Recognition of condensed zones is important because they may be marker horizons that are useful in exploration. Such a zone is demonstrated by the occurrence of ammonites belonging to 12 species and 9 genera from the middle Ozan Formation (lower Taylor Marl) in northeast Texas. The 1-foot (0.3-m) thick bed of bioturbated glauconitic biomicrite contains many specimens of disarticulated vertebrates, molluscs, remanie fossils (blackened phosphatic internal molds), and hiatus concretions. Four of 6 midcontinent ammonite range zones proposed by Cobban and others appear to be represented in the fauna, in ascending order, by Baculites aquilaensis Reeside, Delawarella delawarensis (Morton) (= zones of two unnamed species of Baculites), Baculites obtusus Meek, and Trachyscaphites piniger porchi Adkins (= zones of Baculites mclearni and B. asperiformis). Young may be correct in assuming that the occurrence of Delawarella delawarensis and Baculites aquilaensis in the Ozan Formation may mean that rocks of the upper Austin Group and parts of the lower Taylor Group are the same age. If correlation with the midcontinent zonation is correct, then the sediments that formed the condensed zone slowly accumulated from 81 to 79 m.y. (mid early Campanian to early late Campanian). Several species of the fauna are preserved as both normal and remanie fossils, indicating that members of these species lived in the area for an extended period of time, perhaps as a relict fauna. The fauna includes a mixture of cosmopolitan and endemic species (indicating open shelf environment) wit several types of heteromorphs (indicating moderate water depths).

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