How much of evolutionary history is lost because of the unevenness of the fossil record? Lagerstätten, sites of exceptional fossil preservation, provide remarkable, yet distorting insights into past life. When examining macroevolutionary trends in the fossil record, they can generate an uneven sampling signal for taxonomic diversity; by comparison, their effect on morphological variety (disparity) is poorly understood. We show here that lagerstätten impact the disparity of ichthyosaurs, Mesozoic marine reptiles, by preserving higher diversity and more complete specimens. Elsewhere in the fossil record, undersampled diversity and more fragmentary specimens produce spurious results. We identify a novel effect, that a taxon moves towards the centroid of a Generalised Euclidean dataset as its proportion of missing data increases. We term this effect ‘centroid slippage’, as a disparity-based analogue of phylogenetic stemward slippage. Our results suggest that uneven sampling presents profound issues for our view of disparity in the fossil record, but that this is also dependent on the methodology used, especially true with widely used Generalised Euclidean distances. Mitigation of missing cladistic data is possible by phylogenetic gap filling, and heterogeneous effects of lagerstätten on disparity may be accounted for by understanding the factors affecting their spatiotemporal distribution.