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 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, 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. 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 EthogramSuite 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 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. 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. 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 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. 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. St. Catherines Island is located midway on the Georgia Coast. Bishop and nested female loggerhead, McQueens Dunes, St. Catherines Island. 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. Loggerhead laying eggs in backbeach facies; US Fish & Wildlife Service. Eroded Recent sea turtle egg chamber in dune facies; South Beach, St. Catherines Island, Georgia. Scale = 10 cm. Egg chamber discontinuity in backbeach facies; South Beach, St. Catherines Island, Georgia. Scale = 10 cm. Egg chamber discontinuity in backbeach facies; South 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. Nested female loggerhead returning to sea after nesting and exit crawlway in backbeach facies (note v’s opening in direction of crawl); North Beach, St. Catherines Island, Georgia. Scale = 10 cm. 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. Paleogeography of Western Interior Seaway and approximate location of fossilized sea turtle nest structures in Elbert County, Colorado. Loggerhead nest on washover fan with adjacent body pit (bp) and covering pit (cp); North Beach, St. Catherines Island, Georgia. Scale = 10 cm. bp cp Documentation of loggerhead nest 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.