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The title is a bit different to the question, but there's a reason to that. I am currently in the worldbuilding stage of creating an alien planet, mainly the wildlife, and I may have run into a problem. Back to the title, at one point on Earth, Dinosaurs and large reptiles filled the majority of niches (I'm talking specifically land-borne life and excluding invertebrates). After the mass extinction, the empty niches were filled by mammals and birds. focusing on mammals, they were only able to fill those niches once the dinosaurs had died off and left them behind (correct me if I'm wrong).

On my fictional planet, lets call it Xenoterra for now, there are two dominant groups of animals, the mammal-like, quadrupedal group, and the more reptilian, six-limbed group (the niches usually filled by arthropods and other invertebrates on Earth are filled, on this planet, by very similar animals). If we were to think of these two groups as the equivalent of dinosaurs and mammals, (birds falling into dinosaurs because that's what they are) could they have evolved at the same time, filling the niches but not competing to the point of one being dominated? Note: I'm not including the difference in size into the equation as the dinosaur and mammal part is just a comparison, the creatures in question have the same range of size and adaptability. The question uses an Earth example but in no way goes by 'Earth rules' but the ecosystem is similar. My creatures are not dinosaurs and mammals, I'm comparing them to dinosaurs and mammals because of the similar circumstance.

this question is definitely open to editing so if you want more info tell me. I might be slightly late to answer though.

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  • $\begingroup$ I used dinosaurs and mammals as an example of two competitive animal groups, and the animals on my planet are mammal-LIKE and reptile-LIKE, sheerly by coincidence. $\endgroup$ – XenoDwarf Feb 15 '16 at 7:18
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As long as they aren't in direct competition and are able to fill different niches effectively, they can coexist.

We don't know enough about dinosaurs to answer many of the hard questions here - body covering, movement speed, and warm-bloodedness will play a huge role in determining which roles each group is better at overall.

Of course, since we aren't talking about real dinosaurs and mammals here, you can come up with whatever diversification roles you want.

On modern Earth, for example, reptiles thrive in deserts. This is largely because cold-blooded animals need abundant heat to get moving, but on the whole require less food than warm-blooded animals do. Where food is scarce and sunlight is plentiful, they often out-compete mammals.

Large, slow predators can coexist with fast, weak ones if their preferred prey defend themselves with armor/fighting or fleeing, respectively (on our world, lions and cheetahs coexist for this reason - cheetahs hunt what lions can't catch, lions hunt what cheetahs can't fight). Reptiles (at least modern reptiles) are prone to being slower and tougher than mammals overall, so you can have an ecosystem where speedy felines hunt ungulates while hefty tyrannosaurs prey on armored ceratopsians and so on.

My guess is that on a planet such as yours, the reptile-like animals and mammal-like animals would tend to be specialized for whichever role their particular group performs best at, for the same reason that most birds don't take on mammal-like roles; even though exceptions like kiwis and penguins exist, they usually can't out-compete mammals for very long in non-flying niches. So if you diversify them according to the above rule, speedy dinosaurs (like raptors) and heavyweight mammals (like bears) might have a hard time.

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  • $\begingroup$ you seem to have answered all the points here, if this isn't beaten i'll choose it. $\endgroup$ – XenoDwarf Feb 16 '16 at 9:43
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Mammals and dinosaurs coexisted, and during the Triassic there were mammalian creatures which were dog and bear sized, alongside dog and bear sized Dinosaurs and even a wide variety of crocodilian creatures, all swarming into the same evolutionary niches (for example, fully aquatic crocodiles who's legs had evolved into seal like flippers).

Of course by the end of the Triassic, crocodilians were regulated to the shallow waters by the edge of rivers and lakes, while mammals were pushed down to fill the niches filled by small rodents and shrews even today.

So the question is "why". Evidently, you can have multiple species attempting to fill the same nieces for a short while in evolutionary terms. After the extinction of the dinosaurs, several varieties of birds grew to become top predators in some places (the Terror Birds), but eventually they were overtaken by mammalian predators. Similarly, marsupials have or had evolved into many niches, but only in Australia, where they were isolated from competition, did they thrive until recent times (and humans both hunted many to extinction and brought mammalian competitors which displaced many marsupials from their niches).

So in evolutionary terms, the answer would seem to be that one type of creature is more adaptable and more flexible in filling a niche than any other. Dinosaurs seem to have been very adaptable, filling niches on land, sea and air and regulating all competitors into very small and marginal niches. While we usually think of dinosaurs as giants, this is an artifact of how well larger skeletons are preserved and the "wow" factor in displaying them. In reality, there must have been thousands of species ranging from chicken sized to the immense Sauropods.

So the short term answer is "yes" there can be periods where several different groups are trying to establish dominance over environmental niches, but this is only going to be a short term time frame. I would also suggest that it coincides with the end of a previous epoch, where the formerly dominant species has become extinct, opening up multiple niches for the competitors to move into.

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  • $\begingroup$ Or you can have dinosaurs and mammals at different niches in the same ecosystem. Dinosaurs filling up the niches of larger, dominant creatures (with no carnivorous dinosaur smaller than a utahraptor and no herbivore smaller than a stegosaurus) while mammals stay small and live alongside, filling the niches of smaller beta predators (none larger than a tiger) and no herbivorous mammal being larger than a reindeer. $\endgroup$ – Youstay Igo Feb 15 '16 at 12:34
  • $\begingroup$ Dinosaurs did coexist with both mammals and crocodiles after they exploded into the dominant species, but mammals and crocodiles were (by fossil evidence) pushed into very marginal niches. Utahraptors would eventually have branches moving downrange to displace tigers, then wolves, then cats etc. since the bulk of the ecosystem at that time was built around dinosaurs. Only in epoch changes (the beginning of the Triassic) before the new ecosystems had stabilized, could you get such overlap. $\endgroup$ – Thucydides Feb 16 '16 at 1:44
  • $\begingroup$ Yes I am aware of the mesozoic ecosystem. But during most of the mesozoic, crocodilians lived terrestrial lives as opposed to semi-aquatic as those of today's crocs. I wonder if larger carnivores would develop speciation, replacing smaller carnivores with time. For example, T-rexes did not have a family line of smaller T-rexes gradually replacing deinonychus or velociraptor simply because the deinonychus and velociraptors already present at those niches were too hard to push off. Similarly, I think sabertooths and tigers of today are strong and stable enough in their niches. $\endgroup$ – Youstay Igo Feb 16 '16 at 9:34
  • $\begingroup$ Althought there were smaller predatory dinosaurs, there were some branches of the tyrannosaur family which were also getting smaller. And my example about the crocodiles points out that while they and radiated into many niches, by the end of the Triassic they had been displaced by the more adaptable dinosaurs. Dinosaurs simply outperformed both mammals and crocodiles, and became the dominate species because of this. $\endgroup$ – Thucydides Feb 17 '16 at 0:29
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Size and speed are the key, specially when we are discussing the dominant carnivores. Mammals were only able to fill the dominant land animals group because they were never able to reach the size of the mighty dinosaurs (on Earth). Even if today you introduce a small population of deinonychus and utahraptors (20 individuals of each type) in the american wilds, chances are that they will flourish and replace the mammalian carnivores within 10 years, provided that they can fit in with the extreme cold conditions prevailing today in North America.

I have pointed it out in the past too, and would like to stress on the point now again, that in world building process, you are the decision maker, not the laws of Earth. So the question really is not if dinosaurs and mammals can coexist as dominant land animals, but whether you want to put dinosaurs and mammals together in that role or not.

That is, if you do decide to put dinosaurs and mammals together on your planet, you may want to give them equal physical attributes, specially for the carnivores. For example, if you have a 20 foot long carnivorous dinosaur, you would need to put an Andrewsarcus sized mammalian carnivore in the same habitat to equalize things. Similarly, for 40 and 50 long sauropods, you would want to place alongside them.

Or you may want to distribute the ecological roles more distinctly. That is, the larger and more prominent herbivores and carnivores be restricted to dinosaurs while smaller and less prominent roles be handled out to mammalians. That would mean you can go on putting Tyrannosaurus, Albertosaurus, Gorgosaurs, Allosaurus sized prominent carnivores in dinosaur league and put sabertooths, Amphicyon, Arctodus and other such carnivores in the same habitat which fill the niche of smaller sized carnivores. (God have mercy on the herbivores of that planet in that case because it appears such an ecosystem would be a very unstable one.)

So all in all, it is not a question of "if" the two animal types can coexist but "whether" you want them to coexist or not.

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  • $\begingroup$ i like your answer, maybe i should reword some of my answer to emphasize a particular point. $\endgroup$ – XenoDwarf Feb 15 '16 at 2:51
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    $\begingroup$ That is not at all the reason why mammals did not take over during the Mesozoic. It was because when mammals arrived on scene, all of the ecological niches were already filled. It is hysterical that one would believe that 40 individuals of a different era could out-compete hundreds of thousands of individual that have been selected to hunt those same prey items for no less than 8 million years $\endgroup$ – user15036 Feb 15 '16 at 6:02
  • $\begingroup$ @TheoclesofSaturn: Wouldn't it a better idea if you put your thought into a comprehensive answer which is helpful to OP, instead of snapping at me and deeming me hysterical and whatnot? Ecological niches are not something which is advertised in a newspaper. In fact every successful creature creates its own niche in the ecosystem. After the demise of sauropods and huge herbivores like Peraceratherium, no mammalian herbivore rose those gigantic proportions. Why? Where did the niche of extra-large herbivores disappear to? $\endgroup$ – Youstay Igo Feb 15 '16 at 12:27
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Isolation. Different groups underwent adaptive radiation separately filling the niches in isolated areas. Think Australia and New Zealand on Earth.

Later, once a rich variety is established, something more can happen to remove the barrier and allow more intermixing.

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  • $\begingroup$ Yes. But what would happen after the barriers are removed and the two types of animals mix up to form one unified ecosystem? Would they be able to coexist (after some species being wiped out of course) or would one of them (probably dinosaurs) completely wipe out the others and be the only standing class of animals? $\endgroup$ – Youstay Igo Feb 16 '16 at 9:37
  • $\begingroup$ If the organisms are "mature" and well adapted to the niche, you might get some from group A that are better overall for its niche, and some from group B that are best in other niches. Sharks, Crocadiles, and Pidgeons are all successful. $\endgroup$ – JDługosz Feb 16 '16 at 12:49
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In order to do this try to work out what advantages each type of lifeform has and then realize that they will tend to move towards lifestyles that match that advantage.

So for example if you have your dinosaurs cold blooded then they would be sluggish and slow moving during the night and during cold weather, most likely hibernating through winter. Mammals on the other hand would be able to be active at night but do not scale up to the same size and need a lot more food to keep going.

This gives an immediate niche where the dinosaurs tend to be larger predators active during the day and tending to hunt infrequently but in large amounts. On the other hand mammals would tend to hunt at night or in early morning when the dinosaur types are still sluggish. When food is plentiful they would tend to flourish but in times of drought or famine the dinosaurs survive much longer.

Given this you can see how a dynamic system can easily emerge where certain areas are dominated by mammals, certain areas by dinosaurs, and then others have both mammals and dinosaurs competing.

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If, in your world, this is still a legitimate question, you might consider partnerships between the two species. Is there too few dinosaurs for them to hunt sufficiently alone, or could they rely on the partnership on these mammals in some way? Of course, the mammals would have to be getting something equally necessary from the dinosaurs etc.

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