Predator and prey
Dan Quinsey (UK)
Some fossils show evidence of the violent relationships between predator and prey. Below is a brief discussion of a few of such fossils – fossils that seem to preserve the moment when a carnivore has killed its victim or scavenged a dead body for food.
Modern mammals, fish, and reptiles feed on squid and octopus. Therefore, it can safely be assumed that ancient reptiles and fish fed on their ancient relatives – the ammonites. In fact, ammonites exhibiting bite marks are not uncommon. However, such bite marks are often mistaken by the collector as being just the result of random brakes in the shells. On the other hand, there have been claims of hundreds of ammonites with the preserved tooth marks of mosasaurs. Most of these claims are false. The marks on most of the shells are holes bored or dissolved in the ammonite by limpets or other forms of gastropods, rather then holes made from the bite of such reptiles.
The Placenticeras specimen, on the right, is a fine example of a mosasaur-bitten ammonite. The tooth marks have been left in the phragmocone (chambered portion) of the shell. Here, the septa still supports the surrounding shell as the bite did not result in the wide scale collapse of the shell around the holes. The edges of the holes show an irregular, slightly broken shape. This is very different from the smooth, round holes caused by limpet borings as seen in the Placenticeras ammonite on the left. Limpet (inset) holes show a slightly dissolved and smooth edge to the shell around the holes. Limpets and other gastropods that bore through shells stop dissolving the shell when they get through its the surface and reach the septa. They do not dissolve the underlying septa, as there is nothing further to gain by doing so.
Crinoids possess an endoskeleton composed of calcareous plates covered by a thin epidermis. Each plate is a single, very porous calcite crystal. Unfused plates are held together with ligaments or muscles. The skeleton of a crinoid that lives on the sea floor may be divided into four basic parts: the holdfast that anchors the crinoid to the ocean bottom; the stem that raises the calyx above the substrate; the calyx that contains the internal organs; and from five to as many as 200 feeding arms that gather food.
Fossil crinoids are occasionally preserved with another organism attached, commonly a brittlestar entwined around the crown or near the anal pyramid. There is some debate among palaeontologists regarding this attachment. Some believe the relationship was commensal, meaning the brittlestar lived with the crinoid and consumed faecal pellets excreted by the crinoid as waste. However, other palaeontologists believe the brittlestar may have been predatory and fed on the crinoids themselves.
Tooth-damaged dinosaur bone can be recognized by distinctive markings such as grooves or punctures. Although some damage may have been inflicted during dominance fights, most bite marks probably indicate carnivore activity. Identification of damaged bone can tell us that a particular species of dinosaur was eaten, but it generally does not indicate whether the prey was hunted and killed or opportunistically scavenged. However, in some cases, it may be possible to associate different tooth marks with specific predator activities based on the types and distribution of damage. For example, multiple bite marks on the ends of sauropod limb bones are more likely to represent feeding traces as opposed to assault wounds.
The identity of the animal responsible for bite marks is usually difficult to determine because many Mesozoic vertebrates (including crocodiles) were capable of causing generalized tooth damage to bone. Fortunately, well-preserved tooth marks can occasionally exhibit distinctive shapes, spacing, and/or serration marks that allow comparisons with fossil jaws of contemporaneous carnivores.
Parallel striation marks on the hadrosaur bone fragment (above left) indicate that the predator responsible for making these marks had serrated teeth as shown on the albertosaur tooth specimen.
Even more dramatic are the very rare examples of dinosaur teeth actually stuck in the bones of their prey. In Montana, a tyrannosaurid tooth was found embedded in a Hypacrosaurus fibula, providing more indisputable evidence of carnivore activity.