Tracks made by a medium-sized theropod on a slab of rock just outside of Moab, Utah. Photo by author.

Two years ago, I visited the American West for the first time. I was immediately hooked. Seeing the morning sunlight hit the dinosaur-rich Jurassic rock of northern Utah’s Dinosaur National Monument was what really did it for me. When I saw that, I knew that I had to move out West, and a few weeks ago I settled in Salt Lake City to devote myself to writing about the prehistoric past. I now live right in the middle of dinosaur country—some of North America’s most productive and important dinosaur sites are within a day’s drive—and this past weekend I had the chance to visit a few located just a few hours from my new hometown.

At the southern tip of the series of highways making up the Dinosaur Diamond, Moab is right in the middle of dinosaur country. The geologic strata of the area is piled high with sedimentary rock from the heyday of the dinosaurs—from the Late Triassic through the Early Cretaceous in many places—and, at a few spots, vestiges left by dinosaurs can be easily seen. One such place is right along Potash Road, just outside Moab itself.

Left in Navajo Sandstone dating to about 190 million years ago, the Potash Road dinosaur tracks come from a time tens of millions of years before the famous Jurassic fauna of the Morrison Formation. The world was quite different then. Today the tracks rest in two slabs perched on a rocky hill within a stone’s throw of the Colorado River, but when the tracks were made the area was a sandy shore of a lake.

The tracks were left by at least three different size classes of theropod dinosaurs. Two slabs of rock contain relatively small tracks paleontologists have assigned the name Grallator, slightly bigger tracks known as Eubrontes and even larger footprints, according to an interpretative sign at the site, were left by Allosaurus. This last attribution is probably a mistake. Allosaurus lived later in the Jurassic—around 155 million to 150 million years ago—and, unless an animal dies in its tracks, paleontologists can’t be certain what species created them. That’s why tracks are given their own names. In fact, it is possible that at least some of the tracks were made by dinosaurs of the same species but belonging to different ages. We may never know for sure, but the Potash Road tracks are still wonderful monuments from a time when dinosaurs were at home in Utah. I can’t wait to visit more of them.

Modeling a dinosaur den: a half-size PVC model of a Oryctodromeus burrow used to tests scenarios for how they became preserved. From Woodruff and Varricchio, 2011.

Oryctodromeus isn’t exactly a household name. A small, herbivorous ornithopod found in the Late Cretaceous rock of western North America, it was the sort of dinosaur most often depicted as being prey for charismatic carnivores. But there was at least one aspect of Oryctodromeus that made it particularly interesting—this dinosaur may have lived in burrows.

Based on the context of the rocks they are found in, we know that dinosaur bodies were preserved in a variety of different environments. Some bodies were covered up by seasonal floods, other dinosaurs were washed out to sea, and dinosaurs even died in death-traps created by the footprints of even bigger species. But until Oryctodromeus, dinosaurs had not been found in fossilized dens.

The fact that the small dinosaurs had been buried within a burrow was made clear by the details of their den. At the end of an S-shaped tunnel was a large chamber that had been dug into three different layers of mudrock and later filled with sandstone. The fact that an adult and two juvenile Oryctodromeus were found in the sandstone confirmed that this was a den that had been flooded by a slurry of water and sandy sediment.

But were the dinosaurs buried inside their den, or had their bodies just been washed inside? The dinosaur bones were jumbled up rather than lying in articulated poses on the burrow floor. This left the details of their preservation unclear. In order to solve this mystery, paleontologists Cary Woodruff and David Varricchio created a half-scale model of the original burrow with PVC pipes and conducted experiments with rabbits to see what sort of scenario would best account for the way the dinosaur fossils had been preserved.

The paleontologists ran thirteen trials by filling their artificial burrow with a mixture of water, clay, and sand. Rather than using whole rabbits, though, Woodruff and Varricchio only used disarticulated skeletons. This is because no Oryctodromeus bones were found in their natural positions, hinting that the dinosaurs died, decomposed, and had mostly fallen apart before their preservation. By the time the den was flooded, the dinosaurs had already turned into piles of bones (regardless of whether their skeletons were inside or outside the burrow at the time of the event).

Woodruff and Varricchio modeled the different ways the bones could have found their way into the den by running a variety of tests. In some trials the bones were placed in the burrow, while in others they were included in the sediment mix used to fill the artificial den. Each setup produced a different distribution of bones in the PVC chamber.

Six different trials with differing conditions all created the kind of elevated, dispersed assemblage of bones found in the Oryctodromeus burrow. Bones were initially inside the chamber for four of these trials, but were outside the burrow and contained within the sediment, respectively, in the other two. While this evidence supports the idea that the dinosaur bones may have been inside the den when it was flooded, it remains possible that the bones were washed in from outside.

If the dinosaur skeletons really were washed into the burrow from outside, however, Woodruff and Varricchio argue, it is strange that the bones of an adult and two juveniles should be found together. Furthermore, bones transported by sediment-filled floods are often broken and abraded, and there are no signs of such destructive transport on the Oryctodromeus fossils. The hypothesis that the Oryctodromeus bones were already inside the den remains the best-supported idea. Woodruff and Varricchio caution that further investigations are required to understand how these dinosaurs—and other den-dwelling fossil vertebrates—became preserved.

References:

WOODRUFF, D., & VARRICCHIO, D. (2011). EXPERIMENTAL MODELING OF A POSSIBLE ORYCTODROMEUS CUBICULARIS (DINOSAURIA) BURROW PALAIOS, 26 (3), 140-151 DOI: 10.2110/palo.2010.p10-001r

Tracks made by an Iguanodon-like dinosaur at the 100 million year old Dinosaur Ridge tracksite. Image from Flickr user Matthew Saunders.

The Cretaceous dinosaur tracks scattered along Morrison, Colorado’s Dinosaur Ridge have persisted in the fossil record for 1o0 million years, but they are now in danger of being lost forever. Exposed on the surface, the tracks are being eroded away bit by bit, and a local controversy over the aesthetics of the Colorado landscape has complicated efforts to preserve these tracks.

The fossil sites of Dinosaur Ridge come from three different time periods. There is a 150-million-year-old dinosaur bone quarry, a 100-million-year-old track site, and a 68-million-year-old track site. It is the 100-million year-old set of tracks, dominated by footprints made by an Iguanodon-like dinosaur, that is at the center of the debate. Regular freeze-thaw cycles and exposure to the elements have been gradually destroying the tracks. According to an article in the Denver Post, the nonprofit group Friends of Dinosaur Ridge has proposed that a canopy of high-tech fabric be erected over the site to help prevent further damage. The trouble is that this proposal runs counter to Jefferson County’s official Front Range Mountain Backdrop policy which forbids structures that would obscure or detract from views of the mountains. An article on the debate from LJWorld.com reports:

“The plan that they came up with includes structures and it just doesn’t work,” said Kathryn Heider, a spokeswoman for Jefferson County, which owns the land where the tracks are located 15 miles from Denver. “It doesn’t mean we don’t want to preserve the footprints. It just means we don’t want structures on the backdrop.”

Discussions about what can be done to save the tracks are ongoing, but there is not much time left. Based upon damage already done to the tracks, the Friends of Dinosaur Ridge project that the tracks have only about 10 to 15 years before they are lost. Their destruction would rob a natural treasure from scientists and the public alike. I hope that an amenable solution to this dilemma can be found soon.

A snapshot of the world during the Cretaceous, about 90 million years ago. From Wikipedia.

Paleontologists are constantly reminding themselves of the incompleteness of the fossil record. What has been preserved is only a small fraction of all the organisms and environments that have ever existed. This makes detecting evolutionary patterns a bit of a challenge. In a presentation given at this year’s Society of Vertebrate Paleontology conference, Smithsonian paleontologist Matt Carrano dug into the long-standing question of whether changes in sea level triggered changes in dinosaur diversity.

Over the past few decades, paleontologists have produced a number of graphs depicting dinosaur diversity through time. They show a general trend toward increasing diversity from the Late Triassic through the end of the Cretaceous, but with a few fluctuations in between. The rise and the fall of the seas has been proposed as one of the drivers of these changes. Perhaps, it has been hypothesized, high sea levels might have favored dinosaur diversity by fragmenting some terrestrial habitats or isolating one area from another while simultaneously creating more environments where dinosaurs might be preserved. Then again, it has also been suggested that dinosaur diversity might go up when sea levels are low since there would be a larger land area. In order to detect whether any such trends existed, the scientists looked at the occurrence of about 749 dinosaur species through time and space, noting where paleontologists have gone looking for their bones, as well.

What the Carrano and his colleagues found was that the fluctuations in sea level did not influence dinosaur diversity as we know it today. Our perspective of dinosaur diversity is significantly shaped by where paleontologists have gone looking for fossils, the amount of effort expended there, and also by places that have yet to be extensively studied. Dinosaurs might be more plentiful and easier to find in Cretaceous rocks than Triassic ones, for example, which would account for why dinosaur diversity differs between the two time periods. Any scientific work proposing to look at dinosaur diversity has to take these sampling biases into account.

This is not to say that sea level change did not or could not have influence dinosaur diversity, though. Rising sea levels could have created island chains and other geographical pockets that could have driven dinosaur speciation, or low sea levels might have allowed dinosaur species to range more widely. (We know, for example, that the Western Interior Seaway caused Cretaceous dinosaurs to evolve in different ways in the eastern and western parts of North America.) Detecting these signals from the fossil record, however, will require in-depth sampling and a recognition of the way in which our search for dinosaurs skews the picture of their diversity. As stated by the authors of the paper that was the basis for the SVP presentation: “Considerable future work is required to establish how sampling biases may affect proposed long-term diversity trends and mass extinction events in the terrestrial realm.” If paleontologists want to get at the big picture of dinosaur diversity, they need to look at these biases and get digging at places which are still poorly known.

References:

Butler, R., Benson, R., Carrano, M., Mannion, P., & Upchurch, P. (2010). Sea level, dinosaur diversity and sampling biases: investigating the ‘common cause’ hypothesis in the terrestrial realm Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.1754

The shell of the fossil snail Lioplacodes embedded in the coprolite of an herbivorous dinosaur. From the Lethaia paper.

One of the many reasons I love paleontology is that every now and then I stumble across a paper on some aspect of ancient life I had never considered before. There is much more to the science than descriptions of new species, and one of the studies that most recently caught my eye carried the title “Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana.”

As reported in the 2009 study, paleontologists digging at a 76-million-year-old site within the well-known Two Medicine Formation have found more than 130 snail specimens closely associated with—and sometimes even within—the fossilized feces of herbivorous dinosaurs. Scientists had long recognized that the snails were present in the same deposits as the dinosaurs, indicating that they shared the same habitat, but no one had systematically documented interactions between the large vertebrates and the small gastropods. In fact, up to seven different snail taxa were found in close association with the dinosaur coprolites. Apparently dinosaur poo was a regularly-used resource by many species of snail.

The occurrence of snail fossils within the dinosaur dung was also used by the scientists behind the study to reconstruct what kinds of habitats the animals were living in. Since the most common snails on and within the coprolites were terrestrial snails, the authors of the study propose that the dinosaurs left their droppings on dry land before their feces were subsequently flooded (which would have filled in dung beetle burrows also seen in the coprolites). Although they noted that some of the snail shell fragments within the coprolites could have come from snails that were accidentally ingested while the dinosaurs were eating leaves and rotting wood, at least half of the snail fossils were intact and show no signs of being digested. This suggests that the snails made their way to the dino pats after they were deposited, with the dinosaur feces providing warm, wet, food-rich mini-environments that the snails could comfortably exploit.

CHIN, K., HARTMAN, J., & ROTH, B. (2009). Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana Lethaia, 42 (2), 185-198 DOI: 10.1111/j.1502-3931.2008.00131.x

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.

Carnivore, by Leigh Clark.

If late night B-movies have taught me anything, it is that radiation makes things grow very big really, really fast. This is not true, of course, but it is a standard convention of cheesy science fiction, and it is a theme carried on by Leigh Clark’s novel Carnivore.

The story unfolds at a remote Antarctic research station where a team of scientists has brought back a Tyrannosaurus egg they found frozen in ice. At one point someone says “Gosh, we shouldn’t put any of that radioactive waste we have lying around next to that egg or it will grow very fast!” But of course this is just what the human villains of the story do. Before you know it the little Tyrannosaurus is a full-grown terror, gorging itself on the hordes of nameless characters that seem to appear out of nowhere at the outpost.

I would mention the main characters of the novel, but there is not much point. Almost everyone falls prey to the Tyrannosaurus in gruesome fashion. Indeed, Clark’s antagonist is a very messy eater, and it is no wonder that it eats so many people since it can’t seem to keep those it captures in its mouth for very long. If done right the descriptions of blood and gore could have been chilling, but instead the novel jumps from one scene of over-the-top carnage to the next.

Carnivore mostly serves as an excuse to have a Tyrannosaurus munching on scores of hapless victims in the Antarctic, but a more effective thriller is Lincoln Child’s new novel Terminal Freeze. In some ways it is quite similar to Clark’s book (a team of scientists finds a prehistoric killer locked in ice), but Terminal Freeze is more fully developed. The Arctic base where Child’s novel is set is described in vivid detail, making it easy to imagine his monster slinking down the dark, chilled hallways. As it turns out, Child’s creature is not a dinosaur but an unknown kind of mammal, but it is just as terrifying as Clark’s more famous antagonist.

While the idea that dinosaurs (or other monsters) might be preserved alive in ice for millions of years is a bit silly, we do know that dinosaurs inhabited cold habitats within the Arctic Circle. The past year has seen the publication of several papers describing the diversity of dinosaurs in the cold northern reaches of the globe. While novelists still have to figure out how to close gaps of tens of millions of years to bring dinosaurs and humans together, a tyrannosaur trotting through the snow is not such a far-flung idea after all.

A restoration of the tiny theropod Hesperonychus by Nicholas Longrich.

If you visited what is now Alberta, Canada 75 million years ago, you would have to beware of some formidable predators. The large tyrannosaurids Daspletosaurus and Gorgosaurus prowled the landscape while the smaller sickle-clawed killers Dromaeosaurus and Saurornitholestes stalked their prey in the forest. You might be excused, then, if you missed a smaller feathered predator that weighed about as much as a domestic chicken and was named Hesperonychus.

Announced by paleontologists Nicholas Longrich and Philip Currie this week in the journal PNAS, Hesperonychus is the smallest predatory dinosaur yet known from North America (even smaller than the termite-eating Albertonykus, which Currie and Longrich described last year). It still would have been quite large compared to the mammals of its day, however, and it may have been the scourge of our ancient relatives. This fits with the hypothesis that dinosaur predation on mammals kept mammals small, but as Longrich and Currie point out, it could also mean that the occupation of niches by mammals kept dinosaurs from becoming much smaller.

During the Mesozoic, the time when non-avian dinosaurs flourished, there were no large mammals. One of the biggest was Repenomamus, which was about the size of a small dog and lived during the Cretaceous. It was large enough to eat some baby dinosaurs (which fossil evidence has shown it did) but this was unusual. Most mammals were smaller and ate seeds, insects, and fruit. This means that if there were dinosaurs smaller than Hesperonychus they may have come into competition with mammals for food and places to live in the forest. Rather than coming into such direct competition for resources with mammals it seems that the smallest of theropod dinosaurs were just large enough to see mammals as food.

What is even more surprising is that Hesperonychus does not fit in with any other maniraptoran dinosaurs from North America. When Longrich and Currie studied its bones to determine what kind of dinosaur it was, they found that it was most closely related to the microraptorine dinosaurs from China. This group of feathered dinosaurs, which includes Microraptor and Sinornithosaurus, had not been found in North America before. Not only that, but Hesperonychus is about 45 million years younger than the oldest members of this group in Asia. Therefore it extends the range of the microraptorine dinosaurs over both time and geography, hinting at other tantalizing finds yet to be disinterred from the rock.

Alamosaurus. From Wikimedia Commons.

Fossil trackways have shown paleontologists that some sauropod dinosaurs moved together in herds. But how were their herds organized? Were they made up only of particular age groups or were individuals of different ages all mixed together? In a new paper in Palaeogeography, Palaeoclimatology, Palaeoecology, scientists Timothy Myers and Anthony Fiorillo discuss two different sites that suggest that at least some sauropods segregated their herds by age.

Before discussing the fossil sites in detail, Myers and Fiorillo review some of the problems in inferring behavior from fossil trackways alone. A photo included in the paper, for instance, shows the tracks of a human next to those of a grizzly bear. Was this person walking alongside gentle Ben? No, the tracks had been made hours apart. The same principle holds for fossil tracks. The presence of tracks made by two individuals in the same place does not necessarily mean they were there at the same time. Further evidence would be required to show this was true.

There can be difficulties with evidence from bone beds, too. The fossils from Mother’s Day Quarry in Montana are from a herd of sauropod dinosaurs that may have died during a drought. What is strange, however, is that nearly all the bones are from juvenile and sub-adult animals. Immature animals typically suffer higher death rates than adults during droughts, but the question was whether this site represents a herd of immature animals or simply the immature portion of a larger herd. The lack of adults and the fact that the bones had not been transported after the animals died led Myers and Fiorillo to suggest that the Mother’s Day Quarry site represents an actual herd of immature animals separate from adults.

The Big Bend site in Texas differs in that it consists of three juvenile Alamosaurus that died and were buried together. Like the Montana site, this bone bed represents a single event rather than the accumulation of multiple skeletons over time. The fact that no adult bones are found and that no accumulations of multiple Alamosaurus adults are known suggests that these dinosaurs herded together when young but became more solitary as they became mature.

So what do these two sites mean? Factors that might potentially bias the formation of bone beds must be kept in mind, but they appear to suggest that, in at least some sauropods, juvenile individuals formed groups separate from herds of mature individuals. This may have to do with size. The adults were much, much larger than immature individuals and may have had different dietary needs. This may have segregated herds by age with the younger animals grouping together for protection. This type of age segregation was probably not present in all sauropods, but it may have been prevalent among some of the largest species.

A student screening for fossils at Big Brook. From Flickr user Colin Purrington.

You can find dinosaurs in New Jersey, but you have to know where to look. Even though my home state is known for suburban sprawl and peculiar odors today, a little over 65 million years ago much of it was covered by the ocean. Marine crocodiles, plesiosaurs, and gigantic mosasaurs prowled the near-shore waters, and the dinosaurs Hadrosaurus and Dryptosaurus inhabited the land not too far from the ancient beach. When these dinosaurs died, sometimes their bones were washed out into rivers and carried to the boundary of the sea, where they became fossilized along with the remains of marine animals.

Unfortunately some of the most significant fossil sites in New Jersey have been built over or are no longer being examined, but there is one place where anyone can go to find fossils. It is called Big Brook and is well known for the abundance of shark teeth and other small fossils. Every one in a while, though, someone finds a bit of dinosaur bone.

Last December, New Jersey dentist Paul Kovalski found a chunk of brown bone at Big Brook three inches wide by three inches long. It didn’t look like much, but when he took it to the paleontologists at the Academy of Natural Sciences in Philadelphia, they were able to confirm that it came from a dinosaur. It most likely belonged to Hadrosaurus, New Jersey’s state dinosaur and one of the first major dinosaur discoveries in North America.

I have never been to Big Brook, but I’m making plans to make a number of visits there as the weather warms up. I doubt that I will be lucky enough to find any dinosaur bones, but who knows? I just might get lucky.

I think there is a mistake in this article. The mistake is at the second paragraph, on the last line. “growing understanding that they were not cold-blooded creatures.”, and I think the right one should be “growing understanding that they were cold-blooded creatures.” The “not” shouldn’t be in that line. …

In order to answer this question we have to untangle what phrases like “warm-blooded” and “cold-blooded” really mean, especially since they can be more confusing than helpful.

But are these dinosaurs cold-blooded? Courtesy Flickr user Blush Response

Let’s start with the “cold-blooded” animals like fish, amphibians, and reptiles. Their body temperatures fluctuate with that of their surrounding environment, which means they are ecothermic. This does not automatically mean that these animals are sluggish, though. If the temperature of their surrounding environment is high enough they can be very active (meaning that they are literally “warm-blooded” in those circumstances), and some of these animals even have special physiological mechanisms that help them maintain a high body temperature. Great white sharks, for instance, are able to keep their body temperature several degrees Celsius above the temperature of the cold coastal waters they inhabit.

The animals we often refer to as being “warm-blooded,” by contrast, are more aptly described as being “endothermic.” This means that they generate their own body heat and often keep it at a relatively high, constant temperature. Living mammals and birds are the main examples of this kind of physiology, but there are some species that can switch between being endothermic and ectothermic. Some small birds and bats are endothermic for part of a day or part of the year but ectothermic during other parts. They are so small and burn energy so fast that if they were not able to switch their metabolisms, they would have to constantly collect food or they would die.

So, were dinosaurs ectothermic, endothermic, or something else entirely? Read more after the jump.

It is difficult to say, but they definitely were not “cold-blooded” in the sense that they were slow, stupid, and could only survive as long as the global thermostat stayed above 65 degrees Fahrenheit. Given that dinosaurs were a very diverse group of vertebrates it is probable that different groups had different physiologies. The immense sauropods, for instance, were so big that even if they were ecothermic they could have maintained a high body temperature. The bigger an animal is, the harder it is for them to gain or lose heat, so sauropods might have been endothermic when they were young but became more ectothermic as they became larger. A high, internally-generated body temperature is energetically expensive to maintain, and the largest of dinosaurs may have undergone a physiological shift that allowed them to remain active but not have to spend their entire lives eating.

If any dinosaurs were endothermic in the way that living mammals and birds are, however, it was the small predatory dinosaurs closely related to birds. The close association of dinosaurs like Deinonychus and Dromaeosaurus with birds suggests that they might have been endothermic, and this is reinforced by the presence of this kind of dinosaur within the Arctic Circle. Even though the world was warmer in the Cretaceous than it is today, it could still get very cold, cold enough to snow, in the highest latitudes. If dinosaurs were physiologically like crocodiles or lizards they probably could not survive in such a cold place, but discoveries in Siberia and Alaska show diverse communities of dinosaurs might have lived there year round. This suggests that many dinosaurs were endothermic and could internally maintain a high body temperature, especially the small dinosaurs that would more quickly lose heat if they were ectothermic.

Unfortunately we can’t take the temperature or study the physiology of any non-avian dinosaur today, but the evidence suggests that if they were not fully endothermic like most modern birds and mammals, then dinosaurs had another physiological strategy that allowed them to maintain high body temperatures. The idea that they were “cold-blooded” animals just like living lizards has gone extinct.