edited July 8, 2023 by Nick G.

[jdp]

Just to supplement what Nick has said here, there is quite a lot of variation in the size, extent, and even presence of these holes in the sides of the skull within both reptiles and in synapsids. Interestingly enough, what seems to be a bit more important is that there are some rather substantial differences in the internal organization of the skull and associated spaces for soft tissue (jaw muscle groups, brain size and shape, vasculature, middle ear, etc). So, for instance, it is probably likely that early synapsids like Ophiacodon or Dimetrodon opened and closed their jaws in different ways from early reptiles like Captorhinus or Petrolacosaurus. They also likely sensed their environment differently, walked differently, and even developed their teeth in somewhat different ways that contribute to later important differences we see today. So, yes, the earliest synapsids look vaguely reptile-y but they were already laying the groundwork for the rather radical changes that take place in the middle and late Permian to produce animals that appear much much more mammal-like.

[Mahnmut]

Hello together,

without questioning that synapsids and sauropsids went their different ways, I´d like to add some thoughts:

There where sauropsids with teeth (I have heard) and there still are. Beaks are just one specialization of the most successful group of sauropsids today. Fur and feathers are just two solutions to the same problem (to make a complex story simple) and not that dissimilar in that they are ceratinous outgrowths of the skin.

 

Maybe an example to support what Tidgys Dad said about evolution:

Imagine two species of beaked, feathered sauropsids which sing in order to impress potential partners. If a regional group changes their song in a hereditary way (which means starting with a mutation in one germ cell as far as I understand it, but maybe even if they only change their behavior, which would make it culture in a way) that makes them unattractive to the rest of the population, this alone can be the starting point of speciation. No visible change at this point, but all that happens to the two groups from then on are not coincidences, but different lines of causality that may in principle lead to 500 kg flightless giants and to 5 gram pollinators (flightless birds do often have feathers resembling fur in structure by the way).

Thats not how birds, mammals,feathers and fur, moas and hummingbirds evolved according to modern theory, but its a story built from true parts in order to emphasize the point how small differences may begin different paths.

The paths do causally follow from the beginnings, but they are not determined by them.

Best Regards,

J

Edited July 18, 2023 by Mahnmut
clarification

[jdp]

I was talking about things like heterodonty and mammalian-style thecodonty.

[Mahnmut]

17 hours ago, jdp said:

I was talking about things like heterodonty and mammalian-style thecodonty.

I agree that there are significant differences between synapsids and sauropsids, referring to the OPs dichotomy of beaks and molars I  simply wanted to point out that those are just specialisations of certain groups inside those two branches. 

I completely agree that those holes in the head where not standalone features

Best Regards,

J