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The world's evaporite and carbonate rock deposits show a northward offset from the present geographic equator. The northern carbonate boundary has an average position at present lat. 40°N, and the southern boundary has an average position at about lat. 30°S. The width of this carbonate belt has changed through time. During the Mesozoic, the width decreased from about 90° (10,000 km) in Triassic time to about 65° (7,000 km) in Cretaceous time. The median position of the carbonate belt was 20–25°N lat. during the Triassic, 19°N during the Jurassic, and 15°N during the Cretaceous. This median position is interpreted to be the position of the Mesozoic thermal equator.

The Boreal ammonoid realm did not extend farther south than 38°N lat. during the Triassic, Jurassic, and Early Cretaceous. However, the position of the southern border of the Boreal realm was not constant. It shifted between 38°N and 60°N during Mesozoic time. Moreover, the number of Boreal ammonoid taxa decreases from north to south. In contrast, the number of Tethyan-realm ammonoids is greatest around the equatorial belt, and decreases toward both poles. The ammonoid diversity gradients are symmetrical with respect to the present thermal equator.

Hypotheses of continental drift, polar wandering, and earth expansion fail to explain the stable position of the carbonate belt and the constant genus- and speciesdiversity relations of Triassic-Cretaceous ammonoids with respect to the present equator. The constant positions of the diversity gradients and the carbonate and evaporite belts are explained most logically if it is assumed that the continents were in the same positions during Mesozoic time as today.

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