Drapetomaniac wrote:
naturalplastic wrote:
The whole point of being a reptile was the innovation of egg laying that enabled the offspring to develop in a hardshelled little container away from water. That plus having watertight skin on the adult animals. So the first reptiles were by definition already free of the water, and already layed modern bird like eggs. So the ancestors of all reptiles, and all the creatures who later descended from reptiles (dinosaurs, birds, and mammals) were fully functioning egg layers. So what you're saying here isnt even possible, and its virtually the opposite of the truth.
Watertight skin evolved first, its relatively simple, its just a barrier. An egg however its more complicated, it has to be sealed without suffocating the embryo, and being able to hatch. So at some point you had reptile like creatures, that had to lay there eggs in water like an amphibian would. I'm not going to split hairs about what is the definition of a reptile. These were intermediate forms between amphibians and reptiles/mammals, today totally extinct.
Basically Orwell says something similar when he talks about the phylogeny (but he uses jargonese).
Well first of all- David Attenborough said that the mammals at the dawn of the dinosaur era "may have laid eggs".
Second : unlike your book the encyclopedea Britannica does not say "mammals split off before reptiles did" it says " the group of reptiles that first branched off from the stem reptiles was the group that led to mammals". A similar, but not identical statement that I believe is a more accurate way of putting it becaus the fossil record shows that it was a long time and a lot of evolution occured between the time we branched off from the stem reptiles and the time we became primitive mammals.
The reptiles family tree includes groups not closely related. The tutara in New Zealand and turtles lay eggs but on the family tree they are about as far from lizards as are humans.
If unrelated groups of living reptiles lay eggs, and birds lay eggs, and the primitive mammalian duckbill platypus lays eggs, its resonable to assume that the common ancestor of all of those disparate creatures did too- which would me our ancestor did too. That would be far more likely than thinking that the platypus reinvented egg laying itself. It seems more reasonable to think that we went through a phase of being furry, but laying eggs, like the platypus, and then moved on from egg laying. Fur and endothermy came first- then we got our reproductive act together later.
The fossil record shows that in fact that we did go through a long a phase of "looking like modern reptiles" during the Permian. The demitradon ( the lizard like creature with the sail on its back) was related to the ancestors of the much latter mammal-like reptiles ( and even those arent quite mammals) and it was quite reptilian looking.
Actually both statements of who branched off first are oversimplifications.
What happened was that vertibrates came on land, and what we call reptiles split off into several groups. The two most important groups were the diapsids ( creature with two arches in their skulls) and the therapsids ( creatures with three arches in their skulls).
Earth has been a battle ground between the diapsids and therapsids for the last 400 million years.
The diapsids are your basic lizardlike reptiles- lizards snakes gators ( but not turtles- theyre are third group).
The therapsids became demitrodon, and then the Mammallike reptiels who dominated the planet for a long time.
But then the diapsid archasaurs took over and almost drove the therapsids to extinction.
The archosaurs became the dinosaurs, and therapsids spawned the primitive mammals.
Dinosaurs and mammels coexisted for 140 million years but the mammals were the oppressed low life noctrernal rodentlike creatures - the dinosaurs had all of the big animal niches. So the diapsids had the upper hand.
During this time a group of dinosaurs gave rise to the birds.
Then the comet hit. The dinosaurs died out.
The mammals grabed the large animal niches vacated by the dinosaurs.
But the birds took over the skies and actually have more living species than mammals.
But the night sky was siezed by flying mammals ( one sixth of all mammal species are bats).
So even though the post dinosaur era is the called the "aga of mammals" its really still a battle ground between the diapsids (birds) and the therapsids (mammals) with no obviousl winner even after 400 million years!
But the therapsids write the books so we can declare that weve won!
30 Jan 2011, 6:35 pm
