New process helps unravel dinosaur bone chaos
Intermountain West is littered with dinosaur bones. In the rock layers of the Late Jurassic, from New Mexico to Montana, paleontologists have discovered deposits that resemble skeletal traffic jams.
Whether connected or mixed in a stack, the bones of prehistoric icons such as Allosaurus, Stegosaurus, Diplodocus, and more are often found in abundance – the result of Jurassic monsoon flooding that washed several individuals and species together in large piles, covering them with sediment that left them to petrify. What may seem like a scientific godsend, however, can quickly turn into Apatosaurus– a big headache for experts trying to decipher the details of prehistory from these osteological accumulations.
“How many dinosaurs are we looking at?” may seem like a simple question, but paleontologists know it differently. Each dinosaur skeleton, large or small, consists of 200 or more bones. As the layers of Upper Jurassic bone formed, these skeletons did not always stay connected (“articulated”) or close together (“associated”). Decomposition, scavengers and the force of floodwaters carrying sediment fragmented and dispersed the remains. In places such as the Cleveland-Lloyd Dinosaur Quarry in central Utah, there are no full articulated skeletons. Paleontologists believe the site contains the remains of at least 46 Allosaurus“Only because they identified 46 left femora, or thigh bones, of that species over there.
These estimates are only minima, however, as the left femurs of some animals are likely missing. A similar situation is also true for other prehistoric bone beds. “Until now, the primary attribution of bones to an individual has been based on whether the bones have been found articulated or associated,” says Kayleigh Wiersma-Weyand, a graduate student at the University of Bonn. Paleontologists generally assume that bones close to the same species and of comparable size belong to the same animal, but there has been no effective way to test this idea. Now, Wiersma-Weyand and his colleagues are proposing a solution, published in Palaeontologia Electronica: look inside the bones.
Wyoming’s Howe-Stephens and Howe Scott Quarries have long been hot spots for paleontologists. But like other reputedly productive Jurassic bone beds in the West, the remains of these rocks were strewn together before burial. However, by examining the microscopic cellular structure of the bones, the researchers were able to associate isolated bones with identified individuals.
Team members thinly sliced cores of bone from long-necked sauropod limbs for examination under a microscope. (The overall structure of bones can be preserved after this sampling process, if done carefully.) Their study is the first to combine several types of microstructure analysis to refine which bones go with which skeleton. The technique involves examining features such as growth lines, the number of openings for blood vessels in bone tissue, and circular structures where new tissue has grown to replace the old one.
“I think it’s a smart approach to a common problem,” says Adelphi University paleontologist Michael D’Emic, who was not involved in the new study. It can be difficult, especially in historical collections built decades ago, to tell whether a particular bone matches others found at the same site or whether it was buried as a single piece. Some dinosaur skeletons on display in museums have been reconstructed from several isolated bones from the same location, with no way of verifying whether all of these parts belonged to one animal or more. “This article opens up a new approach to determining which individual is which individual,” explains D’Emic – provided that museums allow the necessary bone sampling.
The new study builds on decades of research into how a dinosaur’s bones record its growth and life. This research focused on various bones from many places, says Wiersma-Weyand, “but now we can apply our general knowledge to specific deposits.
In the case of a sauropod nicknamed “Max”, for example, almost all of the bones were found in a disjointed pile. Two of the leg bones were still together – but did the other isolated bones belong to this Galeamopus? The researchers found that the structural details of articulated bones matched those of many disarticulated bones, suggesting they belonged to the same individual. But scientists also found that some bones, previously attributed to Max based solely on their appearance, actually belonged to other animals. Thus, they reduced Max’s precise skeleton more than 148 million years after the dinosaur’s death.
The study has certain limitations. “Different elements of a [single] skeletons have different biomechanical constraints and preserve slightly different biomechanical profiles, ”says Julia McHugh, a museum paleontologist in western Colorado, who was not involved in the new research. While acknowledging this, Wiersma-Weyand notes that sometimes her team’s process is more powerful in determining which bones are doing do not go together. Starting with bones that are still articulated or associated allows to define a basis for the allocation of additional bones. Multiple lines of microscopic analysis work best for testing a hypothesis that bones belong to the same animal.
Using a microscopic structure to identify which bones belong to which dinosaurs has applications beyond the best estimate of the number of individuals in a deposit, says McHugh – and possibly beyond dinosaurs, too. “It could be very useful in determining the age profiles of populations in individual bone beds,” whether it’s Jurassic dinosaurs or fossil mammals, she says.
This approach can also help reveal how these remains got to where they are, says Wiersma-Weyand. In a petrified river channel, for example, matching bones to specific dinosaurs can help paleontologists identify the direction in which the water flowed when the bones were buried. This is essential to reconstruct the formation of the bone banks and to determine whether they register one or more burials.
“It’s pretty exciting!” said Joseph Peterson, a paleontologist at the University of Wisconsin-Oshkosh, who was not involved in the study. “Being able to reconstruct how several disarticulated skeletons in conjunction with the environment in which they are buried would bring aspects of modern forensic and crime scene analysis to paleontology.