Lesson 12 Reptiles(1)

Phylogenetic Classification of Chordates

  • Major Groups: The following groups encompass the main classifications within the phylum Chordata:

    • Cephalochordata (Lancelets)

    • Urochordata (Tunicates)

    • Agnatha (Jawless Fishes)

    • Chondrichthyes (Cartilaginous Fishes)

    • Osteichthyes (Bony Fishes)

    • Amphibia (Amphibians)

    • Reptilia (Reptiles)

    • Aves (Birds)

    • Mammalia (Mammals)

  • Key Features Across Groups:

    • Each major group reflects evolutionary adaptations such as limbs, lungs, jaws, vertebrae, mammals (characterized by mammary glands and hair), and the presence of an amnion in embryos.

    • Importance of the phylogenetic tree in visualizing relationships among these groups.

Amniotes and Their Classification

  • Definition of Amniotes:

    • Amniotes are organisms that reproduce using amniotic eggs, providing a protective environment for the developing embryo. These eggs consist of four extraembryonic membranes:

    1. Amnion: Encloses the embryo in fluid, cushioning it against mechanical shock.

    2. Allantois: A sac for storing metabolic waste (CO_{2} and nitrogenous waste).

    3. Chorion: Works with the allantois to allow gas exchange.

    4. Yolk Sac: Contains a stockpile of nutrients for the developing embryo.

  • Traditional Classification Issues:

    • Traditional classification of tetrapods as paraphyletic means they contain groups with members that can be classified into more than one clade.

  • Modern Classification:

    • New classifications now place birds within the dinosaur lineage, highlighting their relationship with reptiles.

Clade Amniota Overview

  • Major Subclades:

    • Synapsida: Includes mammals.

    • Sauropsida: Includes reptiles, birds, and dinosaurs.

  • Subclass Anapsida: All extinct; turtles placed here but traditionally are classified as diapsids (thus, paraphyletic).

  • Subclass Diapsida: Consists of non-avian reptiles and birds, characterized by specific skull openings.

Lepidosauria: Lizards, Snakes, and Tuataras

  • Order Sphenodontia (Tuataras):

    • Only two surviving species found in New Zealand.

    • Feature a well-developed parietal eye (third eye) with a lens and retina under the skin, used for regulating circadian rhythms.

  • Order Squamata (Lizards and Snakes):

    • Characterized by a kinetic skull with movable joints that allow for significant manipulation of prey.

    • Male squamates possess paired copulatory organs called hemipenes.

    • Lizards: Usually have eyelids and external ears; include diverse forms like monitors, chameleons, and geckos.

    • Snakes: Limbless (though some retain vestigial pelvic girdles), lack eyelids, and have no external ears.

Archosauria: The Ruling Reptiles

  • Definition: A group of diapsids that includes crocodilians, pterosaurs, dinosaurs, and birds.

  • Key Features: Presence of an antorbital fenestra (opening in the skull in front of the eye) and teeth set in deep sockets (thecodont dentition).

Order Crocodilia

  • Diverse Groups: Includes crocodiles, alligators, caimans, and gharials.

  • Anatomical Advancements:

    • Four-Chambered Heart: Unlike other reptiles, crocodilians have a completely separated ventricle, providing efficient oxygen delivery similar to birds and mammals.

    • Secondary Palate: A bony plate that separates the nasal and mouth cavities, allowing them to breathe while submerged with only their nostrils above water.

    • Thecodont Teeth: Teeth are set in sockets, which provides greater strength for gripping large prey.

  • Reproduction and Behavior:

    • Temperature-Dependent Sex Determination (TSD): The sex of the hatchlings is determined by the temperature of the nest during incubation.

    • Parental Care: Unlike most reptiles, crocodilians display complex maternal care, guarding nests and carrying hatchlings to the water.

Characteristics of Reptilian Ancestry

  • General Traits of Reptiles:

    1. Lack of gills; respiration is entirely pulmonary.

    2. Efficient lungs and improved circulation; reptiles utilize negative pressure breathing.

    3. Highly ossified skeletons with a stronger rib cage.

    4. Skin is dry with minimal glands, preventing desiccation.

    5. Three-chambered hearts (except crocodilians and birds, which have four chambers).

    6. Presence of epidermal scales made of beta-keratin.

    7. Uric acid as the main nitrogenous waste product to conserve H_{2}O.

    8. Internal fertilization.

    9. Eggs feature calcareous or leathery shells to resist drying.

    10. Direct development without a larval stage.

    11. Ectothermy: Most reptiles regulate body temperature through behavioral means rather than internal metabolism.

Anatomy and Physiology of Reptiles

  • Nervous System:

    • Increased cerebrum size relative to amphibians.

    • Jacobson's organ (vomeronasal organ) aids in detecting chemical signals.

    • Heat-sensing pits present in some reptiles like pit vipers to detect infrared radiation.

Characteristics of Turtles

  • Order Testudines:

    • Dermal bone plates compose the carapace (dorsal shell) and plastron (ventral shell).

    • Carapace is fused with the ribs and thoracic vertebrae.

    • Turtles lack teeth and possess a sharp, keratinized beak.