Model organisms are a staple of biology. They are taxa that are used to answer larger questions about that group as a whole, or some general biological problem. Model organisms are chosen for their ease of handling, cheap acquisition, generally “generic” structures, or all of the above. Every major class has a model organism to represent it. Just among vertebrates we have:
Mammals with mice (Mus musculus), dogs (Canis familiaris [or Canis lupus familiaris if you lean that way]), cats (Felis catus [or Felis sylvestris catus for the same reason as dogs]), guinea pigs (Cavia porcellus) and rhesus monkeys (Macaca mulatta).
Birds with chickens (Gallus gallus), pigeons (Columba livia), and zebrafinch (Taeniopygia guttata).
Ray finned fish with zebrafish (Danio rerio), swordtails (Xiphophorous) and cichlids (Cichlidae).
Amphibians with the African clawed frog (Xenopus laevis), and axolotol (Ambystoma mexicanum).
Reptiles with anoles (Anolis), fence lizards (Sceloporous), painted turtles (Chrysemys picta) and finally, the American Alligator (Alligator mississippiensis).
Alligators are relatively new to the model organism realm, but they have proven to be extremely informative. They seem to the be most even tempered of extant crocodylians, making them “more safe” for researchers to work with. Hatchlings start off as miniscule 68 gram (0.15 lbs) animals that later can grow to 363 kg (800 lbs) adults, passing through an enormous size range throughout ontogeny. This growth rate is very food dependent, making it possible to raise alligators almost as bonsai trees. Also, with their unique position on the organismal family tree, alligators are one of the closest living relatives to dinosaurs. Along with birds, they have the potential to help constrain our assumptions about dinosaurs; thus making them very popular subjects for paleontological research as well.
Today, alligators get to make one more stamp on human knowledge with the release of the 3D alligator project from the Holliday and Witmer labs.
Researchers from both labs went through the painstaking process of digitizing the skulls of an adult and a hatchling American alligator, and then digitally separated each bone. The result is a 3D model that can have each bone turned on and off at will. The neat thing is that both labs have made these data freely available for anyone to look at, and download as 3D pdfs, wirefusion models, and multiple movies.
So if one every wanted to know just how many bones make up a crocodylian skull, or how each bone aligns to each other, I highly recommend downloading the 3D pdfs of the adult and hatchling. Not only will one learn all the different bones that compose the skull, but by comparing hatchling to adult, one can see just how radically these bones change throughout ontogeny.
It’s neat, free, informative and reptilian. What more can one ask for. 🙂
Two new papers have recently hit the journal circuit. Both of them involve using living crocodylians to gain a better understanding of paleo-life.
The first one comes from Denver Museum of Natural History paleontologist, Dr. Kenneth Carpenter:
Carpenter, K. 2009. Role of Lateral Body Bending in Crocodylian Track Making. Ichnos. Vol.16:202-207. doi:10.1080/10420940802686137.
The study used an adult Caiman sclerops (first use of a large adult reptile for a locomotion study; at least as far as I know) placed in a small room with two 30cm walls placed on either side of it. This restricted any lateral movement, and “funneled” the animal out the singular opening. At this opening, a camera was placed. It would photograph the animal as it left the room. The room itself, had a smoothed mud covering. This muddy floor would record the tracks of the C.sclerops as it walked by.? Several runs were done, and photographs were taken for each run.
This is the first study I have seen that gave a front view shot of an adult crocodylian as it walked along. As Carpenter mentioned in the paper:
This front view is in contrast to most photographic studies which only capture pro?le and top views….
Carpenter also mentioned the potential of there being an ontogenetic change in limb stance as animals move from hatchling to adult. This is something that I have hinted at previously Hatchling crocodylians seem to have weaker femoral adductors than adults. This is understandable given the greater weight that adult femora need to bear. This can result in a skewed view of crocodylian erect stance; with most authors tending to underestimate the degree of “parasagittality.”
That said, I was surprised to read that Carpenter had found the adult Caiman sclerops to have a hip adduction angle of approximately 65? from the horizontal. Judging from figure1B, the hindlimb appears to be much closer to the midline than the forelimb. Fig1D seems even closer to, if not 90?. It is important to point out that much of the hindlimb is blocked by the body in this shot, as the animal is fully laterally extended. A concurrent shot from behind would have been very useful here; as would an x-ray series of shots throughout the walk phase (for instance: see this long video of a Crocodylus acutus walk cycle. Pay special attention to the position of the femur).
Alas, that is not what the paper is about.
The paper is about how lateral movements during locomotion, have substantial effect on trackways. Dr. Carpenter points out how, despite the semi-erect stance of the forelimbs, the track evidence would suggest an animal with a much narrower (parasagittal?) stance. This has bearing on how prehistoric reptiles, in particular: quadrupedal dinosaurs, may have stood.
One might rightfully ask if we should expect dinosaurs to have had any lateral movement to their walking cycle at all. Carpenter points out that lateral body bending, though not quite as exaggerated as that of crocs, is present in most tetrapods. Birds seem to be the sole exception, with their extremely stiff thorax. However birds are also obligate bipeds, and the avian thorax is much shorter and stiffer than that of dinosaurs.
So it would seem to be a likely bet that quadrupedal dinosaurs likely exhibited some degree of lateral body bending.
Carpenter’s work rightfully asks us to caution reconstructions of stance based largely off of trackway evidence. A fine case study that the paper brings up, is ceratopians. This group, more than any other, has received considerable attention for how the forelimbs were oriented. Early work on ceratopians, favoured a hefty sprawl to the forelimbs (e.g.? Gilmore 1905, or Lull 1933). This was critically evaluated during the heyday of the dinosaur renaissance. Authors such as Bakker (1986), Paul and Christiansen (2000), instead favoured a fully erect stance. A large portion of the data supporting this assertion, was trackway based. The results of this study call into question that view. However this was not the first paper to have done so. Thompson and Holmes (2007) also questioned the “erect ceratopid” view, using a half scale model of a Chasmosaurus irvinensis forelimb. Their results come closer to the results from this paper. Though Thompson and Holmes felt that there was no real modern analogue to ceratopian forelimb mechanics.
In the end, Dr. Carpenter reminds future researchers of the importance in incorporating the entire animal when analyzing trackways.
The second paper comes from the Journal of Experimental Biology.
Owerkowicz, T., elsey, R.M. and Hicks, J.W. 2009. Atmopsheric Oxygen Level Affects Growth Trajectory, Cardiopulmonary Allometery and Metabolic Rate in the American Alligator (Alligator mississippiensis). J.Exp.Biol. Vol.212:1237-1247. doi:10.1242jeb.023945.
The authors embarked on a study of how previous paleo-atmospheric oxygen levels might have affected the lives of animals that would have been alive through these times. According to Owerkowicz et al, crocodylians were chosen because:
Given their phylogenetic position and highly conserved morphology throughout their evolutionary history, crocodilians are often thought to retain many characteristics of basal archosaurs.
I do take some issue with this, as prior reviews on crocodylomorph diversity (Naish 2001) coupled with many new discoveries ( Buckley et al 2000,? Clark et al 2004, Nobre & Carvalho 2006)? continually cast doubt on the old view that crocodylians have survived “unchanged” for some 200 million years. Nevertheless, the results of the study are both interesting, and relevant to reconstructions of how paleo-life would have adapted to these wildly different paleo-atmospheres.
Owerkowicz et al raised groups of hatchling American alligators (Alligator mississippiensis) under three different atmospheric conditions. A hypoxic (12% O2) condition reminiscent of paleo-atmospheric models for the late Triassic/Early Jurassic periods. Current atmospheric conditions (21% O2), and a hyperoxic (30% O2) condition reminiscent of paleo-atmospheric models for the Carboniferous and Permian periods.
The results were interesting, though not too surprising. As expected, hypoxic alligator hatchlings were smaller than their normal and hyperoxic counterparts. However, the degree of growth stunting is pretty surprising. Hypoxic hatchlings were about 12% shorter and 17% smaller than normal hatchlings.
Surprisingly, hatching time did not change under any conditions. This suggests a degree of “hard wired” embryological development inside the egg. In the case of the hypoxic hatchlings, they came out “almost done.” While all three groups had remnants of a yolk sac upon hatching, the hypoxic hatchlings actually had the yolk sac still protruding (normal and hyperoxic hatchlings just showed distended bellies). In some cases, the yolk sac was larger around than the hind legs, thus making movement clumsy and cumbersome.
Other interesting results from this study, included notable changes to the cardiopulmonary system. Hypoxic hatchling lungs were actually smaller than the lungs of normal hatchlings; which appears counterintuitive. The heart, meanwhile, showed distinct hypertrophy in hypoxic animals. The authors believe that lack of lung growth in hatchlings may have been due to the fact that lung function does not start until after hatchlings have hatched.? The heart, on the other hand, is hard at work circulating blood just as soon as it is formed; so it would have experienced the challenges of hypoxia at a very early stage.? Bolstering this hypothesis from the authors was the fact that three months after hatching, hypoxic alligators showed a distinct increase in lung growth rate (the lungs appeared to be “catching up” to the heart).? Hypoxic alligators showed shrunk livers as well. No real explanation for this was given, but it was mentioned that reduced liver mass seems to be a common trait in animals raised in hypoxic conditions. It appears to have some bearing on overall metabolic rate.
Hyperoxic hatchlings exhibited “typical” organ growth rates.? Where hyperoxic animals excelled was in breathing and metabolic rate.
Breathing rates were smaller in this group, while metabolism and growth rate were all larger. The explanation by the authors was that these hyperoxic animals were receiving such high amounts of oxygen in each breath, that they were actually hitting saturation at much shallower breaths; hence the shallow breathing. The higher metabolic rate is believed? due to a lack of right-left shunting in the crocodylian heart. This shunting is usually caused by low oxygen levels (like that experienced in diving), and tends to result in metabolic depression to conserve available oxygen stores.? Since these alligators lungs were constantly saturated with oxygen, right-left shunting never occurred, resulting in an elevated metabolism.
Incidentally, Owerkowicz et el give mention of a cardiac shunt known in embryological birds (via the ductus arteriosis). Though only analogous, one can’t help but wonder what this might have meant for all those dinosaurs that lie between these two groups.
Interestingly, hypoxic alligator hatchlings also showed a higher standard metabolic rate. Though these animals would voluntarily eat less than their normal and hyperoxic counterparts, their metabolism was more like hyperoxic hatchlings than they were normal hatchlings.? Owerkowicz et al believe the reason for the increased metabolism was due to the higher cost of breathing in these animals. Despite taking “normal” breaths, hypoxic hatchlings were taking in a larger tidal volume than their normal and hyperoxic siblings. The heart was also working harder to deliver enough oxygen to tissues.
Finally the authors give mention of growth rates in hyperoxic animals. Basically, it is faster. The authors mention that this might be caused by the persistently elevated metabolic rate, or perhaps from channeling saved energy from breathing (which is one of the main energetic costs in reptiles) into biomass.? It could be a mix of both, but I’m more inclined to think that it comes more from channeling energy reserves into other parts of the body. A high metabolism means nothing, if there is not enough free energy to go around. Just look at the hypoxic gators from this study. Despite their high metabolism, they grew slower than their peers.
The results of this study showed how modern animals can acclimate to different atmospheric conditions. They don’t show how animals would adapt and evolve in these conditions, but they do hint at the general directions, and help give us a clearer picture of what life was like millions of years ago.
Bakker, R. 1986. The Dinosaur Heresies. William Morrow. New York. ISBN: 0821756087, 978-0821756089 pps: 209-212.Buckley, G.A., Brochus, C.A., Krause, D.W., Pol.D. 2000. A Pug-Nosed Crocodyliform from the late Cretaceous of Madagascar. Nature. vol.405:941-944.
Clark.J.M., Xu, X., Forster, C.A., Wang, Y. 2004. A Middle Jurassic ‘Sphenosuchian’ from china and the Origin fo the Crocodylian Skull. Nature. Vol.430:1021-1024.
Gilmore, C.W. 1905. The Mounted Skeleton of Triceratops porosus.? Proceedings United States National Museum. Vol.29:433-435.
Lull, R.S. 1933. A Revision of the Ceratopsia, or Horned Dinosaurs. Memoirs of the Peabody Museum of Natural History. Vol.3:1-175.
Naish, D. 2001. Fossils Explained 34: Crocodilians. Geology Today. Vol.17(2):71-77.
Nobre, P.N. and Carvalho, I.S. 2006. Adamantinasuchus navae: A New Gondwanan Crocodylomorpha (Mesoeucrocodylia) from the Late cretaceous of Brazil. Gondwana Research. Vol.10:370-378.
Paul, G.S., and Christiansen, P. 2000. Forelimb Posture in Neoceratopsian Dinosaurs: Implications for Gait and Locomotion. Paleobiology, 26(3):450-465.
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.