REPTILIA 1
1. Geological/ecological setting
a. World today is very different from world when amniotic organisms evolved
i. Today is much dryer and colder than it was then
ii. Continents are in different positions and are widely separated now
b. First amniotes arose in Carboniferous (285-360 mya)
c. In general, radiations were associated with increase in diversity of terrestrial habitats
(increase in diversity of niches)
i. Coal swamps were prevalent on land
ii. Earliest radiation followed major radiation of terrestrial insects
iii. At the end of the Permian period (248 mya) was the largest mass extinction
iv. From late Permian through the Mesozoic, geological changes favored increase in
terrestrial habitat diversity
v. By early Mesozoic, “modern looking” food webs characterized terrestrial
communities: mixed plants, herbivorous insects, and vertebrates, carnivores, etc.
vi. Breakup of Pangea and continental drift continued to increase climate diversity into
the Cretaceous
2. Systematic/taxonomic overview
a. Basic patterns of relationship pretty well understood
b. Major lineages determined by skull characters (note names based on structure of skulls -see fig. 7-2 / 7.5 : 171 / 175)
i. Number of holes in head is primary determinant = temporal fenestration
ii. Anapsida
1. Includes modern turtles, tortoises
2. Skull has no holes (other than eyes)
iii. Synapsida
1. Single hole
2. First major lineage to split from rest of amniotes (by Late Carboniferous)
3. Formed mammals and our ancestors (the “mammal-like reptiles”)
iv. Diapsida
1. Double hole
2. Crocodilia, dinosaurs, and birds
3. Modern lizards, snakes, tuatara
v. Number of holes arose independently in synapsids and diapsids, so two unrelated
groups
c. Taxonomy still in some dispute
i. Traditional classification is artificial and messy, because it separates mammals and
birds from their ancestors
ii. But cladistic classification too cumbersome
iii. Book uses a combination of names that make it relatively confusing
iv. We will use older names simply because they make more sense
v. But keep in mind that relationships are not nearly as neat and tidy as we make them
seem
3. Major trends in amniote evolution (from Carboniferous through Mesozoic)
a. Note that “trends” here refers to the fact that similar kinds of changes were favored in
multiple lineages over time
i. Does not imply linear, “progressive” change
ii. Does not imply that changes happened at same rate across groups
iii. So what we actually see is different combinations of traits in different groups at
different times
b. Result of these “trends” was/is:
i. Increasing ecological diversity
ii. Increased activity levels
iii. Increased size
c. Can divide “trends” into four major categories (but they are all interrelated)
i. 1-Increasing adaptation to terrestrial environments
1. Amniotic egg (shared, derived character that defines the group) has a system
of extra embryonic membranes (and usually a shell)
2. Modification of integument, including “waterproofing”
ii. 2-Increase in locomotor efficiency
1. Early amniotes (and some modern forms) retain basic girdle structure and
locomotor system as amphibians (fig 8.6 : 202)
2. Derived amniotes change to upright posture with limbs beneath the body.
Advantages include
a. Improved locomotor efficiency
b. Frees trunk musculature to be used solely for ventilation of lung
iii. 3-Increased feeding efficiency allows modifications of skull for wide range of food
types; basic changes include
1. Lighten skull by decreasing the number and thickness of bones
2. Improve jaw mechanics (fig 8.14)
iv. 4-Increase in metabolic efficiency = improve ability to acquire, deliver oxygen and
nutrients to tissues
1. Big change to lungs: switch from buccal pump to aspiration lung, which is
much more efficient than buccal pumping lung
2. Modification of heart, circulatory system (fig 8.10)
a. Ventricle incompletely subdivided to provide “functionally” 4chambered heart
b. Allows higher pressure systemic circuit (so improves oxygen,
nutrient delivery)
c. Because incompletely subdivided, permits flexibility in circulatory
patterns, especially for divers (crocs, turtles)
d. Summarize: amniotes “most successful” terrestrial vertebrates; radiations supported by
i. Increase in the complexity of terrestrial habitats
ii. Extinction of most previously dominant terrestrial vertebrates
iii. Series of changes permitting radiation into full range of terrestrial niches
4. Testudinia – Turtles and Tortoises
a. Systematics, diversity, and distribution
i. The earliest fossils of turtles date to the late Triassic (210 mya) in the Mesozoic
Era; relatively little major morphological change has taken place since that time
ii. Modern turtles/tortoises include ~ 250 species in 13 families
iii. Worldwide distribution and variety of habitats
1. Turtles/tortoises can be found on all continents except Antarctica
2. Can also be found in all warm and temperate oceans
3. Occupy a wide diversity of both terrestrial and aquatic habitats
b. Form and function
i. Skull is unique among modern reptiles by having a horny epidermal "beak"
covering the dentary (major bone of lower jaw)
ii. Most obvious characteristic = "shell"
1. A relatively heavy adaptation that can limit diversity
2. Composed of two parts
3.
4.
5.
6.
7.
a. Carapace above
b. Plastron below with 6 plates
c. Two pieces connected via bony processes
Both plastron and carapace have same basic structure
a. Dermal bone beneath
b. Epidermal (protein) scales (scutes) above
Bones of the carapace are fused to the vertebrae and ribs, leaving the ribs
outside the limb girdles – unique compared to ribs inside the girdles on all
other tetrapods)
In a number of species have hinged plastrons: front and rear lobes can be
pulled upward to close openings for protection (in box turtles)
In others, plastron is reduced in size, allowing greater mobility/agility: one
species (musk turtle) can even climb several feet into trees to bask
Variation in shell morphology is correlated with general differences in
habitat and ecology
a. Terrestrial species tend to have
i. High domes
ii. Broad, elephant-like feet
iii. For example, the tortoises in the family Testudinidae
b. Aquatic species tend to have
i. Streamlined shells, often with reduced ossification
ii. Webbed feet for swimming
iii. For example, soft-shell turtles
iv. Marine species show even greater reduction for reduced drag
iii. Respiratory and circulatory systems
1. Heart is very complex
a. Structurally
i. Completely divided atria
ii. Incompletely divided 3-chambered ventricle with flaps that
separate oxygenated and deoxygenated blood
b. Functionally, the arrangement permits
i. Complete separation of oxygenated, deoxygenated blood
during "normal" circulation
ii. High pressure systemic and low pressure pulmonary circuits
(lower resistance)
iii. But also allows shunting of blood between systemic and
pulmonary circuits (during diving for aquatic species)
2. Aquatic species
a. When underwater for long periods
b. May supplement lungs by performing gas exchange with water,
using specialized epithelium of the pharynx
c. Others open cloaca to suck in water
d. Large sacks open from cloacal wall that are highly vascularized
e. Pump water in and out of sacks for respiration
c. Reproduction
i. Turtles are long-lived, with relatively low reproductive rates
ii. All are oviparous, with relatively long incubation periods and no parental care
iii. Many exhibit temperature-dependent sex determination
1. This form of sex determination is found in many turtles, all crocodilians,
and some lizards
2. Temperature during nesting determines sex of offspring
3. In general, the higher temperature produces the larger sex (so in turtles, high
temperatures produce females)
4. Usually, each nest will produce predominantly individuals of one sex;
multiple nests with different temperatures produces the overall sex ratio
(usually 1:1)
iv. All have internal fertilization
v. Clutch size (number of eggs per nest) depends on species
1. Small species is 4-5 eggs
2. Large (especially marine) species is up to 100 eggs
d. Ecology and behavior
i. Migration
1. Finding nesting sites for terrestrial turtles
a. In home range
b. Usually based on knowledge of local landmarks
c. Use sun for orientation
2. For sea turtles
a. Have very strong nest-site fidelity
b. Have remarkable ability to find nest site after foraging thousands of
km away
e. Conservation status and issues
i. At least three groups of turtles are facing major conservation risks
1. Sea turtles
2. Large land tortoises (e.g., gopher tortoises in U.S.; Galapagos and Aldabara
tortoises)
3. A whole variety of species now being exported for food and use in the
exotic pet trade, with Asian species especially affected
ii. Life history traits described above place turtles at risk generally
1. Low reproductive rates, long period before sexually mature, and lack of
parental care mean recruitment of new individuals into populations will be
slow
2. Many species are restricted to a single island; So any time conditions create
high adult mortality, populations will be in trouble
iii. Specific threats include (but not limited to)
1. Habitat loss and degradation
2. Overexploitation for food and the pet trade
iv. According to 1996 Red List
1. Seven species Extinct
2. Two species Extinct in the Wild
3. 14 species Critically Endangered (facing an extremely high risk of
extinction in the wild in the immediate future)
4. 33 species Endangered (not Critically Endangered but facing a very high
risk of extinction in the wild in the near future)
5. 62 species Vulnerable (not Critically Endangered or Endangered but facing
a high risk of extinction in the wild in the medium-term future)
6. One species is Lower Risk/Conservation Dependent (cessation of a taxonor habitat-specific conservation program will result in the taxon qualifying
for one of the threatened categories above within 5 years)
7. 47 species Lower Risk/Near Threatened (don’t qualify for conservation
dependent, but close to qualifying for vulnerable)
8. Note total = 166 out of ~ 250 species = 65% of extant species currently
facing documented risk!