The Biology of Sea Turtles, Volume I

Peter L. Lutz, John A. Musick, Peter L. Lutz, Michael J. Kennish, Peter L. Lutz, John A. Musick

• Éditeur: CRC Press
• Année: 2017
• ISBN:
  9781351411288
• Édition: 1
• Sujet: Sciences biologiques
• Sous-sujet: Biologie marine

Sea turtles have existed for millions of years, making them fascinating subjects of study. In the last 20 years, the science of sea turtle biology has expanded at an exponential rate, leading to major advances in many areas. This book synthesizes the results of these advances and focuses on how these endangered marine reptiles operate in, adapt to, and are dependent upon particular features of their marine environment. New technology in data gathering, such as DNA analyses, remote sensing, and physiological monitoring techniques, has led to a much greater understanding of the biology of the sea turtle at all stages of their life history.

1	Evolution, Phylogeny, and Current Status Peter C. H. Pritchard

CONTENTS

1.1 Introduction

1.2 Marine Turtles

1.3 Historical Classifications of Marine Turtles

1.4 Cladistic Placement of Marine Turtle Groups

1.5 Marine Turtle Families

1.6 Familial Definitions

1.6.1 Cheloniidae

1.6.2 Dermochelyidae

1.6.3 Toxochelyidae

1.6.4 Protostegidae

1.7 Dermochelyid Diversity

1.8 Cheloniid Diversity

1.9 Generic Definitions of Living Cheloniids

1.9.1 Chelonia

1.9.2 Eretmochelys

1.9.3 Lepidochelys

1.9.4 Caretta

1.9.5 Natator

1.10 Affinities of Recent Cheloniid Species: Current Questions

1.11 Survival Status of Living Cheloniids

1.11.1 Legal and Formal Status

1.11.2 Biological Status

1.11.2.1 The Green Turtle, Chelonia mydas

1.11.2.2 The Black Turtle, Chelonia agassizii

1.11.2.3 The Flatback, Natator depressus

1.11.2.4 The Loggerhead, Caretta caretta

1.11.2.5 The Hawksbill, Eretmochelys imbricata

1.11.2.6 The Olive Ridley, Lepidochelys olivácea

1.11.2.7 The Kemp’s Ridley, Lepidochelys kempi

1.11.2.8 The Leatherback, Dermochelys coriacea

References

1.1 INTRODUCTION

Turtles, being reptiles and tied to the terrestrial environment for oviposition (with the single partial exception of the chelid Chelodina rugosa [Kennett, 1993]), are generally assumed to have had terrestrial origins. This assumption is probably correct. Additional evidence is provided by the earliest fossil turtles of the genus Proganochelys and its relatives that had already achieved a broad distribution in the northern continents (then still united as “Laurasia”) by the Triassic, and that appear from their limb structure to have been terrestrial, or, at most, marsh-living forms (Gaffney, 1990).

Authors such as Romer (1945) and Swinton (1958) assumed that the group of early reptiles long known as the Cotylosauria, characterized inter alia by the absence of true temporal fossae, was ancestral to the turtles. Yet intermediate stages were unknown in the fossil record, the alleged “missing link,” Eunotosaurus Seeley 1892, from the Permian of South Africa having now been reinterpreted, and Swinton, although observing that “the Chelonian skull has many features in common with that of Diadectid Cotylosaurs,” cautiously concluded that “a direct relationship is not likely.”

Gaffney (1975) considered the turtles to be the sister group of the Synapsida plus the Diapsida. But he later revised this opinion (Gaffney and Meylan, 1988), and argued that the turtles were the sister group to the Diapsida alone. Subsequently, Lee (1993) conducted a cladistic analysis of the Chelonii and various groups of primitive amniotes and concluded, on the basis of 16 shared derived features, that the pareiasaurs, a group of anapsid, large, terrestrial reptiles with short, heavy bodies, and with large osteoderms possibly anticipatory of the chelonian shell, were evolutionarily close to the earliest turtles. The pareiasaurs were, for a time, cosmopolitan in distribution, but although they flourished in the Late Permian, they were not persistent. Lee considered the evidence linking the chelonians to other traditionally cotylosaurian groups such as the captorhinids and the procolophonoids to be weak.

In view of the terrestrial adaptations of the earliest turtles, it is noteworthy that the subsequent evolution of the Order Chelonii has seen a remarkable proliferation of aquatic taxa, whereas terrestriality has been retained (or reachieved?), amongst modern forms primarily by the single family Testudinidae (with about 40 living species). Some isolated fossil genera (e.g., Meiolania, Zangerlia — see Gaffney [1983] and and Mlynarski [1972] for discussion) also appeared to have been terrestrial, as are a few living emydids and batagurids. Yet eleven of the twelve living families are basically aquatic, although in some cases families that are considered exclusively aquatic today, such as the Pelomedusidae and the Dermatemyidae, once had significant terrestrial representation (Wood, 1985; Mlynarski, 1972). The great majority of the extinct families were entirely aquatic also, as far as is known. Moreover, even specialized aquatic forms, such as the Trionychidae, appeared very early in the fossil record (Hutchison, 1982). Furthermore, as far back as the early Jurassic, a newly described turtle, Kayentachelys, from the Kayenta formation of eastern Arizona, considered to be the sister taxon for all other cryptodires, had a carapace with all of the morphological features of modern, fully aquatic species (Gaffney et al., 1987).

The successful penetration of aquatic niches by both early and modern turtles was probably made possible by a remarkable example of preadaptation. Other living aquatic reptiles, including the sea snakes and other snake species adapted to various degrees for freshwater environments, marine iguanas, crocodilians, etc., as well as the extinct ichthyosaurs, swim (or swam) by means of body and tail undulations not dissimilar to those of typical fishes. Turtles, on the other hand, lost the capacity for this form of propulsion when they developed the shortened, rigid body form and corselet that has characterized the group since the Triassic. This body form offered armored resistance to attack by predators, but the tradeoff was reduced speed and agility, obliging those terrestrial chelonian species surviving in a world with increasingly sophisticated predators to adopt specialized (fossorial, cryptic, insular) life-styles (Pritchard, 1979).

Nonetheless, while the encarapaced, terrestrial body form with columnar, walking-type limbs precluded aquatic locomotion by serpentine undulation, it was remarkably preadapted for very different modes of propulsion, namely swimming by means of alternating thrusts of limbs with webbed digits, or simultaneous strokes with powerful paddlelike forelimbs, and these occurred repeatedly.

The shell required only modest modifications in the transition to aquatic life, these generally taking the form of an overall lower, more streamlined profile, with sharp, tapered edges to the carapace margins, and expansion of the plastral lobes, thus facilitating lateral (swimming) rather than ventral (walking) limb movements (Gaffney et al., 1987).

Only the limbs required profound modifications, and these were indeed forth-coming, the precise form of the newly reshaped limbs being governed by the degree of aquatic specialization appropriate to the new life-style. In the vast majority of aquatic chelonians, the limbs represent a compromise between the needs of swimming and of walking. In most swimming forms the hind limbs are longer, more powerful, and have more extensive webbing than the forelimbs, and they generate the principal propulsive force, although their narrow specialization for this function alone is constrained by the need in all chelonian species to utilize the hind limbs for nest construction.

But in some of the most aquatic chelonians, the forelimbs provide the principal propulsive force. A trend in this direction is evident in the soft-shelled turtles (Trionychidae), in which the forelimbs have become substantially modified into swimming paddles, and the foreclaws (and hindclaws) are reduced to three on each limb. In these turtles, propulsive force is generated to a comparable degree by forelimbs and hindlimbs. Forelimb specialization for swimming is carried further in the family Carettochelyidae (now monotypic, but with an elaborate fossil record and forebears widespread in northern continents [Mlynarski, 1976]), in which the forelimbs are paddlelike and two-clawed, but still flexible.

1.2 MARINE TURTLES

Modification of the forelimbs for swimming reached its apogee in the modern marine turtles of the families Cheloniidae and Dermochelyidae, in which the paddlelike forelimbs achieve a substantial degree of rigidity by having elongate phalanges enmeshed in a continuous matrix of tough connective tissue. This not only makes independent mobility of individual digits impossible, but flexion of the blade of the paddle as a whole is tightly constrained by the combination of the fibrous binding of the digits and nonalignment of the corresponding joints between the individual phalanges of adjacent digits (Zangerl, 1980). In such forms, the forelimb claws are essentially rudimentary, reduced to one, or two at most. But the claw on the first digit has a specialized secondary function in adult male cheloniids, in which it is enlarged and hooklike, and is used for clasping the anterolateral marginal area of the female during copulation.

Such limbs are poorly adapted for terrestrial locomotion in that the proximal parts of the limbs bear almost the entire weight of the animal, but ability to locomote on land cannot be compromised beyond a certain point because of the inescapable marine chelonian committment to terrestrial oviposition. On the other hand, the extraordinary demands for superior swimming ability and endurance made by a marine life incorporating transoceanic migrations ensure that adaptations for aquatic locomotion are paramount and uncompromised, whereas terrestrial locomotion can be allowed to become slow and labored, as long as it is not entirely precluded.

Turtles are an important component of marine ecosystems — primarily tropical and to a lesser degree subtropical ones, but with one species (Dermochelys coriacea) showing remarkable adaptations for survival and function in very cold water (Frair et al., 1972; Greer et al., 1973). Today, however, only seven or eight species of marine turtles survive, the majority distributed unevenly through all three tropical oceans, but with three having relatively restricted distributions (the flatback, Natator depressus, in northern Australia; Kemp’s ridley, Lepidochelys kempi, in the Gulf of Mexico and North Atlantic; and the black turtle, Chelonia agassizii, in the eastern Pacific).

Despite this paucity of species, the living marine turtles are not a relictual group. They have great economic value as well as extreme vulnerability to mankind, at least while nesting, and their inclusion on most lists of threatened or endangered species is a reflection primarily of past overexploitation and current need for better management rather than to inherently poor adaptation to post-Pleistocene conditions.

The documented great diversity of sea turtle taxa in the past probably derives from several causes, including on the one hand the relatively good chances of fossilization of these heavy-boned animals, and on the other hand the disappearance of entire isolated oceans (such as the Niobrara Sea), once populated by diverse and remarkable sea turtle species. Moreover, the overall evolutionary history of the group is paralleled on a mega-scale by the history of life itself (Gould, 1980) or on a human-economy scale by the reduction in the number of automobile manufacturers in the U.S. during the last century even as cars have become more, rather than less popular (Yates, 1996). In these examples, an early blossoming forth of numerous fundamentally “experimental” types (Pritchard and Trebbau [1984] list 27 entirely extinct genera in the Cheloniidae alone) became winnowed down, probably through elimination of overspecialized types with poor response to changing conditions, to a much-reduced diversity of modern survivors, characterized to a considerable degree by being less rather than more specialized than their...