The Leisey Shell Pit

During the late 1970s, Frank Garcia and other amateur fossil collectors often searched the spoil piles of the Leisey Shell Pit located about a mile east of Tampa Bay, Florida.  The Leisey family owned the site and mined the sand and seashells which are used in construction to make concrete and other building materials.  One day, Mr. Garcia discovered a wall of fossils exposed when a bulldozer stripped away a large layer of sand.

excavations at Leisey Shell Pit 1A

Arrow points to a layer of fossils found in the Leisey Shell Pit.

I think this is a photo of Frank Garcia standing next to the wall of fossils found at this site.  The fossils probably accumulated here through a combination of river and tidal action.  A major river that no longer exists may have flowed into Tampa Bay then.

excavations at Leisey Shell Pit 1A

Typical density of the fossils found at the shell pit.  Note the saber-tooth tiger canine in the middle.  The fossiliferous layer of one section was 2000 square meters.  If my math is correct, that’s the equivalent of about a square mile.  The site is so rich, paleontologists could have named a land mammal age after it.  Too bad the Irvingtonian Land Mammal Age already had a name.

After Mr. Garcia’s discovery, professional paleontologists descended on the site and excavated over 50,000 fossils in a decade of work, gaining valuable information about the early Pleistocene.  The majority of the fossils were found in 2 sections, labled IA and 3A, though fossils were also found in 2 smaller sections, on the surface, and in spoil piles.  In 1992 the Leisey family stopped mining sand here.  There was no need to keep operating the pumps, so because the location is slightly below sea level this site became flooded.  Fossil hunting here now requires scuba gear.

Scientists had difficulty dating the age of the fossils found in the Pleistocene strata at this site.  They are too 0ld for radio-carbon dating and nearly too old for uranium series dating.  Scientists can’t use pottassium-argon dating because there is no volcanic ash in Florida.  Instead, scientists had to use inexact indirect methods to estimate the age of the fossils here.  A study of strontium isotope ratios found in mollusc fossils narrowed the time range to between 1 million-2million years BP.  A geomagnetic chronology study determined the fossils were at least 780,000 years old. (See http://www.geo.arizona.edu/palynology/geos462/12paleomag.html for an explanation of geomagnetic chronology).  Using index fossils, scientists narrowed the time frame further to between 1.5 million-1.1 million years BP.  There are no fossils known exclusively from the Blancan Land Mammal Age in the fossil rich strata.  This age ended approximately 1.5 million years ago.  But there are some fossils of species not thought to have survived past 1.1 million years ago, thus explaining why scientists narrowed it down to this time range.

The site is named for the abundant sea shells found all through the sand.  Scientists catalogued 98 species of bivalves, 113 of snails, 16 of bryozoans, 1 worm, 9 arthropods, and 1 starfish.  3% of the species of sea shells found here are extinct.  I looked in vain on google images for a photo of one to put on this blog.  But there are pictures of these obscure extinct species in the publication linked below.  The vast majority of mollusc fossils here are of marine species, but a few are freshwater.  Two of the freshwater species no longer occur in peninsular Florida, becoming extirpated during a sea level rise that inundated much of the state.  Morphology studies show that predatory snails and whelks bored holes in 33% of the bivalves.  I remember showing my ex-brother-in-law a whelk in the process of feeding upon a mussel, and he didn’t believe me–he thought it was part of the whelk shell.

Section IA of the Leisey Shell Pit is thought to represent a paleo-brackish environment.  The most common fossil species of fish found in that section are in order: 1. alligator gar 2. snook 3. mullet 4. bull shark 5. eagle ray 6. porcupine fish.  As I discussed last week, alligator gar no longer occur in peninsular Florida due to a later marine transgression.  Great white sharks are an open ocean species, but fossils of this shark are found here in surprising numbers.  They likely hunted close to shore for monk seals and manatees.  Three extinct species of sharks and 1 extinct species of ray that were common during the Pliocene still swam near the Florida coast 1.1 million years ago, perhaps making their last stand.  They were an extinct species of mako shark (Isuris hestalis), an extinct nurse shark (Ginglymosoma serus), an extinct snaggletooth shark (Hemipristis sp.) and an extinct guitar fish (Rhynochobatus sp.).

Guitar fish still live in the Indo-Pacific Oceans. They are shaped like a guitar, hence the name.  An extinct species used to live in the Atlantic as recently as the early Pleistocene.

Fossil hunters love snaggletooth shark teeth.  Snaggletooth sharks also still live in the Indo-Pacific Oceans but have been extinct in the Atlantic for about 1 million years.

Section 3A of the Leisey Shell Pit is thought to represent a more freshwater environment.  The most common fossil species of fish found in this section are in order: 1. redear sunfish 2. mullet 3. bowfin 4. lake chubsucker 5. brown bullhead 6. Seminole killifish.  Redbreast sunfish and largemouth bass were also common here.  In all, scientists identified 73 species of bony fish and 23 species of sharks and rays at the shell pit.

Scientists catalogued 29 species of reptiles and amphibians among the fossils of the Leisey Shell Pit.  All of them still live in peninsular Florida with the exception of 2 species of giant land tortoises that became extinct and 2 species of freshwater turles that no longer occur in this region–pond sliders and alligator snapping turtles.  Again, rising sea levels eliminated habitat for those 2 species, and they have failed to recolonize the region.

Scientists found the remains of 45 species of birds at the Leisey Shell Pit of which 15 are now extinct.  A new species of extinct condor, Gymnogyps kofordia, was discovered here.  Two species of teratorns scavenged alongside G. kofordia, including Teratornis incredibilis, a spectacular bird with an 18 foot wingspan.  Woodward’s eagle (Amplibuteo woodwardii) was another impressive species.  Possible ancestors of today’s roseate spoonbill and avocet flew here, but extinct species of a stork (Ciconia), a loon, geese, and flamingos left no ancestors.  The oldest known fossil of a trumpetor swan was found in the pit.  Trumpetor swans no longer occur in Florida.

Here’s the list of fossil mammal species found at the site.  Scientists estimated the approximate abundance of each large land species based on their abundance in the fossil composition.  I added that information too.  The authors of the study caution that mastodons, mammoths, and ground sloths may be underrepresented because their bones were too heavy to be transported by water which is how most of these animals ended up deposited in the shell pit.

beautiful armadillo

Pachyarmatherium leiseyi–a large species of extinct armadillo

pampathere–also a large species of extinct armadillo

glyptodont

Wheatley’s ground sloth–the evolutionary ancestor of Jefferson’s ground sloth

Harlan’s ground sloth

Eremotherium sp.–a 4-clawed giant ground sloth as big as an elephant

Nothrotheriops texanus–a ground sloth related to the Shasta ground sloth found in the La Brea tar pits.  All ground sloths together equaled 6% of the total fossil composition of large land mammals.

Armbruster’s wolf

Edward’s wolf

monk seal

raccoon

Arctodus pristinus–the lesser short-faced bear, probable ancestor to Arctodus simus

river otter–earliest record

spotted skunk

scimitar-toothed cat

Smilodon gracilis–saber-tooth cat ancestor to Smilodon fatalis.  All large carnivores equaled 6% of the population of large land mammals with this species the most common carnivore.

bobcat

cheetah

giant beaver

gopher

porcupine

capybara

cotton rat

bog lemming

Townshend’s hare

cottontail

tapir–<1%

horse–14%

donkey–8%

flat-headed peccary–9%

long-nosed peccary–1%

large-headed llama–7%

stout-legged llama–38%

white tailed deer–2.5%

bottlenose dolphin

spinner dolphin

pilot whale

manatee

mastodon–1%

gompothere–a species of proboscidean related to mastodons

Hay’s mammoth–2.5%. ancestor to the Columbian mammoth

Stout legged llamas were the most common large mammal living in Florida then, totaling 38% of the total.  Both species of llamas made up 45% of the total, while both horses and donkeys totaled 22%, making them the next most common.  Note that white-tailed deer only comprised 2.5% of the population.  65% of the herbivores were mixed browsers and grazers, while 35% were strictly grazers. Cottontails and gophers were the most common small mammals.  Smilodon gracilis was by far the most common large carnivore.

Gnaw marks on the bones of llamas and horses match those of wolves.  Gnaw marks on the proboscideans and ground sloths match those of Arctodus pristinus, a short faced bear.  Wolves hunted and scavenged llamas and horses, but bears probably just scavenged sloths and proboscideans which would have been too large or tough for the bears to subdue.  Perhaps the bear took control of the larger carcasses when the beasts died naturally and were able to keep wolves away.  Armbruster’s wolf and Edward’s wolf overlapped in time at this site, but the former eventually replaced the latter and may be ancestral to the late Pleistocene dire wolves.

Illustration of paleolama and Eremotherium.  Paleolama myrifica was the most common large mammal living in Florida then.

A layer of strata at the base of the Leisey Shell Pit did contain fossils of Miocene-age species of 3-toed horses.  Bison fossils found at the shell pit likely date to the late Pleistocene rather than the early Pleistocene.  Bison didn’t colonize North America until about 300,000 BP (probably).

The paleobotanists who examined the fossil pollen and plant macrofossils found here remarked that if they hadn’t known the age of the site, they would have assumed they were studying data from a modern day site.  The plant composition of the early Pleistocene is no different from that of the present day in central Florida.  Pine, oak, hickory, sweetgum, herbs, grass, and composites dominated the pollen assemblage.  Identified plant macrofossils found included loblolly or slash pine cones, live oak, sabal palm, saw palmetto, cypress, wax myrtle, hazel nut, and grape.  The region was interpeted as being an open oak and pine woodland with some wetlands, but little cypress swamp.

Reference:

Hulbert, Richard; Gary Morgan, and David Webb

“Paleontology and Geology of the Leisey Shell Pit, Early Pleistocene of Florida”

Bulletin of the Florida Museum of Natural History 37 (1) 1995

http://ufdc.ufl.edu/UF00095791/00001/1j

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