Mammal domination

[BlueKrono]

I've had this question for years, but I've never met someone who might be able to come up with an answer. Most of the science museums employees I've met didn't know a synapsid from a diapsid. Anyway, my question is: Why didn't mammals (or mammal ancestors) take over from the get-go when they began to appear in the late Permian period? They had most of the traits that enable mammals to be the dominant lifeforms on Earth today, including warm-blooded homeostasis, caring for and having fewer offspring, and propensity for high intelligence. Why didn't early mammals trounce their cold-blooded reptilian competitors and proceed to live in their shadow for 230 million years?

[Yutyrannus]

From what I've heard, part of this is because archosaurs have a much more efficient breathing system than synapsids do, and that helped in the lowered oxygen levels after the end of the Permian.

[Papi-Anon]

From what I've read, another factor was that synapsids used more water when they excreted whereas archosaurs retained more of their body's water when they excreted and this was an important adaptation given how hot the planet had become by the time the Triassic began.

[Lanthanotus]

Dominance in an ecological way of understanding as described here seems to be a quite subjective thing.... what is dominance at all? Number of species or individuals? Total mass? Impact on the environment? Presence in fossil record? .... ? Just my two cents on the terms,...

The question itself is rather interesting though and could probably never really proved in any way. Aside from water and breathing as written before, metabolic rate is a very important cause for evolutionary success. Advanced exothermic physiology and behaviour can be quite a benefit in hard times, while even primitive endothermy often bears harsh drawbacks during exceptional climate changes.

[BlueKrono]

Interesting ideas so far. By dominance I think the closest definition to what I mean would be impact on the environment, or possibly being the top carnivores/ herbivores in an environment in the way that amphibians ruled the Permian Period, reptiles the Mesozoic and mammals today. As far as species diversity and quite likely biomass, insects (especially beetles) have us beat, but it would be hard to argue that humans (and to a lesser extent other mammals) aren't the dominant/ most successful species on Earth today.

[Gwangi]

No doubt reproductive strategies may have contributed as well.

[stargatedalek]

I would argue that barring humans dinosaurs are probably still the planets most dominant lifeforms in terms of "higher niches" in a global ecosystem. Beetles may outnumber even plants in terms of species diversity, but birds have even them beat in terms of niche diversity.

Mammals may be large and noticeable, but they just don't seem to show the same propensity for claiming unusual niches that dinosaurs have. Compare to the mammals strong-suit which would have to be their capacity for generalist behavior. Pigs are a perfect example of this, they can (and unfortunately have) colonized practically every environment in the world simply by doing virtually the same thing everywhere they end up.

Of-course there are some mammals that break from this mold and spread via adapting their behavior and diet to suit different regions, humans are one, orca are another.

And don't forget that some places in the world still have "reptiles" (birds, crocodilians, snakes, tortoises, and monitor lizards) as their largest predators and herbivores.

From what I've read, another factor was that synapsids used more water when they excreted whereas archosaurs retained more of their body's water when they excreted and this was an important adaptation given how hot the planet had become by the time the Triassic began.

While it's definitely true that birds and potentially other ornithodirans excrete very little water in their waste, the Triassic wasn't nearly as dry as is often depicted. It was certainly dryer than the rich inland swamps and waterways of the Permian (which is also often incorrectly depicted as a desert), but it wasn't a global desert itself by any means. Rather there were extensive seasonal forests and floodplains, more comparable to modern scrub swamp or dambo regions than a true desert.

[Halichoeres]

I would argue that barring humans dinosaurs are probably still the planets most dominant lifeforms in terms of "higher niches" in a global ecosystem. Beetles may outnumber even plants in terms of species diversity, but birds have even them beat in terms of niche diversity.

Mammals may be large and noticeable, but they just don't seem to show the same propensity for claiming unusual niches that dinosaurs have. Compare to the mammals strong-suit which would have to be their capacity for generalist behavior. Pigs are a perfect example of this, they can (and unfortunately have) colonized practically every environment in the world simply by doing virtually the same thing everywhere they end up.

Of-course there are some mammals that break from this mold and spread via adapting their behavior and diet to suit different regions, humans are one, orca are another.

And don't forget that some places in the world still have "reptiles" (birds, crocodilians, snakes, tortoises, and monitor lizards) as their largest predators and herbivores.

From what I've read, another factor was that synapsids used more water when they excreted whereas archosaurs retained more of their body's water when they excreted and this was an important adaptation given how hot the planet had become by the time the Triassic began.

While it's definitely true that birds and potentially other ornithodirans excrete very little water in their waste, the Triassic wasn't nearly as dry as is often depicted. It was certainly dryer than the rich inland swamps and waterways of the Permian (which is also often incorrectly depicted as a desert), but it wasn't a global desert itself by any means. Rather there were extensive seasonal forests and floodplains, more comparable to modern scrub swamp or dambo regions than a true desert.

I think I have to disagree in the mammals vs. birds dominance question, at least where the present is concerned. There are no fully marine birds; they all have to give birth on land. Bats give them a run for their money in at least some aerial niches, although birds are definitely better at high-altitude performance (thank the superior respiratory system you mentioned). Nevertheless, whereas there are more bird species than mammal species, the morphological, body size, and fundamental niche disparity of mammals is almost certainly greater. Maybe if you combined all diapsids vs. all mammals (extant) they'd have a shot.

[Neosodon]

Mammals didn't take control during the Mesozoic period because they were not superior to dinosaurs. Mammals took control because they were able to fill the void left by the extinction of dinosaurs. If the great extinction never happened dinosaurs would still be alive today.

[Flaffy]

Mammals didn't take control during the Mesozoic period because they were not superior to dinosaurs. Mammals took control because they were able to fill the void left by the extinction of dinosaurs. If the great extinction never happened dinosaurs would still be alive today.

Dinosaurs never went extinct.

[Neosodon]

Mammals didn't take control during the Mesozoic period because they were not superior to dinosaurs. Mammals took control because they were able to fill the void left by the extinction of dinosaurs. If the great extinction never happened dinosaurs would still be alive today.

Dinosaurs never went extinct.

I wasn't counting birds. Modern birds are in a completely different niche than their large terrestrial ancestors. Birds fulfill a completely different role than mammals. Dinosaurs use to fill the role of large terrestrial herbivores and carnivores - the same role that mammals have taken today. If the great extinction never happened dinosaurs would still fill that role today.

[BlueKrono]

I think it would be helpful to look even more recently, at the Cenozoic. Large carnivorous (and by some recent theories herbivorous) flightless birds were at the top of the food chain, but were outcompeted by the more adaptable mammals. It seems like on an even playing field (one where dinosaurs didn't have a huge head start) mammals do better than reptiles or even birds. Was the Earth's climate really different enough to tip the balance to the reptiles during the Permian?

[stargatedalek]

I think it would be helpful to look even more recently, at the Cenozoic. Large carnivorous (and by some recent theories herbivorous) flightless birds were at the top of the food chain, but were outcompeted by the more adaptable mammals. It seems like on an even playing field (one where dinosaurs didn't have a huge head start) mammals do better than reptiles or even birds. Was the Earth's climate really different enough to tip the balance to the reptiles during the Permian?

You are oversimplifying things a might. Many of them lived in isolated island environments devoid of large mammalian predators and instead were adapted to deal with avian or reptilian predators (if any), the introduction of mammalian predators by humans or tectonics created a power vacuum and to say that giant birds were "inferior" to mammalian counterparts because they lost out is very shortsighted. First of all excluding Phorusrhacidae most large flightless birds were driven to extinction by human hands (if at times indirectly). This includes nearly all of the large herbivorous species.

Phorusrhacidae in contrast died off very gradually, and did an impressive job surviving as long and as well as they did given how specialized they were prior to the connecting of the Americas. They had to contend with changing climate and prey sources that their mammalian counterparts were already largely adapted to, and said mammals tended to be less specialized to begin with. That a number of Phorusrhacidae spread into North America, let alone lasted just long enough not to have cohabitated with early evidence of first nations peoples, is a testament to their ancestry. Don't forget, most large South American mammals fared even poorer.

[Halichoeres]

Maybe the answer would be clearer if we had a better idea of exactly what happened and when. There are several factors that might have contributed to the end-Permian extinction, but let's say that it had to do with runaway warming, some bolide impact, and anoxia. You could call this a sort of productivity crisis. In that situation, as Lanthanotus alluded to, it can be really costly to maintain homeothermy, large body size, an energetically expensive brain, or a high activity level. Things that don't require much food, or are fossorial, or are facultatively aquatic, would probably be at an advantage. What that would mean for the P-T is that more clades of diapsids survived than synapsids (this seems to be an empirical fact, but I'm postulating the mechanism). Post-recovery, it might just be that diapsids had a sufficient advantage in numbers and diversity that they were able to occupy more niches more quickly.

At the K-Pg, a sort of upside-down dynamic obtained, with the dozen or so lineages with the largest average body size all being diapsids, and the mammals being comparatively small and resilient to low productivity. Given molecular evidence that most orders of modern mammals had diverged before the K-Pg, coupled with all the Jehol mammals demonstrating that they were quite diverse even in the Lower Cretaceous, it's possible that mammals had a significant numerical advantage entering the Cenozoic, positioning them to expand fairly quickly.

There's a whole tangent on evolution-as-an-entropy-promoter that I could go into, but I'll only inflict that on folks if asked.

[BlueKrono]

Halichoeres - Maybe I'm just a huge nerd, but I would actually be interested in hearing about that... though perhaps in summary.

[Papi-Anon]

There's a whole tangent on evolution-as-an-entropy-promoter that I could go into, but I'll only inflict that on folks if asked.

I'd be interested in hearing it too. I remember the season 1 episode of Farscape having an epsiode about mad-scientist who was obsessed with becoming a perfect organism, and even stating that evolution was a path to perfection, but I always disagreed with that observation even if it was just a scifi thing.

[stargatedalek]

I just couldn't help myself!

I'd like to hear it too.

[Halichoeres]

Well, since you asked!

The typical definition you hear of "fitness" in the context of natural selection is the relative fraction of the next generation that includes copies of your DNA. Some evolutionary biologists, most notably Leigh van Valen, have considered that definition incomplete. van Valen proposed the notion of ecological fitness, which is instead related to the total amount of entropy arrested by your existence or that of your offspring. In an ecological fitness framework, getting larger increases your fitness even before you reproduce. Living longer increases your fitness. Maintaining a complex physiology increases your fitness. By the second law of thermodynamics, you can only arrest entropy in a system by increasing the entropy outside of the system. So you can get bigger, but only by using a lot of energy; you can live longer, in the same way; you can have lots of babies---you get the idea. Getting larger as a way of arresting entropy (for yourself, remember, while increasing the entropy of your environment) might also help explain Cope's rule, which is the general observation that many lineages tend to produce larger-bodied taxa as their history progresses, until there's a mass extinction. For example, lots of diapsid lineages produced their largest members right around the Campanian and Maastrichtian (pterosaurs, tyrannosaurs, ceratopsians, therizinosaurs, ornithopods, maybe squamates).

Another way to think about this is just, "how much energy has flowed through this organism to get to this point?" This is a sum of its standing biomass, its total metabolic activity, and its offspring.

Relatedly, an MIT physicist has proposed that because life is so good at increasing the total entropy of the universe, it might not only be thermodynamically favored, but inevitable that life will originate in a situation where the appropriate chemical precursors are available. (https://www.quantamagazine.org/20140122-a-new-physics-theory-of-life/) I find this hypothesis really fascinating but I admit I struggle with some of the hairier details.

[BlueKrono]

Very interesting. I always had a more simplified concept of biological fitness - more babies = more successful. I hadn't taken into account size and complexity, which are of course also standards of biological fitness. I think that by those three standards mammals are still the front runner in today's world, and archosaurs were during the Age of Reptiles. However, we have a strong competitor in two of those categories even today in the insects. They have far more offspring and are far more diverse than mammals have ever been. Only during the Age of Insects (the Carboniferous) did insects surpass other groups in size, and this was almost certainly due to the environmental conditions on the planet at the time, with high temperatures, humidity and environmental oxygen. Insects in today's world can't get much bigger than the giant weta due to our atmosphere. I'm putting the picture together in my mind now... the world was better suited for one type of organism during different periods due to environmental factors rather than any group being intrinsically "better" or more fit. Makes me think of Jared Diamond's book Guns, Germs and Steel, which brilliantly explained why Europeans took over the world in the Age of Discovery rather than non-Europeans - simply a matter of having been born into the perfect combination of resources on our planet. His arguments demolished the antiquated idea that human fitness had anything to do with racial attributes.

[stargatedalek]

Actually insects never managed to surpass amphibians, fish, or cephalopods in size.

I'm still not keen on considering size or reproductive rates as measures of success. As this same principle points out larger animals require more energy, which means more time and energy spent gathering food, and that becomes a cycle. If an animal can happen into another way to survive that less drastically increases its energy requirements (IE venom or poison), why is that less successful than the animal that became larger to deter predation?