Possible Causes of Injury Poor Health and Death in Dinosaurs

Typical, popular depictions of dinosaur deaths are melodramatic and limited to two scenarios: mortal combat with other dinosaurs or a direct hit from an asteroid. The latter scenario has been added only recently, dating from 1980 (Chapter 16). Dinosaur deaths, depicted in fiction or other re-enactments, usually focus on dinosaur-versus-dinosaur conflicts, except where anachronistic humans are included to finish them off in some creative way (Chapter 1). But has anyone ever seen a depiction of a dinosaur dying from eating a toxic plant? Or being stung to death by Mesozoic bees and wasps? Or tripping over a log and falling off a cliff?

Or expiring from old age? Once the mind is opened to the realm of possibilities for death, and these are compared to recorded information on actual modes of death for vertebrates, the most likely causes rarely match preconceived notions.

Application of scientific methods can soon show that a hypothesis for a proposed cause of death can be tested, at least partially, for its probability through observations made in modern, natural settings. For example, predation, the hunting of a live animal (prey) by another (predator) for the purposes of killing and eating that animal, has been overstated in its importance in the deaths of terrestrial vertebrates. A study of modern hyena predation in Africa reported that only 1% to 2% of all animals in a prey population actually died as a result of hyenas hunting them. The same application of actual data to causes of human deaths can also yield surprising results. Despite the fear many people have of dying from bites by poisonous snakes, factual information shows that many more people die each year from allergic reactions to bee-stings (hence the previous allusion to dinosaurs dying in the same manner). Similarly, despite much fear in the USA of terrorist attacks, the actual risks of injury or death from this possible source of harm are still far lower than the odds of dying from accidental electric shock, drowning in a swimming pool, or being maimed or killed in a car accident, especially for people living outside of major cities.

For individual dinosaurs, here are some possible causes of death and the evidence supporting these causes:

■ Suffocation by ash and gases from volcanic eruptions, getting stuck in a muddy watering hole, or rapid burial by mass movements of sediment. Evidence: Large accumulations of mostly one species of dinosaur, such as the ornithopod Hypacrosaurus, theropod Allosaurus, and neoceratopsian Psittacosaurus, have been found in near-complete condition in volcanic ash or muddy deposits, respectively; dinosaurs seemingly "frozen" in life-like positions occur in sandy deposits, such as a specimen of the theropod Oviraptor sitting on its nest, or specimens of the theropod Velociraptor and ceratop-sian Protoceratops intertwined with one another.

■ Intraspecific competition, where dinosaurs fought with individuals of the same species for food, mates, or territory.

Evidence: Toothmarks or other bone injuries, seen in some tyrannosaurids and ceratopsians, suggest they were caused by the same species of dinosaur.

■ Predation, where a dinosaur died from injuries inflicted by a predator. Evidence: Remains of a dinosaur have been found in the abdominal area of another animal; toothmarks or dislodged teeth in bones inflicted by a different species of dinosaur (such as tyrannosaurid toothmarks in the ceratopsian Triceratops or Edmontosaurus) or by other potential predators, such as crocodiles (Fig. 7.1).

■ Drowning, either in a river or an open body of water such as a lake or delta. Evidence: Mass accumulations of dinosaurs of the same species (the thero-pod Coelophysis or some ornithopods, such as hadrosaurs, and some sauropods) or nearly complete individual dinosaurs of differing species are preserved in a river or deltaic deposit.

■ Pathogenic conditions, ailments caused by diseases (such as bacterial or viral infections), imbalanced nutrition, or other environmental stresses. Evidence: Diseases are indicated by bone overgrowths in Allosaurus and Triceratops, bone cancer in Allosaurus, or arthritis (gout) and fungal infections in Tyrannosaurus.

■ Injuries (of uncertain causes).

Evidence: Stress fractures are seen in phalanges of ceratopsians and sauropods, fused caudal vertebrae occur in ankylosaurs, and some theropod

FIGURE 7.1 Crocodile toothmarks in hadrosaur dorsal vertebra, Aguja Formation, Late Cretaceous, west Texas. Whether these trace fossils represent predation or scavenging of the hadrosaur is currently unknown. Photograph by Stephen W. Henderson.

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