NESTING TRACES OF SEA TURTLES: A RETROSPECTIVE PERSPECTIVE

1
NESTING TRACES OF SEA TURTLES A RETROSPECTIVE
PERSPECTIVE Gale A. Bishop Museum of Geology and
Paleontology South Dakota School of Mines and
Technology 501 east St. Joseph Street, Rapid
City, South Dakota 57701
Crawlways Crawlways are produced by the female
sea turtle ascending and descending the beach and
by hatchlings scampering to the ocean. The
crawlway morphology (symmetry and nest
morphology) allow the sea turtle conservationist
to identify the species which produced the
crawlway (Witherington, 19xx). Individual turtles
produce identifiable crawlways due to attached
epizoans, flipper pathology, and individual
crawling characteristics. The direction the
turtle was crawling may often be determined by
asymmetrical push marks from rear flippers and by
v-shaped drags made by claws on the front
flippers that open in the direction of crawling.
Because of this, entrance and exit crawlways can
be identified and used in reading the nest. Once
on the beach, the turtle may have to wander to
find a suitable nesting site or might become
disoriented and wander about after nesting trying
to find her way back to the ocean giving rise to
a wandering pattern. Upon emergence, hatchlings
produce radiating arcs of overlapping crawlways
leading from their emergence crater to the ocean.
Occasional misorientation, disorientation, or
catastrophe can be read in their crawlway
patterns.
ABSTRACT Body fossils of sea turtles are present
in rocks dating back through the Cretaceous
traces of nesting activities of sea turtles ought
to have a similar range. Recent loggerhead sea
turtle nests have been extensively studied on St.
Catherines Island, GA. Study and documentation of
over 1450 nests and associated crawlways, and
detailed trenching of more than 50 of these has
led to a three dimensional model of modern sea
turtle nests and the description of the
sedimentary structures associated with nesting of
the species Caretta caretta Linnaeus, 1758. The
sedimentary structures studied present a
recognizable spectrum of preservable traces
dictated by a nesting ethogram consisting of nine
described steps 1) approach to the beach, 2)
ascent of the beach, 3) wandering the beach, 4)
wallowing to damp sand, 5) digging an egg
chamber, 6) depositing eggs, 7) backfilling egg
chamber, 8) covering activity, and 9) crawling to
the sea. Loggerhead nest structures are
elliptical with surface bioturbation masking
underlying nest structures consisting of a body
pit and egg chamber. Crawlways made during
entering and leaving the nest, the thin veneer of
bioturbated sand produced in the covering
activity, and the underlying body pit and egg
chamber are capable of preservation in ancient
rocks. Successful incubation of sea turtle eggs
could result in hatchling stopes, emergence
craters, and fan-shaped patterns of hatchling
crawlways depredation of nests should result in
recognizable sedimentary structures. The lack
of recognition of ancient sea turtle nests may be
partly due to the lack of description of Recent
nests. The spectrum of potential nesting
structures is obfuscated by the spatial
constraints of differing viewpoints of biologists
and geologists the horizontal view of beach
structures normally observed by biologists must
be set in contrast to the vertical orientation of
geologic information of geologists. This
spectrum of potential sedimentary structures and
disparate viewpoints has been partly validated,
and is illustrated by the Cretaceous sea turtle
nest described from the Fox Hills Formation of
CO.
Hatchling loggerhead scrambling to sea after
emergence and hatchling crawlways from emergence
crater to sea in backbeach facies North Beach,
St. Catherines Island, Georgia. Scale 10 cm.
Nested female loggerhead returning to sea after
nesting and exit crawlway in backbeach facies
(note vs opening in direction of crawl) North
Beach, St. Catherines Island, Georgia. Scale 10
cm.
Research Locations Recent loggerhead nesting
structures have been studied on the beaches of
St. Catherines Island, Georgia. Fossil structures
were studied in the Fox Hills Formation of Elbert
County, Colorado.
Body Pits Once the sea turtle senses a change in
temperature from cool to warm as she passes from
tidally-cooled to solar-heated sand at the high
tide line, she will often attempt to nest. This
is initiated by digging a body pit by wallowing
and scraping dry surface sand away from and under
her body ("wallowing down" to damp sand) so she
can excavate an egg chamber in damp sand that
will hold vertical wall due to its cohesion.
Occasionally the turtle will encounter damp sand
from at surface and produce a body pit in it
right at the surface forming a distinctive nest
morphology (a "sand angel" analogous to "snow
angels" produced by children in fresh snow). Sand
angels may also be produced by hatchlings if they
hang up in vegetation or are flipped on their
backs during their rush to the sea.
Documentation of loggerhead nest 95-104 on
backbeach showing two unsuccessful body pits (bp)
and the successful covering pit (cp) South
Beach, St. Catherines Island, Georgia. Scale
1.0 m on smaller scale drawing.
cp
bp
Paleogeography of Western Interior Seaway and
approximate location of fossilized sea turtle
nest structures in Elbert County, Colorado.
St. Catherines Island is located midway on the
Georgia Coast.
Bishop and nested female loggerhead, McQueens
Dunes, St. Catherines Island.
Loggerhead nest on washover fan with adjacent
body pit (bp) and covering pit (cp) North Beach,
St. Catherines Island, Georgia. Scale 10 cm.
Loggerhead nest with covering pit, body pit, and
egg chamber, trenched to investigate nest
morphology and heavy mineral distribution South
Beach, St. Catherines Island, Georgia. Scale 10
cm.
Ichnology of Sea Turtles The possible traces
left by sea turtles are limited to nesting
structures made by females nesting on sandy
beaches in tropical or subtropical regions.
Nesting behaviors are strongly imprinted on
modern sea turtles and described as a nesting
ethogram (Hailman and Elowson, 19xx) .The
ethogram for loggerhead sea turtles (Caretta
caretta) includes nine distinct segments each
component of which results in characteristic
traces of the behaviors that form potential trace
fossils. The traces also provide a clue to the
evolutionary sequencing of the behavioral
segments the presence of a covering activity
(resulting in production of a covering pit) would
imply depredation pressures existed in the past
that led to the development of a hiding strategy.
Although the beach has a low preservation
potential, the abundance of nests year after year
increases the potential for their preservation .
The recognition of these structures in the fossil
record is difficult due to their small size,
cryptic appearance, and lack of experience of
geologists with structures of this sort. The
typical nesting ethogram would produce a suite of
linked structures that can be depicted either
verbally or diagrammatically Loggerhead
Ethogram Suite of Expected Traces Approach to
the Beach None Ascent of the Beach Entrance
Crawlway Wander to find Nest Site Wandering
Crawlway Wallow a Body Pit Body Pit Excavate
the Egg Chamber Egg Chamber Deposit the Eggs
Egg Chamber Backfill the Egg Chamber Egg
Chamber Discontinuity Covering Activity
Covering Pit Return to the Ocean Exit
Crawlway
Egg Chambers Once the sea turtle has wallowed
down to damp sand, she will excavate an egg
chamber using her rear flippers (try this
yourself next time you visit the beach!) in an
alternating scooping motion. The egg chamber is
excavated to the depth to which the turtle can
reach with her rear flippers and may show a
bilateral symmetry in an urn-shaped excavation
about 20-25 cm in diameter. Occasionally turtles
will attempt nesting multiple times as they
encounter subsurface obstructions (logs, wrack,,
or roots) and may leave several open body pits
and egg chambers behind as they scoot forward to
try again. Once a suitable egg chamber is
constructed the eggs will be extruded and the egg
chamber backfilled, and possibly even tamped,
with sand by the turtle's rear flippers. This
filling is brecciated and in beaches with heavy
minerals, will be obvious as a homogeneous,
bioturbated sand cutting vertically through the
horizontally laminated back beach facies. In
horizontal view, if the loose sand of the
covering pit is removed, this biogenic
sedimentary structure will stand in stark
contrast to the contour-like patterns of the back
beach facies. The horizontal cross-sectional area
of the egg chamber is usually less than 1 of the
nest area clearly an adaptation to protect eggs
against depredation.
Egg chamber discontinuity in backbeach facies
South Beach, St. Catherines Island, Georgia.
Scale 10 cm.
Loggerhead laying eggs in backbeach facies US
Fish Wildlife Service.
Egg chamber discontinuity in backbeach facies
South Beach, St. Catherines Island, Georgia.
Scale 10 cm.
Eroded Recent sea turtle egg chamber in dune
facies South Beach, St. Catherines Island,
Georgia. Scale 10 cm.
Map view and cross-section of loggerhead sea
turtle nest St. Catherines Island, Georgia.
Typical nesting structures of loggerhead sea
turtle North Beach, St. Catherines Island,
Georgia. Scale 10 cm.