Fig 3.

Scanning electron micrograph of a longitudinal section of a stalk of Amphiroa species A grown in seawater with the estimated Late Cretaceous Mg/Ca ratio of 1.0. Numbers are mole percentages of magnesium in substitution for calcium in cell wall carbonate. Measurements are at junctures of four cells along cell rows, one of which was added daily. Percentages in the 17.51–19.15 range are for calcification in modern seawater. During 4 days of growth in the Late Cretaceous treatment, the percentage declined progressively, reaching 1.93 before the specimen died.