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Reptilia, Archosauria, Crocodylomorpha, Crocodylia, Alligatoridae, Alligatorinae


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Common Name: Olsenís Alligator

Source of Species Name: Named for Russel Olsen, who prepared the original holotype specimen for the Museum of Comparative Zoology at Harvard University (White, 1942).

Age Range: early Miocene (middle Hemingfordian), from about 18 to 16 million years ago. Possibly extends back to very early Miocene (late Arikareean), about 23-22 million years ago.

Florida Fossil Occurrences:

Florida map with occurrences indicated

Figure 1. Map of Florida, with black circles indicating counties where fossils of Alligator olseni have been found (the circles do not indicate a specific location within the county where the fossils were found, and some counties may have two or more different locations producing this species).

Florida Fossil Sites with Alligator olseni:
Gadsden County—Midway Local Fauna; Milwhite Gunnfarm Mine
Gilchrist County—Thomas Farm
Leon County—Griscom Plantation; Seaboard Local Fauna

Overall Geographic Range: Southeastern United States, with occurrences in southeastern Texas (possibly) and Florida. Type locality is Thomas Farm, near Bell, Gilchrist County, Florida (White, 1942).

Comments: Alligator olseni is an intermediate species nested between primitive and derived members of the genus Alligator (Malone, 1979; Brochu, 1999, 2004, 2011; Snyder, 2001, 2007). It was initially described from Thomas Farm by White (1942), and all conclusive material representing this species has since been collected from the same locality (Brochu, 1999; Snyder, 2001, 2007). Excavations at Thomas Farm continue on a regular basis, and Alligator olseni fossils are still discovered and collected by scientists and volunteers alike at this site (Fig. 2).

Skull of Olsen's alligator

Figure 2. UF 205700, a well-preserved Alligator olseni skull collected in 1999 from the Thomas Farm locality in Gilchrist County, Florida in dorsal (top) and ventral (bottom) views.

The skeleton of Alligator olseni outwardly looks very similar to that of the modern American alligator (Alligator mississippiensis), but they can be distinguished by some subtle characteristics. The splenial bone, found on the internal (medial) surface of the lower jaw in crocodylians, rarely preserves as a fossil. However, the preserved suture lines on the mandible are typically visible and can be used to determine how far anteriorly the splenial extended. In Alligator olseni, the splenial usually extends into the mandibular symphysis (Fig. 3; White, 1942; Malone, 1979; Snyder, 2001, 2007). In Alligator mississippiensis, the splenial does not extend this far, and ends posterior to the mandibular symphysis (Fig. 3).

Comparison of extension of splenial on dentary

Figure 3. Comparison of the left mandibles of UF 176192, Alligator olseni (top), and UF 147700, Alligator mississippiensis (bottom). Note the preserved suture lines indicating participation of the splenial bone in the mandibular symphysis in Alligator olseni and the exclusion of the splenial bone from the mandibular symphysis in Alligator mississippiensis.

In addition, adult Alligator olseni specimens are generally smaller than adult Alligator mississippiensis specimens, with few individuals from Thomas Farm likely exceeding more than about seven to eight feet (2.5 meters) in length (Meylan et al., 2001). Fossils of Alligator mississippiensis from Florida frequently represent individuals that exceeded ten feet (3.3 meters) in length, and are found in much younger deposits (Meylan et al., 2001). The snout of Alligator olseni is also comparatively blunter than the snout of Alligator mississippiensis, and there is often a higher degree of ornamentation on the dorsal skull surface near the orbits in Olsenís alligator than in the American alligator. For more on the osteology of Alligator olseni and comparisons with other named species of Alligator, see White (1942) and Snyder (2007).

There are some other transitional forms potentially separating Olsenís alligator and the American alligator, although their exact placement within the genus Alligator is debated. Brochuís (1999, 2004, 2011) analyses placed Alligator sinensis, the extant Chinese alligator, as a closer relative to the modern American alligator than Alligator olseni, but Snyder (2001, 2007) found it basal to Alligator olseni and Alligator mississippiensis. This debate brings into focus the question of how and when the ancestor of the Chinese alligator dispersed into Asia from North America. See Brochu (1999, 2003) and Snyder (2001, 2007) for more on this topic. The validity of Alligator thomsoni Mook, 1923 and Alligator mefferdi Mook, 1946, two other Miocene Alligator species, are also disputed. Malone (1979) considered both to be synonymous with Alligator mississippiensis, while Brochu (1999) and Snyder (2001, 2007) regarded both to be valid species closely related to the modern American alligator. The placement and validity of different species within the genus Alligator is an ongoing endeavor subject to frequent debate.

Other fossils from Florida assigned to Alligator olseni were collected from the Midway and Milwhite Gunnfarm Mine localities of Gadsden County, as well as the Griscom Plantation and Seaboard localities of Leon County (Auffenberg, 1957; Olsen, 1964, 1968; Stoutamire, 1975). This fossil material consists of isolated teeth, osteoderms, and a few fragmentary vertebrae and skull bones. These sites are estimated to similar in age with, and geographically close to the type locality for the species (Fig. 1; Woodburne, 2004). In addition, the morphology and sizes of the fossils from these sites are consistent with similar elements from Thomas Farm (Auffenberg, 1957). Teeth and partial osteoderms of the unrelated crocodylian Gavialosuchus are also potentially present at some of these sites (Auffenberg, 1957). This complicates the assignment of these isolated remains, but the simplest way to differentiate between the teeth and osteoderms of Alligator and Gavialosuchus is to compare them with specimens of known affinity (Meylan et al., 2001). Osteoderms, the highly pitted dermal bones forming the scutes running along the dorsal surface (back) of crocodylians, are essentially flat and possess no midline dorsal keel in Gavialosuchus. However, in Alligator, the osteoderms possess a pronounced midline dorsal keel and are not flat (Fig. 4). Regarding dentition, the teeth of Gavialosuchus are typically narrower in diameter and taper to a sharper tip, while those of Alligator are usually blunter at the tip and wider in diameter (Fig. 5).

Comparison of osteoderms

Figure 4. Comparison of fossil crocodylian osteoderms from Miocene localities in Florida. A, D, UF 206718, Alligator olseni osteoderm from the Thomas Farm type locality in Florida. B, E, UF/FGS 1439, Alligator olseni osteoderm from the Griscom Plantation locality in Florida. C, F, UF 156780, Gavialosuchus americaus osteoderm from the Haile 5B locality in Florida. Images in top row in dorsal view and images in bottom row appear in anterior view (scale bar changes between the Alligator and Gavialosuchus images). Note the prominent midline dorsal keels present on the Alligator osteoderms contrasted with the flat Gavialosuchus osteoderm.

Comparison of teeth

Figure 5. Comparison of fossil crocodylian teeth from Miocene localities in Florida and Texas. A, UF 248574, Alligator olseni tooth from the Thomas Farm type locality in Florida. B, UF/FGS 10991, Alligator olseni tooth from the Seaboard locality in Florida. C, LSUMG/V-2255, cast of tooth referred to Alligator olseni from the Toledo Bend locality in Texas. D, UF/FGS 10992, Gavialosuchus tooth from the Seaboard locality in Florida. All images appear in lingual view. Note the different shape and texture of the Gavialosuchus tooth compared to the Alligator teeth, as well as the size disparity between the Florida and referred Texas Alligator teeth.

Albright (1994) reported Alligator olseni fossils from the Toledo Bend locality in Texas near its border with Louisiana, although the age does not agree with that of the type locality. This locality is estimated at approximately 23-22 million years ago in the late Arikareean (very early Miocene), which is notably older than the Hemingfordian (early Miocene) age of Thomas Farm (Woodburne, 2004). Albright (1994) used the characteristic splenial to conclude that the material from Toledo Bend represents Alligator olseni, although this was not confirmed by Snyder (2007) or mentioned by Brochu (1999). A cast of an alligatorid tooth (Fig. 4C) from Toledo Bend in the FLMNH collections (the original fossil is housed in the LSU Museum of Geology) is much larger than definitive Alligator olseni specimens from Florida, and may instead represent a different, larger alligatorid taxon. Until the fossils described by Albright (1994) are reexamined, the assignment of the Toledo Bend material to Alligator olseni is questionable. However, if this material does represent Alligator olseni, Albrightís (1994) record would provide significant range extensions in body size as well as geographic and geologic occurrence for the species.

Since its initial discovery and description (White, 1942), Alligator olseni has continued to be an important taxon when attempting to determine evolutionary relationships within Alligator. The Thomas Farm locality regularly produces well-preserved specimens, including several skulls found by museum scientists and volunteers over the past two decades. Although the evolutionary relationships of the species within the genus Alligator are not absolutely resolved and many different evolutionary hypotheses exist (Mook, 1946; Malone, 1979; Brochu, 1999, 2004, 2011; Snyder, 2001, 2007), Alligator olseni continues to be one of the best known extinct species within the genus and an important focus of research by crocodylian specialists.

Database of total available images of this species:

Catalog number Element Available views
UF 205700 skull dorsal   ventral
UF 200362 dentary lateral   dorsal   medial
UF 206920 dentary lateral   dorsal   medial
UF 248574 tooth lateral   medial
UF/FGS 10991 tooth medial
UF 199189 humerus anterior  dorsal   ventral
UF 1361 ulna anterior   posterior   lateral   medial
UF 161099 tibia anterior   posterior   lateral   medial
UF 206926 fibula anterior   posterior   lateral   medial
UF 204241 calcaneum anterior   posterior   lateral   medial
UF 206718 osteoderm dorsal   ventral   lateral
UF/FGS 1439 osteoderm dorsal   ventral   lateral

Scientific Publications and Other References Cited:

Auffenberg, W. 1957. Notes on fossil crocodilians from southeastern United States. Quarterly Journal of the Florida Academy of Sciences, 20(2):107-113.

Brochu, C. A. 1999. Phylogenetics, taxonomy, and historical biogeography of Alligatoroidea. Journal of Vertebrate Paleontology, 19(2):9-100. http://www.jstor.org/stable/10.2307/3889340

Brochu, C. A. 2003. Phylogenetic approaches toward Crocodylian history. Annual Review of Earth and Planetary Sciences, 31:357-397. http://www.annualreviews.org/doi/abs/10.1146/annurev.earth.31.100901.141308

Brochu, C. A. 2004. Alligatorine phylogeny and the status of Allognathosuchus Mook, 1921. Journal of Vertebrate Paleontology, 24(4):857-873. http://www.jstor.org/stable/10.2307/4524781

Brochu, C. A. 2011. Phylogenetic relationships of Necrosuchus ionensis Simpson, 1937 and the early history of caimanines. Zoological Journal of the Linnean Society, 163:S228-S256. http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.2011.00716.x/full

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

Meylan, P. A., W. A. Auffenberg, and R. C. Hulbert Jr. 2001. Reptilia 2: Lizards, Snakes, and Crocodilians. Pp. 137-151 in R. C. Hulbert (ed.), The Fossil Vertebrates of Florida. University Press of Florida, Gainesville.

Mook, C. C. 1923. A new species of Alligator from the Snake Creek beds. American Museum Novitates, Number 73, 13 p. http://digitallibrary.amnh.org/dspace/handle/2246/4668

Mook, C. C. 1946. A new Pliocene Alligator from Nebraska. American Museum Novitates, Number 1311, 12 p. http://digitallibrary.amnh.org/dspace/handle/2246/2307

Olsen, S. J. 1964. The stratigraphic importance of a lower Miocene vertebrate fauna from north Florida. Journal of Paleontology, 38(3):477-482. http://www.jstor.org/stable/1301523

Olsen, S. J. 1968. Miocene vertebrates and north Florida shorelines. Palaeogeography, Palaeoclimatology, Palaeoecology, 5(1):127-134. http://dx.doi.org/10.1016/0031-0182(68)90065-5

Snyder, D. 2001. Evolution of the modern Alligator lineage. M.S. Thesis, University of Florida.

Snyder, D. 2007. Morphology and systematics of two Miocene alligators from Florida, with a discussion of Alligator biogeography. Journal of Paleontology, 81(5):917-928. http://www.jstor.org/stable/10.2307/4498848

Stoutamire, S. 1975. A new middle Miocene vertebrate fauna from the Florida panhandle. M.S. Thesis, Texas Tech University. http://hdl.handle.net/2346/15763

White, T. E. 1942. A new alligator from the Miocene of Florida. Copeia, 1:3-7. http://www.jstor.org/stable/10.2307/1437933

Woodburne, M. O. (editor) 2004. Late Cretaceous and Cenozoic Land Mammals of North America: Biostratigraphy and Geochronology. Columbia University Press, New York, 391 p.

Original Author(s): Evan T. Whiting

Original Completion Date: December 8, 2012

Editor(s) Name(s): Richard C. Hulbert Jr.

Last Up-dated On: April 10, 2013

This material is based upon work supported by the National Science Foundation under Grant Number CSBR 1203222, Jonathan Bloch, Principal Investigator. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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