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A Production of

Alligator mississippiensis, American Alligator
Dr. Timothy Rowe - The University of Texas at Austin
Christopher Brochu, Matthew Colbert, Kyoko Kishi, John Merck
Alligator mississippiensis
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skull
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Texas Memorial Museum (TMM M-983)

Image processing: Dr. Christopher Brochu
Image processing: DigiMorph Staff
Publication Date: 10 Jul 2003

juvenile | adult

ITIS TNS Google MSN

See also The Visible Alligator Skull website.

What is an alligator?

The American alligator (Alligator mississippiensis) is a large crocodylian reptile found throughout the southeastern United States.  Individuals as long as 19 feet (6.5 m) have been reported, but ten or eleven feet (3.5 m) is more typical for large adults. 

Alligators are semiaquatic, spending most of their time in the water but coming out on land to absorb heat periodically.  They eat virtually anything they can swallow, but are not generally considered dangerous to humans - very few attacks have been reported.  Other crocodylians, like the Nile crocodile of Africa and the Indopacific saltwater crocodile, are genuine maneaters.

There is another kind of alligator - the Chinese alligator (Alligator sinensis) - found in China. (To see a picture of a Chinese alligator, you can access the Crocodilian Photo Gallery.) Closely related to the alligators are the caimans, which are today found through much of Latin America.  Together, the alligators and caimans form a group called Alligatoridae.  The word "alligator" is sometimes used to refer to any member of Alligatoridae.

Like other crocodylians, female alligators lay their eggs on land.  They guard their nests and, when the young alligators hatch, they protect the hatchlings.  Baby alligators make a grunting sound to attract their mothers when they sense danger.  Alligators are about nine inches (22.5 cm) long when they hatch.  If fed well, young alligators grow quickly and can be 3 feet long (1 m) by the time they are three years old.

Like other reptiles, alligators are ectothermic ("cold-blooded") and cannot generate their own body heat.  For this reason, they spend part of their time basking in the sun to absorb heat.

Why are alligators important?

In the Florida everglades, adult alligators excavate deep pits in the marshland.  These are called alligator holes, and during the dry season they remain filled with water.  They can be the only remaining standing water over a large area, and as such alligators help maintain this fragile wetland.

Alligators are also the top predator throughout much of their range.  They fill an important role in keeping the populations of other wetland animals from growing out of control.

Alligators can also be economically important.  Wild alligators were nearly hunted to extinction during this century because of demand for their skin, which can be used as exotic leather.  Enlightened conservation policies during the 1970's and 1980's brought the alligator back from the brink of extinction, and alligator skins can now be provided to the leather industry from alligator farms and carefully regulated hunting.

Who are the Crocodylians?

Crocodylia includes living alligators, crocodiles, and gharials, and some of their closest extinct relatives.  There are currently 23 living species within this group, but crocodylian diversity was much higher in the past. 

The definition of Crocodylia has fluctuated in recent years.  Older books may use the word "Crocodylia" to refer to living crocodylians and a large assemblage of extinct animals extending back to the Triassic period (roughly 245 million years ago).  More recently, we have restricted the name Crocodylia to the last common ancestor of living alligators, crocodiles, and gharials, and all of its descendents. This excludes many of the extinct animals formerly considered to be crocodylians; these now belong to a larger group called Crocodyliformes.  The oldest crocodylians are from the Late Cretaceous, roughly 80 million years ago.

You may have noticed that I spell "crocodylian" with a "y" rather than an "i", as in other books (i.e., "crocodilian").  The new spelling indicates that the more restricted defnition is being used.

What's the difference between an alligator and a crocodile?

This is one of the most common questions crocodile researchers face. There are 23 living species of crocodylian, and in fact not all are properly alligators or crocodiles - the Indian gharial (Gavialis gangeticus) is in a separate crocodylian lineage. Moreover, while some books use the word "crocodile" specifically for one of the crocodylids or "true crocodiles" (members of Crocodylus and Osteolaemus), others use "crocodile" to refer to any crocodylian, or even to a larger group of animals including Crocodylia and some extinct creatures not belonging to Crocodylia in the strictest sense. As such, the question "what's the difference between an alligator and a crocodile?" can be difficult to answer.

Systematists are scientists who study the diversity of living things and its organization. As a crocodile systematist, I regard a "crocodile," to be one of the the eleven living species of Crocodylus the single species of West African dwarf crocodile (Osteolaemus tetraspis), and those fossil taxa more closely related to them than to other crocodylians. The question is this: how are these different from alligators?

Many popular books state that alligators have blunt snouts, and crocodiles have narrow, pointed snouts. This sometimes works. Look at these two crocodylians:

Which one is the alligator, and which is the crocodile? The animal on the left is an American alligator (Alligator mississippiensis), and the animal on the right is a Cuban crocodile (Crocodylus rhombifer). In this case, the snout shape criterion works - the alligator does, indeed, have a broader, blunter snout.

But this doesn't always work!

For example, look at these two skulls:

If we can't always use snout shape, what can we do?

Which is the alligator? Which is the crocodile? In this case, the skull on the left is narrow, and that on the right is broad. Is the skull on the left a crocodile? In this case, no. The skull on the right, although broad, comes from a specimen of Morelet's crocodile (Crocodylus moreletii) from Mexico; the skull on the left is a rare form of common caiman (Caiman crocodilus apaporiensis), an alligatorid from Colombia. In this case, the snout shape criterion fails.

Left:Field Museum of Natural History 69812.
Right:Texas Memorial Museum M-4980.

Perhaps the best single distinguishing characteristic between alligators and crocodiles is occlusion - the way the teeth come together when the jaws are closed. In alligators, the lower teeth occlude inside the upper teeth, and there is a complete overbite, as shown by the American alligator skull to the left.

But in crocodiles, the lower and upper teeth occlude along the same line, and the teeth interfinger. As a result, there is no overbite and the lower teeth are visible when the jaws are closed. This skull (Texas Memorial Museum of Science and History M-6801) from a Morelet's crocodile shows this well.

Did the alligator occlusion pattern evolve from the crocodile pattern, or was it the other way around? As it turns out, the common ancestor of alligators and crocodiles had an occlusal pattern "intermediate" between that of its living descendents.

This is the skull of an extinct animal closely related to living crocodylians (U.S. National Museum 22039). It comes from the middle Cretaceous (roughly 100 million years ago) of central Texas. It shows the occlusal pattern seen in the common ancestor of alligators and crocodiles. Note that the teeth toward the back of the jaw show an alligator-like overbite. But the fourth lower tooth, which is enlarged in all crocodylians, passes through a notch on the skull, as in crocodiles. Alligatorids lost this notch, and crocodylids lost the posterior overbite.


About the Species

The scanned skull (TMM M-983) is from the partial skeleton of a young Alligator mississippiensis from the Florida Ever-glades that is housed in the collections of the Vertebrate Paleontology Laboratory of the Texas Memorial Museum of Science and History. The specimen was flensed and dried, defleshed with dermestid beetles, and degreased in dilute ammonia. Sex of the specimen is unknown. A video of the specimen is presented in the "Real Alligator Skull Viewer."

A young specimen was chosen as part of a systematic comparison between juvenile specimens of modern crocodylian species, using high-resolution X-ray CT (Brochu, 1997a b, c, unpubl. data). The ontogenetic immaturity of the specimen at time of death is indicated by many features. Most obviously, the skull length (217 mm from snout tip to back of jaws) is less than half that of the largest known skulls. The laterosphenoids, which arise early in ontogeny from paired ossifications on either side of the braincase (de Beer, 1937), approach each other but remain separated in this specimen ("ls" in tvs. 85-120). In fully mature individuals the two bones are in contact along the midline of the skull. In addition, the basioccipital remains separated from the exoccipitals by a widely open suture (cor. 150-185), whereas in the largest known specimens these bones are fused. In gross shape, larger skulls also have relatively broader snouts (Mook, 1921; Kälin, 1933; Hall and Portier, 1994). In the post-cranium, the neurocentral sutures are open throughout the presacral column and as far back as the sixth caudal vertebra, another indication of immaturity (Brochu, 1996). The vomers and both stapes are missing from this skull, presumably an artifact of preparation. We note that some rostral structures are not bilaterally symmetrical and this may complicate interpretation of some slices.

About this Specimen

This specimen was scanned in the horizontal plane for a total of 135 slices, each 0.48 mm thick with an interslice spacing of 0.48 mm. The dataset displayed on this site was generated in 1995 by Scientific Measurement Systems, Inc., of Austin, Texas, using an SMS 101 scanner that was built in the 1980's. During the scanning, the scanner evidently under went a shutdown-restart when the X-ray path reached approximately one centimeter ventral to the palate. This produced a shift in image contrast in some of the slices at this level, which is most obvious in the coronal slices passing through the lateral pterygoid flanges.

Owing to the orientation of the skull during scanning, the sagittal and coronal synthetic slices deviate from the true sagittal and coronal planes by approximately two degrees. For example, sagittal slice 195 passes directly through the midline at the back of the skull, where one can see the choanal septum, the anterior and posterior channels of the median eustachian opening, and the full extent of the basisphenoid rostrum. However, this slice passes through the right first dentary alveolus rather than the mandibular symphysis. Please note: on this website the term "horizontal" refers to the "transverse" plane of the old website and the published dataset, Alligator: Digital Atlas of the Skull.

About the
Scan
Literature

For general information for the nonspecialist:

Grenard, S. 1991. Handbook of Alligators and Crocodiles. Kreiger Publishing Co., Malabar, Florida, USA.

Guggisberg, C.A.W. 1972. Crocodiles - their natural history, folklore, and conservation. Newton Abbot (Devon, England), 195 pp.

McIlhenny, E.A. 1935. The Alligator's Life History. Boston: Christopher Publishing House.

Neill, W.T. 1971. The Last of the Ruling Reptiles: Alligators, Crocodiles and their Kin. Columbia Univ. Press, New York.

Ross, C.A., and S. Garnett. (Eds.). 1989. Crocodiles and Alligators. Facts On File, Inc., New York.  **perhaps the best book available on this subject.

More technical literature:

(please note: this is NOT an exhaustive list of the available literature. This is largely a list of references used in the completion of the computed tomography CD entitled "Alligator: Digital Atlas of the Skull." For a more complete listing of the literature, we recommend Mason Meers' on-line Bibliography of Crocodilian Biology.

Abel, O. 1928. Allognathosuchus, ein an de cheloniphage Nahrungsweise angepa§ter Krokodiltypus des nordamerikanischen Eozäns. Paläontologisches Zeitschrift, 9:367-374.

Bartels, W.S. 1984. Osteology and systematic affinities of the horned alligator Ceratosuchus (Reptilia, Crocodilia). Journal of Paleontology, 58:1347-1353.

Berg, D.E. 1966. Die Krokodile, insbesondere Asiatosuchus und aff. Sebecus?, aus dem Eozän von Messel bei Darmstadt/Hessen. Abhandlungen des Hessischen Landesamtes für Bodenforschung, 52:1-105 + 6 plates.

Bertau, M. 1935. Zur Entwicklungsgeschichte des Geruchsorgans der Krokodile. Zeitschrift für Anatomie und Entwicklungsgeschichte, 104:169-202.

Brochu, C.A. 1996a. Closure of neurocentral sutures during crocodilian ontogeny: implications for maturity assessment in fossil archosaurs. Journal of Vertebrate Paleontology, 16:49-62.

_____. 1996b. New eusuchian crocodyliforms from the Paleocene of West Texas: biogeographic and phylogenetic implications. Geological Society of America Abstracts with Programs, 28:6.

_____. 1997a. A review of "Leidyosuchus" (Crocodyliformes, Eusuchia) from the Cretaceous through Eocene of North America. Journal of Vertebrate Paleontology 17:679-697.

_____. 1997b. Phylogenetic systematics and taxonomy of Crocodylia. Ph.D. Dissertation, University of Texas at Austin.

Brühl, C.B. 1862. Das Skelett der Krokodilinen. Wilhelm Braumüller, Vienna, 48 pp + 20 plates.

Buffetaut, E. 1985. The place of Gavialis and Tomistoma in eusuchian evolution: a reconciliation of palaeontological and biochemical data. Neues Jahrbuch für Geologie und Paleontologie Monatshefte, 12:707-716.

Busbey, A.B. 1994. The structural consequences of skull flattening in crocodilians; pp. 173-192 in J.J. Thomason (ed.), Functional Morphology in Vertebrate Paleontology, Cambridge University Press, New York.

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Carlson, W. 1993. A brief introduction to computed X-ray tomography; in Rowe, T., W. Carlson, and W. Bottorf (eds.), Thrinaxodon: Digital Atlas of the Skull. The University of Texas Press. CD-ROM.

Case, E.C. 1925. Note on a new species of the Eocene crocodilian Allognathosuchus, A. wartheni. Contributions from the Museum of Geology, University of Michigan, 2:93-97.

Clark, J.M. 1994. Patterns of evolution in Mesozoic Crocodyloformes; pp. 84-97 in N.C. Fraser and H.D. Sues (eds.), In the Shadow of the Dinosaurs, Cambridge Univ. Press, New York.

Clark, J.M., and M.A. Norell. 1992. The Early Cretaceous crocodylomorph Hylaeochampsa vectiana from the Wealden of the Isle of Wight. Amer. Mus. Novitates, 3032:1-19.

Colbert, E.H. 1946. The eustachian tubes in the Crocodilia. Copeia, 1946:11-14.

Conroy, G.C., and M.W. Vannier. 1984. Noninvasive three-dimensional computer imaging of matrix-filled fossil skulls by high-resolution computed tomography. Science, 226:1236-1239.

Cott, Hugh B. 1961. Scientific results of an inquiry into the ecology and economic status of the Nile Crocodile (Crocodilus niloticus) in Uganda and Northern Rhodesia. Transactions of the Zoological Society of London (B), 29:211-357.

Currie, P.J. 1985. Cranial anatomy of Stenonychosaurus inequalis (Saurischia, Theropoda) and its bearing on the origin of birds. Canadian Journal of Earth Science, 22:1643-1658.

_______, and X.I. Zhao. 1993. A new troodontid (Dinosauria, Theropoda) braincase from the Dinosaur Park Formation (Campanian) of Alberta. Canadian Journal of Earth Science, 30:2231-2247.

de Beer, G.R. 1937. The Development of the Vertebrate Skull. Clarendon Press, Oxford.

Dal Sasso, C., and G. Pinna. 1997. Besanosaurus leptorhynchus n. gen. n. sp., a new shastasaurid ichthyosaur from the Middle Triassic of Besano (Lombardy, N. Italy). Paleontologia Lombarda, 4: 00-00.

Densmore, L.D. 1983. Biochemical and immunological systematics of the order Crocodilia; pp. 397-465 in M.K. Hecht, B. Wallace, and G.H. Prance (ed.), Evolutionary Biology, Vol. 16, Plenum Press, New York.

Densmore, L.D., and H.C. Dessauer. 1984. Low levels of protein divergence detected between Gavialis and Tomistoma: Evidence for crocodilian monophyly. Comp. Biochem. Phys., 77B(4):715-720.

Densmore, L.D., and R.D. Owen. 1989. Molecular systematics of the order Crocodilia. Am. Zool., 29:831-841.

Densmore, L.D., and P.S. White. 1991. The systematics and evolution of the Crocodilia as suggested by restriction endonuclease analysis of mitochondrial and nuclear ribosomal DNA. Copeia, 1991:602-615.

Erickson, B.R. 1972. Albertochampsa langstoni, gen. et sp. nov., a new alligator from the Cretaceous of Alberta. Scientific Publications of the Science Museum of Minnesota, New Series, 2:1-13.

Estes, R., and J.H. Hutchison. 1980. Eocene lower vertebrates from Ellesmere Island, Canadian Arctic Archepelago. Palaeogeography, Palaeoclimatology, Palaeoecology, 30:325-347.

Ferguson, M.W.J. 1981. The structure and development of the palate in Alligator mississippiensis. Achives for Oral Biology, 26:427-443.

_____. 1984. Craniofacial development in Alligator mississippiensis; pp. 223-273 in M.W.J. Ferguson (ed.), The Structure, Development and Evolution of Reptiles, Symposia of the Zoological Society of London 52, Academic Press, New York.

_____. 1985. Reproductive biology and embryology of the crocodilians; pp. 329-492 in C. Gans, F. Billett, and P.F.A. Maderson (ed.), Biology of the Reptilia, Vol. 14, John Wiley and Sons, New York.

Gatesy, J., R. de Salle, and W. Wheeler. 1993. Alignment-ambiguous nucleotide sites and the exclusion of systematic data. Molecular Phylogenetics and Evolution, 2:152-157.

Gilmore, C.W. 1946. A new crocodilian from the Eocene of Utah. Journal of Paleontology, 20:62-67.

Grine, F. E. 1997. Dinocephalians are not anomodonts. Journal of Vertebrate Paleontology, 17:177-183.

Groombridge, B. 1987. The distribution and status of world crocodilians; pp. 9-21 in G.J.W. Webb, S.C. Manolis, and P.J. Whitehead (ed.), Wildlife Management: Crocodiles and Alligators, Surrey, Beatty and Sons, Chipping Norton, NSW.

Hall, P.M., and K.M. Portier. 1994. Cranial morphometry of New Guinea crocodiles (Crocodylus novaeguineae): ontogenetic variation in relative growth of the skull and an assessment of its utility as a predictor of the sex and size of individuals. Herpetological Monographs, 8:203-225.

Hass, C.A., M.A. Hoffman, L.D. Densmore, and L.R. Maxson. 1992. Crocodilian evolution: insights from immunological data. Molecular Phylogenetics and Evolution, 1:193-201.

Hasse, C. 1873. Das Gehörorgan der Crocodile nebst weiterem vergleichend anatomischen Bemerkungen über das mittlere Ohr der Wirbeltiere und dessen Annexa. Anatomische Studien, 4:679-750.

Higgins, D.W. 1972. A review of Oligocene alligators from the Big Badlands of South Dakota. M.S. Thesis, South Dakota School of Mines and Technology.

Holman, J.A. 1995. Pleistocene Amphibians and Reptiles in North America. Oxford University Press, New York, 243 pp.

Hochstetter, F. 1906. Beiträge zur Anatomie und Entwicklungsgeschichte der Blutgefä§systems der Krokodile. Reise in Ostafrika, 4:1-139.

Huxley, T.H. 1875. On Stagonolepis robertsoni, and on the evolution of the Crocodilia. Quart. Jour. Geo. Soc. London, 31:423-438.

Iordansky, N.N. 1973. The skull of the Crocodilia; pp. 201-260 in C. Gans and T. Parsons (ed.), Biology of the Reptilia, Vol. 4, Academic Press, London.

Jackson, K., D.G. Butler, and D.R. Brooks. 1996.  Habitat and phylogeny influence salinity discrimination in crocodilians: implications for osmoregulatory physiology and historical biogeography. Biological Journal of the Linnean Society, 58:371-383.

Joeckel, R.M. 1992. Comparative anatomy and function of the leptaucheniine oreodont middle ear. Journal of Vertebrate Paleontology, 12(4):505-523.

Kälin, J.A. 1933. Beiträge zur vergleichenden Osteologie des Crocodilidenschädels. Zoologisches Jahrbucher, 57:535-714.

_____. 1936. Hispanochampsa mülleri nov. gen. nov. sp., ein neuer Crocodilide aus dem unteren Oligocän von Tarrega (Catalonien). Abhandlungen der Schweitzerische Paläontologische Geselschaft, 58:1-40.

 _____. 1939. Ein extrem kurzschnauziger Crocodilide aus den Phosphoriten des Quercy, Arambourgia (nov. gen.) gaudryi de Stefano. Abhandungen der Schweizerischen Palaeontologischen Gesellschaft, 62:1-18 + 3 plates.

Klembara, J. 1991. The cranial anatomy of early ontogenetic stages of Alligator mississippiensis (Daudin, 1802) and the significance of some of its cranial structures for the evolution of tetrapods. Paleontographica, 215:103-171.

Kumazawa, Y., and M. Nishida. 1995. Variations in mitochondrial tRNA gene organization of reptiles as phylogenetic markers. Molecular Biology and Evolution, 12:759-772.

Li, J. 1987. A new species of Alligator from Shanwang, Shandong. Vertebrata PalAsiatica, 7:199-207.

Luo, Z., and D.R. Ketten. 1991. CT scanning and computerized reconstructions of the inner ear structure of multituberculate mammals. Journal of Vertebrate Paleontology, 11:220-228.

Malone, B. 1979. The systematics, phylogeny and paleobiology of the genus Alligator. Ph.D. Dissertation, City University of New York.

Mook, C.C. 1921a. Skull characters of recent Crocodilia with notes on the affinities of the recent genera. Bulletin of the American Museum of Natural History, 44:123-268.

_____. 1921b. Allognathosuchus, a new genus of Eocene crocodilians. Bulletin of the American Museum of Natural History, 44:105-110.

_____. 1932. A study of the osteology of Alligator prenasalis (Loomis). Bulletin of the Museum of Comparative Zoology, 74:19-41.

_____. 1946. A new Pliocene alligator from Nebraska. American Museum Novitates, 1311:295-304.

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_____. 1850b. Monograph on the Fossil Reptilia of the London Clay, and of the Bracklesham and Other Tertiary Beds, Part II: Crocodilia (Crocodilus, etc.). Paleontographical Society, London, 50 pp. + 11 plates.

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Links

IUCN Crocodile Specialist Group

Bibliography of Crocodilian Biology

Crocodilian Natural History and Conservation

Crocodilians on the Internet

Croc Portrait Gallery

Literature
& Links

None available.

Additional
Imagery

To cite this page: Dr. Timothy Rowe, Christopher Brochu, Matthew Colbert, Kyoko Kishi, John Merck, 2003, "Alligator mississippiensis" (On-line), Digital Morphology. Accessed December 19, 2014 at http://digimorph.org/specimens/Alligator_mississippiensis/adult/.

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