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The outline of a Tyrannosaurus showing the measurements used in the PLoS One study.

When dinosaurs were first recognized by European naturalists during the early 19th century, they were interpreted as being immense, lumbering reptiles similar to iguanas and crocodiles. Since that time our understanding of dinosaurs has changed substantially; early paleontologists such as Gideon Mantell, William Buckland, and Richard Owen would not recognize dinosaurs as we know them today. The once revolutionary idea that dinosaurs were dynamic creatures is now the standard view, yet the details of dinosaur physiology are still not completely known. A new study published in the journal PLoS One adds to the ongoing debate about dinosaur biology, and it suggests that dinosaurs might have actually inherited the physiology necessary to lead very active lives.

Most of the debate has centered on whether dinosaurs were endothermic like birds (i.e. internally regulated their body temperature through their metabolism) or ectothermic like living reptiles (i.e. had body temperatures that fluctuated more widely according to their surrounding environment). As some scientists have pointed out, it is not necessary to think that dinosaurs were precisely like living birds or reptiles—they could have had a unique physiology all their own—but the broad questions of whether dinosaurs were more like endotherms or ectotherms has remained.

Given that all the non-avian dinosaurs are extinct, though, we can’t simply stick a thermometer into a dinosaur and take their temperature. (Nor would such an activity be necessarily advisable, at least without wearing a protective suit of armor.)  The questions that remain must be approached more indirectly, and  in the new study scientists Herman Pontzer, Vivian Allen, and John Hutchinson looked at how much energy it would take for dinosaurs to walk and run. If they could figure out the cost of moving around, they reasoned, they could determine whether an ectothermic or endothermic metabolism would be able to provide the amount of energy the dinosaur required.

The team estimated the leg length of the bipedal dinosaurs, as this measurement has been used to estimate the cost of walking and running in living animals. They also estimated the volume of the muscles that would have attached to the leg bones based upon the size of muscles required to move the legs of the dinosaurs. These estimates could then be compared to what has been observed in living animals, providing an indirect way to see whether dinosaurs were more like ectotherms or endotherms.

What the scientists found was that the largest dinosaurs in the study (Plateosaurus, Dilophosaurus, Allosaurus, Gorgosaurus, and Tyrannosaurus) would have required an endothermic metabolism to move around, while the smaller dinosaurs, such as Archaeopteryx, fell more within the range expected for ectotherms. This created something of a paradox as the small, feathered dinosaurs are the ones thought to be most bird-like in terms of physiology.

Size might have made all the difference. While the study produced clear results for the larger dinosaurs the results for the smaller dinosaurs were ambiguous. Even though the smaller dinosaurs in the study (such as Archaeopteryx, Compsognathus, Velociraptor, and Microraptor) had anatomical traits suggestive of endothermy, the study placed them into the ectotherm range. What this probably means, the authors argue, is that energy expenditure in these smaller animals might have been different than in the large dinosaurs, but the technique they used could not successfully distinguish between the two metabolic ranges in the smaller dinosaurs.

More certain were the results of the larger dinosaurs. It had been proposed that large dinosaurs could afford to be ectothermic as their large body size would have allowed them to retain heat, thus living a “warm-blooded” lifestyle without actually being endothermic.  If the new analysis is correct, however, then it is more likely that the largest dinosaurs would have to have been endotherms. And since they evolved from small ancestors, that makes it possible that the smaller dinosaurs were also endotherms. The fact that pterosaurs, close relatives of dinosaurs (which were not included in the present study), also have traits that seem to indicate more bird-like metabolic rates suggests that endothermy either evolved multiple times or that it is an ancestral trait for the common ancestor of both pterosaurs and dinosaurs. Determining which scenario is the case, however, will require further study in combination with other lines of evidence from the fossil record.

A section of the trackway showing a ornithischian dinosaur (green tracks) walking uphill. From the PLoS One paper.

About 199 million years ago, on a small patch of land that is now preserved in the present-day African nation of Lesotho, there was an inclined slope next to a riverbed. Within hours, days, or even weeks of each other, several different dinosaurs climbed up and down the slope, leaving their footprints behind. Their tracks can still be seen there today, and as reported by paleontologists Jeffrey Wilson, Claudia Marsicano, and Roger Smith in the journal PLoS One, these tracks give us some clues as to how those dinosaurs moved.

Dinosaur footprints are effectively bits of fossilized behavior, and the Lesotho tracksite provides a rare look at how dinosaurs walked when moving up or down inclines. The site preserves the tracks of several ornithischian dinosaurs, which may have been similar to Lesothosaurus, and a single theropod dinosaur, which the researchers compare to Dracovenator. They handled the slippy slope in different ways.

The theropod dinosaur tracks show that it was walking parallel to the riverbank on the top of the slope before veering downwards to descend to the water. When it did so it stayed on two feet but it moved more slowly, as indicated by the shorter length between footprints in the portion where it was going downhill. This dinosaur also appears to have gripped into the ground with its foot claws, steadying itself as it moved downhill.

The ornithischians did something different. One of the ornithischian dinosaurs started on the riverbank and moved up the slope, and as it moved it changed the way it walked. On the riverbed it walked on all fours, holding its limbs out to the side and placing its entire foot on the ground. This was a slow-and-steady posture. As it began to move up the slope, however, the dinosaur moved its limbs closer to the midline of the body and stood on its tiptoes. Only when it reached the top of the slope did the dinosaur then stand up on two legs, keeping the same tip-toed posture.

What these tracks show is that the way dinosaurs handled walking on inclined surfaces was constrained by the type of bodies they had. The ornithischians changed their posture to cope with different obstacles and walked on all fours if they had to. The theropod, by constrast, could not do the same. It probably had arms that were too short to assist it in coming down the hill and thus relied on gripping the ground with its feet to stabilize itself.

At a time when we regularly see dinosaurs walking around on television and in movies this might seem kind of humdrum, but I think this description is still impressive. It provides us with a fleeting glimpse into the lives on animals that have been dead for hundreds of millions of years.

A sculpture of the early mammal Morganucodon on display at the Smithsonian. From Flickr user Arbitrary.Marks.

Scientists at Utah’s Dinosaur National Monument have been quite busy this summer. At the beginning of the season they were blasting some sauropod skulls out of the rock for collection, and now the Chicago Tribune reports that they have discovered hundreds of tiny footprints in rock about 190 million years old. These tracks were not made by dinosaurs, though, but possibly by mammals.

Even though many people think of the Mesozoic (about 251 to 65 million years ago) as the “Age of Dinosaurs,” there were plenty of other creatures around during that time. The first true mammals evolved around 190 million years ago, about when the Dinosaur National Monument tracks were made, although mammal-like creatures had been around for tens of millions of years. Once the first mammals evolved, the group began to diversify, giving rise to the ancestors of modern groups as well as lineages that have gone extinct.

According to a Chicago Tribune report, it seems that the creatures that made the tracks lived in a dry, desert-like environment. Each dime-sized track differs in preservation, but together they provide a snapshot into the life of a rat-sized creature scurrying about the ancient dunes. Given that the animal did not die in its tracks, however, we cannot be entirely sure whether the tracks were made by a “true” mammal or a creature closely related to the common ancestor of all mammals. Since the details used to tell the difference between mammals and mammal-like animals are skeletal, it may not be possible to determine which sort of animal made the tracks. Still, though, the tracks are a rare find and I cannot wait until they are published in an academic journal so we can all learn more about them.

A restoration of the North Arlington fossil site by artist Clinton Crowley featuring Woodbinesuchus and Protohadros.

A few months ago I wrote about the rush to study and excavate a Cretaceous fossil site in North Arlington, Texas before developers start construction on the land. University of Texas at Arlington paleontologists and students have been scouring the site to learn what they can, and this week they announced the discovery of a 100-million-year-old crocodylian from the site.

During the time this crocodylian lived, Texas was part of a river ecosystem that eventually spilled out into a seaway that ran through the middle of North America. Turtles, sharks and lungfish swam in the water and the dinosaur Protohadros browsed on vegetation along the shoreline. No doubt this new crocodylian preyed upon some of those fish. But it has yet to be announced just what species this new fossil discovery belongs to.

There was at least one kind of crocodylian present at the North Arlington site, Woodbinesuchus, but might this new fossil represent something new? The researchers studying the site have stated that some of the crocodylian fossils they have found do not match Woodbinesuchus, so perhaps this more recent discovery is something new. I can’t wait to find out what it is.

Theropod tracks. From Flickr user mrmoorey.

Did a three-foot-tall predatory dinosaur species make an ancient 2,500-mile migration between what is now Wyoming and the UK’s Isle of Skye about 170 million years ago? According to Hunterian Museum paleontologist Neil Clark, quite possibly yes. In the 1980s, a number of theropod footprints were found on the Isle of Skye. They closely resemble tracks that were later found in Wyoming. To see if they were made by the same kind of dinosaur, the tracks from Wyoming and the UK will be digitally scanned so they can be compared in detail.

As Brent Breithaupt of the University of Wyoming has noted, though, the tracks more likely mean that similar dinosaurs were living at similar latitudes at about the same time. A 2,500-mile migration, especially across an ancient sea, is a little hard to swallow and would require extraordinary evidence. Indeed, dinosaur tracks are usually given their own scientific names as they usually cannot be attributed to a particular species with certainty (that is, unless we find a dinosaur that literally died in its tracks). The scientists will continue to compare the tracks from Wyoming and the Isle of Skye, but a lot more evidence will be needed to confirm the idea that these theropods were migrating.