Could a predatory relationship between two ancient species reveal an early driving force of evolution? Absolutely, according to Mark Wilson, professor of geology at The College of Wooster, and Paul Taylor of The Natural History Museum in London.
In an article from the July issue of Geology, titled "Predatory Drill Holes and Partial Mortality in Devonian Colonial Metazoans," Wilson and Taylor explain how a 380-million-year-old animal, known as a hederellid, reacted to repeated attacks by an unknown assailant. Hederellids are extinct colonial animals that made skeletons of branching tubes. According to the two scientists, new evidence shows that hederellids responded to these predators, who drilled through their tubes (most likely with a radula-like device), by plugging the holes with skeletal patches secreted by internal tissues. They also closed off damaged sections with skeletal plugs.
"It's similar to the modular response in ships whose hulls have been punctured," says Wilson. "The hole can be repaired, or that area may be sealed off by bulkheads, and the ship, or in this case, the hederellid, continues to function."
Using an optical microscope, Wilson and Taylor examined approximately 350 hederellid colonies from paleontological collections of the Natural History Museum in London and one colony from the University of Michigan Museum of Paleontology. What they discovered was that almost all of the drill holes were along the upward-facing surfaces of the tubes, meaning that the predator penetrated from above. Furthermore, it is clear that the holes were patched while the hederellid was alive.
"We are convinced that these are predatory drill holes," say the two scientists, "because they: (1) were excavated from the outside, as shown by the evidence of incomplete drill holes; (2) damaged the living hederellid as shown by the existence of patch-like repairs; (3) always penetrated directly into the skeletal tubes; and (4) showed a preference for particular hederellid species, despite the availability of other hederellid species encrusting the same substrates."
What makes this discovery noteworthy is the age of the fossils (in the Paleozoic era), which provides the earliest evidence of this type of predator-prey relationship. "The drill holes in Devonian hederellids are the earliest unambiguous examples of selective predation on a colonial metazoan, and these may be the most ancient examples of a prey organism successfully repairing drilling damage to its skeleton," say Wilson and Taylor.
"One of the primary reasons to study predation in the fossil record is to ascertain whether predators were a significant selective force in the evolution of prey organisms and vice versa," say the authors. "In this case, we see a successful response of a prey species to the attention of predators. This supports the hypothesis of 'escalation' in the evolution of marine animals. It is an early example of an arms race between a prey animal evolving better ways to defend itself against a predator that is improving its own offensive abilities."
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