Results 1 to 5 of 5

The Evolution of Snakes

  1. #1
    Join Date
    Mar 2013
    Location
    Nottingham, UK
    Posts
    50

    The Evolution of Snakes


    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 hypotheses 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.




    Last edited by Cat001; 11-15-2017 at 04:49 PM.

  2. #2
    Join Date
    Mar 2013
    Location
    Nottingham, UK
    Posts
    50
    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.


  3. #3
    Join Date
    Mar 2013
    Location
    Nottingham, UK
    Posts
    50
    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.

  4. #4
    Join Date
    Mar 2014
    Location
    New Hampshire
    Posts
    6,024
    if your theory on evolution is correct what happened to the middle ones, from what they start out as to what they finish off as? take the giraffe for instance, if it started out with a small neck and over time the neck got longer, what happened to the ones with a mid size neck, the'd be to tall to bend successfully to eat grass and not tall enough to eat on the trees higher up.
    he is your friend and protector, he will love you unconditionally, you owe it to him to be worthy of such devotion

  5. #5
    Join Date
    Mar 2013
    Location
    Nottingham, UK
    Posts
    50
    Evolution is a scientific "Theory", meaning that it is supported by facts and has been repeatedly observed and validated. The colloquial word "theory" is not used in the same context as the scientific meaning, but rather implies a hypothesis. Other examples of scientific theories include; Newtons Theory of Gravitation, Germ Theory of Disease and Theory of Heliocentrism. The origin, early evolution, and relationships of tetrapods form the focus for the interaction of several disciplines. Paleontology (study of fossils) taphonomy (how creatures died and became fossilized) and paleobiogeogreaphy (where creatures lived and how they were distributed in time and space) as well as modern zoology, anatomy, physiology, and developmental and molecular genetics, all contribute various aspects to the understanding of past life. Evolution is not a single persons idea but the independent conclusion of many different disciplines.

    How the giraffe evolved, for example, can be explained by environmental pressures within the specific niche the animal is in. Those which reached the leaves further up on the tree had a survival advantage (access to more food) over those who could not, meaning they were more likely to pass on their advantageous genes to their offspring. A slightly taller neck was the advantage at the time, animals do not aim to reach an end result so you can't start with the conclusion, but rather start at the beginning and see the population changes made which were appropriate for the environment they were living in at the time. Given enough time and a constant pressure to further adapt, those genes would dominate within the populations gene pool and given further time (hundreds of thousand of years) would result in a population of animals that looked noticeably different from their ancestors. Environments always change, whether its from savannahs to deserts, or rainforests into marshlands, the animals populations are required to adapt or face extinction. The fact that life is still on this planet is evidence itself of the sheer adaptability of life, given that the earth is nothing like it was a thousand years ago, let alone a million years ago or older. More than 99% of all the species that have lived on the planet are now extinct, this is because extinction, speciation and evolution are all important processes for sustainable Biodiversity.

    Animals are never 'finished' but are constantly adapting to their ever changing environments. One recent and fascinating example of rapid adaptation to an environmental change is of the gecko species Gymnodactylus amarali, which has recently undergone rapid changes in the space of 15 years. The disappearance of one of their main competitors has resulted in the availability of a new and previously inaccessible food source. The Geckos have developed enlarged heads to be better cope with eating bigger termites but the bodies have not changed size (likely a trade-off as a bigger body would mean greater energy requirements). The animals will continue to adapt to better 'fit' their environment.

    https://newatlas.com/geckos-evolving...ctivity/50721/

    If you want to read further on the subject "Gaining Ground: The Origin and Evolution of Tetrapods" is a good book.
    Last edited by Cat001; 11-15-2017 at 03:22 PM.

Please reply to this thread with any new information or opinions.

Similar Threads

  1. Most Venomous Snakes
    By Alpha1 in forum Reptiles and Amphibians
    Replies: 0
    Last Post: 04-17-2015, 05:17 PM
  2. Snakes?
    By MyImmortalWolf in forum Reptiles and Amphibians
    Replies: 13
    Last Post: 11-22-2014, 08:31 PM
  3. Snakes Everywhere
    By Reptile person in forum Humor
    Replies: 0
    Last Post: 04-29-2014, 07:11 AM
  4. Understanding How Snakes FLY
    By Alpha1 in forum Reptiles and Amphibians
    Replies: 2
    Last Post: 03-13-2014, 03:43 AM
  5. Facts About Snakes
    By Alpha1 in forum Reptiles and Amphibians
    Replies: 1
    Last Post: 08-28-2013, 12:24 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Family & Health Forums: Mom Forum - Senior Forums - Health Forum - Low Carb Forum