Archosaurs - The Ruling Reptiles

            Two hundred and fifty-two million years ago, life on Earth looked like it was coming to a close. The Permo-Triassic extinction, the greatest the world had ever seen, had destroyed almost all living things – 70% of land species and a staggering 96% of all marine species were wiped out. Even insects, renowned for being great survivors, could not escape its deadly grasp. However, as it always does, life persevered. Within a few million years, the survivors had already repopulated the world. In the place of the synapsids, which had ruled the Permian, dicynodonts were able to diversify and dominate the Early Triassic. Though life had returned, the world was still very barren. The great reefs of the oceans had been destroyed, and the only sea life at the time was small colonies of shelled invertebrates. Small patches of ferns were spread across the deserts of Pangaea. While the dicynodonts were starting their brief rule of the planet, another group of animals was evolving, a group that would one day take the place of the dicynodonts and become the dominant species of the Earth for millions and millions of years – the archosaurs.

            Before we delve deeper into the story of the archosaurs, we have to first define what exactly an archosaur is. The ruling clade of the Permian had been the synapsids, mammal-like reptiles such as Dimetrodon and Gorgonops, which were mainly characterized by having a single temporal fenestra, an opening in the skull behind the eye. Archosaurs were a subgroup of the diapsids, which had two temporal fenestrae. The main characteristic of the archosaurs was the antorbital fenestra, an opening in front of the eye. Other characteristics included a narrow skull, a mandibular fenestra (opening in the jawbone), teeth in sockets, a smaller fifth toe, and expanded sinuses. The earliest archosaurs had scutes (bony plates) and developed the calcaneal tuber, or heel bone. While the scutes were lost in later species, the calcaneal tuber stayed (and also evolved separately in mammals). The clade-specific traits end there, however. Archosaurs are divided into two different groups – ornithosuchians and pseudosuchians.

            The main defining feature between the two groups was the structure of the ankle. The pseuodusuchian ankle, known as the crutotarsal (“crocodile-normal”) ankle, is very complex and flexible, allowing for two methods of movement: a sprawling walk similar to that found in most present-day reptiles, and an erect walk like that of a dinosaur. The present-day psuedosuchians are crocodiles, alligators, and gavials. Prehistoric pseudosuchians included rauisuchians, phytosaurs, and aetosaurs – relatives of crocodiles all adapted for different lifestyles.

In contrast, the ornithosuchian or mesotarsal ankle is much less complex and only allows movement in an erect posture. This forces ornithosuchians to walk parasagittally, meaning the limbs move parallel to the spine. It is not fully understood as to what the advantages and disadvantages are to the parasagittal walk, but the general consensus is that it improves agility and maneuverability, but decreases stability. Modern ornithosuchians are limited to birds, whereas the group has historically included dinosaurs, pterosaurs, and various other reptiles known as dinosauromorphs.

There is much debate as to when the archosaurs actually originated. Some scientists believe that the archosaur line actually began in the Permian, and survived the extinction into the Triassic and onwards. Others say that archosaurs did not appear until the Triassic, but classify many Early Triassic reptiles such as prolacertiformes (long-necked lizard-like reptiles) and rhynchosaurs (strange, pig-like reptiles) as archosaurs. The general scientific consensus is that the archosaurs first appeared around 245 million years ago, and that their relatives (like the prolacertiformes and rhyncosaurs) are part of a broader group known as archosauromorphs.

This brings us back to the Early Triassic, where a small reptile known as Euparkeria first appeared. Euparkeria was a relatively small animal, about 70 centimeters long. It would have looked no different than any of the other diapsid reptiles around, a crocodilian animal with a slightly sprawling stance and a row of scutes down its back. On the inside, however, Euparkeria was extremely unique in many ways. Most important of all, its skull featured an antorbital fenestra, the main defining characteristic of an archosaur. It is also believed that due to the structure of Euparkeria’s ankle, it would have had a slightly more erect walk than its contemporary species and possibly been able to walk upright for brief periods of time. Though these small differences were insignificant at the time, Euparkeria was, in the opinion of many scientists, the very beginning of the animals that would dominate the world for millions and millions of years – the archosaurs.

The Late Triassic saw the end of the rhynchosaurs and many other archosauromorphs. However, the descendants of Euparkeria and its relatives had diversified and began to dominate Pangea. The world had changed since the Permian Extinction, and lush forests now existed in many places. Populating the forests and deserts were the earliest pseudosuchians. The rauisuchians, large “running crocodiles” such as Postosuchus and Saurosuchus, dominated the landscape as the apex predators. The phytosaurs, which looked almost exactly like modern crocodiles but were actually distant relatives, lurked in rivers and lakes as ambush predators. Aetosaurs, strange herbivorous crocodilians, also lived around freshwater. Sphenosuchians, which looked like small dinosaurs but were actually more closely related to crocodiles, also first appeared during the Late Triassic. It was a good time for the psuedosuchians.

Another type of reptile first appeared during the Late Triassic. Descended from the dinosauromorph archosaurs like Marasuchus, they too had antorbital and mandibular fenestrae, calcaneal tubers, and all of the other archosaur traits. They would have looked similar to any other animal living in the Triassic world. Upon closer inspection, however, one would notice the interesting aspects of these new creatures. First of all, they walked upright on two legs, a posture very rare in contemporary animals. Their ankle was mesotarsal, a structure before then only found in the dinosauromorphs. They also had a large upper-arm muscle structure and extra vertebrae in the hip region for better balance and support. For all of the Triassic period, these new animals would remain inferior to the rauisuchians. But in a few million years, after the next great extinction had wiped away the mighty pseudosuchians, these creatures would quickly adapt and become the next ruling clade – the dinosaurs.

The earliest dinosaurs were saurischian, or lizard-hipped. At first, they would have been limited to small carnivores, such as Eoraptor, one of the oldest known dinosaurs. Soon, however, some evolved to be larger, like Herrerasaurus and Coelophysis. It was not until much later in the Triassic that the dinosaurs were really able to diversify. The theropods (bipedal, carnivorous dinosaurs) stayed relatively small and would have only been able to scavenge larger kills or hunt smaller animals. Prosauropods, relatives of the mighty sauropods, also evolved in the Late Triassic, and were the first herbivorous dinosaurs to exist. Near the very end of the period, dinosaurs had begun to dominate. Many large theropods, like Lillientsternus, grew to sizes of up to 20 feet long. While the prosauropods were still the most common herbivores, the first sauropods appeared, along with the earliest ancestors of an entire new order of dinosaurs, the ornithopods. Once the Triassic-Jurassic extinction did away with many of the giant pseudosuchians, the dinosaurs really started to thrive.

Yet another new type of reptile evolved in the Late Triassic – the pterosaurs. Reptiles had been “experimenting” with flight throughout the Triassic, but none had gotten past gliding from tree to tree – until the pterosaurs, of course. Before then, flight had been a movement limited only to insects. Now the archosaurs, who had already conquered the land, had taken to the sky. Although they were able to fly, the first pterosaurs were not completely built for it. While the rhamphorynchids of the Jurassic would evolve vaned tails to use for steering, the primitive pterosaurs did not have the correct muscle structure to use their tails for anything. They also had needle-like teeth, a feature lost by the later azhdarchid and ornithocheiroid pterosaurs. Interestingly enough, the pterosaurs did not evolve from the gliding reptiles of the Early Triassic, but were more closely related to early archosaurs like Euparkeria. This is mostly shown in the fact that, while the “wings” of the gliding reptiles were simply flaps of skin connected to the midsection or the limbs, pterosaur wings are actually formed out of an extremely long finger bone, similar to bats and birds.

The Triassic-Jurassic extinction wiped out the aetosaurs, phytosaurs, and countless other species of animals. The rauisuchians went extinct, but their descendants, the crocodylomorphs, thrived on. With the large predators out of the way, the dinosaurs began to diversify even further. The prosauropods were still the most common herbivores, but sauropods gradually became more common. Many of the Early Jurassic sauropod species were relatively small, but some, such as Barapasaurus, were already reaching massive size. The theropods, similarly, grew in size. The dilophosaurs, descendants of the coelophysids like Lilliensternus, became the first large predatory dinosaurs, able to take down animals single-handedly that would have taken a pack of coelophysids. The ornithischian dinosaurs, meanwhile, were also diversifying. Many of the ornithschians of the Triassic and the Early Jurassic were small, bipedal herbivores that looked similar to the theropods, known as heterodontosaurs. In the Early Jurassic, however, the beginnings of a new ornithschian group arose – the thyreophorans, better known as the armored dinosaurs. These first thyreophorans were simple quadrupeds with scutes on their backs, still needing to run from predators like the heterodontosaurs, but would one day become the great stegosaurs and ankylosaurs.

The other archosaurs were not doing as well. Pterosaurs were going strong, but were evolving much more slowly than the dinosaurs. Aside from slight differences and diversification, they were still very similar to the Triassic pterosaurs. The pseudosuchians were now limited to crocodylomorphs (which includes sphenosuchians), and were no longer the apex predators. Both the pterosaurs and pseudosuchians, however, would continue to exist throughout the Mesozoic, and some survived longer.

The dinosaurs diversified even further in the Late Jurassic. Although the dilophosaurs had gone extinct, several new groups of theropods had arisen. The allosaurs, very closely related to the dilophosaurs, grew even larger than their ancestors and were some of the first dinosaurs to be able to prey upon sauropods and stegosaurs. The first abelisaurs evolved, starting as smaller species such as Ceratosaurus. Small theropods like Compsognathus first appeared. Though they were overshadowed at the time by the allosaurs and abelisaurs, they were the ancestors of what would become the dominant theropods in the Cretaceous.  Herbivorous dinosaurs, meanwhile, were beginning to be able to rely on strategies other than running from predators. While the prosauropods had gone extinct, the sauropods had gotten even larger, with the diplodocids reaching lengths of over 80 feet and macronarians (such as Brachiosaurus) growing to massive heights of over 50, able to kill theropods with whiplike tails. The thyreophorans had split into two groups – the stegosaurs and the much less common ankylosaurs. A new type of ornithschian, the iguanodontids, first appeared, though they too were very uncommon and limited to smaller species such as Camptosaurus, which may have relied on the protection of larger herbivores to survive. The pterosaurs were doing very well, and were starting to make the transition from the long-tailed, toothy ramphorhynchids to the beaked, short-tailed ornithocheiroids. It was the time of the ornithosuchians.

Another new type of ornithosuchian appeared during the Late Jurassic – birds. The earliest birds, like Archaeopteryx, looked similar to dinosaurs, but with wings and feathers. It would take millions of years for any species recognizable as a bird to evolve. These first birds were probably unskilled fliers, and would have only flown in emergencies. The pterosaurs owned the Jurassic skies.

There was no Jurassic-Cretaceous extinction, and it is hard to define the border between the two periods. Much of it is defined biologically, based off of changes across all forms of life. The allosaurs were mostly extinct, but their descendants, the carcharodontosaurids, had taken their place as the top predators. The descendants of the small Jurassic theropods like Compsognathus had diversified and begun to grow. The spinosaurs - massive, sail-backed, waterside predators - rivaled the carcharodontosaurids in size. Dromaeosaurs, like Utahraptor and Deinonychus, first arose in the Early Cretaceous, along with their close relatives the therizinosaurs and troodontids. Yet another theropod group appeared in the Early Cretaceous, though it was then limited to small species that could have only hunted small, weak prey – the tyrannosaurs. It was the beginning of the golden age of the dinosaurs.

The ornithschians were also evolving. The Early Cretaceous saw the end of the stegosaurs, leaving the ankylosaurs as the surviving thyreophorans. Most of the sauropods had gone extinct, leaving space for new herbivores to come in – a space quickly occupied by new types of ornithschian dinosaurs. The iguanodontids, such as Ouranosaurus and Tenontosaurus, diversified quickly and spread across the world. An entirely new group appeared – the marginocephalians. Although they started as small species like Psittacosaurus, they would soon evolve into two of the most dominant types of ornithschian – the ceratopsians and pachycephalosaurs.

The dinosaurs only evolved and diversified further in the Late Cretaceous. While the spinosaurs and carcharodontosaurids had gone extinct, other theropods had taken their place. The tyrannosaurs grew to massive sizes, becoming some of the largest predators of all time. Abelisaurs dominated the southern hemisphere, hunting enormous sauropods like Argentinosaurus. The dromaeosaurs actually grew smaller and more bird-like, changing from terrifying terrestrial predators to harmless, arboreal gliders – in some cases. Other dromaeosaurs, like the famous Velociraptor, stayed on the ground and in their niche as extremely effective, feathered predators. The oviraptorosaurs and alvarezsaurs – beaked, crested relatives to the dromaeosaurs - both appeared in the Late Cretaceous. Omnivorous theropods, the therizinosaurs and ornithomimosaurs, became more and more common. While the diplodocids and macronarians had died out, the titanosaurs (more advanced sauropods like Argentinosaurus and Dreadnoughtus) took their place, becoming the largest creatures to ever walk the Earth. It was, truly, the golden age of the dinosaurs.

The Cretaceous was a time for gigantism. The azhdarchid pterosaurs, the largest flying creatures ever, thrived during the Early Cretaceous. Other pterosaurs - which no longer looked like their toothy, tailed ancestors – existed in all shapes and sizes, and truly dominated the air. Several types of giant crocodile, like Deinosuchus, competed against the spinosaurs in the Cretaceous rivers and lakes. In the ocean, the metriorhynchids (marine crocodylians) were also thriving. Herbivorous pseudosuchians made a comeback, too, in the form of the notosaurs – very different from the primitive aetosaurs of the Triassic. Most birds still looked very similar to the dromaeosaurs, but a few began to look like the birds of today, such as Hesperonis. The Cretaceous was not just the time for dinosaurs, but also the heyday for archosaurs in entirety.

The ornithschians did not miss out on this golden age. The ankylosaurs grew larger and more diverse. The iguanodontids not only continued to thrive, but also gave rise to a new group of dinosaurs, the hadrosaurs. The marinocephalians changed dramatically from their small, Early Cretaceous ancestors. Famous ceratopsians like Triceratops and Styracosaurus ruled the Cretaceous world, able to take down even the largest tyrannosaurs. The pachycephalosaurs, strange, dome-headed dinosaurs, arose and spread across the world. All living groups of dinosaurs greatly diversified and evolved in the Cretaceous.

The dinosaurs were ruling the planet, outshining any other group of creature. No other group on the planet has ever achieved such a long, dominant, and diverse rule. It is important to remember that the dinosaurs were not a class, like birds or mammals are. They were simply a clade, a group defined by the descendants of a common ancestor – much more limited than the other ruling groups. Archosaurs, too, are only a clade, though a considerably larger one than the dinosaurs. But they alone managed to fill almost every ecological niche on the planet for millions and millions of years – predators and prey, herbivores, carnivores, and omnivores, terrestrial, arboreal, aerial, and aquatic. Only the mammals have ever come near the range of dominance displayed by the archosaurs in the Mesozoic, and they have only been dominant for the better part of the past 66 million years, only slightly over half the length of the 186-million-year rule of the archosaurs. From the tiny Triassic reptiles like Euparkeria, the archosaurs survived and evolved, becoming the largest creatures to ever walk or fly, ruling the Earth for millions of years, and persevered through not one but two mass extinctions, truly earning them their name – the ruling reptiles.

The Cretaceous, like any time period, came to an end – in the form of a mass extinction. Though not as terrible as the Permain Extinction, where the story of the archosaurs began, the Cretaceous-Neogene extinction decimated life on Earth. The dinosaurs, of course, went extinct, along with the pterosaurs and other dinosauromorphs – besides birds. The sphenosuchians and metriorynchids also met their demise. There are many theories as to what actually caused this extinction, but the generally accepted one is that an asteroid struck what is now Chicxulub, Mexico. The impact sent dust and debris rocketing into the atmosphere, causing a worldwide blackout. This caused many plants to die, resulting in a lack of food for the herbivores, resulting in a lack of food for the carnivores. There is evidence, however, that some dinosaurs actually survived the extinction and lived on into the Neogene for a short time. Nonetheless, the K-Pg (Cretaceous-Paleogene) extinction marked the end of the reign of not only the dinosaurs, but also the archosaurs.

There is not much to tell of the story of the archosaurs past the Cretaceous. The birds continued on as the last surviving orithosuchians, and were some of the top predators of the Cenozoic until the Pleistocene (Ice Age) in the form of the phorusrhacids, or “terror birds”. The crocodylomorphs and champsosaurus (ancestors of gavials) survived and continued to thrive, but experienced little change from then to the modern day. The day of the archosaurs was over, but they were not gone. They live on today, significantly different and less dominant, but they still survive, unlike so many clades of animals wiped out by time and natural forces.

There are a few extra things that I would like you to remember. First of all, much of this essay is written about topics that are highly debated in the scientific community and subject to change. I have written the opinion that is most commonly accepted by the community. Science, of course, is always changing, and tomorrow there could be a fossil discovered that shows that archosaurs actually evolved in the Permian period, or that aetosaurs never went extinct and modern crocodilians are direct descendants of them. Palaeontology, like any science, changes. The other point that is important to remember is that modern archosaurs are barely representative of their ancestors. It is often said that crocodiles have not changed since they first evolved, but if you went looking for a crocodile in the Early Triassic, you would instead find long-legged terrestrial predators and semi-aquatic herbivores in their place. Likewise, it is fairly evident that most dinosaurs do not look anything like birds. Because, like any group of animals, archosaurs change. And who knows what the future holds for them?

References

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Editors of Encyclopedia Britannica. (n.d.). Archosaur (reptile subclass). In Encyclopedia Britannica (p. 1). Encyclopedia Britannica.

Gee, H., & Rey, L. V. (2003). A field guide to dinosaurs. Hauppauge, N.Y.: Barron's.

Hone, D. (n.d.). [What are archosaurs?]. Retrieved September 16, 2014, from Dave Hone's Archosaur Musings website: http://archosaurmusings.wordpress.com

Hutchinson, J. R., Speer, B. R., & Wedel, M. (1995, October). Archosauria. Retrieved September 12, 2014, from University of California Museum of Paleontology website: http://www.ucmp.berkeley.edu/taxa/verts/archosaurs/archosauria.php

When dinosaurs ruled. (2014). National Geographic, Special (When Dinosaurs Ruled), 1-128.