The Presence of the Extinct Pleistocene Giant Tortoises (Hesperotestudo sp.) is Evidence of Open Environments but not of Warmer than Present Day Climates

The extinct giant tortoises of North America are the most poorly studied species of Pleistocene megafauna.  A google search of the largest species–Hesperotestudo crassicutata–yields a blog article I wrote several years ago as the top result.  As far as I can determine, there has been no original research of the Hesperotestudo genus in the past 15 years.  I am unaware of any scientist who currently focuses their research on the Hesperotestudo genus.  The 2 foremost experts on this genus–the late William Auffenberg and the late Claude Hibbard–have been dead for decades.  It’s a shame few researchers are studying the paleoecology of these tortoises because they were probably keystone species as important as mammoths and mastodons in shaping the landscapes where they lived.

There were 2 species of tortoises in the Hesperotestudo genus living in southeastern North America during the Pleistocene–H. crassicutata, a large species, and H. incisa, a species intermediate in size between H. crassicutata and the extant gopher tortoise (Gopherus polyphemus).  The Hesperotestudo genus is considered to be in the same monophyletic clade as the gopher tortoise.  In 1960 Claude Hibbard wrote the presence of giant tortoises in the fossil record indicated mostly frost free climates.  He believed their presence meant warmer than present day climates in the southeast…during the Ice Ages.  His assumption has been repeated in dozens if not hundreds of scientific papers without question.  I challenge this assumption, and as far as I know, I’m the only person who does.  I believe tortoises in the Hesperotestudo genus burrowed in the ground and could escape freezing temperatures by retreating into their burrows.  William Auffenberg referred to these tortoises as “non-burrowing,” but he never conducted an anatomical study to determine whether or not they could burrow into the ground.  No one has.  (Please email me if I’ve missed something in my research.)  The gopher tortoise, the closest living relative of the Hesperotestudo tortoises, digs extensive burrow systems.  Therefore, it’s a better assumption to hypothesize the Hesperotestudo tortoises did as well.  Hibbard and Auffenberg thought the Hesperotestudo tortoises were too large to dig burrows.  Recently, a reader of my blog alerted me to an African species of tortoise, Geochelone sulcata, that weighs up to 200 pounds.  This species does dig burrows, proving that size is not an obstacle to digging burrows.  The African spurred tortoise uses burrows to escape from the heat of the desert sun rather than frosts which don’t occur in the region where they live.

African spurred tortoise at burrow entrance

The African spurred tortoise digs extensive burrows to escape temperature extremes.  I propose the extinct American giant tortoises (Hesperotestudo sp.) also dug burrows and could use them to survive freezing temperatures.

During the Pleistocene climate changed much more rapidly than it has since the beginning of the Holocene ~10,000 BP.  Frequent frosts must have struck the south during the coldest climate cycles.  The Hesperotestudo line of tortoises could not have avoided extinction for millions of years, if they were incapable of surviving freezing temperatures.  I just do not accept Hibbard’s weak assumption.  Moreover, giant tortoises probably also made use of burrows dug by ground sloths and pampatheres.  Their burrows dotted the landscape as well.  (See: https://markgelbart.wordpress.com/2012/10/10/some-giant-ground-sloths-dug-long-burrows/ )

The presence of giant tortoises does indicate the existence of open environments.  Giant tortoises eat the kinds of forbs and other plants that grow in sunny conditions. They were more common on the coastal plain where a combination of fire, hurricane winds, megafauna foraging, and xeric soils contributed to open forest canopies.  However, fossil evidence of H. crassicutata has been found as far north as Bartow County, Georgia; suggesting pockets of open habitat extended into the ridge and valley region of the Appalachians.  Apparently, a jaguar gnawed on the tortoise bones which were found at Ladds.

Numerous other species of vertebrates and invertebrates made use of giant tortoise burrows.  The tortoises undoubtedly influenced the composition of plants in the environment by consuming some species, avoiding others, and perhaps spreading seeds in their dung.  Their tunnels aerated the soil and influenced the character of the landscape.

Giant tortoises favored drier environments within their range because this is where the forest canopy would have been more open.  This preference explains why so many different species of giant tortoises colonized islands far into the sea.  Beach habitats resemble desert scrub due the dearth of fresh water.  Giant tortoises inhabiting xeric beach habitats were at risk to be swept out to sea during storms.  But they float and have the ability with their slow metabolism to survive long periods without food or fresh water.  For a while during the Pleistocene a tortoise from the Hesperotestudo genus (H. burmudae) lived on Bermuda.  Bermuda was a much larger island during the low sea levels of Ice Ages, and the North American continent was closer because dry land extended onto the continental shelf.  H. burmudae colonized the island after some individuals floated out to sea following some storm event(s) during the low sea levels of an Ice Age.  H. burmudae became extinct when sea level rose and inundated its favored habitat during an interglacial 300,000 years ago.  Overhunting by man is the most likely reason the 2 continental species became extinct.

Reference:

Meyland and Steyer

“Hesperotestudo (Testudines: Tetudonidae from the Pleistocene of Bermuda, with comments on the phylogenetic position of the genus”

Zoological Journal of the Linnean Society 2000

Advertisements

Tags: , , , , , ,

One Response to “The Presence of the Extinct Pleistocene Giant Tortoises (Hesperotestudo sp.) is Evidence of Open Environments but not of Warmer than Present Day Climates”

  1. Erik Ringdal Says:

    It should also be noted that the largest tortoise recorded, Colossechelys atlas from Asian pleistocene had a rather flat carapace, could be a burrowing form.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


%d bloggers like this: