Fig. 1: These hard limestones were once the bottom of a warm and shallow sea in the northern Caribbean.
Carcharodon megalodon Agassiz, 1843 is a giant version of the great white shark (Carcharodon carcharias) of our modern oceans, and one of the top predators of its time. It is also one of the most recognized fossils, and one of the most coveted by fossil collectors. In Cuba, fossils of this extinct shark are interestingly common on limestone of the Habana-Matanzas carbonate hills, where local quarrymen frequently find them among whales, dugongs, and other shark fossils (fig. 2) (Iturralde et al., 1996).
Fig. 2: Carcharodon megalodon from the Guines formation.
Overall the fossilized teeth are superbly preserved. Many can be found together with other teeth, invertebrate fossils, or alone encased in the rock matrix. This same matrix holds microscopic clues to the environment - the paleoecology of the ancient seas where these and other creatures lived - plus clues to how they came to be preserved in such way for millions of years.
On closer inspection, the limestone of the Guines fm. are composed of small microscopic organisms, large mollusks (gastropods and bivalves), sea urchins, and very few corals (fig. 3-6). This type of limestone is called fossiliferous limestones for its abundance of fossils. It is also known as fine-grained, often recrystallized biodetritic limestone because the minerals that make up the rock have changed their structure after many millions of years of exposure to water and other conditions. It is biodetritic because the larger clasts, were once parts and bits of living organisms, such as the mollusk mentioned above, that became part of the sediment when this was a sea.
|Fig. 3. Gastropods and bivalve molds and casts on limestone of the formation.|
For instance, fig. 3 shows a rock face where many benthic organisms that were once living within the seafloor are preserved. The half moon structures are casts of bivalve shells, now filled in with minerals. The rounded structures are gastropods and parts of sea urchins. Shark teeth are found in such mix.
|Fig. 4: Schizaster cf. guirensis? A regular echinoderm - a sea urchin of this Miocene ancient sea.|
The Guines formation dates to the middle-late Miocene and is well over 10 million years old. Over the last 100 years, it has had several descriptions and names. It was once called the Yumuri Limestone by DeGoyler (1918) and Bermudez and Hofftetter (1959), or the Yumuri Formation. Judoley and Furrazola considered this formation just a variation or member (1971). The detailed studies of Iturralde (1969) clearly establish it as a distinct Miocene formation, covering extensive areas in meridional western Cuba (Franco et al, 1992).
(Trivia: The Miocene epoch was named by one of the first geologists Sir Charles Lyell).
|Fig. 5: A single scleractinian coral polyp Scolymia cf. cubensis Edwards and Haime, 1849 alongside bivalve clams. The Scolymia is a fossil index of the middle-late Miocene (~ 13 to 5 million years ago).|
Under the microscope, and after making a very thin slice of the rock (thin enough to allow light to go through the rock), one can see microscopic shells of tiny water snails called pteropods, miliolid, and amphistaginid foraminifera, within a matrix of the minerals calcite (CaCO3), and dolomite CaMg(CO3)2 which has grown later. In thin section as it is called, the bioclasts have long been eroded and dissolved, leaving behind empty spaces in the shapes of the organisms that were once in the matrix known as ghosts (fig. 5).
|Fig. 6: Thin slide showing the ghosts of bivalve shells, a planktic and miliolid foraminifera in the center field of view.|
The sparitic crystals suggest recrystallization (see the prismatic crystals inside the ghosts).
The fauna and the minerals suggest that the Guines limestone formed in relatively shallow and warm waters, teeming with bottom living and burrowing lifeforms in its carbonate substrate. The large vertebrate remains indicate the presence of large sharks and whales, and all of them an intricate warm water environment, likely forming open water channel. Such an environment was suggested by previous research Iturralde (1969) and visible in the paleo reconstructions of MacPhee and Iturralde (2000). Figure 7.
|Fig. 7: Paleoreconstruction of the Miocene seaway that separated what is now the region of Havana-Matanzas.|
Form MacPhee and Iturralde, 2000.
Fig. 8: A large fragment of bone encased on the limestone. This is large enough to be from a Miocene vertebrate,
maybe even a whale!
But what do the rocks and fossils say about that past? The fossil fauna of the Guines fm. suggests that this given region was once a shallow marine environment, open, with very few reef banks. The frequent fossil shark fossils suggest that this area could have been a shark nursery. I will speculate further and think that it was so because it supported marine mammals and fish to feed these large sharks, and because the basin was warm and confined enough to be a nursery (fig. 7-8). More evidence, as always, is needed.
Stay tuned for more !
Bermudez, B., and Hofftetter (1959). Stratigraphic Lexicon of Cuba. Lexique Stratigraphique International, 5, Amerique Latine, Fasc. 2c, Cuba et lles Adjacents, 140 pág.
DeGoyler (1918). The geology of Cuban petroleum deposits. American Association of Petroleum Geologists Bulletin, 2: 133-167.
Franco-Alvarez, G.L. et al. (1992). Léxico Estratigráfico de Cuba. Centro de Nacional de Información Geológica, La Habana, 658 pág.
Iturralde-Vinent, M. (1969). El Neogeno de la Provincia de Matanzas, Cuba. Publicacion Especial Instituto Nacional de Recursos Hidraulicos, 7: 3-30.
Iturralde-Vinent, M., G. Hubbell, and R. Rojas Consuegra. 1996. Catalog of Cuban fossil Elasmobranchii (Paleocene-Pliocene) and Paleoceanographic Implications for their Lower-Middle Miocene Ocurrence. Boletin de la Sociedad Jamaicana de Geologia, 31: 7-21.
Judoley and Furrazola (1967). La posición de Cuba en la estructura geológica de la región del Caribe. Tecnológica, 5 (6).
MacPhee, R.D.E., e Iturralde-Vinent, M., (2000). A short history of Greater Antillean land mammals: biogeography, paleogeography, radiations, and extinctions. Tropics, 10 (1): 145-154.
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