December 12, 2012
Dinosaurs never cease to surprise. Even though documentaries and paleoart regularly restore these creatures in lifelike poses, the fact is that ongoing investigations into dinosaur lives have revealed behaviors that we never could have expected from bones alone. Among the most recent finds is that dinosaurs were capable of digging into the ground for shelter. Burrows found in Australia and Montana show that some small, herbivorous dinosaurs dug out cozy little resting places in the cool earth.
But when did dinosaurs develop burrowing behavior? The distinctive trace fossils found so far are Cretaceous in age, over 100 million years after the first dinosaurs evolved. That’s why a new PLoS One paper by paleontologist Carina Colombi caught my eye. In the Triassic rock of Argentina’s Ischigualasto Basin, Columbi and coauthors report, there are large-diameter burrows created by vertebrates that lived approximately 230 million years ago. Archaic dinosaurs such as Eoraptor and Herrerasaurus roamed these habitats–could dinosaurs be responsible for the burrows?
Colombi and colleagues recognized three different burrow forms in the Triassic rock. Two distinct types–differentiated by their diameter and general shape–were “networks of tunnels and shafts” that the authors attributed to vertebrates. The third type showed a different pattern of “straight branches that intersect at oblique angles” created by the burrowing organism and the plant life. The geology and shapes of the burrows indicate that they were created by living organisms. The trick is figuring out what made the distinct tunnel types.
In the case of the first burrow type, Colombi and collaborators propose that the structures were made by small, carnivorous cynodonts–squat, hairy protomammals. In the other two cases, the identities of the burrow makers isn’t clear. The second type included vertical shafts that hint at a vertebrate culprit. Dinosaurs would have been too big, but, Colombi and coauthors suggest, other cynodonts or the bizarre, ancient cousins of crocodiles–such as aetosaurs or protosuchids–could have created the burrows. Unless remains of these animals are found associated with the burrows, it is impossible to be sure. Likewise, the third type of trace might represent the activities of animals that burrowed around plant roots, but there is no clear candidate for the trace-maker.
As far as we know now, Triassic dinosaurs didn’t burrow. Even though they were not giants, they were still too large to have made fossils reported in the new research. Still, I have to wonder if predatory dinosaurs such as Herrerasaurus, or omnivores like Eoraptor, dug poor little cynodonts out of their burrows much like the later deinonychosaurs scratched after hiding mammals. There’s no direct evidence for such interactions, but, if small animals often sheltered from heat and drought in cool tunnels, perhaps predators tried to nab prey resting in their hiding places. One thing is for sure, though: we’ve only just started to dig beyond the surface of Triassic life.
Colombi, C., Fernández, E., Currie, B., Alcober, O., Martínez, R., Correa, G. 2012. Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications. PLoS ONE 7,12: e50662. doi:10.1371/journal.pone.0050662
December 5, 2012
For the past twenty years, Eoraptor has represented the beginning of the Age of Dinosaurs. This controversial little creature–found in the roughly 231-million-year-old rock of Argentina–has often been cited as the earliest known dinosaur. But Eoraptor has either just been stripped of that title, or soon will be. A newly-described fossil found decades ago in Tanzania extends the dawn of the dinosaurs more than 10 million years further back in time.
Named Nyasasaurus parringtoni, the roughly 243-million-year-old fossils represent either the oldest known dinosaur or the closest known relative to the earliest dinosaurs. The find was announced by University of Washington paleontologist Sterling Nesbitt and colleagues in Biology Letters, and I wrote a short news item about the discovery for Nature News. The paper presents a significant find that is also a tribute to the work of Alan Charig–who studied and named the animal, but never formally published a description–but it isn’t just that. The recognition of Nyasasaurus right near the base of the dinosaur family tree adds to a growing body of evidence that the ancestors of dinosaurs proliferated in the wake of a catastrophic mass extinction.
In March of 2010, Nesbitt and a team of collaborators named a leggy, long-necked creature from the same Triassic rock unit in Tanzania they named Asilisaurus kongwe. This creature was a dinosauriform–a member of the group from which the first true dinosaurs emerged–and, even better, appeared to to be the closest known relative to the Dinosauria as a whole. The find hinted that the dinosaur lineage had probably split off from a common ancestor by this time, meaning that the most archaic dinosaurs may have already existed by 243 million years ago. Roughly 249-million-year-old footprints of dinosauriforms found among Poland’s Holy Cross Mountains, described by different researchers later the same year, added evidence that the dinosauriforms were diversifying right from the beginning of the Triassic–not long after the catastrophe that decimated life on earth at the end of the Permian, around 252 million years ago.
Nyasasaurus is another step closer to the first true dinosaurs, and is just as old as Asilisaurus. To find an animal with such distinctive, dinosaur-like traits in the Middle Triassic indicates that dinosaurs already existed, or their ancestral stem was already established. Either way, Eoraptor and kin from South America can no longer be considered as the first dinosaurs, but rather a later radiation of forms. Even though our knowledge of Nyasasaurus is only fragmentary–the dinosaur is represented by a right humerus and a collection of vertebrae from two specimens–the dinosauriform nonetheless marks an additional 12 million years of dinosaur time that paleontologists are only just starting to explore.
Whether or not we ever achieve a more complete view of Nyasasaurus depends on the luck and the caprices of the fossil record. In the new paper, Nesbitt and coauthors point out that the rare, fragmentary nature of the remains found so far reflects that dinosauriforms–and early dinosaurs–were marginal parts of the ecosystems they inhabited. Dinosaurs did not dominate from the very start. They were relatively meek, small animals that lived in a world ruled by archosaurs more closely related to crocodiles. It was only in the Late Triassic and Early Jurassic, when their archosaurian competition was diminished, that dinosaurs became dominant. That means the earliest dinosaurs and their ancestors are few and far between in the Triassic record.
Still, when I asked Nesbitt what Nyasasaurus might have looked like, he cited other dinosauriforms and early dinosaurs as templates to constrain our expectations. Nyasasaurus may have looked quite like Asilisaurus–a leggy animal with an elongated neck–although Nyasasaurus may have been bipedal. Future finds will test this idea, but the fact remains that paleontologists are closing in on what the very first dinosaurs were like. As paleontologists uncover more early dinosaurs and dinosauriforms, the dividing line between the two disappears–scientists are starting to smooth out the evolutionary transition between the first dinosaurs and their ancestors. What role Nyasasaurus played in that transformation isn’t yet clear, but the creature is a signal that over 10 million years more of uncharted dinosaur history remains in the rock.
Nesbitt, S., Sidor, C., Irmis, R., Angielczyk, K., Smith, R., Tsuji, L. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464, 7285: 95–98. doi:10.1038/nature08718
Nesbitt, N., Barrett, P., Werning, S., Sidor, C., Charig, A. 2012. The oldest dinosaur? A middle Triassic dinosauriform from Tanzania. Biology Letters. http://dx.doi.org/10.1098/rsbl.2012/0949
November 20, 2012
Dinosaur giants are among the most famous Mesozoic celebrities. Yet the dinosaur growth spurt didn’t start just as soon as Eoraptor and kin evolved. For most of the Triassic, the first act in their story, dinosaurs were small and gracile creatures, with the first relatively large dinosaurs being the sauropodomorphs of the Late Triassic. Even then, Plateosaurus and kin didn’t come close to the truly enormous sizes of their later relatives–such as Diplodocus and Futalognkosaurus. Discerning when dinosaurs started to bulk up is difficult, however, and made all the more complicated by a set of enigmatic bones found in England.
The fossils at the heart of the in-press Acta Palaeontologica Polonica study, as described by University of Cape Town paleontologist Ragna Redelstorff and coauthors, have been known to researchers for a long time. During the mid-19th century, naturalists described at least five large, incomplete shafts found in the Late Triassic rock of southwest England’s Aust Cliff. Two of these fossils were later destroyed, but, drawing from the surviving specimens and illustrations of the lost bones, paleontologist Peter Galton proposed in 2005 that they came from large dinosaurs that lived over 200 million years ago. In particular, two of the bones resembled stegosaur bones, which would have extended the origin of the armored dinosaurs further back than previously thought.
Not everyone agreed with Galton’s proposal. The bone shafts could be from as-yet-unknown sauropods, some paleontologists argued, while other researchers pointed out that the lack of distinctive features on the bones were unidentifiable beyond the level of “tetrapod” (the major group of vertebrates descended from fish with limbs, similar to Tiktaalik). The bones came from big creatures–possibly more than 20 feet long, based on comparisons to other fossils–but the identity of the Aust Cliff animals is unknown.
Since the outside of the bone shafts provide so little information about their identity, Redelstorff and collaborators looked to the microstructure of two specimens for new clues. While the histological evidence appears to show that the sampled bones belonged to the same species, the authors argue, each individual shows different growth strategies. One bone shaft came from a slightly bigger, rapidly growing individual, while the smaller bone represents an older animal that regularly experienced temporary halts in growth (visible as lines called LAGs in the bone). Why this should be so isn’t clear, but Redelstorff and coauthors suggest individual variation, differences between the sexes or ecological factors as possible causes.
But what sort of animals were the Aust Cliff creatures? When the researchers compared their sample with three kinds of dinosaurs–sauropods, archaic sauropodomorphs and stegosaurs–and Triassic croc cousins called pseudosuchians, the pseudosuchians seemed to be the closest match. Indeed, while the researchers concluded that the “Aust Cliff bones simply do not offer a good match with any previously described histologies,” the specimens appeared to share more in common with those of croc-line archosaurs than with dinosaurs.
This isn’t to say that the Aust Cliff animals were definitely large psuedosuchians, like the recently named Smok. As the researchers point out, the specimens contained a type of bone tissue not previously seen in pseudosuchians–either these animals were not pseudosuchians, or these pseudosuchians were a previously unknown histology. And, Redelstorff and collaborators point out, the bones might be attributable to a sauropodomorph named Camelotia that is found in the same deposits. Studying the bone microstructure of Smok and Camelotia for comparison would be a logical next step in efforts to narrow down the identity of the Aust Cliff animals. Until then, this early “experiment” in gigantism–as Redelstorff and colleagues call it–remains an unresolved puzzle.
Still, the study highlights the importance of building a deep database of paleohistological samples. Had the researchers sampled just one bone, they may have come to the conclusion that all bones of that type would exhibit the same life history–either rapid, continuous growth or a stop-and-go pattern, depending on which they studied. Together, the bones show variations in the natural history of what is presumably the same species, which brings up the question of how quirks of environment, biology and natural history are recorded in bone. If we are going to understand the biology of dinosaurs and other prehistoric animals, we need to cut into as many bones as we can to understand how variable and biologically flexible the creatures truly were.
Redelstorff , R., Sander, P., Galton, P. 2012. Unique bone histology in partial large bone shafts from Aust Cliff (England, Upper Triassic): an early independent experiment in gigantism. Acta Palaeontologica Polonica http://dx.doi.org/10.4202/app.2012.0073
November 17, 2011
November has been a good month for sauropodomorph fans. Earlier this week I wrote about Leyesaurus, a newly named dinosaur that was part of a diverse cast of creatures preceding the mighty, long-necked sauropods. Now paleontologist Sergio Cabreira and colleagues have named another, even older relative of this peculiar group: Pampadromaeus barberenai. This animal may provide some hints about what the ancestral dinosaur might have been like.
Attendees at the 71st annual Society of Vertebrate Paleontology meeting got a preview of Pampadromaeus courtesy of study author Max Langer a few weeks ago. The study published in Naturwissenschaften goes into more detail. The newly described dinosaur is remarkable for both the location of its discovery and its placement in the dinosaur evolutionary tree. While many of the earliest known dinosaurs, such as Eoraptor and Panphagia, have been found in the Late Triassic strata of Argentina, Pampadromaeus was excavated from roughly 230- to 228-million-year-old, Late Triassic deposits in southern Brazil. Most of the skeleton was found, including the majority of the skull.
But what truly makes Pampadromaeus stand out is the dinosaur’s intermediate place between some of the earliest known dinosaurs and the later, more specialized sauropodomorphs such as Leyesaurus and Plateosaurus. While the skull of Pampadromaeus is long, low and generally resembles those of sauropodomorphs, the newly described dinosaur had different kinds of teeth in the jaw. Leaf-shaped teeth thought to correspond to herbivory were set in the front, while an array of short, recurved teeth often associated with carnivory followed toward the back of the mouth. Perhaps Pampadromaeus was an omnivorous dinosaur not yet fully committed to a life of chewing on plants. The anatomy of the rest of the dinosaur’s approximately four-foot-long body is consistent with a unique and varied lifestyle. Pampadromaeus had long legs and comparatively short arms, which hint that the dinosaur was an obligate biped. It seems unlikely that Pampadromaeus switched between walking on two legs and all fours as in later sauropodomorphs.
Taken together, the skeletal traits may indicate that Pampadromaeus retained features of what is thought to be the ancestral dinosaur archetype: a bipedal carnivore or omnivore similar to Eoraptor. Exactly where the dinosaur fits in relation to sauropodomorphs is difficult to ascertain, however. Several analyses in the new study place Pampadromaeus just outside the sauropodomorph group, which may indicate that the dinosaur represents a “stem” lineage from which the true sauropodomorphs evolved. Further discoveries and analyses are required to provide the context necessary to understand where Pampadromaeus belongs in relation to these dinosaurs. Still, Pampadromaeus is more closely related to the early sauropodomorphs than to the forerunners of theropod dinosaurs. By comparing the anatomy of such a creature to theropod foreunners such as Herrerasaurus and Staurikosaurus, perhaps paleontologists will be better able to understand what the common ancestor of the sauropods and theropods was like and reconstruct one of the great splits in the evolutionary history of dinosaurs.
Cabreira, S., Schultz, C., Bittencourt, J., Soares, M., Fortier, D., Silva, L., & Langer, M. (2011). New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil Naturwissenschaften DOI: 10.1007/s00114-011-0858-0
August 24, 2011
Sometimes fossils aren’t what they initially seem.
Back in 2008, paleontologists Jerzy Dzik, Tomasz Sulej and Grzegorz Niedźwiedzki presented what they believed was a big predatory dinosaur from an approximately 200-million-year-old Late Triassic site in Lisowice, Poland. They gave it the nickname “The Dragon of Lisowice,” and in a short summary of the find, Sulej and Niedźwiedzki speculated that the then-unnamed creature “may have initiated the evolutionary line that would eventually culminate in the famous super-predator Tyrannosaurus rex.” The Dragon was thought to signify the dawn of truly terrifying theropod dinosaurs, but it turns out this carnivore may have been a particularly imposing member of a very different lineage.
When first mentioned in the 2008 paper, the predator from prehistoric Poland was said to be known from various skeletal elements that may represent two individuals. An dinosaur-like skeleton was reconstructed on the basis of these partial remains, and now an in-press version of a paper describing the animal by the same researchers has become available through the journal Acta Palaeontologica Polonica. Now the “dragon” has a name: Smok wawelski.
The new paper records some changes from the earlier report. For one thing, the various skeletal scraps found at the excavation site are said to belong to a single individual and not two as originally hypothesized. More significantly, though, the proposed family relationships of Smok have been changed.
Despite being touted as an Allosaurus ancestor shortly after its discovery, in the new paper Smok is simply called “a new large predatory archosaur.” This is a very general statement. The Archosauria is a huge group of vertebrates whose first members evolved more than 240 million years ago; it includes crocodylians, pterosaurs, dinosaurs and various extinct lineages closely related to these major groups. Of these, Niedźwiedzki and colleagues state that Smok shows resemblances to both theropod dinosaurs and a group of extinct, land-dwelling crocodile cousins called rauisuchians, though distinguishing which lineage the animal should be assigned to is difficult. Smok is definitely some kind of archosaur, but precisely what branch of the archosaur family tree it belongs on has not yet been fully resolved.
Exactly what Smok is requires further research to sort out—the paper states that Niedźwiedzki is working on this issue as part of his PhD thesis—but the prospect that it was a dinosaur doesn’t look good. As Bill Parker and others have commented elsewhere, Smok is almost certainly more closely related to crocodile-line archosaurs than to dinosaurs. Characteristics of the skull and hips, especially, underscore this as the most likely possibility. The overall resemblance of Smok to large predatory dinosaurs is a result of evolutionary convergence, or the independent evolution of characteristics in distantly related groups, and the dinosaurian appearance of the reconstructed skeleton was primarily created through using a dinosaur-like template for the known remains.
This isn’t the first time a carnivorous croc-relative has been mistaken for an ancestor of big, bad theropod dinosaurs. In 1985, paleontologist Sankar Chatterjee described a large Triassic predator he called Postosuchus. Described as “the arch predator of its time,” Postosuchus was correctly identified as being a rauisuchian, but Chatterjee also believed that the predator also exhibited traits that foreshadowed those seen in Tyrannosaurus many millions of years later. In fact, Chatterjee proposed that many major characteristics seen in tyrannosaurs were present in Postosuchus, and therefore the Triassic creature “may [have been] close to the ancestry of tyrannosaurs.”
Chatterjee was wrong about Postosuchus being the rootstock for tyrannosaurs. Rauisuchians had nothing to do with the ancestry of Tyrannosaurus or any other dinosaurs—they were a unique group of creatures more closely related to crocodiles which overlapped in time with early dinosaurs. (The tyrannosaurs, instead, originated from small, feather-covered coelurosaurian dinosaurs which looked quite different from the latest and most famous members of the group.) The resemblances Postosuchus and Smok share with large predatory dinosaurs are the results of convergence and are not true signals of close evolutionary relationships. Dinosaur or not, though, these rauisuchians were still formidable and terrifying predators. A terrestrial, 16-foot, carnivorous crocodile-like predator is not something I would like to meet in a dark alley (or anywhere else, really).
Chatterjee, S. (1985). Postosuchus, a New Thecodontian Reptile from the Triassic of Texas and the Origin of Tyrannosaurs Philosophical Transactions of the Royal Society B: Biological Sciences, 309 (1139), 395-460 DOI: 10.1098/rstb.1985.0092
Dzik, J., Sulej, T., & Niedźwiedzki, G. (2008). A Dicynodont-Theropod Association in the Latest Triassic of Poland Acta Palaeontologica Polonica, 53 (4), 733-738 DOI: 10.4202/app.2008.0415
Niedźwiedzki, G., Sulej, T., Dzik, J. (2011). A large predatory archosaur from the Late Triassic of Poland Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0045