Lecture 9, Amniote and Reptile Origins

Amniotes: Definition and Key Groups

Amniotes are a significant group of tetrapods characterised by the presence of an amniotic egg, which is a pivotal evolutionary innovation that allows these organisms to reproduce in terrestrial environments without the dependence on water for early developmental stages. This group comprises living mammals and reptiles, including birds, as well as their most recent common ancestor and all of its descendants. Amniotes are divided into two major groups which highlight their evolutionary lineage and morphological features:

Major Groups:

  1. Synapsida: This group exclusively includes mammals and their extinct relatives. The synapsid lineage is characterised by a single temporal fenestra in the skull, which is a key aspect of their cranial morphology.

  2. Reptilia: This group encompasses all reptiles, including birds. Reptiles are distinguished by their scaly skin and typically lay eggs with hard shells.

Within the Reptilia, we can further recognise several important subgroups:

Sauria (Crown Reptiles):

Also known as crown reptiles, this group includes two main lineages that showcase the diversity of reptilian adaptations:

  • Lepidosauromorpha: This clade encompasses tuataras, lizards, and snakes. Members of this group possess a unique scale structure and a variety of foraging and reproductive strategies.

  • Archelosauria: This group contains turtles and archosaurs; the latter includes the highly adapted crocodilians and birds. The evolutionary adaptations of archosaurs have allowed them to thrive in diverse ecological niches.

In addition to these major groups, there are several extinct stem-reptile groups that provide insights into the evolutionary history and development of modern reptiles. Their fossil records help illustrate the transitions that occurred over millions of years.

Key Derived Features of Crown Amniotes

Amniotes are distinguished from their amphibian relatives by several key evolutionary innovations, which have facilitated their adaptation to terrestrial life. The most important derived features include:

  1. The Amniotic Egg:

    • The amniotic egg is considered the defining feature of crown amniotes, as it allows for a more advanced form of reproductive adaptation in terrestrial environments. The amniotic egg is composed of several extraembryonic membranes that are notably absent in non-amniotic organisms:

      • Amnion: This membrane forms a fluid-filled cavity that acts as a protective cushion for the developing embryo, preventing desiccation and mechanical shocks.

      • Allantois: This membrane plays a crucial role in storing nitrogenous waste produced by the embryo and facilitates gas exchange, contributing to the embryo's metabolic needs.

      • Chorion: This outer membrane envelops the egg and, in combination with the allantois, creates a surface for gas exchange, ensuring the embryo receives sufficient oxygen during development.

    • The amniotic egg is typically also equipped with a yolk sac that stores nutrients required for the embryo's growth. Additional structures include:

      • Albumen: This provides a water and protein reservoir that supports the embryo.

      • Porous eggshell: This adaptation allows for gas exchange while providing protection, essential for an egg laid in a terrestrial environment.

    • The development of the amniotic egg facilitates internal fertilisation, and its dry outer covering greatly reduces water loss. This reduction in moisture requirements removes the necessity for a larval stage and the need for an aquatic environment to reproduce, thus enabling amniotes to occupy fully terrestrial habitats.

  2. Keratinised Skin:

    • Amniotes possess skin featuring a thickened outer layer known as the stratum corneum, which is composed of dead cells filled with the fibrous protein keratin. Keratin is a vital component of the integumentary systems in vertebrates, providing protection and structural integrity.

    • There are two types of keratin present:

      • Beta keratin: Found primarily in reptiles (including birds), this version forms durable structures such as scales, claws, and feathers, providing not only protection but also aiding in locomotion and other functions.

      • Alpha keratin: More commonly found in mammals, this type is softer and offers flexibility.

    • The keratinised skin plays a critical role in reducing water loss from the body, which is essential for survival in terrestrial environments where water availability may be limited.

  3. Costal Ventilation:

    • Unlike amphibians, which utilise a positive pressure buccal pump mechanism for breathing, amniotes employ a negative pressure aspiration pump to draw air into their lungs. This method of breathing is far more efficient and allows for increased oxygen intake.

    • The evolution of costal ventilation is facilitated by the mobility of the ribs relative to the vertebrae. Ribs are connected to the sternum and are enhanced by the presence of intercostal muscles which are located between the ribs.

    • When the intercostal muscles contract, they raise the ribs, effectively increasing the volume of the thoracic cavity. This action creates negative pressure within the lungs, allowing air to flow in and maximise the efficiency of gas exchange.

The Evolutionary Assembly of the Amniote Body Plan

The assembly of the amniote body plan evolved in a stepwise manner along the amniote stem, signifying a gradual adaptation to terrestrial life:

The Split:

  • The divergence between the amphibian total group (Temnospondyli) and the amniote total group (Reptiliomorpha) occurred around 355 million years ago during the early Carboniferous Period. This period marked significant evolutionary developments as these groups adapted to different ecological niches.

  • This watershed moment in evolutionary history took place during Romer’s Gap, a period in the fossil record stretching from about 360 to 345 million years ago, characterised by a scarcity of fossils, making the study of early amniote evolution challenging.

Stem Amniotes:

Several extinct groups contribute to our understanding of the early evolution of amniotes:

  1. Anthracosaurs:

    • Lived from the early Carboniferous to the early Permian and were predominantly aquatic in their habitats. They displayed features such as elongated trunks, long tails, and small limbs, indicating adaptations for an aquatic lifestyle.

    • They show early indications of amniote skeleton characteristics, including a larger pleurocentrum relative to the intercentrum in their vertebral structures.

  2. Seymouriamorphs:

    • These were predominantly terrestrial organisms from the Permian that showcased robust skeletons and limbs with fully formed joints, allowing them to navigate various terrains. Despite their terrestrial adaptations, they had to return to aquatic environments to reproduce because their larval forms were still reliant on gills.

    • They display an important amniote characteristic of a dominant pleurocentrum in their vertebrae, indicative of their evolutionary lineage.

  3. Diadectomorphs:

    • Known from the late Carboniferous to the late Permian, this group included both terrestrial and semi-aquatic forms, with many species evolving into large herbivores.

    • They likely possessed features such as costal ventilation and keratinised skin and are viewed as the sister group to crown amniotes, highlighting their evolutionary significance.

Early Reptilian Evolution and Diversification

The initial evolution of reptiles involved several key groups that illustrate the transition towards the crown group:

  1. Paleothyris and Hylonomus:

    • These species are among the earliest known amniotes, dating back to the late Carboniferous. They were traditionally regarded as the oldest crown amniotes; however, recent analyses have suggested that they may be placed higher on the amniote stem due to their distinctive features.

    • Both exhibit a solid skull morphology without temporal openings (anapsid), and they have a stapes that serves to brace the braincase. Their jaw mechanics were enhanced by a derived feature of a lateral flange on the pterygoid bone, which indicates the evolution of a pterygoideus muscle, allowing for complex jaw movement.

    • They further showcase derived postcranial features, including a modified second cervical vertebra allowing for enhanced head mobility, two sacral ribs, and an astragalus bone in the foot, indicative of their adaptations.

  2. Captorhinids:

    • This group of reptiles featured an anapsid skull and is seen as traditional stem reptiles, although recent classifications place them on the amniote stem. Their existence ranged from the late Carboniferous to the Permian, exhibiting a diverse diet that varied from omnivory to herbivory.

  3. Parareptiles:

    • Parareptiles reside on the reptile stem, although there is ongoing debate regarding whether they form a distinct clade. Key members include aquatic Mesosaurs, renowned for possessing the oldest known amniotic embryos. They displayed elongated bodies with needle-like teeth that equipped them for a predatory lifestyle.

    • Their geographical distribution has provided vital evidence supporting the theory of continental drift, indicating their widespread presence across oceans.

    • The group known as Procolophonia represents diverse terrestrial and fossorial animals, displaying varied diets ranging from insectivory to herbivory, while being the only stem-reptile clade to survive the Permo-Triassic extinction event.

  4. Diapsids:

    • Diapsids are characterised by possessing two temporal openings in their skulls, a feature that allows for muscular and skeletal adaptations benefiting their ecological strategies. Notable lineages include Weigeltisauridae, such as Coelurosauravus, which exhibited elongated, rod-like bones on the body, likely supporting gliding membranes for assisted mobility.

    • Another important group within diapsids is Younginiformes. This group features uncertain classifications and includes both terrestrial and aquatic forms, showcasing diversity in morphology and adaptations. Examples range from lizard-like Youngina to aquatic forms like Claudiosaurus and Hovasaurus.

  5. Sauria (Crown Reptiles):

    • The crown group of reptiles includes the Lepidosauromorpha, represented by tuataras, lizards, and snakes, and the Archelosauria, which encompasses turtles, crocodilians, and birds. Notably, these adaptations allowed these groups to diversify successfully into numerous habitats and roles in contemporary ecosystems.

Summary of Key Points

  • Amniotes represent a diverse lineage of tetrapods defined by the presence of the amniotic egg, granting them the ability to reproduce in fully terrestrial environments.

  • Key evolutionary innovations such as the amniotic egg, keratinised skin, and costal ventilation have enabled the amniotes to occupy terrestrial niches effectively and efficiently.

  • The evolutionary assembly of the amniote body plan unfolded in a systematic manner, with extinct groups like anthracosaurs and diadectomorphs contributing vital insights into the transition from amphibian-like ancestors to fully terrestrial amniotes.

  • Early members such as Paleothyris and Hylonomus illustrate the blend of primitive and derived traits that characterise the early evolutionary steps of amniotes.

  • The lineage of diapsids showcases numerous forms that highlight adaptations for varied ecological roles, including gliding flight and aquatic forms.

  • The Sauria crown group is exemplified by modern reptiles, including birds, illustrating the remarkable diversification of this lineage from their ancient relatives.