That biological complexity has increased through geological time seems almost too obvious to be worth stating: Once there was bacteria, now there is New York city! When roughly surveying the tree of life, it appears that this complexity has slowly accumulated through time, giving the impression of an unstoppable “evolutionary law”. But how complexity appears within lineages is surprisingly tricky to address because complexity itself is fiendishly difficult to measure. In this work we use the serially-repeating elements of the vertebral column to directly measure complexity in mammals, whose vertebrae are especially complex, as an example of its evolutionary acquisition. Based on exceptionally-preserved fossils, we look back at the forerunners of mammals to ask: How did their backbone become so complex?
While the vertebral column may be the perfect tool for measuring complexity, the remains of complete vertebral columns from transitional fossils are extremely rare! Therefore, we traveled all over the world to hunt down fossil vertebrae in museum collections, including collections in Europe, South Africa, and North America. Using microCT technology and painstaking manual segmentation, we studied the morphology of individual vertebrae from animals spanning “Pelycosaurs”, the mammal ancestors that most closely resemble basal amniotes, to cynodonts, the group from which mammals themselves emerged. Using this dataset, we are addressing many interesting questions about how the mammalian backbone functioned and evolved (for example, how did mammal regions arise?).
In this study we tested seven alternative hypotheses for how the vertebral column of mammal may have become complex. To account for the sparseness of the fossil record, we applied a statistical approach that takes taxonomic sampling into account while testing each hypothesis. The traditional idea of gradual and steady increase could be easily rejected based on the data. Instead our data point to distinct evolutionary steps in complexity at multiple points in the tree, and two separate lines of evidence suggest a link between complexity and the evolution of high activity levels in mammals.
First, the increase in complexity in the fossil record coincides with evidence for the evolution of endothermy and elevated activity in Triassic cynodonts. Second, the degree of vertebral complexity in living mammals correlates with their basal metabolic rate, a measure of their level of activity. Together these data support the hypothesis that evolution of the axial skeleton may be influenced by its dual function in breathing and locomotion. The specialization of the mammalian respiratory system for increased activity in the Triassic allowed separation of these functions, providing new opportunities for vertebral complexity and adaptation. Therefore, broad-scale evolutionary trends may belie much more complicated patterns on specific branches of the tree of life.
Cover image: Edaphosaurus, Museum of Comparative Zoology, Harvard University