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Crocodylia

Article by: Savage, Jay M. Department of Biology, University of Miami, Coral Gables, Florida. Cash, W. Ben Department of Biology, Maryville College, Maryville, Tennessee. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.168610 (http://dx.doi.org/10.1036/1097-8542.168610)

Content

• Morphology • Feeding • Reproduction • Classification • Bibliography • Additional Readings

An order of the class Reptilia (infraclass Archosauria) whose aquatic species include the alligators, caimans, crocodiles, and gavials. The group has a long fossil history from the Late Triassic, and its members are the closest living relatives of the extinct dinosaurs and the birds. The 23 living species are found in tropic areas of Africa, Asia, Australia, and the Americas. One form, the saltwater crocodile (Crocodylus porosus), has traversed oceanic barriers from the East Indies as far east as the Fiji Islands. See also: Archosauria (/content/archosauria/048900); Reptilia (/content/reptilia/581800)

Morphology

The order is distinguished from other living reptiles in that it has two temporal foramina, an immovable quadrate, a bony secondary palate, no shell, a single median penis in males, socketed teeth, a four- chambered heart, and an oblique septum that completely separates the lung cavities from the peritoneal region.

Certain of these unique features and other salient characteristics of the Crocodylia are intimately associated with their aquatic life. The secondary palate, composed of medial expansions of premaxillary, maxillary, palatine, and pterygoid bones, divides the mouth cavity into two separate regions. The area above the bony palate forms an air passage extending from the external nostrils at the tip of the snout posteriorly to near the orbital region. The lower space retains the usual functions of the mouth and is bordered above by the secondary palate, and below by the lower jaw. A special pair of fleshy flaps are found at the posterior end of the mouth cavity and form a valvular mechanism which separates the mouth from the region where the air passage opens into the throat (Fig. 1). This complex arrangement allows crocodylians to breathe even

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though most of the head is under water, or the mouth is open holding prey or full of water. Inasmuch as these great reptiles are extremely active when submerged and may remain under water for considerable periods, the increased efficiency of respiration provided by the oblique septum is an obvious adaptation. The significance of the four-chambered heart also lies in its contribution to rapid cellular respiration through an increased efficiency of circulation. Interestingly, all three of these structural characteristics are also found in mammals, although differing in detail. The Crocodylia, however, are not closely related to any of the reptiles that ultimately gave rise to the mammal stock.

Fig. 1 Sagittal section of crocodylian head to show secondary palate and air passage.

Other aquatic adaptations include valvular external nostrils that are closed during immersion, a recessed eardrum that can be covered by a skin flap underwater, webbed toes, and a long, compressed, muscular tail that propels the crocodylian through the water with strong lateral thrusts. In addition, the eyes, adapted for nocturnal activity by a vertically elliptical pupil, and the nostrils are mounted in raised areas of the head so that the animal may see and breathe without exposing much of its body above water. The method of locomotion on land is by progression on all fours, with the belly and head held off the ground and the tail dragging behind. In the water, movement is produced by lateral undulations of the tail and the forelimbs are held flat against the sides of the body.

Feeding

Crocodylians are carnivorous and prey upon insects, mollusks, fishes, and mammals. Usually only the larger adults attack terrestrial prey, which is usually caught at the water's edge and after being grasped in the powerful jaws is dragged down to be drowned. The jaws, numerous sharp teeth, and the habit of twisting and rolling underwater after the prey is seized all contribute to the particular reputation crocodilians share. Even on land they are imposing creatures, especially when utilizing the muscular tail for defense. As with any large predator, crocodilians should be treated with respect and caution should be used in approaching individuals in the wild.

Reproduction

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During the breeding season, male crocodilians set up territories on land which they defend against intruders of the same species. During this period, their loud roars are frequently heard at night. Fertilization is internal and the shelled eggs are deposited in excavations in the sand or in large nests of decaying vegetation, depending upon the species. In some forms, the female guards the nest and several females may take turns in protecting a communal nest. It has been reported that when the young hatch the female may liberate them from the nest and lead them to the nearest water.

Classification

The living species of crocodilians are placed in two families and eight genera. The family Crocodylidae contains two subgroups: the true crocodiles, Crocodylinae, including the genera Crocodylus found in all tropic areas, Osteolaemus in central Africa, and the false gavial (Tomistoma) in Malaya and the East Indies; and the alligators and caimans, Alligatorinae, including the genera Alligator of the southeastern United States and near Shanghai, China, Caiman from Central and South America, and Melanosuchus and Paleosuchus of South America. The gavial (Gavialis gangeticus) of India and north Burma is the only living member of the family Gavialidae. Crocodiles differ most obviously from alligators and caimans in head shape and in the position of the teeth, although other technical details also separate them (Figs. 2 and 3). The gavial differs from other living forms in its extremely long and narrow snout, with 27 or more teeth on the upper jaw and 24 or more on the lower. Even the false gavial (Tomistoma), which resembles the gavial in head shape, has no more than 21 teeth on the upper jaw and 20 on the lower. See also: Alligator (/content/alligator/024200); Crocodile (/content/crocodile/168600); Gavial (/content/gavial/282900)

Fig. 2 Comparative lateral view of crocodylian snouts, showing tooth arrangement of (a) Gavialidae and Crocodylinae, and (b) Alligatorinae.

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Fig. 3 Dorsal view of crocodylian heads.

Jay M. Savage W. Ben Cash

Bibliography

E. H. Colbert, The Age of Reptiles, 1965

E. Jarvik, Basic Structure and Evolution of Vertebrates, 2 vols., 1981

S. P. Parker (ed.), Synopsis and Classification of Living Organisms, 2 vols., 1982

F. Pough, Herpetology, 3d ed., Prentice Hall, Upper Saddle River, 2004

A. S. Romer, Vertebrate Paleontology, 3d ed., 1966

K. P. Schmidt and R. F. Inger, Living Reptiles of the World, 1957

S. W. Williston, Water Reptiles of the Past and Present, 1914

G. R. Zug, L. J. Vitt, and J. P. Caldwell, Herpetology, Academic Press, San Diego, 2001

Additional Readings

R. W. McDiarmid, M. S. Foster, and C. Guyer (eds.), Reptile Biodiversity, University of California Press, London, 2012

J. K. Njau and R. J. Blumenschine, Crocodylian and mammalian carnivore feeding traces on hominid fossils from FLK 22 and FLK NN 3, Plio-Pleistocene, Olduvai Gorge, Tanzania, J. Hum. Evol., 63(2):408–417, 2012 DOI: 10.1016/j.jhevol.2011.05.008 (http://dx.doi.org/10.1016/j.jhevol.2011.05.008)

M. Stein et al., Humeral morphology of the early Eocene mekosuchine crocodylian Kambara from the Tingamarra Local Fauna southeastern Queensland, Australia, Alcheringa, 36(4):473–486, 2012 DOI: 10.1080/03115518.2012.671697 (http://dx.doi.org/10.1080/03115518.2012.671697)

D. B. Weishampel and C. M. Jianu, Transylvanian Dinosaurs, The Johns Hopkins University Press, Baltimore, MD, 2011

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American Society of Ichthyologists and Herpetologists (http://www.asih.org/)

Center for North American Herpetology (http://www.cnah.org/)

Crocodile Specialist Group (http://www.flmnh.ufl.edu/natsci/herpetology/crocs.htm)

Society for the Study of Amphibians and Reptiles (http://www.ssarherps.org/)

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