Early land animals skipped the tadpole phase
Early land animals skipped the tadpole phase
早期陆生动物跳过了蝌蚪阶段
For decades, biologists thought that early tetrapods, ancient vertebrates that started conquering the land over 300 million years ago, developed like modern amphibians—beginning their lives as purely aquatic tadpoles and then metamorphosing into terrestrial adults. 几十年来，生物学家一直认为早期四足动物（3亿多年前开始征服陆地的古代脊椎动物）的发育方式与现代两栖动物相似——即生命初期是纯水生的蝌蚪，随后变态发育为陆生个体。
“A lot of that comes from this old ‘scala naturae’ idea that you had fish that evolved into the next stage up, which were amphibians, and then amphibians evolved into the next stage up, which were reptiles that evolved into birds and mammals,” said Jason Pardo, a research associate at the Field Museum. 菲尔德博物馆（Field Museum）的研究助理杰森·帕多（Jason Pardo）表示：“这很大程度上源于古老的‘自然阶梯’（scala naturae）观念，即鱼类进化到下一个阶段是两栖动物，两栖动物再进化到下一个阶段是爬行动物，进而进化为鸟类和哺乳动物。”
We’ve never had evidence that early tetrapods had an amphibian lifestyle; we have assumed it because it made intuitive sense. “It’s easier to make the transition from water to land if you’re already making that transition as part of your life cycle,” Pardo said. 我们从未有过证据表明早期四足动物拥有两栖动物的生活方式；我们之所以这样假设，是因为它在直觉上讲得通。帕多说：“如果你在生命周期中已经包含了从水到陆的转变，那么这种过渡会更容易。”
But now, a new Science study that Pardo co-authored with Arjan Mann (the Field Museum’s assistant curator of early tetrapods) shows our most basic assumptions about the first tetrapods that started living on land might be wrong. 但现在，帕多与菲尔德博物馆早期四足动物助理馆长阿詹·曼（Arjan Mann）共同发表在《科学》杂志上的一项新研究表明，我们关于首批开始陆地生活的四足动物的最基本假设可能是错误的。
Baby monsters
幼年怪兽
The researchers’ study focused mainly on embolomers, an extinct group of large predators that lived roughly 300 million years ago. Embolomers looked like a cross between a crocodile and an eel, with large skulls full of sharp teeth, followed by long, eel-like bodies. 研究人员的研究主要集中在栓椎类（embolomers）上，这是一类生活在大约3亿年前的已灭绝大型掠食者。栓椎类动物看起来像是鳄鱼和鳗鱼的结合体，有着布满尖牙的大头骨，后面连接着长长的、类似鳗鱼的身体。
It had short, stocky limbs adapted mainly for paddling in water, but also capable of powering brief, clumsy excursions on land. They are thought to be one of the first vertebrates that made a partial transition from an aquatic to a terrestrial lifestyle. 它们拥有短而粗壮的四肢，主要适应在水中划行，但也能够支撑它们在陆地上进行短暂而笨拙的移动。它们被认为是第一批从水生生活方式向陆生生活方式进行部分过渡的脊椎动物之一。
These animals could reach over three meters in length, but to understand the very beginning of their life cycle, scientists focused on examining some of their centimeter-scale babies. 这些动物体长可超过三米，但为了了解它们生命周期的起点，科学家们重点研究了一些厘米级的幼体。
One of the most important specimens in the study was an embolomere fossil designated FMNH PR 1082, which has been sitting in the collections at the Field Museum for decades. “I think it’s been there for about 50 or 60 years. It was originally found by a collector,” Mann said. 研究中最重要的标本之一是编号为 FMNH PR 1082 的栓椎类化石，它在菲尔德博物馆的馆藏中已经存放了几十年。曼说：“我想它已经在那里存放了五六十年了。它最初是由一位收藏家发现的。”
The fossil was initially classified as a different tetrapod species due to its very small size. But armed with modern imaging techniques like electron microscopy, Pardo and Mann realized the fossil was in fact a very young embolomere that died before it could even consume its first meal. 由于体型极小，该化石最初被归类为另一种四足动物。但借助电子显微镜等现代成像技术，帕多和曼意识到，这块化石实际上是一只非常年幼的栓椎类动物，它在还没来得及吃第一顿饭之前就死亡了。
“We were able to identify features that linked it to embolomers like the shape of the vertebrae, the radial spines on the tail, and the nice little fangs,” Mann said. 曼说：“我们能够识别出将其与栓椎类动物联系起来的特征，比如椎骨的形状、尾部的放射状棘刺，以及那些可爱的小尖牙。”
The team also realized that this young embolomere looked like a miniature adult. What’s more, the individual had an abdominal yolk, a portion of the egg’s yolk sac that an embryo internalizes into its body cavity just before hatching to use as an energy reserve. 研究小组还意识到，这只幼年栓椎类动物看起来就像是成年个体的缩小版。更重要的是，该个体体内有腹部卵黄，这是胚胎在孵化前吸收到体腔内作为能量储备的卵黄囊的一部分。
(According to the researchers, this internal yolk mass suggests the ancestral tetrapod egg was relatively large and nutrient-dense, like the ones laid by reptiles and birds, as opposed to the small eggs laid by amphibians and fish.) （据研究人员称，这种内部卵黄团表明，原始四足动物的卵相对较大且营养丰富，类似于爬行动物和鸟类产下的卵，而不是两栖动物和鱼类产下的小卵。）
Despite having hatched recently, the specimen lacked the external gills that tadpoles should have, and there were signs of ossification in its bones—it was a youngling, but it was no tadpole. 尽管刚孵化不久，但该标本缺乏蝌蚪应有的外鳃，且骨骼已有骨化迹象——它确实是幼体，但绝非蝌蚪。
The lack of a tadpole phase so early in life suggested there was no metamorphosis in this animal’s life cycle, which led Pardo and Mann to hypothesize the development of early tetrapods was direct, as it is in the reptiles or mammals that appeared much later. 生命早期缺乏蝌蚪阶段表明，该动物的生命周期中不存在变态发育，这促使帕多和曼推测，早期四足动物的发育是直接发育，就像后来出现的爬行动物或哺乳动物一样。
Then they started looking at other fossils to make sure the FMNH PR 1082 was not some kind of outlier. It turned out it wasn’t. 随后，他们开始研究其他化石，以确保 FMNH PR 1082 并非某种特例。事实证明，它并非特例。
Skipping the tadpole
跳过蝌蚪阶段
To confirm this wasn’t an isolated case, the team looked at a second, even smaller embolomere hatchling that also showed no signs of external gills. Once this was confirmed, the team started checking fossil collections in other museums across America, looking for youngsters of other ancient lineages to see whether the missing tadpole phase was a broader evolutionary trend. 为了确认这并非孤例，研究小组观察了第二只更小的栓椎类幼体，它同样没有外鳃迹象。确认这一点后，研究小组开始检查美国各地博物馆的化石馆藏，寻找其他古老谱系的幼体，以观察“缺失蝌蚪阶段”是否是一种更广泛的进化趋势。
They found that a tiny hatchling of Phlegethontia longissima, a strange group of early tetrapods that lost their limbs entirely. These had a head dominated by large eyes and a partially ossified jaw—but again, no external gills. 他们发现了一种名为 Phlegethontia longissima 的微小幼体，这是一类完全失去四肢的奇特早期四足动物。它们的头部以大眼睛和部分骨化的颌骨为特征——但同样没有外鳃。
Then, the researchers started looking further out along the evolutionary tree at earlier animals that hadn’t yet fully made the transition from fins to limbs. They focused on fossils that had historically been thought to be larval lungfish. 随后，研究人员开始沿着进化树向更远的地方探索，寻找那些尚未完全完成从鳍到肢过渡的早期动物。他们重点研究了历史上被认为是肺鱼幼体的化石。
Pardo and Mann’s re-examination revealed that these were young megalichthyids, early finned tetrapodomorphs that predated emobolomeres by 20 to 30 million years. The smallest of these specimens measured just two centimeters. 帕多和曼的重新检查显示，这些其实是巨鱼类（megalichthyids）的幼体，它们是比栓椎类动物早 2000 万到 3000 万年的早期有鳍四足形类动物。这些标本中最小的仅有两厘米长。
In a few cases, multiple individuals were preserved together in the same concretion, indicating that these animals may have remained in groups for some time after hatching. But even this species showed signs of direct development, with gradually ossifying bones rather than undergoing an abrupt, amphibian-like metamorphosis. 在少数情况下，多个个体被保存在同一结核中，这表明这些动物在孵化后可能成群生活了一段时间。但即使是该物种也表现出直接发育的迹象，骨骼是逐渐骨化的，而不是经历两栖动物那种突发性的变态发育。
“We looked at a number of different species that represent different lineages in the transition from fish to tetrapods, and what we found is that none of them have anything that looks remotely like a tadpole,” Pardo said. 帕多说：“我们研究了代表从鱼类到四足动物过渡过程中不同谱系的多个物种，结果发现它们中没有任何一个看起来像蝌蚪。”
Without the tadpole phase and metamorphosis, though, the transition from water to land was probably way tougher than we thought. 然而，如果没有蝌蚪阶段和变态发育，从水到陆的过渡可能比我们想象的要艰难得多。
Amphibian innovation
两栖动物的创新
“We have lots of assumptions in our field that are based on relatively limited data,” Pardo said. One of those assumptions was that a distinct aquatic larval stage made the water-to-land transition easier for early tetrapods. 帕多说：“我们领域中有很多假设是基于相对有限的数据。”其中一个假设是，独特的幼体水生阶段使早期四足动物更容易完成从水到陆的过渡。
While direct development, without undergoing a radical metamorphosis early in the animal’s lifecycle, might appear to be a simpler solution, it likely made the lives of young embolomers considerably harder. The first challenge they faced was being… 虽然直接发育（即在生命早期不经历剧烈的变态发育）看起来是一个更简单的方案，但这很可能使幼年栓椎类动物的生活变得更加艰难。它们面临的第一个挑战是……