The Lack of Pleistocene Crocodylus acutus Fossils in Southeastern North America and its Climatic Implications

The American crocodile (Crocodylus acutus) has a range limited to the tropics.  The American alligator (Alligator mississippiensis) can survive subfreezing air temperatures by  finding refuge in deep water holes, and they can even remain active in water as cold as 45 degrees Fahrenheit, but these temperatures kill crocodiles.  The unusual freezing temperatures that struck south Florida in 2009 killed 150 crocodiles–roughly 8% of the population there.  Freezing temperatures are the main limiting factor on the American crocodile’s range.  Scientists refer to this as the “winter air isotherm.”

Range of  Crocodylus acutus.

The American crocodile is an ancient species.  The oldest known fossils of this species were found in the Rio Tomayate River, El Salvador, and they date to about 1.5 million years BP.  It is an adaptable animal capable of swimming from Cuba to Florida.  Although most females have strict nesting ranges, some males are capable of impressive long distance dispersal.  One individual was tracked, captured repeatedly, and released from various locations after it traveled over 60 miles following each release.  Another straggler made its way as far north as Isle of Palms, South Carolina one summer.  So it’s clear that if it were not for sub-freezing winters, the American crocodile would have a much greater range.  Yet, as far as I can determine, it is absent from the Pleistocene fossil record of southeastern North America.  There are only 2 Pleistocene-aged specimens in the University of Florida Museum database, and these originated in Jamaica. 

Fossils of the extinct giant tortoise (Hesperotestudo crassicutata) have been found as far north as northwestern Georgia, and it apparently was common on the coastal plain of southeastern North America.  Many paleoecologists cite the presence of the giant land tortoise as evidence that there were no freezing temperatures in this region during the time the chelonians roamed the south.  They assume this species couldn’t survive subfreezing temperatures.  However, I’ve proposed that this close relative of the extant gopher tortoise (Gopherus polypherus), a burrow dweller, also dug burrows where they could escape frosts.  (See:https://markgelbart.wordpress.com/2011/04/15/the-extinct-pleistocene-giant-tortoise-hesperotestudo-crassicutata-must-have-been-able-to-survive-light-frosts/)  Or they utilized burrows dug by giant ground sloths. (See also:https://markgelbart.wordpress.com/2012/10/10/some-giant-ground-sloths-dug-long-burrows/)  If there were no freezing temperatures on the Atlantic coastal plain during the Ice Ages, American crocodile fossils should be at least occasionally found alongside the giant tortoise remains, but instead they are completely absent–evidence the region was never frost free during any climate phase of the Pleistocene.

American crocodiles may be maladapted for cold climates, but they are well adapted to live in saline environments.  They can survive on islands that are completely devoid of fresh water.  They prefer coastal swamps where they can nest on the borders of brackish canals.  They formerly nested on Miami Beach and the Florida Keys, but man rubbed out all the crocodiles on those prime real estate locations.  By 1970 there were fewer than 300 left in south Florida, but since they’ve been protected, the population here has bounced back to 2000.  They primarily inhabit the waters of the Turkey Point nuclear plant, and the numerous small islands in the southern part of the Everglades National Park.  They can grow up to 20 feet long but mostly feed on fish and birds.  They rarely attack large mammals.  American crocodiles hybridize with Cuban crocodiles in areas where their ranges overlap. 

Visual comparison between an alligator and an American crocodile.  Note the more narrow snout of the crocodile.

The Rio Tomayate Fossil Site

Paleontologists are especially interested in the Rio Tomayate fossil site because it is located in Central America where the Great American Faunal Interchange took place about 3 million years ago when a landbridge formed between North and South America.  There’s no shortage of potential fossil sites in the region but due to political instability the area has been understudied.  But in 2002 Juan Cisneros was able to collect fossils that emerged above water level in the Rio Tomayate, El Salvador.  The fossil bones were embedded in claystone that emerges above water level during the dry season.  The city of San Salvador uses the river as a sewer, so Dr. Cisneros had to make sure and wash his hands frequently during excavation.

The most common fossils were large bones of gompotheres (Cuvieronius tropicalis) and Ermeotheriums.  The former were close relatives of mastodons; the latter were  enormous ground sloths as big as elephants.  Both ranged as far north as South Carolina’s coastal plain during warm climate phases of the Pleistocene.

Mixotoxodon.

Gompothere.

Dr. Cisneros also found fossils of a mixotoxodon, an extinct notoungulate that ranged no farther north than Mexico.  It was a forest dweller about the size and build of a large rhino.  Fossils of other large species found at this site include a Megalonyx type of ground sloth, glyptodont, horse, 2 species of llamas, red brocket deer, white-tail deer, an unidentified large canid, giant tortoise, and American crocodile.  Llamas originally evolved in North America, then colonized South America during the Great American Faunal Interchange before becoming extinct in their land of origin.  This is the only site in Central America where fossil llamas have been found.  They also became extinct here.  The unidentified canid fossil consisted of a cheek bone with teeth.  Based on the size of the tooth, it was larger than a timber wolf.  It may belong to an unknown species or it could be from a Theriodictis platansis–an extinct, heavy-bodied dog known from South American fossil sites.

Not many fossils of small animals were found at this site.  The deposition favored the accumulation of large bones.  Water currents washed away most of the smaller bones.  However, Dr. Cisneros did find remains of mud turtles, a duck, and a rabbit hip bone.

Reference

Cisneros, J.C.

“New Pleistocene Vertebrate Fauna from El Salvador”

Revista Brasiliena de Paleontologica 8 (3) 2005