An organism's body size entails both physiological and ecological costs. Furthermore, as a parameter in analyzing organisms, it represents a fundamental and essential morphometric character. Reductions in size following mass extinction is a commonly observed phenomenon in the fossil record. This study examines the evolutionary significance of this phenomenon termed the: 'Lilliput Effect' by proposing that it represents a rapid evolutionary response to altered selection pressures during a mass extinction. This primary hypothesis is evaluated against two additional hypotheses of size reduction: 1) stunted growth as a response to stressed ecosystems, and/or 2) mass extinctions are size selective. These hypotheses were tested using data from shell size measurements and morphology primarily from molluscs and brachiopods from both North America and Denmark.. Morphological differences were evaluated using Elliptical Fourier Analysis (EFA) of outline shape in conjunction with Principle Components and Canonical Variate Analysis. The first part of this study provides a detailed methodology for data collection and analysis. New methods were developed which display promise in improving the degree to which differences and similarities in shape can be elucidated using EFA. These methods were then employed to test hypotheses of morphological change through minor events of local significance in the Florida Neogene and following the K/Pg mass extinction. Data sources for the K/Pg mass extinction were from high resolution (10 cm intervals) collection of bulk samples from the Brazos River in Texas as well as reposited museum specimens for the Braggs locality in Alabama and Danish samples. Study of size and evolution through more minor local events in the fossil record entailed measurements and shape analysis of left valves of bivalves from the genus Chine. Various environmental changes occurred at variousmpoints during the Neogene in Florida Neogene, most profoundly documented during the Plio-Pleistocene with accompanying faunal turnover. TheChione specimens analyzed were derived from a discontinuous sequence encompassing ~18 Ma and represent material from the Miocene Chipola Formation through to the Recent. The size of Chione increased from the Miocene to the Pliocene and then decreased from the Pliocene into the Pleistocene possibly due to lower primary productivity. The later part of the Pleistocene into the Recent was characterized by increased size relative to the early Pleistocene and size was stable through this interval. Morphologically, Chione changed in shape from the Miocene to the Pliocene, but remained in stasis from the Pliocene to the Recent suggesting that stabilizing selection may work well during periods of rapid, minor, environmental perturbations. There were a number of global changes occurring within the late Maastrichtian pr and the results of size measurements of molluscs demonstrate a decrease in size prior to the K/Pg mass extinction in Texas likely in response to a number of global scale events occurring towards the close of the Cretaceous that was also associated with morphological evolution in the small bivalve Breviarca webbervillensis. Paleocene material from Texas was dominated by smaller, newly evolved allochthonous grazing gastropods. These gastropods are thought to be newly evolved lineages as their first occurrence is marked in the Paleocene. Smaller sized nuculid bivalves were also a prominent feature in the Texas Paleocene and showed a rapid size beginning 40 cm above the boundary clay. At Braggs, Alabama, two groups, gastropods and oysters showed decreases in size across the boundary, and these changes are most likely a product of animpoverished Paleocene ecosystem. The pectinids were the only group of bivalve mollusc to xii reduce in size following extinction in Denmark most likely in response to a reduction in bryozoan substrate which resulted in a very soft coccolith-derived mud substrate in the mass extinction's aftermath. Size measurements from the Danish brachiopods showed reductions in size across the boundary in all genera except Rugia. There was a marked size reduction in specimens from the upper Maastrichtian at Stevns Klint as compared to the lower Maastrichtian at Mons Klint. The reasons for this are most likely due to lower temperatures at Stevns based on isotope data as well as lower productivity evidenced by lower δ13C values. There was a change in morphology for Terebratulina chrysalis in the earliest Paleogene due to shifted selective pressures favoring increased surface area as an adaptation to softer substrates. Size reduction within Danish sequences is evolutionary in nature as the Danian is characterized by different species within existing Maastrichtian genera and two new genera. The results of this study demonstrate that changes in body size can be a result of evolution from changing selective pressures as well as ecological perturbations. Distinguishing evolutionary forcings from ecological requires the collection and understanding of morphological data. Changes in size for Terebratulina chrysalis could have easily been interpreted as ecological were it not for morphological results showing the changes in surface area resulting from changing selective pressure. Late Cretaceous changes in climate and sea level produced observable changes in size and morphology in Breviarca webbervillensis. Potential size selectivity of the K/Pg mass extinction failed to account for any of the patterns observed in the data. Gryphaeid oysters in Denmark would have been a perfect candidate to support conclusions of size selectivity as they were the largest molluscs measured in this study and survived mass extinction.