February 13, 2014
Vertebrate Study Reveals The Evolution Of The Face
[ Watch the Video: Animation Sequence of Romundina Fish Fossil ]
Brett Smith for redOrbit.com - Your Universe Online
A study published on Wednesday in the journal Nature has revealed new details on the evolution of the jaw – a major defining structure in the evolution of the face.
In the study, a team of European scientists used X-ray images of a series of fossils to reveal details on the incremental assembly of the face as jawed vertebrates diverged from jawless vertebrates.
Today, jawed vertebrates significantly outnumber jawless vertebrates, with lampreys and hagfishes being the only jawless vertebrates. Previous research has shown that jawed vertebrates evolved from jawless ones, a process that essentially turned the face inside out.
As jawless vertebrates develop, sections of tissue expand forward on both sides of the brain, meeting in the midline at the front to make a big top lip around a single "nostril" that lies just in front of the eye. In jawed vertebrates, this same development moves forward in the midline under the human brain, pushing in between the left and right nasal sacs, which open independently to the exterior, resulting in two nostrils as opposed to a single nasal hole in the middle. The front area of the brain is also considerably longer in jawed vertebrates, resulting in a nose that is positioned at the front of the face as opposed to back between our eyes.
To learn more about the intermediary steps in this development, the study team examined the skull of a 410-million-year-old jawed, armored fish called Romundina that inhabited what is now arctic Canada. The ancient fish has separate left and right nostrils, like our own. However, the nostrils sit far behind an upper lip, like the “nose” of a jawless vertebrate.
"This skull is a mix of primitive and modern features, making it an invaluable intermediate fossil between jawless and jawed vertebrates,” said study author Vincent Dupret, an evolutionary biologist at Uppsala University.
The study team said their X-ray images of Romundina, generated at the European Synchrotron (ESRF) in Grenoble, France, show that its skull contained a brain with a short front end, which resembled that of a jawless vertebrate.
"In effect, Romundina has the construction of a jawed vertebrate but the proportions of a jawless one", said study author Per Ahlberg, an expert in vertebrate evolution at Uppsala University. "This shows us that the organization of the major tissue blocks was the first thing to change, and that the shape of the head caught up afterwards.”
By determining how Romundina fits in with other ancient fish, the study authors said they were able to map out all the main steps of the transition from jawless to jawed vertebrates.
"Without the intense X-rays produced at the ESRF, we would not have been able to create a virtual representation of the internal structures of the skull," said study author Sophie Sanchez, a biologist at Uppsala and the ESRF.