Abstract
Changes in body size have been the subject of numerous palaeontological and neontological studies, but despite several general postulated "rules", the underlying processes controlling them are still incompletely understood, and their broad applicability is debated. Here we utilise morphological and ecological data from the Jurassic marine bivalve Chlamys textoria (Schlotheim, 1820) to analyse spatial and temporal trends in body size and ornamentation. We find: (1) fluctuations in body size during the Jurassic and no support for Cope's rule (the tendency to increase body size over geological time within an individual lineage); (2) a gradual increase in the average height to length ratio of the valves during the Jurassic. In the absence of any obvious adaptive advantage we suggest genetic drift as the causal mechanism; (3) a significantly larger mean body size in mid-palaeolatitudes than in the Jurassic tropics, providing evidence for the validity of Bergmann's rule (the assertion that body mass increases with latitude); and (4) a complex relationship between the number of plicae and the environment, which we explain as an improvement towards camouflaging the shell. doi:10.1002/mmng.201200002
Highlights
Body size is among the most obvious characteristics of an organism, and it is closely related to the biotic and abiotic environment as well as life-history traits
Extra-tropical specimens are larger than those from the tropics on average, and – in accordance with Bergmann’s rule – we find some support for a significant positive linear trend of body size across latitude
Detailed analysis suggests that the established patterns and trends reflect true biological/evolutionary signals and are not artifacts of an uneven representation of environmental and lithological categories in the data set
Summary
Body size is among the most obvious characteristics of an organism, and it is closely related to the biotic and abiotic environment as well as life-history traits. Numerous museum collections of fossil marine invertebrates facilitate body-size research over broad temporal intervals, making it possible to recognize distinct patterns in the evolution of body size Because they are characterised by high preservation potential, global distribution, and high abundance, fossil marine bivalves have been, and continue to be, the subject of numerous studies on body size In contrast, focuses on a single bivalve species: the Jurassic marine pectinid Chlamys textoria (Schlotheim, 1820; Fig. 1) This should be seen as an attempt to minimise taxonomic “noise” by considering a single species that is likely to exhibit similar changes in body size in response to ecological stimuli throughout its range. In a study on the influence of temperature on body size, there might be no visible trend because of differing long-term morphologic/evolutionary responses of the various species studied
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