October 24, 2013

People with high cheekbones and strong jawlines may have an ancient fish to thank for these desirable signs of beauty, according to a study of a 419-million-year-old jawbone found in China. The exceptionally well-preserved fossil, from a fish named Entelognathus primordialis, represents the most primitive vertebrate (animal with a backbone) with what humans would recognize as a modern jaw and face. It suggests that the three-boned jaw used for chewing by modern jawed vertebrates, including humans, may have evolved much earlier than scientists had thought. “This is like finding the nose of a space shuttle in a hay wagon from the Middle Ages,” one of the scientists involved in the study told USA Today. Scientists published their description of the remarkable fossil, unearthed in Yunnan Province in southern China in 2010, in the online edition of the journal Nature. The name Entelognathus primordialis means primordial complete jaw.

The scientists recognized Entelognathus as a member of early group called placoderms that first appeared around 440 million years ago. The placoderms were large fish with heads covered in a heavy armor of thick bony plates and scales. These creatures had powerful, yet simple, beak-like jaws with sharp boney plates that served as teeth. Entelognathus was about 8 inches (20 centimeters) long. The placoderms died out by the end of the Devonian Period, about 360 million years ago, leaving no descendants.

A fish that lived 58 million years ago left its image in this fossil. Such fossils reveal many details about fish that are now extinct. Scientists study fossils to discover how fish developed through the ages. (World Book photo)

Paleontologists have long thought that modern boney fish evolved from another primitive group of jawed fish that were probably more like sharks, with a skeleton made of cartilage rather than bone. This unknown group gave rise to the sharks and rays, whose skeletons are built mostly of cartilage. This group was also the immediate ancestors to the bony fishes and their descendants, including amphibians, reptiles, birds, and mammals. Boney fish and land vertebrates are all recognized by a distinctive three-boned jaw composed of a lower jawbone called the dentary and two upper jaw bones called the premaxilla and the maxilla. The first modern boney fish do not appear in the fossil record until the Mesozoic Era, about 251 million years ago.

The fossils of Entelognathus show anatomical details of the jaw in much finer detail than the fossils of most known placoderm specimens. In the newly discovered fossils, the scientists clearly saw a complex jaw, composed of interlocking bones similar to those of modern boney fish and land vertebrates, even though they are millions of years older than these familiar groups. This finding raises the possibility that the ultimate origins of modern jawed vertebrates, including humans, are found among the fierce-looking placoderms, like Entelognathus, and not the more modern-looking groups that appeared later as most paleontologists had thought.

Additional World Book articles:

• Fish (The Development of Fish)
• Prehistoric animal