개구리는 한자어로 와(蛙)라고 한다. 무당개구리·두꺼비·청개구리·맹꽁이·개구리 등의 각과가 이에 포함된다. 그 중에서도 몸체가 작고 다리가 긴 종류를 개구리라 하며, 또 예로부터 울음소리를 즐기기 위해 사육한 기생개구리도 개구리라 하였다.
19세기 초까지는 어류나 파충류의 무리로 취급되었는데, 이것은 어류와 파충류로 진화하는 도중에 있다는 것을 의미한다. 개구리는 화석 어류인 총기류(總슝類:실러캔스 등)에서 진화되어 처음으로 지상생활이 가능하게 된 네 다리를 가진 동물이다.
고생대 쥐라기에 출현하였으며, 그 조상형은석탄기와 트라이아스기에 볼 수 있다. 석탄기의 지층에서 화석으로 발견되는 유미양서류(有尾兩棲類) 가운데 견두목(堅頭目)은 현존하는 폐어(肺魚)나 경골어류와 머리뼈의 구조가 비슷하며, 한편 골격의 분화가 덜 이루어진 파충류와 비슷하므로 이와 같은종류로부터 한쪽은 파충류로, 다른 한쪽은 양서류로 진화된 것으로 추정된다.
1. 형태및발생
양서류의 형태는 파충류나 조류 등에 비해 비교적 그 변화가 단순하다. 그 중에서도 개구리 무리는 화석에서 볼 수 있는 것과 비교하면 거의 변화가 없었다. 단지유미류에 비해 뒷다리가 길어져 땅 위로 도약하며, 발가락 사이에 물갈퀴가 발달하여헤엄치는 데 편리하게 되었다는 점 등의 분화가 진행되었다. 자세히 살펴보면 생활환경에 적응하는 형태로 변화되었다는 것을 알 수 있다.
일반적으로 수생(水生)에서 육생(陸生)으로 변화되었으므로 피부의 변화가 크다. 건조한 지역에 서식하는 것은 두껍고 건조한 피부를 가지나, 일반적으로는 습한 피부로 호흡을 잘 하고 건조한기후에는 약하며, 독선(毒腺)이 발달된 것도 있다. 비늘은 보통 없으나, 두꺼비의 일종은 다리에 굳은 부분이 있는데, 이것은 비늘이 퇴화된 것으로 추정된다.
개구리의 몸체는 머리·가슴·다리의 3부분으로 되어 있다. 좌우대칭이며, 전후·등배도잘 분화되어 있다. 머리는 편평하며 삼각형에 가깝고, 상하의 눈꺼풀이 있는 눈이 있으나 위쪽 눈꺼풀은 움직이지 않는다. 눈의 뒤쪽에는 원형 또는 타원형의 고막이 있다. 입은 크고 입을 열면 위턱의 가장자리에 작은 턱니가 나란히 있으나, 두꺼비에는 턱니가 없다. 이 이빨은 먹이를 씹는 데 쓰지 않고 먹이를 놓치지 않도록 잡는 데 이용된다.
앞다리·뒷다리는 각기 3부분으로 나누어져 있으며, 앞다리에는 5개의발가락이 있는데, 첫째발가락이 작고 피하에 숨겨져 있어 보통 4개로 센다. 뒷다리에는 5개의 발가락이 보통이나 6개인 것도 있다. 특히 수컷의 앞다리의 둘째발가락 안쪽에 색소가 풍부한 피부선(皮膚腺)이 비후되어 있고, 생식기의 발달이 뚜렷하다.
북한은 최근 신의주에서 1억 5천만년 전 것이라고 추정되는 개구리 화석을 발견했다고 발표했다.
얼마 전 시조새 화석이 발견되었던 쥐라기 층에서 발굴되었던 개구리 화석은 척추골과 앞 다리뼈, 골반 등이 잘 발달되었다고 소개했다.
If you could time-travel back to the Jurassic period, you would see all sorts of dinosaurs, but you might also glimpse a more familiar creature - a frog, according to a new finding by University of Pennsylvania evolutionary biologist Neil Shubin.
After years of painstakingly putting together tiny bone fragments, Shubin and colleague Farish Jenkins from Harvard University have reconstructed the skeleton of a frog that hopped among the dinosaurs nearly 200 million years ago.
This finding of the earliest true frog will help paleontologists understand how the frog evolved its unique jumping ability - an adaptation that they believe allowed the frog to spread around the world and persist, barely changed, for so many millions of years.
"The frogs hit on this successful design very early," says Shubin. The fossil shows that when the creature was alive, it looked like a frog and it hopped more or less like a frog, says paleontologist Robert Carroll of McGill University, in Montreal. "Anyone over 5 years old would look at this and say, 'Oh, that's a frog.'"
Shubin says he was still a graduate student at Harvard when he and Jenkins dug up the fossil in 1982, at a site in the Arizona desert near Flagstaff. It was an unrecognizable scattering of bone fragments that Shubin describes as "road kill."
It took several years just to figure out that it was a frog and then a few more years to reconstruct its anatomy, he says.
Frog anatomy images:
Organ System
Digestive System
Nervous System
3-D Views of Internal Structures
They knew the age of the frog because it was embedded in a layer of rock that contained other easily identified fossils from the Jurassic era. The same rock layer is found in other parts of the world as well. As a specialist in the Jurassic period, and the period before it, the Triassic, Shubin has traveled to both Morocco and Greenland in search of other fossils that reveal the fauna of these periods.
The One and Only Morocco.
The reason the same deposit exists at such diverse points on the globe is that, during the Triassic and Jurassic periods, all the continents were jammed together into one supercontinent now called Pangaea.
"Back then, you could walk from Atlantic City to Casablanca without crossing water," Shubin says.
By the late Jurassic, fossils show that frogs had spread to what is now Europe and Africa. Today there are more than 4,000 species of frogs living around the world.
Fossils found in the same layer of rock with Shubin's frog show that the dinosaurs shared their turf with a variety of other wildlife. During the Triassic period, there were many other reptiles and amphibians.
Most of these competitors to the dinosaurs died out in a massive extinction that took place at the boundary between the Triassic and Jurassic eras.
Learn more about The Impact Catastrophe and Mass Extinction.
Species that survived into the Jurassic included ancestral turtles, crocodiles, and tiny rodent-like mammals. Also carrying over were the frog's predecessors. Shubin and Jenkins compared their frog with these earlier fossils to figure out how frogs evolved.
Shubin says he believes the frog descended from a salamander-like amphibian with a long tail. Fossils of such creatures are found dating back about 260 million years.
But by 225 million years ago, the creatures started to look more leggy, as shown by a fossil that dates from that period, found in Madagascar. Around that time, the pelvis was becoming elongated and lined up with the spine, and the tail was receding inside the pelvis.
Eventually, the tail vertebrae fused and became a structure called the urostyle, which Shubin says is crucial in transferring forces from the legs to the body during a jump.
And while other animals have a pelvis that is fused to the spine, frogs developed a movable joint that allows the pelvis to slide up and down the backbone, a configuration that helps catapult frogs into the air.
"This pelvic attachment is seen in no other animal," Shubin says. The Madagascar frog shows a step toward this feature, which is completely developed in the new Arizona frog, which Shubin and his colleague named Prosalirus Bitis. Prosalirus is derived from the Latin "to leap." Bitis is the Navajo word for "high over it," to honor the fact that the fossil came from Navajo land. Many researchers believe that jumping evolved to help frogs to escape dinosaurs and other predators.
Journey Around The Navajo Nation.
Some of the frogs today have evolved more advanced jumping mechanisms while others are considered "primitive" - meaning they are little changed from their ancestors. The more primitive frogs have little control over their jumping, says McGill's Carroll.
It's sort of random, but it usually lands them in water. "Even if they don't know where they are going, they usually end up safer than they would be on land," he says.
Frogs survived through the age of dinosaurs and hung on through the mass extinction during which dinosaurs perished. But frogs are now disappearing all over the world, says Shubin, and no one knows why. Some blame acid rain, others the thinning ozone layer.
"It's really amazing," says Shubin, "because frogs have been so successful for so many millions of years."
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