page_title: Crocodilia
section_title: Evolution
hierarchical_section_title: Crocodilia / Taxonomy and classification / Evolution
caption_attribution_description: Protosuchus, pencil drawing. Protosuchus was a genus of carnivorous crocodylomorph from the Early Jurassic.
context_page_description: Crocodilia is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the order's total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, and diversified during the Mesozoic era. The order Crocodilia includes the true crocodiles, the alligators and caimans, and the gharial and false gharial. Although the term 'crocodiles' is sometimes used to refer to all of these, crocodilians is a less ambiguous vernacular term for members of this group.
  Large, solidly built, lizard-like reptiles, crocodilians have long flattened snouts, laterally compressed tails, and eyes, ears, and nostrils at the top of the head. They swim well and can move on land in a "high walk" and a "low walk", while smaller species are even capable of galloping. Their skin is thick and covered in non-overlapping scales. They have conical, peg-like teeth and a powerful bite.
context_section_description: The main distinguishing characteristic of diapsid tetrapods is the presence of two openings (temporal fenestrae) on either side of the skull behind the eye. Living diapsids include modern reptiles and birds. The feature that distinguishes archosaurs from other diapsids is an extra pair of openings in the skull (antorbital fenestrae) in front of the eye sockets. Archosauria is the crown group containing the most recent common ancestor of crocodilians and birds and all its descendants. It comprises the Pseudosuchia, the "false crocodiles", and the Ornithosuchia, which in turn comprises the dinosaurs and their relatives, the pterosaurs and the birds. Pseudosuchia is defined as living crocodilians and all archosaurs more closely related to crocodilians than to birds. The Pseudosuchia–bird split is assumed to have occurred close to the Permian–Triassic mass extinction event. In modern crocodilians, the antorbital fenestrae are walled off externally and exist merely as sinuses. They were present in most of their fossil ancestors as small openings.
  The crocodylomorphs are the only pseudosuchians to have survived the Triassic–Jurassic extinction event, 201.3 million years ago. During the early Jurassic period, the dinosaurs became dominant on land, and the crocodylomorphs underwent major adaptive diversifications to fill ecological niches vacated by recently extinguished groups. Unfolding fossil evidence shows that Mesozoic crocodylomorphs had a much greater diversity of forms than modern crocodilians. Some became small fast-moving insectivores, others specialist fish-eaters, still others marine and terrestrial carnivores, and a few became herbivores. The earliest stage of crocodilian evolution was the protosuchians, which evolved in the late Triassic and early Jurassic. They were followed by the mesosuchians, which diversified widely during the Jurassic and the Tertiary. Another group, the eusuchians, appeared in the late Cretaceous 80 million years ago and includes all the crocodilians living today.
  Protosuchians were small, mostly terrestrial animals with short snouts and long limbs. They had bony armor in the form of two rows of plates extending from head to tail, and this armor is retained by most modern crocodilians. Their vertebrae were convex on the two main articulating surfaces, and their bony palates were little developed. The mesosuchians saw a fusion of the palatine bones to form a secondary bony palate and a great extension of the nasal passages to near the pterygoid bones. This allowed the animal to breathe through its nostrils while its mouth was open under the water. The eusuchians continued this process with the interior nostrils now opening through an aperture in the pterygoid bones. The vertebrae of eusuchians had one convex and one concave articulating surface, allowing for a ball and socket type joint between the vertebrae, bringing greater flexibility and strength. The oldest known eusuchian is Hylaeochampsa vectiana from the lower Cretaceous of the Isle of Wight in the United Kingdom. It was followed by crocodilians such as the Planocraniidae, the so-called 'hoofed crocodiles', in the Palaeogene. Spanning the Cretaceous and Palaeogene periods is the genus Borealosuchus of North America, with six species, though its phylogenetic position is not settled.
  The three primary branches of Crocodilia had diverged by the end of the Mesozoic. The earliest-known members of the group is Portugalosuchus from the Cenomanian (95 million years ago) and after are alligatoroids and gavialoids that lived in North America and Europe during the Campanian (around 83.6–72.1 million years ago). The first known crocodyloids appeared in the Cenomanian (around 72.1–66.0 million years ago), that lineage must have been present during the Campanian, and the earliest alligatoroids and gavialoids include highly derived forms, which indicates that the time of the actual divergence between the three lineages must have been a pre-Campanian event.
caption: Restoration of early crocodylomorph Protosuchus
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS3 Windows", "Image DateTime": "2013:11:26 23:05:15", "Image ExifOffset": "164", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "302", "Thumbnail JPEGInterchangeFormatLength": "3725", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "774", "EXIF ExifImageLength": "420"}