Evolution and classification

Paleontology

Like all reptiles, crocodiles are diapsids—i.e., they have two openings on each side of the skull. Crocodiles also show the most important characteristics of the group that includes the dinosaurs (subclass Archosauria). The skull exhibits distinctly developed upper and lower temporal fenestrae (that is, openings behind the eye sockets); the teeth arise from sockets, and the roof of the skull lacks an opening for the parietal organ—a median, dorsal outgrowth of the brain (see tuatara: Form and function). Within the Archosauria, the crocodiles are a separate order, since they have developed a secondary bony palate, which encloses the nasal passage from the exterior nasal openings to the choanae (internal nostrils). These features occur even in the most primitive representatives of the crocodilian group, namely the Protosuchia of the Late Triassic Epoch (237 million–201.4 million years ago); but their muzzles were very short, and the choanae were relatively far forward on the palate.

As crocodiles continued to evolve, the openings of the choanae tended to be located farther back. In the Mesosuchia of the Jurassic (201.4 million–145 million years ago) and Cretaceous (145 million–66 million years ago) periods—to which the long-snouted ocean crocodiles also belong—the choanae were already located at the posterior part of two bones of the skull (palatines). In the true crocodiles (Eusuchia), which appear in the Upper Jurassic, the choanae are entirely enclosed by the pterygoids (that is, the paired bones on the lower part of the cranium). In modern species they are found at the posterior border of the palate. The Sebecosuchia, which existed from the Late Cretaceous (100.5 million–66 million years ago) to the Miocene Epoch (23 million–5.3 million years ago), possess skulls with flattened sides, and the choanae lie in a depression in the anterior part of the pterygoids. Modern genetic studies of crocodilian DNA have been used to identify species differences and, in some cases, indicate natural hybridization.

Distinguishing taxonomic features

The families and genera of the order Crocodylia are differentiated primarily by the anatomical peculiarities of their skulls. The classification of each species is based mainly upon external characteristics, such as the proportions of the snout, the bony structures on the dorsal side of the snout, the number of teeth, the number and arrangement of the large knobs on the nape of the neck, and the characteristics of the dorsal plates. For instance, alligators possess a broad, U-shaped snout and have an “overbite”—that is, all the teeth of the lower jaw fit within (are closer to the tongue than) the teeth of the upper jaw, whereas crocodiles have a narrow, V-shaped snout. The large fourth tooth on each side of the alligator’s lower jaw fits into a socket in the upper jaw; typically, no lower teeth are visible when the mouth is closed. In contrast, the large fourth tooth on each side of the crocodile’s lower jaw projects outside the snout when the mouth is closed.

Annotated classification

Extinct groups represented only by fossils are indicated by a dagger (†).

• Order Crocodylia, or Crocodilia

  Archosaurs with a secondary palate; heavy cylindrical body; large, triangular head; legs short, toes webbed; long, muscular tail; large flat plates on belly, keeled ones on back; heart 4-chambered.

  • †Suborder Protosuchia

    Upper Triassic; muzzle very short; choanae (internal nostrils) in region of palatine bones.

  • †Suborder Mesosuchia

    Jurassic to Upper Cretaceous; choanae in posterior part of palatine bones.

  • †Suborder Sebecosuchia

    Upper Cretaceous to Miocene; skull laterally flattened; choanae in depression in anterior part of pterygoids.

  • Suborder Eusuchia

    Upper Jurassic to Recent; choanae entirely enclosed by pterygoids.

    • Family Alligatoridae (alligators and caimans)

      4 genera and 8 species; teeth of lower jaw fit inside those of upper jaw.

    • Family Crocodylidae (true crocodiles)

      3 genera and 17 species; teeth of upper and lower jaws form one interdigitating row when mouth is closed.

    • Family Gavialidae (gavial)

      2 genera and 2 species; extremely long snout, more than 22 teeth in each jaw; nasal bones separated from premaxillaries.

Critical appraisal

Widely different views prevail concerning the classification of the living groups of Eusuchia—the alligators and caimans, the true crocodiles, and the gavials. The three groups have been treated here as distinct families within suborder Eusuchia. Of these, the alligators and the true crocodiles are more closely related to each other, and they are sometimes regarded as constituting two subfamilies of the family Crocodilidae. Some authors regard the gavials as a third subfamily. Tomistoma schlegelii, the Malaysian false gharial, has been placed by some authors in Crocodilidae and by others with the Gavialidae.

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In addition, one authority has separated the order Crocodilomorpha into two suborders, Crocodylia (or Crocodilia) and Paracrocodylia. According to this scheme, the Crocodylia include as infraorders those groups given above as suborders. This scheme also contains a suborder, Thalattosuchia.