The Biology and Diversity of Extant Reptiles.
The Biology and Diversity of Extant Reptiles.
The word reptile itself does not describe a monophyletic group of vertebrates, like the mammals or birds. It is used to classify a polyphyletic group of animals that are a subset of the larger group of Amniotes. The term is best described through a cladogram:
Dr. Paul M. Barrett 2001
The reptiles first appeared in the early Carboniferous, having evolved from labyrinthodont amphibians. They had several distinct advantages over the amphibians that allowed them to flourish during the cold, dry Permian period. These lead them to be the most successful group of vertebrates and this period was referred to as the "age of reptiles." They exploited all terrestrial niches, a few marine and even one group took to the air. No other group of vertebrates had ever displayed such diversity and mammals today are their only match. They were the first group of tetrapods to successfully overcome the major problems of terrestrial life, namely desiccation, thermoregulation and respiration.
Adaptations to withstand desiccation
Unlike amphibians reptiles are able to tolerate dryer conditions and avoid desiccation. This allowed them to exploit more niches and therefore achieve greater radiation. They did this in a number of ways:
Skin. Reptilian skin is impermeable to water due to its unique structure. It has a thick dermis and a thick epidermis. The relative thickness of these layers helps somewhat in water conservation but it is the components of the epidermis that make the skin waterproof. See diagram:
The ?-keratin makes up the lower layer and is softer and more pliable then the darker harder ß-keratin layer. This gives flexibility and strength as well as providing a waterproof cuticle. Because of this reptiles were able withstand osmotic water loss and re-invade the sea. The Chelonia (turtles) have become the most prolific marine reptile and they also have an interesting skin that is worth mentioning. The have a shell covering their body, made out of a horny layer attached to a denser bony layer. It comes in two pieces, the carapace, for dorsal protection and the plastrom for ventral protection.
2 Excretion. Reptiles have modified their excretory products to conserve water. Because they do not have a complex enough kidney to produce urea they create uric acid, a large molecule that has a very low water content. Although the water content is lower than urea the trade off is that the molecule is expensive to make. Reptiles also reduce the water content of their faeces through active removal of NaCl. This creates an osmotic gradient to draw water out and desiccate the excrement. The excess NaCl is then excreted through the nose.
3 Creation of the cloaca. This is a cavity into which the urinary, alimentary and genital ducts open. It has three sections, the coprodaeum where faeces collect and enter, the urodeum where the ureter empties out and water is removed and finally the proctodaeum, the collection point for uric acid and faeces. This centralisation of all the excretory products allows for greater water retention as well as providing for the next point.
4 Internal Fertilisation. Since the air would dry out gametes produced by the reptiles having a site for internal fertilisation prevents this and opens up ...
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3 Creation of the cloaca. This is a cavity into which the urinary, alimentary and genital ducts open. It has three sections, the coprodaeum where faeces collect and enter, the urodeum where the ureter empties out and water is removed and finally the proctodaeum, the collection point for uric acid and faeces. This centralisation of all the excretory products allows for greater water retention as well as providing for the next point.
4 Internal Fertilisation. Since the air would dry out gametes produced by the reptiles having a site for internal fertilisation prevents this and opens up the possibility for different types of carrying the young, e.g. oviparity to viviparity.
5 Invention of the amniote egg. Arguably the most important of all of the adaptations, the amniote egg has released the reptiles from being bound to water for reproduction as now their eggs can be laid on land without fear of desiccation. I will not go into the structure of the egg in great detail as I do not feel it relevant. However I will say that the cleidoic egg (shelled egg) prevents water loss but still allows for gaseous exchange this combined with extraembryonic membranes make the reptiles much better suited for a terrestrial life. Some marine reptiles have managed to overcome this tie to land be becoming ovoviviparous or even viviparous, though most (e.g. leatherback turtles) have to return to land in order to lay eggs.
Adaptations for thermoregulation.
Thermoregulation is important as it allows for a greater tolerance to environmental conditions and therefore greater niche differentiation. Reptiles have a partially separated heart allowing for incomplete double circulation. Oxygenated and Deoxygenated blood are less mixed, making for a more effective and higher metabolism to be attained. However this is still not effective enough to become endothermic like the mammals or birds. Reptiles are termed ectothermic as although they are unable to maintain a constant internal body temperature through metabolic processes they instead carefully control the amount of heat entering their body. They can do this in 5 ways.
Behaviourally by choosing a suitable place in the environment.
2 Orientate themselves either toward or away from the sun during basking to regulate the amount of heat entering the body.
3 Physiologically. They can cover themselves in urine and cool themselves by having it evaporate. Crocodiles open their mouths to dissipate heat.
4 Change colour, allowing them to reflect different wavelengths of light and therefore heat.
5 Vary their heart rate greatly in order to prevent cooling down.
Adaptations for Respiration.
Reptiles have adapted to make more use of their lungs as the main organ for gaseous exchange. They use their ribs and costal muscles to inflate their lung by swinging the rib cage in and out. This means that the lung can be large and independent of the buccal cavity, there is no mixing of the stale and the new air, allowing the buccal cavity to develop separately. Overall a higher metabolic rate is now possible.
Diversity of the reptiles
The reptile group can be firstly divided into two main sections, the parareptilia (anapsids) and the diapsids. What sets these two groups apart is that the diapsids have two pairs of temporal fenestrae (holes) one upper and one lower. These allow for the attachment of muscles for the jaw although some reptiles have lost one or even both through their evolution. Parareptiles (anapsids) have no such fenestrae.
Parareptiles
Chelonia (Turtles and Tortoises)
This is the only group alive today and comprises of about 250 species. They have a long evolutionary history, the earliest known turtle being Proganochelys, arising in the Late Triassic of Europe and southern Africa. They form two lineages based on the way they retract their head into their shell, the Cryptodira (80% of species) bend their neck in a straight line into their shell whilst the Pleurodira bend their neck sideways in the bilateral plane. They have exploited many niches from terrestrial and marine to freshwater. They are very immobile animals because of their large shells and the fact they have their pelvis girdles inside their ribs, a condition unique to Chelonia. Also their ribs are fused to their carapace, which prevents them from using costal movements to ventilate their lungs. Instead they respire through buccal pumping and complex gut shifting to force air in and out of their lung. They also rely more on cutaneous respiration, up to 15%. Some marine turtles (Rheodytes) can breathe a bit through their anus. They have no teeth, instead their jaws are lined with a horny beak that allows them a wide diet and is self sharpening. To coincide with their beak they have very strong jaws. The muscles extend over the braincase or palate in order to increase the moment and so mechanical advantage. They show no parental care for their offspring.
Diapsids
Of the extant groups of diapsids today the group is split into two. The Lepidosauria and Archosauria. This division occurred in the carboniferous period. Lepidosaurs are the Sphenodons and Squamates and are distinguished by their teeth being in a simple groove along the edge if the jaw and having lost the bar of bone from the lower temporal fenestra. Archosaurs are the Crocodylia and have teeth in individual sockets in the jaw and two additional openings in the skull, one in front of the eye, the antorbital fenestra, and one in the lower jaw, the mandibular fenestra.
Lepidosauria
Sphenodons
These are living dinosaurs, there is only one extant species found in New Zealand. They are nocturnal creatures and so very rarely seen . They have a very low metabolic rate, living up to 100 years. Their eggs take 1 year to develop and hatch.
Squamates
Lizards
Appearing around the middle Jurassic, lizards have been very successful in exploiting multiple niches, arboreal, burrowing, cracks in rocks and even marine. There are about 370 known species found mainly in the tropics and subtropics. Some are herbivorous but most are insectivorous or carnivorous. They have highly flexible (kinetic) skulls allowing more effective handling of prey items. They can shed their tails in times of stress, known as tail autonomy. They have a sprawling gait and modifications to their limb girdle make them more agile. It is a slow and expensive form of movement but it makes them very stable on the substrate, which is ideal. Finally they possess many reproductive modes ranging from oviparity to viviparity.
Snakes
These are the descendants of lizards, the current consensus being that they evolved from, or related to the monitor lizards (varanids). They are found in tropic and temperate habitats and are all ambush predators. Their skulls have lost many individual bones resulting in an "open" skull, which is much more flexible than lizards. Their upper jaws can move independently of each other and their lower jaws can e separated and dislocated in order to handle larger prey. They show a complete loss of limbs pectoral girdles and pelvic girdles. Extant boas and pythons though do retain vestigial pelvic bones. They have moderately long tails and extremely elongated trunk sections, created by gene duplication. Some snakes have up to 200 vertebrae. Venom glands have evolved in some snake lineages. The venom contains digestive enzymes to help break down the food and powerful neurotoxins to help disable it.
Archosauria
Crocodylia (crocodiles alligators and gavials)
This is the only extant group of Archosaurs alive, the extinct one being the plesiosaurs. Today there are only about 20 species found in the tropics and they are not very morphologically diverse. However they were very diverse in the Mesozoic and tertiary faunas. As with the snakes they are all ambush predators apart from some larger crocodiles, which will approach prey. In contrast with the snakes they have very heavily built skulls with a closed antorbital fenestra, which strengthens the snout whilst it is being twisted during feeding. They have long snouts, conical teeth and large jaw muscles. Their eyes and nose are on the dorsal side of the skull perfectly adapted for an aquatic lifestyle. They can change gait as they walk. For slow movements or whilst in water they use a sprawling gait. To move quickly or over long distances they adopt a "high walk" a semisprawling gait. There are some examples of galloping and even some of them becoming bipedal. They have bony plates underneath their scales along their back called scutes. These are connected to the backbone via a series of tendons and help brace the crocodiles back whilst it is on land, something like a suspension bridge. Unlike the chelonia, crocodiles do show a great deal of parental care, they build and guard nests, help their young to hatch and acre for their young in nursery pools for several weeks after hatching.
Summary
As seen from this essay the diversity and subsequent biology of reptiles is very wide and encompasses many niches. However reptiles today are nowhere near as prolific as they were in the Mesozoic era where they ruled the earth and exploited hundreds, if not thousands, more niches. The real diversity and biology of reptiles was seen about 60 million years ago, before the mammals got a hold. But the only way to investigate that is through the fossil record and that will never give us all the information needed to draw a complete picture of reptile diversity.
Chris Holland Jesus College
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