Fossil mammals and evolutionary change - stepping cautiously beyond the obvious
Friday seminar by Mikael Fortelius, University of Helsinki
The intuitive understanding that dental form has functional meaning can be traced back at least to Aristotle and is most likely of palaeolithic origin. It served Cuvier well as a foundation of the earliest scientific reconstructions of fossil mammals and was an important component in the thinking of early evolutionists, Darwinian and Lamarckian alike. The increasingly sophisticated methods that have been developed in the last decades to quantify and analyse dental form still rely on such intuitive understanding and analogue with simple mechanical tools, but when applied in a context of large fossil datasets with good spatial and temporal resolution they become powerful tools for analysing the deep time context and dynamics of mammalian evolution. The general term ‘ecometrics’ has been proposed for the use of functional traits in this manner, both for macroecological and macroevolutionary analysis. Mean hypsodonty has been used to compute rainfall (humidity) patterns in the global biosphere today and to map palaeoprecipitation in the geological past.
Recently several discoveries have been made that take ecometrics beyond the initial phase of charting simple patterns in space and time. Together with cutting edge count, mean hypsodonty allows mapping of primary productivity of ecosystems in the present as well as the past, and therefore allows mapping of palaeobiomes in humidity-temperature climate space. Ecometric analysis has revealed that the later Cenozoic shift towards drier and cooler climates drove trends in relative abundance and community structure favouring herbivores with a dental capability for feeding in circumstances conducive of increased tooth wear. It also shows that trophic level strongly influenced the pattern of regular rise and fall of species investigated over their entire temporal range. At the level of communities, dental ecometrics has shown that species assemblages (chronofaunas) show both a pattern of regular rise and fall and a shifting spatial distribution that reflects shifting climatic conditions at the continental scale. Very recently dental ecometrics that do not rely directly on functional analogy but simply relate holistic morphometrics to observed function have shown promise both for understanding the developmental side of dental evolution and for resolving morphologies that lack counterparts among living mammals.
Mikael Fortelius, Dept. of Geosciences and Geography, University of Helsinki