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 20, 2011
Baby dinosaurs are hard to find. While the bones of large, adult dinosaurs were often sturdy enough to survive the processes involved in fossilization, the bones of young dinosaurs were small and delicate and have rarely made it into the fossil record. In many cases we just don’t know what baby dinosaurs looked like. Now a pair of well-preserved eggs has allowed paleontologists a rare look into the early growth of one species.
The eggs in question were part of a clutch found in the approximately 200-million-year-old, Lower Jurassic rock of South Africa. They were discovered in 1976, but the tiny skeletons inside two of the eggs were found only during more recent preparation of the fossils. The analysis of these tiny dinosaurs was published late last year in the Journal of Vertebrate Paleontology.
Although the species of dinosaur that laid the eggs cannot be known with absolute certainty, the most likely candidate is Massospondylus carinatus. This was one of the long-necked sauropodomorph dinosaurs which were early cousins to later giants such as Apatosaurus. Based on the anatomy of the newfound embryos, though, these dinosaurs underwent some major changes between hatching and adulthood.
Compared to skeletons seen in other dinosaur eggs, the Massopondylus embryos were well-developed and were probably close to hatching. They did not look very much like their parents. Instead of being long and slender, these baby dinosaurs were relatively short and squat. Their heads were huge compared to their body size, and their eye sockets were proportionally quite large. They also probably started life walking around on all fours—only later did they gain the ability to switch between walking on two legs and all four.
In fact, many dinosaurs appear to have undergone major changes during their lives. Baby dinosaurs were not just smaller copies of adults. As dinosaurs grew, their proportions changed, and thanks to finds like these paleontologists are beginning to understand how these impressive animals got their start in life.
Reisz, R., Evans, D., Sues, H., & Scott, D. (2010). Embryonic skeletal anatomy of the sauropodomorph dinosaur Massospondylus from the Lower Jurassic of South Africa Journal of Vertebrate Paleontology, 30 (6), 1653-1665 DOI: 10.1080/02724634.2010.521604