Abstract

Among the numerous ammonites of the Cretaceous of Colombia, the Pulchelliidae are of great importance because of their abundance and, sometimes, their good preservation. Since L. VON BUCH, Colombian representatives of this family have been repeatedly treated. However, so far, the knowledge of the group has not been completely satisfactory.

Based on of 609 specimens, the author attempted to revise the previously described forms and describe the unknown ones. Within this material, 32 species and 6 subspecies (13 species and 5 new subspecies) are distinguished, classified in the four genera Nicklesia, Psilotissotia, Pulchellia, and Heinzla. The genus Pulchellia, to which 16 species and subspecies belong, is subdivided into 6 new subgenera. The 7 species and subspecies of the genus Heinzia are distributed in three subgenera named after the genera of HYATT.

For classification, the shape of the ventral side and the width of the umbilicus are considered first. The suture line, ribbing, and presence of nodules are of little value for characterizing ultra-specific units. We often observe that some species are clearly distinguishable in a relatively young state, while adult specimens show a certain "convergence" in shell shape. This "convergence" is considered as an indication of kinship.

In Colombia, the Pulchelliidae are found almost exclusively in the Barremian; only one subspecies occurs in the lowermost Aptian. According to the Pulchellias they contain, the Barremian layers of Colombia can be subdivided into the following zones: Upper Barremian, Middle Barremian, and Lower Barremian.

Some first and fourth subzone species are distributed from Peru to the eastern part of the Mediterranean region. In contrast, the second and third subzone species are almost restricted to Colombia, with only two species found in Trinidad, which are identical or closely related to Colombian forms. From this fact, it is deduced that essential phases of the phylogenetic development of this family took place in the Colombian territory.

The material studied contains two juvenile specimens of Nlcklesia, indicating that this genus is derived from Desmocerataceae. Smooth Nicklesias probably represent the root of the whole family from which the Nicklesias ornatas and later the Pailotissotias developed. Simultaneously with the latter appear the first Pulchellias with concave belly and narrow navel. The Pulchellias reach their maximum fullness in the Middle Barremian. At the end of this stage appears in Colombia the genus Heinzia, with concave belly and open navel, which probably formed in the Mediterranean region and spread to South America, possibly because of adaptation to life in relatively shallow water. The Colombian Heinzias belong to the largest and most rustic forms of the family, representing its senile stage.

We do not yet know of layered Pulchelliidae that belong with certainty to the Upper Barremian. However, in the lowermost Aptian Psilotissotia chalmasi andina, a delayed representative of this otherwise extinct family, is sporadically obtained.