With all the attention given to the Terrible Lizards throughout their history, it's often easy to forget they weren't the only beasts terrorizing the world during the Mesozoic. In fact, the majority of reptiles of that era weren't dinosaurs at all, though they fell under the Archosaur lineage: Pterosaurs ruled the skies, whilst a variety of marine reptiles and crocodilians ruled the waters. This domination of other habitats by non-dinosaurs helps answer a nagging question about our favorite toothy monsters: with the exception of the avian-line dinos, why were dinosaurs limited only to the land?
It's essential to lay down ground rules at this point, otherwise everything will become confused. First, as many a second-grader can (and will) tell you, pterosaurs and ichthyosaurs, reptiles of air and water respectively, were not dinosaurs. In fact, dinosaurs are pretty much defined by a land-based lifestyle and body plan, including hollow bones and an erect posture. Only one dinosaur genus, the Spinosaurs, are thought to be significantly amphibious (and one strange outlier, the small, swanlike Halszkaraptor escuilliei). But what of birds? Aren't birds dinosaurs? We'll make an exception, since avian dinos had been flapping around since the Jurassic; but this was a very small portion of the dinosaur lineage, and - like contemporary mammals - these species remained small and limited in their range. Until a large, open-sky avian flyer is found from this period, it's safe to assume pterosaurs held utter dominion over the air.
Even on the ground dinosaurs had some fierce competition: Crocodilians, of course, grew enormous to take advantage of the tons of flesh stooping by the waterhole. There were even "walking crocs" that didn't need water, and the especially strange Poposaurs, croc cousins that nearly beat the dinosaurs at their upright-walking game.
Instead of looking at the Mesozoic from a dino-centric perspective, we must view the Dinosaur clade as merely another type of creature in a vast menagerie of Mesozoic beasts. Their dominance of land habitats was not some kind of "conquest", but based upon the same evolutionary pressures that limited marine reptiles to the oceans and pterosaurs to the skies. The largest difference between the Mesozoic world and our own is that, while birds and mammals have adapted to fill aquatic and aerial habitats, ancient reptiles were extremely segregated. Just as there were no aquatic dinosaurs, there were no land-living icthyosaurs, or flightless pterosaurs.
So wherefore this strange locking-in of superorders? Why did only one kind of tetrapod dominate each habitat in the Mesozoic? In today's world, the spread is much more complex; birds have the air, certainly, but mammals spread between land, sea, and air. While it is only the bats among mammals that have true flying capability, the number of species in the bat family is certainly significant. And even birds exhibit numerous forays into the aquatic and terrestrial sphere. So why was the habitat spread in the Mesozoic so conservative?
There are a number of factors at play here. Firstly, there's a question of time and climate. Mammals and birds have only been prominent on the earth for about 65 million years, a time frame featuring wild swings in global temperature, from the Paleocene-Eocene Thermal Maximum (55 mya) to the Ice Ages (100,000-10,000 ya). Dinosaurs, by comparison, had 186 million years of relatively stable temperatures. Wild temperature swings can be devastating in the short term, but can also supercharge evolution as organisms find ways to cope with the temperature changes, invading new ecosystems as the old inhabitants weaken. At the same time, the relative brevity of the Age of Mammals means different organisms have had less time to settle into their niches. Assuming human activity doesn't wipe out a significant portion of mammalian and avian genera, they might eventually shake out into purely terrestrial and purely aerial organisms, with little to no overlap. Only time - a lot of time - will tell.
The second aspect to this mystery is related, but refers more to the triggering event for each "Age" of dominant organisms: the Extinction Events preceding the Mesozoic and the Cenozoic. The circumstances surrounding each were vastly different, and their aftermaths spooled out quite differently. Preceding the Mesozoic was the catastrophic Permian Extinction Event, also known as the Great Dying. Over 70% of land species, and 90% of marine species, were extinguished over a period of 15 million years; it wasn't quite a single destructive event as a massive, eon-long dwindling, a winding down of life on earth. The world was becoming much dryer due to the amalgamation of landmasses into the supercontinent Pangaea, whose vast interior limited the amount of moisture that could enter from the coast. Simultaneously, the Dekkan Traps in Siberia, a mighty chain of volcanoes, began to erupt all at once, spewing billions of tons of silicates into the atmosphere and likely darkening the sky and poisoning the sea. The fossil record indicates a vast die-off of plant life, which in turn caused oxygen levels to plummet. The world was literally choking to death. By the time the Dekkan Traps stopped erupting, the biological bottleneck was so severe that 90% of terrestrial organisms on the planet consisted of only one genera, the Dicynodonts - the very last of the so-called "Mammal-like Reptiles". When the dust settled (literally), the only organisms left were those adapted to low oxygen levels; perhaps high-altitude land life, and marine life suited to brackish waters.
Contrast this with the K-Pg Extinction Event. Most scientists believe the abrupt change in the fossil record was caused (or at least catalyzed) by a gigantic asteroid, which slammed into the Yucatan Peninsula in Mexico and created the 100-mile-wide Chicxulub Crater. While the Earth's ecosystem was already fragile, what with supervolcanoes in India and several smaller cosmic impacts rattling the planet and throwing the climate out of whack, the Chicxulub asteroid was the final blow that wiped out the Terrible Lizards - in as little as 30,000 years, according to a recent estimate. The largest of the dinosaurs were the first to go, followed by the meat-eaters (who would have initially gorged on the tons of available flesh); the size of dinosaurs would have halved, and halved again, until finally only the avian dinosaurs (birds) remained. This makes sense, in that birds would have been the only dinosaurs with enough range to find food in the wasteland which followed, and adaptable enough to cope with the sudden nuclear winter and subsequent greenhouse event. The majority of animals that survived were small, and able to either migrate to find food or wait out the bad times. Birds and mammals fit these respective bills. With 75% of animal life wiped out by the event, whole new niches were opened up to the survivors.
I believe this was the real difference between the beginning of the Mesozoic and the Cenozoic: while the Mesozoic began with a drawn-out, lingering extinction, from which animal life was slow to recover, the Cenozoic literally started off with a bang. The sudden catastrophe, while spectacular, was not thorough enough to erase the myriad genera of tetrapods we know today. Thus the birds and mammals, already primed for adaptability and opportunism by living in the shadows of larger reptiles, were able to very quickly evolve to as many habitats as they wanted, with little or no competition.
The last piece in this puzzle is the very fact of physical adaptability in dinosaurs - their "Evolutionary Plasticity". A peek into the fossil record shows that, once dinosaurs had established their various families, basic bone and dental structure changed very little. Dinosaur body structure was locked in for millions of years, and while these vehicles were elegant, they didn't seem to allow for much variation. Dinosaurs began as very simple-boned Archosaurs, and gradually radiated out into the genera we know from the fossil record. Among dinosaurs, only the birds evolved relatively quickly - and under the feathers, exhibit little variation beyond beaks and wings.
Contrast this with the mammals. It's important to remember that mammals were evolving alongside the dinosaurs, and while they were relatively small, they quickly developed a suite of features their reptilian overlords couldn't begin to imagine. They had very strong evolutionary plasticity; as soon as the dinosaurs were gone, they began to radiate into myriad body plans. From simple, four-footed omnivores arose creatures as distinct as whales and elephants, dugongs and cheetahs, seals and bats. And all this in little over 60 million years.
Of course my argument about evolutionary plasticity in mammals brings up an important question: don't organisms evolve because they are forced to? If dinosaurs evolved slowly, that just meant their body plans were suited for their environment, and they didn't need to change in order to adapt. The argument that mammals evolved because they were "better suited" to evolve seems a little disingenuous.
It is important to consider that biological evolution - that is, the accumulation of tiny genetic mutations over time which gives rise to new species of organisms - is both an internal and external phenomenon, a positive and negative feedback. Organisms are mutating all the time. Every time offspring are produced, the genetic material of both parents is combined, swapped, replicated, and re-encoded; genes activated in the parent organism may remain activated in the offspring. Evolutionary change is a constant of life. Environmental triggers are known to switch genes on and off. The upshot of all this constant mutation is that exterior forces are merely a catalyst for the rise of new species, not the direct cause. The motto of evolution is not quite "Adapt or Die"; it's better to think of it as, "Diversify and Exploit". The more an organism can diversify, developing into an array of new species to fit many different environments, the better chance that organism has of surviving an extinction event.
The crux of my argument, therefore, is that mammals mutate faster. I doubt this is a function of "more adaptable" genes (I'm not sure if there is such a thing). Rather, it's a function of rapid reproductive turnover, resulting in a higher accumulation of mutations; and a more complex skeleto-dental structure. In a reproductive contest between Archosaurs and mammals, mammals win hands-down. While reptiles produce many more offspring (in the form of eggs), only a tiny percentage of these last to adulthood; birds have slightly better success, but usually produce fewer offspring at any given time. Small mammals, meanwhile, have an extremely high success rate with their offspring, churning out babies, which then reach sexual maturity at a very young age and begin pumping out offspring of their own. Small mammals are evolutionary engines; it is from such a small, basal mammal that mammalian life radiated out into its myriad forms, filling in the chinks of ecological niches and then expanding to dominate them when the opportunity arose.
The second part of my argument about the speed of mammalian mutation is the complexity of their skeletal and dental structures. Compared to relatively smooth-boned, simple skulled reptiles, mammalian bones are covered in flanges, burrs, and odd gestalt structures, especially in the skulls. A hallmark of mammal-hood is the complexity of their teeth, with several different tooth types serving different functions in the same mouth. Mammal lineages "lose" certain teeth, only to have a different set evolve to serve the same function as what was lost - consider the lack of I-teeth in the megalonicnid ground sloths, whose premolars then lengthened into formidable fangs; or the incisors of the Thylacoleonid marsupials, which became formidable killing weapons. Mammalian teeth are perhaps their most adaptable feature of all, allowing them to use any available food source.
This adaptability-from-complexity model runs counter to our experience. When we think of a tool as "adaptable" to different uses, we often imagine the simplest tool - a rock, for instance, which can smash, cut, weigh down, or roll as needed. As the rock is shaped to fit a specific function, it becomes less and less adaptable to its previous functions - for instance, an axe can never function like a wheel, though both tools arose from the same source. In mammals, this rings true: in the case of bats, the complexity of their flying structures means they are nearly useless on the ground. However, I think there is a confusion of "complex" with "hyper-developed". Whales are hyper-developed for ocean life, but their skeletal structures are actually less complex than terrestrial mammals. In order to evolve into purely aquatic organisms, they had to de-evolve many hallmark mammalian structures. The ancestors of mammals were less complex, and thus had more adaptive potential, but this is because they were less developed than their descendants. By evolving a more complex skeletal structure, they developed a Swiss Army knife of features, which then allowed for greater adaptability into myriad different environments. Comparatively, dinosaurs were not only much less complex, but hyper-developed into this state: there was not much room for changes to skeletal structure. The exception to this rule was in the Therapoda, which gave rise to birds; but once arms developed into wings, there was a severe limitation on how those limbs could further evolve - no known bird has re-evolved grasping front limbs. The only use for wings is to move through fluid, whether air or water, and when they no longer serve this purpose they quickly dwindle into display structures or disappear altogether.
These, then, are the reasons dinosaurs are almost purely terrestrial, while the dominant tetrapods of our time have adapted to exploit every habitat: firstly, the amount of time dinosaurs and mammals were dominant on planet Earth, and the climatic conditions during that time; secondly, the kind of extinction event which kickstarted the Mesozoic and the Cenozoic, and how life recovered from each extinction event; and lastly, the structural adaptability of dinosaurs when compared to mammals, and how it affected their ability to diversify. Each factor, when combined, created evolutionary conditions which affected how dinosaurs and mammals responded to their habitats: in the case of dinosaurs, they were limited to only one habitat - land - while mammals adapted to exploit land, sea, and even the air.
Rick Out.
It's essential to lay down ground rules at this point, otherwise everything will become confused. First, as many a second-grader can (and will) tell you, pterosaurs and ichthyosaurs, reptiles of air and water respectively, were not dinosaurs. In fact, dinosaurs are pretty much defined by a land-based lifestyle and body plan, including hollow bones and an erect posture. Only one dinosaur genus, the Spinosaurs, are thought to be significantly amphibious (and one strange outlier, the small, swanlike Halszkaraptor escuilliei). But what of birds? Aren't birds dinosaurs? We'll make an exception, since avian dinos had been flapping around since the Jurassic; but this was a very small portion of the dinosaur lineage, and - like contemporary mammals - these species remained small and limited in their range. Until a large, open-sky avian flyer is found from this period, it's safe to assume pterosaurs held utter dominion over the air.
Even on the ground dinosaurs had some fierce competition: Crocodilians, of course, grew enormous to take advantage of the tons of flesh stooping by the waterhole. There were even "walking crocs" that didn't need water, and the especially strange Poposaurs, croc cousins that nearly beat the dinosaurs at their upright-walking game.
Instead of looking at the Mesozoic from a dino-centric perspective, we must view the Dinosaur clade as merely another type of creature in a vast menagerie of Mesozoic beasts. Their dominance of land habitats was not some kind of "conquest", but based upon the same evolutionary pressures that limited marine reptiles to the oceans and pterosaurs to the skies. The largest difference between the Mesozoic world and our own is that, while birds and mammals have adapted to fill aquatic and aerial habitats, ancient reptiles were extremely segregated. Just as there were no aquatic dinosaurs, there were no land-living icthyosaurs, or flightless pterosaurs.
So wherefore this strange locking-in of superorders? Why did only one kind of tetrapod dominate each habitat in the Mesozoic? In today's world, the spread is much more complex; birds have the air, certainly, but mammals spread between land, sea, and air. While it is only the bats among mammals that have true flying capability, the number of species in the bat family is certainly significant. And even birds exhibit numerous forays into the aquatic and terrestrial sphere. So why was the habitat spread in the Mesozoic so conservative?
There are a number of factors at play here. Firstly, there's a question of time and climate. Mammals and birds have only been prominent on the earth for about 65 million years, a time frame featuring wild swings in global temperature, from the Paleocene-Eocene Thermal Maximum (55 mya) to the Ice Ages (100,000-10,000 ya). Dinosaurs, by comparison, had 186 million years of relatively stable temperatures. Wild temperature swings can be devastating in the short term, but can also supercharge evolution as organisms find ways to cope with the temperature changes, invading new ecosystems as the old inhabitants weaken. At the same time, the relative brevity of the Age of Mammals means different organisms have had less time to settle into their niches. Assuming human activity doesn't wipe out a significant portion of mammalian and avian genera, they might eventually shake out into purely terrestrial and purely aerial organisms, with little to no overlap. Only time - a lot of time - will tell.
The second aspect to this mystery is related, but refers more to the triggering event for each "Age" of dominant organisms: the Extinction Events preceding the Mesozoic and the Cenozoic. The circumstances surrounding each were vastly different, and their aftermaths spooled out quite differently. Preceding the Mesozoic was the catastrophic Permian Extinction Event, also known as the Great Dying. Over 70% of land species, and 90% of marine species, were extinguished over a period of 15 million years; it wasn't quite a single destructive event as a massive, eon-long dwindling, a winding down of life on earth. The world was becoming much dryer due to the amalgamation of landmasses into the supercontinent Pangaea, whose vast interior limited the amount of moisture that could enter from the coast. Simultaneously, the Dekkan Traps in Siberia, a mighty chain of volcanoes, began to erupt all at once, spewing billions of tons of silicates into the atmosphere and likely darkening the sky and poisoning the sea. The fossil record indicates a vast die-off of plant life, which in turn caused oxygen levels to plummet. The world was literally choking to death. By the time the Dekkan Traps stopped erupting, the biological bottleneck was so severe that 90% of terrestrial organisms on the planet consisted of only one genera, the Dicynodonts - the very last of the so-called "Mammal-like Reptiles". When the dust settled (literally), the only organisms left were those adapted to low oxygen levels; perhaps high-altitude land life, and marine life suited to brackish waters.
Contrast this with the K-Pg Extinction Event. Most scientists believe the abrupt change in the fossil record was caused (or at least catalyzed) by a gigantic asteroid, which slammed into the Yucatan Peninsula in Mexico and created the 100-mile-wide Chicxulub Crater. While the Earth's ecosystem was already fragile, what with supervolcanoes in India and several smaller cosmic impacts rattling the planet and throwing the climate out of whack, the Chicxulub asteroid was the final blow that wiped out the Terrible Lizards - in as little as 30,000 years, according to a recent estimate. The largest of the dinosaurs were the first to go, followed by the meat-eaters (who would have initially gorged on the tons of available flesh); the size of dinosaurs would have halved, and halved again, until finally only the avian dinosaurs (birds) remained. This makes sense, in that birds would have been the only dinosaurs with enough range to find food in the wasteland which followed, and adaptable enough to cope with the sudden nuclear winter and subsequent greenhouse event. The majority of animals that survived were small, and able to either migrate to find food or wait out the bad times. Birds and mammals fit these respective bills. With 75% of animal life wiped out by the event, whole new niches were opened up to the survivors.
I believe this was the real difference between the beginning of the Mesozoic and the Cenozoic: while the Mesozoic began with a drawn-out, lingering extinction, from which animal life was slow to recover, the Cenozoic literally started off with a bang. The sudden catastrophe, while spectacular, was not thorough enough to erase the myriad genera of tetrapods we know today. Thus the birds and mammals, already primed for adaptability and opportunism by living in the shadows of larger reptiles, were able to very quickly evolve to as many habitats as they wanted, with little or no competition.
The last piece in this puzzle is the very fact of physical adaptability in dinosaurs - their "Evolutionary Plasticity". A peek into the fossil record shows that, once dinosaurs had established their various families, basic bone and dental structure changed very little. Dinosaur body structure was locked in for millions of years, and while these vehicles were elegant, they didn't seem to allow for much variation. Dinosaurs began as very simple-boned Archosaurs, and gradually radiated out into the genera we know from the fossil record. Among dinosaurs, only the birds evolved relatively quickly - and under the feathers, exhibit little variation beyond beaks and wings.
Contrast this with the mammals. It's important to remember that mammals were evolving alongside the dinosaurs, and while they were relatively small, they quickly developed a suite of features their reptilian overlords couldn't begin to imagine. They had very strong evolutionary plasticity; as soon as the dinosaurs were gone, they began to radiate into myriad body plans. From simple, four-footed omnivores arose creatures as distinct as whales and elephants, dugongs and cheetahs, seals and bats. And all this in little over 60 million years.
Of course my argument about evolutionary plasticity in mammals brings up an important question: don't organisms evolve because they are forced to? If dinosaurs evolved slowly, that just meant their body plans were suited for their environment, and they didn't need to change in order to adapt. The argument that mammals evolved because they were "better suited" to evolve seems a little disingenuous.
It is important to consider that biological evolution - that is, the accumulation of tiny genetic mutations over time which gives rise to new species of organisms - is both an internal and external phenomenon, a positive and negative feedback. Organisms are mutating all the time. Every time offspring are produced, the genetic material of both parents is combined, swapped, replicated, and re-encoded; genes activated in the parent organism may remain activated in the offspring. Evolutionary change is a constant of life. Environmental triggers are known to switch genes on and off. The upshot of all this constant mutation is that exterior forces are merely a catalyst for the rise of new species, not the direct cause. The motto of evolution is not quite "Adapt or Die"; it's better to think of it as, "Diversify and Exploit". The more an organism can diversify, developing into an array of new species to fit many different environments, the better chance that organism has of surviving an extinction event.
The crux of my argument, therefore, is that mammals mutate faster. I doubt this is a function of "more adaptable" genes (I'm not sure if there is such a thing). Rather, it's a function of rapid reproductive turnover, resulting in a higher accumulation of mutations; and a more complex skeleto-dental structure. In a reproductive contest between Archosaurs and mammals, mammals win hands-down. While reptiles produce many more offspring (in the form of eggs), only a tiny percentage of these last to adulthood; birds have slightly better success, but usually produce fewer offspring at any given time. Small mammals, meanwhile, have an extremely high success rate with their offspring, churning out babies, which then reach sexual maturity at a very young age and begin pumping out offspring of their own. Small mammals are evolutionary engines; it is from such a small, basal mammal that mammalian life radiated out into its myriad forms, filling in the chinks of ecological niches and then expanding to dominate them when the opportunity arose.
The second part of my argument about the speed of mammalian mutation is the complexity of their skeletal and dental structures. Compared to relatively smooth-boned, simple skulled reptiles, mammalian bones are covered in flanges, burrs, and odd gestalt structures, especially in the skulls. A hallmark of mammal-hood is the complexity of their teeth, with several different tooth types serving different functions in the same mouth. Mammal lineages "lose" certain teeth, only to have a different set evolve to serve the same function as what was lost - consider the lack of I-teeth in the megalonicnid ground sloths, whose premolars then lengthened into formidable fangs; or the incisors of the Thylacoleonid marsupials, which became formidable killing weapons. Mammalian teeth are perhaps their most adaptable feature of all, allowing them to use any available food source.
This adaptability-from-complexity model runs counter to our experience. When we think of a tool as "adaptable" to different uses, we often imagine the simplest tool - a rock, for instance, which can smash, cut, weigh down, or roll as needed. As the rock is shaped to fit a specific function, it becomes less and less adaptable to its previous functions - for instance, an axe can never function like a wheel, though both tools arose from the same source. In mammals, this rings true: in the case of bats, the complexity of their flying structures means they are nearly useless on the ground. However, I think there is a confusion of "complex" with "hyper-developed". Whales are hyper-developed for ocean life, but their skeletal structures are actually less complex than terrestrial mammals. In order to evolve into purely aquatic organisms, they had to de-evolve many hallmark mammalian structures. The ancestors of mammals were less complex, and thus had more adaptive potential, but this is because they were less developed than their descendants. By evolving a more complex skeletal structure, they developed a Swiss Army knife of features, which then allowed for greater adaptability into myriad different environments. Comparatively, dinosaurs were not only much less complex, but hyper-developed into this state: there was not much room for changes to skeletal structure. The exception to this rule was in the Therapoda, which gave rise to birds; but once arms developed into wings, there was a severe limitation on how those limbs could further evolve - no known bird has re-evolved grasping front limbs. The only use for wings is to move through fluid, whether air or water, and when they no longer serve this purpose they quickly dwindle into display structures or disappear altogether.
These, then, are the reasons dinosaurs are almost purely terrestrial, while the dominant tetrapods of our time have adapted to exploit every habitat: firstly, the amount of time dinosaurs and mammals were dominant on planet Earth, and the climatic conditions during that time; secondly, the kind of extinction event which kickstarted the Mesozoic and the Cenozoic, and how life recovered from each extinction event; and lastly, the structural adaptability of dinosaurs when compared to mammals, and how it affected their ability to diversify. Each factor, when combined, created evolutionary conditions which affected how dinosaurs and mammals responded to their habitats: in the case of dinosaurs, they were limited to only one habitat - land - while mammals adapted to exploit land, sea, and even the air.
Rick Out.
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