About 220 million years ago, or the end of the Triassic period, marks the emergence of the turtle. Through evolutionary changes and periodical risks of extinction, the turtle has survived and is one of the closest living relatives of the dinosaur. With more than 250 species worldwide, turtles come in a variety of sizes and colors.
Turtles are classified into one of three categories; aquatic, terrestrial and semiaquatic. All modern turtle species look similar in appearance, but each turtle has species specific traits that differ one from another. Studying turtle chronology, Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, Beijing, claims to have found a missing link in the evolution of the turtle. He suggests that turtles looked quite different -- possibly without a shell -- during the Triassic period.
Born from eggs, most turtles emerge into the world at about 2 to 3 inches. Once fully grown, various anomalistic lengths are measured when describing turtle size -- typically the length and width of its head and the length and width of its carapace.
Under the Sea
Sea turtles (Chelonia mydas) are classified as hard-shelled (cheloniid) and leather-shelled (dermochelyid) species. Sea turtles are aquatic and spend their entire lives in the ocean. For camouflage, sea turtles are muted greens and browns. Full-grown sea turtles measure anywhere from 25 to 58 inches. Like all turtles, sea turtles are reptiles, which means they are cold-blooded vertebrates with scaled skin, use lungs for respiration and have a three-chambered heart. Like most turtles, sea turtles lay eggs on land, but reproduction encounters take place at sea. The turtle's outer shell is called the carapace, and the bottom shell is called the plastron. The carapace is fortified with hard scales called scutes. The sea turtle familia has seven sub-species: flatback, green, olive ridley, hawksbill, Kemp's ridley, leatherback and loggerhead.
Box turtles are the only indigenous land turtle in North America, with sub-species existing worldwide. The highly squared dome shape of the carapace gives the box turtle its name. Box turtles are colored with orange and yellow carapace stripes on an olive-green and brown background. Box turtle species vary in length from 3.5 to 8 inches. Currently, four genus species are classified as box turtles, with another 12 orders distinguished. Often confused with the tortoise, box turtles are closer in relation to sea turtles. Box turtles are territorial, with a home range of about 750 square feet. While not water turtles, box turtles will seek out decaying logs, leaf piles, mud and shallow water pools to assuage the dangerous mid-day heat.
Water and Land Dwellers
The most recognizable species of semiaquatic turtles is the red-eared slider (Trachemys scripta elegans.) This turtle was once sold worldwide as a popular first pet for children. A 1970s government ruling put a stop to the sale of this turtle due to a salmonella scare. Semiaquatic turtles are popular again with pet owners because of their distinct coloration and defined markings. Most semiaquatic turtles have bright red and yellow lines on their carapace and external derma. Full-grown red sliders reach lengths of 7 to 9 inches. Omnivorous feeders, semiaquatic turtles prefer a daily diet of vegetables, insects, fish, meat and worms. Spending a lot of their time in the water, semiaquatics come on land to forage sustenance and garner body heat from the sun.
- National Oceanic and Atmospheric Administration: The Anatomy of Sea Turtles, by Jeanette Wyneken, Ph.D.
- Smithsonian National Zoological Park: Reptiles and Amphibians-Box Turtle
- Indiana Department of Natural Resources: Eastern Box Turtle
- University of North Carolina: An Overview of Common Semi-Aquatic Turtles, by David T. Kirkpatrick, Ph.D
- Wildlife and Natural History: Care-Aquatic and Semi-Aquatic Turtles
- Euro Turtle: Sea Turtle Comparisons
- Davidson College Herpetology Lab Page: The Eastern Box Turtle
- Comstock Images/Stockbyte/Getty Images