The Classification of Dinosaurs
The intention of this article is to provide a more comprehensive overview of the familiar concept of dinosaurs, without repeating well-known notions such as "how agile and fast the Tyrannosaurus was" or "how the bony plates of the Stegosaurus worked", but rather to focus attention on scientific topics perhaps of a more general nature, but certainly indispensable for a broader understanding within the context of what is known regarding the very familiar giants of the Mesozoic.
A bit of history
The Dinosaur word means "terrible lizard" and dates back to the distant 1841 when Richard Owen, one of the fathers of paleontology, coined it to classify a particular group of spectacular fossils that were beginning to emerge more and more numerous both in England and elsewhere in the world.
This is what he announced:
"The combination of these characters (teeth set in sockets, symphysis of the lumbar vertebrae, graviportal limbs) and all manifested in creatures that far surpass the largest of existing Reptiles will, I believe, be sufficient reason to establish a distinct group, a suborder of Lizards (now superorder of the subclass Archosauria, Diapsida), for which I would propose the name Dinosauria".
At that time, scientific rigor was certainly not as strict as it is today, but even the most relevant scientists in the field then made colossal mistakes, such as mixing bones belonging to different species, or worse, irreparably destroying precious specimens just to be the first in a real bone-hunting race, even going as far as to commit petty acts of sabotage against rivals!
Fossilization
"Terrible lizards" were certainly some dinosaurs, but it is an unfair term when we consider how many species so diverse have alternated for 180 million years in this heterogeneous group of which, despite the discoveries of new specimens being almost countless now (we are around a thousand), however, what we possess and therefore know about dinosaurs are just a few crumbs, between fossils and fossil evidence, compared to what they represented for hundreds of millions of years. This is because fossilization is an extremely rare and fortuitous event, especially when we talk about animals, dinosaurs in particular, that lived exclusively on land where.
On land, unlike in the marine depths, the possibility of preserving the remains of an organism after death is a phenomenon that occurs only in exceptional cases, such as near a swamp, where it is the mud that fulfills the task of preventing total decomposition, thus allowing for the gradual process of molecular replacement of organic matter with inorganic matter, until creating a true "cast" of the bones, transformed into stone.
What is NOT a dinosaur
Thanks to the fame they have managed to achieve especially in recent years, very few people can now say they don't know what a dinosaur is, and yet, precisely because of excessive information too often projected towards sensationalism, in the end, we end up calling dinosaurs also some flying reptiles such as Pterosaurs, or marine creatures like Mosasaurs, Plesiosaurs, Ichthyosaurs, Placodonts, and Nodosaurids, which have nothing in common with dinosaurs, except being reptiles too, having lived during the same Era, and being just as spectacular as they are fascinating.
Dinosaurs all belong to the class of reptiles, that group of organisms which, towards the end of the Paleozoic era, definitively broke free from the aquatic environment typical of fish but also amphibians, through the invention of the egg with a shell, a waterproof skin that prevents dehydration as well as the definitive loss of the aquatic larval phase with gill respiration.
If Ichthyostegalids can therefore be considered the link between fish and amphibians, then Seymouriamorphs are the link between amphibians and reptiles. And none of these, naturally, are dinosaurs.
To this era also belong the Synapsid reptiles Pelicosauri such as Dimetrodon and Edaphosaurus, known for the large thermoregulatory sail they had on their back, which was then reinvented by certain dinosaurs like Spinosaurus or Ouranosaurus and similarly also Stegosaurus. From the Pelicosauri, in the Triassic period, derived the Therapsids (the so-called mammal-like reptiles), with their peculiar dentition, covered in fur and probably capable of giving birth to live young (no longer needing eggs) and even nursing them... And they too, like the Pelicosauri, are not dinosaurs. From one of their phylogenetic lines, instead, all extinct and extant mammals derived, from mice to Baluchitherium, from the eohippus to the whale, although they had to wait for the Mesozoic era to end and with it the dominance of the dinosaurs to be able to prevail and establish themselves, filling the niches left empty by their disappearance.
It's a bit more challenging to recognize that certain bipedal Thecodonts like Euparkeria and Saltopus are not dinosaurs. They were the ancestors of dinosaurs, Pterosaurs, and crocodiles (and for some, even birds), reptiles themselves but less evolved and less specialized, which soon gave way to the dominance of dinosaurs from the beginning of the Triassic with the appearance of Prosauropods.
Even more difficult for scientists is to recognize in certain feathered animals like Archaeopteryx where a dinosaur ends and a bird begins. The solution to the controversy lies in calling dinosaurs both this group, which is still certainly poorly adapted to flight (lacking a keeled sternum), with teeth instead of a beak and clawed fingers free from the wing edge, and all birds, extinct and living, which, undoubtedly, as transformed and specialized as they are, are the most direct descendants and the only survivors.
Some scholars even associate crocodiles or the Komodo dragon with dinosaurs, which they have nothing to do with. Therefore, at this point, it is necessary to briefly discuss taxonomy to bring some order among the various classes of vertebrates and in the reptile class to which dinosaurs belong, as well as crocodiles, turtles, snakes, ichthyosaurs, plesiosaurs, dimetrodons, and pterosaurs.
Vertebrates
The phylum is the broadest grouping into which living beings can be placed, sharing certain characteristics that inevitably also place them on the same evolutionary path. This means that, if genetics is not just an opinion, they all have a distant common ancestor, of which they all retain the main characteristics. In the specific case of vertebrates, it means having a dorsal cord to support the underlying organs, which evolved from fish-like creatures like the amphioxus into a backbone divided into vertebrae articulated with each other, with additional bones joining anteriorly to form a cranial box to protect the main sensory organs and the brain, and laterally two belts each with a pair of appendages for locomotion called limbs.
Adding a rib cage, we have, very succinctly, what we can define as a vertebrate from the osteo-morphological point of view.
Continuing by affinity, the phylum of vertebrates is usually divided into the following classes:
- true fish
- amphibians
- reptiles
- birds
- mammals
But nothing in biology is really too simple.
Birds are dinosaurs, so the classes of reptiles and birds could merge into one without so many clear differences (also thanks to the contribution of the new "linking fossils" found in China); even fish must be separated into two distinct classes, namely Chondrichthyans (with cartilaginous skeletons, such as sharks, rays, skates, and chimaeras) and Osteichthyans (all other fish, with bony skeletons), without forgetting their ancestors, other extinct fish-like vertebrates such as Ostracoderms and Placoderms, with more or less armored bodies.
In order to avoid writing a treatise on comparative anatomy to learn what allows us to assign an organism to one class rather than another, let's set aside all the other classes and focus only on that of reptiles, in order to define what a reptile is in broad terms and subsequently distinguish the substantial (and not just apparent) difference between a crocodile, a Tyrannosaurus, or an Ichthyosaur.
Reptiles
An organism is undoubtedly classified as a reptile not arbitrarily, but because it possesses certain peculiar characteristics that assign it to this class and this class alone. In the skull skeleton, for instance (which becomes essential in paleontology, as we can rarely say anything about the physiology of soft parts), a reptile has a single occipital condyle, lacks a secondary palate, has two particular bones in the jaw joint that allow it only to open and close the mouth without being able to chew laterally as we mammals do. Moreover, it lays eggs, has a basal metabolism called "cold-blooded" (meaning it needs to compensate by absorbing thermal energy from the environment), its body is covered in scales (the scales are those of fish), and to not complicate things too much, we can stop here.
However, with these few data and what we are able to discover thanks to paleontology, we can easily realize that some inconsistencies already arise. For example, in Synapsids (the mammal-like reptiles), we find that the occipital condyles split, as will be the rule in mammals, hair appears instead of scales, and perhaps a primitive placenta and mammary glands (plausible, but they are only hypotheses) end up completing the confusion: where do reptiles end and mammals begin?
This happens because in nature, evolution does not proceed in watertight compartments or in sudden leaps (apart from some controversial theories that we are not discussing here), but through more or less gradual transformations due to entirely random mutations. If from fossils they appear as distinct stages, it is because, as already mentioned, the specimens we find are the fortunate result of a very rare event that brings to light only an infinitesimal part of all the life forms that existed in the past, and that is why we strive so much in the continuous search for missing linking fossils. Therefore, just as there are reptiles and mammals living or extinct, among the latter there will have been more or less numerous intermediate stages that presented characteristics equally intermediate between the two classes, as that particular phylogenetic line of Synapsids evolved into something new and "more evolved" destined to replace the dinosaurs just as on a parallel level the birds were doing the same.
Another apparent inconsistency regarding the very concise definition of reptiles given earlier concerns the same chewing mechanism. If a crocodile or a lizard indeed chew in a "primitive manner" (and let's omit the kinetic skull of snakes), the same cannot be said for certain more specialized dinosaurs that appeared towards the end of the Cretaceous Period, such as Ceratopsians (those with horns) or Hadrosaurs (the "duck-billed" ones). Both groups invented entirely new ways of chewing, with "scissor-like teeth" or "even pivoting" on the palate, of which we find nothing similar among organisms still alive today, but it was lost forever following their extinction.
Also, on the matter of laying eggs, we cannot exclude that some groups of dinosaurs, which perhaps we will never even know about, were ovoviviparous (but so are certain snakes) or even viviparous, like Ichthyosaurs; even more controversial, finally, is defining all reptiles as "ectothermic", that is, cold-blooded. Without delving into the intricacies of physiology, it is the opinion of many now that certainly not all but some dinosaurs, whose physical prowess and agility leave no doubt (Jurassic Park presented Velociraptor and Ornithomimus quite well), were if not outright "homeothermic/endothermic" (i.e., warm-blooded like mammals and birds - and these last ones we remind once again are dinosaurs...) certainly very close to this definition, probably through an intermediate and entirely new mechanism of which, most likely, we will never know the exact biochemical nature unless we go back in time to capture some specimens.
The subclasses of Reptiles
The first reptiles, the most primitive ones derived from Amphibians Seymouriamorphs, had a massive skull, without any of the temporal fenestrae yet that would have lightened the structure without compromising its strength.
These were the Anapsids, the first of the subclasses into which the class of Reptiles is commonly divided. From the Anapsids, order Cotylosaurs, all the other subclasses with cranial fenestrations will evolve, leaving as their only modern descendants the well-known turtles (order Chelonia).
Without delving too much into which cranial bones the aforementioned fenestrae opened, we can distinguish the further subclasses:
- Synapsids
- Euryapsids
- Parapsids
- Diapsids Lepidosaurs
- Diapsids Archosaurs
The first three had a single fenestra differently located, the last two a pair vertically adjacent.
The Synapsids include the aforementioned orders of Pelicosauri and Therapsids that gave rise to mammals. The Euryapsids are reptiles with marine habits, such as Placodonts and the better-known Plesiosaurs; the Parapsids are Ichthyosaurs, also marine, while the Lepidosaurs include both the extinct Tanystropheids and Mosasaurs (from which the well-known Komodo dragon derives) and almost all current reptiles, namely the orders of Saurians (lizards, geckos, iguanas, chameleons...) and Ophidians (snakes).
Finally, there are the Archosaurs, which are the ones that interest us most, because they include precisely the dinosaurs.
The subclass of Archosaur Diapsids is further divided into the following orders:
- Thecodonts (from which all other orders originated)
- Armored reptiles (all crocodiles, including those extinct which, apart from their enormous size, have not changed much in their appearance and physiology, thus surviving the great extinction at the end of the Cretaceous and reaching the present day)
- Pterosaurs (i.e., all flying reptiles from which neither birds nor bats originated!)
- Saurischians
- Ornithischians
These last two orders include all dinosaurs, which some group into the superorder called Dinosauria.
What is a dinosaur
Dinosaur is something well-defined, which allows us to group them all into two distinct groups (Saurischians and Ornithischians), but with the same characteristics found in the other orders of the same subclass, foremost being the two-fenestra skull typical of Archosaurs. What distinguishes them from the other orders, however, is a fundamental characteristic that undoubtedly determined their evolutionary success: the erect posture of the limbs.
Unlike crocodiles, or even better, their ancestors, the Thecodonts, dinosaurs have limbs positioned close to their bodies and perpendicular to the ground. A crocodile (Archosaurs), as well as a lizard (Lepidosaurs), walks with its legs projecting from the sides of the body, with the humerus and femur pointing outwards, and not immediately downwards, as in mammals. This results in a slithering and serpentine gait, the opposite of the upright and stable walk that a dog or an Iguanodon may have.
Adopted by the early Prosauropods, it led with evolution to the phenomenal agility characteristic of Celurosaurs, such as Ornithomimids or Maniraptora, capable of running fast (Struthiomimus, Gallimimus), or even jumping and balancing on one leg while tearing apart prey with the sickle claw of the other (Deinonychus, Utahraptor). All stabilized by a tail made rigid by special bony ligaments.
Studies of their footprints (ichnology) applied to skeletal conformation have allowed us to derive precise mathematical formulas that tell us at what speed a large Sauropod or Teropod could move, as well as a small Iguanodont or a massive Ankylosaurus. And this results in a picture worthy of scenes from the African savannah, with wildebeests grazing or gazelles running pursued by leopards or lionesses.
It has been discovered that some carnivorous dinosaurs hunted in groups, while others, even different species, nested together and grazed advancing in herds with the young at the center to protect them from predators; yet others adopted sophisticated parental care worthy of birds or mammals. There were even dinosaurs capable of modulating a whole range of sounds to warn of danger, keep in touch between young and parents, compete acoustically to win a mate, or, if necessary, engage in head-butting fights, like present-day deer and ibexes.
All this were the dinosaurs, and certainly much more than we will ever be able to derive from the scarce fossil evidence that has reached us, so much so that someone has proposed to call them "logosaurs," or intelligent lizards, venturing the fantasy-biological hypothesis that if they had not become extinct, perhaps evolution would have led some particularly brainy Celurosaurs (Stenonychosaurids) towards a form of reptilian intelligence similar to humans...
Dinosaurs Systematics
Evolving from a common ancestor, dinosaurs took two paths, branching into two distinct groups, corresponding to the orders of Saurischia and Ornithischia. The substantial difference between these two groups lies in the conformation of two pairs of pelvic bones, the ischium and pubis. In Saurischia, the two pubic bones are pointed forwards, producing that characteristic bulge on the belly that is noticeable in the most faithful illustrations and reconstructions, especially in certain bipeds such as Tyrannosaurus, for example; the ischium, on the other hand, was pointed backward.
In Ornithischia, both pairs of bones pointed backward, as will happen later by evolutionary convergence in birds, even though they derive from the Saurischia Celurosaurs, with divergent ischium and pubis.
Saurischians are usually divided into the suborders of Prosauropods, Sauropods and Theropods, or according to other classifications, into Theropods and Sauropodomorphs, where the latter includes the infraorders of Prosauropods and Sauropods. Theropods are also divided into the infraorders of Carnosaurs and Coelurosaurs, but systematic classification is in continuous turmoil due to the upheavals caused by increasingly frequent discoveries. Thus, we find other infraorders such as Deinonychosaurs and Ornithomimosaurs placed separately outside Coelurosaurs or even grouped into Maniraptora, and so forth. The same happens with Ornithischians. These include the suborders of Ornithopods, Ankylosaurs, Ceratopsians and Stegosaurs. But we can add Pachycephalosaurs, separated from Ornithopods and no longer considered as their family, Scelidosaurs, the ancestors of Ankylosaurs, and Nodosaurids (also armored with spikes and bony plates, but without the club at the end of the tail), separated from the latter into a different family.
The concept of species
All systematic classification aims to group animals based on morphological, physiological, and evolutionary similarities into increasingly larger sets up to the phyla level. However, each of these groups must necessarily start from the basic biological unit, which is the species.
Since the mid-1700s, with Linnaeus being its originator, species have been indicated by two names placed together, the genus (always written in uppercase) and the species (always written in lowercase). The species name can never be indicated separately from the genus since there can be identical nouns for different genera, perhaps even belonging to the plant kingdom! For example, Panthera tigris is the tiger, while Cypraea tigris is a Gastropod Mollusk. So, if we want to indicate our domestic dog, we say Canis lupus, where Canis is the genus and lupus indicates the species. However, lupus alone means nothing if it is not preceded by Canis, the genus to which it belongs.
Furthermore, the concept of species has a precise biological meaning, as well as a taxonomic one: a species is defined as a group of organisms whose two sexes (if they exist) in the adult stage, when mating with each other, are capable of reproducing and producing fertile offspring. Fertile means that the offspring must also adhere to the same law as the parents, namely being able to perpetuate their own genetic heritage through a new generation, thereby fulfilling the purpose of life, which is to allow the various DNA molecules specific to each species to duplicate and multiply indefinitely, preserving themselves in the genetic pool of populations (and we'll skip the discussion of evolution, to which we'll give a brief mention later).
Organisms belonging to distant species, such as a leech and an elephant, or even of the same genus, such as Canis aureus (golden jackal) and Canis latrans (coyote), do not meet this definition of species. They cannot mate with each other, and even if they did, they would waste energy and resources because they are not genetically compatible, so their union would not produce anything. This concept also applies to rare instances in nature, such as when organisms from different species, like the donkey (Equus asinus) and the horse (Equus przewalskii), are capable of producing offspring (mule and hinny). However, these hybrids are sterile and incapable of producing fertile offspring. To further complicate matters and distinguish one breed (or subspecies) from another, a third name (always written in lowercase) must be added after the species.
Back to the case of our domestic dog, we say Canis lupus familiaris to distinguish all breeds of dogs (natural, such as the dingo, or those created by humans) from the Canis lupus lupus breed, which is the European wolf. At this point, it is permissible to apply this concept to dinosaurs as well. So, when we commonly speak of Tyrannosaurus, Protoceratops, or Hypsilophodon, we are actually indicating genera and not the individual species that composed them.
More accurately, we should say: Tyrannosaurus rex, Protoceratops andrewsi, or Hypsilophodon foxi. Alternatively, within the same genus, we can have more than one recognized species (where, from the study of fossil remains, there are no doubts due to differences such as polymorphism within populations, differences between sexes, or between young and adult individuals that might mistakenly lead to the belief that fossil remains, perhaps incomplete, belong to specimens of different species). Thus, within the genus Stegosaurus, we have Stegosaurus ungulatus, Stegosaurus armatus, and Stegosaurus stenops, and within the genus Lambeosaurus, we have Lambeosaurus clavinitialis, Lambeosaurus lambei, and Lambeosaurus magnicristatus, and so on.
Evolution and natural selection
We previously mentioned that the purpose of life, if we insist on finding a cause-effect purpose, is merely chemical, i.e., to allow the various DNA molecules specific to each species to duplicate and multiply indefinitely, preserving themselves in the genetic pool of populations. Although this duplicative mechanism was established billions of years ago randomly and without "double purposes," that is, to differentiate into millions and millions of different species, nothing is immutable and eternal, and the most complex things end up being imperfect. This means that inevitable mutations in the genome, i.e., alterations of DNA base pairs due to various factors (pertaining to molecular biology, which we won't delve into here), even if kept in check by repair mechanisms, can produce, among many harmful and unsuccessful mutations, small or more significant alterations within a species. If these alterations are favorable to life, they can be passed on through generations, accumulate with other mutations, and eventually create something new, something different, which environmental factors may entirely isolate from the original lineage, resulting in a new organism, or rather, a new species. This mechanism is always at play.
Consider the breeds of dogs we have created. They are still the same species, genetically compatible. But a dachshund, separated from a wolf, along with other dachshunds, perhaps on a deserted island, is likely to give rise to some strange carnivore adapted to digging tunnels, or perhaps over time, to one with flippers like seals. This is evolution, a continuous change and adaptation to equally changing environmental conditions, and this is where the other determining factor comes into play: natural selection. Under its rigorous and constant control, the environment determines which species can survive and which will become extinct, which will replace the less suitable ones in the inevitable competition that ensues. This has been happening for billions of years, slowly, so much so that everything seems immutable to our eyes. But fossils prove otherwise, and the biodiversity of this planet is the most evident proof, unless one wants to believe that all species were created together or through spontaneous generation, like worms appearing in an apple when it rots.
If, instead of creationism, we credit Darwin and evolutionism, then we know that multicellular organisms originated from microscopic marine unicellular organisms. Some learned to harness solar energy and followed the path leading to algae and plants (autotrophs), while others preferred to obtain what they needed at the expense of others (heterotrophs). Over time, from crawling worms, forms adapted to swimming emerged, fins gave rise to limbs, lungs replaced gills, and eggs with shells enabled vertebrates to conquer the land, alongside plants and arthropods. Thus, from fish, amphibians originated, from these reptiles, and from the latter birds and mammals. So, it's not silly to say that we derive from worms, that from a turtle ancestor, mice emerged, and from them, monkeys, and thus humans. And so, even the dinosaurs, so different and spectacular, obeyed the same rules of life that prevail on this planet: mutation and evolution, natural selection, and extinction. Species compete, they replace each other, they last more or less than others in a continuous turnover that allows life to persist on Earth, despite glaciations, continents drifting apart, eruptions, or human devastation. But sometimes destiny oversteps its bounds, and entire ecosystems are swept away forever.
Mass extinctions
Mass extinctions are crucial events in the history of life on Earth, and understanding why some groups survive while others disappear is fundamental to understanding the workings of evolution and natural selection.
Sharks, mammals, birds, and other groups that have survived mass extinctions generally have characteristics that have made them more adaptable to changing environments or better able to recover after a catastrophic event. For example, sharks have a large genetic diversity and a wide range of habitats, making them more resilient to environmental impacts. Mammals and birds, on the other hand, have demonstrated a great capacity for adaptation and expansion into new habitats.
When it comes to comparing dinosaurs with other extinct groups such as plesiosaurs, ichthyosaurs, and pterosaurs, there are various hypotheses that seek to explain why some became extinct while others did not. One of the most widely accepted theories concerns the effect of a meteorite impact, which may have caused global fires, sky darkening, and sudden climate changes that would have put pressure on terrestrial and marine ecosystems. Other factors such as massive volcanic eruptions, changes in sea level, and global temperatures may also have contributed to the extinction of dinosaurs and other groups.
However, it is important to note that not all dinosaur species became extinct simultaneously. Some groups appear to have been more vulnerable to environmental impacts than others, and their biological characteristics, such as body size, diet, and reproductive behavior, may have influenced their survival during and after the extinction event.
In summary, by studying the biological characteristics of extinct groups and comparing them with those that have survived, we can gain a better understanding of how evolution and natural selection have shaped life on Earth and how catastrophic events have influenced its history.
The meteorite fell, whether we like it or not, corroborating the claims of American catastrophists or disagreeing with Nobel Prize winner Luis Alvarez regarding the layer of iridium deposited in the K-T boundary (Cretaceous-Tertiary) in various parts of the globe. However, the Chicxulub crater in the Yucatan exists, even if buried under kilometers of sediments, and has been dated precisely to 65 million years ago when a 10 km diameter meteorite collided with Earth at a speed of 90,000 km per hour. Moreover, it seems that it wasn't the only one, but another one also fell in the North Sea, or even a true bombardment, and probably more will fall in the not too distant future, with a periodicity that follows Kepler's laws governing the motion of all celestial bodies. There have been numerous mass extinctions on our planet, not just at the end of the Cretaceous: just think of the one that marked the end of the Paleozoic, compared to which, in terms of the number of species disappeared, the extinction of dinosaurs seems quite trivial.
Let's just say that the dinosaurs were very unfortunate because their decline was already inexorably marked, always by astronomical laws, due to which the great summer that allowed many dinosaur species to evolve and specialize in a semi-tropical environment almost stable with essentially eutrophic characteristics, was ending, and the meteorite only dealt the final blow, accelerating what climate changes had already begun to alter for some time. Just to give an example, the ichthyosaurs had already disappeared, 25 million years before the dinosaurs; but above all, the flora itself was changing: many plants, on which the diet of the giant sauropods was based (already in evident decline towards the end of the Cretaceous, replaced by more specialized ornithischians), became extinct, replaced by angiosperms, flowering plants.
Then the Earth was shaken by the cosmic cataclysm and its long-term effects: an endless night of darkening due to the dust suspended in the atmosphere, the greenhouse effect followed by a long decrease in temperature throughout the planet, planet-wide fires due to the fall of incandescent material, acidification of rain and therefore of the seas... Plants and plankton were the first to suffer, and many food chains were interrupted. The first terrestrial animals to be affected were all those that were too large to hide underground or otherwise unable to do so. Even in the sea, there was no escape from shock waves and the death of plankton that followed. Small mammals, lizards, insects, still had enough to survive, even with the reduction of food resources. Opportunists and detritivores therefore prevailed. But even among large animals, there were exceptions: monitor lizards feed on rotting carrion and are immune to decomposition bacteria; among archosaurs, only crocodiles proved to be the most prepared, as they can go without eating for up to two years. Obviously, unlike sharks, which appeared on Earth long before reptiles, plesiosaurs were not physiologically predisposed to face such a change. And even less so the fragile pterosaurs, the giant herbivorous dinosaurs, and the large carnivores that depended on them.
Thus, all those animals that already possessed in their genes the capabilities that could allow them to survive remained. And they inherited the Earth.
