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FIGURE 7.9 Protoceratops (left) and Velociraptor (right) in close proximity to one another (nicknamed "The Fighting Dinosaurs") from the Late Cretaceous of the Gobi Desert, Mongolia. These specimens likely represent rapid burial and autochtonous fossils. Transparency No. 18973, Courtesy Department of Library Sciences, American Museum of Natural History.

FIGURE 7.9 Protoceratops (left) and Velociraptor (right) in close proximity to one another (nicknamed "The Fighting Dinosaurs") from the Late Cretaceous of the Gobi Desert, Mongolia. These specimens likely represent rapid burial and autochtonous fossils. Transparency No. 18973, Courtesy Department of Library Sciences, American Museum of Natural History.

Rapid burial of dinosaur remains has been mostly attributed to deposition by river floods, but sandstorms have also been proposed as a mechanism. The unusually well-preserved Late Cretaceous ceratopsians, ankylosaurs, and theropods of the Gobi region in Mongolia are often cited as examples of such rapid burial (Chapters 9, 12, and 13).

With reference to the Gobi, the most famous example of individual dinosaurs that were likely interacting with one another when they were buried comes from the Late Cretaceous. In this instance, the complete skeletons of the theropod Velociraptor and ceratopsian Protoceratops were found with the Velociraptor lying on its right lateral surface with a fore limb in the mouth region and a hind limb in the abdominal region of the Protoceratops. The latter dinosaur is oriented with its dorsal side up (in normal "life" position). This extraordinary co-occurrence, found by one of the Polish expeditions (Chapter 3), has been nicknamed "The Fighting Dinosaurs" and indeed such an interpretation is quite reasonable in the light of the evidence (Fig. 7.9). Another dinosaur that seemed to have been stuck in time, frozen in a life position from the Late Cretaceous, is a specimen of Oviraptor that shows a body posture consistent with that of a brooding female over a nest of eggs. Preservation of this posture implies that the dinosaur was likely buried while protecting its nest. Just to show that this find, as startling as it might seem, is probably not unusual in terms of the behavior it implies, a similar specimen of Oviraptor situated above a nest of eggs was also found in strata of the same age in Mongolia. Moreover, most of the skeleton of a small theropod from the Late

Cretaceous of Montana, Troodon, was found on a clutch of eggs that contained embryonic remains of the same species (Chapter 9).

The taphonomic problem with such apparent "snapshots" from the Mesozoic is that they show little to no signs of struggle from whatever burial process preserved the dinosaurs. In other words, if the dinosaurs in "life position" were buried alive, then the burial was so rapid that they died before they could react to being buried or they were unable to move once buried. The "fighting dinosaurs," the brooding Oviraptor, and other numerous, beautifully preserved Late Cretaceous dinosaurs of the Gobi all show that burial was instantaneous and death must have been immediately before or simultaneous with burial. The traditional explanation for the extraordinary preservation of the Gobi dinosaurs is that they were buried rapidly by fierce sandstorms, which still regularly pound the Gobi Desert today. Perhaps these modern dramatic changes in weather influenced such interpretations. Beginning with paleontologists of the American Museum expeditions of the 1920s, they would have found such storms a useful (and harrowing) modern analogue (Chapter 3). However, these interpretations incorporated little detailed sedimentological evidence that convincingly demonstrated sandstorms during the Late Cretaceous caused rapid burial of dinosaurs.

A recently proposed alternative hypothesis invokes the sudden deposition of wet sand (associated with rain storms) onto the hapless animals within alluvial fans. An equivalent volume of wet sand is much heavier than dry sand, which would have limited the movement of any animal trapped in it and allowed for a minimum of struggling before they succumbed to suffocation. Other evidence arguing against sandstorm burial is the lack of physical structures and sediments that typify wind-blown (eolian) deposits in the thick deposits containing some of the best dinosaur remains. Furthermore, many of the dinosaur tracks and invertebrate trace fossils that have been found in what are interpreted as eolian deposits indicate some breaks in sedimentation. These facies also lack abundant dinosaur body fossils. Consequently, the new hypothesis is that stable dunes existed first, where dinosaurs lived in fruitful abundance during times of moist climate, and these animals were buried later by alluvial fans caused by rainstorms. Although other facies in Late Cretaceous deposits of Mongolia clearly reflect wind deposition, probably from sandstorms, the rocks of undisputed eolian origin do not contain abundant skeletal remains, let alone the complete skeletons found in other facies.

Another way for individual dinosaurs to have accumulated and been preserved temporarily before burial would have been through deposition at the bottom of a lake, swamp, lagoon, or marsh. In this scenario, burial could have been slow in comparison to burial rates in other environments. Modern examples of these aquatic environments have more oxygenated water toward the air-water interface but less oxygenated (anaerobic) water toward the sediment-water interface. The stagnant nature of these environments allows little circulation of oxygenated water toward their bottoms. The anaerobic conditions prevent many animals from living at the bottom, preventing scavenging and aerobic bacterial decay. Accordingly, this results in a high degree of fossil articulation and, in some cases, soft-tissue preservation. For example, the small, well-preserved Lower Cretaceous theropods of China and Italy (Chapters 5 and 9) come from lacustrine and marsh deposits, respectively.

Even if dinosaur bodies or body parts were rapidly buried or buried slowly under anaerobic conditions, they had to be buried deeply enough that scavengers could not dig them up or they were not susceptible to being uncovered by rapidly moving water or wind. No doubt many dinosaurs died in the many catastrophic ways described previously in this chapter, but they did not ultimately make it into museums unless they reached the next taphonomic step, diagenesis.

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