Natural History of Vertebrates
Lecture Notes
Chapter 8 - Amniote Origins
Chapter 10 - 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. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).
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 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 8-24) 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 surronding 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 membrane, especially the allantois to serve as a respiratory
organ, would 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.
Skulls are one character that is relied on more than anything else in describing fossil remains to
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
10-218-25).
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 8=26). In addition, the fenestra
allowed the jaw muscles to have a greater area of origin and to be bigger in cross-section (figure
8-27).
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.
Turtles
Turtles originated from the earliest anapsid amniotes in the Carboniferous. 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.
Pleurodires
(side-necked turtles)- only in Southern Hemisphere, about 50 species in two families
Cryptodires
(hidden-necked turtles) - 200 species in 10 families
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 unique is 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 10-4).
Some species have shells with a hinge or two in the plastron. This allows a turtle to draw into its
shell and then close the shell as protection against predation (for example a box turtle). 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 10-5).
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 10-5 and 10-7).
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
10-6). A few turtles use other structures for respiration when under water; pharynx in soft-
shelled 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.
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 10-7).
Reproduction and behavior
All turtles are oviparous (lay eggs). The eggs are covered by a leathery membrane, 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 10-8).
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 10-9). 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.
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 10-10 and 10-11). They can find very small beaches or islands
from thousands of miles away. They use light, wave action, and 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.
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, increase 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 10 March 2003
Provide comments to Dwight Moore at mooredwi@emporia.edu
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