November 3, 2012
Cope’s Rule Applies To Some Dinosaurs, But Not Others
Alan McStravick for redOrbit.com - Your Universe Online
Before noted paleontologist and ichthyologist Edward Cope passed away in 1897, he had devised a theory that has, to this day, its proponents and detractors. His theory, known today as Cope´s Rule, stated that animals will, in their own voyage through the process of evolution, grow ever larger. This evolutionary trend has been noted across the animal kingdom. Researchers from the National Museum of Natural History (NMNH) want to explore Cope´s Law on a group of animals that haven´t lived on this Earth for some time.
Gene Hunt and other researchers from the NMNH in Washington, D.C. are testing Cope´s Rule, via some of the latest statistical modeling methods, to see if and how it might have applied to dinosaurs. From research conducted, they have found that Edward Cope and his rule were absolutely right. Except when it wasn´t.
Cope´s rule has long been demonstrated in many instances, though it has been recognized that it doesn´t necessarily hold true at all taxonomic levels. While a larger body size is typically associated with an increased fitness of an animal to its particular environment, there are also some disadvantages that have been recognized, both on an individual and a clade level. Clades that are comprised of larger individuals are more prone to extinction. This fact may act to limit the maximum size of an organism.
As a clade is subjected to this selectional pressure, a series of advantages are presented, both in terms of mating success and survival rate. As an example, larger organisms tend to have fewer predators and find it easier to catch and kill their own prey or competitors. Other benefits, such as better thermal efficiency, increased intelligence and a longer lifespan have also been noted.
But all is not wine and roses for the larger species. The larger an organism grows the more food and water it requires in order to survive. Also, as size increases in a species, it will inevitably shift from r to K-selection.
In the field of ecology, the r/K selection theory relates to the selection of combinations of traits in an organism that tradeoff between the quantity and the quality of the offspring produced. The r-selection is typically reserved for organisms that live in unstable environments. These organisms will produce many offspring and with greater frequency than their K counterparts in an effort to propagate the species. K-selection exists typically in larger animals that reside in more stable environments. There is much more of a parental investment required for a K-selection offspring. Biologically speaking, neither mode of propagation is considered superior to the other.
Specific to mammals, Blaire Van Valkenburgh of UCLA stated, “Cope´s rule, or the evolutionary trend toward larger body size, is common among mammals.” She continues, addressing larger carnivores, ““¦interspecific competition for food tends to be relatively intense, and bigger species tend to dominate and kill smaller competitors. Progenitors of hypercarniverous lineages may have started as relatively small-bodied scavengers of large carcasses, similar to foxes and coyotes, with selection favoring both larger size and enhanced craniodental adaptations for meat eating.”
Cope´s rule is not without its detractors, however. Many paleobiologists have been and remain skeptical, stating the rule may merely represent a statistical artifact. Many purported examples of Cope´s rule will often assume that the stratigraphic age of a fossil is proportional to its clade rank, a measure of how far derived they are from their ancestral state. Paleobiologists have claimed this relationship is, in fact, quite weak. They also point out that counterexamples to Cope´s rule exist and are common throughout geological time. While an increase in size occurs more often than not, it is not a universal trend.
The idea that, if left unfettered, the trend of ever-larger size would produce organisms of gargantuan proportions does introduce the fact that there must be some factors that present themselves to limit the process. One such theory states that a clade´s increase in size could produce organisms so large that it would leave it vulnerable to extinction. Add to this certain physically imposed limits to size of organisms. For example, insects must stay small enough for oxygen to diffuse to all parts of their bodies, while flying birds must remain light enough to fly.
Hunt, who is also the curator for the Department of Paleobiology at the NMNH stated, "For a long time, dinosaurs were thought to be the example of Cope's Rule." Hunt points out other animal groups, mammals in particular, offer viable proof of the rule, as well.
Hunt, along with colleagues Richard FitzJohn of the University of British Columbia and Matthew Carrano, also of the NMNH, focused their research by utilizing the femurs of dinosaurs to establish animal size. The data they were able to retrieve from the femurs was then used in their statistical modeling. The team was looking specifically for two things: the directional trends in size over time and whether or not there were any detectable upper limits for the dinosaurs body size.
"What we did then was explore how constant a rule is this Cope's Rule trend within dinosaurs," said Hunt. They did this by exploring the historical record of dinosaurs, finding some clades of dinosaurs did trend larger over time. In particular, ceratopsids and hadrosaurs show significant increases in size rather than decreases over time. And while birds are known to have evolved from theropod dinosaurs, the team intentionally excluded them from the study because they recognized the evolutionary pressure birds were faced with in order to lighten up and decrease in size, allowing them to fly more effectively.
The team, focusing on the upper limits to size, found that Cope´s rule applied sometimes but not others. Sauropods, which were four-legged and ornithopods presented no discernible indication to exactly how large they might have been able to evolve. These two groups contained the largest land animals that have ever lived on Earth.
The theropods, whose most famous member was the Tyrannosaurus rex, did present a limit to how large they might have eventually evolved. But, as Hunt says, this is not particularly surprising because the theropods were bipedal and there are physical limitations placed on how massive a creature can grow while still being able to move around on two legs.
Hunt concedes that why Cope´s Rule works at all is really not very well understood. “It does happen sometimes, but not always.” He goes on to say that the traditional idea that somehow “bigger is better” because a bigger animal is less likely to be preyed upon is plainly naÃ¯ve. This is because before a creature was able to evolve into the larger and even gigantic versions of itself, it started off as a small animal that was, for several evolutionary generations, vulnerable to climatic variations and larger predators.