In this week’s lab, we examined Order Testudines, better known as the turtles. We examined the general anatomy of turtles, and then walked through the worldwide and local diversity of turtle families and species.
Skeleton: We began lab by looking at the skeletal anatomy of the Testudines. Turtles have a shell that is composed of a carapace, covering the dorsal side of the animal, and a plastron, covering the ventral side of the animal. These structures are completely unique among extant reptiles. Unlike all other living reptiles, the ribs of turtles (which make up part of the carapace) enclose the pectoral and pelvic girdles. In addition to expanded ribs, the carapace is made up of dermal bones that complete the carapace to form a single bony shield. The plastron is made up of derived remnants of the sternal elements of the skeleton, as well as dermal bones that may be derived from gastralia (ventral “ribs” like those seen in the alligator skeleton from the first week of lab).
Scutes: The bones of the carapace and plastron are covered by thickened epidermal scales, called scutes. Although they overlay the bones of the carapace and plastron, the shapes of the scutes do not correspond to the shapes of the underlying bones. As epidermal structures, scutes are periodically shed.
Girdles: Despite their bizarre shell, the limb girdles of turtles are fairly similar to those seen in other tetrapods. The pelvic girdle is composed of ilia, ischia, and pubes, which is typical for tetrapods, and is fused to the plastron in the Pleurodira.
Skull: The turtle cranium is unique among extant reptiles, in that it exhibits the anapsid condition, meaning that it possesses no temporal fenestrae. Instead, the turtle skull has temporal emarginations, which are deep inscriptions at the posterior end of the skull that allow space for muscles related to jaw function.
Reproduction: The shell presents unique a unique problem for mating in turtles, given that they exhibit the amniote condition of internal fertilization and the male must deliver his sperm to the female’s cloaca. To accomplish this, male turtles have a large intromittent organ that is erected from the cloaca by vascular pressure, which they use to transfer sperm to the female. The presence of this intromittent organ means that the tails of male turtles are typically noticeable fatter than those of females of the same species. In addition, the vent of male turtles is generally positioned well posterior to the posterior margin of the carapace, and tends to be anterior to or in line with the posterior margin of the carapace in females. Males also tend to have a plastron that is somewhat concave to prevent them from slipping off the females during mating.
Cryptodira vs. Pleurodira
These are moderately large, pleurodiran river turtles with broad, domed, streamlined shells. They are active swimmers and herbivorous and opportunistically carnivorous.
These are relatively small, pleurodiran turtles with a large heads and a domed carapace and large plastron that may be hinged. They are primarily carnivorous, and are semiaquatic to aquatic, often walking along the bottom in slow-moving water.
These are marine, cryptodiran turtles that only return to land to lay eggs. They are specialized for a marine lifestyle, with flat, streamlined shells covered with epidermal scutes (unlike the Dermochelyidae) and swim using forelimbs that modified into flippers. Most are dietary specialists.
These cryptodiran turtles possess an oblong, moderately domed carapace. Relative to body size, they have large heads. They commonly have hinged plastrons. They are primarily aquatic, but hibernate and may forage on land. The stinkpot turtle, Sternotherus odoratus, is native to Ohio.
These are cryptodiran turtles with a broad, flat carapace that is jagged at the posterior edge. They have a greatly reduced, cruciform plastron, as well as the longest tail (relative to body size) of any turtle. They are almost exclusively aquatic, and are opportunistic omnivores. The common snapping turtle, Chelydra serpentina, is native to Ohio.
These cryptodiran turtles are flattened and have a reduced plastron and carapace that are covered in thick skin rather than epidermal scutes. They lack scales and possess long necks and elongated, snorkel-like snouts. Subfamily Cyclanorbinae has femoral flaps on the plastron. They are opportunistic omnivores that dwell on the bottom or actively swim to locate food and prey. The spiny softshell turtle, Apalone spinifera, and smooth softshell turtle, Apalone mutica, are native to Ohio.
These cryptodiran turtles have high domed shells (with the exception of the pancake tortoise), elephantine feet. They are all terrestrial, and most are herbivorous, though some are opportunistically omnivorous.
These cryptodiran turtles exhibit a variety of ecologies from aquatic to terrestrial. They have a large plastron and are generally moderately-sized turtles. Females are often twice the size of males in some species. The midland painted turtle, Chrysemys picta marginata; spotted turtle, Clemmys guttata; Blanding’s turtle, Emydoidea blandingii; map turtle, Graptemys geographica; false map turtle, Graptemys pseudogeographica; hieroglyphica river cooter, Pseudemys concinna; eastern box turtle, Terrapene carolina; and red-eared slider, Trachemys scripta, are native to Ohio
(leatherback sea turtle, monotypic genus): These are marine, cryptodiran turtles that are similar to the cheloniids in many respects. They have forelimbs modified into flippers and only return to land to lay eggs. Unlike the cheloniids, they have a broad, streamlined shell that lacks epidermal scutes. Due to their high surface-area-to-volume ratios, they are inertial endotherms. They are also specialists on jellfish, and have anatomical specializations for this diet.
These cryptodiran turtles have a heavy carapace with a high dome that lacks epidermal scutes. Their bodies are scaleless. They have forelimbs modified into flippers that possess two large claws. Their forelimbs and swimming motions appear intermediate (structurally and functionally, not evolutionary) between trionychids (softshell turtles) and chelonioids (sea turtles). They have a nose that is pig-like, and are highly aquatic in rivers and estuaries.
These cryptodiran turtles have an oblong, smooth, slightly domed shell. They have a small, pointed head. They live in slow moving rivers, float at the surface to bask, and feed on vegetation in the streamside at night.
These cryptodiran turtles have a head that is extremely large relative to body size. They have a moderately large plastron with a flexible ridge. The temporal emarginations in the skull are minimal. They are nocturnal and live in small, rocky streams in the mountains.
These cryptodiran turtles represent the most speciose family of turtles. They range from small to large, and have a variety of ecologies and range from aquatic to terrestrial and carnivorous to herbivorous.