Paleontologists have long been fascinated by bizarre structures on prehistoric animals. The horns of Styracosaurus, the sail of Dimetrodon, the crest of Tupuxuara and more—these odd ornaments raise the questions, "what were those structures used for, and how did they evolve?" In a recent review of dinosaur evolution, paleontologists Kevin Padian and Jack Horner suggested that such structures—among dinosaurs, at least—often had more to do with species recognition than anything else. But another new study, by Joseph Tomkins, Natasha LeBas, Mark Witton, David Martill and Stuart Humphries in the journal American Naturalist, proposes that conspicuous ornaments on other prehistoric animals may have played a role in how prehistoric animals paired up.
Among the most popular hypotheses for the evolution of big, showy structures has been that they were used in thermoregulation (in other words, to regulate body temperature). By turning its huge sail towards the morning sun, for example, the Dimetrodon would have been able to warm up faster than its prey—giving it an edge while herbivores were still sluggish—and it could have turned its sail out of the sun's rays to dump excess heat during the warmest parts of the day. Interestingly, though, one of its herbivorous cousins—Edaphosaurus—had a similar sail, as have other animals during the course of prehistory (including the dinosaurs Spinosaurus and Ouranosaurus). The crests of pterosaurs have also proven to be enigmatic. Their origin and function have been attributed to everything from thermoregulation to sexual selection to their use as rudders to steer, but which of these ideas is correct?
The paleontologists behind the new study looked at published measurements of the fossil genera Dimetrodon (a synapsid) and Pteranodon longiceps (a pterosaur). Enough specimens of both have been found to see how certain traits—particularly their ornaments—differ, and the scientists looked at how the shape of those structures changed with increasing body size. If the sails of the synapsids and the crest of the pterosaur seemed to become proportionally bigger on larger animals and two different sexes could be distinguished on the basis of the extents to which the traits were expressed, then it would be likely that the evolution of the traits can at least be partially attributed to sexual selection—the evolution of particular traits due to competition for mates.
The results of the study appeared to confirm that thermoregulation was not the driving factor behind the evolution of the structures in question. In the case of Pteranodon, the crests of bigger individuals were disproportionately large for their size, a finding in accord with what the researchers expected under the sexual selection hypothesis. Figuring out the relationship between sail size and body size in Dimetrodon, however, was a little more difficult. There were not enough individuals of any one species to compare to one another, so instead the paleontologists compared measurements taken from seven species. In a similar fashion to Pteranodon, larger species of Dimetrodon had increasingly larger sails compared to their body size, which the authors took to suggest that this trait, too, was sexually selected. (They also extended this trend to the sail-backed synapsid Edaphosaurus, which was not actually examined in this paper.)
The trouble with detecting the influence of sexual selection in the fossil record, however, is that it is difficult to associate it with sexual dimorphism, or significant difference in form between males and females. If a trait was sexually selected it should appear more prominent in one sex than the other, but determining the sex of extinct animals is very tricky. It is usually assumed that individuals with spectacular ornaments are males, but specimens that are be assumed to be female might really be a juvenile or even a member of a different species. Nevertheless, the scientists state that the degree of crest development in large Pteranodon longiceps is so strong that it only can be understood as a result of sexual selection, especially as other competing hypotheses (such as its use as a rudder) do not hold up to scrutiny. As far as Dimetrodon goes, the authors reject the hypothesis that the sails evolved for the purpose of thermoregulation because even small individuals—animals so small that the sails would not have provided any benefit for heating up quickly—have large sails, and so the increasingly large sails on increasingly large animals is again interpreted as a sign of sexual selection.
What the authors conclude from all this is that the crest of Pteranodon and the sails of Dimetrodon and Edaphosaurus did not evolve to help these animals regulate their body temperature, but were instead display ornaments possibly used in the competition for mates. Just how these animals used these organs to compete with each other is not touched on in the study—though, in a BBC news story, co-author Joseph Tomkins offer some speculations—and further research will be required to further test these hypotheses. If males and females from individual species of Pteranodon and Dimetrodon could be distinguished, for example, it would be a wonderful test of the hypotheses summarized in the new paper, but such a study requires a more complete sample of these extinct animals than we presently have.
Tomkins, J., LeBas, N., Witton, M., Martill, D., & Humphries, S. (2010). Positive Allometry and the Prehistory of Sexual Selection The American Naturalist DOI: 10.1086/653001