Why Did Synapsids Become Mammals?

[Gelatinous squid]

What was so special about having a synapsid? Did it confer some advantage that encouraged reptiles to eventually become mammals?

[siteseer]

First, none of the synapsids were technically reptiles. They had an ancestral group separate from those of the modern forms and various extinct groups. Even among biologists, for a long time "reptile" was a convenient umbrella for all scaly-skinned, ectothermic tetrapods but it wasn't a very useful grouping in distinguishing the actual closeness of relationships of those groups. Snakes descended from lizards and tuatara is somewhat related to lizards but both are not that closely related to crocodilians and none of those groups are closely related to turtles. They have been grouped together because they are all the scaly-skinned tetrapods that humans have ever seen alive. Numerous other groups died out in the millions upon millions of years before the first dinosaurs while others died out across the Mesozoic.

Scientists have become more interested in organizing groups in terms of ancestors and descendants rather than simply dumping together groups with similar superficial features. It's part of a general thing that science shoots for - increased precision in terms and descriptions.

The skull openings in early synapsids such as pelycosaurs (like Dimetrodon) allowed for more powerful biting which combined with specializations along the tooth row (development of incisors for nipping, canines for a faster killing bite, molars for more finely shredding meat) led to not just grabbing small prey and eating it whole but also biting into larger prey and cutting out bite-sized chunks. Other tetrapods of the time could only feed on what they could eat whole or perhaps twist off with a great expense of energy. This led to the ability in later synapsids to chew rather than just cutting up the prey allowing for faster and easier digestion. This turned out to be a winning adaptation because it was a more efficient way of processing what was eaten.

This isn't the most complete explanation but it's the general idea as I understand it.

What was so special about having a synapsid? Did it confer some advantage that encouraged reptiles to eventually become mammals?

Edited December 20, 2014 by siteseer

[ashcraft]

The only thing that is distinctly different in mammals from other groups are mammary glands. We are a group within the synapsids, meaning we have one hole on the sides of our head, I believe the only one still surviving. The synapsids are a group within the amniotes, of which several still survive.

It could be that holes allowed our ancestors some benefit, or maybe it simply was being in the right place at the right time when the asteroid hit, when many niches simultaneously opened up. Survivors rapidly filled the niches, and excluded other species from moving into it.

I don't know that I have answered, or even really addressed, your question. In my defense, my "holiday cheer" has made me a bit loquacious, and y'all can't get up and walk away, unlike my wife and family.

Brent Ashcraft

[Rodney]

I have always tried to understand the asteroid part that caused the big extinction. When you have certain groups of animals that died off or became extinct before this and after certain events, and you continued to have animals go extinct way after the asteroid impact, is it possible that the big dinosaurs just died off from combinations of events and not just this one asteroid event that started it?

Rodney

[ashcraft]

Due to the change in the flora/fauna, there was a large die-off of many species at this time. It wasn't only dinosaurs, it wasn't only animals, and it wasn't only on land. The cause raises the hackles of many. Based on my understanding of the evidence, and my understanding of how evolution progresses, I think it was a single cataclysmic event (asteroid impact). Others, Auspex for example, see it as a chain of events capped off by a final "coup de gras".

There is a constant turnover of species through time. When one species replaces another, it usually is from the same group. Sabertooths replaced thylacosmilus as the apex predator in South America when it joined with North America. They were both mammals. At the end of the cretaceous, we see replacement of dinosaurs with mammals. The only way this could have happened was by whole-sale removal of all species that were better adapted to those niches then mammals.

Mammals were small generalists at this time, hardly "ready" to be an apex predator. When Europeans came here, they removed the apex predators (wolves, mountain lions). The niches were filled by other mammals (coyotes, bobcats), not by birds or lizards. Birds and lizards could have filled these slots, but they couldn't evolve fast enough to exclude the mammals from these niches. If all of the mammals were removed (and it will happen), whatever survives will fill the niches.

Brent Ashcraft

[Rodney]

Ok thanks Brent, I was trying to understand how all this took place with the big asteroid event. If this big die off of species was caused by this asteroid event and directly related to flora/fauna die off, I was thinking that the large species die off would take place quickly like less than one year for all the species that were in this food chain, but you have a die off span the best I can tell that lasted thousands of years, so I still don`t understand it really.

Thanks,,

Rodney

[ashcraft]

Ok thanks Brent, I was trying to understand how all this took place with the big asteroid event. If this big die off of species was caused by this asteroid event and directly related to flora/fauna die off, I was thinking that the large species die off would take place quickly like less than one year for all the species that were in this food chain, but you have a die off span the best I can tell that lasted thousands of years, so I still don`t understand it really.

Thanks,,

Rodney

I personally think the die-off was quite rapid, maybe in minutes. Others think that the ecosystem was degrading, and species diversity was declining as individual species were going extinct at a faster rate then they were forming, then the asteroid hit and really mucked it all up. Either way, evidence suggests that around 80% of all species were gone at the end of the cretaceous. Since it takes maybe 100 individuals to maintain a species, that implies that 99% of all individuals were wiped out.

Plants were effected, but they recover quickly from such events because of their seeds. So you ended up with all of this vegetation (food source), and not many herbivores, and even fewer predators. Insects were also effected, but because of their life cycles, also recovered quickly. Mammals were generalist feeders/insectivores at this point, so they were able to rapidly evolve to feed on these sources of nutrients. It is only a short step from an insect eating shrew to a gazelle eating lion.

In my opinion (which along with $1.50 will get you a soda), the cretaceous extinction event is pretty well understood. The mechanics are still studied/debated, but we have the gist of it. Other extinction events, not so much. There have been at least 5, and maybe 6. I add that because the Cambrian Explosion looks alot like the recovery from an extinction event. The Permian Extinction particularly really set life back on its heals, and effects of it seemed to last through the Jurassic, when another extinction event occured.

Then there is the extinction of the megafauna at the end of the last ice age. Humans? Meteors? Climate? Combination? Here in North America, we saw white-tailed deer responding to this event by evolving into many "subspecies", from the northern variety at 300 pounds, to the tiny Key's deer at 50 pounds. Who knows where it would have gone if we (Europeans), hadn't arrived.

Interesting stuff, at least to me.

Going to the Dutchtown Formation to cut a whatzit out of the rock.

Brent Ashcraft

Edited December 26, 2014 by ashcraft

[Auspex]

I have long thought that a confluence of circumstances have to align to cause a mass turn-over of taxa.

The Chixulub impactor certainly raised havoc, but some species were already in decline at the time, and new technology has narrowed down the dating of the massive flood basalt event that created the Deccan Traps in India to a time contemporaneous with the impact. I also think that the flowering plant revolution had already upset the global ecological applecart.