Marcelo in his office at the American Museum of Natural History.

One of Marcelo's current research interests is the systematic relationships of freshwater stingrays, with a focus on the relationships among different genera of stingrays--their diversity and how the different species are related in evolutionary terms. His research includes looking at both fossil and living stingrays. In particular, he studies fossil stingrays from extinct freshwater lakes in Wyoming and extinct coral reefs in Italy. He then compares the characters of these extinct fossil species to the characters of stingray species still alive today. By studying the form, or anatomy, of these animals, Marcelo builds a picture of their evolutionary relationships.

The characters in stingrays that most often elucidate relationships between genera are found in their skeletal anatomy, sensory pore pattern, and muscles. A fossil provides a preserved two-dimensional skeletal pattern that contains valuable anatomical information. This is a difficult process because stingrays are anatomically conservative, that is, there is not a great deal of variation in the anatomies of different species. When Marcelo manages to find a character that unites two groups of stingrays, it is an important finding. For example, in one study, Marcelo found that the cartilaginous support for the pectoral fins of two genera of Australian stingrays are derived, or unique. In other words, only those two stingray genera have this particular character in common. In evolutionary terms, they are more closely related to each other than to any other genera.

A large part of Marcelo's work, and the topic of his dissertation, focuses on the systematics of electric rays. There are some fifty described species of electric rays all over the world. However, the systematics of electric rays is in a state of chaos, needing much work. In the past five years alone, Marcelo has discovered some twelve new species of electric rays. In addition, Marcelo is producing a complete study of the fossil rays from the Eocene of Monte Bolca. These rays existed 52 million years ago in a coral reef environment in what is now northeastern Italy. It is one of the richest localities for fossil rays in the world.