BIOGEOGRAPHIC AND EVOLUTIONARY SIGNIFICANCE OF TURRITELLID GASTROPODS FROM THE MIOCENE OF NORTHERN COLOMBIA

The Neogene sedimentary section of the Cocinetas Basin in northern Colombia preserves a rich record of marine invertebrates, especially mollusks. Molluscan fossils are highly diverse and often well preserved, offering a window into the rapidly changing paleoenvironments and biogeography of northern South America. The basal unit in the Neogene succession, the Uitpa Formation has a generally poorly preserved invertebrate assemblages. The conformably overlying Jimol and Castilletes formations, however, contains a varied suite of depositional environments with better-developed shell beds and thicker successions of intervening siltstone. Biostratigraphic assessment and strontium isotopic results from these units indicate that they were deposited in the Early Miocene through Middle Miocene (ca. 17-14 Ma). These results help to place the shallow marine assemblages of Cocinetas Basin into a wider geologic context that aids our understanding of how these faunas relate to the broader evolutionary and biogeographic history of the southern Caribbean during the Neogene.

Turritellid gastropods are diverse and abundant Neogene fossils across the southern Caribbean, especially in Panama and Venezuela. Much less has been known about the group in Colombia, where previous work suggested only 2-3 species per formation. As part of a wider study of Neogene turritellids in the region, we have begun an intensive examination of turritellid species from the Jimol and Castilletes formations. Preliminary results indicate that the Jimol contains as many as 4 turritellid species, including Turritella coccoditana, T. curamichatensis, T. larensis, and T. matarucana. The Castilletes may contain as many as 8-10 species, including Turritella altilira, T. berjadinensis, T. coccoditana, T. cornellana, T. falconensis, T. gatunensis, T. gilbertharrisi, T. larensis, T. matarucana, and T. montanitensis. Most of these species are also known from Panama and/or Venezuela, and their presence in northern Colombia therefore fills in geographic and stratigraphic gaps in their known ranges. Species for which apical whorls are preserved (T. gatunensis and T. matarucana so far) are consistent with most fossil and Recent turritellid species: relatively small paucispiral protoconchs suggestive of short-term planktonic development.