华盛顿，10月27日电 两栖动物依靠渗透性皮肤进行呼吸和饮水等关键生理功能，这是一种在淡水中极佳的适应性特征，却会被海水中的盐分所破坏。因此，无论是已灭绝还是现存的两栖动物，均无完全海洋生活方式的物种被发现。直到现在。
WASHINGTON, Oct 27 - Amphibians rely on their permeable skin for the crucial functions of breathing and drinking, a wonderful adaptation in freshwater that is subverted by the salt in seawater. As a result, there has been no known amphibian — extinct or extant — with a fully marine lifestyle. Until now, that is.
研究人员在中国贵州省发现了两具近乎完整的两栖动物骨骼化石，它们生活在约2.37亿年前三叠纪的海洋中，拥有扁平、盘状且带装甲的身体，以及宽大的嘴巴，显示为一种栖息底栖、靠吸食伏击猎物的捕食者。
Researchers have discovered in China's Guizhou Province two nearly complete skeletons of an amphibian that lived in the sea about 237 million years ago during the Triassic Period, with a flat, disc-shaped and armored body and a wide mouth indicative of a bottom-dwelling suction feeder that ambushed its prey.
这种生物被命名为“海生革胸螈”，体长约30英寸（75厘米），早于恐龙出现。
The creature, named Gerrothorax maritimus, measured about 30 inches (75 cm) long and predated the dinosaurs.
“最重要的发现是，存在一种早期两栖动物全职生活在海洋中，”加州大学戴维斯分校古生物学家本间良介（Ryosuke Motani）说，他是发表在《自然·生态与进化》期刊上的这项研究的主要负责人之一。
"The most important finding is that there was an early amphibian that lived in the sea full-time," said paleobiologist Ryosuke Motani of the University of California, Davis, one of the leaders of the study published in the journal Nature Ecology & Evolution.
化石出土于由海洋沉积物形成的岩层中，并在一具名为“贵州鱼龙”的鱼龙化石附近被发现，这种海生爬行动物体长约23英尺（7米）。鱼龙是当时的海洋顶级掠食者，这也解释了该两栖动物为何拥有骨质身体装甲。
It was unearthed in rocks that formed from ocean sediment, and was found near fossils of an ichthyosaur — a type of marine reptile — called Guizhouichthyosaurus that was about 23 feet (7 meters) long. Ichthyosaurs were the apex marine predators of the time, and justified the amphibian's bony body armor.
根据其嘴部形状和喉部结构推测，它采用吸食捕食方式。吸食捕食是指水生动物迅速扩张口腔制造负压，将水和猎物一同吸入。
Its use of suction feeding is suggested by the shape of its mouth and the structure of its throat. Suction feeding occurs when an aquatic animal quickly expands its mouth cavity to create a drop in pressure, drawing water and prey inside.
“头骨扁平宽大，嘴巴很大，长有许多细小的圆锥形牙齿，”中国北京大学古生物学家、该研究合著者蒋大勇说。
"The skull is flat and wide with a large mouth, having many small conical teeth," paleontologist and study co-author Da-yong Jiang of Peking University in China said.
“它是一种底栖动物，一种生活在泥质至粉砂质海底的吸食捕食者。它很可能捕食任何游过身边的生物，如鱼类、蠕虫，或许还有头足类动物，”蒋大勇说，指的是鱿鱼的亲属。
"It is a bottom animal, a suction feeder that lived on a muddy to silty seafloor. It would likely capture anything that swam by, such as fish, worms and maybe cephalopods," Jiang said, referring to squid relatives.
莫塔尼表示，一种体型相当、栖息于底层并以吸食方式进食的鲨鱼——须鲨，是一个很好的现存类比。
The wobbegong, a bottom-dwelling and suction-feeding shark of comparable size, is a good present-day analog, Motani said.
研究人员称，海生革胸螈是从有装甲的淡水祖先演化而来的。此前曾发现一种关系密切的有装甲淡水两栖动物的化石。
The researchers said Gerrothorax maritimus evolved from armored freshwater ancestors. Fossils of a closely related armored freshwater amphibian were previously discovered.
开创性的两栖动物 现存的两栖动物（如青蛙、蝾螈和无足的盲蛇）有8000多个物种。其中一些能耐受咸淡水——即河流等水体入海处海水与淡水混合的水域。但不足2%的物种表现出任何耐盐性，且没有任何物种生活在完全的海洋环境中。此外，此前没有任何两栖动物化石显示过海洋生存的迹象。
A PIONEERING AMPHIBIAN There are more than 8,000 species of living amphibians such as frogs, salamanders and legless caecilians. Some can withstand brackish water — saltwater and freshwater mixing where bodies of water like rivers meet the sea. But less than 2% exhibit any salt tolerance, and none live in fully marine environments. In addition, no previous amphibian fossil indicated a marine existence.
两栖动物的皮肤具有多种功能，除其他作用外，还是呼吸和吸收水分的主要通道。
An amphibian's skin is multifunctional, acting among other things as the main gateway for breathing and water intake.
莫塔尼说：“然而，它们的皮肤在海水中的运作方式并不相同。当青蛙在海水中游泳时，会因渗透压作用，海水通过可渗透的皮肤吸走体内水分，导致其失去体水。出于同一原因，它也无法饮水。结果，它会失去体重，不得不回到淡水中休息恢复。这一关键特征阻碍了两栖动物成为完全的海洋生物。”海生革胸螈之所以能做到这一点，是通过一种称为“预适应”的现象，即为某种功能而演化的生物性状被用于另一种不同的功能。
"Their skin, however, does not work the same way in seawater," Motani said. "When a frog swims in seawater, it loses its body water because the seawater sucks out the body water through the permeable skin through osmotic pressure. It cannot drink water either for the same reason. As a result, it loses body weight and will have to rest in freshwater to recover. This key feature prevents amphibians from becoming fully marine." Gerrothorax maritimus was able to do so through a phenomenon called exaptation, in which a biological trait that evolved for one function is repurposed for a different function.
莫塔尼说：“曾有一种淡水两栖动物，恰好拥有殖民海洋所需的‘装备’——身体装甲。这副装甲在淡水中保护动物免受捕食者侵害，但当其谱系殖民到水系中盐度更高的部分时，也可用来保护携带者免受海水侵害，就像人穿着潜水服进行水下探险一样。”
"There was a freshwater amphibian that happened to have the equipment needed to colonize the sea — the body armor. The armor protected the animal from predators in freshwater but it could also be used to protect the bearer from seawater when the lineage colonized the saltier part of the water system," Motani said, like a person wearing a dive suit for underwater exploring.
这副装甲本会抑制海水通过皮肤被吸收，迫使动物通过嘴巴饮用海水，再利用鳃过滤掉盐分。
The armor would have inhibited seawater absorption through the skin, requiring the animal to drink saltwater through its mouth and then filter out the salt with its gills.
在化石记录中，有陆生动物和淡水水生动物尸体死后被冲入大海的例子。但研究人员表示有信心认为这种两栖动物是真正的海洋生物，部分原因在于其骨骼呈现出三叠纪海水的化学特征，表明该动物在海洋中发育和生长。
In the fossil record, there are examples of land animals and freshwater aquatic animals whose bodies were carried out to sea after death. But the researchers expressed confidence that this amphibian was a genuine marine creature, in part because its bones exhibited a chemical signature of Triassic seawater, showing the animal developed and grew in the ocean.
“非常奇特，自有其魅力，”茂谷谈到这个新物种时说。“它的魅力在于独特、略显滑稽的外表和生理特征。我从未想过会有两栖动物与鱼龙共存同地。仅这一点就给我留下了深刻印象。”
"Very odd and charming in its own way," Motani said of the new species. "Its charm is in its unique, somewhat comical appearance and physiological peculiarity. I did not ever imagine that there was an amphibian that shared time and space with ichthyosaurs. That alone made a big impression on me."