Prehistoric sea dragons kept themselves warm

By Ed Yong | June 10, 2010 2:00 pm

Ichthyosau

When dinosaurs ruled the land, other groups of prehistoric reptiles dominated the waters. Their bones have also fossilised and they reveal much about how these ‘sea dragons’ lived. They tell us about the shape of their bodies, the things they ate and even how they determined their sex. And according to Aurélien Bernard from the University of Lyon, they can tell us whether these reptiles could control their body temperature.

The majority of reptiles are ‘cold-blooded’. Unlike mammals and birds, they can’t generate and retain their own heat, and their body temperature depends on their surroundings. But Bernard thinks that at three groups of marine reptiles – the dolphin-shaped ichthyosaurs, the crocodile-shaped mosasaurs, and the the paddle-flippered plesiosaurs – bucked this trend. Whether in tropical or cold waters, they could maintain a constant body temperature that reached as high as 35-39 degrees Celsius.

Bernard estimated the body temperature of these ocean-going predators by studying their teeth. He took samples from 40 plesiosaurs, ichthyosaurs and mosasaurs as well as several prehistoric fish. The specimens came from five continents, and a range of periods from the Triassic to the Cretaceous.  In every tooth, he measured the amount of different oxygen isotopes, a value that depends on the animal’s body temperature and the composition of the water it swallows.

The data from the fish helped to calibrate the reptile data. By and large, fish body temperatures reflect the temperatures of the surrounding seawater. If the reptiles’ teeth had the same composition of oxygen isotopes as those of the fish, their bodies were also similarly as warm as their surroundings and they were probably cold-blooded. Any differences reflect a different means of regulating body heat.

Plesiosaur_Mosasaur

Using a mathematical model, Bernard calculated that both ichthyosaurs and plesiosaurs managed to keep a constant balmy body temperature from around 24-35°C, even when swimming through waters as cold as 12°C. The abilities of mosasaurs were less clear, but it seems that they had at least some control over their body temperature.

These results fit with the portraits of ichthyosaurs and plesiosaurs as active, fast-swimming hunters, which needed warm bodies for their fast chases and deep dives. Likewise, the ambiguity around the body temperature of mosasaurs is consistent with the idea that they were ambush predators, whose sit-and-wait strategies wouldn’t have demanded such high metabolisms.

Other lines of evidence support Bernard’s conclusion. In an earlier study, Ryosuke Motani from the Royal Ontario Museum suggested that the ichthyosaur Stenopterygius had a cruising speed and metabolic rate similar to today’s tuna. Other scientists noted that ichthyosaurs grew incredibly quickly after birth, another sign of a high metabolic rate. And finally, fossils that probably came from plesiosaurs and ichthyosaurs have been recovered from southeastern Australia, a region that would have been bitterly cold when these animals were swimming about.

However, it’s still unclear how these giant reptiles managed their body heat. Today, the giant leatherback turtle is sort of warm-blooded – its massive size allows it to retain heat more effectively than its smaller cousins, an ability known as gigantothermy. If leatherbacks can pull of this trick, it’s entirely likely that even bigger animals like the mosasaur Tylosaurus did something similar.

Modern fish, including some of the ocean’s top predators, use different tricks to warm their blood. Swordfish can temporarily raise the temperature of their brains and eyes, which gives it an edge when hunting fast-moving prey.

Tuna go one step further. Like all fish, its hard-working muscles heat up the blood that flows through them. In other fish, that heat would be lost as the blood returns to the gills for a fresh load of oxygen. But the tuna’s blood vessels are arranged so that the warm blood flowing from the muscles travels past, and heats up, cold blood coming in from the gills. This set-up keeps the heat generated by the tuna’s muscles inside its own body. Some sharks rely on a similar heat exchanger; perhaps plesiosaurs and ichthyosaurs did the same.

Reference: Science http://dx.doi.org/10.1126/science.1187443

Photos by Captmondo, Sebastian Bergmann and Piotrus

More on prehistoric marine reptiles:

How prehistoric sea monsters sorted males from females

Comments (5)

  1. Metabolic rates of tuna — that’s HIGH. Very cool stuff, thanks for bringing it.

  2. Zachary Miller

    Very interesting. One wonders how readily ectothemic vertebrates switch over to endothermy. Seems like plenty did–I think the current thinking is that Archosauria was, by and large, endothemic. Scott Sampson proposed an interesting “middle ground” solution for dinosaurs that would not require full mammal-scale endothermy.

    For mosasaurs, I wouldn’t be surprised if they turned out to be gigantotherms. I say this because their immediate ancestors–monitors and agilisaurs–were probably ectothermic. But even monitor lizards get around traditional ectothermy somehow, and have very fast exertion recovery rates.

  3. Jesse

    Zach;

    In regards to monitor lizards, if I’m not mistaken, most (all?) monitors have an advanced gular pump consisting of thin bones framing the neck attached to powerful muscles that they can use to literally force air into their lungs, which is one of the reason they recover so quickly.

    I’m a layman here so hopefully someone with a bit more knowledge will chime in but it is generally believed that one of the chief limiters of reptile metabolism is Carrier’s Constraint, because of the sprawling sideways shifting walk most reptiles have the movement of the body at high speeds actually compresses one lung, then the other back and forth, forcing stale air to shift to the uncompressed lung instead of being totally expelled and replenished.

    Monitors get around this buy using their gular pump to literally force open both lungs, allowing them to be rapidly replenished, they also affect a slightly higher walking posture when moving at high speeds that also helps with this.

    I’ve also heard that they, and crocodilians, also have more advanced hearts but my Google-fu is weak today and I can’t find the research to back that up…

  4. JMW

    @Jesse #3.

    I’m also a layman, so I have to ask: would Carrier’s Constraint apply to the undulating motion of swimming icthyosaurs and plesiosaurs? To my mind, the mososaur’s sit-and-wait-and-ambush strategy makes Carrier’s Constraint less of a concern.

  5. Jesse

    JMW,

    I would imagine that it wouldn’t be much of a concern in marine reptiles, though I have no proof of this, here is my reasoning;

    Carrier’s Constraint basically boils down to an issue of not being able to refill the lungs properly during exertion.

    Marine reptiles hunted primarily underwater, and, like modern marine mammals, held their breath while hunting, or at least I’m not aware of any research indicating they had gills and would be quite shocked if some was pointed out to me (though it would be interested to know if they could absorb air directly some other way the way some turtles do through their cloacal bursae).

    Since during their peak levels of activity they wouldn’t be attempting to refill their lungs anyways I’d imagine Carrier’s Constraint would be less of an issue. Whatever adaptations they had to allow them prolonged periods of stay underwater probably also would’ve limited or nullified the effects of Carrier’s Constraint.

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