Southeastern Giant Beavers of the Pleistocene have been Declared a Distinct Species from Northern Giant Beavers

The giant beaver of the Pleistocene was semi-aquatic like its modern living cousin (Castor canadensis), but it ate different plant foods, and therefore occupied a different ecological niche.  Giant beaver fossils are fairly common throughout the midwest but have also been found at numerous localities in the southeast, particularly Florida.  Scientists formerly thought southern giant beavers were the same species (Casteroides ohioensis) that ranged throughout the midwest and northeast.  The reason for this misconception was the lack of complete skulls in the collections of southern museums.  Skulls of giant beavers were excavated from the Leisey Shell Pits in Florida, but this site dates to the early Pleistocene, and paleontologists thought they represented a species that was ancestral to the late Pleistocene giant beaver, thus explaining the differences in skull characteristics.  However, a complete skull resembling those early Pleistocene giant beavers was discovered in the Cooper River in South Carolina, and this was from a late Pleistocene deposit.  Recently, paleontologists got their hands on 2 more giant beaver skulls dating to the late Pleistocene of Florida.  Scuba divers found 1 in Lake Rousseau, and the other was found in the Aucilla River.  After a careful anatomical analysis, scientists determined the late Pleistocene giant beaver of Florida, coastal Georgia, and coastal South Carolina was a different species than the giant beaver of the midwest and northeast.  They gave it the scientific name Casteroides dilophidus.

Photo: Giant Beaver, Castoroides ohioensis.

Size comparison between the Pleistocene giant beaver and the extant beaver.  The 2 species co-existed for 2 million years.  Scientists recently realized there were 2 different species of giant beaver–a northern and a southern.

Casteroides dilophidus had a shorter ridge on the top of its skull than C. ohioensis.  This ridge is known as the saggital crest.  One of its skull sutures bears in a different direction than that same suture on C. Casteroides, and C. dilophidus’s cheek row teeth are located differently in relation to the zygomatic arch.  The projection of the frontal bone of the eye socket is “better developed” in C. dilophidus than in C. casteroides, according to the study.  Some C. dilophidus specimens have grooves in their teeth that are never found in C. casteroides, but this can’t always be used as a distinguishing characteristic.  The authors of the study don’t have enough data to determine whether giant beaver fossils found in the mid-south (Alabama, Arkansas, Mississippi, and Tennessee) belong to C. dilophidus or C. ohioensis, so they suggest classifying those specimens as Casteroides sp.  Many fossil specimens (mostly teeth) of Casteroides have been found in the mid-south but no complete skulls.

Casteroides preferred treeless freshwater marshes where they could feed on the aquatic plants that flourished in full sunlight.  Wooded swamps and bottomlands are too shady for the plants they liked to eat.  Much erroneous speculation surmises modern extant beavers outcompeted Casteroides.  Instead, the smaller species of beaver created habitat favorable to Casteroides.  Extant beavers fell trees and open up the forest canopy, allowing succulent shade-intolerant vegetation to thrive.  Old beaver ponds eventually become filled with sediment and are converted to the wet treeless marshes Casteroides required.  Moreover, Casteroides co-existed with Castor canadensis for 2 million years.

An ecological cycle of alternating beaver species during the Pleistocene is apparent.  Castor canadensis converted wooded swamps and bottomlands to treeless marshes.  When trees became scarce, Castor canadensis would abandon the locality while Casteroides would move in. But Casteroides didn’t eat trees, allowing the forest to grow back. Castor canadensis would then recolonize the location as Casteroides moved away in search of a more open habitat.  The presence of both species in the fossil record reflects a varied environment and a much more diverse ecosystem than exists today.

Reference:

Hulbert, R. C.; A. Kirne, and G.S. Morgan

“Taxonomy of the Pleistocene Giant Beaver Casteroides (Rodentia: Casteroidae) from Southeastern U.S.”

Bulletin of the Florida Museum of Natural History 53 (2) 2014

 

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