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
September 19, 2012
When British anatomist Richard Owen coined the term “Dinosauria” in 1842, there were nowhere near as many dinosaurs known as there are today. And even among that paltry lot, most specimens were isolated scraps that required a great deal of interpretation and debate to get right. The most famous of these enigmatic creatures were Megalosaurus, Iguanodon and Hylaeosaurus–a trio of prehistoric monsters that cemented the Dinosauria as a distinct group. But they weren’t the only dinosaurs that paleontologists had found.
Almost 20 years before he established the Dinosauria, Owen named what he thought was an ancient crocodile on the basis of a tooth. He called the animal Suchosaurus, and only recently did paleontologists realize that the dental fossil actually belonged to a spinosaur, one of the heavy-clawed, long-snouted fish-eaters such as Baryonyx. Likewise, other naturalists and explorers discovered remnants of dinosaurs in North America and Europe prior to 1842, but no one knew what most of these fragments and fossil tidbits actually represented. Among these discoveries was the sauropodomorph Thecodontosaurus–a dinosaur forever connected with Bristol, England.
Paleontologist Mike Benton of the University of Bristol has traced the early history of Thecodontosaurus in a new paper published in the Proceedings of the Geologists’ Association. The story of the dinosaur’s discovery began in 1834, when reports of remains from “saurian animals” started to filter out of Bristol’s limestone quarries. Quarry workers took some of the bones to the local Bristol Institution for the Advancement of Science, Literature and Arts so that the local curator, Samuel Stutchbury, could see them. Yet Stutchbury was away at the time, so the bones were also shown to his paleontologist colleague Henry Riley, and when he returned Stutchbury was excited enough by the finds to ask quarrymen to bring him more specimens. He wasn’t the only one, though. David Williams–a country parson and geologist–had a similar idea, so Stutchbury teamed up with paleontologist Henry Riley in an academic race to describe the unknown creature.
All three naturalists issued reports and were aware of each other’s work. They collected isolated bones and skeletal fragments, studied them and communicated their preliminary thoughts to their colleagues at meeting and in print. In an 1835 paper, Williams even went so far as to suppose that the enigmatic, unnamed animal “may have formed a link between the crocodiles and the lizards proper”–not an evolutionary statement, but a proposal that the reptile slotted neatly into a static, neatly-graded hierarchy of Nature.
Riley, Stutchbury and Williams had become aware of the fossils around the same time in 1834. Yet Stuchbury and Williams, especially, were distrustful of each other. Stutchbury felt that Williams was poaching his fossils, and Williams thought Stutchbury was being selfish in trying to hoard all the fossils in the Bristol Institution. All the while, both parties worked on their own monographs about the animal.
Ultimately, Riley and Stuchbury came out on top. Williams lacked enough material to match the collection Riley and Stutchbury were working from, and he didn’t push to turn his 1835 report into a true description. He bowed out–and rightly felt snubbed by the other experts who had higher social standing–leaving the prehistoric animal to Riley and Stutchbury. No one knows why it took so long, but Riley and Stutchbury gave a talk about their findings in 1836, completed their paper in 1838 and finally published it in 1840. All the same, the abstract for their 1836 talk named the animal Thecodontosaurus and provided a short description–enough to establish the creature’s name in the annals of science.
But Thecodontosaurus was not immediately recognized as a dinosaur. The concept of a “dinosaur” was still six years away, and, even then, Richard Owen did not include Thecodontosaurus among his newly-established Dinosauria. Instead, Thecodontosaurus was thought to be a bizarre, enigmatic reptile that combined traits seen in both lizards and crocodiles, just as Williams had said. It wasn’t until 1870 that Thomas Henry Huxley recognized that Thecodontosaurus was a dinosaur–now known to be one of the archaic, Triassic cousins of the later sauropod dinosaurs. Thecodontosaurus only held the faintest glimmerings of what was to come, though. This sauropodomorph had a relatively short neck and still ran about on two legs.
The tale of Thecodontosaurus was not only a story of science. It’s also a lesson about the way class and politics influenced discussion and debate about prehistoric life. Social standing and institutional resources gave some experts an edge over their equally enthusiastic peers. Paleontologists still grapple with these issues. Who can describe certain fossils, who has permission to work on a particular patch of rock and the contributions avocational paleontologists can make to the field are all areas of tension that were felt just as acutely in the early 19th century. Dinosaur politics remain entrenched.
For more information, visit Benton’s exhaustively-detailed “Naming the Bristol Dinosaur, Thecodontosaurus” website.
Benton, M. (2012). Naming the Bristol dinosaur, Thecodontosaurus: politics and science in the 1830s Proceedings of the Geologists’ Association, 766-778 DOI: 10.1016/j.pgeola.2012.07.012
April 26, 2012
Paleontologists are naming new dinosaurs at an extremely rapid pace. This past week alone, we’ve seen the announcement of Philovenator and Ichthyovenator, and the next new dinosaur is undoubtedly only a few days from publication. But we have also lost a few dinosaurs. Some of these, such as Dracorex, Anatotitan and Torosaurus, might get folded into other genera thanks to our changing understanding of how dinosaurs grew up. And as paleontologist Bill Parker pointed out at Chinleana, creatures once thought to be dinosaurs have been recategorized as very different, distantly related sorts of archosauriforms (the major group to which dinosaurs, crocodiles and many related lineages belong). Shuvosaurus, for example, was originally described as a Triassic iteration of the “ostrich mimic” dinosaurs such as Ornithomimus but turned out to be a strange, bipedal creature that was more closely related to crocodiles. And Revueltosaurus, an animal originally cast as a dinosaur because of its teeth, is now known to be more closely related to the well-armored “armadillodile” aetosaurs.
Yet reinterpretations can go the other way. Parker points out that a paper just published in Vertebrata PalAsiatica reports that a fossil thought to represent a superficially crocodile-like animal is actually part of a dinosaur jaw.
In 1947, paleontologist Yang Zhongjian—better known to many by the name C.C. Young—mentioned a fragment of a sauropodomorph dinosaur’s snout discovered in the roughly 195-million-year-old, early Jurassic deposits near Lufeng, China. He referred the specimen to Lufengosaurus, one of the many long-necked, small-skulled dinosaur cousins of the more famous sauropods. A few years later, Young changed his mind. He redescribed the battered fragment as a piece of a phytosaur skull. These archosaurs, found in older Triassic strata, generally resembled crocodiles but were actually a different group. (The easiest way to tell the difference is that the nasal openings of phytosaurs sat far back on their snouts, near their eyes.) Young named the animal Pachysuchus imperfectus, and although heavily damaged, the fragment became an important milestone for phytosaurs. The fossil was discovered in early Jurassic rock, so it lived millions of years after phytosaurs disappeared elsewhere. Young’s phytosaur seemed to represent the last of these trap-jawed aquatic predators.
Not everyone agreed with Young’s conclusion. While some paleontologists followed Young’s phytosaur ID, others said that the fragment was too uninformative to tell exactly what kind of archosaur it belonged to. The specimen was somehow lost in the collections of China’s Institute of Vertebrate Paleontology and Paleoanthropology, hindering efforts to figure out exactly what sort of animal Pachysuchus was.
Paul Barrett and Xu Xing relocated and re-examined Pachysuchus, but they didn’t see a phytosaur. Young was much closer to the mark with his original determination. The damaged skull piece exhibits many traits never seen in phytosaurs but that closely match what paleontologists have documented among sauropodomorph dinosaurs. Exactly what species of dinosaur the jaw belonged to is impossible to say—the appropriate traits for a species identification may be missing—but the best fit is certain some variety of sauropodomorph.
There were no Jurassic phytosaurs in Asia. And the proposed occurrences of Jurassic phytosaurs elsewhere are highly questionable, at best. These creatures, which lived alongside and probably preyed on early dinosaurs, were wiped out at the end of the Triassic, just before dinosaurs rose to global dominance.
Barrett, P. M., and X. Xu. 2012. The enigmatic reptile Pachysuchus imperfectus Young, 1951 from the lower Lufeng Formation (Lower Jurassic) of Yunnan, China. Vertebrata PalAsiatica 50:151-159
January 25, 2012
Two years ago, paleontologist Robert Reisz and colleagues revealed that the Early Jurassic dinosaur Massospondylus started off life as an awkward little thing. An exceptional set of eggs recovered from South Africa in 1976 contained the well-preserved skeletons of these baby dinosaurs, and the infants did not look very much like their parents. A roughly 20-foot-long adult Massospondylus had an extended neck and a long, low skull and it walked on two legs. But a baby of the same dinosaur had a short neck, a big head for its body, and it walked on all fours. The change between baby and adult was fantastic, and now, in a new PNAS paper, Reisz and colleagues provide an even more detailed look at how Massospondylus started life.
In 2006, Reisz and collaborators located the site where the Massospondylus eggs had been discovered in South Africa’s Golden Gate Highlands National Park. They found more eggs and baby dinosaurs, but not just that. About 190 million years ago, this place was a nesting ground that multiple Massospondylus used from one season to the next.
The paleontologists have discovered bones, eggshell fragments and ten egg clutches—the largest has 34 eggs—within a six-and-a-half-foot swath of siltstone. These nest sites were not all found in the same level, demonstrating that this particular place was used multiple times by Massospondylus moms. Despite the fact that this place was a nesting ground, however, there does not appear to be any evidence that the parent dinosaurs made special accommodations for the eggs—no clear sign of bowl-shaped depressions or other hints of nest construction were found.
Exactly how much parental care adult Massospondylus offered their babies is unknown. Crocodylians and many birds—the closest living relatives of dinosaurs—often attend their nests from the time the eggs are laid and guard their offspring for at least a short interval after their babies hatch. Massospondylus may have done the same, and small tracks found in siltstone blocks indicate that hatchling dinosaurs remained in the nesting site after emerging from their eggs. The tiny hind- and fore-foot tracks are about twice the size of what would be expected for a newly-hatched Massospondylus, and so it seems that the babies stayed at the site until they doubled in size, at least.
The setting of the nesting site allowed all these intricate details to be preserved. In the time of Massospondylus, the site was a relatively dry habitat near the margin of a prehistoric lake. Relatively gentle flooding events covered up the nest site with fine-grained sediment, and afterwards the area dried out. This was a regular, seasonal cycle, and the bad timing of some expectant dinosaur parents resulted in the good fortune of the paleontologists.
With this new data point, Reisz, Evans, and co-authors looked at the big picture of dinosaur reproduction to see which traits might be widely shared and which might be specializations. It seems that communal nesting sites that were used over and over again was an old, common aspect of dinosaur behavior. And, regarding sauropodomorphs specifically, the Massospondylus site may provide some insight into the evolution of different reproductive behavior among its larger sauropod cousins. Evidence from some sauropod nesting sites has been taken to suggest that exceptionally large long-necked dinosaurs did little more than lay eggs and leave their offspring to fend for themselves. What the Massospondylus site might indicate is that the “lay ‘em and leave ‘em” strategy was not the ancestral state for these dinosaurs, but instead was a reproductive specialization related to increasing body size.
So far, this is the oldest known dinosaur group nesting site. Similar sites created by hadrosaurs and sauropods are about 100 million years younger—a vast expanse of time. Potentially earlier nest site finds have not been well studied. One such Late Triassic site in Argentina has yielded multiple infant and juvenile specimens of the sauropodomorph Mussaurus. I asked David Evans, a paleontologist at the Royal Ontario Museum and one of the co-authors of the new study, about the possibility that the Mussaurus locality is an even older nesting ground. “[E]vidence of any form of extensive nesting site [at the Mussaurus localities] is very scant,” he said, but noted that “given our luck in South Africa, I would not at all be surprised if there are a bunch of nests similar to what we have [found] at the Mussaurus localities too—someone just needs to look and document.”
Pol, D., & Powell, J. (2007). Skull anatomy of Mussaurus patagonicus (Dinosauria: Sauropodomorpha) from the Late Triassic of Patagonia Historical Biology, 19 (1), 125-144 DOI: 10.1080/08912960601140085
Reisz, R., Evans, D., Roberts, E., Sues, H., & Yates, A. (2012). Oldest known dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1109385109
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