Tetrapods and AmniotesAmphibians and other non-Amniote TetrapodsA Look at the Earliest TetrapodsWhat is classically referred to the amphibia is a paraphyletic group
including the ancestors of the amniotes as well as the ichthyostegids
which were ancestral to all the tetrapods. The living amphibians are
monophyletic, along with various other extinct forms we can get away
with calling them amphibians. Thus, the lissamphibia which include the
living amphibians and the temnospondyls, along with the leposphondyls
(also a questionable group) make up the amphibians. Ichthyostegids
Temnospondyls
Lissamphibians
Lepospondyls
"Anthracosaurs"
Seymouria at U of Arizona Tree of Life Phylogeny and tetrapods at university of texas Someone elses lecture on "amphibians" at U of Michigan more stuff on earlly "amphibians" Baphetidae Berkely site for tetrapods Amniote Eggs: Freedom From the Swampamphibian eggsRequire water to survive, and amphibians hatch as aquatic larvae (tadpoles). Thus, before the development of the amniote egg, tetrapods had to return to the water to reproduce.What is an Amniote Egg?Amnion is a membrane surrounding the embryo which contains salt-water solution (amniotic fluid). Attached to the embryo is a yolk sack for feeding it, and an allontois for waste. Surrounding these things is the albumin, or egg white, which cushions the embryo. This in turn is surrounded by the chorion, which is a membrane just under the shell. The amniote egg is like a space ship for the embryo, containing all the necesary life support systems.Without this egg, the tetrapods could never be called truly terrestrial. Holes in the Head: the Fenestration of the Amniote SkullSkull Within a Skull.Before all this stuff about holes in the head can really be understood, you should understand something about the primitive tetrapod skull: it was really two skulls arranged in russian doll fashion. The inner skull is the brain case (cranium). The outer skull is much reduced in the back of your own head, and is represented primarily by the cheek bones, or zygomatic arch. In the front it is represented by other facial bones, particularly those in front of the sinuses. In primitive tetrapods the outer skull was the most obvious part of the skull, and the cranium was difficult to see (it was really small too).The muscles that move the jaw are found under these outer skull bones (put your finger against your temple and bite hard--you feel one of these muscles, which is inserted into the jaw from under the zygomatic arch). As tetrapods evolved, many lines reduced the outer skull in several ways to accommodate the growth of these muscles. These skull reductions all started with holes or fenestrae. The amniotes have been primarily classified by the arrangements of holes in the skull with respect to the skull bones. Names for the Different HolesThe bones in question are the postorbital bone just behind the eye and the squamosal bone near the back of the skull. The anapsid condition refers to the lack of any fenestration of the skull. The synapsid condition is when the opening appears below this pair of bones. The diapsid condition refers to the presence of two openings, one below and one above the bones. The euryapsid condition refers to one opening above the two bones.The anapsid condition is thought to be primitive,
therefore of dubious help in sorting out relationships, particularly
among the early amniotes. Modern classification tends to label reptiles
somewhat after the first amniotes. In the sense that it is used
by your text, the reptiles are a paraphyletic group. It is thought that
the mammal ancestors branched off of the amniote line before reptiles
were evolved. Earliest Amniotes, Paleozoic "Reptiles"Diagnostic Features first Amniotes:
History
Anapsida systematics Synapsids: Mammal-like "Reptiles"Diagnostic features
History
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