Variety is the Spice of Life

This past week, I attended the 72nd Society of Vertebrate Paleontology meeting in Raleigh, North Carolina. I’m still on the mend from the science hangover. Still, while my memories are relatively fresh, I want to mention one of the important themes that ran through some of the meeting’s sessions.

Paleontologists are drawing more information from prehistoric bones than ever before. A fossil femur or petrified pelvis isn’t just an anatomical object, but a time capsule entirely composed of more subtle clues about ancient life. Among other lines of evidence, a bone’s microstructure – or histology – preserves clues about an individual animal’s growth and physiology. In the ongoing discussion about dinosaur biology, for example, histology has helped paleontologists better understand how quickly Triceratops and company grew up, and how radically the fossil celebrities changed as they aged.

As informative as histology can be, though, the conclusions we draw from bone must be tempered by how much remains unknown. As paleontologist Sarah Werning aptly pointed out in her presentation on marsupial histology, what we know about mammal growth is constrained by an unbalanced dataset biased towards animals of economic important and zoo animals. Some groups, such as marsupials, have been almost entirely ignored, and this inhibits our ability to understand basic aspects of mammal biology.

How, then, can we accurately interpret prehistoric evidence when we don’t fully understand living species? Consider a study on mammal bone published this year regarding regular slowdowns in growth, recorded in parts of the skeleton as rings called “lines of arrested growth” (LAGs). These features were previously thought to be a characteristic of ectothermic animals – crocodiles, lizards, and the like – whose body temperatures are regulated by the surrounding environment. Since dinosaur bones show LAGs, too, some researchers made the case that the great saurischian and ornithischian tribes were either ectothermic or had some kind of metabolism halfway between the “typical” mammal and reptile. Yet, as the new study showed, even endothermic mammals lay down LAGs in response to cold or dry seasons, and this therefore obliterates the tie between LAGs and a set physiological profile. If the present is truly the key to the past, we need to do a much better job of sampling modern animals.

But even among fossil creatures, paleontologists should make a concerted effort to cut into more bones. In a presentation of research conducted with Werning, graduate student Zachary Morris highlighted the extreme variability in a weird, dragon-like Triassic creature called Vancleavea. The type of bone tissue in each of the fossils, the number of LAGs, and the growth patterns varied across the seven individual animals that were sampled. There was no single biological profile that was consistent within the species. If paleontologists only sampled one femur or humerus, they might come to broad conclusions about how Vancleavea lived and totally miss the animal’s inherent variability.

Other sessions and posters on paleohistology also underscored how meager our sample sizes are. We’re really only just beginning to draw out the details of prehistoric lives from bone. Rather than reconstructing prehistoric lives on the basis of a single sectioned bone, paleontologists should take as many samples as possible to examine variation within a population or species. After all, variation is grist for the evolutionary mill, and we cannot hope to fully understand the evolution of life on earth if we preoccupy ourselves with forming new typologies on the basis of single specimens. Variation is an essential part of paleobiology, and I hope that researchers continue to cut, grind, and zoom in on bone to better understand the true nature of vanished life.

[On a tangential note, I appreciate that the organizers of SVP stepped up to advertise the #2012SVP Twitter hashtag, and, for most of the conference, arranged for free wi-fi. I’m thrilled to see SVP become a more open conference where experts can communicate freely with each other and the public. I hope that next year’s meeting in Los Angeles sees even more paleontologists online.]