The Metazoic Era
It is no secret that I wanted to make the Metazoic era the true Age of Mammals. This is where mammals are at their greatest diversity. Some of the mammals you may encounter in this website may look strange. Some may look familiar. I do use a lot of parallel evolution in my theories. But some may look very strange to you as well. Most however, do have modern counterparts.
When I first started with the mammals, I had an agenda in mind. I wanted to create a world that was literally dominated by mammals, fazing out the birds, reptiles and other large, major groups. Though I am keeping some to represent those groups, as well as many others. Though the seas I figure will be dominated by various cephalopods, the air will be dominated by bats, particularly pteropods, and the trees will be dominated by lemurs. The ground levels though will have a myriad of different creatures running around.
Mammals of the Metazoic are much like they are today. They still feed their offspring with milk secreted by special glands in their nipples. Most, with the exception of the platypus and echidnas, give birth to live young. Their bodies are still covered with fur, much like mammals of today are. Though the composition of the fur is slightly different than it is in today’s mammals. Particularly in the pteropods. To lighten their load, the pteropods’ fur fibers hollowed out. This became a unique feature among Metazoic mammals. Today, we see this only in polar bears and golden moles. This is also the source of the vibrant colors that appear in the Pteropod family during the Metazoic.
The mammals of the Metazoic also differ from modern mammals in quite a number of other ways. In many groups, the testes and scrotum of the males are either greatly reduced, or completely retreated into the pelvis. Tomorrow, it would be much more difficult to differ males from females just by looking at them. This is possible because the body temperature of most Metazoic mammals is just slightly lower than that of birds, which also do not have external genitalia. Very few Metazoic mammal families still have the large genitals like we see in modern mammals. They are mostly the felines and the Ailurocyonids. The bone structure of Metazoic mammals also differs, by being slightly less massive than those of modern mammals, which enables them to grow to such massive sizes during the Metazoic. During the Cenozoic, that was a problem, as large mammals reproduce slowly. But the young in Metazoic mammals develop slightly faster than modern mammals once they are born. In most families, the gestation period is less than 9 months. The young also are born much smaller than modern mammals, in proportion to body size. For example, a mammal in the Metazoic that is the size of an elephant, may carry a baby for about 9 months, and the young that are born may weigh no more than 5-10 pounds, and about the size of a dachshund. A massive reduction to the 200-pound size of a modern newborn baby elephant.
Carnivorous mammals in the Metazoic are also designed to go for longer periods without eating. On average, they may eat 3 times a week. Though vegetarian mammals like deer may eat regularly every day. This is an advantage since in some regions like the desert, large enough food items may not be as numerous or easy to find enough to sustain them. These mammals may retain their energy though for several days after eating. Like modern mammals, they tend to store that energy in their fat cells.
When I write about mammals of the Metazoic in book form, I do not tend to want to separate them in order, like you might see in works like Walker’s Mammals of the World. Mostly what I do is separate them by type. Usually a very generalized form of type, like the large grazers, the small grazers, the carnivores, the Pentadactyls (formerly known as primates), and the winged mammals. I classify the large grazers as being family groups where more than 50% of the species in that family are over 5 feet tall. The small grazers are exactly the opposite, only for them, more than 50% of their members are less than 5 feet tall. Though this may not be true for all members of that family. The kangaroos for example. Most of the members of the kangaroo family in the Metazoic are over 5 feet tall. But there are some that are 2 to 4 feet tall, and even some that are under one foot tall. But I class the family as “large grazers” because of the majority of species in that family being more than 5 feet tall.
The carnivores of the Metazoic, not all of them are classified the same as modern carnivores. The actual term “carnivore” means one who eats meat, and also refers to the meat-crushing molars, called “carnissals”, in the back of the mouth. Only true carnivores today have them. However, that is not so true in the Metazoic, as several families not classified as “carnivores” today, also developed crushing teeth for grinding meat in the Metazoic. I just took the basic term “carnivore” and applied it to all families in the Metazoic that are mostly meat-eaters. The Pentadactyls are also slightly different. During the Cenozoic, for a mammal to have 5 fingers on each foot is not so unusual. But during the Metazoic, most mammals lose one finger, and have between one and four toes. But not the Pentadactyls. They still need all 5 fingers for grasping tree branches, where most of the species make their homes. In a sense, they did not change. But I drop the name “primate” for Metazoic species and prefer to refer to them as “Pentadactyls”.
The flying mammals is usually the last group I talk about. They are all mammals that have developed wings. Most are direct descendants of modern bats, mostly the Pteropods. They change though during the Metazoic. Metazoic bats have developed longer legs with stronger musculature for standing. They no longer hang upside down like modern bats. Instead, this allows them to perch like modern birds, which has allowed the bats to take over many of the ecological niches left by birds in the air. The wings are used in both walking and flying. The wings are no longer totally naked, but now have very short, fine, soft hairs that are able to grab air flow and allow Metazoic bats to fly for longer distances without tiring or even so much of the wings. Not all the species in this classification are bats. There are also some descendants of lemurs that have developed wing-like structures for long-distance gliding.
The main point I had in mind when creating the mammals of the future was to streamline them. More so than they are today. I wanted to treat the mammals of this project as if they were really real animals around today. So, I separated them into orders, families, genera and species. That is what this portion of this book is going to show you, the results of over 25 years of labor and hard thinking. Though in this book, I don’t want to put them in any particular order. I will go by families though that I have completed. They will be labeled the same as the animals I did in the previous chapters. So, sit back and enjoy the works of my Metazoic Mammal Project.
I have often heard debates about which group of animals in Earth’s history was the most successful. Some think the dinosaurs were the most successful animals that ever lived. They ruled for 3 epochs; the Triassic, the Jurassic and the Cretaceous. Some say it was the trilobites, who lived 521 to 251 million years ago, had many species, and are related to today’s arthropods. The closest relatives of theirs's alive today are the horseshoe crabs. But they are only very distantly related and trilobites have no other kin around today.
However, from the late Carboniferous, 311 million years ago, to the late Triassic, another group of reptilians was dominating. These were the Synapsids. These are the earliest form of the order that would eventually become mammals. Once thought to be merely “mammal-like reptiles”, this has since been debunked. Synapsids are not reptiles, but do share a common ancestor, the Amniotes, whose lineage split apart around 318 million years ago to also form the Saurapsids. The Synapsids were the most dominant life forms from the late Palaeozoic to the early Mesozoic. They were the largest land creatures to roam the planet in the later half of the Permian period. They are represented by several groups, among them was the Therapsids, which appeared in the mid-Permian to the late Triassic period, and would be the group that would become the true mammals.
It was during the Triassic when the Therapsids took over the world that Synapsids became what we know today as typical mammals. The legs moved under the body, for a more upright position, and they began to develop fur. The evolution of fur probably came along with the evolution of warm-bloodedness. Especially among the smaller Therapsids. Because small animals lose heat faster than larger animals, the small Therapsids needed to evolve methods of keeping their body temperature in check. One of these ways was to develop hair, or fur. Mammals are also unique in that the jaw is comprised of 2 major bones, the maxilla and the mandible. In the Permian synapsids, their jaws were more reptilian-like, with 5 major bones, and only 1 bone in the inner ear, the cochlea, which is responsible for balance and detecting noise vibrations. By the mid-Triassic era, 3 of the bones in the jaws had shrunk and become the bones that would make up the inner ear of all mammals, the malleus, incus and stapes.
Animals that are warm-blooded must constantly eat, which is another way of stoking their body temperature. Milk production is believed to have evolved with much earlier Synapsids. Possibly around 300 million years ago. Modern salamanders lay across their eggs and secrete a liquid from glands in their skin to keep their eggs moist when needed. It is believed that this is how mammary glands, which are specialized sweat glands, evolved in mammals. Some animals, like platypus and echidnas, do not even have nipples. They simply sweat milk from these specialized glands and the young laps it off the mother’s skin.
Some scientists even have argued, in the case of dinosaurs, that the upright, bird-like posture is the key to success. It may be true for both dinosaurs and synapsids. Having an upright posture allows an animal to see over tall vegetation, and prevents them from being eaten, or allows them to find food.
Synapsids were believed to lay eggs that more or less resembled modern lizard eggs. That is, they were not hard and had no calcified outer shell, but soft and leathery. Modern monotremes also lay these kind of eggs. But other than them, no other mammals today lay eggs. How young are kept by the mother is also up for conversation. We have no idea if synapsids had pouches, like marsupials, or kept their young huddled in an underground burrow, or nest. No fossilized eggs of synapsids have ever been discovered, as leathery shells do not fossilize very well. The echidna mother creates a fold of skin when she has babies that makes a makeshift pouch. But this is a very cumbersome method, which is why echidnas do not move very fast. Platypus, on the other hand, lay their nests in underground burrows.
Among the many clades of therapsid groups, the one that is the most important are the Cynodonts. These were closest to all modern mammals, and appeared in the fossil record about 225 million years ago, the last years of the Triassic. This was also when dinosaurs first appeared on the scene. Cynodonts were small, burrowing animals for the most part. By this time, most Synapsid groups, who had been dominant up to this point, were being out-competed by the Sauropsids. The only family of synapsids that would survive the Triassic/Jurassic extinction event were the Cynodonts. Cynodonts developed the mixed dentition mammals today have. Most animals have a combination of canine teeth, incisors and molars. Whereas other Synapsid groups did not have that.
During the millions of years the dinosaurs took over, there was very little diversity among mammals. It is believed that mammals got their present-day beginning in South America, with the discovery of Brasilodon, which is believed to be the very first placental mammal. No one knows how mammals switched from laying eggs to giving birth to live young. But it is believed that the method evolved so mammals could spread to different kinds of habitats because they gave birth. This is possibly what allowed mammals to assort, rather than stay confined to an entirely underground existence.
After the Cretaceous period was over, some 65 million years ago, mammals seized their opportunity, just as the Synapsids did before them. In the Cenozoic era, which is considered the age of man/mammals, mammals took over all open niches left by the dinosaurs. While dinosaurs had become birds. While there are indeed more bird species than mammals, and they are very successful animals, they do not get as big as their close relatives, the dinosaurs got. Neither do most mammals in the Cenozoic. It is believed that the reason birds never got as big as dinosaurs is because today’s mammals are holding them down. Just as dinosaurs held down mammals during the Mesozoic. But birds did not totally lose out. Being small gives birds the ability to fly, which allows them to make a quick get-away from predators, in an atmosphere that has 25% less oxygen than in the Mesozoic.
Some changes were made to mammals during the domination of the dinosaurs. For one thing, the lifespan of mammals is believed to have gotten shorter due to dinosaurs. I saw an article about this the other day. It's called the Longevity Bottleneck Hypothesis, and was first argued by Professor Joao Pedro de Magalhaes from the University of Birmingham. This hypothesis explains how mammals have such short lifespans, whereas similar-sized birds, like parrots, can live up into their 80s. It is believed that mammals evolved to live shorter lives because all they really need to do is reproduce. It was too dangerous for Mesozoic mammals to leave their burrows and go off looking for a mate, copulating and birthing. The danger was being eaten by a ravenous dinosaur. While some reptiles and amphibians show no significant signs of aging, all mammals—including humans—show a marked aging process. Professor de Magalhaes' hypothesis suggests that during the Mesozoic Era, mammals faced persistent pressure for rapid reproduction during the reign of dinosaurs, which over 100 million years led to the loss or inactivation of genes associated with long life, such as processes associated with tissue regeneration and DNA repair.
So, what makes an “age of mammals”? Or an “age of reptiles”? The answer: it is which group dominates. Often that means the group with the greatest diversity and size. The group that covers all niches and ecosystems. This is why there is an “age of mammals” and the preceding “age of reptiles”. And if you ask me what group of animals throughout Earth’s history is (or was) the most successful, I’d have to say the mammals are.
When I first started with the mammals, I had an agenda in mind. I wanted to create a world that was literally dominated by mammals, fazing out the birds, reptiles and other large, major groups. Though I am keeping some to represent those groups, as well as many others. Though the seas I figure will be dominated by various cephalopods, the air will be dominated by bats, particularly pteropods, and the trees will be dominated by lemurs. The ground levels though will have a myriad of different creatures running around.
Mammals of the Metazoic are much like they are today. They still feed their offspring with milk secreted by special glands in their nipples. Most, with the exception of the platypus and echidnas, give birth to live young. Their bodies are still covered with fur, much like mammals of today are. Though the composition of the fur is slightly different than it is in today’s mammals. Particularly in the pteropods. To lighten their load, the pteropods’ fur fibers hollowed out. This became a unique feature among Metazoic mammals. Today, we see this only in polar bears and golden moles. This is also the source of the vibrant colors that appear in the Pteropod family during the Metazoic.
The Golden moles (Family: Chrysochloridae), have fur that is almost unique among mammals. Uncommon today, but quite common in the Metazoic.
The mammals of the Metazoic also differ from modern mammals in quite a number of other ways. In many groups, the testes and scrotum of the males are either greatly reduced, or completely retreated into the pelvis. Tomorrow, it would be much more difficult to differ males from females just by looking at them. This is possible because the body temperature of most Metazoic mammals is just slightly lower than that of birds, which also do not have external genitalia. Very few Metazoic mammal families still have the large genitals like we see in modern mammals. They are mostly the felines and the Ailurocyonids. The bone structure of Metazoic mammals also differs, by being slightly less massive than those of modern mammals, which enables them to grow to such massive sizes during the Metazoic. During the Cenozoic, that was a problem, as large mammals reproduce slowly. But the young in Metazoic mammals develop slightly faster than modern mammals once they are born. In most families, the gestation period is less than 9 months. The young also are born much smaller than modern mammals, in proportion to body size. For example, a mammal in the Metazoic that is the size of an elephant, may carry a baby for about 9 months, and the young that are born may weigh no more than 5-10 pounds, and about the size of a dachshund. A massive reduction to the 200-pound size of a modern newborn baby elephant.
Carnivorous mammals in the Metazoic are also designed to go for longer periods without eating. On average, they may eat 3 times a week. Though vegetarian mammals like deer may eat regularly every day. This is an advantage since in some regions like the desert, large enough food items may not be as numerous or easy to find enough to sustain them. These mammals may retain their energy though for several days after eating. Like modern mammals, they tend to store that energy in their fat cells.
When I write about mammals of the Metazoic in book form, I do not tend to want to separate them in order, like you might see in works like Walker’s Mammals of the World. Mostly what I do is separate them by type. Usually a very generalized form of type, like the large grazers, the small grazers, the carnivores, the Pentadactyls (formerly known as primates), and the winged mammals. I classify the large grazers as being family groups where more than 50% of the species in that family are over 5 feet tall. The small grazers are exactly the opposite, only for them, more than 50% of their members are less than 5 feet tall. Though this may not be true for all members of that family. The kangaroos for example. Most of the members of the kangaroo family in the Metazoic are over 5 feet tall. But there are some that are 2 to 4 feet tall, and even some that are under one foot tall. But I class the family as “large grazers” because of the majority of species in that family being more than 5 feet tall.
The carnivores of the Metazoic, not all of them are classified the same as modern carnivores. The actual term “carnivore” means one who eats meat, and also refers to the meat-crushing molars, called “carnissals”, in the back of the mouth. Only true carnivores today have them. However, that is not so true in the Metazoic, as several families not classified as “carnivores” today, also developed crushing teeth for grinding meat in the Metazoic. I just took the basic term “carnivore” and applied it to all families in the Metazoic that are mostly meat-eaters. The Pentadactyls are also slightly different. During the Cenozoic, for a mammal to have 5 fingers on each foot is not so unusual. But during the Metazoic, most mammals lose one finger, and have between one and four toes. But not the Pentadactyls. They still need all 5 fingers for grasping tree branches, where most of the species make their homes. In a sense, they did not change. But I drop the name “primate” for Metazoic species and prefer to refer to them as “Pentadactyls”.
The flying mammals is usually the last group I talk about. They are all mammals that have developed wings. Most are direct descendants of modern bats, mostly the Pteropods. They change though during the Metazoic. Metazoic bats have developed longer legs with stronger musculature for standing. They no longer hang upside down like modern bats. Instead, this allows them to perch like modern birds, which has allowed the bats to take over many of the ecological niches left by birds in the air. The wings are used in both walking and flying. The wings are no longer totally naked, but now have very short, fine, soft hairs that are able to grab air flow and allow Metazoic bats to fly for longer distances without tiring or even so much of the wings. Not all the species in this classification are bats. There are also some descendants of lemurs that have developed wing-like structures for long-distance gliding.
The main point I had in mind when creating the mammals of the future was to streamline them. More so than they are today. I wanted to treat the mammals of this project as if they were really real animals around today. So, I separated them into orders, families, genera and species. That is what this portion of this book is going to show you, the results of over 25 years of labor and hard thinking. Though in this book, I don’t want to put them in any particular order. I will go by families though that I have completed. They will be labeled the same as the animals I did in the previous chapters. So, sit back and enjoy the works of my Metazoic Mammal Project.
How Mammals Ruled The World, Twice
I have often heard debates about which group of animals in Earth’s history was the most successful. Some think the dinosaurs were the most successful animals that ever lived. They ruled for 3 epochs; the Triassic, the Jurassic and the Cretaceous. Some say it was the trilobites, who lived 521 to 251 million years ago, had many species, and are related to today’s arthropods. The closest relatives of theirs's alive today are the horseshoe crabs. But they are only very distantly related and trilobites have no other kin around today.
Left: The oldest known synapsid, Archaothyris. Picture by Nobu Tamura (palaeoexibit.blogspot.com).
Right: The oldest known placental mammal, Brasilodon. Picture by Anatomical Society/Wiley.
However, from the late Carboniferous, 311 million years ago, to the late Triassic, another group of reptilians was dominating. These were the Synapsids. These are the earliest form of the order that would eventually become mammals. Once thought to be merely “mammal-like reptiles”, this has since been debunked. Synapsids are not reptiles, but do share a common ancestor, the Amniotes, whose lineage split apart around 318 million years ago to also form the Saurapsids. The Synapsids were the most dominant life forms from the late Palaeozoic to the early Mesozoic. They were the largest land creatures to roam the planet in the later half of the Permian period. They are represented by several groups, among them was the Therapsids, which appeared in the mid-Permian to the late Triassic period, and would be the group that would become the true mammals.
Skull of Dimentrodon, showing the typical synapsid dentition, with molars that lack cusps. This would not develop until the Cynodonts emerged.
It was during the Triassic when the Therapsids took over the world that Synapsids became what we know today as typical mammals. The legs moved under the body, for a more upright position, and they began to develop fur. The evolution of fur probably came along with the evolution of warm-bloodedness. Especially among the smaller Therapsids. Because small animals lose heat faster than larger animals, the small Therapsids needed to evolve methods of keeping their body temperature in check. One of these ways was to develop hair, or fur. Mammals are also unique in that the jaw is comprised of 2 major bones, the maxilla and the mandible. In the Permian synapsids, their jaws were more reptilian-like, with 5 major bones, and only 1 bone in the inner ear, the cochlea, which is responsible for balance and detecting noise vibrations. By the mid-Triassic era, 3 of the bones in the jaws had shrunk and become the bones that would make up the inner ear of all mammals, the malleus, incus and stapes.
Animals that are warm-blooded must constantly eat, which is another way of stoking their body temperature. Milk production is believed to have evolved with much earlier Synapsids. Possibly around 300 million years ago. Modern salamanders lay across their eggs and secrete a liquid from glands in their skin to keep their eggs moist when needed. It is believed that this is how mammary glands, which are specialized sweat glands, evolved in mammals. Some animals, like platypus and echidnas, do not even have nipples. They simply sweat milk from these specialized glands and the young laps it off the mother’s skin.
Some scientists even have argued, in the case of dinosaurs, that the upright, bird-like posture is the key to success. It may be true for both dinosaurs and synapsids. Having an upright posture allows an animal to see over tall vegetation, and prevents them from being eaten, or allows them to find food.
Synapsids were believed to lay eggs that more or less resembled modern lizard eggs. That is, they were not hard and had no calcified outer shell, but soft and leathery. Modern monotremes also lay these kind of eggs. But other than them, no other mammals today lay eggs. How young are kept by the mother is also up for conversation. We have no idea if synapsids had pouches, like marsupials, or kept their young huddled in an underground burrow, or nest. No fossilized eggs of synapsids have ever been discovered, as leathery shells do not fossilize very well. The echidna mother creates a fold of skin when she has babies that makes a makeshift pouch. But this is a very cumbersome method, which is why echidnas do not move very fast. Platypus, on the other hand, lay their nests in underground burrows.
Among the many clades of therapsid groups, the one that is the most important are the Cynodonts. These were closest to all modern mammals, and appeared in the fossil record about 225 million years ago, the last years of the Triassic. This was also when dinosaurs first appeared on the scene. Cynodonts were small, burrowing animals for the most part. By this time, most Synapsid groups, who had been dominant up to this point, were being out-competed by the Sauropsids. The only family of synapsids that would survive the Triassic/Jurassic extinction event were the Cynodonts. Cynodonts developed the mixed dentition mammals today have. Most animals have a combination of canine teeth, incisors and molars. Whereas other Synapsid groups did not have that.
During the millions of years the dinosaurs took over, there was very little diversity among mammals. It is believed that mammals got their present-day beginning in South America, with the discovery of Brasilodon, which is believed to be the very first placental mammal. No one knows how mammals switched from laying eggs to giving birth to live young. But it is believed that the method evolved so mammals could spread to different kinds of habitats because they gave birth. This is possibly what allowed mammals to assort, rather than stay confined to an entirely underground existence.
After the Cretaceous period was over, some 65 million years ago, mammals seized their opportunity, just as the Synapsids did before them. In the Cenozoic era, which is considered the age of man/mammals, mammals took over all open niches left by the dinosaurs. While dinosaurs had become birds. While there are indeed more bird species than mammals, and they are very successful animals, they do not get as big as their close relatives, the dinosaurs got. Neither do most mammals in the Cenozoic. It is believed that the reason birds never got as big as dinosaurs is because today’s mammals are holding them down. Just as dinosaurs held down mammals during the Mesozoic. But birds did not totally lose out. Being small gives birds the ability to fly, which allows them to make a quick get-away from predators, in an atmosphere that has 25% less oxygen than in the Mesozoic.
Some changes were made to mammals during the domination of the dinosaurs. For one thing, the lifespan of mammals is believed to have gotten shorter due to dinosaurs. I saw an article about this the other day. It's called the Longevity Bottleneck Hypothesis, and was first argued by Professor Joao Pedro de Magalhaes from the University of Birmingham. This hypothesis explains how mammals have such short lifespans, whereas similar-sized birds, like parrots, can live up into their 80s. It is believed that mammals evolved to live shorter lives because all they really need to do is reproduce. It was too dangerous for Mesozoic mammals to leave their burrows and go off looking for a mate, copulating and birthing. The danger was being eaten by a ravenous dinosaur. While some reptiles and amphibians show no significant signs of aging, all mammals—including humans—show a marked aging process. Professor de Magalhaes' hypothesis suggests that during the Mesozoic Era, mammals faced persistent pressure for rapid reproduction during the reign of dinosaurs, which over 100 million years led to the loss or inactivation of genes associated with long life, such as processes associated with tissue regeneration and DNA repair.
So, what makes an “age of mammals”? Or an “age of reptiles”? The answer: it is which group dominates. Often that means the group with the greatest diversity and size. The group that covers all niches and ecosystems. This is why there is an “age of mammals” and the preceding “age of reptiles”. And if you ask me what group of animals throughout Earth’s history is (or was) the most successful, I’d have to say the mammals are.
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