The variability of the ammonite Dufrenoya texana Burckhardt
DOI:
https://doi.org/10.32685/0120-1425/bolgeol4.2-3.1956.174Keywords:
Upper Aptian, Eastern Cordillera, stratigraphic distribution, specimens, ontogenic
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Copyright (c) 1956 Servicio Geológico Colombiano
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Abstract
Dufrenoya texana Burckhardt is a rather common ammonite in the Upper Aptian of the Eastern Cordillera of Colombia. However, it shows considerable variation; specimens with extreme ornamentation may be classified as belonging to different species.
For a more complete study of the variability of this species, we selected the 36 best- preserved specimens from a single population, i.e., from the Upper Aptian basal horizon of Mesa de Los Santos (Santander Dept.). Table 1 shows the main characters of these specimens using diagrams. The observed differences are independent of shell size since the same degree of variability can occur in all ontogenetic stages. Within the investigated population we can distinguish three main groups of different shapes, which are connected by all degrees of transition.
Dufrenoya texana Burckha. H.DT with narrow umbilicus and numerous ribs, which are strongly pronounced in the ventral region, and with a relatively complicated suture line.
Dufrenoya texana sanctorum n. var. with wide umbilicus, with few smooth ribs in the ventral region, and with a relatively simple suture line.
Dufrenoya texana media n. var. shows a combination of characteristics or an intermediate form between the two groups mentioned above.
In the juvenile stage, the suture line of Dufrenoya texana and Dufrenoya texana sanctorum is equally simple and cannot be distinguished from each other. However, w ith advancing ontogenetic development, the suture of the var. texana becomes considerably more complicated, while Dufrenoya texana sanctorum hardly changes the simple form of its youth. For this reason, we consider the variety sanctorum as a more primitive stage and the variety texana as a more advanced one.
The stratigraphic distribution confirms this: so far, we know Dufrenoya texana sanctorum only from the basal horizon of the Upper Aptian, whereas the middle and Texan varieties also occur in higher horizons, with the Texan variety becoming progressively more prominent. In the uppermost Aptian, the species Dufrenoya texana is represented exclusively by the variety Dufrenoya texana.
References
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RIEDEL, L. Amonitas del Cretácico Inferior de la Cordillera. Oriental.- Estudios geológicos y paleontológicos sobre la Cordillera Oriental de Colombia II , Min. dé Industrias y Trabajo, Bogotá 1938.
