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JOHN D. ORCUTT AND SAMANTHA S. B. HOPKINS


ecosystems but by using the fossil and paleo- environmental records to trace chronoclines, following ecological change through time. By applying a four-dimensional perspective to ecology, biotic responses to environmental conditions that do not exist in modern eco- systems can be observed and potential causal factors that are currently tightly tied to one another can be teased apart as they vary through time. This approach has historically played a small part in our understanding of ecological drivers of biological trends, in large part because of the perceived incompleteness of the fossil record and inaccuracy of paleo- environmental reconstructions. However, many taxa are represented by very large fossil samples, and many regions have been the subject of rigorous paleoecological study, allowing robust reconstructions of trends along chronological transects and, at least in certain cases, the identification of causal factors. A great deal of paleontological research along these lines has focused on Cenozoic fossil mammals of North America, which are represented by an extremely rich fossil record that has been extensively collected for well over a century. These studies have, for the most part, tracked either mammal diversity (Lillegraven 1972; Alroy et al. 2000; Prothero 2004) or body size (Koch 1986; Alroy 1998; Gingerich 2003; Smith et al. 2010; Orcutt and Hopkins, 2013; Saarinen et al. 2014) through time. Others have examined patterns within the same variables at different intervals through time (Rose et al. 2011; Lyons et al. 2013; Fraser et al. 2014). Chronocline analysis is especially well


suited to address one of the longest-standing ecological questions: What drives mammal body-size evolution? This question was first raised by Bergmann (1847), who observed that latitudinal body-size gradients were visible within most mammal taxa at several taxo- nomic levels, with larger taxa or individuals tending to live at higher latitudes and smaller taxa or individuals living at lower latitudes. However, trying to tie body size to any other biotic or climatic variable has proven difficult. Bergmann himself suggested that the gradients he observed were a product of temperature, as large animals are better able to retain heat due


to their small surface area–to–volume ratio, while smaller animals are more effective at shedding it. Some analyses, most notably that of Geist (1987), have suggested that not only is Bergmann’s rule sensu stricto invalid but that the monotonic latitudinal body-mass gradients on which it is based do not exist. However, other studies have confirmed the patterns observed by Bergmann, finding body-size gradients within most mammal taxa (Ashton et al. 2000; Meiri and Dayan 2003; Blackburn and Hawkins 2004) and faunas (Rodríguez et al. 2008). While some authors have supported Bergmann’s rule sensu stricto, others have suggested that other ecological variables play a more direct role than tempera- ture in driving body-size evolution. Some of the proposed mechanisms posit biotic drivers. Primary productivity may limit the size to which herbivores can grow (Rosenzweig 1968), while the size and abundance of prey may influence body size in predators (McNab 1970; Erlinge 1987). Size trends in island taxa suggest that competition may play an important role in shaping body-mass patterns, but the effects of competition appear to vary between size classes (Damuth 1993), while predation pres- sure may select for larger prey taxa (Korpimäki and Norrdahl 1989). Besides temperature, two other climatic variables have been posited to play a major role in body-size evolution: precipitation (large animals have a greater capacity for storing water and will be selected for in arid climates; James 1970) and season- ality (large animals have a greater capacity for fat reserves and will be selected for in seasonal climates; Millar and Hickling 1990). Several paleontological studies have tested


Bergmann’s rule, either explicitly or indirectly (Gingerich 2003; Smith et al. 2010; Meachen and Samuels 2012; Lovegrove and Mowoe 2013; Lyons and Smith 2013; Orcutt and Hopkins, 2013; Saarinen et al. 2014). These studies have ranged fromlocal to global in scope and, as with neontological analyses, have reached divergent conclusions. Gingerich (2003) examined condy- larth and perissodactyl body-mass trends across the Paleocene/Eocene Boundary inWyoming’s Bighorn Basin, finding that all the taxa in question showed body-mass spikes during the Paleocene–Eocene Thermal Maximum (PETM).


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