11. Dinosaurs and Dragonflies

Life Evolves and Thrives According to its Environment

Stromatolites found in Western Australia These Stromatolites found in Western Australia are believed to be one of the first forms of life on Earth.
(Western Australia Now and Then)

Three and a half billion year old fossilized microorganisms and stromatolites give evidence of the one cell organisms that evolved not long after the formation of Earth. Over the next few billion years these one cell organisms evolved greater complexity until about 451 million years ago they were able to reach the next higher level of life: multicellular organisms. This sudden appearance of multicellular life, known as the Cambrian explosion, marks the beginning of the Phanerozoic eon. These first multicellular organisms were marine plants and animals; the first terrestrial animals did not evolve for another hundred to two hundred million years.

This chapter explores the relationship between the changing global atmospheric environment and the evolution of species. While the search for a reasonable explanation for how the gigantic dinosaurs and pterosaurs grew so large is what led to realizing that the atmosphere has changed considerably, these scientific paradoxes were only two of numerous paradoxes that became apparent because earlier scientists had simply assumed that Earth’s atmospheric environment was more or less the same as the present. Now, with the understanding that the Earth’s atmosphere was going through vast changes over time, we can go explore and understand the evolution of species from the prospective of whether the atmosphere was thick or thin, and in doing so we are capable of resolving these paradoxes. While the changing thickness of the atmosphere affected both terrestrial and marine, the effect was usually more apparent with the terrestrial species since these species physically existed in the atmosphere’s environment. Hence, most of this discussion will be centered on how the thickness of the atmosphere affected the terrestrial species.

dragonfly Dear Harry Potter fans, please do not mistake this dragonfly for the golden snitch.

Species evolve to fill available ecological niches and these ecological niches can only exist within physical and biological constraints. As such, the fossils of animals and plants give evidence of the environment that existed when these species were alive. For most species the reasons why they are successful are obvious, and yet there are still many other species that make us wonder why they have such a strange form or appearance. While nature may never run out of science puzzles for us to ponder, the vast majority of these mysteries can be solved by following the guide that there must be a rational scientific explanation for why it is the way it is.

Much of this chapter involves drawing conclusions regarding the atmospheric environment based on the fossil evidence of what species existed at that time. Conversely once we recognize the state of the atmosphere during a given geological time it then becomes much easier to make sense of the species that evolved in these environments. Besides a species need to fit within its physical environment we much also not forget that each species must also successfully compete with the other species in its environment as well. Besides the physical constraints we must consider the biological constraints as well. Being successful in correctly resolving current scientific paradoxes involves keeping our imagination open to what is possible as opposed to turning a blind eye to the evidence that challenges our assumptions.

DinosaurTheory began with the idea that the unique size and form of dinosaurs and pterosaurs is evidence of their environment. This evidence, along with the geological evidence, has made it clear that the Earth had and extremely thick atmosphere during the Mesozoic era. In addition, the fossil and geological evidence also showed that before the Mesozoic era there was a thin atmosphere period, the Permian period, sandwiched between the Mesozoic era and earlier times when the atmosphere was extremely thick. Now that the fossil evidence has been so helpful in identifying when the Earth’s atmosphere was either thick or thin, it is now possible to turn this perspective around to solve numerous scientific paradoxes regarding the life that existed during the Phanerozoic eon.

Gigantic Insects, Millipedes, and Spiders Oh My
Carboniferous and Permian Periods, 360 to 245 mya

animals classification chart For more information on animal classification go to www.earthreminder.com

Dinosaurs and flying pterosaurs are not the only giant oddities of the prehistoric world. About 400 million years ago, long before dinosaurs and other vertebrates, the first arthropods crawled out of the water and on to the land. Many of these animals soon grew to be many times larger than most arthropods that we see today.

In the Carboniferous period, an early centipede or millipede, known as Arthropleura, grew up to 2.6 meters long. Also during the Carboniferous, there existed a 70 centimeters long air-breathing scorpion known as Pulmonoscorpius. Seventy centimeters is also the wingspan of the gigantic ancestor dragonflies known as the Meganeura: the top flying predator of the Carboniferous and Permian periods. Of course, let us not forget about giant cockroaches; these nasty insects were among the first to crawl on the Earth, and they will probably still be here long after Homo sapiens are gone.

Since a much thicker atmosphere was so successful in explaining how the dinosaurs grew so large there may be a temptation to believe that an extremely thick atmosphere might also work for explaining how the late Paleozoic era arthropods grew so large. However this is a flawed idea since the late Paleozoic era was actually a time when the Earth’s atmosphere was relatively thin. So why did these arthropods grow to be so much larger than what is common today?

A higher level of atmospheric oxygen is the most popular explanation given for what allowed the late Paleozoic era arthropods to grow so large. This hypothesis has merit since, as we will see, larger arthropods have difficulty obtaining enough oxygen and so higher levels of atmospheric oxygen would help in addressing this problem. Nevertheless, while the higher level of atmospheric oxygen hypothesis deserves honorable mention, the actual reason for why terrestrial arthropods were able to grow so much larger during the late Paleozoic era is because of the lack of competition with vertebrates; for about the first hundred million years after the arthropods evolved onto the land there were no terrestrial vertebrates.

It is a bit of irony that the primary reason that terrestrial arthropods of the Carboniferous period were able to grow so large is because they started out so small. In contrast to vertebrates, there is almost no limit to how small arthropods can be and in fact there are countless invertebrate species that are microscopic in size. In regards to evolution, the smaller the individuals the shorter the species generation cycle and likewise the quicker a species can evolve into new species. More than anything else this gave arthropods the advantage over vertebrates in overcoming the challenges of evolving into terrestrial animals. Once they evolved as terrestrial animals the land was their and theirs alone and so without competition from the vertebrates they were able to grow large as they filled all the environmental niches that were available.

This changed once vertebrates evolved onto the land. With few exceptions, wherever the vertebrates went there were no longer giant size arthropods. For the terrestrial arthropods, there was no longer any advantage in being large since being large just made it more difficult to hide from carnivorous vertebrates. Whenever the two met it was far more likely that vertebrate would prey on the arthropod and not the other way around. To understand why it works out this way we need to look at the differences between vertebrates and arthropods.

Cicada Insect next to its Shell After Molting Cicada Insect next to its Shell After Molting

The most obvious difference between vertebrates and arthropods is that vertebrates have their support structure on their insides while arthropods have their support structure on their exterior. Structurally speaking one could claim that an exterior support structure is actually superior to an interior support structure: it is more difficult to bend a thick walled tube than it is to bend a rod. An addition benefit of the arthropods’ exterior exoskeleton is that it acts like an armored suit in protecting the animal’s interior muscles and organs from cuts and abrasions. Yet the advantages of the arthropods’ exterior exoskeleton ends once we take into consideration the fact that vertebrates and arthropods are growing organisms.

Bones are living tissue and so throughout the life of the vertebrate its bones are continuously growing and being rejuvenated along with the other organs of the animal. In contrast to this, the exoskeleton of an arthropod is simply a harden chitin, calcium, and carbonate material that is incapable of expanding with the growth of the animal. Consequently in order to keep up with its growth an arthropod must periodically break out of its old exoskeleton and then wait for the hardening of its next larger exoskeleton to form. Most insects molt five to seven times throughout their growth stage and during each of these molting processes the arthropod body momentary lacks a supporting structure as it exits is old exoskeleton. Quite often something will go wrong or a predator will take advantage of the arthropods vulnerability during this stage such that animal does not survive the transition. In contrast to the steady and uneventful growth process of vertebrates, the dangers associated with molting causes many if not most arthropods died before reaching the stage of being an adult.

Caterpillar Caterpillar showing nine small black holes along its side that are spiracles

Another major difference between vertebrates and arthropods is the way these different groups of animals breathe. Arthropods have a mostly passive breathing system consisting of numerous air ducts called trachea. Air comes in through openings called spiracles and then travels down the trachea before diffusing into the cells.

There is weakness to this design in that the cells that are closes to the exterior openings receive the most oxygen while the cells that are the deepest inside the interior receive the least. Still this respiratory system works fine for animals that are extremely small; it only becomes inefficient when these animals are a bit larger. A closer look at the interior of these larger arthropods shows that they are struggling to get enough oxygen. Even after enlarging the air ducts to deliver more air to the interior there would still be a noticeable reduction in the metabolism of these arthropods. This is important since slow moving animals have difficulty with either defending themselves or running away from predators and so they often end up being the prey.

Another feature of arthropods that does not scale well is their underdeveloped circulatory system. Arthropods have an open circulatory system which means that the blood just pours out around the organs and then slowly works its way back to the heart. This is not nearly as efficient as the closed circulatory system used by vertebrates. With a closed circulatory system a network of arteries and veins is highly effective in transporting oxygen and nutrients to the cells throughout the body. For small arthropods the open system works just fine but if arthropods want to be the same size as vertebrates and they also want to compete at their level then they will need to evolve a closed circulatory system.

To summarize, while the unique features of arthropods – the exoskeleton, passive oxygen intake, and open circulatory system – all work well if an animal is small, these features do not work so well as the animal becomes comparable in size to most vertebrates. Because of this large arthropods are usually unsuccessful at hiding from, running away from, or the defending themselves from the larger and faster vertebrates. Hence whenever vertebrates enter an environment that contains large arthropods it is not long before the vertebrates have eaten all of the large arthropods.

frog Here is a frog looking up at the ‘pie in the sky’.
If you can’t fly then the next best thing is to evolve a long sticky tongue. During the Permian period the air must have been thick with flying invertebrates because there were no flying vertebrates to prey on them.

During the late Carboniferous period there were no terrestrial vertebrates and this is why some terrestrial arthropods were able to grow so exceptionally large. This age of the giant terrestrial arthropods lasted for many millions of years until finally during the Permian period certain vertebrates completed their evolution journey from fish to amphibian. Amphibians took out many of the giant terrestrial arthropods but some still survived because amphibians could not wander far from the marshes and swamps. This problem was solved a few more million years later when the reptiles evolved from the amphibians. During the Permian period the amphibians and reptiles wiped out the giant crawling terrestrial arthropods. The only giant arthropods that were left were the one that flew overhead.

The best known giant flying insect of this time was the ancestor dragonfly known as the Meganeura. This species evolved midway through the Carboniferous period and it was still flying overhead at the end of Permian period. The reason the Meganeura lasted longer than the other giant terrestrial arthropods is because the vertebrates could not chase after it: during this time the vertebrates could not fly.

Flying vertebrates did not evolve until the Mesozoic era. Flying reptiles or pterosaurs first evolved in the Triassic period and then birds evolved in the late Jurassic period, while flying mammals – bats – did not evolve until the early part of the Cenozoic era. Of these flying vertebrates, birds have shown themselves to be incredible effective in keeping both the size and number of insects in check, while bats have done to part at limiting the number of insects flying during the night. Our world today would be a very different place if vertebrates had not evolved the ability to fly.

To summarize, the gigantic millipedes, scorpions, cockroaches, and dragonflies of the late Carboniferous and Permian periods did not grow large because the atmosphere was thicker: the atmosphere during this time was just as thin as it is today. Instead the reason these arthropods were able to grow so large is because when they evolved onto the land or evolved wings to fly, there were no vertebrates in these environments that would prey on them. But once the vertebrates evolved onto the land and evolved the ability to fly, the arthropods that survived were only the ones that stayed small and learned how to avoid the vertebrates.

The Evolution of Flight: How Evolution Works

There is no mystery on how insects evolved the ability to fly. It takes nothing more than a slight breeze to toss the vast majority of small and light insects into the air. In fact, in the perspective of small insect, the greater challenge is staying put. For what we might call a nice breeze or slightly windy day, must seem like a hurricane to a small insect. While many may hide from the wind, others could find it beneficial to use the wind to expand their travels. From here, it is only a series of small steps to evolve the ability to fly. Evolving the ability to fly is easier for animals that are small. When we take note of how many different ways that flying insects evolved, we get a sense of how easy it was for insects to complete this step. In contrast to the insects, how the vertebrates evolved the ability to fly has always been a scientific puzzle.

Gliders Examples of living gliding vertebrates: A) Chinese flying frog, B) Southern flying squirrel, C) Flying paradise snake, D) Sugar glider. Figure created by Manning, 2020. Images: D. Gordon E. Robertson; Kim Taylor; The Daily Conversation; Remon Knaap.

Before going further it will be helpful to have a good definition or understanding of what is flying. Currently there are numerous vertebrates that have flying in their name – flying dragon, flying snake, flying fish, flying squirrel, flying lemur, flying gecko – and yet all of these listed vertebrates are actually gliders: not flyers. To fly an animal needs more than just wings or a flat surface to slow down its fall to the ground: it needs a means of propelling itself forward with at least enough power to maintain level flight. Airplanes make this distinction clear by having both wings for generating lift and a propulsion system, such as a jet engine or an engine and propeller, for moving the airplane forward. Birds and other flying animals combine these two tasks by using their muscles to produce the forward propulsion by flapping their wings. In the act of flapping their wings, most flying vertebrates need a relatively high power output to achieve and maintain level flight.

For many decades, scientists have been trying various hypotheses on how vertebrates such as the pterosaurs, birds, and bats could have evolved the ability to fly. The problem is that if one assumes that the Earth's atmosphere has always been relatively thin like it is today, then it is difficult to imagine a plausible pathway for animals to gradually evolve wings that could provide enough power for flying. While scientists have proposed various questionable hypotheses in the hope of solving this problem, critics of science have used this problem as a means of attacking the Theory of Evolution by asking “what good is a half formed wing?” While these critics are correct in pointing out that these scientists hypotheses on how flight evolved do not make sense, they are wrong in trying to claim that the problem lies with the Theory of Evolution. To clear up this confusion, let’s review how evolution works and apply this to the evolution of flight.

There are numerous advantages to flying: an animal can escape a predator by flying away, a high flying predator can use it’s ‘bird’s eye view’ to locate its prey and then use the speed gained from the altitude to make its kill, and if the weather is uncomfortable then the animal can simply fly to another climate. However, just because there are numerous advantages to flying, a species cannot will itself towards becoming a flying animal; that’s not how evolution works.

There is no guiding plan to evolution. Thus, even though there are many advantages to flying a species cannot direct itself towards evolving the features that it needs to fly. The only reason why a species might evolve is if there is some immediate positive benefit that comes from making the change. Furthermore, evolutionary change takes place through numerous small steps. Each new generation represents another chance for the species to take another small step closer to evolving a new feature. Not only must the new feature be beneficial but there must also be some benefit all along the way towards evolving this feature.

Think of evolution as being like water that is flowing downhill on its way to the ocean. The water does not think about what path it should take nor does it have the ocean or anything else in mind as its objective. It just flows so that it is continuously going downhill to a lower elevation. Furthermore, if there is no adjacent location that is at a lower elevation then the water stops flowing and collects so as to form a lake. Comparably, instead of flowing downward like water, evolution is always trying to flow towards greater benefit. If a partial new feature enhances a species ability to thrive then as long as this is so the species will continue to evolve this new feature. If there is no immediate benefit that comes from have a new feature then the species will not evolve that feature.

Evolution of Pterosaurs

Flying Dragon Lizard Draco Lizards or Flying Dragons do not actually fly but instead they glide between trees as they search for food or avoid predators.

Today in Southeast Asia there are gliding reptiles called Draco Lizards or Flying Dragons. These small reptiles are capable of gliding from tree to tree by extending their elongated skin covered ribs out to their sides to create a flat aerodynamic surface. Being able to glide from one tree to another saves these arboreal insectivores considerable amount of time as they search for food. In addition, gliding from one tree to another helps these lizards avoid predators that may be on the ground and it is also a great way to escape from snakes or other predators that may be able to climb the trees. Besides being able to cover horizontal distances as they fall, these lizards are capable of controlling their travels so that they are able to hit their intended target: the trunk of a nearby tree. These gliding lizards are perfectly positioned for evolving into flying reptiles if only the atmosphere was substantially denser.

Flying Rhamphorhynchoid known as Dimorphodon macronyx Rhamphorhynchoids were the first pterosaurs. They evolved during the late Triassic and went extinct during the Cretaceous. Drawing created by Dmitry Bogdanov - dmitrchel@mail.ru.

The earliest gliding reptiles are known as Weigeltisaurus jaekeli and they existed 255 million years ago near the end of the Permian period. These gliding reptiles were extremely similar to the modern day Flying Dragons of Southeast Asia. Like the Flying Dragons of Southeast Asia these gliding reptiles of the late Permian period were constantly gliding from one tree to another, but they could not fly. Similar to what it is today, the atmosphere was thin during the Permian period and so these cold blooded reptiles could not generate enough power to fly.

The Permian-Triassic Extinction that came at the end of the Permian period is what made flight possible. This ‘mother of all extinctions’ wiped out 96% of all marine species: the marine species that were removing the carbon dioxide from the oceans and likewise the atmosphere. Meanwhile volcanoes continued to exhaust their carbon dioxide into the atmosphere, and consequently over the next several million years the chemical makeup and the thickness of the atmosphere dramatically changed. By the end of the Triassic period life had recovered and once again there were reptiles gliding between trees. But this time was different because with the much thicker atmosphere the power requirements for flight were much lower and so it did not take long for some of these gliding reptiles to evolve into being true flyers.

pteranodon skeleton mount Pteranodons were large pterosaurs that evolved in the late Jurassic period and went extinct at the end of the Cretaceous period: 160 to 66 million years ago.

The gliding arboreal reptiles of the late Triassic period evolved 230 million years ago; it was only two million years later that Earth had its first flying vertebrates. There is a strong resemblance between the ancestral gliding reptiles and these first pterosaurs; the main difference is that these first pterosaurs were significantly larger than the gliding reptiles and of course the wings of the flyers are now attached to fore limbs thus allowing them to power their flight. These first pterosaurs, known as Rhamphorhynchoids, flew between about 215 and 145 million years ago.

The various species of pterosaurs generally grew larger from the time they first evolved to when they went extinct. The first pterosaurs, the Rhamphorhynchoids, evolved during the late Triassic period about 215 and 145 million years ago. Besides being among the smallest pterosaurs, the Rhamphorhynchoids had teeth and a long tail. By the late Jurassic period several new species of pterosaurs evolved that are known as Pterodactyloids. While the Pterodactyloids had shorter tails than the Rhamphorhynchoids, the Pterodactyloids tended to be much larger than the Rhamphorhynchoids; most Pterodactyloids had about a meter long wingspan (about three feet). Before the extinction of the Pterodactyloids in the early Cretaceous period the next larger pterosaurs had evolved: the Pteranodons. The wingspans of many Pteranodons were about five to seven meters. Pteranodons were also different from Pterodactyloids in that they had a long robust neck, prominent cranial crest, and they had a beak instead of a jaw filled with teeth. Pteranodons first evolved in the late Jurassic period and by the later Cretaceous period they were thriving. Finally one of the largest pterosaurs of all time was the Quetzalcoatlus, a pterosaur that flew during the late Cretaceous period. Quetzalcoatlus was similar to the Pteranodons except it was substantially larger: it stood as tall as a giraffe and it had wingspan comparable to most small recreational airplanes.

Ptroubling Pterosaurs

*Paleontologists are still sorting out the classification of pterosaurs such that some reference sources may not agree with the pterosaurs classification given here. The one thing that paleontologists do agree on is that pterosaurs were not dinosaurs, and yet even this is still confusing to the public since most non-academia sources continue to refer to pterosaurs as being flying dinosaurs.

Pterosaur* Time Period Wingspan (m) Notes
Rhamphorhynchoids Late Triassic
and Jurassic
0.25 to 1.0 smallest pterosaurs
long tail, teeth
Pterodactyloids Late Jurassic 1.0 to 1.5 medium size
short tail, teeth
Pteranodons Cretaceous 3 to 7 large, toothless, robust
neck, prominent crest
Quetzalcoatlus Late Cretaceous 11 one of the largest flying
animals of all time

It was only a few million years after Pteranodons appeared that the smaller Pterodactyloids went extinct. However it incorrect to assume that one type of pterosaur drove out the other. Instead what happened is that birds had evolved and it was this new comer that displaced the pterosaurs. Being the same size and within the same environment as the birds, the Pterodactyloids could not compete with the superior flying warm-blooded birds. Meanwhile the larger Pteranodons hung on but only by avoiding direct competition with the birds. While the birds flew through the trees or just above them in search of their prey or seeds to eat, the Pteranodons were soaring high above: either migrating with the seasons or possibly circling like vultures as they waited for something to die. Soaring requires little movement and so the slower cold-blooded metabolism of pterosaurs actually works to their favor for this type of flying.

The pterosaurs, dinosaurs, and birds coexisted throughout the Cretaceous period and even much earlier during the Jurassic period. At the end of the Cretaceous period, the end of Mesozoic era, the pterosaurs went extinct along with the dinosaurs.

Theories on How Pterosaurs Were Capable of Flying

Considering the high metabolism required for flying, if we assume that the flying environment of the Mesozoic era was the same or comparable to the present flying environment then it should not be possible for a group of Mesozoic era reptiles to evolve the capability of flying, and since relative power requirements for flying increase with size of the flyer, we would certainly not expect that some of these flying reptiles could be capable of becoming the largest flyers of all time.

Paleontologists have struggled in their efforts to explain how the pterosaurs flew.

Superman Paleontologists claim that pterosaurs were able to fly by making a super leap into the sky.

The level of a person’s understanding of physics determines one’s appreciation of the two most apparent scientific paradoxes of the Mesozoic era; while even a child understands that there is something odd about dinosaurs being so large, a person has to have at least some understanding of the requirements of flight to appreciate how ludicrous it is to believe that giant pterosaurs could have flown in a thin atmosphere. For those who wish to review these arguments, the derivation of the flight equations was given in chapter three: The Science of Flight and the Paradox of Flying Pterosaurs. The calculations showed that even with the large pterosaurs treated as if they were warm blooded animals they would not have even ten percent of the power needed to fly in a thin atmosphere similar to what currently exist. On the other hand, when we rework the flight equations while accounting for the atmosphere being much thicker, the calculations show that even the largest cold-blooded pterosaurs would have no difficulty flying.

To meet the requirements for flight an animal needs to have wings, a means of guiding their flight, and enough power to create the lift needed to overcome the downward force of gravity. While animals that glide between trees are usually capable of fulfilling the first two requirements, while airborne they typically cannot generating power or at least enough power so as to achieve true flight. How much power is needed so that a glider can evolve into a flyer depends on the density of the atmosphere.

Let’s look at the flight equations that show us how the Mesozoic’s thick atmosphere made the evolution of the flying pterosaurs possible.

The much thicker atmosphere made it easier for the pterosaurs to fly in two distinct ways: first, just as with the dinosaurs, the extremely thick atmosphere produced a buoy force that reduced the effective weight of the pterosaurs and second, because lift is produced by throwing air down if the air is denser then less power is needed to thrown this air down to produce this lift. We start by first calculating the effective weight.

By summing up the forces acting on the pterosaur submerged in the thick atmosphere fluid and applying a little algebra, we arrive at the following equation giving the effective weight:

N = Fg (1 - ρF / ρS)

Where N is the effective weight or in this case the required lift, Fg is the weight, ρF is the density of the fluid (the thick atmosphere), and ρS is the density of the pterosaur. While the density of these pterosaurs is unknown, it was probably not so different from the density of most other vertebrates at being close to the density of water or about 1000 kg/m3. Filling in the other values for Quetzalcoatlus having an estimated weight of 7000 N and living during the Cretaceous period when the air density was approximately 660 kg/m3, the Quetzalcoatlus effective weight is calculated as being 2380 N.

The greatest potential for error in making these calculations comes from estimating is the pterosaurs metabolism. While there is agreement that reptiles have a lower metabolism than mammals, the fundamental differences between exothermic reptiles and endothermic mammals makes this little more than an educated guess. For now the claim will be that the metabolism of reptiles is typically about five times less than the same size mammals. Using this value, the available power for the Quetzalcoatlus is 0.36 kW.

Inserting these corrected values into our flight equations we are able to calculate our thick atmosphere value for the Quetzalcoatlus relative power.

Flyer Weight
Front Area
Drag Coefficient
(Front Area)
for least Power
Minimum Power
for Flight
Available Power
Power Ratio
Thin Atm
7000 2.5 0.50 12 18 20 1.8 0.09
Thick Atm
2380 2.5 0.50 12 0.47 0.17 0.36 2.2

As explained in the flight chapter, the bare minimum power ratio for flight is 1.0 and so airplanes and flying animals need to have a power ratio greater than this so that they can maneuver and maintain altitude in the usual non ideal weather conditions. The 2.2 power ratio of the Quetzalcoatlus shows that in it was a capable flyer as it flew in the extremely thick Mesozoic atmosphere.

While this analysis has primary focused on explaining how the large pterosaurs could have generated enough power to fly, the results from the calculation solves and additional problem: how the Quetzalcoatlus could have taken off or landed.

Generally the larger the flying vertebrate or airplane is then the faster it needs to be going to either takeoff or make a safe landing. So while a small bird or bat needs to do nothing more than hop into the sky before flapping its wings, the largest birds often have to run as fast as they can in order to take off, and the much larger airplanes usually need to be traveling over a hundred mph before they can lift off. Similar statements matching increasing weight with speeds can be said about the landings. Furthermore, while the wheels of airplanes work well for traveling at high speeds; the short legs of large vertebrates limit how fast these animals can run. Because of the difficulty in reaching minimum takeoff and landing speeds many large birds look comical as they attempt to either takeoff or land.

The thick Mesozoic atmosphere lowered the power requirements enough that the gigantic Quetzalcoatlus could fly, and in addition the thick atmosphere also made it easy for Quetzalcoatlus to takeoff and land. Imagine the impossibility of Quetzalcoatlus trying to run at 40 mph, or how many bones a Quetzalcoatlus would break if it were to try to land with a speed of 18 m/s (40 mph). Such problems disappear when the atmosphere is much thicker. In the thick Mesozoic atmosphere the Quetzalcoatlus needed to be moving at about a half a meter per second to either take off or land.

From the time when they first evolved 215 mya until when they went extinct 66 mya, the pterosaurs went from being small flying reptiles to becoming the largest animals that ever flew. It was an amazing evolutionary development that was possible only because the Earth’s atmosphere was so much thicker during the Mesozoic era.

Details about Dinosaurs

Were dinosaurs warm or cold blooded?

The analysis of the spacing of dinosaur tracks strongly suggests that many of these large animals were trotting rather than walking along at a slow lumbering pace. Based on this evidence of a higher metabolism, some paleontologists have suggested that dinosaurs may have been w arm-blooded (endothermic) animals. However, while a vertebrate needs to be endothermic to achieve a high metabolism in a cold environment, the elevated temperature of an endothermic metabolism is unnecessary and can even be undesirable in climate that is consistently warm. For example, the cold-blooded (ectothermic) fish that swim in warm ocean water show high activity because the water that surrounds them keeps their bodies at a consistent warm temperature. Likewise, it is logical that dinosaurs would also be highly active if they were submersed in an extremely thick and consistently warm atmosphere.

Not only is the endothermic dinosaur hypothesis solving a problem that may not exist, the warm-blooded dinosaur hypothesis is possibly creating its own set of problems. Extremely large endothermic animals need to have some way to dissipate their thermal energy so that they do not overheat. Among the large modern mammals, elephants flap their big ears and spend much of their time soaking in water to stay cool, while the even larger whales have a strong preference to swimming in cooler ocean waters. If dinosaurs were actually warm-blooded, then the larger dinosaurs should have a stronger preference for the cooler higher latitudes than the smaller dinosaurs. If such a distinction exists, paleontologists have yet to recognize it.

Generally, an ectothermic metabolism works best in climates that are consistently warm while an endothermic metabolism is superior in climates that lean towards being cooler and have wide fluctuations in daily or seasonal temperatures. The global Mesozoic climate consisted of steady warm temperatures, a climate that favors the belief that the dinosaurs were ectothermic vertebrates.

Brachisaurous Blood Pressure

T-Rex bone

Image / diagram showing and explaining difference between Canard and standard airplane.

Image of Pterodactyloid along with the 'Don't Break Your Neck' story

Dinosaur’s synchronized movement of rear legs with tail

The Diminishing Atmosphere's Effect On Terrestrial Vertebrates
Cenozoic Era 65 mya to Present Time

Sixty five million years ago the K-T mass extinction marked the end of the Mesozoic era and the beginning of the Cenozoic era. When this event occurred, the atmosphere was extremely thick and after the extinction and throughout all of the Cenozoic era the atmosphere transition from being extremely thick to being the relatively thin atmosphere that we have today. Mostly it was just the smaller terrestrial animals that survived the extinction: the dinosaurs were dead. Similar to other mass extinctions, several million years when by before the land was once again suitable for the evolution of larger terrestrial life.

The geological evidence gives us an idea of how the Earth transitioned from a thick to thin atmosphere throughout the Cenozoic era. For the first 25 million years, between 65 and 40 million years ago, there was no ice at the poles, the same as what it was during the Mesozoic era. Then, starting at 40 million years ago, ice at the poles came and went as the atmosphere's thickness made its unsteady decline. This went on until just over two million years ago when the Earth began a new ice age.

The dinosaurs did not make it through the K-T mass extinction and this left some big open slots that the mammals filled. Many of these mammals grew large in comparison to today’s mammals but they were still far short of being dinosaur size. This is because by the time terrestrial life had fully recovered the atmosphere had already diminished considerably and so this atmosphere was not nearly as effective in reducing the effective weight of these large mammals. In addition, these large terrestrial mammals of the Cenozoic era were not shaped like the dinosaurs because exceptionally large rear legs and a strong tail were no longer useful in the thinner atmosphere. Along with the diminishing atmosphere, from the middle of the Cenozoic era to the present time we see this general trend of these terrestrial animals shrinking in size until we reach the size of the present day animals.

Paraceratherium The Paraceratherium was about sixty percent larger than the present day largest terrestrial animal the African elephant. Created by Sameer Prehistorica.

The largest of these terrestrial animals was a tall giant rhinoceros like mammal named Paraceratherium (besides Paraceratherium this species has had numerous other names that have come and gone in popularity: Indricotherium, Baluchitherium, Dzungariotherium, and Aralotherium). While it is taller than a giraffe and about three times heavier than an African Elephant, its exact size is unclear because of the incompleteness of the fossil bones. The Paraceratherium went extinct about twenty three million years ago.

Besides the Paraceratherium there were numerous other large terrestrial species that existed during the middle and even late Cenozoic era. These include the giant ground sloth, the saber tooth tiger, giant otters, giant beavers, giant armadillos, and many others. While generally terrestrial animals shrunk in size during the Cenozoic era this is not always the case for every species. The reason for this is because biological competition within an environment can be every bit as important as physical constraints in determining how large an animal can be, and so we need to carefully consider what is actually limiting the size of an animal. For example, what is there that limits the size of beavers? If there are no bears in a wooded environment what is there to keep beavers from evolving to the size of bears or even larger? Hence the reason that beavers are much smaller today appears to be due to differences in the biological competition rather than having anything to do with the thinning of the atmosphere.

External Links / References

Species Interactions With Each Other and the Environment: Ecology

Monster Terrestrial Arthropods of the Carboniferous and Permian Periods

Largest Insects and Spiders

Difference between Terrestrial Arthropods and Terrestrial Vertebrates

Evolution of Flying Vertebrates

Leaping Lizards


Endothermic and Ectothermic (Warm and Cold Blooded) Vertebrates

Geological Ages

Ecological Niche

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