主龙形类（Archosauriformes）又名初龙形类、初龙型类，在希腊文里的意思是“占优势的蜥蜴”与“外形”，是双孔类爬行动物的一个演化支，在晚二叠纪（约2亿5000万年前）从原始主龙形下纲祖先演化而来。这些主龙形类，包括古鳄科与更先进的物种，是外表上类似鳄鱼、半水生的掠食动物，身长约1.5公尺，拥有长口鼻部，站立时手肘往外伸展。不像同时代兽孔目动物巨大的体积，古鳄科在二叠纪-三叠纪灭绝事件中存活下来，原因也许因为牠们是食腐动物，也许因为牠们能潜到水中躲避过热的气候。以上状况都是假设的；唯一肯定的是这些动物在牠们的新环境是高度成功的，并且快速演化。

Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Late Permian (roughly 250 million years ago). It was defined by Jacques Gauthier (1994) as the clade stemming from the last common ancestor of Proterosuchidae and Archosauria; Phil Senter (2005) defined it as the most exclusive clade containing Proterosuchus and Archosauria. These reptiles, which include members of the family Proterosuchidae and more advanced forms, were originally superficially crocodile-like predatory semi-aquatic animals about 1.5 meters (5 ft) long, with a sprawling elbows-out stance and long snouts. Unlike the bulk of their therapsid contemporaries, the proterosuchids survived the catastrophe at the end of the Permian, perhaps because they were opportunistic scavengers or because they could retreat into water to find respite from an overheated climate. Any such scenarios are hypothetical; what is clearer is that these animals were highly successful in their new environment, and evolved quickly. Within a few million years at the opening of the Triassic, the proterosuchids had given rise to the Erythrosuchidae (the first sauropsids to totally dominate their environment), which in turn were the ancestors of the small agile Euparkeriidae, from which a number of successfully more advanced families – the archosaurs proper – evolved rapidly to fill empty ecological niches in the devastated global system. The Archosauria includes crocodylians, birds, and their extinct relatives.