Do brachiopods show substrate‐related phenotypic variation? A case study from the Burgess Shale

As sessile, benthic filter feeders, brachiopods share an intimate relationship with their chosen substrate. Individuals of Micromitra burgessensis in the Burgess Shale Formation are preserved in life position, attached to a range of hard substrates, including skeletal debris, conspecific brachiopods, sponges and enigmatic tubes. Here we investigate the phenotypic variability of M. burgessensis associated with differing substrate attachments. We apply geometric morphometrics to test for variation by plotting landmarks on the exterior of ventral and dorsal valves of M. burgessensis specimens that are preserved attached to different substrates. Using principal component, canonical variate analyses and anova , we determine that there is some variation in shape related to substrate. Canonical variate analyses, for ventral valves and dorsal valves, indicate that specimens attached to the same substrate are recognizable in shape from specimens attached to other substrate types. The strength of differentiation however, is not robust and combined with our discriminate analysis of separate populations suggests that there is the potential for substrates to exercise only weak control over the morphology of Brachiopoda.     

Studying morphological integration and modularity at multiple levels: concepts and analysis

Although most studies on integration and modularity have focused on variation among individuals within populations or species, this is not the only level of variation for which integration and modularity exist. Multiple levels of biological variation originate from distinct sources: genetic variation, phenotypic plasticity resulting from environmental heterogeneity, fluctuating asymmetry from random developmental variation and, at the interpopulation or interspecific levels, evolutionary change. The processes that produce variation at all these levels can impart integration or modularity on the covariance structure among morphological traits. In turn, studies of the patterns of integration and modularity can inform about the underlying processes. In particular, the methods of geometric morphometrics offer many advantages for such studies because they can characterize the patterns of morphological variation in great detail and maintain the anatomical context of the structures under study. This paper reviews biological concepts and analytical methods for characterizing patterns of variation and for comparing across levels. Because research comparing patterns across level has only just begun, there are relatively few results, generalizations are difficult and many biological and statistical questions remain unanswered. Nevertheless, it is clear that research using this approach can take advantage of an abundance of new possibilities that are so far largely unexplored.     

Cranial distinctiveness in the Apennine brown bear: genetic drift effect or ecophenotypic adaptation?

Molecular studies highlighted a strong genetic affinity between the remnant and isolated population of the Apennine brown bear and other southern European populations. Despite this genetic closeness a recent morphometric study revealed a marked phenotypic distinctiveness of the Apennine population, supporting the reinstatement of a distinct taxon (Ursus arctos marsicanus). By building upon previous morphological analyses, we adopted geometric morphometrics to better investigate the skull morphology of the Apennine brown bear with reference to the other, closely related southern European populations. Both skull shape and size differences confirmed the strong divergence of U. arctos marsicanus. In particular, the Apennine bears are characterized by an enlargement of the supraorbital apophysis and a larger distance across the zygomatic arches. Furthermore, our analyses highlighted significant shape differences of the first upper molar in the Apennine bears. Our results suggest that the Apennine bears underwent a rapid morphological change, possibly driven by genetic drift and local selective pressures. Because the greatest morphological differentiation is likely to be related to the muscles involved in mastication, we hypothesize that local selective pressures might be related to a shift in food habits, with highly reduced depredation and feeding on large carcasses in favour of vegetation and hard mast (beech nuts and acorns). These results suggest an adaptive distinctiveness of the Apennine bears, which should be carefully considered in any management and conservation action addressed to this highly endangered population. Although more in‐depth molecular studies are required to better assess the taxonomic and genetic status of the Apennine brown bear population, our study emphasizes the importance of morphological analyses as a complementary tool for a more thorough characterization of variation and divergence in endangered taxa. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Mammal disparity decreases during the Cretaceous angiosperm radiation

Fossil discoveries over the past 30 years have radically transformed traditional views of Mesozoic mammal evolution. In addition, recent research provides a more detailed account of the Cretaceous diversification of flowering plants. Here, we examine patterns of morphological disparity and functional morphology associated with diet in early mammals. Two analyses were performed: (i) an examination of diversity based on functional dental type rather than higher-level taxonomy, and (ii) a morphometric analysis of jaws, which made use of modern analogues, to assess changes in mammalian morphological and dietary disparity. Results demonstrate a decline in diversity of molar types during the mid-Cretaceous as abundances of triconodonts, symmetrodonts, docodonts and eupantotherians diminished. Multituberculates experience a turnover in functional molar types during the mid-Cretaceous and a shift towards plant-dominated diets during the late Late Cretaceous. Although therians undergo a taxonomic expansion coinciding with the angiosperm radiation, they display small body sizes and a low level of morphological disparity, suggesting an evolutionary shift favouring small insectivores. It is concluded that during the mid-Cretaceous, the period of rapid angiosperm radiation, mammals experienced both a decrease in morphological disparity and a functional shift in dietary morphology that were probably related to changing ecosystems.