Horns, Clubs, Plates and Spikes: How Did They Evolve?

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As a group, dinosaurs were certainly well-ornamented animals. Horns, spikes, crests, plates, sails, clubs and other strange structures marked the bodies of many dinosaurs, but figuring out why these dinosaurs had these structures in the first place has often been difficult to figure out. Numerous hypotheses for different structures have been proposed over the years. Were the horns of Triceratops used for defense, one-on-one combat between members of the species, or as a way to identify members of a group? Were the plates along the back of Stegosaurus just for display, or did they play some role in regulating body temperature? Were the crests of some hadrosaurs used as snorkels, or did they allow the dinosaurs to make low calls which resonated across the landscape?

Some of these ideas—such as the snorkeling hadrosaurs—have been abandoned over the years, but in many cases the strange features of dinosaurs remain mysterious. More than that, the reason such features might have evolved in the first place is often unclear, and as paleontologists Kevin Padian and Jack Horner point out in a new review of these structures published in the Journal of Zoology, no hypothesis can be taken as a default explanation for why a certain kind of structure evolved. Instead, the paleontologists propose, a new approach must me taken—one that explicitly views dinosaurs within their evolutionary context.

Figuring out the function and origin of a particular structure is a complicated process. Something like the horns of a Styracosaurus may have been used for both defense and social displays, for example, but even if the functions of the horns can be identified it does not mean that horns originally evolved for these reasons. Instead horns may have evolved due to one kind of evolutionary pressure and been co-opted for another at a different time, so there can be a disparity between why a structure evolved and what it ends up being used for. This is why understanding the evolutionary history of a particular lineage of dinosaurs is so important.

After looking at groups of dinosaurs famous for having strange structures—such as ankylosaurs and the horned dinosaurs—Horner and Padian identified only weak trends. The latest ankylosaurs were better armored than the earliest ankylosaurs, for example, but the patterns of armor varied so widely among the later forms that it seems as if display may have been more important than defense. If defense were the sole factor in determining the pattern of ankylosaur armor then it might be expected that different species would show very similar arrangements that were optimized for protection against predators, but the variation suggests that defense was not the sole factor shaping ankylosaur armor. Similarly, even though some horned dinosaurs almost certainly did lock horns in combat, there is no sign that horns evolved for this purpose—the dinosaurs' ability to joust with each other was the consequence of having horns evolved for another reason.

What Padian and Horner propose is that species recognition might have played a more important role in the evolution of strange structures than has otherwise been appreciated. Strange structures may have begun to evolve to allow members of a species to identify each other, particularly potential mates, and only later been co-opted for other uses. If this is correct, they predict, then the pattern of evolutionary change should not have a straightforward direction. If the armor of ankylosaurs had evolved solely for defense, for example, we would expect to see a straightforward evolutionary trajectory in which the protective function of the armor gets better and better over time with little variation. If species recognition was more important, however, the pattern would increasingly vary as it would only be important for species to differ from one another. Additionally, this hypothesis would be strengthened if several closely-related species were living in the same place at the same time and their structures showed divergence into new forms, making it easier to tell species apart.

According to Padian and Horner, the overall evolutionary pictures of many groups of dinosaurs is consistent with their hypothesis, but the paper focuses on proposing a new way of looking at the fossil record rather than providing flat answers. The reexamination of old material and the discovery of new fossils will be essential to testing their ideas, especially as more specimens of rare dinosaur species are uncovered. (Relatively few dinosaur species are well-represented enough to look at these patterns, especially among theropod dinosaurs.) Furthermore, it is still worthwhile to try to determine the function of structures in particular dinosaur species. If the mysteries of these structures can be unlocked and then viewed in the context of the dinosaur evolutionary tree then it may become possible to gain insight into how those structures originated and changed over time. This is not something that can be accomplished in a year or even a decade, but as we learn more about each dinosaur species we can gain a greater appreciation for the patterns which marked their evolution.

Padian, K., & Horner, J. (2010). The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? Journal of Zoology DOI: 10.1111/j.1469-7998.2010.00719.x

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