Date: April 2, 2018
Source: Cell Press
Summary:
Researchers reporting
in Current Biology on April 2 have evidence that an extinct species
of monitor lizard had four eyes, a first among known jawed vertebrates. Today,
only the jawless lampreys have four eyes.
The third and fourth eyes refer
to pineal and parapineal organs, eye-like photosensory structures on the top of
the head that play key roles in orientation and in circadian and annual cycles.
The new findings help to elucidate the evolutionary history of these structures
among vertebrates.
The photosensitive pineal organ
is found in a number of lower vertebrates such as fishes and frogs, the
researchers explain. It's often referred to as the "third eye" and
was widespread in primitive vertebrates.
"On the one hand, there was
this idea that the third eye was simply reduced independently in many different
vertebrate groups such as mammals and birds and is retained only in lizards
among fully land-dwelling vertebrates," says Krister Smith at the
Senckenberg Research Institute in Germany. "On the other hand, there was
this idea that the lizard third eye developed from a different organ, called
the parapineal, which is well developed in lampreys. These two ideas didn't
really cohere.
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