Tuesday, December 3, 2013

Back to the western Caribbean, Pt. 2

The previous post was to give you a brief introduction into the geology of the Chagres Formation, which is where we are focusing part of our collecting efforts. This next post is to give you an idea of what happens when we find fossils of large marine vertebrates in the Chagres.

Often, when we find fossils in the Chagres these are on the rock exposed in the wave-cut platform along the beach (although there are exceptions). The benefit of this is the relatively easy access, the downside, is that we usually have only about 4 hours (spanning between the period before, during and after, the low tide) to collect the fossil. Back in April, during one of our trips to the Caribbean coast, we saw a partial marlin mandible exposed on one of the wave-cut platforms (picture below). That day we were prospecting, that is, looking for new localities and new fossils that were exposed so that we could plan to collect them in the near future. This was one of those finds.
A partial marlin mandible exposed on the wave-cut platform (anterior to the left).
The near future ended up being July. And so it was that together with Carlos De Gracia (a STRI intern whose main interest are fossil fishes), and my summer 2013 interns, Chris, Christina, and Silvia (previously featured here) we set out to the Caribbean to collect that fossil marlin.

Collecting the fossil took us three days. The first day was cut short and we couldn't do much work as the weather turned bad, and we had to leave after about an hour of work. Day two was purely devoted to digging a trench around the fossil so that we could wrap it in a protective plaster jacket for proper removal and transportation back to STRI (see picture below). This is a near obligatory task and method of proper collection of fossil vertebrates and has been used at least  since the late 1800's. While digging around it, we realized that we had more than we thought, the fossil consisted of mandible and skull, which meant we had to dig deeper, and wider around the fossil to be able to remove it completely.
Day 2 of the excavation. From left to right: Carlos De Gracia (STRI intern) and my summer 2013 interns, Christina, Chris and Silvia, make the trench around the marlin skull and mandible.
After we dug a deep enough trench around the fossil, we were ready to put a plaster jacket around it. Except that by then it was late, and the tide was coming back in, meaning we had to jacket the specimen another day. Finally, on our third day at this particular site, were were able to put a jacket around the fossil (see pictures below).
Day 3 of the excavation. Christina and Silvia are finish the jacket protecting the marlin skull and mandible.
This is the locality, if you click on the image you can see Silvia (near the center) and Christina (to the right); behind her is the jacket (the white blob on the ground).
After the plaster dried we undercut the block and flipped it so that we could remove excess rock and make it a bit lighter to carry back to the truck and then drive back to STRI (see pictures below). This specimen, as well as several other marlins that have been collected from the Chagres (including the one mentioned here), will be prepared and studied by Carlos.
After we popped and overturned the jacket,  me, Chris, and Carlos removed some of the excess rock in order to make it lighter.
Liftoff!! Even after trimming some of the rock off, this was still a pretty heavy block.
Other fishes known from the Chagres include a variety of bony fishes (mainly know from otoliths, which I mentioned in Part 1) as well as a several species of sharks (of which cookie-cutter sharks are one of the most common). Marlins are also relatively common and easy to recognize in the field; in fact, this year alone, we have collected several other skulls, mandibles and vertebrae.

Because they are common, the fossil marlins of the Chagres have not gone unnoticed, unlike the marine mammals. In 1978, Harry L. Fierstine described the first fossil marlin known from Panama. The fossil, consisting of a nearly complete skull, represented a new species, which he named Makaira panamensis. Modern relatives of Makaira panamensis include the black and the blue marlins. A second fossil marlin from Panama was described in 1999 (Fierstine, 1999). This one was found in the late Miocene Gatun Formation, which underlies the Chagres Fm. The fossil consisted of a partial rostrum, and was identified as Makaira cf. M. nigricans, the same species as the blue marlin (Fierstine, 1999). This implies that this particular species has been around for several million years, or more likely, that some extant members of this group are morphologically conservative and show very little differences from its fossil relatives.  

I'm sure we'll learn more about the fossil fishes of the Chagres and Gatun formations in the not so distant future. For now, stay tuned as this series is not yet over!

Literature Cited

Fierstine, H. L. 1978. A new marlin, Makaira panamensis, from the Late Miocene of Panama. Copeia 1978:1-11.

Fierstine, H. L. 1999. Makaira sp., cf. M. nigricans Lacépede, 1802 (Teleostei: Perciformes: Istiophoridae) from the Late Miocene, Panama, and its probable use of the Panama Seaway. Journal of Vertebrate Paleontology 19:430-437.

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