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1 Natural History of Vertebrates Lecture Notes Chapter 9 - Amniote Origins Chapter 12 - Turtles These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. Be sure to study the List of Terms The Origin of the Amniotes The amniotes represent the groups that we think of as reptiles, birds, and mammals. Amniota refers to a group of organisms that have an amniotic egg. Radiation began in the Early Carboniferous. The nonamniote tetrapods were slowly replaced in the fossil record as the amniotes evolved through the Permian. From the Early Triassic on we see a great diversity and number of amniotes, while the nonamniote tetrapods declined in number. Amniotic Egg An amniotic egg (figure 9-12) has a shell that may be flexible and leathery (turtles) or rigid and calcified (birds). Initially, the developing embryo is on one end of the large yolk. The first membrane to develop is the yolk sac, which is part of the developing gut, and blood vessels transport nutrients from the yolk to the embryo. This is also the pattern that we see in amphibians and fish. The difference is in the three new membranes. The chorion develops from the body wall and spreads outward around the embryo. The amnion also develops from the body wall and spreads outward around the embryo. The chorion and amnion meet and merge and leave the outer membrane as the chorion and the inner membrane as the amnion, both surrounding the embryo. The allantois develops from the posterior hindgut and eventually forms its own pocket in which nitrogenous wastes are stored. It also functions for the exchange of gases and thus serves as a respiratory organ. The selective advantage of the amniotic egg was probably its ability to nourish a larger fetus on land. A larger egg, without the three membranes, especially the allantois to serve as a respiratory organ, may not be able to exchange gases with the atmosphere at a sufficient rate for development. This larger egg allowed the amniotes to become larger and radiate into a variety of niches previously unavailable to nonamniotes. It is also possible, though not likely, that the three membranes evolved in a live-bearing ancestor to increase the efficiency of nutrient exchange between mother and fetus and that a descendent of this ancestor returned to egg-laying mode giving rise to the amniotic egg. 2 All vertebrates that have the embryo develop within an amniotic egg have lost the external gills and the lateral line system of non-amniote vertebrates and thus no aquatic larval stage is possible. Skull morphology Skulls are one character that is relied on more than anything else in describing fossil remains of species. Thus skull morphology has been used to divide the amniotes into different groups or clades. The arches of the skull, which give rise to fenestra (holes) in the skull, have primarily been used to name these groups (figure 9-13). Anapsid - no arches and thus no fenestra. The skull is solid as seen in turtles. Synapsid - single arch and thus one fenestra. This skull is typical of mammals. Diapsid - double arch and thus two fenestra. This skull is seen in reptiles and bird. Coupled with the changes in the skull were changes in the muscles that move the jaws. The result was increased force to rapidly close the jaw and the ability to generate forces at different angles so that the animal could chew and crush its food (figure 9-14). In addition, the fenestra allowed the jaw muscles to have a greater area of origin and to be bigger in cross-section (figure 9-15). This also allows the amniotes the ability to closed the jaw and then continue to apply force with the mouth closed (static pressure feeding) Skin is much less permeable to water, mostly as a result of increased amounts of lipids in the skin. Amniotes also have more keratinized structures in the skin such as feathers, hair, or scales. Amniotes also costal (rib) ventilation of the lungs. Turtles Turtles originated from the earliest amniotes in the Carboniferous. It is possible that turtles evolved from within the diapsid lineage and then secondarily lost the two skull fenestra thus have an anapsid skull. The earliest fossils that are clearly turtles are from the Late Triassic and turtles have changed little from that time. Two major groups of turtles. (13 families, 300 species) (table 12-1) • Pleurodira (side-necked turtles)- only in Southern Hemisphere, 3 families, 79 species • Cryptodira (hidden-necked turtles) - 10 families, 220 species Shells All turtles have a shell. It is divided into two parts; carapace, which is the upper shell, and the plastron, which is the lower shell. The carapace is composed of dermal bone that grows from 59 different centers of ossification. These centers of ossification give rise to several series of dermal bones (peripherals, costals, neurals) in the carapace. The neural bones are fused to the vertebrae while the costals are fused to the ribs. The pelvic and pectoral girdles are inside the ribs, which is unique among the vertebrates. The plastron is also derived from dermal bone, except for the part that is derived from the clavicles and interclavicle. The epidermal scutes, which cover the bony shell do not correspond to the dermal bones underneath (figure 12-5). Some species have shells with a hinge or two in the plastron. This allows a turtle to draw into its 3 shell and then close the shell as protection against predation (for example Terrapene). The exact number and position of the hinges vary among the species of turtles and these kinetic shells have evolved a number of different times among the lineages of turtles. Circulation Turtles (like amphibians and most reptiles) have a three chambered heart (figure 12-6). There are two circuits in tetrapods; the systemic circulation that carries blood through the body and the pulmonary circulation that carries blood through the lungs. Because there is only one ventricle, turtles (like amphibians and other reptiles) can shunt blood from the pulmonary to the systemic circulation, if the lungs are not being used for respiration (for example, during diving or hibernation). Even though the ventricle is a single chamber there are three separate compartments or regions in the ventricle and through a series of ridges, a turtle can shunt blood from the pulmonary into the systemic circulation or can maintain the blood in the systemic circulation separate from the blood in the pulmonary circulation. A muscular ridge divides the ventricle into the cavum pulmonale, which opens into the pulmonary artery, and the cavum venosum, which opens into the right and left aortic arches. Across the top of this ridge is the interventricular canal which connects the right side of the ventricle (cavum pulmonale) with the left side of the ventricle (cavum venosum). Normally, the right atrium get deoxygenated blood from the systemic circulation and then passes it to the cavum venosum, through the arterioventricular valves. The median side of the A-V valve covers the interventricular canal so that the blood must flow from the cavum venosum into the cavum pulmonale and then into the pulmonary arteries of the pulmonary circulation. Blood from the left atrium passes through the A-V valves into the cavum arteriosum. As the ventricle contracts, blood started flowing from the cavum venosum to the cavum pulmonale but as the ventricular contraction continues, the muscular ridge closes the passage from the cavum venosum to the cavum pulmonale and allows blood to flow from the cavum arteriosum into the cavum venosum and then into the aortic arches. The timing of blood flow through the heart prevents the mixing of oxygenated blood coming from the pulmonary circulation with deoxygenated blood coming from the systemic circulation (study figures 12-6 and 12-8). Respiration is primarily by lungs. Because the ribs are immovable, ventilation is by a visceral pump, in which the viscera are pushed against the pleural cavity to force air out of the lungs. The viscera are then pulled down to draw air into the lungs. Muscular activity is used for both inhalation and exhalation (figure 12-7). A few turtles use other structures for respiration when under water; pharynx in softshelled turtles and the cloaca in several diving turtles. In both cases, the turtles pump water in and out of the pharynx or cloaca and can exchange oxygen and carbon dioxide across the membranes of these structures. Intracardiac shunts Turtles (like squamates and crocodiles) have the ability to shunt blood from the pulmonary circulation (and bypass the lungs) to the systemic circulation. It would do this during periods of apnea (no breathing) when the lungs are not being ventilated and there would be no oxygen to be taken up into the blood. This would occur for several reasons, but diving is probably the most common reason. 4 Because the turtle's heart only has three chambers, there is a potential for mixing blood from the systemic and pulmonary circulation. Under normal condition, the pressure in the pulmonary circulation is less and blood flows out of the cavum venosum through the interventricular canal and into the cavum pulmonale and then into the lungs before the oxygenated blood from the cavum arteriosum would flow into the cavum venosum and then into aortic arches. During diving, however, muscles that line the walls of pulmonary arterioles constrict the size of the blood vessels in the lungs and thus raise the pressure in the pulmonary circulation. This rise in pressure causes the blood in the cavum venosum to flow into the aortic arches, which open from the cavum venosum. This effectively cause some of the blood from the right atrium to flow directly into the systemic circulation. This is called a right-to-left intracardiac shunt, because blood in shunted directly from the right side (right atrium) into the flow of blood that normally comes from the left side (left atrium) (figure 12-8). Reproduction and behavior All turtles are oviparous (lay eggs). The eggs are covered by a leathery membrane (though calcified in a few species) , which would prevent sperm from reaching the egg. Thus fertilization is internal before the shell is produced to coat the egg. To facilitate internal fertilization and insure that only individuals of the same species mate, there is at least some courtship or other species recognition signals present. Turtles employ visual, olfactory, tactile, and auditory cues during courtship. Many species have a complex series of lines on the face that are used as species recognition signals. Several species have glands in the male that enlarge during the breeding season and produce pheromones that are used to mark substrates within a territory. Males often engage in combat that involves biting the head of an opponent or ramming him and trying to overturn him. Large tortoises often live in herds and a large male is often dominant. Fighting among individuals serves to establish the dominance hierarchy (figure 12-9). Eggs are laid in a nest that is dug by the female. After this, there is no parental care. The condition of incubation determines the sex of the turtles. Higher temperature usually leads to the development of the larger sex, which is usually female. Lower temperatures usually yield males, though this pattern is not always true for every species. The range in temperature where the sex changes is fairly narrow (3 - 4 C) (figure 12-10). Because of this mechanism for sex determination, most of the hatchlings from a given nest will be one sex or the other. Across the population, there will be about a 1:1 sex ratio produced. Hatchling behavior of marine turtles has been better studied than other turtles. There are about 100 eggs in a typical nest and hatching is almost simultaneous. Vocalizations help to get all of the nestmates synchronized for hatching. Then enmass they dig their way to surface of the sand. During the night, all of the turtles emerge from the nest at once and race to the ocean. Along a stretch of beach there will likely be many other nests of turtles emerging at the same time. This simultaneous emergence of thousands of hatchlings serves to saturate the predators that quickly respond to this abundant food supply. Navigation Marine turtles display amazing abilities to navigate from natal beaches to adult feeding grounds and back to the natal beach of their birth to lay eggs (figures 12-11 and 12-12). They can find very small beaches or islands from thousands of miles away. They use light, wave action, and 5 migration to locate their position. Loggerhead turtles use the Earth's magnetic field to locate their latitude on the surface of the Earth. Thus they know where they are in relation to the ocean currents. The turtles feed on turtle grass that grows on sandy flats but typically in places that are not close to good beaches for laying eggs. Thus adult female turtles must migrate every couple of years between the feeding grounds and the nesting area. Olfaction is also an important cue when the turtles get close to the beaches. Temperature Regulation Turtles are ectotherms, that is, they have a body temperature that is primarily determined by their environment. Turtles regulate their body temperature behaviorally. On sunny days, they bask in the sun to increase body temperature. The increase in temperature increases the rate of various metabolic functions, for example, growth, egg development, and digestion. For aquatic turtles, basking helps to kill algae and rid themselves of leeches. Marine turtles are, to some extent, endothermic in that they have a countercurrent exchange systems of blood vessels in the flipper to conserve heat. This system transfers heat from the blood in the proximal part of the flipper to the blood that is returning to the body from distal end of the flipper. The result is that heat is conserved in the body and not lost through the extremities. Last updated on 12 March 2012 Provide comments to Dwight Moore at [email protected]