Dinosaurs - warm blooded vs low oxygen levels

I was watching ‘miracle planet’ last night…

I was under the impression that dinosaurs being thought of as reptiles was wrong. That they were actually warm blooded and should be thought of as ‘pre-birds’ and just as warm blooded and not reptiles.

This made sense, as mammals and dinosaurs came about at the same time and dinosaurs out-competed mammals.

However, miracle planet seemed very confident that dinosaurs were reptiles but outcompeted mammals due their being low oxygen levels on Earth during their time. (something like 10% rather than 20%)

The dinosaurs were cold blooded but were much more efficient than mammals in the low-oxygen environment - therefore mammals were dominated.

They brought it up as if it were fact…a done deal.

Is this true? Is it commonly acknowledged dinosaurs were cold blooded and there was low oxygen levels?

I can’t picture a Brontosaurus being cold blooded. I’m not a Paleobiologist so I do not know. But I’d think dinos were warm blooded, able to regulate their own body temps. Otherwise how would a massive animal of 20+ tons have the energy to eat tons of vegitation a day, ***and *** sun itself to maintain it’s core temp?

Wait a second… “low oxygen”? Weren’t oxygen levels much higher then? I know the levels fluctuated, but weren’t some of our planet’s highest oxygen levels during the age of dinosaurs?

:Wanders off to locate reference:

I swore oxygen levels were actually higher. This is what allowed larger insects to thrive. Perhaps my timeline is a little off.

The debate over warm-blooded vs. cold-blooded is hardly resolved in the paleontology world. The debates appear to get quite heated. The shear size of the Apatosaurus and its cousins is what many argue, allowed a cold-blooded creature to remain energetic. The large size allowed them to store heat better than their smaller cousins.

Some of the debates seem to indicate that the Ornithischia (bird-hipped) were warm blooded like their descendents the birds and the Saurischia (lizard-hipped) were cold blooded.

The current consensus is that dinosaurs are endothermic*. Some argue that large dinosaurs lost heat relatively slowly to the surrounding air, and could have been inertial homeotherms**. Smaller Dinosaurs would not have this ability.


  • Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering, fat burning, and panting (Greek: endo = “within,” therm = “heat”). Some writers restrict the meaning of “endothermy” to mechanisms which directly raise the animal’s metabolic rate in order to produce heat. The opposite of endothermy is ectothermy.

** Animals that are warmer than their environments through sheer size rather than through special adaptations like those of birds or mammals.

I swore O2 levels were higher also…which took me by surprise.

However, I have a tendency to let things that interest me ‘slide’ for years, then pick it up again…such as finding out about inflation theory around 1990 after not having read anything about astronomy since 1980ish…

I figured maybe this was the like…and that it was a done deal and I was way out of touch.

I can’t find a good reference offhand, but oxygen levels in the atmosphere peaked at about 30% (sea level) during the Carboniferous–Permian Transition and descending to 16% or less in the late Permian, apparently coincident with the devastating Permian–Triassic Extinction Event about 250 Myr ago. (However, when I say “conicident” I’m talking about within a few million years, so whether it was a case of the P-T extinction or an incidental result of the root cause is unknown and hotly debated.) Oxygen levels have fluctuated more moderately in between those extremes since Earth’s reducing atmosphere first began to become oxygenated by photosynthetic organisms somewhere between 2 Byr and 2.5 Byr ago. (The composition and distribution of Earth’s atmosphere is poorly understood and largely speculative beyond 1 Byr ago.)

Also, even if the composition of the atmosphere were less oxgyen rich, this doesn’t necessarily favor the hypothesis that dinosaurs were cold-blooded. It’s true that modern homothermic organisms in classes mammalia and aves are certainly accusomed to current oxygen levels, while the remaining sauropods have a variety of ingenous (if often primitive in comparison to mammalian) mechanisms for respiration which offer many an advantage in restricted oxygen environments, but this reflects more the particular niches to which they’re adapted and/or their evolutionary heritage than being better suited as a class to a low oxygen atmosphere. A larger body size requires more energy–and thus more respiration–for constant thermoregulation, but on the other hand also has less surface area to lose heat to begin with. A mammal with sloth-like resting habits could readily adapt to a lower oxygen environment, and indeed, there are a number of large ungulates that function fine in the low partial oxygen pressure at high altitudes where large reptiles are not found. And there’s a great spectrum of thermal regulation capability between homeothermy and poikilothermy.Anatomically, dinosaurs as a general group are most resembled by modern aves, including skeletal features that support thermoregulation; hence, dinosaurs were probably not as sophisticated as modern aves or mammals, but likely had signficant means to affect body temperature regulation.

It’s also probably a gross oversimplification to group “dinosaurs” together as a single class of similiar anatomical and phenotyptical construction. The term dinosaurs represents a very broad spectrum of animals from the size of mice to several times the mass of the largest mammal, and they no doubt had a great diversity in metabolism, thermoregulation, behavior, et cetera. We can only develop rough speculations based upon evidence of fossil remains as to what their behavior was like, but to group them all as “cold-blooded” based upon one alleged environmental characteristic isn’t just a thin rationale, it’s positively anerexic. And while this isn’t particularly germaine to the discussion, it’s simply not true that “dinosaurs outcompeted mammals”; while large mammals didn’t develop until after the K-T extinction event (likely as a result of mass extinction of dinosaurs which left niches for strict predators and herbivores) mammals of the day were quite successful in their own domain (detrius scavengers and opportunistic carnivores), in which they were not really competing with dinosaurs as a supposed class. This would be like suggesting that insects are outcompeted by mammals because there are no 100kg insects. With a few notable exceptions, large mammals tend to be specialized toward a particular environment and diet, and the vast majority of large mammal species have become extinct or evolved dramatically with relatively short periods of existance.

So the rationale presented for dinosaurs being cold-blooded was:[list=a][li]spurious,[]overly simplistic, and[]likely wrong.[/list]Thus be warned about getting accurate science information from the televisor, which as far as I can tell is mostly a tool for marketing sugared breakfast cereal and feminine hygiene products.[/li]

The definition of “reptile” is not “cold-blooded tetrapod”. Dinosaurs are very much reptiles, by pretty much any taxonomic definition you can come up with. Some were very likely “warm-blooded” (particularly those closest to birds), others may have been “mass endotherms” (e.g., the very large beasties like sauropods), which simply by virtue of being so large maintained heat much better than their smaller kin. We have fossil evidence that at least some theropod species were covered in feathers, during at least part of their life (e.g., tyrannosaurs might have been feathered as juveniles, but not as adults). These feathery coverings imply a means of regulating heat, which in turn implies endothermy.

Further, you have the fact that dinosaurs and pterosaurs are closely related. Pterosaurs were the first vertebrate fliers, and some of the smaller ones were seemingly the equal of modern birds in terms if maneuverability and capability. Fossils also suggest that pterosaurs also had some sort of integumentary covering, akin to hair. Which, coupled with their active lifestyles, implies endothermy there, as well.

So, we have close relatives of dinosaurs being likely endothermic, and at least some dinosaurs themselves being endothermic. A reasonable conclusion might be that the “in between” critters were also endothermic. Of course, we have no direct evidence for most dinosaur species.

Oxygen levels were very low at the beginning of the Triassic (recovery from the permian extinction was just beginning). But neither dinosaurs nor mammals appear until the late Triassic, a good 25 million years after the beginning of the period. Oxygen levels were increasing during the Triassic, and reached 18% by the end of the Cretaceous.

It’s difficult to parse out exactly what time period you mean here, since you’re using present tense, but there are no “remaining sauropods” at this time.

Urk…I mean Sauropsida (reptiles).


Nitpick: my understanding is that birds descended from the saurischia, not the ornithischia.

Another Straight Dope perennial.

See, the trouble is that “warm blooded” and “cold blooded” are not well defined terms, and the more we learn about animal metabolism the more we learn that things are much more complicated than they appear.

There are animals that maintain constant body temperatures, either high or low, and these are called homeothermic (“same temperature”). There are animals that have varied body temperatures, and these are called poikilothermic…

Wait a second. No need to reinvent the wheel. Just read these:


I like how the articles break out homeo-vs poikilo thermy, endo vs exo thermy, and tachy vs brady metabolism.

See, a giant Jurassic sauropod probably didn’t have a mammal-like or bird-like metabolism, but that doesn’t mean it therefore had a crocodile-like or lizard-like metabolism. A sauropod that sits in the sun eating during the day isn’t going to cool off at night like a lizard does, because it’s surface area to volume ratio is much lower. That’s gigantothermy. Animals can be warm blooded at certain times and cold-blooded at rest, that’s heterothermy, like bats who assume room temperature when they sleep. Animals can have certain parts of their bodies warm and other parts at ambient temperature. Animals can have one metabolic system during one part of their life cycle, but change at a different part of their life cycle–that is, small juveniles will need different methods of thermoregulation than gigantic adults.

Now, about oxygen levels. Sure, giant insects thrived during a period of high oxygen levels. But oxygen levels weren’t “higher in the past”, they have been both higher and lower. And dinosaurs and birds had much more efficient lungs than mammals, and it’s theorized that extremely low oxygen levels at the end of the Permian preferentially killed off the mammal-like reptiles in favor of the proto-dinosaurs. But the really big winner of the PT extinction was the mammal-like reptile Lystrosaurus. Lots of early Triassic strata contain nothing but Lystrosaurus…with all the other large animals extinct it must have multiplied like cockroaches once the plants returned. Peter Ward theorized that Lystrosaurs was a mountain species that was pre-adapted to low oxygen levels. Or perhaps it was just luck of the draw that enabled it to survive. http://en.wikipedia.org/wiki/Lystrosaurus.

No cite, but IIRC the most important thematic change relating to metabolism was the emergence of nutritious, fatty seeds as a concentrated food source, improving upon the predominant food source of pulpy, fibrous vegitation that required slow metabolism and long digestion. So, before seeds, vegetarian animals were like modern elephants and cattle, eating huge quantities of poor quality calorie sources and leaving admirable piles of fibrous dung.

There was a discussion on this on CBC Radio this weekend (repeating an episode from 12/16/2006). Scroll down to “Out of Thin Air”. You can listen to the discussion, and there is a reference to a book that addresses this question.

That’s almost certainly Peter Ward’s book:


The probelm with this hypothetsis is threefold.

Firstly it ignores altogether that most of the world’s ecosystems today are based on animals that eat leaves. Think about the African savnannas or the Amazon rainforest. The animals that live primarily on fruiting bodies are very, very thin on the ground. >90% of the primary consumers are living on leaves. That does not in any way decrease the productivity of those systems or prevent animals with high metabolic rates such as birds, monkeys and horses from thriving on a leaf diet. That alone would seem to falsify any claim that high metabolic rates can’t evolve in systems dependant on fibrous material.

The next problem is is that when you say “before seeds” you are really talking about the very early carboniferous when there were only mosses and liverworts. There were no terrestrial vegetarian animals to speak of in those days. Even the giant clubmosses produced starch or oil rich fruiting bodies, and they probably produced nectar as well. Certainly the ferns and seed ferns produced numbers of species with starch rich fruiting bodies. You shouldn’t think of the seedless plants we have today as being typical. The modern survivors are those that have managed to survive in the face of far more efficient reproductive strategies fom seed plants. IOW the modern seedless plants are largely those that have abandoned any attempt at producing seeds in favour of other strategies that enable them to compete with seed plants. Nonetheless there are still a few seedless plants that produce starchy fruiting bodies in sufficient quantities to make them worthwhile for huamns to harvest for food. Before the evolution of seed plants such structures were ubiquitous.

The third problem with the hypotehsis is that high quality foods don’t have to be fruiting bodies. Plants have always needed ways to store energy in adverse condtions, and seeds plants often do this in the form of seeds. However before the evolution of seeds alternative methods would have evolved such as carbohydrate or oil rich tubers and stems. These methods are still important today, and large parts of our caloric intake comes from non-fruiting sources such as sugar cane, potatoes and sago. The rhizomes of bracken fern have been used as a source of stacrh worlwide for millenia. Once again I think the problem stems from a assumtpoion that modern seedless plants are phsyically similar and occupy the same niche as ancients seedless plants. This simply isn’t the case, and when seedless ferns comprised the rain forests and desert flora and filled every niche in between they would have probably produced as much high quiality food as seed plants

>Plants have always needed ways to store energy in adverse condtions, and seeds plants often do this in the form of seeds.

Really? How do they get the energy back out? Aren’t seeds basically lost to the plant after forming?

Depends on how you look at it. Sure, the parent plant can’t access the energy stored in seeds, but that energy is stored for later use by the seedlings. So the plant puts all its stored resources into the seeds and then dies. The seeds with their stored energy stay dormant until conditions improve.