Download and Chapter 12

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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.
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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
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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.
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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
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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]