The Mesozoic (M-sequence) block model derived by Larson and Hilde [1975] from the Hawaiian magnetic anomaly lineations has been the template for the Late Jurassic-Early Cretaceous GPTS. Most published timescales [e.g. Kent and Gradstein, 1985; Harland et al., 1990] utilize this block model with a constant spreading rate assumption to interpolate between age tie-points. The M0 to M25 magnetic anomaly sequence of Larson and Hilde [1975] was extended to M29 [ Cande et al., 1978] and subsequently to M38 [ Handschumacher et al., 1988]. For anomalies older than M25, amplitudes are reduced, possibly signifying reduced geomagnetic paleointensities in this part of the Late Jurassic [ Cande et al., 1978]. In the last few years, new block models from the Japanese, Phoenix and Hawaiian lineations have become available for the M0-M29 interval [ Nakanishi et al., 1989, 1992; Channell et al., 1994].