33.1: Dinobook or Saurogram? - Dinosaur Social Behavior

There is a lot of debate over the social-networking skills of dinosaurs. On one hand, the living descendants of dinosaurs (birds) are highly social. On the other hand, social interactions are inherently difficult to preserve in the fossil record, so direct evidence is hard to come by. The two main ways that social behaviors can be inferred is through trackways and mass mortality sites.

Figure \(\PageIndex{1}\): Cal Orcko, Boliva is known for its enormous limestone wall that is covered by one of the longest stretches of dinosaur footprints ever found. In this picture, these sauropod tracks are seen starting in the left middle of the image then trending to the top of the wall.

Dinosaur track sites are known in many places around the world. Lots of useful information can be gained from them, including speed and gait. Some of them have enough tracks to represent multiple individuals and even multiple species. When this happens, paleontologists can infer social behaviors that would never be preserved otherwise. Each dinosaur group has a characteristic style of tracks, though in many cases, the exact dinosaur that laid them down is hard to distinguish. Trace fossils like tracks are also given their own nomenclature that follows the same rules as naming a biologic organism, complete with geniuses and species.

Figure \(\PageIndex{2}\): Theropod trackway from the Ardley Quarry. (Photo by J.J. Day.)

A good example of a dinosaur track site which has a reasonable interpretation at behavior is the Ardley Quarry in the UK. It not only shows a large group of animals walking the same direction (including possible prey and predators), but even different ages all together, similar to what is seen in large mammal herds today. This does not prove that dinosaur herds and modern mammal herds had the same structure, but it is a good indication. It has even been used to make evolutionary inferences about the extent of radiation of different dinosaur groups.

Another famous example is Dinosaur Stampede National Monument in Australia. There is still debate as to what exactly happened at this site, where a lone large individual was walking through the mud with many different smaller individuals. Regardless of the exact circumstances, the large variation in the smaller tracks give an indication that the group was a herd with different-aged animals.

Figure \(\PageIndex{3}\): Drawing of an Iguanodon skeleton by G. Lavette as it was found nearly articulated.

Mass mortality sites also help get at the behavior of dinosaurs, with the general thought that if they lived together, they died together. The first of these was discovered in Belgium in 1878 when nearly 40 articulated Iguanodons were found. In 1974, at least 14 Pachyrhinosaurus individuals were found together in an ancient Alberta riverbed, most likely washed up together when trying to cross a flooded river. There are countless others, but just to highlight some important ones: In 2008, a find in Inner Mongolia, China was announced that had more than 20 Sinornithomimus animals that were all immature sub-adults, between one and seven years old. It is proposed that they were a social group, perhaps left to their own devices while adults cared for the infants.

Figure \(\PageIndex{4}\): Deinonychus reconstruction (by Fred Wierum via Wikimedia.)

Deinonychus, the dinosaur that the raptors from the Jurassic Park movies were based on, has a lot of interesting evidence of social behavior. North American fossil sites have found a lot of individual animals of varied ages in concentrated sites. Also, they have been found in close proximity to a possible prey animal, called Tenontosaurus. Deinonychus was about 1/20th the size at best, so how did they bring them down? Many have speculated that this is an indication of pack hunting. Certainly, their proximity in the fossil record could have been related to scavenging, but pack hunting is an intriguing interpretation.

Figure \(\PageIndex{5}\): Maybe the greatest fossil find ever, from Mongolia: a Protoceratops and Velociraptor engaging in battle. The Velociraptor’s sickle claw is lodged in the side of its prey, while the Protoceratops grabs the Velociraptor’s hand in its mouth. (Photo by Matt Affolter.)

The most unambiguous dinosaur fossil depicting behavior is the famous fighting dinosaurs. Found in the Gobi in 1971, it is a fossil that has a Protoceratops and a Velociraptor (the smaller relative of Deinonychus) forever locked in combat. The Velociraptor has its killing claw lodged into the side of the Protoceratops, while it fights back by grabbing the attacker’s arm in its beak. There is nothing yet found that comes close to depicting behavior like this fossil, which is why many consider it to be the greatest find ever.

Rearing behavior

Another aspect of social behavior is parental investment. Many reptiles are not known for being great mothers and fathers, with sea turtles famously laying their eggs on a beach and leaving. The closest relatives of dinosaurs (crocodiles and especially birds), however, are noteworthy parents in the animal kingdom. So where do dinosaurs fall on this spectrum?

Figure \(\PageIndex{6}\): Citipati fossil found in a brooding position over eggs. (Photo by Dinoguy2 via Wikimedia.)

The first noteworthy discovery on this front was nests. In the early 1920s, an expedition led by Roy Chapman Andrews in Mongolia found what is believed to be the first dinosaur nest and eggs. Near the nest was a skeleton, described and named by Henry Osborn as Oviraptor. Initially, the nest was thought to belong to a common dinosaur in the stratum Protoceratops. Oviraptor, which means “egg thief,” was assumed to be stealing the eggs and eating them with its toothless beak. Even Osborn at the time cautioned the use of such an interpretation in its name, but it stuck. In the 1990s, more discoveries, like the related animal Citipati found fossilized in a brooding stance over a nest helped clear the image of Oviraptor, though it is too late to change the name.

Figure \(\PageIndex{7}\): Fossil reconstruction of a Maiasaura and hatchlings.

Maiasaura, the “good mother lizard,” with the femanine ‘saura’ used for “lizard,” was described by Jack Horner in the 1970s. Its discovery has done more to advance our understanding of parental behavior than any other dinosaur. Instead of finding just one dinosaur or one nest, an entire nesting ground was discovered, spaced closer together than the length of one adult, as some birds do today. The eggs were arranged in a spiral and placed on top of rotten vegetation to keep them warm (instead of using the parents’ warmth). The most important find was of hatchlings, which had underdeveloped legs which made walking unlikely, and yet, had worn teeth. It is estimated that the hatchlings might have spent a year in the nest, and although the mortality rate was high, it was the parents of these babies that fed them and protected them.