Cleary, T.J., Moon, B.C., Dunhill, A.M. & Benton, M.J. 2015 The fossil record of ichthyosaurs, completeness metrics and sampling biases . Palaeontology 58: 521–536 doi:10.1111/pala.12158
Ichthyosaur completeness varies greatly through the Mesozoic, but there are trends within the data. High levels of completeness are associated particularly with Lagerstätten, as is to be expected, medium-sized specimens tended to be more complete than the larger or smaller size groups, and specimens from the northern hemisphere were more complete than from the southern hemisphere.
At least some of this may be due to availability of specimens: more specimens are collected from Lagerstätten in the northern hemisphere and are medium-sized; and so more of these specimens were included in the data. There are other potential biases related to the preservation and collection potential of these specimens that should be taken into account in studied of the diversity of ichthyosaurs and other fossil groups.
Beardmore, S.R., Orr, P.J., Manzocchi, T. and Furrer, H. 2012. Float or sink: modelling the taphonomic pathway of marine crocodiles (Mesoeucrocodylia, Thalattosuchia) during the death–burial interval. Palaeobiodiversity and Palaeoenvironments 92 (1): 83–98. doi:10.1007/s12549-011-0066-0
Mannion, P.D. and Upchurch, P. 2010. Completeness metrics and the quality of the sauropodomorph fossil record through geological and historical time. Paleobiology 36 (2): 283–302. doi:10.1666/09008.1