What are the atmospheric requirements for large dinosaurs? and are the atmospheric constituents for supporting large dinosaurs any different from the atmosphere today?
Oxygen levels have changed greatly throughout Earths's history starting with very little atmospheric oxygen and gradually increasing as photosynthetic activity appeared until all the oxygen sinks such as minerals had been filled, then taking off, reaching as high as 35% of the atmosphere during the carboniferous period 3. The reasons behind this sudden increase in O₂ has to do with the development of bark covered wooden plants, a feature made possible by the molecule lignin, which strengthens the trunks of trees and makes tall plants possible. However, lignin was hard for microbes to digest, and the dead trees built up, never returning their carbon to the atmosphere as CO₂. So the oxygen they had made while alive stayed in the air 4.
The plot shows the upper (red) and lower (green) estimates for O₂ concentration over time. The data for this plot was taken from a review of analysis of sulfur, carbon, iron, and manganese deposits 2. If you're interested in how isotopic analysis can give information about atmospheric oxygen, read that paper, it's detailed.
That spike in the plot is the carboniferous. This period in history is interesting because of the appearance of large invertebrates, such as 3 foot wide dragonflies and 6 foot long centipedes 5. Since these animals move oxygen through a series of tubes called trachea, where air just moves through by diffusion, it was speculated that the higher concentration of oxygen allowed this growth. Recent research indicates that insects can force air through their trachea, and that competition between predators and prey could have explained most of the size 6.
So how does this affect dinosaurs? The carboniferous period was about 350-300 million years ago, which corresponds with the spike on the graph. Dinosaurs appeared in the Triassic, which was about 250-200 million years ago, but oxygen levels were at 16% of the atmosphere at the time 7 ( current O₂ levels are about 20% ) If we move into the Jurassic, about 200-150 million years ago, oxygen levels rise to about 26% 8. In the Cretaceous, about 150-65 million years ago, O₂ was at about 30% 9. The O₂ percentages are taken from the infoboxes on the right side of the wikipedia articles, which in turn take the number from a plot very similar to the plot I posted here, despite being in German 10.
I compiled lists of heaviest and lightest dinosaurs by going through the wikipedia article on dinosaur size 11 and going to the article on each dinosaur and checking if it was Triassic, Jurassic, or Cretaceous. If we look at the heaviest dinosaurs, most of them appeared during the Cretaceous:
Heaviest Dinosaurs Theropods Metric Tons Period Spinosaurus aegyptiacus 7-20.9 Cretaceous Carcharodontosaurus saharicus 6.1-15.1 Cretaceous Giganotosaurus carolinii 6.1-13.8 Cretaceous Tyrannosaurus rex 6-9.5 Cretaceous Oxalaia quilombensis 5-7 Cretaceous Acrocanthosaurus atokensis 5.6-6.2 Cretaceous Tyrannotitan chubutensis 4.9-5.6 Cretaceous Suchomimus tenerensis 2.7-5.2 Cretaceous Therizinosaurus cheloniformis 5 Cretaceous Tarbosaurus bataar 4-5 Cretaceous Sauropods Metric Tons Period Amphicoelias fragillimus 122.4 Jurassic Argentinosaurus huinculensis 60-90 Cretaceous Antarctosaurus giganteus 69-80 Cretaceous Mamenchisaurus sinocanadorum 75 Jurassic Sauroposeidon proteles 40-60 Cretaceous Paralititan stromeri 59 Cretaceous Brachiosaurus altithorax 28.7-56.3 Jurassic Puertasaurus reuili 50 Cretaceous Ruyangosaurus giganteus 50 Cretaceous Alamosaurus sanjuanensis 50-60 Cretaceous Futalognkosaurus dukei 38.1-50+ Cretaceous Turiasaurus riodevensis 40-50.9 Jurassic/Cretaceous Camarasaurus supremus 47 Jurassic Diplodocus hallorum 30-42.5 Jurassic Supersaurus vivianae 32-40.2 Jurassic Elaltitan lilloi 42.9 Cretaceous Tehuelchesaurus benitezii 41.3 Jurassic Apatosaurus louisae 18-41.3 Jurassic Ornithopods Metric Tons Period Shantungosaurus giganteus 9.9-22.5 Cretaceous Iguanodon seeleyi 15 Cretaceous Saurolophus angustirostris 6.6-9 Cretaceous Iguanodon bernissartensis 8.3-8.6 Cretaceous Edmontosaurus annectens 3.2-7.6 Cretaceous Brachylophosaurus canadensis 4.5-7 Cretaceous Saurolophus osborni 6.6 Cretaceous Lanzhousaurus magnidens 6 Cretaceous Parasaurolophus walkeri 3-5.1 Cretaceous Charonosaurus jiayinensis 5 Cretaceous Barsboldia sicinskii 5 Cretaceous Ceratopsians Metric Tons Period Triceratops horridus 5-14 Cretaceous Triceratops prorsus 6.5-11 Cretaceous Titanoceratops ouranos 6.5-11 Cretaceous Eotriceratops xerinsularis 10 Cretaceous Pentaceratops sternbergii 3-4.8 Cretaceous Pachyrhinosaurus canadensis 3-4.4 Cretaceous Styracosaurus albertensis 4.2 Cretaceous Agujaceratops mariscalensis 2.6 Cretaceous Centrosaurus apertus 1.1-2.5 Cretaceous Coronosaurus brinkmani 2 Cretaceous Rubeosaurus ovatus 2 Cretaceous Achelousaurus horneri 2 Cretaceous Pachyrhinosaurus lakustai 2 Cretaceous Chasmosaurus belli 2 Cretaceous Thyreophorans Metric Tons Period Dacentrurus armatus 5-7.4 Jurassic Ankylosaurus magniventris 1.7-6 Cretaceous Stegosaurus stenops 2.6-5.3 Jurassic Cedarpelta bilbeyhallorum 5 Cretaceous Hesperosaurus mjosi 3.5-5 Jurassic Tuojiangosaurus multispinus 4.8 Jurassic Wuerhosaurus homheni 4 Cretaceous Niobrarasaurus coleii 4 Cretaceous Stegosaurus ungulatus 3.5 Jurassic Gobisaurus domoculus 3.5 Cretaceous Nodosaurus textilis 3.5 Cretaceous Palaeoscincus costatus 3.5 Cretaceous Sauropelta edwardsi 3 Cretaceous Edmontonia rugosidens 3 Cretaceous Edmontonia schlessmani 3 Cretaceous Edmontonia longiceps 2.3-3 Cretaceous Tuojiangosaurus multispinus 2.8 Jurassic Euoplocephalus tutus 2-2.7 Cretaceous Jiangjunosaurus junggarensis 2.5 Jurassic
This isn't exactly a fair way to do it, but I didn't want to calculate average dinosaur mass over time. I know that some groups, such as the ceratopsians didn't appear until the late Jurassic, skewing the results, and I didn't make any attempt to differentiate between late or early periods. However I also noticed that many of the smallest dinosaurs appeared in the Cretaceous as well. Pay attention to the units, even the smallest sauropods are measured in metric tons.
Lightest Dinosaurs Theropods kg Period Parvicursor remotus 0.137 Cretaceous Epidexipteryx hui .164-.391 Jurassic Compsognathus longipes 0.26 Jurassic Ceratonykus oculatus 0.3 Cretaceous Juravenator starki 0.34-0.41 Jurassic Ligabueino andesi 0.35 Cretaceous Microraptor zhaoianus 0.4 Cretaceous Sinosauropteryx prima 0.55-0.99 Cretaceous Rahonavis ostromi 0.58 Cretaceous Mahakala omnogovae 0.76-0.79 Cretaceous Xiaotingia zhengi 0.79 Jurassic Mei long 0.85 Cretaceous Microraptor gui 0.95-1.50 Cretaceous Sauropods Metric Tons Period Pleurocoelus nanus 0.5 Cretaceous Magyarosaurus dacus 0.75 Cretaceous Europasaurus holgeri 0.8-1 Jurassic Bonatitan reigi 1 Cretaceous Lapparentosaurus madagascariensis 1.4 Jurassic Lessemsaurus sauropoides 1.8 Triassic Lirainosaurus astibiae 1.8-4 Cretaceous Shunosaurus lii 2.2-6.7 Jurassic Ampelosaurus atacis 2.5 Cretaceous Amargasaurus cazaui 2.6-3.8 Cretaceous Hypselosaurus priscus 2.7-8 Cretaceous Euhelopus zdanskyi 3.4 Cretaceous Neuquensaurus australis 3.5-6.1 Cretaceous Rinconsaurus caudamirus 4.1 Cretaceous Atacamatitan chilensis 4.3 Cretaceous Dicraeosaurus hansemanni 4.4-5 Jurassic Ornithopods kg Period Gasparinisaura cincosaltensis 1-13 Cretaceous Yueosaurus tiantaiensis 3.9 Cretaceous Fulgurotherium australe 6 Cretaceous Notohypsilophodon comodorensis 6 Jurassic Yandusaurus hongheensis 6.6-7.5 Jurassic Hypsilophodon foxii 7-21 Cretaceous Thescelosaurus sp. 7.9-86 Cretaceous Valdosaurus canaliculatus 10 Cretaceous Haya griva 11 Cretaceous Agilisaurus louderbacki 12 Jurassic Drinker nisti 20 Jurassic Changchunsaurus parvus 20 Cretaceous Qantassaurus intrepidus 20 Cretaceous Zephyrosaurus schaffi 20 Cretaceous Oryctodromeus cubicularis 20 Cretaceous Orodromeus makelai 20 Cretaceous Ceratopsians kg Period Liaoceratops yanzigouensis 2 Cretaceous Yamaceratops dorngobiensis 2 Cretaceous Psittacosaurus sinensis 4.1 Cretaceous Psittacosaurus lujiatunensis 5 Jurassic Yinlong downsi 5.5 Cretaceous Micropachycephalosaurus hongtuyanensis 5.9 Jurassic Chaoyangsaurus youngi 6 Jurassic Xuanhuaceratops niei 6 Jurassic Bagaceratops rozhdestvenskyi 7 Cretaceous Psittacosaurus meileyingensis 8 Cretaceous Psittacosaurus neimongoliensis 8-8.4 Cretaceous Archaeoceratops oshimai 10 Cretaceous Psittacosaurus mongoliensis 12.1-20 Cretaceous Psittacosaurus sibiricus 15 Cretaceous Thyreophorans kg Period Scutellosaurus lawleri 3 Jurassic Emausaurus ernsti 50 Jurassic Scelidosaurus harrisonii 64.5-270 Jurassic Animantarx ramaljonesi 300 Cretaceous Struthiosaurus transylvanicus 300 Cretaceous Struthiosaurus austriacus 300 Cretaceous Gargoyleosaurus parkpinorum 300 Jurassic Mymoorapelta maysi 300 Jurassic Minmi paravertebra 300 Cretaceous
So what does this mean for the relationship between oxygen levels and dinosaur size? Higher oxygen levels in the Jurassic and Cretaceous probably allowed for larger animals to exist, however as time went on Dinosaurs became more diverse in general, including size. Many of them evolved to become smaller, despite being allowed to become larger by higher levels of oxygen. We should also note that even though oxygen levels were still lower than they were during the carboniferous, the animals were much larger, most likely due to their more efficient oxygen transport systems (lungs, 3(4?) chambered hearts, sealed circulatory systems, hemoglobin).
If we think about what could drive species to become larger, competition for food and protection from predators comes to mind. Sauropods that could reach the leaves in taller trees had distinct advantages over those who could not. Prey animals stood a better chance at fighting off predators if they were bigger. Likewise, predators could kill larger prey if they became larger too. Oxygen didn't drive them to become larger, it simply allowed them to.
To actually answer the question, could the modern atmosphere support animals as large as dinosaurs? It probably could. The heaviest living animal today is the blue whale, at an average mass of 110 metric tons, is heavier than all but 1 of the sauropods on the list I compiled. The heaviest land animal living today is the African Elephant, with average mass of 4.9 metric tons and as high as 10 metric tons, would be a respectable competitor with many of the dinosaur groups I listed 12.
Another interesting question is not how oxygen affected the size of organisms, but how it affects it their metabolism. Cold blooded animals don't need as much oxygen because they don't spend as much energy. Warm blooded animals must constantly expend energy to maintain body heat, and this requires greater consumption of oxygen. Obviously being large and warm blooded would force you to need even greater levels of oxygen than 1 or the other. It may be that the advantages of being warm blooded outweighed the advantages of large size, but I don't want to dig into that question right now.
I'm not so sure that larger land animals require that much more oxygen. The problem is metabolism's waste heat.
Anyone remember the square-cube law? All other things being equal, double an object's size, and its area quadruples, while its volume octuples.
Broadly speaking, an object loses heat in proportion to surface area, while an animal produces heat in proportion to volume. For our double-sized creature, heat production per unit area would double.
Other things being equal, to reduce overheating, metabolic rates would have to go down, thus reducing the oxygen demand.
Conversely, consider the opposite: a creature half the size of the original. It now produces half the heat per unit area. If it needs to maintain a minimum body temperature, then (other things being equal) its metabolic rate will need to go up. This will certainly be easier if the oxygen concentration is higher.
Of course, other things are never equal, so this is only one piece of the puzzle. But it seems that, for land animals that are already large, increased oxygen levels alone should push size downward.
Or am I missing something?