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New Ideas About Old Sharks A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders New Ideas About Old Sharks A rare fossil sheds light on the poorly understood relationship between early sharks and bony fishes Susan Turner and Randall F. Miller he world’s oceans, lakes and riv- of the fossil led to surprising implica- bony fish succeeded in nature’s great Ters harbor about 30,000 species of tions. First, it became clear that early gamble where other creatures fell by fish. In addition to a diverse array of sharks were more diverse than scien- the wayside. In large part, their success bony fishes, scientists have described tists had imagined. Next, the presence began with the evolution of jaws. some 830 species of Chondrichthyes— of pectoral spines on the new fossil Jaws enable a predator to grasp sharks, skates, rays and other cartilagi- suggests that relationships among live prey while consuming it. Teeth nous fishes. Many paleobiologists are bony and cartilaginous species might allow the prey’s body parts to be cut working to decipher the fossil record be somewhat closer and also more or chewed for easy digestion. Such a left by the bony fish—our ancestors complex than had been thought. Thus, useful adaptation greatly increased the after all, and what we generally find this type of shark could help scientists evolutionary opportunities for early on the end of a fishing line. Others are work out the evolution of bony fishes, fishes and sharks. Jawless fishes, the concerned with the mysterious and the Osteichthyes. This and other fos- forerunners of today’s lampreys and closely connected story of how the sils being discovered around the world hagfishes, hung on, but in an evolu- sharks and their relatives came to be. look set to revolutionize our view of tionary backwater as parasites and Chondrichthyans are remnants of an early shark and fish evolution. scavengers. Without jaws and teeth, it ancient lineage, rare survivors from an seems, modern land vertebrates would explosion of forms that mostly disap- A Lot More Fish in the Sea never have arisen. Because of the im- peared before the end of the Devonian We begin our story with the earliest portance of this transition, paleontolo- Period, 359 million years ago. fish, going far back to a time when the gists have longed to have a complete Thanks to the rarity of intact shark planet was already old, but vertebrate picture of how primitive ancestral fossils, the transition from primitive life was just getting started. There were species evolved into the sharks whose fish to sharks and bony fishes is poorly neither sharks nor fishes as we know fictional counterparts terrorize movie- understood. Following the discovery them today. Instead, creatures that goers today. of a relatively complete early shark defy simple classification were swim- How sharks and bony fishes are fossil, paleontologists hope to better ming through the Earth’s seas. related is a key part of the puzzle. understand this important evolution- During the Ordovician Period, some Complete fossil fishes from this era ary progression. Initial examination 488 to 443 million years ago (mya), the are exceedingly rare, but sharks’ teeth first jawless fishes, or agnathans, arose. are common. But from teeth alone it’s Susan Turner is a senior research associate at the During the next geological period, the difficult to infer taxonomic classifica- Queensland Museum and Monash University, Silurian (443–416 mya), agnathans di- tions. So the hunt for good fossil fishes Australia. She has recently been an Australian versified. Through an elegant modi- continues. On July 4, 1997, a team led Research Council Fellow and Australian represen- fication of gill arches (specialized gill by one of us (Miller) found a fossil fish tative on the UNESCO-IUGS Scientific Board for IGCP (now International Geoscience Programme). structures), some evolved into the first in Canada that was a very special catch Her research concentrates on Paleozoic and early jawed fishes, which in turn diversified indeed: the most complete shark fossil Mesozoic fishes, especially sharks and shark ances- during the Devonian (416–359 mya) specimen from the early Devoni- tors. She also studies the use of fish microfossils into all the fishes known to - an. Identified as Doliodus problemati- to date rocks and understand the Earth’s past d a y. (One branch split off to become cus, a species known previously only geography. Randall F. Miller is curator of geol- land-dwellers and eventually hu- from teeth, this specimen has enabled ogy and chair of the Natural Sciences Division at mans.) Among these jawed fishes were close study of the body shape and den- the New Brunswick Museum, Canada, where he shark-like creatures, at the time but tal characteristics of a species thought studies various fossil groups including Paleozoic one kind of a bizarre array of marine to be among the earliest sharks—per- arthropods. The focus of his research has been on fauna. Also exploring the evolution- haps a missing link between sharks Quaternary late-glacial environments and beetle fossils. He is also an adjunct professor in the ary possibilities in the sea at this time proper and the so-called spiny sharks, Department of Geology at the University of New were myriad strange vertebrates and or acanthodians. So it was of particular Brunswick. Address for Turner: School of Geosci- invertebrates including placoderms interest that the fossil’s teeth or tooth ences, Monash University, Victoria 3800, Austra- (armored fish), trilobites and ammo- rows, were found in situ—a unique lia. Internet: [email protected] noids—all now extinct. The sharks and circumstance for a fossil this old. The 244 American Scientist, Volume 93 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. Chris Fallows/apexpredators.com Figure 1. Sharks, especially the white shark, occupy a special place in popular culture. Paleontologists, like moviemakers, focus particular attention on these ancient animals, whose teeth are abundantly distributed in the fossil record. Studies of shark origins promise to yield infor- mation on critical steps in early evolution that remain puzzling, such as the transition from jawless fishes to sharks and bony fishes. A recent finding of an intact fossil of an early shark may revise the story of these fascinating creatures. fossil shows that 400 million years ago, Sir Arthur Smith Woodward, doyen took on a new significance in early sharks had already developed the com- of paleontology at the Natural His- shark evolution. plex battery of teeth that made Jaws so tory Museum in London, formally Problems with classification aren’t frightening. described Doliodus (as Diplodus) teeth the only kind to have plagued stu- in 1892. Ramsey Traquair, his counter- dents of the “problematic deceiver.” Long in the Tooth part at the Royal Museum of Scotland Paleogeographers were puzzled too. It’s not easy to reconstruct fossil ani- in Edinburgh, revised the description Its fossils are found primarily in rocks mals. And among the most conten- the following year and changed the in Canada, from the Devonian part of tious fossils are those belonging to the name to Doliodus. Traquair realized the a continental agglomeration called Eu- specimen we found. Its name, Doliodus teeth were arranged in tooth rows, the ramerica. But most early shark fossils, problematicus, alerts you to its troubled conveyor-belt arrangement of teeth including some that looked like Dolio- history: It means “problematic deceiv- that enables elasmobranchs—mod- dus, are known from the Gondwana er.” These and many other fossil teeth ern sharks—to constantly replace lost supercontinent, which lay thousands have since confused paleontologists. teeth and agreed with Woodward that of miles to the southeast. So was Do- Typically millimeter-sized, Doliodus Doliodus was a shark. Succeeding gen- liodus not a shark, or were sharks a lot teeth were first found by a British collec- erations of paleontologists erroneously more widespread than thought? With tor known to us only as Mr. Jex. Despite changed Woodward’s and Traquair’s so little evidence to go on, it’s hard this character’s historical evanescence, classification. Taxonomically, then, to decide. So goes the study of early his fossils persisted in museums and Doliodus fossils have been homeless, shark evolution: Every tooth counts. other collections, and more Doliodus passed back and forth from the sharks In the case of Doliodus, scientists were teeth have been uncovered by others. to the acanthodians. It was not until keen to discover more. Indeed, before Although they have received plenty of recently that one of us (Turner) finally the Canadian team set out, one of us attention from eminent scientists, they recognized that Doliodus teeth indeed (Turner) advised them to “Keep an eye puzzle paleontologists to this day. belonged to a shark. Thus the species open for shark teeth.” Little did we www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2005 May–June 245 with permission only. Contact [email protected]. �������� ���������� �������� �������� ������������� ������� �������� ��������
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