Turtles of the World: A Guide to Every Family

By Jeffrey E. Lovich and Whit Gibbons

A lavishly illustrated guide to the world's turtles that covers every family and genus

Turtles of the World reveals the extraordinary diversity of these amazing reptiles. Characterized by the bony shell that acts as a shield to protect the softer body within, turtles are survivors from the time of the dinosaurs and are even more ancient in evolutionary terms than snakes and crocodilians. Of more than 350 species known today, some are highly endangered. In this beautiful guide, turtle families, subfamilies, and genera are illustrated with hundreds of color photographs. Each genus profile includes a population distribution map, a table of information, and commentary that includes notable characteristics and discussion of related species.

• More than 250 beautiful color photos
• Each profile features a distribution map, table of information, and commentary
• Broad coverage includes every family and genus

• Language: English
• Publisher: Princeton University Press
• Release date: Dec 7, 2021
• ISBN: 9780691229034

Book preview

INTRODUCTION

Turtles are arguably the most successful vertebrates to have ever lived. These iconic animals have been symbolized, memorialized, utilized, and revered by cultures throughout the world for thousands of years. They have changed little in appearance over 200+ million years due to the origin and retention of a morphology and lifestyle unique among vertebrates but which has endured the test of time. Instantly recognizable by their trademark bony shell, turtles are the only vertebrates, living or extinct, to have their limb girdles (hips and shoulders) inside the shell formed by their rib cage. On the one hand, the turtle shell appears to be an ingenious form of protective armor against many predators and the elements, while on the other the shell would appear to be a liability from the standpoint of mobility and other life functions.

Turtles have a variety of traits humans consider to be of interest. The sex of hatchlings is often determined by incubation temperatures of eggs in the nest, not genetically by X or Y sex chromosomes. Some female turtles have the ability to store viable sperm for several years after one mating, and embryos can have multiple sires in the same clutch. In northern latitudes, turtles basically hold their breath for several months during hibernation under ice. Some species have the ability to breathe through their cloaca as if it were a gill. The longevity of some turtles is widely recognized. Turtles are truly amazing animals!

The beautiful Diamond-backed Terrapins of North America live only in brackish water habitats and are the only members of their subfamily with spots instead of stripes.

Interest in turtles is growing rapidly as measured by popular and scientific publications (Lovich and Ennen 2013). Unfortunately, this rise in curiosity is paralleled by dramatic declines in turtle populations and extinctions of turtle species around the world. Turtles are unquestionably under siege on local, regional, and global scales, with a growing number of species threatened with extinction. More than half of the world’s turtles require some form of conservation action to protect them – the proportion of turtles in trouble eclipses virtually all other major vertebrate groups except primates. They survived the extinction of dinosaurs, drifting continents, and numerous ice ages punctuated by rising sea levels. Whether they will survive humans remains to be seen.

A major purpose of this book is to increase appreciation for these successful creatures, largely unchanged since the mists of time, and expand awareness of their plight in the modern world. We provide a broad overview of general biology, fossil history, and distribution patterns of turtles. We also give details of the ecology and behavior of each of the 14 living families and 95 genera, highlighting some of the remarkable adaptations of selected species. Despite their antiquity, biological knowledge of many turtle species remains incomplete, but we have gleaned material for this book from a large and growing scientific literature base. We provide a list of suggested references for anyone interested in pursuing further details of particular species.

WHAT IS A TURTLE?

TURTLES VS. TERRAPINS VS. TORTOISES

People call turtles by various names including tortoises and terrapins. Technically, all animals with a bony shell and a backbone are turtles, even tortoises and terrapins. Just like foxes are dogs and lions are cats, tortoises and terrapins are turtles. Tortoises are decidedly terrestrial turtles with club-like hind feet. The word terrapin is derived from a Native American word meaning turtle. Simply put, all tortoises and terrapins are turtles, but not all turtles are tortoises or terrapins.

Common names of turtles vary regionally and we have adhered mostly to those recommended in Turtles of the World Annotated Checklist and Atlas of Taxonomy, Synonymy, Distribution, and Conservation Status (8th edition; 2017). Scientific names of turtles are generally reliable for distinguishing between species but are in a state of flux because of phylogenetic reinterpretations and descriptions of new species. We use the classification system for families, genera, and species accepted internationally by the majority of turtle biologists at the time of writing, as specified in the checklist mentioned above. We have augmented the list by including several species undescribed at the time of that publication. Taxonomy is an everchanging field, with more changes forthcoming. Turtles have not changed for centuries, but what we call them continues to be a moving target.

CONTINENTAL SPECIES DIVERSITY OF TURTLES

1 2 3 4 5 6

1

North America

99 species

2

South America

58 species

3

Europe

8 species

4

Africa

59 species

5

Asia

95 species

6

Australasia

39 species

Species diversity of non-marine turtles found on six continents and their associated island systems, based on the Turtle Taxonomy Working Group, 2017. The exact numbers change every year as new species are described but general continental relationships remain comparable, with the highest diversities being in North America and Asia.

The Pig-nosed Turtle of Australia and New Guinea is the only species in its family and the only freshwater turtle with flippers.

TURTLE BIODIVERSITY

The greatest concentrations of turtle biodiversity in the world are in the southeastern USA and southern Asia. Buhlmann et al. (2009) listed the total number of non-marine species for the USA as 53, while Asia has 77 species. The Mediterranean region of Europe has 14 native species, Australia 35, Africa 48, and Central and South America 51 and 48 respectively but includes overlap of some species among these regions. Notable concentrations of non-marine turtle species also occur in the Galapagos Islands (13) and on Madagascar (nine). These estimates vary from more recent compilations, including ours (see map opposite), due to recognition of new species and taxonomic disagreements.

The number of kinds of turtles recognized by the scientific community has steadily increased for decades as new species have been discovered in previously understudied regions. For example, the number of new species described per year worldwide has been almost ten times greater in recent years than in the USA where turtle research and descriptions of new species have been in progress much longer (Gibbons and Lovich 2019). Species diversity in Australia is also now better understood and unlikely to change much compared to Asia or Africa where extensive research programs are becoming established.

In addition, our knowledge of turtle phylogenetic relationships continues to change due to scientific advances, often providing recognition that the diversity of species or genera is greater than previously perceived. Turtle biologists have made numerous efforts to report on the fast-growing body of information on turtle taxonomy in various books and technical publications.

TURTLE EVOLUTION, SYSTEMATICS, AND TAXONOMY

Turtles are distinctive. Living species can be recognized and distinguished from all other animals by the presence of a bony carapace and plastron attached on the sides by a bony or ligamentous bridge. Turtles do not have movable ribs like other vertebrates. Instead the ribs are fused to form the structure of the carapace, providing shell integrity and armor. To breathe, turtles use elongate muscles in the abdomen, whereas most other reptiles breathe by moving the ribs to expand and contract the lungs. A common feature of all living turtles, whether Cryptodira (hidden-neck) or Pleurodira (side-neck), is the presence of eight neck vertebrae.

During their long evolutionary history turtles passed through two major extinction events, at the ends of the Triassic and Cretaceous periods. However, not all survived as there are over 700 species of extinct turtles known, double the number of living species. The basic turtle body plan changed little, overall, but no modern turtle has teeth, although several ancient fossil turtles or turtle ancestors did; for example, Pappochelys had marginal teeth (Middle Triassic, Germany), Odontochelys had marginal teeth (Late Triassic, China), and Proganochelys had no marginal teeth but small teeth on the vomer, palatine, and pterygoid bones (Late Triassic, Germany, Thailand, Greenland). The phylogenetic relationship of these various proto- or stem-turtles (here) to modern turtles continues to be debated among paleontologists. All possess some skeletal traits characteristic of today’s turtles, but none appear to be a direct ancestor of all modern turtles. Nonetheless, the case is clear that the lineage of turtles in the broadest sense began eons ago as they plodded alongside the Jurassic dinosaurs.

Paleontologists have proposed numerous evolutionary relationships of turtles based on fossils, such as this Odontochelys from China, preserved over eons.

Turtles have a rich fossil record because their bony shell, skull, and appendages are easily fossilized.

THE EARLIEST TURTLES AND THE FOSSIL RECORD

Numerous hypotheses have been proposed about the evolutionary origin and phylogenetic relationships among living and extinct turtles. Original interpretations were based solely on comparative morphology of modern turtles with those in the fossil record. During recent decades, DNA analyses have resolved many of the relationships among the 14 living families, although total agreement among authorities is yet to be achieved.

All cryptodire and pleurodire turtles are encompassed within the Testudines and may be descended from a common ancestor in the Jurassic, or earlier, before the two major groups of modern turtles diverged. The oldest known fossil turtle to be a cryptodire was described from Late Jurassic deposits in China. Pleurodires and cryptodires are hypothesized to have diverged at an earlier time. Among the 11 families of cryptodires, the softshell turtles (Trionychidae) and pig-nosed turtles (Carettochelyidae) constitute a separate evolutionary branch from the other nine of those 11 families.

WHAT WERE THE LARGEST TURTLES?

The largest turtles known have been extinct species found as fossils, with estimations that body sizes of some may have approached 10 feet (3 meters) in carapace length. However, precise linear measurements are often difficult on fossil specimens and can only be estimated. The largest confirmed sizes are those of Stupendemys geographicus, a freshwater species from the Miocene of South America, and the comparably sized Archelon ischyros found in marine sediments in North America. Stupendemys was a side-necked turtle in the family Pelomedusidae. Archelon was not a direct line ancestor of any modern turtles although their phylogenetic position is debated. Based on a few fossil specimens, both were larger than the still extant Leatherback Sea Turtles.

Leatherback Sea Turtles (center) are larger than humans, but intact fossils of some ancient sea turtles, such as Archelon (left), were even larger.

PHYLOGENETIC RELATIONSHIPS

One statement that all turtle biologists and paleontologists will agree on is that no presentation of phylogenetic relationships among fossil and modern turtles is accepted by everyone. In making phylogenetic trees, biologists refer to stem versus crown groups of organisms. Living species are referred to as the crown group because they are positioned at the top, or in the tips of the branches of the phylogenetic tree. However, the crown group also contains all extinct species, moving down the tree to the oldest common ancestor of all the living species in the crown.

Extinct species that did not directly give rise to those living today are referred to as stem species because they are lower in the tree. Research by Shaffer et al. (2017) resulted in a backbone tree (see opposite) showing the phylogenetic relationships of the 14 families of crown turtles we recognize and serving as a hypothesis of family relationships for future research and evaluation. Based on their molecular clock analyses they estimated that archosaurs (extinct dinosaurs and their modern relatives, birds and crocodilians) split off from primitive turtles 322 million years ago (MYA) during the Carboniferous Period. The oldest common ancestor of crown turtles has been estimated by some researchers to be 220 MYA during the Late Triassic Period.

CLASSIFICATION OF MODERN TURTLES

The advent of modern DNA analyses has unquestionably provided valuable information on patterns of ancestry and relationships among species. Grouping species into families is a comfortable concept for virtually all non-scientists. In fact, most people like schemes that maintain a prescribed order, even though the system for any large group of entities is seldom perfect. For the practical aspect of sorting out the 95 genera and 354 species of living turtles for discussion, we group them according to families. A finer distinction of relationships among genera in some families is the category of subfamilies. We list on this here the subfamilies currently recognized but do not dwell on them in detail in the remainder of the text because the situation is fluid.

Continued studies will lead to new interpretations of phylogenetic relationships that will result in reassortments among families, genera, and species as well as proposed name changes. But the fundamental phylogenetic relationships are unlikely to change the current basic classification.

Recognizing all living and certain extinct turtles as belonging to the order Testudines within the class Reptilia also seems like a rational approach to maintain a system that has worked well for more than two centuries. We will leave for academic consideration the issue of how to shift public perception regarding the esoteric conundrum that birds should be incorporated into the Reptilia as modern dinosaurs. For our purposes, everyone would agree that turtles have had their distinctive line of identity across geologic time by any phylogenetic interpretation.

PHYLOGENETIC RELATIONSHIPS AMONG LIVING TURTLES

Acanthochelys spixii Phrynops hilarii Erymnochelys madagascariensis Podocnemis sextuberculata Pelomedusa subrufa Pelusios subniger Carettochelys insculpta Dogania subplana Pelodiscus sinesnsis Lissemys scutata Chelonia mydas Eretmochelys imbricata Dermochelys coriacea Chelydra serpentina Macrochelys temminckii Dermatemys mawii Kinosternon baurii Staurotypus triporcatus Chrysemys picta Emys blandingii Emys marmorata Platysternon megacephalum Cuora galbinifrons Rhinoclemmys punctularia Kinixys erosa Manouria emys emys Chelidae (partial) Podocnemididae Pelomedusidae Carettochelydidae Trionychidae Cheloniidae Dermochelyidae Chelydridae Dermatemydidae Kinosternidae Emydidae Platysternidae Geoemydidae Testudinidae

This chart shows one hypothesis of the phylogenetic relationships among genera and families of living turtles, based on Shaffer et al. (2017).

ANATOMY AND PHYSIOLOGY

A turtle’s trademark is its bony shell. No other vertebrate, living or extinct, shares this body plan enclosed in a box of bone. All of the internal organs and systems (e.g., skeletal, circulatory, respiratory, excretory), which are comparable to those of other vertebrates, are enclosed within the space between carapace and plastron. Because of the shell, over 80 percent of the dry weight of some turtles is bone.

SHELL MORPHOLOGY

The nomenclature used to describe various elements of the carapace and plastron vary greatly from source to source. We use the system of Ernst and Lovich (2009). The typical turtle has a carapace with about 50 bones arranged like an intricate geometric design. Included are 11 small peripheral bones around the edges of each side. A nuchal bone divides the right and left peripherals over the neck region and a pygal bone divides them over the tail. Moving inward, there are two rows of eight costal bones divided down the midline by a series of neural bones attached to the neural arches of the vertebrae. These are followed by two suprapygals. The other bones, including the nuchal, pygal, and suprapygals, are articulated to their neighbors but not the vertebral column. Many exceptions and variations on this generalized arrangement exist among species.

The typical turtle plastron has nine bones including, from anterior to posterior, a pair of epiplastron bones, a single entoplastron bone, and pairs of hyoplastra, hypoplastra, and xiphiplastra bones. Side-necked (Pleurodira) turtles of the genera Pelusios, Pelomedusa, and Podocnemis have a pair of mesoplastra between the hyoplastra and hypoplastra. This condition is seen, although rarely, in some hidden-necked (Cryptodira) turtles like the sea turtle Lepidochelys and the Pancake Tortoise Malacochersus. Several genera have hinged plastrons, which in Cuora, Terrapene, and Kinosternon allow complete or nearly complete retraction of their head, limbs, and tail into the safety of a tightly closed shell. Tortoises of the genus Kinixys have a hinge on the posterior of their carapace.

The bony shell elements of softshell turtles differ greatly from those of hard-shelled species. For example, peripheral bones are absent. Instead the carapace is a bony disk composed of a nuchal, neurals, and costals covered with leathery skin. The plastron of softshells is reduced and modified anteriorly (no entoplastron) allowing more flexibility when the head is retracted. Another exception to the typical turtle body plan is seen in Leatherback Sea Turtles, which have lost all shell bones except for the nuchal and those rimming the plastron. Instead, they have what is referred to as a mosaic of bony plates called osteoderms embedded in the leathery skin of the carapace.

Some turtles, like this Red-eared Slider, shed their epidermal scutes intermittently or annually, replacing them with new ones.

SHELL SHAPE AND STRUCTURE

nuchal neural (n) suprapygal (s) pygal peripheral (p) costal (c) peripheral (p) epiplastron entoplastron hyoplastron mesoplastron hypoplastron xiphiplastron

Shell bones of typical turtle (carapace left, plastron right)

preplastron epiplastron hyoplastron hypoplastron xiphiplastron costal (c) nuchal neural (n)

Shell bones of softshell turtles (plastron left, carapace right)

cervical vertebral (v) marginal (m) pleural (p) gular humera axillary pectoral abdominal inguinal femoral anal

Scutes covering a typical hard-shelled turtle (carapace left, plastron right)

Most hard-shelled turtles also have a covering of large scales, called scutes and made of keratin, over the carapace and plastron. Although the arrangement of scutes is roughly analogous to those of bony elements, the contact seams on the scutes do not align with the underlying bony sutures, but instead are offset, which presumably increases structural strength. Overlying the peripheral bones of the carapace are 12 marginal scutes on each side (11 in the Kinosternidae). The costal bones are covered by four