October 17, 2012
If there’s one group of dinosaurs that needs better PR, it’s alvarezsaurs. They’re among the strangest dinosaurs to have ever evolved, yet outside of dinosaur die-hards, few people have ever heard of them. They’re not one of those classic forms–the sauropods, tyrannosaurs, stegosaurs, or ceratopsids–that have been cherished for the past century. Paleontologists only recently began to uncover their bones. Alvarezsaurus itself was named in 1991, but it and its close relatives didn’t quite get swept up in the same wave of dinomania as their other Mesozoic cousins.
Alvarezsaurs weren’t big, toothy, or menacing. That’s part of makes them so special. Alvarezsaurus, Mononykus and their relatives from Cretaceous Asia, South America and North America were small dinosaurs–these feathered dinos ranged from the size of a pigeon to about the size of a turkey. In fact, these dinosaurs were so avian in nature that there was once a debate about whether alvarezsaurs were non-avian dinosaurs or birds that had lost the ability to fly. Since those early debates, numerous studies have confirmed that they were non-avian dinosaurs that were closely related to the strange therizinosaurs and ostrich-like ornithomimosaurs.
But the strangest thing of all is the mystery of what alvarezsaurs ate.
Despite being short, alvarezsaur arms weren’t wimpy. Not at all. Alvarezsaur forelimbs were very stout and included one robust finger tipped in a big claw. (Among these dinosaurs, the total number and development of the fingers varied, but they’re connected by having one finger that was bigger than the others.) In contrast, these dinos often had a reduced number of very small teeth. Paleontologists thought they saw a connection between these traits and a life feeding on social insects. Mammals such as pangolins and ant-eaters also have stout, heavy-clawed arms and are toothless–a functional pairing that goes with a life of tearing into ant and termite nests to slurp up the scurrying insects in their nests.
Could alvarezsaurs have done the same? So far, it’s the most popular hypothesis for their bizarre nature. In a 2005 paper, paleontologist Phil Senter proposed that Mononykus would have been capable of the kind of scratch-digging needed to rip open social insect nests. Then, in 2008, Nicholas Longrich and Philip Currie described the alvarezsaur Albertonykus in deposits that also contained traces of Cretaceous termites. Alvarezsaurs seemed to have the right equipment and live at the right time to be social insect predators.
But we don’t really know. No one has published any direct evidence that Albertonykus or any other alvarezsaur ate ants or termites. The hypothesis is certainly a reasonable one, but we still need a test of the idea. Fossil feces may eventually hold the answer.
If paleontologists eventually uncover dinosaur dung of appropriate size that contains ants or termites and comes from a habitat shared by alvarezsaurs, that discovery would strengthen the ant-eating hypothesis. A cololite would be even better. While coprolites are petrified feces that have already been excreted, cololites are fossil poop preserved inside the prehistoric creature’s body prior to expulsion. If paleontologists found an alvarezsaur with a cololite containing termites, that would be direct evidence that these dinosaurs truly did snarf down hordes of insects. For now, though, we can only hope that some lucky fossil hunter makes such a discovery.
July 20, 2012
In 1994, paleontologists made a discovery that turned one dinosaur’s name into an irony. That dinosaur was Oviraptor – the so-called “egg thief” discovered several decades before, but that turned out to be a caring mother.
The story starts in 1923. In that year, an expedition from the American Museum of Natural History discovered dinosaur eggs in the Cretaceous rock of Mongolia’s Gobi Desert. At the time, the paleontologists thought that the eggs had been laid by Protoceratops – a small horned dinosaur that commonly found in these deposits – but there was another dinosaur associated with one nest. The AMNH team also discovered the skull of a toothless theropod dinosaur on top of a clutch of eggs. When paleontologist Henry Fairfield Osborn described the dinosaur in 1924, he presumed that the theropod’s jaws were well-suited to crushing eggs, and that this dinosaur was killed in the act of robbing another dinosaur’s nest. Oviraptor seemed like a fitting name for the Cretaceous looter.
Only, that Oviraptor was probably brooding over the nest. In 1993, fieldwork at another Gobi Desert site uncovered similar eggs, and, fortuitously, same of the eggs preserved the delicate skeletons of near-term embryos. The most spectacular baby was the little skeleton of an Oviraptor-like dinosaur, curled up inside its egg. Even better, the shape of this egg matched the supposed Protoceratops eggs discovered years before. Osborn’s Oviraptor wasn’t stealing eggs, but watching over them, and this conclusion was sooner supported by beautiful skeletons of oviraptorosaur skeletons preserved on their nests, their arms spread to encompass the eggs.
But there was something else very curious about the embryo described by Mark Norell and colleagues in 1994. In the same nest, the paleontologists discovered the partial skulls of two little dromaeosaurids – sickle-clawed dinosaurs such as Velociraptor. These two tiny dinosaurs were either embryos or hatchlings, but why should they be preserved in the same nest with a totally different species?
Norell and co-authors suggested several possibilities. The baby dromaeosaurids could have been the prey of adult oviraptorosaurs, might have been trying to prey on oviraptorosaur eggs, or, after death, could have been transported a short distance into in oviraptorosaur nest. The most tantalizing possibility, though, is that one of the two dinosaur taxa was a nest parasite. Perhaps, when no one was looking, a mother Velociraptor – or similar dinosaur – added a few eggs to an oviraptorosaur’s nest, passing off her parenting duties. Then again, the scenario could have played out the other way around (although I would not envy a baby oviraptorosaur born into a family of vicious raptors).
Frustratingly, we may never know why these two species of dinosaurs were preserved together in the same nest. But I have to wonder if some non-avian dinosaurs were brood parasites. After all, some species of birds – the one lineage of living dinosaurs – sneak their eggs into the nests of other birds, so it’s not inconceivable that this behavior has much deeper, Mesozoic roots. Perhaps, as paleontologists continue to collect and study dinosaur eggs, someone will find more direct evidence of sneaky oviraptorosaurs, raptors, or other dinosaurs.
Norell MA, Clark JM, Demberelyin D, Rhinchen B, Chiappe LM, Davidson AR, McKenna MC, Altangerel P, & Novacek MJ (1994). A theropod dinosaur embryo and the affinities of the flaming cliffs dinosaur eggs. Science (New York, N.Y.), 266 (5186), 779-82 PMID: 17730398
April 6, 2012
Every kid knows how Easter eggs wind up in their yard. According to the canonical weirdness that is the holiday tradition, the Easter bunny delivers the colorful eggs overnight. But the origin of the eggs themselves is hardly ever mentioned. According to a well-timed press release from the University of Leicester, non-avian dinosaurs are the best candidates for some of the candy eggs hidden away on lawns.
There is some real science behind the silliness. In the latest issue of Palaeontology, researchers Nieves López-Martínez and Enric Vicens described a new type of dinosaur egg discovered in the Cretaceous strata of northeastern Spain. The roughly 70-million-year-old eggs, given the name Sankofa pyrenaica to distinguish them from other egg forms previously found, exhibited a strange combination of features.
Many non-avian dinosaur eggs are more or less symmetrical in shape, regardless of whether they are spherical or elongated. Bird eggs, such as those from hens, often have an asymmetrical, tear-drop shape in which one end of the egg is more pointed and the other more rounded. There is some overlap in form, though. Eggs attributed to the small, sickle-clawed dinosaur Troodon and to the beaked oviraptorosaur Citipati are both elongated and taper to one end, and so seem similar to bird eggs.
The profile of the newly described egg type is most similar to that of archaic birds. But, according to López-Martínez and Vicens, the microscopic structure of the Sankofa eggs shares more in common with those laid by non-avian dinosaurs such as Troodon than with birds. The Sankofa eggs exhibit a mix of characteristics seen in both non-avian dinosaurs and archaic birds. Without fossils of the chicks developing inside the eggs, or even associated bones of adult animals, exactly what sort of creature laid this egg is ambiguous.
As part of the study, López-Martínez and Vicens created a chart of non-avian and avian dinosaur egg shapes. And, while it didn’t resolve the true identity of Sankofa, University of Leicester paleontologist Mark Purnell employed the comparative diagram in an attempt to track what sorts of dinosaur—avian or otherwise—could have laid the colorful eggs offered in England’s shopping districts. While many treats had the traditional hen egg shape, Purnell found, at least one resembled the Sankofa egg shape. I have to wonder if other Easter egg variations fit within the non-avian dinosaur range. Perhaps an expedition to the supermarket is in order.
LÓPEZ-MARTÍNEZ, N., & VICENS, E. (2012). A new peculiar dinosaur egg, Sankofa pyrenaica oogen. nov. oosp. nov. from the Upper Cretaceous coastal deposits of the Aren Formation, south-central Pyrenees, Lleida, Catalonia, Spain Palaeontology, 55 (2), 325-339 DOI: 10.1111/j.1475-4983.2011.01114.x
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
January 23, 2012
Imagine a dinosaur as massive as Apatosaurus sitting on a nest. It doesn’t really work, does it? We know without a doubt that these large sauropod dinosaurs laid eggs, but there is no conceivable way that the gargantuan dinosaurs could have sat on their grapefruit-sized eggs without crushing them all. There must have been some other way that the eggs could have been kept safe and warm enough to develop properly. One special site in Argentina suggests that some sauropods had a geological solution to the problem.
Two years ago, paleontologists Lucas Fiorelli and Gerald Grellet-Tinner announced the discovery of a unique nesting site that sauropods returned to multiple times. During a stretch between 134 million and 110 million years ago, expectant mother sauropods came to this site to deposit clutches of up to 35 eggs within a few feet of geysers, vents and other geothermal features. This basin held naturally heated dinosaur nurseries.
A new, in-press paper about the site by Fiorelli, Grellet-Tinner and colleagues Pablo Alasino and Eloisa Argañaraz reports additional details of this site. To date, more than 70 clutches of eggs have been found across an area spanning more than 3,200,00 square feet in a section of rock about four feet thick. Rather than focusing on the habits of the dinosaurs, however, the new study fills out the geological context of the place as a possible explanation for why the dinosaurs came here.
On the basis of geological features and minerals, the authors suggest that the site may have resembled the Norris Geyser Basin of present-day Yellowstone National Park. A series of underground pipes and tubes fed geysers, hot springs and mud pots scattered across an ancient terrain crossed by rivers. The fact that the egg clutches are consistently found near the heat-releasing features is taken by Fiorelli and co-authors as an indication that parent dinosaurs were seeking out these spots to lay their eggs. And this site isn’t the only one. Fiorelli and collaborators also point out that similar sauropod egg sites have been found in South Korea.
Exactly what happened to preserve so many nests is not immediately clear, but the eggs were buried in sediments at least partly produced by the surrounding geothermal features. The eggs were eroded and thinned by the acidic nature of the entombing sediment. Some eggs were destroyed by these and other processes, but others held out and became preserved in place.
Not all sauropod dinosaurs selected such sites for nests. Particular populations near geothermal features may have received a benefit from the natural heat, but how did other populations and species far removed from these hot spots lay and protect their nests? We still have much to learn about how baby sauropods came into the world.
Fiorelli, L., Grellet-Tinner, G., Alasino, P., & Argañaraz, E. (2011). The geology and palaeoecology of the newly discovered Cretaceous neosauropod hydrothermal nesting site in Sanagasta (Los Llanos Formation), La Rioja, northwest Argentina Cretaceous Research DOI: 10.1016/j.cretres.2011.12.002