它们看起来就像是远古哺乳动物的脑袋，然而这些在东非发现的1亿年前的化石遗骸却属于鳄鱼家族中的一位成员。
 
美国俄亥俄大学的Patrick M. O’Connor与同事，在坦桑尼亚西南Rukwa Rift盆地的白垩纪沉积物中发现了一种新的、体型较小的鳄形动物化石，显示了关于趋同性形态演化的进一步证据。
 
研究人员将这种远古鳄鱼命名为Pakasuchus。他们注意到，它们的牙齿是最不像鳄鱼的，倒与那些温血的食肉动物牙齿非常相像——同样具有突出的犬齿、锋利的前磨牙以及扁平的臼齿。
 
研究人员指出，所有这些特征使得这些史前鳄鱼能够咬伤、撕裂并咀嚼它的猎物。与此形成对照的是，现代鳄鱼用来捉住并吞噬猎物的牙齿都是均匀的锥形。Pakasuchus具有能够进行牙冠至牙冠接触——这种接触是咀嚼的特征——的牙系， 并且具有几乎全部被认为属于哺乳动物的牙齿特点。
 
根据对新发现的鳄鱼化石的其他部分进行的分析，包括长而直的四肢，科学家断定，这条鳄鱼生前曾在非洲的稀树大草原上游荡，以大型昆虫，例如蜻蜓，或新孵化的恐龙幼崽为食。
 
研究人员指出，在哺乳动物的祖先纷纷开拓北方大陆时，这种鳄鱼很有可能填补了南半球的小生境。这意味着Pakasuchus能够像最近在非洲发现的其他远古鳄鱼一样，在离开水后找到一种非常安逸的生活方式。
 
研究人员在8月5日出版的《自然》杂志上报告了这一发现。

原文出处：

Nature doi:10.1038/nature09061

The evolution of mammal-like crocodyliforms in the Cretaceous Period of Gondwana
Patrick M. O’Connor,Joseph J. W. Sertich,Nancy J. Stevens,Eric M. Roberts,Michael D. Gottfried,Tobin L. Hieronymus,Zubair A. Jinnah,Ryan Ridgely,Sifa E. Ngasala& Jesuit Temba

Fossil crocodyliforms discovered in recent years1, 2, 3, 4, 5 have revealed a level of morphological and ecological diversity not exhibited by extant members of the group. This diversity is particularly notable among taxa of the Cretaceous Period (144–65?million years ago) recovered from former Gondwanan landmasses. Here we report the discovery of a new species of Cretaceous notosuchian crocodyliform from the Rukwa Rift Basin6 of southwestern Tanzania. This small-bodied form deviates significantly from more typical crocodyliform craniodental morphologies, having a short, broad skull, robust lower jaw, and a dentition with relatively few teeth that nonetheless show marked heterodonty. The presence of morphologically complex, complementary upper and lower molariform teeth suggests a degree of crown–crown contact during jaw adduction that is unmatched among known crocodyliforms, paralleling the level of occlusal complexity seen in mammals and their extinct relatives7, 8, 9, 10, 11, 12. The presence of another small-bodied mammal-like crocodyliform in the Cretaceous of Gondwana indicates that notosuchians probably filled niches and inhabited ecomorphospace that were otherwise occupied by mammals on northern continents.