Apr. 12, 1999 April 9, 1999--Shedding new light on the evolutionary origins of the jaw, John Maisey, a curator in the Department of Vertebrate Paleontology at the American Museum of Natural History, disclosed the first detailed description of a 400-million-year-old primitive shark relative from Bolivia named Pucapampella.
This new fossil discovery contradicts the belief that chondrichthyans, or sharks and their relatives, are primitive due to their jaw characteristics, and points to an advanced specialization in shark evolution. It also provides a missing link in the understanding of how jawed vertebrates evolved from the jawless state -- a crucial initial step toward human evolution.
The development of the jaw is one of the most significant evolutionary events in early vertebrate history. Little is known about the jaw's origins, however, due to a poor fossil record of the critical time when the first jaws evolved, sometime before the Devonian period (412-354 million years ago). Until now, a 370-million-year-old shark called Cladoselache provided the paradigm of jaw evolution because good fossils of it have been available to study for more than a century.
Maisey's paper on Pucapampella, presented today at a conference on early vertebrate evolution hosted by the Natural History Museum of London, reveals evidence of jaw evolution that pre-dates Cladesolache by roughly 30 million years. "This is the earliest shark braincase that we can actually study in any detail," said Maisey. "The way we view the early evolution of the jaw now has to change."
Pucapampella's phylogenetic position lies at the base of the chondrichthyan lineage. Through detailed morphological analysis, Maisey found that Pucapampella's upper jaw was attached to the braincase in a way that was atypical for a chondrichthyan, and more like that of an osteichthyan, or bony fish. In evolutionary terms, bony fish have been considered to have a more advanced jaw structure than sharks. However, Pucapampella suggests that the converse it true. The fact that a shark as primitive as Pucapampella had a bony fish-like jaw attachment suggests that modern shark jaws are an more advanced characteristic than the jaws of bony fish. This closer evolutionary relationship between sharks and bony fish, in turn, influences how science may now view the relationship between jawed and jawless vertebrates. "This discovery removes one of the problems of deriving a jawed vertebrate from a jawless one by saying the jaw has a corresponding structure in a lamprey, which is jawless," explained Maisey.
Fossils of Pucapampella have only been found in Bolivia and South Africa, which were geographically closer during the Devonian period than they are today. This part of the Southern Hemisphere was covered by a cold, shallow ocean that dramatically contrasts the warm, tropical waters modern sharks prefer.
In addition to expanding our knowledge of how fish evolved, Maisey's discovery also expands science's understanding of our own origins. He points out that many characteristics of human anatomy, such as the jaw, originated in our fishy ancestors, even though we may not want to admit it. "The psychology of evolution is interesting," he said. "People don't mind being called a primate or a mammal, but they don't like being called a fish." With further investigations of fossil fish, vertebrate paleontologists may find themselves rewriting the early chapters of our evolutionary history.
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