St. Catherines Island Sea Turtle Conservation Program: Life History: Geologic History of Sea Turtles

Life History: Geologic History of Sea Turtles

Sea turtles have been present in the Earth’s oceans at least since the Early Cretaceous (~105 mybp) (Hirayama, 1998; Kear and Lee, 2006), and possibly since the Late Jurassic Period when they lived at the same time as the dinosaurs. Throughout their 100 million year history sea turtles have adapted to changing conditions as seas rose and fell against the continents, continents drifted apart and collided, and glaciers formed and melted.

Fossil evidence of sea turtles includes hard parts preserved as fossils, body fossils, and evidence of sea turtle activities left as sedimentary structures in the sediment, trace fossils.

Body fossils of sea turtles consist of a few complete specimens and many more partial specimens consisting of disassociated skeletal elements. Because dead sea turtles tend to bloat and float after death and are a ready source of food for scavenging animals, their carcasses are torn apart and decompose allowing bones to fall off as they drift and are rocked by the surge of waves. Scavengers often leave evidence of their activity as bitten or broken bones. Gastric residues and coprolites (fossil dung) have been found preserved with several small Albian sea turtles, protostegid turtles (cf. Notochelone), from South Australia (Kear, 2006) indicating that that turtle's diet consisted primarily of the Cretaceous bivalve Inoceramus.

Because sea turtles nest on sandy beaches, they leave behind evidence of this activity as a suite of traces or markings on the beach, consisting of crawlways and nests. The beach is a very ephemeral environment in the geologic record, seldom preserved in the rock record. However, the duration and intensity of sea turtle nesting during the last 105 million years assures that when and where marine shoreline sediments are preserved, there could be sedimentary structures (markings in the rock) made by nesting sea turtles. This hypothesis was presented to scientists in 1996 (March and Bishop, 1996) and resulted in the discovery of the world's first documented fossilized sea turtles nest in the Cretaceous Fox Hills Sandstone near Limon, Colorado in 1997 (Bishop et al., 1997).

During the Early Cretaceous (Albian Stage) there were already five lineages of sea turtles in the seas (toxiclelids, ctenochelids, chelonids (which are ancestral to modern Hawsbills, Greens, Ridleys, and Loggerheads), protostegids, and dermochelids (which are ancestral to modern Leatherbacks)). Three of these lines of decent are represented only by by fossil material, including the toxichelids, the ctenochelids, and the protostegids. Ancestors of the toxichelids and ctenochelids were probably small, about the size of many modern sea turtles such as loggerheads; while giant forms, classified as the protostegids, including the genera Santochelys, Notochelone, and Bouiachelys swam in the Albian Cretaceous oceans. By mid-Cretaceous time, the Turonian through Campanian Stages), marine sediment preserve body fossils of toxichelids (Toxochelys) , ctenochelids (Ctenochelys), protostegids (Desmatochelys, Chelosphagis, Protostega, and Archelon), and early dermochelids (Corsochelys and Mesodermochelys). The protostegid lineage becomes extinct by mid-Campanian, while the toxichelids (Toxochelys), ctenochelyids (Ctenochelys), and dermochelyids (Mesodermochelys) live into the latest Cretaceous (the Maastrichtian), where these species become extinct before the end of the Cretaceous. By the end of the Cretaceous, the modern sea turtle linages are well formed, the chelonids being represented by Euclastes, Pappigerus, and ancestors of advanced chelonids) (the modern Green, Kemp's, Hawksbill, and Loggerhead) and the dermochelyids by ancestral forms of the advanced dermochelyids (Leatherbacks) are present in the oceans. Zangerl (1960) described Corsochelys haliniches from the Selma Chalk of Alabama as an ancestral chelonid.

The Cenozoic history of sea turtles is still less well known, but will undoubtedly become clear as researchers apply analytic methods to known Cenozoic remains. A brief summary is presented below as a first attempt to bring some of the information together.

Lytoloma elegans from the Late Cretaceous/Early Paleocene phosphate bed deposits near Khouribga, Morocco is figured by photographs on commercial sites. Additional specimens of these coastal and marine turtles have been found in deposits of West Africa, the interior of the Sahara, North Africa, and Western Europe. Hirayama?reviewed the systematics of several Cretaceous and Paleogene cheloniids, including Catapleura repanda (Cope, 1868) from the Upper Cretaceous and Paleocene of North America and Belgium (revised as a valid taxon of a primitive grade), Dollochelys casieri, D. atlantica, and D. coatesi were regarded as junior synonyms of C. repanda. Catapleura arkansaw Schmidt, 1944 from the Upper Cretaceous of North America was judged as a distinct genus from C. repanda and assigned to Zangerlchelys n. gen. Weems discussed Paleocene turtles from the Aquia and Brightseat formations, with a discussion of their bearing on sea turtle evolution and phylogeny.

The Dermochelyidae reached a peak of diversity during the Eocene period about 50 million years ago with at least six species of leatherbacks lived in the oceans worldwide, their fossils found in Africa, Europe, North America, and New Zealand. (Spotilla, 2004).

Hans-Volker Karl & Gottfried Tichy described remains of Oligocene (Rupelian) sea turtles from the Mainz Basin near Alzey. Included in this discovery are bone remains of turtles and a tooth of a crocodile. The new tribe Rupelchelyini, which includes the new genus Rupelchelys and the species R. breitkreutzi n.sp. are new taxa. New turtles material from the Doberg near Bünde in Northern Germany is also reported.

The Oligocene fossil sea turtle Carolinochelys, possibly ancestor of the living loggerhead turtle Caretta caretta is exhibited in a newly mounted (2003) skeleton of Carolinochelys at the Science Museum of Minnesota. This exhibit represents the first articulated specimen of its kind. It is a composite cast, mounted in a swimming pose, produced from a number of specimens housed in the Charleston Museum, the Science Museum of Minnesota, and specimens from private collections. The exhibit provides a view of the plastron and most of the internal skeleton. It is about one meter long. Carolinochelys belonged to a late Oligocene (28 million years old) fauna collected from the Chandler Bridge Formation of South Carolina. The fauna includes whales; a dugong Crenatosiren; sea birds; the crocodile Gavialosuchus; other turtles; and fishes.

Zangerl and Turnbull (1955) described three or four specimens of a sea turtle (Procolpochelys grandaeva fron the Miocene of New Jersey. Robert E. Weems described Syllomus aegyptiacus, a Miocene Pseudodont Sea Turtle from Egypt and Middle Miocene Sea Turtles (Syllomus, Procolpochelys, Psephophorus) from the Calvert Formation.

Alexander O. Averianov published a list of 51 localities of fossil sea turtles from the Middle Jurassic, Early and Late Cretaceous, Paleocene, Eocene and Miocene of Russia, Uzbekistan, Kazakhstan, Tajikistan, and Ukraine. Two new taxa, Dollochelys rogovichi sp.nov. (Cheloniidae, Eocene, Ukraine) and Turgaiscapha kushmurunica gen. et sp.nov. (Chelonioidea indet., cf. Dermochelyidae, Late Cretaceous, Kazakhstan) were described.

C.K. Dodd, Jr., and G.S. Morgan (1992) described fossil sea turtles from the early Pliocene Bone Valley Formation, Central Florida and G. R. Zug (2001) described the Turtles of the Lee Creek Mine.

The one agent of change, which is universally driving sea turtles toward extinction, is their interactions with humans. Rapidly growing human populations and an ever increasing standard of living has led to many negative impacts for sea turtles including their use as food, oceanic pollution, urbanization of coasts and their encasement in seawalls and revetments, increasingly hazardous technology (such as boat propellers), and increasingly efficient fisheries. Thus, it is humans who are driving the sea turtles toward the brink of extinction, including you and me.

Once a lineage of organisms is extinct, it is gone forever from the face of the Earth. Although most organisms have become extinct during the past, the rate and numbers of plants and animals being driven toward extinction by humans is alarming. Because the Earth’s ecosystems are finely tuned to all plants and animals, it is important for us to minimize detrimental effects to them caused by humans.

Sea turtles have become a test case to see if humans can responsibly interact with other organisms on the Earth.