Lab 3: Reptile Info Flashcards
Evolution and Phylogenetic Relationships of Reptiles
Class Reptilia
The old group, “Class Reptilia” is now viewed as a “hodge-podge” of organisms that are not especially closely related to each other and were combined based on primitive, ancestral traits rather than shared, derived features. See your textbook for recent assessments of the phylogeny of the early amniotes (tetrapods that produce amniotic eggs) and their descendants. Some authors have suggested that the term “reptile” still be applied informally to turtles, lizards, and snakes, but not archosaurs and mammal-like synapsids.
Evolution and Phylogenetic Relationships of Reptiles
Amniotes
It is certain that amniotes evolved from earlier tetrapods, but there is insufficient evidence to indicate conclusively from which particular group they arose. Some theories:
a. One hypothesis is that the amniotic egg preceded the evolution of fully terrestrial tetrapods. Earlier reptiles were aquatic but laid their eggs on land. There was a selective advantage in laying eggs on the land because most predators were aquatic.
Also, climatic fluctuations (droughts and flooding) would probably favor the elimination of aquatic development and a free-swimming larva.
b. Another hypothesis proposed that the first amniotes were primarily terrestrial animals. Fossil remains of Hylonomus from Nova Scotia were preserved in 315 million year old tree hollows. This animal had a highly ossified skeleton and well developed limbs, thus reflecting a marked adaptation to terrestrial life. Westlothiana from Scotland, being 38 million years older that Hylonomus, is considered to be the oldest amniote but shows similar skull and limb morphology.
Characteristics of Reptiles – the Skull
Reptile Skull versus the Amphibian Skull:
a. One occipital condyle vs. two in amphibians.
b. A greater degree of ossification.
c. Reduction of loss of certain bones; modification of others.
Characteristics of Reptiles – the Skull
Skull types in amniotes
Classification of amniote lineages is based heavily on skull morphology, especially the number and position of temporal openings in the cheek region associated with the jaw musculature.
a. Anapsid = no opening. Possibly, first amniotes and living turtles.
b. Synapsid = one opening, low on the skull. Mammal-like reptiles and their mammal descendents.
c. Diapsid = two openings; one high, one low. Tuatara, lizards, snakes, archosaurs (including birds), and extinct marine reptiles. The basic diapsid pattern gave rise to several variants, including skulls in which one of the openings was secondarily lost.
Characteristics of Reptiles – Body Form
- Ancestral body form, long body and long tail, short, sprawling limbs – seen in lizards.
- Long body, reduced or absence of limbs, and tail not clearly defined – seen in snakes and limbless lizards. Evolved from “body form 1”.
- Short body encased in a bony shell – seen in turtles. Evolved from “body form 1”
Integumentary adaptations
a. Well-developed epidermal scales whose main function is waterproofing. This allows reptiles to inhabit marine environments, disperse to oceanic islands, and to be common in arid habitats (unlike amphibians). Scales are arranged in very definite patterns. In most snakes, ventral scales are greatly enlarged into broad, transverse bands – scutes.
b. Modified epidermal scales:
i) Elongated spines or “horns”, e.g., horned lizards and some iguanas.
ii) Rattle of rattlesnakes.
iii) Egg tooth (used by hatching young to open egg shell).
c. Well-developed dermal structures, i.e., bony plates in turtles.
d. Few glands.
Anti-Predation Mechanisms
- Glands for the production of venom and teeth for delivering venom.
- Behavioral warning devices. Use of posture, movement or sound production to warn potential predators that they risk injury or death in attacking the displaying reptile.
- Caudal autotomy. The ability to voluntarily shed all or part of the tail to escape from the grip of a predator or to distract it. The tail is later regenerated.
- Withdrawal of head and legs inside a bony shell, i.e., turtles.
- Adaptive coloration. Anti-predation coloration is well developed in lizards and snakes, including camouflage, deflective coloration, aposematic coloration and mimicry
Communication
Like most vertebrate groups, reptiles use a variety of sensory pathways to communicate. Use of visual signals is very well developed in lizards for courtship and defending territories. Snakes rely heavily on chemical and tactile signals.
Reproduction and Development
Amniotic Egg.
The amniotic egg is characteristic of reptiles, birds, monotremes, and, in a modified form, of therian mammals as well.
The shell, which may be leathery or calcified, provides mechanical protection while allowing movement of respiratory gases and water vapor.
The albumin gives further protection against mechanical damage and provides a reservoir of water and protein.
The yolk is the energy supply for the developing embryo.
The real significant advance over anamniotic eggs (fish and amphibians) is the presences of extra-embryonic membranes – the chorion, allantois, and amnion.
— The chorion, the outermost membrane, surrounds the embryo and yolk sac and assists in gas exchange and in forming blood vessels.
— The allantois develops as an outgrowth of the hind gut and lies within the chorion. It functions as a respiratory organ as well as a storage place for nitrogenous wastes.
— The amnion is the fluid filled innermost extra- embryonic membrane surrounding the embryo, forming the amniotic cavity and containing amniotic fluid.
Reproduction and Development
Female and male reproductive systems.
a. Ovaries are paired, thin-walled sacs with eggs at different stages of maturation. Mature eggs enter the oviducts and are fertilized here by spermatozoa. Fertilized eggs are then surrounded by albumen and a shell; they are deposited after they reach the cloaca (or may “hatch” prior to entering the cloaca in ovoviviparous and viviparous forms).
b. Testes are linked by ureters of embryonic mesonephros; these ureters form sperm ducts carrying sperm to the copulatory organs.
i. Cloaca is modified to form a penis or hemipenes in all forms except Tuatara.
ii. Snakes and lizards have paired hemipenes, found adjacent to the cloaca.
Reproduction and Development
Breeding Habits.
Breeding seasons in reptiles are primarily affected by temperature conditions and photoperiods.
a. In snakes, the sexes associate only during the breeding period.
b. There are three features that may occur in the breeding habits of many lizards:
i. Fierce fighting among males for the possession of the females.
ii. Brief and vigorous copulation where the male grasps the female by the neck with his jaws while his hind limbs hold her still as he inserts one hemipene at a time.
iii. Frequent homosexuality among males.
c. Turtles always mate in the water; the male climbs on to the female and scratches, bites, etc. to incite the female. During copulation the male is almost vertical on the female’s back, just long enough to insert his penis.
Reproduction and Development
Parental Care and the Number of Eggs Produced.
a. Fertilization in reptiles is always internal. There are three major types of embryonic development in reptiles, oviparous, lecithotrophic, and matrotrophic.
i. Tortoises, many lizards, and snakes are oviparous, laying eggs like birds. The size of the egg and the clutch size depend on the species. The incubation period is variable; the young reptile breaks out of the egg using an egg-tooth.
ii. Some other reptiles retain their eggs instead of laying them; the young undergo part, or all, of their development in the oviducts. If the young develops using only its yolk as it source of nutrition this is known as lecithotrophy. If hatching occurs in the oviducts and the yolk is not large enough to support the developing young, then there must be some other food source for the newborn, likely coming from secretions from the oviducts. Evidence of a placenta-like structure has also been reported in some snakes (e.g., red-sided garter snake, Thamnophis sirtalis). Generally, the brood size is much smaller with lecithotrophy and matrotrophy than with oviparity.
b. Since the eggs can be well hidden in a terrestrial environment, there is less predation and, therefore, typically fewer eggs are produced.
c. The number of eggs produced is affected by:
i. Duration of time of intrauterine development,
ii. Amount of parental care after deposition of eggs.
iii. Age and size of female.
Reproduction and Development
Sexual Dimorphism
In all reptilian groups there are many examples of sexual dimorphism, with either males or females achieving greater size.
Large female size is often attributed to the greater reproductive output of this sex and the additional space required for gamete production and egg/embryo storage.
Larger male size is often related to sexual selection and predominates in species that are territorial or in which competition for mates occurs.
Dimorphism may also relate to non-reproductive parameters, such as differences in the diet of males and females.
