REPTILES: CHARACTERISTICS, CLASSIFICATION AND REPRODUCTION - BIOLOGY - 2025

The reptiles are a group of vertebrate animals parafilética including turtles, lizards, snakes, crocodiles and Tuataras. In addition, they include several extinct groups of large dinosaurs, plesiosaurs, pterosaurs, among others. Its most prominent feature is the thick skin with scales.

These living things are ectothermic, as they lack the ability to internally regulate their body temperature. Therefore, their distribution is almost restricted to regions with warm climates, where they are very abundant.

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Turtles are organisms with a thick shell and ancestral morphology. They do not have teeth and are all oviparous. The greatest diversity of the group is found in the lizards, a group that has flourished immensely. Most are oviparous and some are viviparous.

Two groups of reptiles, the snakes and the amphisbenidae have suffered a total reduction of the limbs. In addition, snakes have a kinetic (moving) skull that allows them to consume huge prey. Some can inject poison.

Tuataras are endemic animals to New Zealand. Its characteristics are reminiscent of the reptiles that inhabited the earth about 100 million years ago.

Crocodiles are the only non-avian representatives of archosaurs, a lineage that gave rise to the now extinct dinosaurs and current birds.

In reptiles a significant adaptation arises that managed to make reproduction independent of water: the amniotic egg. The egg consists of a structure covered with a calcareous or leathery structure with extra-embryonic membranes called amnion, chorion, yolk sac and allantois. Larval stages of aquatic life are not observed in reptiles.

General characteristics

Reptiles form a paraphyletic group of approximately 8000 species. These organisms are abundant and are found in highly varied habitats, both terrestrial and aquatic, generally with a warm climate.

They are covered by characteristic structures called scales. Most have two pairs of limbs, with five fingers on each. In snakes and some lizards the limbs have degenerated or are severely reduced.

Reptile skin is thick, providing protection against desiccation. The epidermal structures of these animals are made up of a special keratin, called beta keratin. The ossification of the skeleton is complete and the skull is characterized by having a single occipital condyle.

Regarding temperature regulation, all members of the group are ectothermic animals; they are not able to regulate their temperature through metabolic pathways.

In contrast, they can regulate their temperature through their behavior. That is, they move to sunny regions if they need to increase it, or they are located in shady regions to lower body temperature.

The amniotic egg

Sea turtle hatching from egg. Author: Mayer Richard. Wikimedia Commons.

The amniotic egg represents one of the most surprising adaptations of the animal kingdom, since it managed to make the reproductive processes independent of bodies of water.

The egg has four extra embryonic layers or membranes called amnion, allantois, chorion, and yolk sac.

The amnion is the layer that surrounds the embryo. Inside we find fluid that cushions the developing organism and provides an aqueous medium for its growth. Waste substances are stored in the allantois.

The chorion surrounds the entire contents of the egg, and is highly vascular, like the allantois. These two extra embryonic layers participate in respiration, being a vital component for the exchange of oxygen and carbon dioxide. In the yolk sac are the nutritive substances that the embryo will use.

Most amniotic eggs are surrounded by a fairly flexible mineralized shell or covering. This barrier strikes a balance between the protection of the embryo and the exchange of substances, since it acts as a semi-permeable barrier.

Classification

Living reptiles are represented by the following orders: Testudines (turtles), Squamata (lizards and snakes), Sphenodonta (New Zealand tuátaras), and Crocodilia (crocodiles and allies).

Additionally, groups of reptiles are distinguished by the number of openings in their skull. First, we will explore the classification of the different amniotic skulls, and then we will explore the taxonomic classification of the four groups of living reptiles.

Anapsids, synapsids and diapsids

A = Annasids, B = Synapsids, C = Diapsids. Skull_anapsida.svg: Preto (m) Skull_synapsida.png: Preto (m) Skull_diapsida.png: Preto (m) derivative work: Petter Bøckman

In the classification of reptiles - and of the rest of tetrapod vertebrates - it is usual to apply the terms anápsids, diapsids and synapsids. This terminology refers to the pattern of openings (fenestra) at the level of the temporal region in the skull of these animals.

Anapsids are organisms whose temporal region lacks any opening. This morphology is considered primitive and it is thought that the first amniotes that appeared in the course of evolution possessed this anatomical pattern. Within the current species, turtles have an anapsid skull.

However, the case of turtles is very particular. According to current molecular evidence, it has been concluded that these reptiles acquired the anapsid condition in a secondary way, since they come from an ancestor with diapsid temporal openings.

From this ancestral condition, two anatomical variants were derived: the diapsids and the synapsids. In the diapsid skull we find two temporal openings. The diapsid skull is present in current reptiles (including birds and with the exception of turtles).

The third pattern of temporary openings is synapsid, where there are only one pair of temporary openings. Mammals have this type of skull.

1. Order Testudines (Chelonia)

The order Testudines is made up of turtles. These organisms have changed very little throughout evolution, largely maintaining the characteristic morphology of the group.

The most prominent feature is the presence of a dorsal carapace and a ventral plastron. This shell is made up of two layers: an external one composed of keratin and an internal one of bony nature.

The inner layer is a union of bones, fused vertebrae, and other ossified dermal elements. As a unique characteristic among vertebrates, turtles are the only organisms that possess the limbs and girdles within the ribs.

The mandible of chelonians lacks dental structures. Instead, there are a kind of keratin dishes that allow the manipulation and grinding of the food.

From an ecological perspective, turtles have managed to conquer a diversity of niches. There are totally aquatic species (except when the eggs are laid) and totally terrestrial.

In terms of the senses, turtles do not have particularly good hearing. To counteract this disadvantage, they have a very sensitive taste and vision system. In fact, vision is in color and is comparable to the human photoreceptor system.

2. Order Squamata

This order is made up of lizards and snakes. It is very broad and diverse, including almost 95% of all living non-avian reptile species.

Snakes exhibit a number of unique and distinctive characteristics. The body has undergone significant elongation, leading to internal reorganization of the organs; the limbs have been lost and there are adaptations for the consumption of other large animals.

Most lizards have the ability to move their eyelids. In contrast, snakes often have a permanent, transparent coating over their organs of vision. The vision of some species is adapted to sunlight and they have color vision, while other nocturnal species do not.

The skull of the members of this class is called the kinetic skull, since it has mobile properties that allow them to consume and manipulate prey of considerable sizes. Although the skull of lizards is kinetic, in snakes the phenomenon is much more marked.

Formerly, the Squamata order consisted of three suborders: Sauria, Serpentes, and Amphisbaenia. Today, the last group is considered as modified members of Sauria.

Suborder Sauria

It includes the larks, an extremely diverse group of forms that inhabit from terrestrial and aquatic environments to trees, underground areas and even the air. Among the most popular representatives are geckos or geckos, iguanas, skinks, monitors and chameleons.

For most species of lizards, hearing does not represent a relevant sense. However, in a few species the songs are an important element in the search and choice of a mate.

The environments inhabited by this group of organisms are usually arid and dry. Their thick, glandless skin largely prevents water loss. Furthermore, reptile urine is almost solid and contains significant amounts of uric acid. These mechanisms allow them to avoid desiccation.

As reptiles lack thermal regulation and depend on the temperature of the environment, there are few species of reptiles in cold areas, since these conditions limit their growth.

Amphisbenids or blind shingles

Amphisbenids are a very particular group of lizards (Order Squamata), whose morphology resembles that of a snake or an earthworm. Its body is vermiform and the limbs have been lost.

The group's name describes its peculiar ability to move both forward and backward. That is, they can move in the direction of the head and tail with the same efficiency.

Their lifestyle is underground and they have various adaptations related to life underground: vermiform morphology to move around; the skull is hard and robust, allowing excavation; the eyes are atrophied and practically indistinguishable from the body (as they are covered by a layer of skin) and there is no external ear opening.

Suborder Serpentes

Snakes are reptiles that lack limbs and, in most groups, also lack the pectoral and pelvic girdles. Their body is made up of numerous vertebrae that allow them to move along the ground through a series of S-shaped wave movements.

As we mentioned, the skull of snakes is highly kinetic, which means that it can consume huge prey. This characteristic may be responsible for the remarkable success of snakes. Certain species of snakes - about 20% - are capable of injecting venom into their prey.

With regard to the senses, only a small group of snakes with arboreal habits possess remarkable vision. They do not have a tympanic membrane or external ear. For the detection of chemical stimuli, snakes are equipped with the Jacobson's organ, also known as the vomeronasal organ.

3. Order Sphenodonta

The order Sphenodonta is composed exclusively of two living species - the only surviving species of the lineage - belonging to the genus Sphenodon, endemic to New Zealand. They are commonly known as tuataras.

Tuataras have been a study model that has aroused the interest of evolutionary biologists. These species have changed very little in the course of evolution and have a primitive diapsid skull.

4. Order Crocodilia

Crocodiles, alligators, and alligators are robust, semi-aquatic reptiles. They are known primarily for being large predators. Their jaws are powerful and allow them to effectively capture their prey.

The body of the animal consists of an elongated head, body and tail, and short but strong limbs. The latter allow mobility on land, although activities in the land environment are not very common. Together with birds they form the clade Archosauria.

Digestive system

The digestive system of reptiles is made up of a series of organs and elements that orchestrate the passage of food and the extraction of nutrients.

Ingestion begins with the oral cavity, which is extensively modified in reptiles, particularly in kinetic skulls of snakes, and to a lesser extent in lizards. Only crocodiles have a secondary palate.

Followed by the oral cavity, we find the pharynx, then the esophagus and the stomach. They are followed by intestines that empty into a cloaca.

The gastrointestinal tract of most reptiles is short, a typical characteristic of a carnivorous diet. Herbivorous variants have longer intestines that allow them to digest plant matter. Some of them exhibit the behavior of swallowing stones to promote digestion.

Circulatory system

Heart of an iguana bisected through the ventricle. Hartmacl

Circulation in reptiles is made up of two circuits: one of them takes the blood and carries it to the lungs, while the second path orchestrates the passage of blood through the rest of the body.

With the exception of crocodiles, reptiles have a heart with an incomplete partition, which allows little contact between oxygenated and non-oxygenated blood.

In crocodiles the heart is more complex and reminiscent of mammals and birds, since it has two atria and two ventricles. This complete separation increases the efficiency of the process.

Nervous system

In general, the reptilian nervous system has all the basic structures that we find in amphibians, but larger - particularly the brain and cerebellum.

However, compared to birds and mammals, the brain is relatively smaller. For example, the brain of turtles is small - it never exceeds 1% of the total mass of the animal. However, the cerebellum does have a significant size in this group of reptiles.

Most of the sense organs are well developed, although there are several exceptions.

Respiratory system

Respiration in reptiles occurs through the lungs. The skin of these organisms is thick, so it does not play a relevant role in gas exchange, as occurs in amphibians with thin and moist skin.

Most of the members have paired lungs, with the exception of snakes and amphisbenids.

In turtles, there is an additional complication of the respiratory process: the organs are confined in the shell of the animal. To solve this problem, turtles use the abdominal and pectoral muscles as a diaphragm.

The species of turtles that inhabit bodies of water manage to meet their oxygen demands thanks to the pumping they perform in their oral cavity - which is highly vascularized - or in the cloaca. This phenomenon allows them to remain underwater for significant periods of time.

Crocodiles possess an important innovation: the secondary palate. It is made up of a series of skin folds that separates the mouth from the nasal passages. That is, they can breathe while the animal's mouth is open.

Excretory system

All reptiles have paired, lobed kidneys of approximately the same size. In snakes, the kidneys are located in the caudal region of the coelomic cavity.

As in the rest of vertebrates, the functional unit of the kidney is the nephrons. In reptiles, each kidney is made up of a few thousand nephrons, which is small compared to the nearly two million nephrons in the human kidney.

The reduction in the size of the glomerulus is considered as an adaptation to avoid the loss of water. In addition, these are very little vascularized.

The waste product of reptiles is uric acid; for this reason, reptiles are known as uricotelians. This appears to be an adaptation to reduce water loss.

Uric acid is a water insoluble substance. Thus, the product precipitates and the excreted substance consists of a white, semisolid mass.

In contrast, species that live in aquatic environments often excrete ammonia, a substance with high levels of toxicity. As the excretion product is toxic, it needs to be diluted with a greater quantity of water than is necessary for the excretion of uric acid.

Reproduction

Reptiles have internal fertilization and the sexes are separated. Males have two testes and females two ovaries. Males perform copulation with a special organ called the hemipenis. It is even in almost all reptiles, with the exception of turtles and crocodiles.

Turtles are internally fertilized and oviparous. They invest energy in creating a nest for their young, and once the eggs are deposited and covered by the substrate, the parents disregard their future offspring.

A large number of snakes are oviparous and a small group is ovoviviparous. Viviparity has evolved within reptiles nearly a hundred times by separate evolutionary pathways. This phenomenon generally occurs due to an increase in the time that the eggs are retained within the oviduct.

Animals with this reproductive modality are usually associated with cold climates, and it can be an adaptive characteristic that allows reptiles to maintain an optimal temperature for their offspring. Inside the mother's body, the temperature is optimal for the development of the young.

Determination of sex

One of the peculiarities of reptiles in terms of their reproduction is the influence of temperature in determining the sex of animals. This phenomenon has been reported in turtles, lizards, and crocodiles.

In turtles, low temperatures are associated with the production of males. Similarly, high temperatures translate into female production. As temperature is the determining factor, these species lack sex chromosomes.

Are reptiles a true group?

Historically, terrestrial vertebrate animals are classified into three well-defined classes: reptiles, birds, and mammals. Reptiles are cold-blooded organisms with scales; the birds are warm-blooded and have feathers; while mammals are warm-blooded, they are heavy and milk-producing.

Cladist perspective

With this simple description, it seems easy to group organic beings into categories corresponding to classes.

For the cladist school, the reptiles represent a paraphyletic group, that is, they do not form a clade.

From the perspective of cladism, valid groupings include the most recent common ancestor and all his descendants. In the case of reptiles, not all descendants are considered, since this grouping excludes birds.

Birds evolved from a group of dinosaurs called dromaeosaurs. Therefore, if you want to consider a monophyletic group - the most recent common ancestor and all its descendants - you must take into account the birds within the reptiles.

Some biologists who want to refer to these classes correctly use the term avian and non-avian reptiles.

Shared derived characters

In a very superficial way, groups of reptiles and birds may seem very different from each other. However, both share a series of shared derived characters or synapomorphies, as they are known in cladist terminology.

Both “traditional” birds and reptiles have shared anatomical characteristics at the skull and ankle levels. In addition, both have beta keratin in their structures. These characters unify both groups in a clade (monophyletic group).

Birds and crocodiles are sister groups. A crocodile is more related to a hummingbird than to a lizard or a snake. In other words, crocodiles and birds share a more recent common ancestor, and are considered a monophyletic group (remember that monophyletic groups can be nested).

References

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