Discovery sheds light on how vertebrates see
New research led by the University of Leicester has overturned a long-standing theory on how vertebrates evolved their eyes by identifying remarkable details of the retina in the eyes of 300 million year-old lamprey and hagfish fossils.
|Professor Sarah Gabbott on fieldwork in Illinois digging for new fossil specimens. In her hand is |
a hard nodule formed 300 million years ago on a warm, shallow sea floor
[Credit: Thomas Clements]
The researchers examined the eye tissue in two fossil jawless fish species - Mayomyzon (a lamprey) and Myxinikela (a hagfish) found in the Carboniferous age Mazon Creek fossil bed, Illinois.
Using a high-powered scanning electron microscope to magnify the eye 5,000 times they could see that the fossil retina is composed of minute structures called melanosomes - the same structures that occur in human eyes and prevent stray light bouncing around in the eye allowing us to form a clear visual image.
This is the first time that such details in fossil vertebrate eyes have been brought to bear on the tricky problem of how their eyes evolved.
The eye is a complex structure and must have evolved through small step-by-step changes but these are not recorded in living animals and until now it was thought that these anatomical details could not be preserved in fossils.
The details of the retina in the fossil hagfish indicates that it had a functional visual system, meaning that living hagfish eyes have been lost through millions of years of evolution, and these animals are not as primitively simple as we originally believed. As a result they are not the most appropriate model for understanding eye evolution.
Professor Gabbott added: "Sight is perhaps our most cherished sense but its evolution in vertebrates is enigmatic and a cause celebre for creationists. We bring new fossil evidence to bear on an iconic evolutionary problem: the early evolution of the vertebrate eye. We will now scrutinize the eyes of other ancient vertebrate fossils to see if we can finally build a picture of the sequence of events that took place in early vertebrate eye evolution."
The team also found the earliest evidence of skin pigment patterning in a fossil.
She added: "This heralds the realistic possibility of inferring details of the ecology and behavior of our ancient ancestors. Animals today have stripes for many reasons from camouflage to sexual display- we now have the potential to understand behavior in long extinct vertebrates."
Source: University of Leicester [August 02, 2016]