Matt Kolmann

Matt Kolmann

Jun 11, 2016

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Part Dos - Why the hell are there stingrays in the Amazon?!

The South America that the first ancestors of the freshwater rays found was very different than the one we see today.  The Andes didn’t start to form until around 90 million years ago (mya).  The combined Amazon and Orinoco Rivers flowed North, emptying into the Caribbean somewhere near modern day Lake Maracaibo (or alternatively, in the Los Llanos floodplain).  And these weren’t necessarily rivers – the Amazon at different points in time was a large, shallow inland sea called the Pebas Mega-Wetland (28mya) – a huge, partly saline marsh.  At some point, the rise of the Andes altered the flow and angle of these rivers, marshes, and lakes – casting their waters in a more easterly direction. 

How do scientists know these things?  By collecting a wealth of data spanning fossils, preserved ancient pollen (palynology), plate tectonics, what types of sediments comprise certain regions, and even models of isotopic decay to characterize ancient water and climate chemistry.  Another tool that researchers like myself use is the hereditary relationships between species as illuminated by each’s unique genetic code.  Species’ DNA are compared and analyzed to make phylogenetic trees – charts which show how species evolved from their ancestors, and how modern species are related to one another.  We can use the rate of mutational change from these sequences to estimate how long ago certain species diverged from one another.  Coupled with fossils and geology, these molecular ‘trees’ have allowed researchers like Nathan Lovejoy to roughly estimate when stingrays and other marine fishes invaded the Amazon.

So where were we again?  Stingrays, that’s right… Well molecular work by Nathan Lovejoy placed the origin of these stingrays sometime between 6-38 mya, right about the time that the Pebas Mega-Wetland existed.  Fossil stingray teeth from the middle of South America place these rays within arguably ancient freshwaters at around 41 mya… So who’s right? 

Where these fossil rays the same as modern day freshwater stingrays?  Why haven’t we talked about feeding – isn’t this project about feeding?  How did freshwater rays adapt to eat new types of prey?

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About This Project

I'm interested in how stingrays, with jaws made of cartilage, consume tough or stiff prey like insects, crabs, and mollusks. I use high-speed videography and measure bite forces to analyze how rays use their jaws to eat tough prey. These freshwater rays invaded South America 30+ million years ago and diversified to feed on a variety of prey. How does feeding specialization evolve and what does it look like?

Blast off!

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