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, 1992). 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 predation 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||Nest Diagram|
|1. Approach to the Beach
2. Ascent of the Beach
3. Wander to find Nest Site
4.Wallow a Body Pit
5. Excavate the Egg Chamber
6. Deposit the Eggs
7. Backfill the Egg Chamber
8. Covering Activity
9. Return to the Ocean
2. Entrance Crawlway
3. Wandering Crawlway
4. The Body Pit
5. The Egg Chamber
6. The Egg Chamber
7. Egg Chamber Discontinuity
8. Covering Pit
9. Exit Crawlway
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) allows the sea turtle conservationist to identify the species that produced the crawlway (Witherington, 1992).
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.
Once the sea turtle senses a change in temperature from cool to warm (Stoneburner and Richardson, 1981) 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.
|Body pit of a loggerhead attempting to nest in dry surface sand, outline of body with head, front flippers and rear flippers clearly visible; turtle moved forward and to her left and completed a nest [Nest 06-011, South Beach, St. Catherines island].||"Sand angel" of nested loggerhead sea turtle on washover fan, Seaside Spit, St. Catherines Island. Covering pit closely approximates shape and size of turtle that nested and covered by scooting forward.|
|A Loggerhead sea turtle nest on South Beach, St. Catherines Island; trenched along the scarp to show white quartz sand overlying 30 cm heavy mineral deposit and backfilled egg chamber near GSA scale. This nest [97-072]was sketched as part of the 1997 Notebook.||Bioturbated sand filling body pit of Cretaceous sea turtle near Limon, CO, Fox Hills Sandstone, Late Cretaceous. Bottom of body pit discontinuity cutting across horizontally laminated backbeach sand is situated near base of GSA scale.|
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 predation.
Once the egg chamber is backfilled, the turtle enters a covering behavior characterized by front flipper sweeps (Hailman and Elowson, 1992) pushing and throwing sand backwards and propelling the turtle forward. As the turtle moves forward with successive flipper sweeps, she almost always rotates 180 dgrees either clockwise of counterclockwise and then exits the covering pit to crawl back to the ocean. The resultant sedimentary structure is a thin bioturbated layer (20-30 cm thick) covering the egg chamber and body pit .The structure exhibits an elliptical shape approximately 3.0 m long and 1.5 m wide bounded by flipper scarps, with an uneven surface, trails of loose, thrown sand, connected to entrance and exit crawlways. The covering pit and crawlways are exceedingly ephemeral structures on the surface, rapidly erased by wind, rain, and tides.
|Loggerhead involved in covering behavior on backbeach, obstructed nest [98-086].||Leatherback involved in covering behavior on washover fan obstructed nest [05-070].|
|Loggerhead covering pit built by turtle above in obstructed, backbeach nest [98-086] and crawlways from low tide line, North Beach, St. Catherines Island.||Leatherback covering pit built by turtle above in obstructed, washover fan nest [05-070], North Beach, St. Catherines Island.|
After approximately sixty days of incubation in the solar-heated sand, the eggs will hatch and the hatchling turtles will emerge from their eggshell. The hatchlings begin to dig themselves out by a crawling motion of flippers, loosening the sand around and above them which falls through the mass of hatchlings under the influence of gravity, often forming a "stope" or air chamber above the mass of wiggling little turtles. This stoping activity continues as long as the hatchlings are in cool sand beneath the surface. When they near the surface with its solar-heated hot sand, the hatchlings stop their activity and become lethargic, until the sand cools during the night when activity is renewed and the hatchlings emerge from their nest and scamper toward the sea, forming hatchling crawlways. Near the surface the sand is dry and not cohesive, so the surface layer may fall into the stope forming a surface "dimple" announcing a imminent emergence. The final emergence often is en masse, although multiple emerges on successive nights is the norm. Synchronous emergence of large numbers of hatchlings often forms an emergence crater as the surface sand collapses into the void left by the hatchlings. Upon emergence at night the hatchlings head to the sea (downhill, away from the island silhouette, and toward the light and noise of the sea) forming a pie-shaped arc of anatomizing crawlways broadening the sea. Reading these crawlways often allows the documentation of interactions with predators on the beach and an estimate of the number of emergent hatchlings. These are exceedingly ephemeral traces easily destroyed by wind, rain, and tide.
|Emergence crater of Nest [06-047a] indicating the nest has hatched.||Hatchling crawlways leaving emergence crater of nest [93-020] and leading to the sea.||Upon emergence hatchlings scamper across the dark beach to begin their life in the sea.|
Traces of nests depredated by large predators are highly variable and generally consist of strongly disrupted areas that cut across stratigraphic lamination forming crater-like large pits. Eggs may or may not remain in the egg chamber (depending upon the predator and circumstances) and egg shell fragments commonly surround the depredation crater. Small predators (crabs, ants, etc.) leave smaller traces and do not destroy the integrity of the nesting structures. In either case, tracks or trails of the predator may often be associated with the evidence of depredation.
The Fossil Record of Sea Turtle Traces
Because sea turtles nest in a dynamic environment with a low probability of preservation and their modern traces are largely unknown to most geologists and paleontologists, their ichnoloy still remains poorly known. However, by using modern sea turtle traces it is possible to predict what analogous trace fossils will look like when preserved in sedimentary rocks of Cretaceous to Holocene age. The abundance of body fossils of sea turtles attests to heir presence in many regions from Early Cretaceous to present; and indicates that they nested on sandy beaches in those regions during the same interval. Even though the preservation potential of back beach sediments is low, the very number of sea turtles and their multiple nesting behavior would be expected to produce a high potential for preservation where back beach sediments are preserved. This hypothesis was promulgated at the annual meeting of the Rocky Mountain Association of the Geological Society of America (Marsh and Bishop, 1996) by bringing modern loggerhead sedimentary nesting structures to the attention of western geologists who might have seen, or be expected to see, nesting structures of sea turtles in rocks in the Western Interior Seaway.
|Sea Turtle Ichnofossil Record|