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Post by cat001 on Jun 24, 2016 23:43:00 GMT
An image to demonstrate a few of the diverse species of modern snakes.Snakes are a highly diverse group of animals, consisting of over 3000 known species worldwide, occupying a wide variety of niches including fossorial, arboreal, terrestrial and aquatic environments, and living in climates ranging from arid deserts to moist rainforests and open oceans, yet we still know surprisingly little about their origins. Snakes are a highly specialised group of legless, elongated animals, which share their ancestry with the lizards, but unfortunately the fragile and delicate bones of the snake do not fossilize well, leaving scientists with very few, and largely incomplete fossils to draw conclusions from. The lizard ancestors of modern snakes lived some 120 million years ago during the Cretaceous era, but one mystery that doesn’t seem to have a clear answer is why did snakes lose their limbs? Snakes are thought to be close relatives of Monitor Lizards, Beaded Lizards and Gila Monsters due to their anatomical similarities and evolutionary history. A modern example of a burrowing lizard is the Earless Monitor Lizard of Borneo (Lanthanotus borneensis), which possesses reduced eyes and limbs, scaley bodies and no external ears. This modern lizard shares many similarities to snakes and may possibly resemble their early ancestors. Earless Monitor Lizard - (Lanthanotus borneensis)
There are two theories that persist as to how snakes lost their limbs and become so long; 1. They evolved from an aquatic reptile. 2. They evolved from burrowing lizards. Both scenarios would nurture the characteristics of snakes, requiring a streamline body to propel the animal through water or to navigate small tunnels underground. Attempts have been made to answer this mystery by examining the few fossils of proto-snakes that have been discovered. Proto-snakes evolved elongated backs and streamline bodies to help them propel themselves without the use of feet, however, the earliest snakes still possessed small back legs. Some notable fossils of stem snakes include Dinilysia, Najash rionegrina, and Coniophis precedens which were all unambiguously terrestrial, providing compelling evidence against the marine origin hypothesis for snakes. These species may have been able to swim but did not posses specialised adapted features for it. There have been several species of ancient marine snakes discovered, including Haasiophis terrasanctus, Eupodophis descounsis, and Pachyrhachis problematicus, however, phylogenetic analyses have indicated these hind-limbed snakes to be a branching of Alethinophidia rather than representing stem snakes. A representation of Najash rionegrina and Pachyrhachis problematicus
Tetrapodophis amplectus, an unusual snake fossil from the cretaceous period (120 million years ago) recently gained a lot of attention for possessing four distinct limbs instead of just two. These limbs featured comparatively long toes in relation to its small arms and legs. It is speculated that these limbs were repurposed by evolution for grasping of prey or for mating purposes, and were not likely used for walking. The body seemed to lack adaptations for life in a marine environment but was proportioned more like a burrowing animal, lending weight to the theory that snakes evolved on land. A representation of Tetrapodophis amplectus alongside fossil images
It would seem that the strongest possible scenario which may have driven the evolution of snakes is a switch to a more fossorial lifestyle. Readily available food sources underground would have been reached by these ancient cretaceous lizards through burrowing and searching through tunnels and narrow crevices. Living underground would also gain the animals protection from predators. The animals would have adapted to this underground lifestyle over time through the development of characteristics previously described with the Earless Monitor Lizard. Although it is often implied that the anatomical structure of the snake (e.g. reduced limbs and long bodies) is an adaptation for a burrowing lifestyle (fossoriality), other studies have demonstrated that the ancient snakes may have in fact been surface-dwelling and were likely nocturnal stealth hunters that foraged widely for soft-bodied prey in warm, mild, well-watered, and well-vegetated ecosystems. An interesting look at the development of venom in snakes and their early lizard ancestors.
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Post by cat001 on Jun 25, 2016 10:37:56 GMT
Additional: A brief look at the evolution of snake eyes... The structure of snake eyes is particularly interesting due to its unique composition compared to all other vertebrates, even differing from its closest living relatives, the lizards. The reason for the unique nature of snake eyes may be linked to the snakes evolutionary history. The lizard or lizard like ancestor of snakes would have likely been nocturnal or a burrowing (fossorial) species and lived in an environment which did not require colour vision, nor required especially good vision at all. In these ancestral fossorial species the size of the eyes are thought to have been reduced prompted by the low light conditions, and for the cones (which are only useful in daylight) to have regressed. Gordon Walls in the early 1940’s suggested that after this extended fossorial phase, more modern snakes would have moved back to above ground habitats. The photoreceptors that regressed during the fossorial phase were in effect rebuilt to accommodate the requirements of the species moving into a day-time lifestyle. The eye was effectively re-constructed making way for some interesting and unique features. For instance, lizards focus their eyes by changing the shape of the lens, as do other vertebrates, snakes instead move the lens forward and backward. Lizards typically have three eyelids; upper, lower and nictitating membrane. Snakes lack eyelids but the eye is instead covered by a spectacle, a transparent 'lens' that is fixed in position. In the retinas of diurnal lizards, distinct rods and cones are present, but in snakes, the “cones” seem to be modified rods that serve the same function as cones in colour perception.
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Post by scallywag on Jun 25, 2016 11:51:14 GMT
A very passionate subject of yours cat001 thanks for taking the time to post all those lovely photo's too
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Post by cat001 on Jun 25, 2016 17:37:34 GMT
I'd also like to touch on the topic of convergent evolution occurring between snakes and other animal groups (including skinks, the legless lizards, amphisbaenians and caecilians) in regards to the elongation and limb loss. This process has occurred in these other animal groups independently from the snakes but all share ancestry with lizards, excluding the caecilians, which are amphibians. Here's a unique example of a fore-limbed amphisbaenian (bipes), a group of reptile related to lizards. Most amphisbaenians are completely limbless. A Yellow-bellied Skink demonstrating reduced limbs and an elongated body A Legless Lizard, often mistaken for snakes but are a separate group of animals within the order Squamata An amphibious caecilian, note the shovel shaped face, reduced eyes and blunt tail, characteristics of a burrowing animal which may have also occurred in ancient snakes. It's a fascinating process which has given rise to some really diverse groups of animals. The rapid speciation of snakes I find particularly interesting. Legless caecilians, amphisbaenians and lizards number fewer than 350 species compared to the 3000+ species of snake.
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Post by tonib on Jun 26, 2016 17:47:21 GMT
This is a subject that you have obviously done a lot of research into cat001 & thanks for posting a such a clear summary. I found the eye information particularly interesting - I didn't realise that snakes do not have eyelids. I also wonder how many other creatures alter their focus by moving the lens forward & back instead of what we would descibe as "normal" shape changing. Intigured as to why rods should become adapted to function as cones whilst others creatures develped cones for colour seeing or is that just our understanding how colour is seen by other creatures (including humans)? It makes you wonder how scientists found out about cones for colour seeing etc.
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Post by cat001 on Jun 27, 2016 14:24:10 GMT
The structure of the snake eye appears to be fairly unique amongst the vertebrates, arisen from their specific evolutionary circumstance. They possess limited colour vision due to the historical loss of cone light receptors, while other reptiles have fairly advanced colour vision.
I'll recap on the eye as it's interesting stuff...
Vision is made possible through the existance of two types of photoreceptors in the eye:- rods and cones, which are distinguished from eachother by their shapes (from which their names are derived). These photoreceptors are found within the retina, located at the back of the eye and are responsible for converting light signals into electronic signals allowing us to visually perceive the world around us.
Rods are sensitive to the detection of shapes and movement and are sensitive to light so are responsible for vision in low light situations such as at night. However, rods have very low spatial acuity, meaning vision from these photoreceptors lacks sharpness. Furthermore, they do not allow for colour vision. Rods are in thier greatest numbers around the 'periphery' of the retina, meaning they're most active in our peripheral vision, the corner of our vision. Conversely, cones work best in brighter conditions as they are relatively insensitive to light. They have very high spatial acuity meaning vision is sharp and focused, colour vision is also made possible with the presence of cones. The central part of the eye (the fovea) is populated exclusively by cones, processing the information of what you are directly looking at as opposed to what's percived in the corner of your eye.
Retinas of all vertebrates posses rods but not all vertebrates have cones. Some amphibians possess colour vision, many reptiles do but comparatively few mammals see in colour with the exception of humans and some other higher primates. Although reptiles do possess colour vision, they do not have the same level of acuity as humans. The proportion of rods and cones varies considerably between different species. Animals active in bright conditions (during the day), possess both rods and cones. Photoreceptor cells have a pigment with high light-absorbing capabilities. The pigments are composed of a protein, opsin, which is combined with the light absorbing derivative of vitamin A. Human eyes have three types of cones sensitive to three different spectra, these are Red, Green and Blue receptors which allow us to perceive a number of colours within the visible light spectrum. Reptiles have an additional receptor, totalling four in number which allows herptiles (and their cousins, the birds) to see within the Ultraviolet spectrum. This allows the animal to see wavelengths beyond those that humans can perceive and may allow the animal to distinguish colours that appear identical to humans.
So although reptiles may not have the same visual acuity as humans (i.e. their vision isn't as sharp) their colour perception is more advanced than in mammals. The early fish (ancestors of the tetrapods - all land animals) evolved 4 types of cone cells, this state still persists in most amphibians, reptiles and birds, but the normal state for mammals is only possessing 2 types, therefore mammals typically have limited colour vision. Higher primates (including humans) evolved a third colour receptor, the aforementioned Red, Green and Blue channels. The ancestor of snakes, during its fossorial phase did not require great vision at all so these colour receptors were lost (evolved out) but as the snakes began to speciate and adopt a more diurnal lifestyle, eyesight became relevant to the species so the mechanism for colour vision and visual acuity was slowly evolved. This was achieved by repurposing the rods to replace the missing cones. The fact that this occurs in all snakes would imply that they all share a single common ancestor, in which this process first occurred.
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