This paper is available free on line at:
http://mbe.oxfordjournals.org/cgi/reprint/msj124v1
PTN
This was discussed earlier on this list, but this is the official paper...
Kerryn E. Slack, Craig M. Jones, Tatsuro Ando, G. L. (Abby) Harrison, R. Ewan Fordyce, Ulfur Arnason and David Penny (2006) Early Penguin Fossils, Plus Mitochondrial Genomes, Calibrate Avian Evolution. Molecular Biology and Evolution 23: 1144-1155