• Tag Archives lizards
  • Modern-day paleo myths: Dinosaurs as lizards

    Paleomyths

    In this day and age there are no shortage of books, websites, and videos dedicated to debunking classic paleo myths. The majority of this mythbusting focuses on myths about dinosaurs. As the poster children for paleontology, this isn’t that surprising. With so many takes on this subject it comes as no surprise that all of the classic dinosaur myths have long since been debunked, such as dinosaurs as low-energy tail draggers, walking around like Godzilla, being evolutionary failures, inferiority to mammals, being pee brained monsters, etc.

    However, as quickly as these classic dinosaur myths have been eradicated, new ones have come and taken their place. These myths/misconceptions are routinely cited today without any question despite being just as erroneous as the myths that preceded them.

    This is the start of a new series I want to cover on the site: dispelling modern myths in vertebrate paleontology. Given the bent of my website, these myths/misconceptions will largely stay focused on reptile-related animals, though I am open to taking the occasional foray into other animal groups if the myths are egregious enough (which is to say that suggestions are welcomed).

    The seminal installment for this series is one that I see mentioned time and again:

    “Dinosaurs were once thought of as big lizards.”

    Continue reading  Post ID 1592


  • So, why go bipedal?

    As human beings this might come off as somewhat of a “duh” question.

    “To free our hands up, of course.”

    Ah, but like most things in life, the common sense answer is not the right one.

    Consider all the bipedal animals alive today. We have humans, obviously; kangaroos, birds, a few lemurs, and a whole swath of lizards. Of these, the “free up the forelimbs” argument really only holds true for humans, lemurs and birds. What of kangaroos and all those lizards? Exactly what does one benefit from when going bipedal?

    Do you go faster, run longer, or gain a better vantage point?

     

    C.kingii during a typical foraging run.

    All these questions were asked, and somewhat answered in a recent paper by Clemente et al.

    Clemente, C.J., Withers, P.C., Thompson, G., Lloyd, D. 2008. Why Go Bipedal? Locomotion and Morphology in Australian Agamid Lizards.J. Exp. Bio. 211: 2058-2065

    Abstract:

    Bipedal locomotion by lizards has previously been considered to provide a locomotory advantage. We examined this premise for a group of quadrupedal Australian agamid lizards, which vary in the extent to which they will become bipedal. The percentage of strides that each species ran bipedally, recorded using high speed video cameras, was positively related to body size and the proximity of the body centre of mass to the hip, and negatively related to running endurance. Speed was not higher for bipedal strides, compared with quadrupedal strides, in any of the four species, but acceleration during bipedal strides was significantly higher in three of four species. Furthermore, a distinct threshold between quadrupedal and bipedal strides, was more evident for acceleration than speed, with a threshold in acceleration above which strides became bipedal. We calculated these thresholds using probit analysis, and compared these to the predicted threshold based on the model of Aerts et al. Although there was a general agreement in order, the acceleration thresholds for lizards were often lower than that predicted by the model. We suggest that bipedalism, in Australian agamid lizards, may have evolved as a simple consequence of acceleration, and does not confer any locomotory advantage for increasing speed or endurance. However, both behavioural and threshold data suggest that some lizards actively attempt to run bipedally, implying some unknown advantage to bipedal locomotion.

    The conclusions reached, were interesting and quite unexpected.

    I’d say the most surprising part would have to be the discovery that both endurance and speed were found to be inconsequential. Those were the two forces that I figured would have driven the push towards bipedalism. Apparently this is not the case.

    In fact, the only correlate that the authors found was that a switch to bipedalism resulted in an increase in acceleration. Short of that, the authors viewed bipedalism as more of a side effect of speedy locomotion, rather than anything else.

    As one authour put it: “The lizards were pulling a wheelie.”

    There are some gripes (niggles if you will) with the paper. For one, the authors assert that a switch to bipedalism allowed birds to incorporate their forelimbs into wing design. While being bipedal certainly allowed for this, it could not have been the cause. Birds descended from dinosaurs, and very, very very few dinosaurs had wings. Theropods were sporting freed forelimbs for some 80 million years, or so (probably longer given the proposed ancestors of dinosaurs). Wings were not the cause, simply a benefit. Something else had to have spurred the evolution of bipedality.

    Side note: What the heck were theropods doing with those forelimbs anyway? Most paleo artists tend to draw theropods with their arms tucked to the side, yet work by Carpenter (2002) has shown that there was some considerable range of motion in theropod forelimbs. They weren’t brachiating from tree to tree, or anything, but they could certainly do a heckuvalot more than just tuck their arms to the side. Even T.rex with its embarrassingly short forearms, had a surprisingly large range of motion to them. So what’s up paleo art guys? Let’s see some theropods putting their arms to use.

    The largest SNAFU in the paper comes from the cladogram that the authors chose. They chose to go with the broken molecular tree used by Townsend et al, which asserts that Iguanians are actually Scleroglossan lizards. This might sit all fine and well when looking at molecules, but it utterly falls apart upon a morphological assessment. In order for Iguanians to fit in the Scleroglossan family tree, they had to have undergone a tonne of morphological reversals, including the re-softening of the tongue and the re-evolution of both postorbital bars (surprisingly, the latter is actually not out of the realm of impossibility as tuataras have apparently done just that).

    Due to this tree choice, the authors erroneously concluded that bipedalism evolved only once in the lacertilian tree and was lost a multitude of times, with a putative re-acquirement in varanids.

    Another minor complaint comes from the very slight use of Chlamydosaurus kingii; the only lizard known to be a “true” biped (see: Shine & Lambeck 1989). Given that the authors were trying to spot differences between bipeds and quadrupeds, I can understand the use of lizards like Ctenophorus, with their greater spectrum of gaits. However, in doing so they should have qualified their conclusions better in regards to how lizards obtain a bipedal stance. In Chlamydosaurus, bipedal trotting is attained from a standing start. Quadrupedal stance is only seen when stopping to eat. Furthermore, foraging runs and escape runs use two different gaits, with the latter gait more akin to that of other facultatively bipedal lizards. Judging from the stats given in the paper, it seems apparent that the C.kingii used in this study were mostly running away.

    The final complaint comes from the supplementary movies given.

    Frankly the movies are just too short. I have to go super slow-mo just to see anything. So that’s a bummer.

    Overall the paper is rather good. The authors discovered that Lophognathus gilberti runs bipedally 85% of the time. This suggests that C.kingii is not the only truly bipedal lizard out there.

    The authors also observed that, despite any advantage in speed, or endurance, some lizards intentionally push their center of mass towards their hips early on in the running phase in order to more quickly obtain a bipedal gait. The reasons behind this are unclear, but do suggest that bipedalism confers some advantage not discovered during this experiment.

    One advantage alluded to, but never really elaborated on, was the faster acceleration noted in bipeds. Though maximum speed was no different than in a quadruped, this speed was obtained faster. Ecologically I could see this being very advantageous. When one is trying to avoid a predator, maximum top speed is probably less important than reaching that top speed as fast as possible.

    If one lizard has a top speed of 12km/hr and another has a top speed of 8km/hr, and the goal (burrow) is only 50 meters away, then that extra speed isn’t going to mean much. Especially if the slower lizard is able to hit its top speed faster.

    Apparently lizards “pull a wheelie” because in their ecosystem it pays to be a drag racer, rather than a Daytona 500 car.

    ~Jura

    References

    Carpenter, K. 2002. Forelimb Biomechanics of Nonavian Theropod Dinosaurs in Predation. Concepts of Functional Engineering and Constructional Morphology. Vol. 82(1): 59-76.

    Shine, R., & Lambeck, R. 1989. Ecology of Frillneck Lizards, Chlamydosaurus kingii (Agamidae), in Tropical Australia. Aust. Wildl. res. Vol. 16: 491-500.

    Townsend T, Larson A, Louis E, Macey JR. 2004. Molecular Phylogenetics of Squamata: The Position of Snakes, Amphisbaenians, and Dibamids, and the Root of the Squamate Tree. Syst Biol. Vol. 53(5):735-57.


  • Lizards prove evolution can happen rapidly.

    I’m a little behind on this now 3 day old story. It’s been hard to get back into the proverbial swing of things. I believe part of it has to do with the large dearth of nothing that occurred during my “vacation.” The other part probably has to do with being bloody busy. >:)

    Here’s the story.

    And here’s a brief excerpt:

    In 1971, scientists transplanted five adult pairs of the reptiles from their original island home in Pod Kopiste to the tiny neighboring island of Pod Mrcaru, both in the south Adriatic Sea.

    After scientists transplanted the reptiles, the Croatian War of Independence erupted, ending in the mid-1990s. The researchers couldn’t get back to island because of the war, Irschick said.

    The transplanted lizards adapted to their new environment in ways that expedited their evolution physically, Irschick explained.

    The big and most interesting part of this story is how these amazing little saurians evolved new behaviours, a majour change in diet and they evolved a new physical characteristic (cecal valves in the intestine).

    All of this happened in only 36 years!


    Italian wall lizard

    Podarcis sicula; a representative of the species used in the study. Also, a bit of a show-off.

    That’s not just amazing, it’s bloody phenomenal!

    Prior to this study, the only thing that even came close (in tetrapods) was an “earlier” study by Jonathan Losos on Caribbean Anolis. In one study, Losos found rapid evolution of longer hindlimbs in introduced species of Anolis in as little as 14 years. The results were interesting, but were more a case of an amazing case of Natural Selection changing the frequency of a naturally occurring variation. This latest study is a little different. The kind of changes noticed here are normally the ones talked about in insects, or bacteria (i.e. large phenotypic changes).

    It has been argued by some that this finding might be viewed as more fuel for the nutty creationist movement. Proving that evolution can occur quickly, means that creationists can argue better for a Young Earth. However, since the core tenet of creationism is that life does not evolve, this would do quite the opposite for them.

    What this does bolster evidence for is Gould and Eldredge’s theory of Punctuated Equilibria.

    The theory states that most of the time, populations of organisms remain fixed and stable, with very little evolution occurring. Then, when a portion of the population gets segregated and/or are forced to adapt to a different environment, evolution kicks into high gear. Change happens rapidly, and the new population becomes a new species.

    That was a bit simplified, but you get the point.

    Gould and Eldredge used fossil trilobites to support their case. Studies on plants and insects have also shown that this type of speedy evolution can happen. Now we have proof in a relatively large vertebrate as well.

    I think these kinds of observations are good for the biological and paleontological sciences. I think that there is a tendency for paleontologists to “ride the brake” with evolution. Always insisting on measuring evolutionary change in millions and tens of millions of years. I don’t have a problem with this type of thinking when one is looking at changes in whole families and genera, but it seems very unlikely that many of the species seen in the fossil record were evolving so slowly from species to species (e.g. Tyrannosaurs, or mosasaurs). Showing that speedy evolution does happen, helps to better support a more “punctuated” view of evolution. One in which whole genera can arise in as little as a few thousand (or hundred thousand) years.

    Geologically speaking; that’s pretty damned fast.

    ~Jura



    Herrel A, Huyghe K, Vanhooydonck B, Backeljau T, Breugelmans K, Grbac I, Van Damme R, Irschick DJ. (2008) Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource. Proc Natl Acad Sci U S A. 105(12):4792-4795.


  • Site update

    So I’ve been working on dragging the rest of my site into the 21st century. While most of this work has been just transcribing of old code to new code, I didn’t want to just wind up rehashing all of my old info.

    I mean there are only so many ways that one can remix the same stuff.

    As the meat of the site was meant to contain species information, I thought it would be nice if my first revamped species page, was a new one.

    So I went and made one.

    I had been meaning to write about Chlamydosaurus kingii (i.e. the frilled lizard) for a long time now. They have been a lacertilian favourite of mine. It’s rather sad that there is not much written about these guys, save for the occasional herpetocultural article, or the rather glib Wikipedia article.

    Now, while I could have just gone and updated Wikipedia with this info, I wanted to have it on my site first. I mean, if I’m going to be the person to flesh out this species, then it’s only fair that I give my site first dibs.

    Much of the article was culled from data in Shine & Lambeck, 1989. This was, and still is the most comprehensive review of this species. It is interesting to note that this reference is used in the Wikipedia article, but not for any of the more interesting info.

    For example, as can be read in the article, frilled lizards are unique in being the only extant reptile that is an obligate biped. You’d think that would get more attention. Hmmph, it must be that distracting frill. 🙂

    There you go. A new update with actual new content.

    This might just become a pattern. 🙂

    ~Jura


  • Alligators can shift their lungs and lizard ecology determines movement.

    There were two new papers released today in the Journal of Experimental Biology.

    The first one is the biggest, as it received a news story.


    Uriona, T.J., and Farmer, C.G. 2008. Recruitment of the diaphragmaticus, ischiopubis and other respiratory muscles to control pitch and roll in the American alligator (Alligator mississippiensis). J. Exp. Biol. Vol. 211: 1141-1147 doi: 10.1242/jeb.015339

    Abstract

    We used electromyography on juvenile American alligators to test the hypothesis that the following muscles, which are known to play a role in respiration, are recruited for aquatic locomotion: M. diaphragmaticus, M. ischiopubis, M. rectus abdominis, M. intercostalis internus, and the M. transversus abdominis. We found no activity with locomotion in the transversus. The diaphragmaticus, ischiopubis, rectus abdominis and internal intercostals were active when the animals executed a head-down dive from a horizontal posture. Weights attached to the base of the tail resulted in greater electrical activity of diaphragmaticus, ischiopubis and rectus muscles than when weights were attached to the head, supporting a role of this musculature in locomotion. The diaphragmaticus and rectus abdominis were active unilaterally with rolling maneuvers. Although the function of these muscles in locomotion has previously been unrecognized, these data raise the possibility that the locomotor function arose when Crocodylomorpha assumed a semi-aquatic existence and that the musculoskeletal complex was secondarily recruited to supplement ventilation.

    Scientists at the University of Utah have discovered the unique internal subtleties that allow crocodylians to sink, rise, pitch and roll; all without disturbing the water (much). It turns out that the main muscles used for breathing, are also used to actually shift the lungs within the body!

    That’s just crazy awesome. Uriona & Farmer’s work raises the question of how prevalent this ability is in other semi-aquatic animals (e.g. seals, terrapins, manatees). By shifting the lungs further back in the body, crocodylians are able to change their local density. This allows the front, or back of the animal to rise and sink separately from the rest of the body. So too does moving the lungs from side to side allow for rolling in the water. All of this can occur without the need to move any external body parts. This means no extra turbulence gets created in the water, thus allowing crocodylians to better sneak up on their fishy, or fleshy prey.




    Baby crocodiles exhibiting their unique pulmonary powers.

    If anything, it sure speaks to why crocodyliformes have held dominion over the semi-aquatic niche for over 200 million years. Uriona and Farmer do suggest that the ability of these respiratory muscles to do this might not be an exaptation. Rather, this might have been the initial impetus behind the evolution of these muscles. Only later would they have been exapted to help with breathing on land. Though the authours provide some good parsimonious reasons for why this may be (basically it would take less evolutionary steps to accomplish than the other way around), it doesn’t really jive with the fossil evidence. Part of the reason why the crocodylian diaphragm works, is because the pubis (the forepart of the hip bone in most animals, and the part that juts out so prominently in theropod dinosaurs), is mobile. This mobility occurred early on in crocodyliforme evolution, with the crocodylomorph Protosuchus having a pubis that was almost mobile. The problem arises when one looks at this early crocodylomorph. Protosuchus was obviously terrestrial. If Protosuchus was evolving a mobile pubis already, then it was doubtful that it was being used to allow lung shifting in the body (an ability that is helpful when underwater, but pretty pointless on land). Furthermore, Crocodylia proper is the umpteenth time that crocodyliformes have returned to a semi-aquatic existence. It is doubtful that all the numerous land outings that occurred during crocodyliforme evolution, would have retained the ability to move the lungs to and fro. It seems far more likely that this was an ability that evolved in Crocodylia, or somewhere close by on the evolutionary tree, in some taxa that was still semi-aquatic.


    Protosuchus

    Protosuchus richardsoni. An example of an early crocodylomorph.

    Of course it is also possible that crocodyliforme phylogeny is just all f-ed up. With the amount of convergence rampant in that lot, this remains a distinct possibility.

    Either way this is a cool discovery, and one worthy of adding to the crocodylian pages.

    The second paper also comes from the Journal of Experimental Biology. This one involves lizards.


    McElroy, E.J., Hickey, K.L., Reilly, S.M. 2008. The correlated evolution of biomechanics, gait and foraging mode in lizards. J. Exp. Biol. Vol. 211: 1029-1040. doi: 10.1242/jeb.015503

    Abstract

    Foraging mode has molded the evolution of many aspects of lizard biology. From a basic sit-and-wait sprinting feeding strategy, several lizard groups have evolved a wide foraging strategy, slowly moving through the environment using their highly developed chemosensory systems to locate prey. We studied locomotor performance, whole-body mechanics and gaits in a phylogenetic array of lizards that use sit-and-wait and wide-foraging strategies to contrast the functional differences associated with the need for speed vs slow continuous movement during foraging. Using multivariate and phylogenetic comparative analyses we tested for patterns of covariation in gaits and locomotor mechanics in relation to foraging mode. Sit-and-wait species used only fast speeds and trotting gaits coupled with running (bouncing) mechanics. Different wide-foraging species independently evolved slower locomotion with walking (vaulting) mechanics coupled with several different walking gaits, some of which have evolved several times. Most wide foragers retain the running mechanics with trotting gaits observed in sit-and-wait lizards, but some wide foragers have evolved very slow (high duty factor) running mechanics. In addition, three evolutionary reversals back to sit-and-wait foraging are coupled with the loss of walking mechanics. These findings provide strong evidence that foraging mode drives the evolution of biomechanics and gaits in lizards and that there are several ways to evolve slower locomotion. In addition, the different gaits used to walk slowly appear to match the ecological and behavioral challenges of the species that use them. Trotting appears to be a functionally stable strategy in lizards not necessarily related to whole-body mechanics or speed.

    I haven’t had a chance to read much more than what was written already. I do take a bit of offense to the authours referring to scleroglossan foraging technique as “slow,” but what are you going to do?

    I do find it interesting that lizards seem to have lost the ability to “walk” numerous times. That almost seems bizarre. The study points out that ecology produces heavy pressures on lizards in terms of their locomotion type. This is extremely pertinent given how often one hears the old (and wrong!) adage about “reptiles” being incapable of intense aerobic activity.

    According to the above study (among others), it all depends on the animals being tested.

    There we go. Two really cool papers on reptiles, being released in one day.

    ~Jura

    Yes, I know. I used jive. I’m sorry.