Another in an occasional series
In part one we left off at the Jurassic Age. Which is when the mammals first began to diversify.
The Jurassic is when the three modern groups of mammals first appeared. The monotremes (more on which later), the marsupials, and the placentals. There were other groups, none of which survived to modern times.
In the history of life there have been only three major mammalian groups, the marsiupials, the placentals, and the multituberculates. The last small, egglaying mammals that filled essentially the same niche the rodents do today. As a matter of fact, it was the rise of rodents with the evolution of grain bearing plants that drove the multituberculates into extinction. According to available evidence the monotremes have always been rare. But unlike other small mammalian groups, adaptable enough to survive to the present day.
[sidebar]
b]Are Monotremes Mammals?[/b]
Officially they are. But, monotremes do have a number of reptilian features. For instance, the platypus has sprawling limbs much like a lizard, and both the platypus and echidna have basal temperatures that vary greatly during the course of the day. The platypus's more so than the echidna's. Then you have the cloaca. There are other reptilian traits, but those are the ones I could recall off-hand. There are even some reptilian features to the monotreme jaw hinge. For these reasons some people say the monotremes should be classifed as therapsids.
And since I'm on the subject of monotremes, they are not true egg layers. As with some species of snake, they retain their eggs inside their bodies, laying said egg a few days before they are about to hatch. The monotreme eggshell is a 'shell' of a thing. A translucent shell that would collapse under its own weight were it to be layed when first formed.[/sidebar]
[sidebar]
How Far Back do the Mammals Go?
Officially the first mammals appear during the middle Triassic, but there are a few who say that the first true mammals appeared in the late Permian, with the appearance of the mammalian jaw hinge. The thing is, this jaw hinge appeared alongside the reptilian jaw hinge in the same animals, and wouldn't disappear until the middle Triassic. At the same time there are others who think the very earliest mammals should be counted among the mammal-like reptiles because they exhibit reptilian traits.[/sidebar]
The immediate ancestors of the marsupials are unkown. It is known that the marsupials gave rise to the placentals. Among the marsupials the reptilian cloaca disappeared, replaced by the colon and the uretha among males, and the colon, uretha, and vagina in females. In addition, the egg disappeared, to be replaced with a rudimentary placenta through which the embryo is fed for a short time before birth.
But, the pouch for which marsupials are famed was a late development. The very first marsupials likely cared for their young much as monotremes now do. In later species folds of skin appeared which held the young against the mother's body -the possums of Australia have those skin folds, the American opossums in contrast have a true pouch. But these folds only develop during pregnancy, and disappear when the young are old enough to go out on their own. It is with the rise of the wombats, koalas, and roos that the permanent pouch first appeared.
The placentals went the marsupials one further. It is in this group that the true womb appeared, along with a placenta capable of supporting a fetus full term. This isn't as unique as one may think. There are species of placental snakes, and even a placental bee. But, none of them have anything that corresponds to the placental mammal's womb.
[sidebar]
Placental Snakes?
Yep. Snakes as a group range from egg layers to live bearers. True live bearers. There are some species where certain specimens are egg layers, while others are live bearers. Then there are those that follow one of the methods below:
Egg Retention: The female produces fully formed eggs, but retains them in her body until they are just about to hatch. In some cases the young actually hatch out while still inside mom.
Egg Retention, Vestigial Egg: Here the mother forms an egg, but the egg shell is thin and translucent. In some cases the chorion (the egg layer's version of a placenta) penetrates the shell and may form connections with the mother's circulatory system. A nascent placenta in a sense.
Fetus Retention, Pseudo Placenta: The eggshell is lost, the yolk sack may be vestigal or even missing entirely. The chorion still forms, but it connects with the mothers circulatory system. The young are fed through this, but it does not carry off waste. Snakes that use this system are included among the live bearers, but provisionally.
Live Bearing: These snakes bear live young. Instead of a chorion through which the young are fed mother and child form intervowen networks of veins and arteries. A true placenta. Nutrients are provided to the fetus and waste is carried away. some species of corn snake, a few of the rattlers, and the advanced sea snakes are true live bearers.
As you can see, there are placental snakes.
[/sidebear]
At this point I must close this, because the sidebars took up more space than I thought they would. In part three we'll deal with the placental mammals up to the end of the Cretaceous.
In part one we left off at the Jurassic Age. Which is when the mammals first began to diversify.
The Jurassic is when the three modern groups of mammals first appeared. The monotremes (more on which later), the marsupials, and the placentals. There were other groups, none of which survived to modern times.
In the history of life there have been only three major mammalian groups, the marsiupials, the placentals, and the multituberculates. The last small, egglaying mammals that filled essentially the same niche the rodents do today. As a matter of fact, it was the rise of rodents with the evolution of grain bearing plants that drove the multituberculates into extinction. According to available evidence the monotremes have always been rare. But unlike other small mammalian groups, adaptable enough to survive to the present day.
[sidebar]
b]Are Monotremes Mammals?[/b]
Officially they are. But, monotremes do have a number of reptilian features. For instance, the platypus has sprawling limbs much like a lizard, and both the platypus and echidna have basal temperatures that vary greatly during the course of the day. The platypus's more so than the echidna's. Then you have the cloaca. There are other reptilian traits, but those are the ones I could recall off-hand. There are even some reptilian features to the monotreme jaw hinge. For these reasons some people say the monotremes should be classifed as therapsids.
And since I'm on the subject of monotremes, they are not true egg layers. As with some species of snake, they retain their eggs inside their bodies, laying said egg a few days before they are about to hatch. The monotreme eggshell is a 'shell' of a thing. A translucent shell that would collapse under its own weight were it to be layed when first formed.[/sidebar]
[sidebar]
How Far Back do the Mammals Go?
Officially the first mammals appear during the middle Triassic, but there are a few who say that the first true mammals appeared in the late Permian, with the appearance of the mammalian jaw hinge. The thing is, this jaw hinge appeared alongside the reptilian jaw hinge in the same animals, and wouldn't disappear until the middle Triassic. At the same time there are others who think the very earliest mammals should be counted among the mammal-like reptiles because they exhibit reptilian traits.[/sidebar]
The immediate ancestors of the marsupials are unkown. It is known that the marsupials gave rise to the placentals. Among the marsupials the reptilian cloaca disappeared, replaced by the colon and the uretha among males, and the colon, uretha, and vagina in females. In addition, the egg disappeared, to be replaced with a rudimentary placenta through which the embryo is fed for a short time before birth.
But, the pouch for which marsupials are famed was a late development. The very first marsupials likely cared for their young much as monotremes now do. In later species folds of skin appeared which held the young against the mother's body -the possums of Australia have those skin folds, the American opossums in contrast have a true pouch. But these folds only develop during pregnancy, and disappear when the young are old enough to go out on their own. It is with the rise of the wombats, koalas, and roos that the permanent pouch first appeared.
The placentals went the marsupials one further. It is in this group that the true womb appeared, along with a placenta capable of supporting a fetus full term. This isn't as unique as one may think. There are species of placental snakes, and even a placental bee. But, none of them have anything that corresponds to the placental mammal's womb.
[sidebar]
Placental Snakes?
Yep. Snakes as a group range from egg layers to live bearers. True live bearers. There are some species where certain specimens are egg layers, while others are live bearers. Then there are those that follow one of the methods below:
Egg Retention: The female produces fully formed eggs, but retains them in her body until they are just about to hatch. In some cases the young actually hatch out while still inside mom.
Egg Retention, Vestigial Egg: Here the mother forms an egg, but the egg shell is thin and translucent. In some cases the chorion (the egg layer's version of a placenta) penetrates the shell and may form connections with the mother's circulatory system. A nascent placenta in a sense.
Fetus Retention, Pseudo Placenta: The eggshell is lost, the yolk sack may be vestigal or even missing entirely. The chorion still forms, but it connects with the mothers circulatory system. The young are fed through this, but it does not carry off waste. Snakes that use this system are included among the live bearers, but provisionally.
Live Bearing: These snakes bear live young. Instead of a chorion through which the young are fed mother and child form intervowen networks of veins and arteries. A true placenta. Nutrients are provided to the fetus and waste is carried away. some species of corn snake, a few of the rattlers, and the advanced sea snakes are true live bearers.
As you can see, there are placental snakes.
[/sidebear]At this point I must close this, because the sidebars took up more space than I thought they would. In part three we'll deal with the placental mammals up to the end of the Cretaceous.
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