The Lone Ranger
08-03-2008, 04:52 AM
An Introduction to Zoology
Chapter Three: Anatomical Terminology and a History of Animal Life:
Anatomical Terminology:
When describing regions of an animal’s body or the positions of body structures, we use a number of paired technical terms. One thing that’s important to remember is that most of these terms are relative, and make sense only when you’re using them to make comparisons. For instance, it makes no sense to say “the elbow is proximal.” You must specify what it’s proximal to.
[b]Superior/Inferior:
A body structure or region is superior to another if it is above it. Similarly, a body structure or region is inferior to another if it is below it. Consider a cat. Normally, at least, the cat’s backbone is superior to its belly – that is, the cat’s backbone is higher above the ground. This, of course, means that the cat’s belly is inferior to its backbone.
In humans, because we walk upright, “superior” typically means “toward the head,” and “inferior” typically means “toward the feet.” Because of our unusual posture, the terms “superior” and “inferior” are quite useful in describing human anatomy, but they’re rarely used when discussing the anatomy of non-human species that typically walk on all fours (or all sixes, or all eights, or whatever).
Anterior/Posterior:
Anterior means “toward the front of the body.” The terms cranial and cephalic have much the same meaning and are sometimes used as synonyms for anterior, but “anterior” is the preferred term. This is because “cranial” and “cephalic” specifically mean “toward the head,” and many animals lack a distinct head. “Anterior,” on the other hand, always means “toward the front of the body.”
Posterior means “toward the rear of the body.” Caudal is sometimes used as a near-synonym, as it means “toward the tail.” Strictly speaking, though, “caudal” is used only when you’re describing vertebrate animals.
Again, because of our upright stance, the terms “anterior” and “posterior” have slightly different meanings when used to describe humans than when they’re used to describe a typical non-human animal. Regardless of what kind of animal you’re talking about, “anterior” always means “toward the front of the body,” but in humans, “toward the front of the body” means toward the face or belly. In most other animals, “toward the front of the body” means toward the head. Similarly, when you’re describing a human, “posterior” means “toward the back,” but when you’re describing a non-human animal, “posterior” effectively means “toward the tail.”
[b]Dorsal/Ventral:
Dorsal means “toward the back.” You’ve no-doubt noticed that in humans (again, because of our upright stance), the terms “dorsal” and “posterior” are synonymous. That’s not true of other animals. Ventral means “toward the belly.” In humans (but not other animals, of course), “ventral” and “anterior” are synonymous.
Proximal/Distal:
These terms refer to structures that project out from the main portion of the body. A body part is proximal to another if it is closer to the point of attachment or to the trunk of the body. A body part is distal to another if it’s farther away from the point of attachment or to the trunk of the body. For example, your elbow is proximal to your wrist, but distal to your shoulder.
http://www.freethought-forum.com/forum/gallery/files/5/0/anatomical_directions_and_axes_original.jpg
Anatomical directions and body axes.
[b]Superficial/Deep:
A body part is superficial (or external) to another if it is closer to the surface of the body. A body part is deep (or internal) to another if it is farther from the surface of the body. For example, your ribcage is superficial to your heart, but deep to your pectoral muscles.
Medial/Lateral:
The “medial line” of the body is an imaginary line that runs right down the middle of the body, dividing it into right and left mirror-image halves. A body structure is medial to another structure if it’s closer to the midline of the body. A structure is lateral to another structure if it’s farther away from the body midline. For example, your eyes are lateral to your nose, but medial to your ears.
http://www.freethought-forum.com/forum/gallery/files/5/0/lateralmedial_original.jpg
This young lady has graciously consented to demonstrate the
difference between “medial” and “lateral.”
A Brief History of the Kingdom Animalia:
The animal kingdom contains a great diversity of species to be sure, but something like 99% of all the animal species that have ever existed are now extinct. So if you could take a trip into the distant past, the animals you’d encounter would certainly look quite different from the ones that you see today.
[b]Animal Origins:
Surprising though it might seem, fungi and animals are actually fairly closely-related. Certainly, fungi and animals are more closely related to each other than either of them is to plants. The genetic data suggests that the split between the ancestors of fungi and the ancestors of animals occurred only about one billion years ago. (To put that into perspective, the Earth formed about 4.5 billion years ago, and life has existed on Earth for about 4 billion years.)
According to the genetic evidence, the most recent common ancestor of all surviving animal groups lived somewhere between 675 – 875 million years ago. Certainly, animals lived before that time; what is meant by “most recent common ancestor” is the most recent organism that was ancestral to all the animals that have survived to the present day.
But where did animals come from? Both morphological and genetic evidence strongly suggest that animals evolved from a group of protozoans known as the choanoflagellates. Surviving choanoflagellate protists often live in colonies, and it’s likely that the first animals evolved from colonial choanoflagellates.
http://www.freethought-forum.com/forum/gallery/files/5/0/choanoflagellate.jpg
A choanoflagellate, a very (very) distant cousin of yours and mine.
A choanoflagellate can be easily recognized by the distinctive “collar”
that partially surrounds the flagellum.
http://www.freethought-forum.com/forum/gallery/files/5/0/sphaeroeca_choanoflagellate_colony.jpg
A colony of choanoflagellates.
[b] The Four Eras of Animal Life:
Animals have existed for four geological eras, the Neoproterozoic Era, the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era.
The Neoproterozoic Era began when the first animals evolved. The oldest-known fossils of organisms that were clearly animals date back to about 565 – 550 million years ago, but genetic evidence suggest that the first animals evolved about 1 billion years ago, shortly after the split between the fungus/animal lineages. The Neoproterozoic Era ended about 542 million years ago, when the first complex animals resembling modern animals appeared in the fossil record.
The late Neoproterozoic Era is referred to as the Ediacaran, and the very early animals that lived then made up part of what’s known as the Ediacaran biota. For the most part, the soft-bodied animals that lived during the Ediacaran bore little resemblance to modern animals. Sadly almost all of those weird, wonderful animal taxa are long-since extinct.
Even so, if you could go back in time to the Ediacaran, you would see some familiar-looking animals. There were sponges living during the late Neoproterozoic, as well as animals that seemed to resemble modern jellyfishes and their relatives.
http://www.freethought-forum.com/forum/gallery/files/5/0/spriggina_floundensi_original.jpg
Spriggina floundensi lived about 550 million years ago. It may have been
an ancestor of the trilobites that dominated the Earth’s seas many millions of years later.
http://www.freethought-forum.com/forum/gallery/files/5/0/charniodiscus_arboreus_original.jpg
Charniodiscus arboreus lived around 560 million years ago. It may
have been related to modern jellyfishes.
http://www.freethought-forum.com/forum/gallery/files/5/0/ediacaran_trace_fossil.jpg
Fossilized burrows of worm-like animals that lived during the Ediacaran.
[b]The Cambrian Explosion and the Paleozoic Era :
The fossil record suggests that in the last few million years of the Neoproterozoic Era, the diversity of animal life began to increase. The ensuing Paleozoic Era lasted from about 542 – 251 million years ago, and marks the first appearance in the fossil record of abundant, complex animals that were clearly ancestral to modern animals.
The first portion of the Paleozoic Era was the Cambrian Period. Near the beginning of the Cambrian, from about 535 – 525 million years ago, the pace of animal evolution seems to have increased dramatically. Animal diversity apparently increased so rapidly during this time that the period is often referred to as the Cambrian Explosion. During this relatively short period, animals that clearly belonged to almost half of all modern animal phyla first appeared in the fossil record. These organisms include the earliest-known animals that were clearly arthropods, mollusks, echinoderms, and chordates. (We are chordates, so our phylum has been in existence for over 520 million years.)
By any reckoning, the Cambrian Explosion was a major evolutionary event. Before that time, most organisms were either unicellular or simple colonies of cells. True, some simple animals had evolved during the Neoproterozoic, but the rate of animal evolution seems to have accelerated by an order of magnitude or more during the Cambrian.
http://www.freethought-forum.com/forum/gallery/files/5/0/cambrian_original.jpg
Life in the Cambrian. Though most of the critters probably look unfamiliar, many of them were
clearly related to modern animals. Sponges were abundant, and the fish-like animals
swimming near the sponges were early chordates. Many different kinds of arthropods were
present, including trilobites. Then, as today, brachiopods, mollusks, and echinoderms crawled
along the ocean floor. The large animal in the center-top portion of the image, Anomalocaris,
may have been the Earth’s first large predator.
[b]Why Did the Cambrian Explosion Occur?:
What triggered the Cambrian Explosion is one of the great mysteries regarding the history of life. Three principle explanations have been proposed. These explanations are not mutually exclusive, and it’s entirely possible (indeed, it’s quite likely) that a multitude of factors made the Cambrian Explosion possible.
The first hypothesis has to do with the appearance of large, apparently predatory animals like Anomalocaris. Their appearance during the early Cambrian is often cited as a possible explanation for the Cambrian Explosion. According to this hypothesis, the rapid evolution of new body plans that occurred during the Cambrian may have been driven by predator/prey interactions, because once the first truly effective predators evolved, this would have initiated an evolutionary “arms race” between predators and prey.
According to this “arms race” hypothesis, prey animals evolved better senses and protective armor plating (arthropods) or shells (brachiopods, mollusks) in response to the evolution of efficient predators like Anomalocaris. As prey species evolved to be better protected, more sensitive to their environments, and more agile, predators, in turn, were forced to evolve to become faster, larger, and more powerful that they might overcome their prey’s defenses. Not only does the Cambrian mark the first time that animals with shells and body armor appear in the fossil record, it also marks the first time that animals with well-developed eyes appear in the fossil record.
If this hypothesis is correct, the rapid evolutionary innovations during the Cambrian Explosion were driven by the constant need for predators and prey to adapt to each other. As prey animals became more difficult to capture and subdue, predators evolved to become more formidable. As predators became more formidable, prey animals evolved to be even harder to catch and subdue. This created selection pressure for predators to become even more formidable. And so on, and so on.
[break=Oxygen and Animal Origins]
The second hypothesis regarding the Cambrian Explosion has to do with the amount of oxygen in the atmosphere. Plants had not yet evolved by the time of the Cambrian Explosion, but photosynthetic cyanobacteria and algae had been around for well over a billion years by then. During that time, they had been slowly adding oxygen to the atmosphere.
Animals, of course, need oxygen to power their metabolic processes. It’s likely that the Cambrian was the first time in the Earth’s history that atmospheric oxygen levels had become high-enough to support large and active animals.
This “sufficient oxygen” hypothesis is related to the first hypothesis regarding the Cambrian Explosion. If this hypothesis is correct, then the evolution of large, active predators like Anomalocaris did indeed trigger the Cambrian Explosion, but they didn’t “just happen” to evolve at that time. According to this hypothesis, the Cambrian was the first time in the Earth’s history that enough oxygen was available to permit such animals to evolve.
http://www.freethought-forum.com/forum/gallery/files/5/0/anomalocaris.jpg
Anomalocaris may have been the Earth’s first truly effective predator.
In this illustration, it is pursuing a trilobite, an early arthropod. In the
top left portion of the picture is an early chordate.
[break=Homeobox Genes]
The third hypothesis regarding the cause of the Cambrian Explosion has to do with the evolution of an extremely important set of genes that are found in almost all animals. These genes are known as Homeobox genes, and they are crucially important because they regulate developmental processes. If, as seems likely, the first Hox gene complexes evolved in animals just before the time of the Cambrian Explosion, this would have made the Cambrian Explosion possible. Hox genes, once they evolved, gave the animals bearing them far more developmental flexibility than had been present in their ancestors, allowing for the evolution of larger and more complex body forms.
So, the Cambrian Explosion was likely due, at least in part, to the evolution of a new gene complex that made it possible for animals to build larger and more complex bodies than had heretofore been possible.
[break=The Conquest of Land]
Whatever the reasons for the Cambrian Explosion, animal diversity continued to increase throughout the remainder of the Paleozoic Era. Arthropods such as trilobites dominated the seas during the early Paleozoic. Later, larger and more formidable arthropods such as sea scorpions came to dominate the seas. Early chordates developed internal skeletons and became the first vertebrates, the fishes. Eventually, fishes replaced arthropods as the largest marine animals.
Early plants colonized the land during the early Paleozoic. Arthropods soon followed them onto the land. Some time later, vertebrates, too, moved onto the land. The first land-living vertebrates were the amphibians; they first appeared in the fossil record about 360 million years ago.
The amphibians gave rise to the reptiles, which became the dominant land animals in the late Paleozoic.
http://www.freethought-forum.com/forum/gallery/files/5/0/jaekelopterus_rhenaniae.jpg
Jaekelopterus rhenaniae, a sea scorpion.
400 million years ago, these things ruled the seas.
http://www.freethought-forum.com/forum/gallery/files/5/0/ichthyostega_original.jpg
Ichthyostega, an early amphibian – one of the first vertebrates to come out of the water and onto land.
http://www.freethought-forum.com/forum/gallery/files/5/0/dimetrodon.jpg
In the late Paleozoic, reptiles such as these Dimetrodon dominated the land.
[break=The Mesozoic Era]
[b]The Mesozoic Era :
The Paleozoic Era ended 251 million years ago, with the largest-known mass extinction event in the Earth’s history. A “mass extinction event” occurs when a relatively large percentage of the Earth’s species go extinct in a relatively short period of time.
How significant was the Permian-Triassic Mass Extinction Event, the one that ended the Paleozoic Era? By some estimates, more than 95% of the species on the planet became extinct. Entire taxa (such as the trilobites, for instance) vanished. Despite its enormous effect on the course of life on Earth, we still don’t know for sure what caused the Permian-Triassic extinction event!
The mass-extinction event that ended the Paleozoic marked the beginning of the Mesozoic Era, which lasted from 251 million years ago to 65 million years ago. This was the age of the dinosaurs. The Mesozoic, like the Paleozoic before it, ended with a mass-extinction event. The Cretaceous-Tertiary Mass-Extinction that ended the Mesozoic was nowhere near as devastating as had been the Permian-Triassic Mass-Extinction, but even so, it wiped out perhaps 60% of then-living species. It claimed all of the non-avian dinosaurs, the pterosaurs, the ammonites, the ichthyosaurs, the mosasaurs, and numerous other taxa.
The Mesozoic is divided into three periods, the Triassic, the Jurassic, and the Cretaceous. During the Triassic, the first dinosaurs and the first mammals appeared in the fossil record. A common misconception is that mammals first evolved near the end of the dinosaurs’ reign, but in fact, mammals and dinosaurs evolved at about the same time.
If anything, the misconception is exactly backwards. The ancestors of mammals were initially dominant over the ancestors of the dinosaurs. The “mammal-like reptiles” were replaced as the dominant land animals by the early dinosaurs. For the next 150 million years or so, the dinosaurs were an incredibly successful group that completely dominated the Earth’s land ecosystems.
While the dinosaurs dominated the land, pterosaurs dominated the skies. Ichthyosaurs, mosasaurs and plesiosaurs dominated the seas. Because the land, the seas and the skies were all dominated by reptiles during the Mesozoic, the era is often referred to as the “Age of Reptiles.”
http://www.freethought-forum.com/forum/gallery/files/5/0/eoraptor.jpg
Eoraptor, one of the earliest dinosaurs, lived during the mid-Triassic.
[break=The Jurassic]
The Jurassic was when the dinosaurs were at their peak. This is when the largest animals to ever walk the Earth’s surface lived. Also of note is that the first birds appear in the fossil record during the late Jurassic.
http://www.freethought-forum.com/forum/gallery/files/5/0/brachiosaurus.jpg
Brachiosaurus lived during the late Jurassic.
http://www.freethought-forum.com/forum/gallery/files/5/0/archaeopteryx_original.jpg
Archaeopteryx lithographica, one of the first birds. It lived during the late Jurassic.
[break=The Cretaceous]
The Cretaceous was when some of the most famous of the dinosaurs lived – animals such as Tyrannosaurus rex, Deinonychus and Velociraptor. It was the last era of the dinosaurs’ reign, however, as the Mesozoic, like the Paleozoic, ended with a Mass Extinction Event. The Cretaceous-Tertiary Mass Extinction killed off the non-avian dinosaurs, paving the way for the mammals to rise to prominence. It’s widely believed (but by no means proved) that the Cretaceous-Tertiary Mass Extinction was caused by a collision between the Earth and a large asteroid or comet.
http://www.freethought-forum.com/forum/gallery/files/5/0/tyrannosaurus.jpg
Tyrannosaurus rex: If you could build a time machine and travel back in time some
70 million years, this is probably[b] not what you would want to greet you upon your arrival.
http://www.freethought-forum.com/forum/gallery/files/5/0/k-t_extinction.jpg
The last moments of the Cretaceous likely looked something like this.
[b]The Cenozoic Era :
The end of the Mesozoic was a disastrous time for dinosaurs, mosasaurs, and many other animal taxa. But the extinction of the non-avian dinosaurs and other ruling reptiles left many vacant ecological niches which the mammals quickly began to fill. Accordingly, the Cenozoic, which began with the Permian-Tertiary Extinctions 65 million years ago and continues to the present, is often known as the “Age of Mammals.”
http://www.freethought-forum.com/forum/gallery/files/5/0/lion_zebra.jpg
With the extinction of the dinosaurs, mammals have come to dominate terrestrial ecosystems.
http://www.freethought-forum.com/forum/gallery/files/5/0/geological_time.jpg
A brief history of life on Earth.
http://www.freethought-forum.com/forum/gallery/files/5/0/geologic_original.jpg
The Geological Time Scale.
[break=What’s Next]
In the next chapter, we will consider the different body plans that have evolved in the Animalia. For all the diversity to be found in the animal kingdom, the basic body plans have changed very little in more than 500 million years.
Chapter Three: Anatomical Terminology and a History of Animal Life:
Anatomical Terminology:
When describing regions of an animal’s body or the positions of body structures, we use a number of paired technical terms. One thing that’s important to remember is that most of these terms are relative, and make sense only when you’re using them to make comparisons. For instance, it makes no sense to say “the elbow is proximal.” You must specify what it’s proximal to.
[b]Superior/Inferior:
A body structure or region is superior to another if it is above it. Similarly, a body structure or region is inferior to another if it is below it. Consider a cat. Normally, at least, the cat’s backbone is superior to its belly – that is, the cat’s backbone is higher above the ground. This, of course, means that the cat’s belly is inferior to its backbone.
In humans, because we walk upright, “superior” typically means “toward the head,” and “inferior” typically means “toward the feet.” Because of our unusual posture, the terms “superior” and “inferior” are quite useful in describing human anatomy, but they’re rarely used when discussing the anatomy of non-human species that typically walk on all fours (or all sixes, or all eights, or whatever).
Anterior/Posterior:
Anterior means “toward the front of the body.” The terms cranial and cephalic have much the same meaning and are sometimes used as synonyms for anterior, but “anterior” is the preferred term. This is because “cranial” and “cephalic” specifically mean “toward the head,” and many animals lack a distinct head. “Anterior,” on the other hand, always means “toward the front of the body.”
Posterior means “toward the rear of the body.” Caudal is sometimes used as a near-synonym, as it means “toward the tail.” Strictly speaking, though, “caudal” is used only when you’re describing vertebrate animals.
Again, because of our upright stance, the terms “anterior” and “posterior” have slightly different meanings when used to describe humans than when they’re used to describe a typical non-human animal. Regardless of what kind of animal you’re talking about, “anterior” always means “toward the front of the body,” but in humans, “toward the front of the body” means toward the face or belly. In most other animals, “toward the front of the body” means toward the head. Similarly, when you’re describing a human, “posterior” means “toward the back,” but when you’re describing a non-human animal, “posterior” effectively means “toward the tail.”
[b]Dorsal/Ventral:
Dorsal means “toward the back.” You’ve no-doubt noticed that in humans (again, because of our upright stance), the terms “dorsal” and “posterior” are synonymous. That’s not true of other animals. Ventral means “toward the belly.” In humans (but not other animals, of course), “ventral” and “anterior” are synonymous.
Proximal/Distal:
These terms refer to structures that project out from the main portion of the body. A body part is proximal to another if it is closer to the point of attachment or to the trunk of the body. A body part is distal to another if it’s farther away from the point of attachment or to the trunk of the body. For example, your elbow is proximal to your wrist, but distal to your shoulder.
http://www.freethought-forum.com/forum/gallery/files/5/0/anatomical_directions_and_axes_original.jpg
Anatomical directions and body axes.
[b]Superficial/Deep:
A body part is superficial (or external) to another if it is closer to the surface of the body. A body part is deep (or internal) to another if it is farther from the surface of the body. For example, your ribcage is superficial to your heart, but deep to your pectoral muscles.
Medial/Lateral:
The “medial line” of the body is an imaginary line that runs right down the middle of the body, dividing it into right and left mirror-image halves. A body structure is medial to another structure if it’s closer to the midline of the body. A structure is lateral to another structure if it’s farther away from the body midline. For example, your eyes are lateral to your nose, but medial to your ears.
http://www.freethought-forum.com/forum/gallery/files/5/0/lateralmedial_original.jpg
This young lady has graciously consented to demonstrate the
difference between “medial” and “lateral.”
A Brief History of the Kingdom Animalia:
The animal kingdom contains a great diversity of species to be sure, but something like 99% of all the animal species that have ever existed are now extinct. So if you could take a trip into the distant past, the animals you’d encounter would certainly look quite different from the ones that you see today.
[b]Animal Origins:
Surprising though it might seem, fungi and animals are actually fairly closely-related. Certainly, fungi and animals are more closely related to each other than either of them is to plants. The genetic data suggests that the split between the ancestors of fungi and the ancestors of animals occurred only about one billion years ago. (To put that into perspective, the Earth formed about 4.5 billion years ago, and life has existed on Earth for about 4 billion years.)
According to the genetic evidence, the most recent common ancestor of all surviving animal groups lived somewhere between 675 – 875 million years ago. Certainly, animals lived before that time; what is meant by “most recent common ancestor” is the most recent organism that was ancestral to all the animals that have survived to the present day.
But where did animals come from? Both morphological and genetic evidence strongly suggest that animals evolved from a group of protozoans known as the choanoflagellates. Surviving choanoflagellate protists often live in colonies, and it’s likely that the first animals evolved from colonial choanoflagellates.
http://www.freethought-forum.com/forum/gallery/files/5/0/choanoflagellate.jpg
A choanoflagellate, a very (very) distant cousin of yours and mine.
A choanoflagellate can be easily recognized by the distinctive “collar”
that partially surrounds the flagellum.
http://www.freethought-forum.com/forum/gallery/files/5/0/sphaeroeca_choanoflagellate_colony.jpg
A colony of choanoflagellates.
[b] The Four Eras of Animal Life:
Animals have existed for four geological eras, the Neoproterozoic Era, the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era.
The Neoproterozoic Era began when the first animals evolved. The oldest-known fossils of organisms that were clearly animals date back to about 565 – 550 million years ago, but genetic evidence suggest that the first animals evolved about 1 billion years ago, shortly after the split between the fungus/animal lineages. The Neoproterozoic Era ended about 542 million years ago, when the first complex animals resembling modern animals appeared in the fossil record.
The late Neoproterozoic Era is referred to as the Ediacaran, and the very early animals that lived then made up part of what’s known as the Ediacaran biota. For the most part, the soft-bodied animals that lived during the Ediacaran bore little resemblance to modern animals. Sadly almost all of those weird, wonderful animal taxa are long-since extinct.
Even so, if you could go back in time to the Ediacaran, you would see some familiar-looking animals. There were sponges living during the late Neoproterozoic, as well as animals that seemed to resemble modern jellyfishes and their relatives.
http://www.freethought-forum.com/forum/gallery/files/5/0/spriggina_floundensi_original.jpg
Spriggina floundensi lived about 550 million years ago. It may have been
an ancestor of the trilobites that dominated the Earth’s seas many millions of years later.
http://www.freethought-forum.com/forum/gallery/files/5/0/charniodiscus_arboreus_original.jpg
Charniodiscus arboreus lived around 560 million years ago. It may
have been related to modern jellyfishes.
http://www.freethought-forum.com/forum/gallery/files/5/0/ediacaran_trace_fossil.jpg
Fossilized burrows of worm-like animals that lived during the Ediacaran.
[b]The Cambrian Explosion and the Paleozoic Era :
The fossil record suggests that in the last few million years of the Neoproterozoic Era, the diversity of animal life began to increase. The ensuing Paleozoic Era lasted from about 542 – 251 million years ago, and marks the first appearance in the fossil record of abundant, complex animals that were clearly ancestral to modern animals.
The first portion of the Paleozoic Era was the Cambrian Period. Near the beginning of the Cambrian, from about 535 – 525 million years ago, the pace of animal evolution seems to have increased dramatically. Animal diversity apparently increased so rapidly during this time that the period is often referred to as the Cambrian Explosion. During this relatively short period, animals that clearly belonged to almost half of all modern animal phyla first appeared in the fossil record. These organisms include the earliest-known animals that were clearly arthropods, mollusks, echinoderms, and chordates. (We are chordates, so our phylum has been in existence for over 520 million years.)
By any reckoning, the Cambrian Explosion was a major evolutionary event. Before that time, most organisms were either unicellular or simple colonies of cells. True, some simple animals had evolved during the Neoproterozoic, but the rate of animal evolution seems to have accelerated by an order of magnitude or more during the Cambrian.
http://www.freethought-forum.com/forum/gallery/files/5/0/cambrian_original.jpg
Life in the Cambrian. Though most of the critters probably look unfamiliar, many of them were
clearly related to modern animals. Sponges were abundant, and the fish-like animals
swimming near the sponges were early chordates. Many different kinds of arthropods were
present, including trilobites. Then, as today, brachiopods, mollusks, and echinoderms crawled
along the ocean floor. The large animal in the center-top portion of the image, Anomalocaris,
may have been the Earth’s first large predator.
[b]Why Did the Cambrian Explosion Occur?:
What triggered the Cambrian Explosion is one of the great mysteries regarding the history of life. Three principle explanations have been proposed. These explanations are not mutually exclusive, and it’s entirely possible (indeed, it’s quite likely) that a multitude of factors made the Cambrian Explosion possible.
The first hypothesis has to do with the appearance of large, apparently predatory animals like Anomalocaris. Their appearance during the early Cambrian is often cited as a possible explanation for the Cambrian Explosion. According to this hypothesis, the rapid evolution of new body plans that occurred during the Cambrian may have been driven by predator/prey interactions, because once the first truly effective predators evolved, this would have initiated an evolutionary “arms race” between predators and prey.
According to this “arms race” hypothesis, prey animals evolved better senses and protective armor plating (arthropods) or shells (brachiopods, mollusks) in response to the evolution of efficient predators like Anomalocaris. As prey species evolved to be better protected, more sensitive to their environments, and more agile, predators, in turn, were forced to evolve to become faster, larger, and more powerful that they might overcome their prey’s defenses. Not only does the Cambrian mark the first time that animals with shells and body armor appear in the fossil record, it also marks the first time that animals with well-developed eyes appear in the fossil record.
If this hypothesis is correct, the rapid evolutionary innovations during the Cambrian Explosion were driven by the constant need for predators and prey to adapt to each other. As prey animals became more difficult to capture and subdue, predators evolved to become more formidable. As predators became more formidable, prey animals evolved to be even harder to catch and subdue. This created selection pressure for predators to become even more formidable. And so on, and so on.
[break=Oxygen and Animal Origins]
The second hypothesis regarding the Cambrian Explosion has to do with the amount of oxygen in the atmosphere. Plants had not yet evolved by the time of the Cambrian Explosion, but photosynthetic cyanobacteria and algae had been around for well over a billion years by then. During that time, they had been slowly adding oxygen to the atmosphere.
Animals, of course, need oxygen to power their metabolic processes. It’s likely that the Cambrian was the first time in the Earth’s history that atmospheric oxygen levels had become high-enough to support large and active animals.
This “sufficient oxygen” hypothesis is related to the first hypothesis regarding the Cambrian Explosion. If this hypothesis is correct, then the evolution of large, active predators like Anomalocaris did indeed trigger the Cambrian Explosion, but they didn’t “just happen” to evolve at that time. According to this hypothesis, the Cambrian was the first time in the Earth’s history that enough oxygen was available to permit such animals to evolve.
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Anomalocaris may have been the Earth’s first truly effective predator.
In this illustration, it is pursuing a trilobite, an early arthropod. In the
top left portion of the picture is an early chordate.
[break=Homeobox Genes]
The third hypothesis regarding the cause of the Cambrian Explosion has to do with the evolution of an extremely important set of genes that are found in almost all animals. These genes are known as Homeobox genes, and they are crucially important because they regulate developmental processes. If, as seems likely, the first Hox gene complexes evolved in animals just before the time of the Cambrian Explosion, this would have made the Cambrian Explosion possible. Hox genes, once they evolved, gave the animals bearing them far more developmental flexibility than had been present in their ancestors, allowing for the evolution of larger and more complex body forms.
So, the Cambrian Explosion was likely due, at least in part, to the evolution of a new gene complex that made it possible for animals to build larger and more complex bodies than had heretofore been possible.
[break=The Conquest of Land]
Whatever the reasons for the Cambrian Explosion, animal diversity continued to increase throughout the remainder of the Paleozoic Era. Arthropods such as trilobites dominated the seas during the early Paleozoic. Later, larger and more formidable arthropods such as sea scorpions came to dominate the seas. Early chordates developed internal skeletons and became the first vertebrates, the fishes. Eventually, fishes replaced arthropods as the largest marine animals.
Early plants colonized the land during the early Paleozoic. Arthropods soon followed them onto the land. Some time later, vertebrates, too, moved onto the land. The first land-living vertebrates were the amphibians; they first appeared in the fossil record about 360 million years ago.
The amphibians gave rise to the reptiles, which became the dominant land animals in the late Paleozoic.
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Jaekelopterus rhenaniae, a sea scorpion.
400 million years ago, these things ruled the seas.
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Ichthyostega, an early amphibian – one of the first vertebrates to come out of the water and onto land.
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In the late Paleozoic, reptiles such as these Dimetrodon dominated the land.
[break=The Mesozoic Era]
[b]The Mesozoic Era :
The Paleozoic Era ended 251 million years ago, with the largest-known mass extinction event in the Earth’s history. A “mass extinction event” occurs when a relatively large percentage of the Earth’s species go extinct in a relatively short period of time.
How significant was the Permian-Triassic Mass Extinction Event, the one that ended the Paleozoic Era? By some estimates, more than 95% of the species on the planet became extinct. Entire taxa (such as the trilobites, for instance) vanished. Despite its enormous effect on the course of life on Earth, we still don’t know for sure what caused the Permian-Triassic extinction event!
The mass-extinction event that ended the Paleozoic marked the beginning of the Mesozoic Era, which lasted from 251 million years ago to 65 million years ago. This was the age of the dinosaurs. The Mesozoic, like the Paleozoic before it, ended with a mass-extinction event. The Cretaceous-Tertiary Mass-Extinction that ended the Mesozoic was nowhere near as devastating as had been the Permian-Triassic Mass-Extinction, but even so, it wiped out perhaps 60% of then-living species. It claimed all of the non-avian dinosaurs, the pterosaurs, the ammonites, the ichthyosaurs, the mosasaurs, and numerous other taxa.
The Mesozoic is divided into three periods, the Triassic, the Jurassic, and the Cretaceous. During the Triassic, the first dinosaurs and the first mammals appeared in the fossil record. A common misconception is that mammals first evolved near the end of the dinosaurs’ reign, but in fact, mammals and dinosaurs evolved at about the same time.
If anything, the misconception is exactly backwards. The ancestors of mammals were initially dominant over the ancestors of the dinosaurs. The “mammal-like reptiles” were replaced as the dominant land animals by the early dinosaurs. For the next 150 million years or so, the dinosaurs were an incredibly successful group that completely dominated the Earth’s land ecosystems.
While the dinosaurs dominated the land, pterosaurs dominated the skies. Ichthyosaurs, mosasaurs and plesiosaurs dominated the seas. Because the land, the seas and the skies were all dominated by reptiles during the Mesozoic, the era is often referred to as the “Age of Reptiles.”
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Eoraptor, one of the earliest dinosaurs, lived during the mid-Triassic.
[break=The Jurassic]
The Jurassic was when the dinosaurs were at their peak. This is when the largest animals to ever walk the Earth’s surface lived. Also of note is that the first birds appear in the fossil record during the late Jurassic.
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Brachiosaurus lived during the late Jurassic.
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Archaeopteryx lithographica, one of the first birds. It lived during the late Jurassic.
[break=The Cretaceous]
The Cretaceous was when some of the most famous of the dinosaurs lived – animals such as Tyrannosaurus rex, Deinonychus and Velociraptor. It was the last era of the dinosaurs’ reign, however, as the Mesozoic, like the Paleozoic, ended with a Mass Extinction Event. The Cretaceous-Tertiary Mass Extinction killed off the non-avian dinosaurs, paving the way for the mammals to rise to prominence. It’s widely believed (but by no means proved) that the Cretaceous-Tertiary Mass Extinction was caused by a collision between the Earth and a large asteroid or comet.
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Tyrannosaurus rex: If you could build a time machine and travel back in time some
70 million years, this is probably[b] not what you would want to greet you upon your arrival.
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The last moments of the Cretaceous likely looked something like this.
[b]The Cenozoic Era :
The end of the Mesozoic was a disastrous time for dinosaurs, mosasaurs, and many other animal taxa. But the extinction of the non-avian dinosaurs and other ruling reptiles left many vacant ecological niches which the mammals quickly began to fill. Accordingly, the Cenozoic, which began with the Permian-Tertiary Extinctions 65 million years ago and continues to the present, is often known as the “Age of Mammals.”
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With the extinction of the dinosaurs, mammals have come to dominate terrestrial ecosystems.
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A brief history of life on Earth.
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The Geological Time Scale.
[break=What’s Next]
In the next chapter, we will consider the different body plans that have evolved in the Animalia. For all the diversity to be found in the animal kingdom, the basic body plans have changed very little in more than 500 million years.