Invertebrate Paleontology in the mid-Miocene: A trip to Lago Alajuela

A visit to Panama City by paleontologists Cristina Robins and Ian Cannon from the University of Florida this past week meant several field days focused on sampling invertebrate fossils. The goal: to obtain a better picture of the diversity of invertebrate communities within the formations in the Panama Canal, and increase collections of crustaceans and mollusks to be studied back at the University of Florida. Most exciting was our visit to a site outside the boundary of the Canal Excavation, to sample from the Alajuela Formation. Pictured below, Lago Alajuela, a man-made lake created along the Chagres River and major reservoir within the Canal watershed.

Invertebrate Paleo. collection team, July 15, 2015. Starting with the back row and moving left to right, Cristina Robins, project coordinator PCP-PIRE; Michael Ziegler, PCP-PIRE Intern; Ian Cannon, University of Florida; Jorge Moreno, PCP-PIRE Field Leader; Gina Roberti, PCP-PIRE Intern, Summer 2015.

Invertebrate Paleo. collection team, July 15, 2015, Lago Alajuela. The terraced shorelines and extremely low lake levels reflect record lows in rainfall during June, the third driest June on record in Panama in the last 100 years. So much exposed shoreline makes for fantastic fossil hunting. Starting with the back row and moving left to right, Cristina Robins, project coordinator PCP-PIRE; Michael Ziegler, PCP-PIRE Intern; Ian Cannon, University of Florida; Jorge Moreno, PCP-PIRE Field Leader; Gina Roberti, PCP-PIRE Intern, Summer 2015.

The calcareous sandstones and fossil-rich marine limestones of the Alajuela formation are slightly younger than the rocks which outcrop in the Canal Zone. Here, it is much more common to find sharks teeth, similar to those found in the Gatun Formation (of Megalodon pride, aged approximately 5-10 million years younger than the formations within the canal).

PCP-PIRE Intern Michael Ziegler proudly displays a well-preserved shark tooth of the Alajuela Formation.

PCP-PIRE Intern Michael Ziegler proudly displays a well-preserved shark tooth of the Alajuela Formation. Much of the excavations of the PCP-PIRE project have focused primarily on formations which outcrop within the canal (the 18-20 million year-old Culebra, Cucaracha and Las Cascadas most popular). Alajuela is a more recent project site and, thus far, incredibly rich in its fossil yield.

But, this week the focus was on organisms that often garner much less attention: invertebrates.

Cast of a scallop shell, preserved in calcareous limestone of the Alajuela Formation.

Cast of a scallop shell, preserved in calcareous limestone of the Alajuela Formation. Oysters and scallops are in the same family, and have heartier shells than other bivalves and mollusks. Thus, it is much more common to find these species than others more delicate, a preservation bias in the fossil record.

To sample for invertebrate diversity required cutting into the sandy-limestone matrix, to access fossils that were better preserved.

Scallop shell preserved in fossiliferous limestone of the Alajuela Formation. The white color is a result of the calcium-rich carbonate material, from which the shells are made.

Scallop shell preserved in fossiliferous limestone of the Alajuela Formation. The white color is a result of the calcium-rich carbonate material, from which the shells are made. The most diagnostic features for bivalves such as scallops, oysters, clams are along the margins and at the hinge point. (And for scallops, the wings). If you can find fossils that preserve these features that is best for identification!

Many of the sections in which we searched were chaotic beds of oysters and other bivalves (two-part shells), depositional settings in which sea-surface critters grew and accumulated upon each other in every which direction.

Cutting into the bedrock to discover more complete preservation inside. A photograph of a freshly cut surface, with tools for scale.

Cutting into the bedrock to discover more complete preservation inside. A photograph of a freshly cut surface, with tools for scale.

The area around Lago Alajuela as well as the larger Chagres watershed is preserved as a National Park (Parque Nacional). The watershed surrounding the canal, of which the Chagres is a major tributary, is important in providing water for both the canal (of billion-dollar yearly economic value), and also supplies water to three of the largest cities in Panama: Panamá City, Chorrera and Colón (as well as generating water for la ciudad de Panamá and Colón!). The locks of the canal alone use 52 gallons of freshwater a year to operate.

More information on the park and conservation initiatives in Panama can be found here: Parque Nacional Chagres.

To work within the serenity of such an expansive lake (though constructed) is quite a privilege, though the threat of crocodiles has kept most out of the water.

 

Lago Alajuela

Levels in Lago Alajuela are exceptionally low for the rainy season, making it easier to access terrane outside the reach of tropical vegetation.

 

Photography and text by Gina Roberti, PCP-PIRE Intern.

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