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:
In another example of slow-cooked science, this paper was the culmination of over three years worth of work collecting data on tegus. For the study, the authors looked at adult black and white tegus (Salvatore merianae). Tegus are an interesting group of lizards. They are the largest members of the family Teiidae and are often referred to as the monitor lizards of the new world, due to their convergent lifestyles (highly predaceous, active foragers). Besides their varanid-like demeanor, tegus are also known for their enormous jowls, especially in the males. The jowls hold the pterygoideus muscles, the big jaw snappers, which have been shown to increase in size for males during the breeding season (Naretto et al. 2014). As reptiles, tegus have been assumed to follow the standard ectothermic lifestyle of requiring external sources of heat to warm their bodies and maintain stable body temperatures. Looking at the natural history of the animals, tegus appear to fit the mold pretty well. They have distinctive winter and summer activity levels. In the summer, the animals regularly maintained body temperatures of 32â€“35Â°C, and in the winter they let their body temperatures drop to the temperature of their burrows (15â€“20Â°C). This is all fine and good for a bradymetabolic, ectothermic lizard, but when the researchers tracked body temperatures over time they discovered something completely unexpected.
One of the quintessential depictions of prehistoric times is that of an ancient, often volcano ridden, landscape full of animals bearing large showy sails of skin stretched over their backs. Sailbacked animals are rather rare in our modern day and age, but back in the Mesozoic and Paleozoic there were sails a plenty.
By far the most popular sailbacked taxa of all time would be the pelycosaurs in the genus Dimetrodon. These were some of the largest predators of the Permian (up to 4.6 meters [15 feet] long in the largest species). Dimetrodon lived alongside other sailbacked pelycosaurs including the genus Edaphosaurus. These were large herbivores (~3.5 m [11.5 ft] in length) that evolved their sails independently from Dimetrodon. The Permian saw many species of sphenacodontids and edaphosaurids, many of which sported these showy sails (Fig. 1. [1â€“8]).
However sails were hardly a pelycosaur novelty. The contemporaneous temnospondyl PlatyhystrixÂ rugosus (Fig. 1 ) also adorned a showy sail.
Fast forward 47 million years into the Triassic and we find the rauisuchians Arizonasaurus babbitti,Lotosaurus adentus, Xilousuchus sapingensis, and Ctenosauriscuskoeneni ,Â all bearing showing sails on their backs (Fig. 1 [10â€“13]). Much like in the Permian, many of these taxa were contemporaneous and, while related, many likely evolved their sails separately from one another.
There are currently no fossils of sailbacked tetrapods in the Jurassic (as far as I know. Feel free to chime in in the comments if you know of some examples). However the Early Cretaceous gaveÂ us a preponderance of sailbacked dinosaurs (Fig. 1 [14â€“19]) including the cinematically famous theropod Spinosaurus aegyptiacus, the contemporaneous hadrosaur Ouranosaurus nigeriensis, the gharial-mimic Suchomimus tenerensis, the potentially dual sailed sauropod Amargasaurus cazaui, as well as the allosauroids Acrocanthosaurusatokensis, and Concavenator corcovatus. Lastly, the discovery announced last year (and just now coming to light in the news) of better remains for the giant ornithomimid DeinocheirusÂ mirificus have revealed that it too may have sported a small sail along its back.
Once again we find a group of related, largely contemporaneous, animals, most of which probably evolved their sails separately.
Such a huge collection of sailbacked animals all living around the same time (and sometimes the same place) has begged for some type of functional explanation. The usual go-to for large, showy surfaces like these or the plates of Stegosaurus has been thermoregulation. The thinking being that blood pumped through a large surface area like this, when exposed to the sun, has the ability to warm up faster than other areas of the body. Conversely when the sail is placed crosswise to a wind stream, or parallel to the orientation of the sun, heat will radiate out into the environment faster than other areas of the body. That most sailbacked dinosaurs were “localized” to equatorial areas, coupled with the large sizes of all the taxa (1-10 tonnes depending in species) has favoured a cooling mechanism function for dinosaur sails. Whereas a heating function has been presumed to be the primary function for sails in Dimetrodon and Edaphosaurus. No real function has been ascribed to the sails in rauisuchians or Platyhystrix, though this is probably due to a lack of knowledge/interest in these groups.
Alternate functions proposed for these sails have included a self-righting mechanism for swimming, sexual signaling and other presumed display functions. In certain cases, namely Spinosaurus aegyptiacus and Ouranosaurus nigeriensis, it has even been argued that the enlarged spines did not support a sail, but rather were supports for a large, fatty hump akin to that of camels or bison (Bailey 1996, 1997).
Given the wealth of hypotheses for potential sail functions it would be beneficial to first understand what extant sailbacked taxa use their sails for. Unfortunatelyâ€”though unsurprisinglyâ€”there are few if any scientific studies on sail use in extant sailbacked animals. This has lead to the apparent assumption that there are no extant vertebrates with sailbacks.
There are, in fact, quite a few sailbacked animals alive today. These include various fish, amphibians and even reptile species. Learning what these taxa use their sails for may offer us a glimpse at what extinct animals were doing with their sails. Continue reading → Post ID 1592
Continuing my trend of “catching up,” an article in the November issue of Natural History magazine, talks about a new study in the Quarterly Review of Biology, that finds group nesting to be very common place among extant reptiles.
That study would be:
Doody, J.S., Freedberg, S., Keogh, J.S.? 2009. Communal Egg-Laying in Reptiles and Amphibians: Evolutionary Patterns and Hypotheses. Quart. Rev. Biol. Vol.84(3):229-252.
In the paper, Doody et al (no laughing) did a massive search through the herpetological literature (both technical journals, and hobbyist magazines) to look at instances of communal egg laying in reptiles and amphibians (herps). I’m not being hyperbolic here either, as the paper states:
In total, our assembled database was gathered from 290 different sources, including 176 different scientific journals, 72 books or book chapters, 29 unpublished reports, and 13 unpublished theses. We also have included a number of reliable personal communications from herpetologists.
What the authors found was that group gatherings of herps are vastly more common than previously believed. Group egg laying was found to be present in 345 reptile species. Now you might be thinking 345 really isn’t all that much for a group composed of some 8700 species.
Well then aren’t you a Debbie Downer?
Seriously though, the authors address this by mentioning:
Although the difficulty in locating nests hampers our ability to determine the actual frequency of communal egg-laying among species, we can better estimate this proportion by dividing the number of known communally egg-laying species by the total number of species, excluding those for which eggs have not been found. We conducted such a calculation for the three families of Australian lizards known to include multiple communally egg-laying speciesâ€”Gekkonidae, Pygopodidae, and Scincidaeâ€”as gleaned from the Encyclopedia of Australian Reptiles database (Greer 2004). Proportions of these lizard families known to lay communally were 4â€“9%, but, when we exclude species for which nests are not known, these values rise dramatically to 73â€“100%
The biggest take home message to get from Doody et al’s review, is just how much we don’t know about extant reptiles.
…the present review highlights our inadequate knowledge of the nests and/or eggs of reptiles. For instance, the eggs or nests are known in only 7% of Australian lizards of the three families that commonly lay communally (N = 411 oviparous spp.) (Greer 2004).The extent of this knowledge for Australian lizards is probably similar to that for reptile eggs on other continents, particularly South America, Africa, and Asia, where the reproductive habits of reptiles are poorly known. This is in stark contrast to other vertebrates such as birds, for which complete field guides to the eggs and nests are available for several continents
Indeed, just by doing the brief research run needed to compile this blog post, it was apparent that communalism is much more common in reptiles than anyone ever thought. However, because so many of these reports are either anecdotal, or buried in obscure journals, it is easy to miss all the many cases where it is known.
This discovery lead the authors to the inevitable follow up question of: “why?” What benefit do mothers gain by nesting communally?
Numerous hypotheses for why animals nest communally, have been proposed.
Saturated habitat (only so many suitable nest sites)
Sexual selection (choice of males that live in a particular area)
Artifact of grouping for other reasons
Attack abatement (easier to hide a bunch of eggs in one site, than in multiple sites. Less chance that your eggs will be the ones that are eaten).
Maternal Benefits (save time and energy finding a suitable nest site by “freeloading”)
Reproductive success (if the nest site worked once before…)
The authors reviewed all of these possible reasons for communal egg laying in herps. In the end, they found evidence for both the maternal benefits hypothesis, and the reproductive success hypothesis, though they felt a mixed model better explained things.
Sadly, though the authors mentioned how a lack of information on the natural history of most reptiles is largely responsible for this sudden revelation about their nesting behavior, they nevertheless make repeated mentions of how “social interactions are generally less complex in reptiles and amphibians than in other tetrapods” or how herp sociality forms “relatively simple systems“.
The reality is that the old view of simplistic “loner” reptiles that only come together to mate, is not accurate. This is especially true for parental care in reptiles.
The popular view (among the public, and the scientific community) is that reptiles are? “lay’em and leave’em” types when it comes to reproduction. Despite all the herpetological knowledge to the contrary that has been acquired in the past 50 years, it is still popular to spout the party line about reptiles being “uncaring parents.”
Zoologist Louis Somma took issue with this view of reptilian (in particular, chelonian and lepidosaurian) parenting. He conducted a literature search to see how often mentions of parental care in reptiles are recorded. In the end he wound up finding 1400 references to parental care in reptiles (Somma 2003)!
Somma’s survey covered various aspects of parental care. He found reported evidence of nest building and / or guarding in tortoises like Manouria emys (McKeown 1999), Gopherus agassizii (Barrett & Humphreys 1986) and 4 other species of chelonian.
Turning to lepidosaurs, Somma found parental behaviour to be present in 133 species of lizards and 102 species of snakes. Even a species of tuatara (Sphenodon punctatus) is known to guard its nests (Refsnider et al. 2009). Though these numbers appear small compared to the total amount of species that have been described; much like the Doody et al. paper, this is just based off of species whose nesting behaviours we do know. That these taxa all span a wide phylogenetic range, suggests that parental care is more commonplace than initially thought.
Nest guarding is usually a maternal trait, but some squamates exhibit nest guarding behaviour in both parents, such as some cobra and crotaline snakes (Manthey and Grossman 1997) , as well as tokay geckos (Zaworski 1987).
Not only active guarding of the nest, but actual brooding of the eggs is also commonly reported in squamates such as various python species (Harlow & Grigg 1984, Lourdais et al. 2007), and skinks (Hasegawa 1985, Somma & Fawcett 1989). Some species are even known to groom their newly hatched young (Somma 1987).
More interesting still are various reports and observations of parental feeding in some reptile species, such as the skink Eumeces obsoletus (Evans 1959), and the cordylid lizard Cordylus cataphractus (Branch 1998). Not to mention recent evidence of parental feeding in captive crocodylians.
This leads me to the only reptile group where parental care is well publicized: that of the 23 extant crocodylian species. I could, at this point, list references for parental care in crocodylians. However because this behaviour is so well documented for this group, it would seem unnecessary. It is? better to shed light on the many (MANY) examples of parental care in other reptile species. I also didn’t include related examples like placental evolution in the skink genus Mabuya, or instances of egg binding in captive reptile mothers; due to a lack of appropriate substrate to lay their eggs.
In the end, the paper by Doody et al. adds to a growing body of evidence which suggests that the “lay’em and leave’em” reptile species of the world, are the exceptions? and not the rule.
Next time: Biomechanics of running suggest “warm-blooded” dinosaurs. Or: why the aerobic capacity model needs to die already.
Barrett, S.L. & Humphrey, J.A. 1986. Agonistic Interactions Between Gopherus agassizii (Testudinidae)
and Heloderma suspectum (Helodermatidae). Southwestern Naturalist, 31: 261-263.
Branch, B.. 1998. Field Guide to Snakes and Other Reptiles of Southern Africa. Third revised edition. Sanibel Island: Ralph Curtis Books Publishing.
Doody, J.S., Freedberg, S., Keogh, J.S.? 2009. Communal Egg-Laying in Reptiles and Amphibians: Evolutionary Patterns and Hypotheses. Quart. Rev. Biol. Vol.84(3):229-252.
Evans, L.T. 1959. A Motion Picture Study of Maternal Behavior of the Lizard, Eumeces obsoletus Baird and Girard. Copeia, 1959: 103-110.
Harlow, P and Grigg, G. 1984. Shivering Thermogenesis in a Brooding Python, Python spilotes spilotes. Copeia. Vol.4:959?965.
Hasegawa, M. 1985. Effect of Brooding on Egg Mortality in the Lizard Eumeces okadae on Miyake-jima, Izu Islands, Japan. Copeia, 1985: 497-500.
Lourdais, O., Hoffman, T.C.M., DeNardo, D.F. 2007. Maternal Brooding in the Children’s Python (Antaresia childreni) Promotes Egg Water Balance. J. Comp. Physiol. B. Vol.177:560-577.
Manthey, U. and W. Grossman. 1997. Amphibein & Reptilien S?dostasiens. Natur und Tier Verlag, M?nster.
Mckeown, S. 1999. Nest Mounding and Egg Guarding of the Asian Forest Tortoise (Manouria emys). Reptiles, 7(9): 70-83.
Refsnider, J.M., Keall, S.N., Daugherty, C.H., & Nelson, N.J. 2009. Does nest-guarding in Female Tuatara (Sphenodon punctatus) Reduce Nest Destruction by Conspecific Females? Journal of Herpetology. vol.43(2):294-299.
Somma, L.A. 1987. Maternal Care of Neonates in the Prairie Skink, Eumeces septentrionalis. Great Basin Naturalist, 47: 536-537.
Somma, L.A. & Fawcett, J.D. 1989. Brooding Behaviour of the Prairie Skink, Eumeces septentrionalis, and its Relationship to the Hydric Environment of the Nest. Zoological Journal of the Linnean Society. Vol.95: 245-256.
Somma, L. 2003. parental Behavior in Lepidosaurian and Testudinian Reptiles: A Literature Survey. Krieger Publishing Company. 174pgs. ISBN: 157524201X
Zaworksi, J.P. 1987. Egg Guarding Behavior by Male Gekko gecko. Bulletin of the Chicago Herpetological Society, 22: 193.
After spending? a few years collecting and looking at the weirdness that is Gondwanan crocodyliformes, Dr. Paul Sereno has finally started to unveil stuff. With the help of National Geographic comes When Crocs Ate Dinosaurs. It appears to be a special that focuses on the remarkable – and often underrated – diversity seen within this group of animals. The highlight of the program (at least in my opinion) is the focus on all the very un-crocodile like crocodyliformes.
The National Geographic website has a special section that shows off the various, apparently unnamed, taxa. For now, there are just placeholder names that will likely hurt the eyes and ears of anyone who had to deal with the aftermath of The Land Before Time.
The artwork is by artist Todd Marshall. I’ve always enjoyed his portrayals of prehistoric reptiles (he tends to get almost too fanciful with dewlaps and spikes though). Sadly the accompanying animations do not do Marshall’s incredible artwork justice.? It will be interesting to see how it all gets integrated into the television show.
Also airing tonight is a special on NOVA entitled: Lizard Kings. It features the work of Dr. Eric Pianka; a well known and respected lizard ecologist who has focused on monitors for much of his career.? The special looks to be very interesting. Especially given that it appears to have taken years for the film crew to get the footage they needed. As you read this the special has already aired. However, PBS does make their shows avaialable to watch online for free, on their website. The show should also be viewable on Hulu by tomorrow.
I realize that both of these options are only available in the states. To date there seems to be no international options. At best there are some workarounds.
Still, for those that can get them, both shows should prove to be entertaining.
Just announced today in the journal: PNAS, is a new comprehensive study on Komodo dragon feeding ecology. The comprehensive nature of the paper is the result of the contributions from around 28 individuals from all over Australia, as well as the Netherlands, and Switzerland.
The paper is only six pages long, which downplays just how much work must have gone into this project. The authors used Finite Element Analysis, MRIs, and traditional biochemical and dissectional techniques to look deep into the venom apparatus of the living Komodo dragon (V. komodensis).
For those who may have missed it on the first go around, it has recently been discovered that venom is more widespread among squamates than previously thought (Fry et al 2005). The authors of that paper (a few of whom are on this paper) found the presence of specific glands at the base of the mandible in numerous lizard species. These glands were found to release salivary proteins that were, in fact, venom.
It was a “primitive” venom for the most part, with little denaturing, or tissue destroying properties, but enough that it seemed to warrant the construction of a new clade of squamates named: Toxicofera (Fry et al 2005, Vidal & Hedges 2009). Though the discovery of incipient venom production in many squamates, was an intriguing surprise, the resultant cladogram has proven problematic, and controversial. The authors found iguanians (iguanas, chameleons, most pet lizards) to be deeply nested within scleroglossa (skinks, snakes, varanids); a view that flies in the face of every morphological study ever done on this group (e.g. Romer 1956, Pianka and Vitt 2003). In order for Toxicofera’s current associations to be valid, iguanians would have to have re-evolved both their temporal bars, as well as a fleshy tongue. While possible (few things in evolution are impossible), it is extremely unlikely; kind of like expecting snakes to re-evolve limbs.
Despite this contentious relationship, the discovery of venom glands in animals like monitor lizards, was a surprise. This new study by Fry et al is the first to really look at the venom secreting abilities of this gland, and what it means to Komodo dragon ecology.
It turns out that the mandibular venom gland in V.komodoensishas six different compartments that open between the teeth of the lower jaw. Unlike venomous snakes and helodermatid lizards, the venom does not travel through any grooves in the teeth. Rather, it appears to pool at their base; bathing the teeth of the lower jaw prior to biting a prey animal. It’s a crude method of venom delivery, but one that might explain why Komodo dragons have such thick gums (which the teeth erupt through during a bite).
According to the authors, the mandibular venom gland of a 1.6m (5.25ft) Komodo dragon has enough fluid to produce 150mg of venom; 30mg of which would be available for delivery. That’s a fair amount of venom, but how does that translate to toxicity?
Though the delivery method is crude, the venom is fairly potent. According to the authors it only takes 0.1mg/kg of venom in the blood stream to cause pronounced hypotension, and only 0.4mg/kg to cause hypotensive collapse (fainting).
To put this into perspective, I weigh approximately 76kg (168lbs). It would take approximately 7.6mg of Komodo dragon venom to make me light headed, and 30mg to knock my arse out.
Hmm, maybe I should reconsider that Komodo island trip?
Fry et al go on to discuss how V.komodoensis goes about using this venom delivery system during predation. It was at this point that I became a bit hesitant.
Komodo dragon feeding ecology has been the subject of much misconception. Much like dinosaurs, earlier work on these beasts was more accurate than the work that soon followed. When Komodo dragons were first discovered, they were thought to be scary top predators of their respective habitat. This was quickly downgraded to obligate scavenger; possibly due to the animal’s willingness to eat prekilled meat, but more likely from general incredulity that a large reptile can actively hunt mammals (see table 10-2 of Auffenberg 1981 for examples). It really wasn’t until Dr. Walter Auffenberg spent some 13 months in the wild with Komodo dragons, that this myth was officially dispelled, and some 20 years after for it to become common knowledge. However, once it was discovered that animals lucky enough to escape from an initial V.komodoensis attack were found to die hours/days later, the view of Komodo dragons as “bite and release” predators was born (e.g. Bakker 1986).
Auffenberg’s work did show that there is something septic about the bite of oras. This was originally attributed to bacterial flora living in the fairly dirty mouths of these predators. Indeed one study (Gillespie et al 2002) found 54 potentially pathogenic bacteria living in the mouths of oras!
However, and this is the part that always seems to get glossed over: there has never been a reported case of a komodo monitor using this “bite and release” killing strategy. Despite spending over a year living with these animals, Auffenberg never once found an animal bitten, released and then later tracked down after it died. Komodo dragon attacks were quite the opposite in fact. Small, to relatively large prey (goats, boar) were often killed on the spot using violent side to side shaking to snap the neck, while large prey like water buffalo were hamstringed (Achilles tendon severed), followed by abdominal evisceration of the now paralyzed (and often still alive) animal.
Despite the gruesome detail in which Auffenberg described ora attacks, as well as the sheer lack of evidence for a viper style feeding strategy; one can still read about how Komodo dragons “avoid confrontation with their prey” by allegedly employing this method of killing (for instance).
So one can forgive my trepidation over what was to be written about next in the Fry et al paper.
The authors do discuss the alleged “bite and release” hunting style posited for V.komodoensis, but are quick to point out (as I just did) that there has never been a documented case of this hunting strategy being used on dragon prey.Dr. Fry went went one step further in an interview for Science News:
What’s more, rare sightings of the lizards hunting didn’t fit with this method. Victims typically died quickly and quietly after going into shock, the authors say. “No one’s actually seen a Komodo dragon track a prey for three days until it dies of septicemia,” Fry says. “It’s an absolute fairy tale.”
This was very comforting to see. One can only hope that the other news outlets don’t miss this point when doing their write ups (Edit: so muchfor hope).
Fry et al then went on to dispel the myth that the mouth of dragons contain toxic microflora. Though there have been studies that have shown the presence of potentially pathogenic bacteria in wild oras, none of these studies found a consistent microflora between individuals. In fact, the authors point out that some of the bacteria found in Komodo dragon mouths, were the same bacteria found in the guts of most lizards.
That venom must be playing an important role in predation was determined by looking at the evolution of venom in squamates. The authors point out that:
We have shown that in the species that have developed secondary forms of prey capture (e.g., constricting) or have
switched to feeding on eggs, the reptile venom system undergoes rapid degeneration characterized by significant atrophying of the
glands, reduction in fang length, and accumulated deleterious mutations in the genes encoding for the venom proteins (9, 26,
27). This is a consequence of selection pressure against the bioenergetic cost of protein production (28). The robust glands
and high venom yield in V. komodoensis thus argue for continued active use of the venom system in V. komodoensis.
So, while the venom of Komodo dragons is not the primary means by which dragons dispatch their prey, it still must play a pretty important role in prey acquisition. Since envenomated prey tend to become docile and quiet (Auffenberg, 1981, and this paper), it may just play a role in initiating shock, and reducing retaliatory actions by prey. It may also serve as a good “failsafe” in the event of a missed kill. Bitten prey that are “lucky” enough to escape an initial attack, tend to find themselves easily preyed upon shortly thereafter. This is similar to hunting tactics seen in Canadian lynx (the only mammalian carnivores known to have a septic bite) when hunting caribou (Auffenberg 1981).
Using Finite Element Analysis, the authors compared the bite and skull strength of V.komodoensis with that of a similar sized saltwater crocodile (Crocodylus porosus). The results they obtained agreed with previous FE work on Komodo dragons (Moreno et al 2008), which found the bite of oras to be remarkably weak on its own, thus requiring the aid of the postcranial musculature in delivering much of the force. Ora skull strength is at its greatest during bite and pull behaviour. This data agrees well with field observations showing oras biting and pulling back on their prey. Coupled with their recurved and serrated teeth, this results in the creation of large, gaping wounds, which would aid in venom delivery as the ora’s venom would be spread throughout; quickly entering the bloodstream and speeding up shock.
Finally the authors extrapolated their work to the monstrous lacertilian behemoth Varanus (Megalania) prisca. Using the extant phylogenetic bracketing method (Witmer 1995, 1998), they were able to determine the likelihood of venom being present in Megalania. If true, this would make Megalania the largest venomous carnivore to have ever lived.
I’m not sure I buy this part. As Fry et al mentioned in the paper, the venom apparatus tends to degrade quickly when not used. Megalania was a big animal (over 2,000 kg according to the authors, though Molnar 2004 places it as just under 2,000kg for the largest individuals). Any hole that V(M)prisca would create when attacking its prey, would have been devastating enough without the need for anticoagulating venom.
Like the other members of this unique varanid lizard clade, the jawbones of V. prisca are also relatively gracile compared with the robust skull and the proportionally larger teeth similarly serrated (Fig. 3).
I’d be careful about this assumption, as there is only one fairly complete maxilla (upper jaw bone), and portions of the dentary (tooth bearing lower jaw bone), known for Megalania. This makes comparison with extant monitors, rather hard to do. What little skull bones do exist, show that the skull of Megalania was stronger (or at least, less flexible) than that of other monitor lizards (Molnar, 2004).
As it stands right now, there are frustratingly too few fossils of Megalania (especially the skull) to accurately say one way, or the other in regards to venom delivery.
Of course that doesn’t make it any less interesting to speculate about. 🙂
Auffenberg, Walter, 1981, The Behavioral Ecology of the Komodo Monitor, Florida University press, pgs: 406.
Bakker, R. 1986. The Dinosaur Heresies. William Morrow. New York. ISBN: 0821756087, 978-0821756089 pgs: 481.
Fry, B.G., Vidal, N., Norman, J.A., Vonk, F.J., Scheib, H., Ryan Ramjan, S.F., Kuruppu, S., Fung, K., Hedges, S.B., Richardson, M.K., Hodgson, W.C., Ignjatovic, V., Summerhayes, R., Kochva, E. 2005. Early Evolution of the Venom System in Lizards and Snakes. Nature. Vol.439:584-588.
Gillespie, D., Fredekin, T., Montgomery, J.M. 2002. “Microbial Biology and Immunology” in: Komodo Dragons: Biology and Conservation. James Murphy, Claudio Ciofi, Colomba de La Panouse and Trooper Walsh (eds). pgs: 118-126. ISBN: 1588340732/978-1588340733
Molnar, R.E. 2004. Dragons in the Dust: The Paleobiology of the Giant Monitor Lizard Megalania. Indiana University Press. 210pgs. ISBN: 0253343747/978-0253343741
Moreno, K., Wroe, S., Clausen, P., McHenry, C., D’Amore, D.C., Rayfield, E.J., Cunningham, E. 2008. Cranial Performance in the Komodo Dragon (Varanus komodoensis) as Revealed by High-Resolution 3-D Finite Element Analysis. J.Anat. Vol.212:736-746.
Pianka, E.R., and Vitt, L.J. 2003. Lizards Windows to the Evolution of Diversity. U.Cal.Press. 333pgs. ISBN: 0520234014/9780520234017
Romer, A.S. 1956. The Osteology of the Reptiles. Krieger Publishing 800pgs. ISBN: 089464985X/978-0894649851
Vidal, N. and Hedges, S.B. 2009 The Molecular Evolutionary Tree of Lizards, Snakes, and Amphisbaenians. Biologies. Vol.332(2-3):129-139.
Witmer, L. 1995. “The Extant Phylogenetic Bracket and the Importance of Reconstructing Soft Tissues in Fossils” in: Functional Morphology in Vertebrate Paleontology. Jeff Thomason (ed). Cambridge Univ. Press. Cambridge, UK. pgs 19-33. ISBN 0521629217/978-0521629218
Witmer, L. M. 1998. Application of the Extant Phylogenetic Bracket (EPB) Approach to the Problem of Anatomical Novelty in the Fossil Record. J.Vert.Paleo. Vol.18(3:Suppl.): 87A.
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?
All these questions were asked, and somewhat answered in a recent paper by Clemente et al.
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 author 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.
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.
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.
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. >:)
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!
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.
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.
Goldman et al studied geckos of the species Cosymbotus platyurus. Lizards were ran on vertical surfaces that had various degrees of traction. To induce slippage, some of the surfaces were equipped with a section of dry erase board which had been covered with dry erase ink.
Apparently geckos can’t grab onto everything after all.
By placing the lizards at various sections of these surfaces, the researchers were able to get them to either slightly slip, almost fall, or completely.
The results were interesting.
For starters, lizards ran up the surfaces with their tails off the “ground” the entire time. That is, unless they slipped. Immediately upon slipping, the tail would come down and act as a brace to keep the body from tilting back and falling off. The most dramatic case of this came from an experiment in which the researchers had dropped the lizards down straight on the dry erase board. The gecko in question fell backward, caught itself with its hindlegs, and tail. The body fell away from the wall about 60? before the tail fully caught the animal, and it was able to right itself again (see photo above).
By far the neatest test performed involved getting the lizards and placing them in a supine (belly up) position on a light polyethylene foil held together with four fishing lines. Then by gently shaking the platform (or waiting for the lizards to slip), they were able to dislodge the geckos and send them plummeting to their doom.
Doom, in this case, being an embellishment for safely padded landing area.
The lizards fell 2 meters down. High speed cameras recorded the first 23cm of that trip. These little guys were able to go from fully upside down to rightside up in only 106 milliseconds.
Let me make sure I’m getting the full effect of that result across:
These geckos were able to reorient themselves in 106 THOUSANDTHS of a second, or .00106 seconds!!
Geckos now hold the record for fasting righting time of a vertebrate without the aid of wings.
This unique feat is accomplished by rotating the substantial tail on these animals. As they fall, the tail is rotated counterclockwise. Physics does the rest. Conservation of angular momentum takes over and the entire body winds up turning clockwise, thus reorienting the animal. This phenomenon is known as: air righting with zero angular momentum. It’s the same effect that cats are often lauded for.
Cats, and other air righting mammals (e.g. rats) accomplish their air righting maneuvers by flexing and twisting their backs. Evolution removed the need/use for a large powerful tail in mammals, hundreds of millions of years ago.
No so with lizards. Thanks to this “fifth appendage” all the geckos needed to do was use their tail like a little propeller. There is, however, a caveat.
Cosymbotus platyurus, like most geckos, is capable of caudal autotomy (i.e. voluntary tail loss). Would lizards that have lost their tails still be able to right themselves?
Jusufi et al tested this scenario too. They carefully elicited the loss of the tails in some of their experimental animals, and then subjected them to the same tests as before. The results were striking. Tailless lizards were unable to keep themselves from falling in the vertical slip tests. When they were dropped supine, they still righted themselves, but the rate at which they did it was much slower. Tailless geckos relied on the kind of back flexion and twisting seen in mammals.
Taking things one step further, the authours decided to see how much of a role the tail plays during free fall. They placed the lizards in vertically oriented wind tunnels and “set them free.” The results were unequivocal; the tail acts as the main rudder in these guys. Geckos would rotate their tails counterclockwise to turn left, and clockwise to turn right, while the body remained a stationary airfoil.
The overall results demonstrated the incredible importance of tails in geckos. This is interesting given that so many geckos are also willing to part with their tails when in danger.
It seems that in the natural world it’s better to risk knocking oneself out from a fall, than to risk getting eaten by a bodypart that stubbornly stays on.
It’s unfortunate that the researchers didn’t test the air-righting ability of these geckos after their tails had grown back. Judging from the videos it appears that all the work is being generated by the proximal tail muscles; which stay even after autotomy. Theoretically then, it should still work in a regrown tail.
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 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.
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.