CONICET, Museo Paleontológico Egidio Feruglio, Avenida Fontana 140, Trelew, 9100, Chubut, Argentina.
Biol Rev Camb Philos Soc. 2013 Nov;88(4):862-72. doi: 10.1111/brv.12030. Epub 2013 Feb 28.
Extant crocodylians have a limited taxonomic and ecological diversity but they belong to a lineage (Crocodylomorpha) that includes basal and rather generalized species and a highly diverse clade, Crocodyliformes. The latter was among the most successful groups of Mesozoic tetrapods, both in terms of taxonomic and ecological diversity. Crocodyliforms thrived in terrestrial, semiaquatic, and marine environments, and their fossil diversity includes carnivorous, piscivorous, insectivorous, and herbivorous species. This remarkable ecological and trophic diversity is thought only to occur in forms with a completely akinetic skull, characterized by a functionally integrated and tightly sutured braincase-quadrate-palate complex. However, the patterns of evolutionary change that led to the highly modified skull of crocodyliforms and that likely enabled their diversification remain poorly understood. Herein, a new basal crocodylomorph from the Late Jurassic of Patagonia is described, Almadasuchus figarii gen. et sp. nov. The new taxon is known from a well-preserved posterior region of the skull as well as other craniomandibular and postcranial remains. Almadasuchus figarii differs from all other crocodylomorphs in the presence of six autapomorphic features, including the presence of a large lateral notch on the upper temporal bar, an otic shelf of the squamosal that is wider than long, a deep subtriangular concavity on the posterolateral surface of the squamosal, and an elongated pneumatopore on the ventral surface of the quadrate. Phylogenetic analysis focused on the origin of Crocodyliformes places Almadasuchus as the sister group of Crocodyliformes, supported by synapomorphic features of the skull (e.g. subtriangular basisphenoid, absence of basipterygoid process, absence of a sagittal ridge on the frontal, and a flat anterior skull roof with an ornamented dorsal surface). New braincase information provided by Almadasuchus and other crocodylomorphs indicates that most of the modifications on the posterior region of the skull of crocodyliforms, including the strongly sutured braincase, quadrate, and the extensive secondary palate appeared in a stepwise manner, and pre-dated the evolutionary changes in the snout, jaws, and dentition. This indicates that the progressively increased rigidity of the skull provided the structural framework that allowed the great ecological diversification of crocodyliforms during the course of the Mesozoic. The phylogenetic pattern of character acquisition inferred for the strongly sutured (akinetic) skull and the appearance of more diverse feeding behaviours that create high mechanical loads on the skull provides another interesting parallel between the evolution of Mesozoic crocodyliforms and the evolutionary origins of mammals.
现存的鳄类在分类学和生态多样性上都很有限，但它们属于一个谱系（鳄形目），其中包括基础和相当普遍的物种以及高度多样化的鳄形目。后者是中生代四足动物中最成功的群体之一，无论是在分类学还是生态多样性方面。鳄形目动物在陆地、半水生和海洋环境中繁衍生息，其化石多样性包括肉食性、食鱼性、食虫性和食草性物种。这种显著的生态和营养多样性被认为只出现在具有完全不动头骨的形式中，这种头骨的特征是脑壳、方骨和腭骨紧密结合，功能上融为一体。然而，导致鳄形目高度特化的头骨的进化变化模式，以及可能使其多样化的原因，仍然知之甚少。本文描述了来自巴塔哥尼亚晚侏罗世的一种新的基础鳄形目动物，Almadasuchus figarii 属种。新的分类单元是通过头骨的保存完好的后区以及其他颅颌骨和后躯骨骼遗骸来确定的。Almadasuchus figarii 与所有其他鳄形目动物的不同之处在于有六个独特的特征，包括上颞骨上有一个大的侧凹，鳞状骨的耳骨架比长更宽，鳞状骨后外侧表面有一个深的三角形凹陷，以及方骨腹面有一个拉长的气腔。针对鳄形目起源的系统发育分析将 Almadasuchus 置于鳄形目动物的姐妹群中，这得到了头骨的特征（例如，三角形基蝶骨、没有基翼突、额骨上没有矢状脊，以及带有装饰性背表面的平坦前颅顶）的支持。Almadasuchus 和其他鳄形目动物提供的新脑壳信息表明，鳄形目动物头骨后部的大多数变化，包括紧密愈合的脑壳、方骨和广泛的次生腭，都是以逐步的方式出现的，早于吻部、颌骨和牙齿的进化变化。这表明，头骨的逐渐增加的刚性为鳄形目动物在中生代的生态多样化提供了结构框架。强烈愈合（不动）头骨的特征获得的系统发育模式以及出现更多多样化的进食行为，这些行为对头骨造成了高机械负荷，这为中生代鳄形目动物的进化和哺乳动物的进化起源之间提供了另一个有趣的相似之处。
