Beginning around 318 million years ago, a major glaciation occurred that led to a major marine extinction throughout the world.
A mid-Carboniferous drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites especially hard. This sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 318 million years ago, at the onset of the Permo-Carboniferous Glaciation.
Thursday, February 28, 2013
Wednesday, February 27, 2013
320 Million B.C.T. - Reptiles Emerged
Around 320 Million B.C.T., the first reptiles evolved.
Reptiles arose about 310-320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic (outside heat reliant) metabolisms. So defined, the group is paraphyletic, excluding endothermic (internal heat reliant) animals like birds and mammals that are descended from early reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their mammal-like reptile ancestors.
A group is said to be paraphyletic if it consists of all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants, typically just one or two such groups. For example, the group of reptiles, as traditionally defined, is paraphyletic: it contains the last common ancestor of the reptiles — including the extant reptiles as well as the extinct mammal-like reptiles — along with all descendants of that ancestor except for mammals and birds.
Though few reptiles today are apex predators, many examples of apex reptiles have existed in the past. Apex predators (also known as alpha, super, top- or top-level predators) are predators with no predators of their own, residing at the top of their food chain. Reptiles have an extremely diverse evolutionary history that has led to biological successes such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.
Reptiles first arose from amphibians in the swamps of the late Carboniferous Period. Increasing evolutionary pressure and the vast untouched niches of the land powered the evolutionary changes in amphibians to gradually become more and more land based. Environmental selection propelled the development of certain traits, such as a stronger skeletal structure, muscles, and more protective coating (scales) became more favorable; and, thus, the basic foundation of reptiles were founded. The evolution of lungs and legs are the main transitional steps towards reptiles, but the development of hard-shelled external eggs replacing the amphibious water bound eggs is the defining feature of the class Reptilia and is what allowed these amphibians to fully leave water. Another major difference from amphibians is the increased brain size, more specifically, the enlarged cerebrum and cerebellum. Although their brain size is small when compared to birds and mammals, these enhancements prove vital in hunting strategies of reptiles. The increased size of these two regions of the brain allowed for improved motor skills and an increase in sensory development.
The origin of the reptiles lies about 320–310 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorph labyrinthodonts. The oldest known animal that may have been an amniote, a reptile rather than an amphibian, is Casineria (though it has also been argued to be an amphibian). A series of footprints from the fossil strata of Nova Scotia, dated to 315 million years, show typical reptilian toes and imprints of scales. The tracks are attributed to Hylonomus, the oldest unquestionable reptile known. It was a small, lizard-like animal, about 20 to 30 cm (8–12 in) long, with numerous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana (for the moment considered a reptiliomorph amphibian rather than a true amniote) and Paleothyris, both of similar build and presumably similar habit. One of the best known early reptiles is Mesosaurus, a genus from the early Permian that had returned to water, feeding on fish. The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians, such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.
Reptiles arose about 310-320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic (outside heat reliant) metabolisms. So defined, the group is paraphyletic, excluding endothermic (internal heat reliant) animals like birds and mammals that are descended from early reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their mammal-like reptile ancestors.
A group is said to be paraphyletic if it consists of all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants, typically just one or two such groups. For example, the group of reptiles, as traditionally defined, is paraphyletic: it contains the last common ancestor of the reptiles — including the extant reptiles as well as the extinct mammal-like reptiles — along with all descendants of that ancestor except for mammals and birds.
Though few reptiles today are apex predators, many examples of apex reptiles have existed in the past. Apex predators (also known as alpha, super, top- or top-level predators) are predators with no predators of their own, residing at the top of their food chain. Reptiles have an extremely diverse evolutionary history that has led to biological successes such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.
Reptiles first arose from amphibians in the swamps of the late Carboniferous Period. Increasing evolutionary pressure and the vast untouched niches of the land powered the evolutionary changes in amphibians to gradually become more and more land based. Environmental selection propelled the development of certain traits, such as a stronger skeletal structure, muscles, and more protective coating (scales) became more favorable; and, thus, the basic foundation of reptiles were founded. The evolution of lungs and legs are the main transitional steps towards reptiles, but the development of hard-shelled external eggs replacing the amphibious water bound eggs is the defining feature of the class Reptilia and is what allowed these amphibians to fully leave water. Another major difference from amphibians is the increased brain size, more specifically, the enlarged cerebrum and cerebellum. Although their brain size is small when compared to birds and mammals, these enhancements prove vital in hunting strategies of reptiles. The increased size of these two regions of the brain allowed for improved motor skills and an increase in sensory development.
The origin of the reptiles lies about 320–310 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorph labyrinthodonts. The oldest known animal that may have been an amniote, a reptile rather than an amphibian, is Casineria (though it has also been argued to be an amphibian). A series of footprints from the fossil strata of Nova Scotia, dated to 315 million years, show typical reptilian toes and imprints of scales. The tracks are attributed to Hylonomus, the oldest unquestionable reptile known. It was a small, lizard-like animal, about 20 to 30 cm (8–12 in) long, with numerous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana (for the moment considered a reptiliomorph amphibian rather than a true amniote) and Paleothyris, both of similar build and presumably similar habit. One of the best known early reptiles is Mesosaurus, a genus from the early Permian that had returned to water, feeding on fish. The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians, such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.
Wednesday, February 20, 2013
323 Million B.C.T. - The Pennsylvanian Subperiod Began
Around 323 Million B.C.T., the Pennsylvanian subperiod began.
The Pennsylvanian is the younger of two subperiods (or upper of two subsystems) of the Carboniferous Period. It lasted from roughly 323.2 ± 1.3 to 298.9 ± 0.8 million years ago. As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few million years. The Pennsylvanian is named after the American state of Pennsylvania, where the coal-productive beds of this age are widespread.
The division between the Pennsylvanian and Mississippian subperiods comes from North American stratigraphy. In North America, where the early Carboniferous beds are primarily marine limestones, the Pennsylvanian was in the past treated as a full fledged geologic period between the Mississippian and the Permian. In Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous Period. The current internationally used geologic timescale of the International Commission on Stratigraphy (ICS) gives the Mississippian and Pennsylvanian the rank of subperiods, -- subdivisions of the Carboniferous Period.
The Pennsylvanian is the younger of two subperiods (or upper of two subsystems) of the Carboniferous Period. It lasted from roughly 323.2 ± 1.3 to 298.9 ± 0.8 million years ago. As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few million years. The Pennsylvanian is named after the American state of Pennsylvania, where the coal-productive beds of this age are widespread.
The division between the Pennsylvanian and Mississippian subperiods comes from North American stratigraphy. In North America, where the early Carboniferous beds are primarily marine limestones, the Pennsylvanian was in the past treated as a full fledged geologic period between the Mississippian and the Permian. In Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous Period. The current internationally used geologic timescale of the International Commission on Stratigraphy (ICS) gives the Mississippian and Pennsylvanian the rank of subperiods, -- subdivisions of the Carboniferous Period.
Wednesday, February 13, 2013
323 Million B.C.T. - The Mississippian Subperiod Ended
Around 323 Million B.C.T., the Mississippian subperiod came to an end.
The Mississippian is a subperiod in the geologic timescale or a subsystem of the geologic record. It is the earliest/lowermost of two subperiods of the Carboniferous period lasting from roughly 358.9 ± 0.4 to 323.2 ± 0.4 million years ago. As with most other geochronologic units, the rock beds that define the Mississippian are well identified, but the exact start and end dates are uncertain by a few million years. The Mississippian is so named because rocks with this age are exposed in the Mississippi River valley.
The Mississipian was a period of marine ingression in the Northern Hemisphere. The ocean stood so high only the Fennoscandian Shield (Scandinavia) and the Laurentian Shield (Saint Lawrence and Great Lakes region) stood above sea level. The cratons (the geologic shields) were surrounded by extensive delta systems and lagoons, and carbonate sedimentation on the surrounding continental platforms, covered by shallow seas.
In North America, where the interval consists primarily of marine limestones, the Mississippian subperiod was in the past treated as a full-fledged geologic period between the Devonian and the Pennsylvanian. During the Mississippian subperiod an important phase of orogeny (mountain formation) occurred in the Appalachian Mountains.
In Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous system, and sometimes called the Upper Carboniferous and Lower Carboniferous instead.
The Mississippian is a subperiod in the geologic timescale or a subsystem of the geologic record. It is the earliest/lowermost of two subperiods of the Carboniferous period lasting from roughly 358.9 ± 0.4 to 323.2 ± 0.4 million years ago. As with most other geochronologic units, the rock beds that define the Mississippian are well identified, but the exact start and end dates are uncertain by a few million years. The Mississippian is so named because rocks with this age are exposed in the Mississippi River valley.
The Mississipian was a period of marine ingression in the Northern Hemisphere. The ocean stood so high only the Fennoscandian Shield (Scandinavia) and the Laurentian Shield (Saint Lawrence and Great Lakes region) stood above sea level. The cratons (the geologic shields) were surrounded by extensive delta systems and lagoons, and carbonate sedimentation on the surrounding continental platforms, covered by shallow seas.
In North America, where the interval consists primarily of marine limestones, the Mississippian subperiod was in the past treated as a full-fledged geologic period between the Devonian and the Pennsylvanian. During the Mississippian subperiod an important phase of orogeny (mountain formation) occurred in the Appalachian Mountains.
In Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous system, and sometimes called the Upper Carboniferous and Lower Carboniferous instead.
Monday, February 11, 2013
340 Million B.C.T. - The Egg Evolved
Around 340 million years ago, the first egg bearing animals -- the first amniotes -- appeared on the Earth.
The amniotes are a group of tetrapods (four-limbed animals with backbones or spinal columns) that have an egg equipped with an amnios, an adaptation to lay eggs on land rather than in water as anamniotes do. They include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes, and the lack of a larval stage, distinguish amniotes from tetrapod amphibians.
The first amniotes (referred to as "basal amniotes"), such as Casineria, resembled small lizards and had evolved from the amphibian reptiliomorphs about 340 million years ago, in the Carboniferous geologic period. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms.
The amniotic egg represents a critical divergence within the vertebrates, one enabled to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread across the globe, eventually to become the dominant land vertebrates.
Very early in the evolutionary history of amniotes, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is Protoclepsydrops from about 320 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (ca. 306-312 million years ago).
The first amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. Their eggs were small and covered with a leathery membrane, not a hard shell like those of birds or crocodiles. Although some modern amphibians lay eggs on land, with or without significant protection, they all lack advanced traits like an amnion. This kind of egg became possible only with internal fertilization. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land, as species evolved to lay their eggs on land where they were safer than in the water. The ancestors of the amniotes probably laid their eggs in moist places, as such modest-sized animals would not have difficulty finding depressions under fallen logs or other suitable places in the ancient forests; and dry conditions were probably not the main reason the soft shell emerged. Indeed, many modern day amniotes are dependent on moisture to keep their eggs from desiccating.
Amniotes can be characterized in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in amphibians. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis. The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments; or the massive size and yolk content of eggs may have evolved to allow direct development of the embryo to a larger size.
Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois evolved to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta.
The amniotes are a group of tetrapods (four-limbed animals with backbones or spinal columns) that have an egg equipped with an amnios, an adaptation to lay eggs on land rather than in water as anamniotes do. They include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes, and the lack of a larval stage, distinguish amniotes from tetrapod amphibians.
The first amniotes (referred to as "basal amniotes"), such as Casineria, resembled small lizards and had evolved from the amphibian reptiliomorphs about 340 million years ago, in the Carboniferous geologic period. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms.
The amniotic egg represents a critical divergence within the vertebrates, one enabled to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread across the globe, eventually to become the dominant land vertebrates.
Very early in the evolutionary history of amniotes, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is Protoclepsydrops from about 320 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (ca. 306-312 million years ago).
The first amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. Their eggs were small and covered with a leathery membrane, not a hard shell like those of birds or crocodiles. Although some modern amphibians lay eggs on land, with or without significant protection, they all lack advanced traits like an amnion. This kind of egg became possible only with internal fertilization. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land, as species evolved to lay their eggs on land where they were safer than in the water. The ancestors of the amniotes probably laid their eggs in moist places, as such modest-sized animals would not have difficulty finding depressions under fallen logs or other suitable places in the ancient forests; and dry conditions were probably not the main reason the soft shell emerged. Indeed, many modern day amniotes are dependent on moisture to keep their eggs from desiccating.
Amniotes can be characterized in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in amphibians. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis. The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments; or the massive size and yolk content of eggs may have evolved to allow direct development of the embryo to a larger size.
Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois evolved to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta.
Friday, February 8, 2013
360 Million B.C.T. - The Great Fossil Mystery
From 360 Million B.C.T. to 345 Million B.C.T., a gap in the fossil record occurred. It is as though after destroying most of the Creation that occurred during the Devonian Period, God paused for 15 million years, before beginning the Creation of the Carboniferous Period.
Romer's Gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist Dr. Alfred Romer, who first recognised it.
Romer's gap ran from approximately 360 to 345 million years ago, corresponding to the first 15 million years of the Carboniferous Period. The gap forms a discontinuity between the primitive forests and high diversity of fishes in the end Devonian and more modern aquatic and terrestrial assemblages of the early Carboniferous.
There has been long debate as to why there are so few fossils from this time period. Some have suggested the problem was of fossilization itself, suggesting that there may have been differences in the geochemistry of the time that did not favor fossil formation. Also, excavators simply may not have dug in the right places. However, the existence of a true low point in vertebrate diversity has been supported by independent lines of evidence.
While initial arthropod terrestriality was well under way before the gap, and some digited tetrapods might have come on land, there are remarkably few terrestrial or aquatic fossils that date from the gap itself. Recent work on Paleozoic geochemistry has confirmed the biological reality of Romer's gap in both terrestrial vertebrates and arthropods, and has correlated it with a period of unusually low atmospheric oxygen concentration, which was independently determined from the idiosyncratic geochemistry of rocks formed during Romer's gap.
Aquatic vertebrates, which include most tetrapods during the Carboniferous, were recovering from a Late Devonian extinction -- a major extinction event that preceded Romer's gap, one on par with that which killed the dinosaurs. In the Late Devonian extinction, most marine and freshwater groups went extinct or were reduced to a few lineages, although the precise mechanism of the extinction is unclear. Before the Late Devonian extinction, oceans and lakes were dominated by lobe-finned fishes and armored fishes called placoderms. After Romer's gap, modern ray finned fish, as well as sharks and their relatives were the dominant forms. The period also saw the demise of the Ichthyostegalia, the early fish-like amphibians with more than five digits.
The low diversity of marine fishes, particularly shell-crushing predators (durophages), at the beginning of Romer's gap is supported by the sudden abundance of hard-shelled crinoid echinoderms during the same period. The Tournaisian stage -- the first fifteen million years of the Carboniferous Period that corresponds to the period of the Romer's Gap -- has even been called the "Age of Crinoids". Once the number of shell-crushing ray-finned fishes and sharks increased later in the Carboniferous, coincident with the end of Romer's gap, the diversity of crinoids with Devonian-type armor plummeted, following the pattern of a classic predator-prey cycle.
The gap in the tetrapod record has been progressively closed with the discoveries of such early Carboniferous tetrapods as Pederpes and Crassigyrinus. There are a few sites where vertebrate fossils have been found to help fill in the gap, such as the East Kirkton Quarry, in Bathgate, Scotland, a long-known fossil site that was revisited by Stanley P. Wood in 1984 and has since been revealing a number of early tetrapods in the mid Carboniferous; "literally dozens of tetrapods came rolling out: Balanerpeton (a temnospondyl), Silvanerpeton and Eldeceeon (basal anthracosaurs), all in multiple copies, and one spectacular proto-amniote, Westlothiana", Paleos Project reports. However, tetrapod material in the earliest stage of the Carboniferous, the Tournaisian, is typically scarce relative to fishes in the same habitats, which can appear in large death assemblages, and is unknown until late in the stage. Fish faunas from Tournaisian sites around the world are very alike in composition, containing common and ecologically similar species of ray-finned fishes, rhizodont lobe-finned fishes, acanthodians, sharks, and holocephalans.
For many years after Romer's gap was first recognized, only two sites yielding Tournaisian-age tetrapod fossils were known: one in East Lothian, Scotland and another in Blue Beach, Nova Scotia. In 1841, William Logan, the first Director of the Geological Survey of Canada, found footprints from a tetrapod. Blue Beach maintains a fossil museum and displays hundreds of fossils from this period that continue to be found as the cliff continues to reveal new fossils as it continues to erode. In 2012, tetrapod remains from four new Tournaisian sites in Scotland were announced. These localities are the coast of Burnmouth, the banks of the Whiteadder Water near Chirnside, the River Tweed near Coldstream, and the rocks near Tantallon Castle alongside the Firth of Forth. Fossils of both aquatic and terrestrial tetrapods are known from these localities, providing an important record of the transition between life in water and life on land and filling some of the lacunae (missing fossil record) in Romer's gap. These new localities may represent a larger fauna, as all lie within a short distance of each other and share many fishes with the nearby and contemporary Foulden fish bed locality (which has not produced tetrapods thus far).
Romer's Gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist Dr. Alfred Romer, who first recognised it.
Romer's gap ran from approximately 360 to 345 million years ago, corresponding to the first 15 million years of the Carboniferous Period. The gap forms a discontinuity between the primitive forests and high diversity of fishes in the end Devonian and more modern aquatic and terrestrial assemblages of the early Carboniferous.
There has been long debate as to why there are so few fossils from this time period. Some have suggested the problem was of fossilization itself, suggesting that there may have been differences in the geochemistry of the time that did not favor fossil formation. Also, excavators simply may not have dug in the right places. However, the existence of a true low point in vertebrate diversity has been supported by independent lines of evidence.
While initial arthropod terrestriality was well under way before the gap, and some digited tetrapods might have come on land, there are remarkably few terrestrial or aquatic fossils that date from the gap itself. Recent work on Paleozoic geochemistry has confirmed the biological reality of Romer's gap in both terrestrial vertebrates and arthropods, and has correlated it with a period of unusually low atmospheric oxygen concentration, which was independently determined from the idiosyncratic geochemistry of rocks formed during Romer's gap.
Aquatic vertebrates, which include most tetrapods during the Carboniferous, were recovering from a Late Devonian extinction -- a major extinction event that preceded Romer's gap, one on par with that which killed the dinosaurs. In the Late Devonian extinction, most marine and freshwater groups went extinct or were reduced to a few lineages, although the precise mechanism of the extinction is unclear. Before the Late Devonian extinction, oceans and lakes were dominated by lobe-finned fishes and armored fishes called placoderms. After Romer's gap, modern ray finned fish, as well as sharks and their relatives were the dominant forms. The period also saw the demise of the Ichthyostegalia, the early fish-like amphibians with more than five digits.
The low diversity of marine fishes, particularly shell-crushing predators (durophages), at the beginning of Romer's gap is supported by the sudden abundance of hard-shelled crinoid echinoderms during the same period. The Tournaisian stage -- the first fifteen million years of the Carboniferous Period that corresponds to the period of the Romer's Gap -- has even been called the "Age of Crinoids". Once the number of shell-crushing ray-finned fishes and sharks increased later in the Carboniferous, coincident with the end of Romer's gap, the diversity of crinoids with Devonian-type armor plummeted, following the pattern of a classic predator-prey cycle.
The gap in the tetrapod record has been progressively closed with the discoveries of such early Carboniferous tetrapods as Pederpes and Crassigyrinus. There are a few sites where vertebrate fossils have been found to help fill in the gap, such as the East Kirkton Quarry, in Bathgate, Scotland, a long-known fossil site that was revisited by Stanley P. Wood in 1984 and has since been revealing a number of early tetrapods in the mid Carboniferous; "literally dozens of tetrapods came rolling out: Balanerpeton (a temnospondyl), Silvanerpeton and Eldeceeon (basal anthracosaurs), all in multiple copies, and one spectacular proto-amniote, Westlothiana", Paleos Project reports. However, tetrapod material in the earliest stage of the Carboniferous, the Tournaisian, is typically scarce relative to fishes in the same habitats, which can appear in large death assemblages, and is unknown until late in the stage. Fish faunas from Tournaisian sites around the world are very alike in composition, containing common and ecologically similar species of ray-finned fishes, rhizodont lobe-finned fishes, acanthodians, sharks, and holocephalans.
For many years after Romer's gap was first recognized, only two sites yielding Tournaisian-age tetrapod fossils were known: one in East Lothian, Scotland and another in Blue Beach, Nova Scotia. In 1841, William Logan, the first Director of the Geological Survey of Canada, found footprints from a tetrapod. Blue Beach maintains a fossil museum and displays hundreds of fossils from this period that continue to be found as the cliff continues to reveal new fossils as it continues to erode. In 2012, tetrapod remains from four new Tournaisian sites in Scotland were announced. These localities are the coast of Burnmouth, the banks of the Whiteadder Water near Chirnside, the River Tweed near Coldstream, and the rocks near Tantallon Castle alongside the Firth of Forth. Fossils of both aquatic and terrestrial tetrapods are known from these localities, providing an important record of the transition between life in water and life on land and filling some of the lacunae (missing fossil record) in Romer's gap. These new localities may represent a larger fauna, as all lie within a short distance of each other and share many fishes with the nearby and contemporary Foulden fish bed locality (which has not produced tetrapods thus far).
Thursday, February 7, 2013
360 Million B.C.T. - The Carboniferous Period Began
Around 360 million years ago, the Carboniferous Period began.
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 million years ago, to the beginning of the Permian Period, about 299.0 ± 0.8 million years ago. The name Carboniferous means "coal-bearing" and derives from the Latin words carbo (coal) and ferre (to carry), and was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern 'system' names to be employed, and reflects the fact that many coal beds were formed globally during this time. The Carboniferous is often treated in North America as two geological periods, the earlier Mississippian and the later Pennsylvanian.
Terrestrial life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would eventually evolve into reptiles, the first fully terrestrial vertebrates. Arthropods (insects, arachnids, and crustaceans) were also very common, and many (such as Meganeura), were much larger than those of today. Vast swaths of forest covered the land, which would eventually be laid down and become the coal beds characteristic of the Carboniferous system. A minor marine and terrestrial extinction event occurred in the middle of the period, caused by a change in climate. The latter half of the period experienced glaciations, low sea level, and mountain building as the continents collided to form Pangaea.
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 million years ago, to the beginning of the Permian Period, about 299.0 ± 0.8 million years ago. The name Carboniferous means "coal-bearing" and derives from the Latin words carbo (coal) and ferre (to carry), and was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern 'system' names to be employed, and reflects the fact that many coal beds were formed globally during this time. The Carboniferous is often treated in North America as two geological periods, the earlier Mississippian and the later Pennsylvanian.
Terrestrial life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would eventually evolve into reptiles, the first fully terrestrial vertebrates. Arthropods (insects, arachnids, and crustaceans) were also very common, and many (such as Meganeura), were much larger than those of today. Vast swaths of forest covered the land, which would eventually be laid down and become the coal beds characteristic of the Carboniferous system. A minor marine and terrestrial extinction event occurred in the middle of the period, caused by a change in climate. The latter half of the period experienced glaciations, low sea level, and mountain building as the continents collided to form Pangaea.
Wednesday, February 6, 2013
360 Million B.C.T. - The Devonian Period Ends
Around 360 million years ago, the Devonian Period came to an end.
Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian Period, it is not clear over how long a period these extinctions took place, with estimates varying from 500 thousand to 15 million years. Neither is it clear whether it was a single mass extinction or a series of several smaller ones one after the other. Nevertheless, the balance of evidence suggests that the extinctions took place over a period of some three million years beginning about 374 million years ago.
During the late Devonian extinction, as many as seventy percent of all species vanished. Marine species were more severely affected than those in freshwater. Brachiopods, ammonites, and many other invertebrates suffered heavily, as did agnathan and placoderm fish. On land, where plants were diversifying and amphibians were beginning their evolution, there seem to have been far fewer losses.
The causes of the Devonian extinction are unclear. The disproportionate losses amongst warm water species suggest that the Earth's climate changed -- most likely for the cooler. The global cooling was an important factor and it has been suggested that this was associated with (or may have even caused) a drop in the oxygen levels of the shallower waters.
Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian Period, it is not clear over how long a period these extinctions took place, with estimates varying from 500 thousand to 15 million years. Neither is it clear whether it was a single mass extinction or a series of several smaller ones one after the other. Nevertheless, the balance of evidence suggests that the extinctions took place over a period of some three million years beginning about 374 million years ago.
During the late Devonian extinction, as many as seventy percent of all species vanished. Marine species were more severely affected than those in freshwater. Brachiopods, ammonites, and many other invertebrates suffered heavily, as did agnathan and placoderm fish. On land, where plants were diversifying and amphibians were beginning their evolution, there seem to have been far fewer losses.
The causes of the Devonian extinction are unclear. The disproportionate losses amongst warm water species suggest that the Earth's climate changed -- most likely for the cooler. The global cooling was an important factor and it has been suggested that this was associated with (or may have even caused) a drop in the oxygen levels of the shallower waters.
Tuesday, February 5, 2013
370 Million B.C.T. - Amphibians Appeared
Around 370 million years ago, amphibians evolved.
Amphibians are ectothermic (relies on outside heat sources), tetrapod (four-limbed) vertebrates of the class Amphibia. They inhabit a wide variety of habitats with most species living within terrestrial, fossorial (underground), arboreal or freshwater aquatic ecosystems. Amphibians typically start out as larva living in water, but some species have developed behavioral adaptations to bypass this. The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely upon skin. They are superficially similar to reptiles but, along with mammals and birds, reptiles are amniotes (egg layers) and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators and in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians evolved in the Devonian Period from sarcopterygian (lobe-finned) fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia. The three modern orders of amphibians are Anura (the frogs and toads), Caudata/Urodela (the salamanders), and Gymnophiona/Apoda (the caecilians [worm or snake like amphibians]). The total number of known amphibian species is approximately 7,000, of which nearly ninety percent (90%) are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) Chinese Giant Salamander (Andrias davidianus) but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.
The first major groups of amphibians developed in the Devonian period, around 370 million years ago, from lobe-finned fish similar to the modern coelacanth and lungfish, which had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills.
Ichthyostega was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, Eusthenopteron. Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to cope with the increased gravitational effect of life on land. They developed "hands" and "feet" with five or more digits; the skin became more capable of retaining body fluids and resisting desiccation. The fish's hyomandibula (jaw) bone in the hyoid region behind the gills diminished in size and became the stapes (ear bone) of the amphibian ear, an adaptation necessary for hearing on dry land. An affinity between the amphibians and the teleost (ray finned) fish is the multi-folded structure of the teeth and the paired supra-occipital bones at the back of the head, neither of these features being found elsewhere in the animal kingdom.
At the end of the Devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates, though some, such as Ichthyostega, may have sometimes hauled themselves out of the water. It is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the elephant seal. In the early Carboniferous (360 to 345 million years ago), the climate became wet and warm. Extensive swamps developed with mosses, ferns, horsetails and calamites. Air-breathing arthropods evolved and invaded the land where they provided food for the carnivorous amphibians that began to adapt to the terrestrial environment. There were no other tetrapods on the land and the amphibians were at the top of the food chain, occupying the ecological position currently held by the crocodile. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. They were the top land predators, sometimes reaching several meters in length, preying on the large insects of the period and the many types of fish in the water. They still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. It was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their dominance in the period that followed.
During the Triassic Period (250 to 200 million years ago), the reptiles began to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the biosphere. According to the fossil record, Lissamphibia, which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence precludes precise dating, but the most recent molecular study suggests a Late Carboniferous/Early Permian origin of extant amphibians.
The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A 2005 molecular phylogeny, based on rDNA analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs. It also appears that the divergence of the three groups took place in the Paleozoic or early Mesozoic (around 250 million years ago), before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fish. The briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils. There are large gaps in the fossil record, but the discovery of a proto-frog from the Early Permian in Texas in 2008 provided a missing link with many of the characteristics of modern frogs. Molecular analysis suggests that the frog–salamander divergence took place considerably earlier than the palaeontological evidence indicates.
As they evolved from lunged fish, amphibians had to make certain adaptations for living on land including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward but on land, quite different mechanisms were required. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli via the medium of air. They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviors suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.
Amphibians are ectothermic (relies on outside heat sources), tetrapod (four-limbed) vertebrates of the class Amphibia. They inhabit a wide variety of habitats with most species living within terrestrial, fossorial (underground), arboreal or freshwater aquatic ecosystems. Amphibians typically start out as larva living in water, but some species have developed behavioral adaptations to bypass this. The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely upon skin. They are superficially similar to reptiles but, along with mammals and birds, reptiles are amniotes (egg layers) and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators and in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians evolved in the Devonian Period from sarcopterygian (lobe-finned) fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia. The three modern orders of amphibians are Anura (the frogs and toads), Caudata/Urodela (the salamanders), and Gymnophiona/Apoda (the caecilians [worm or snake like amphibians]). The total number of known amphibian species is approximately 7,000, of which nearly ninety percent (90%) are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) Chinese Giant Salamander (Andrias davidianus) but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.
The first major groups of amphibians developed in the Devonian period, around 370 million years ago, from lobe-finned fish similar to the modern coelacanth and lungfish, which had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills.
Ichthyostega was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, Eusthenopteron. Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to cope with the increased gravitational effect of life on land. They developed "hands" and "feet" with five or more digits; the skin became more capable of retaining body fluids and resisting desiccation. The fish's hyomandibula (jaw) bone in the hyoid region behind the gills diminished in size and became the stapes (ear bone) of the amphibian ear, an adaptation necessary for hearing on dry land. An affinity between the amphibians and the teleost (ray finned) fish is the multi-folded structure of the teeth and the paired supra-occipital bones at the back of the head, neither of these features being found elsewhere in the animal kingdom.
At the end of the Devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates, though some, such as Ichthyostega, may have sometimes hauled themselves out of the water. It is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the elephant seal. In the early Carboniferous (360 to 345 million years ago), the climate became wet and warm. Extensive swamps developed with mosses, ferns, horsetails and calamites. Air-breathing arthropods evolved and invaded the land where they provided food for the carnivorous amphibians that began to adapt to the terrestrial environment. There were no other tetrapods on the land and the amphibians were at the top of the food chain, occupying the ecological position currently held by the crocodile. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. They were the top land predators, sometimes reaching several meters in length, preying on the large insects of the period and the many types of fish in the water. They still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. It was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their dominance in the period that followed.
During the Triassic Period (250 to 200 million years ago), the reptiles began to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the biosphere. According to the fossil record, Lissamphibia, which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence precludes precise dating, but the most recent molecular study suggests a Late Carboniferous/Early Permian origin of extant amphibians.
The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A 2005 molecular phylogeny, based on rDNA analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs. It also appears that the divergence of the three groups took place in the Paleozoic or early Mesozoic (around 250 million years ago), before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fish. The briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils. There are large gaps in the fossil record, but the discovery of a proto-frog from the Early Permian in Texas in 2008 provided a missing link with many of the characteristics of modern frogs. Molecular analysis suggests that the frog–salamander divergence took place considerably earlier than the palaeontological evidence indicates.
As they evolved from lunged fish, amphibians had to make certain adaptations for living on land including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward but on land, quite different mechanisms were required. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli via the medium of air. They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviors suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.
Subscribe to:
Posts (Atom)