Since living birds are living dinosaurs, we know that modern dinosaurs often travel in flocks, exhibiting a complex social structure. But did extinct, non-avian dinosaurs do the same? Over the last several decades, numerous claims have been made that they did, but finding unequivocal evidence of herding in the fossil record is difficult.
The most definitive evidence comes from sequences of fossilized footprints called trackways. Several track sites have now been found that suggest herding behavior in some groups of dinosaurs. At the Davenport Ranch site outside San Antonio, Texas, R. T. Bird of AMNH discovered an amazing set of sauropods tracks in 1940. Detailed analyses of these trackways reveal a herd of 23 sauropods rambling over muddy ground at a pace of about 2 meters per second. The pattern of overlapping tracks indicates that the largest adult sauropods led the herd, followed by smaller individuals.
Other trackways in Colorado and Korea show herds of large sauropods moving side-by-side in a horizontal front. At one site in Colorado, the animals' paths undulate from left to right in unison.
Trackways of meat-eating theropods are less common, but one occurrence is Bird's specimen from Glen Rose, Texas, a part of which is on display at AMNH. In all, the Paluxy River trackways, some of which were sent to other institutions, reveal the footprints of at least 12 sauropods and at least three, large, carnivorous dinosaurs moving along a mudflat above the waterline. But it is unclear how long after the sauropods passed that the theropods followed or if they moved in a pack. However, in Bolivia, paleontologist Giusseppe Leonardi discovered eight parallel trackways of six adult and two juvenile sauropods moving along the shore in a front, followed shortly afterward by a pack of at least 32 medium sized, carnivorous theropods, moving in the same general direction along the same front.