Since life on land began, it has consisted of four megadynasties, as they are sometimes called. 自从陆地上开始有生命以来，它由4个所谓的大王朝组成。

　　The first consisted of primitive, plodding but sometimes fairly hefty amphibians and reptiles. 第一个大王朝包括行动缓慢的有时候又相当笨重的原始两栖动物和爬行动物。

　　The best-known animal of this age was the Dimetrodon, 这个年代最著名的动物是异齿龙，

　　a sail-backed creature that is commonly confused with dinosaurs (including, I note, in a picture caption in the Carl Sagan book Comet). 那是一种背部有翼的动物，常常与恐龙相混淆（我注意到，包括卡尔·萨根《彗星》一书中的一处图片说明在内）。

　　The Dimetrodon was in fact a synapsid. 异齿龙实际上是一种下孔亚纲动物。

　　So, once upon a time, were we. 我们从前曾经也是下孔亚纲动物。

　　Synapsids were one of the four main divisions of early reptilian life, the others being anapsids, euryapsids, and diapsids. 下孔亚纲是早期爬行动物的4个主要部之一，其他3个部分别是缺孔亚纲、调孔亚纲和双孔亚纲。

　　The names simply refer to the number and location of small holes to be found in the sides of their owners' skulls. 这些名字只是指在它们的颅骨侧面发现的小孔的数量和位置。

　　Synapsids had one hole in their lower temples; diapsids had two; euryapsids had a single hole higher up. 下孔亚纲在颞颥下部有1个孔；双孔亚纲有2个孔；调孔亚纲只有上部1个孔。

　　Over time, each of these principal groupings split into further subdivisions, of which some prospered and some faltered. 届时，每个主要的部又分成若干分部。其中有的兴旺，有的衰落。

　　Anapsids gave rise to the turtles, which for a time, perhaps a touch improbably, 缺孔亚纲产生了鳖。鳖一度似乎快要处于主宰地位，

　　appeared poised to predominate as the planet's most advanced and deadly species, 成为这个星球上最先进、最致命的物种，虽然这有点儿荒唐可笑。

　　before an evolutionary lurch let them settle for durability rather than dominance. 但是，由于进化比较缓慢，它们保持了长久的生存地位，而不是统治地位。

　　The synapsids divided into four streams, only one of which survived beyond the Permian. 下孔亚纲分成四支，只有一支闯过了二叠纪。

　　Happily, that was the stream we belonged to, and it evolved into a family of protomammals known as therapsids. These formed Megadynasty 2. 幸运的是，我们恰好属于这一支。它进化成为一个原始哺乳动物家族，被称之为兽孔目爬行动物。这类爬行动物构成了第二大王朝。