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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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Figure 2. Fossil discoveries in the deep geological record show that the early fishes diversified during the Ordovician period into several major groups, many of which went extinct by the end of the Devonian. Classified based on the physical characteristics of their fossils, these include the Agnatha (including anaspids, galeaspids and thelodonts), the Placodermi (a group of armored fish) and shark relatives including the Acan- thodii (the so-called “spiny sharks”). Survivors from this time include the true sharks, or Chondrichthyes (of which only the holocephalian and elasmobranch species survive) and the bony fishes, or Osteichthyes, including the lobe-finned sarcopterygians and the ray-finned actinopter- ygians, which comprise most modern fishes. Modern tetrapods, including human beings, are descendants of the sarcopterygians. Contempo- rary with the early jawless fishes were conodonts, a group of uncertain affinity that arose in the Cambrian. In the early Ordovician to late Si- lurian, two major (now extinct) groups arose. Arandaspids are armored, jawless vertebrates that may be the earliest known craniates—animals with a braincase. Their fossils peter out around the end of the Ordovician. Heterostracans lasted longer, until the end of the Devonian.
know how good that advice would Before long, McGovern handed ontologists to fill significant gaps in turn out to be. over a specimen that we quickly real- knowledge. Our group that day consisted of ized was a significant discovery: the The most important feature of this New Brunswick Museum staff and oldest known articulated shark fossil. fossil is its paired pectoral spines. six-year-old Sasha Miller. Sasha turned All its teeth were in place, and almost These suggest that many isolated fossil over slabs of rock, and one of us (Mill- the entire front part of the shark had spines might have belonged to sharks er) worked on their stratigraphy—an- been beautifully preserved, including rather than acanthodians as previously alyzing the rock layers to place the the braincase, jaws and pectoral fins, believed. The adaptive significance of associated fossils in geologic time. all in their anatomically correct posi- these spines is uncertain. They were Meanwhile, Jeff McGovern of the mu- tions. The mudstone had entombed prevalent in acanthodians, placoderms seum staff and Heather Wilson of the the shark’s cartilage, teeth, scales and and these new “sharks,” but are pres- University of Manchester excavated a long, thick spine jutting out along ent today only in a few sharks (on dor- fossil-rich mudstone a few yards away. the front edge of each pectoral fin, a sal fins) and catfish. The spines may Aside from sharks’ teeth, the exotic feature previously unknown in carti- have helped ward off giant eurypter- fossil fauna included jawless cepha- laginous fishes—all this in a specimen ids and other predatory arthropods. laspid fish, placoderms, acanthodians of a species identifiable as Doliodus, If they began as additional enlarged and weird invertebrates such as gi- the deceiver previously known only scales on the leading edges of the later- ant sea scorpions, or eurypterids. The from teeth. By amazing good fortune, al fins, then they might also have been sediment was deposited a little after our colleague Richard Cloutier, from a result of sexual dimorphism, the dif- 407 million years ago (according to the the Université du Québec, recovered ferentiation of male and female body dates of volcanic rocks underneath) part of the braincase a month later. But shape. Until now, most fossil spines in a calm lagoon or estuary; scattered why were the discoveries important? have been attributed to acanthodians, land-plant fossils indicate proximity to Having a complete articulated fossil which are characterized by having an ancient shoreline. (with parts connected) enables pale- spines on all their paired fins.
246 American Scientist, Volume 93 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] So perhaps Doliodus wasn’t a shark at all. Features of the fossil blur the �� ������������������������ �� ��������������������������� distinction between acanthodians and ��������� early chondrichthyans. To put the new �� ���������������������������� fossil into the context of early fish evo- ������� ������� lution, it is necessary to look in more �� ��������������� ����������� detail at the origins of sharks from �� ��������������������� their jawless ancestors. ����������� ����������� Where Do They All Come From? During their early evolution, fishes ������������� diversified into dozens of taxonomic groups, which paleontologists recog- ���������� � nize by studying the fossils and coin- ��������� ��������� � ��������� ing excruciating polysyllabic names. � The names of these groups (the genera � ������ or species, collectively taxa) are given ����� ����������� ���������� based on morphology, or physical char- � acteristics, alone; the biological concept ������ �������� of a species (which rests on the ability of � individuals to produce viable offspring) ������������� cannot be tested directly in extinct ani- � mals. The various groups’ origins are ������� ������ generally regarded as monophyletic, �������� �������� each having arisen from a single com- mon ancestor—separate branches on Figure 3. Paleogeographic reconstruction of early Devonian continents shows Earth domi- the evolutionary tree arising from a nated by two large land masses, which contained bodies of water. Most undisputed shark fos- single stem. sils (numbered markers) come from regions that were part of Gondwana—which is therefore Some paleontologists hypothesize thought to be where sharks originated. The discovery in Canada of a remarkably intact fossil that vertebrates originated in Gond- of Doliodus problematicus (1), an early shark, places a possible origin of sharks in Euramerica, wana, because most fossils come from or at least suggests that sharklike fish were more widespread at an earlier time. (Image based on original by C. R. Scotese, Paleomap Project.) areas that were once part of that paleo- continent. However, tiny cone-shaped fossil teeth from conodonts, eel-like ani- front mals with a finned tail and a possible notochord (a supporting rod later in- corporated into the vertebral column in true vertebrates), are widely found in strata from a broad span of geologic time—the Cambrian to the late Trias- sic, some 500 to 100 mya—in areas that once were also part of Euramerica. On pectoral this basis, others hold that conodonts ﬁn �������� are the most primitive vertebrates and spines ���������� that vertebrates arose in Euramerica. Others refute the classification of con- odonts as vertebrates and say that the fossils are not even teeth. A team led by Degan Shu from the Early Life Institute at Northwest Uni- versity, Xi’an, China, claims the earliest vertebrates based on fossils from the lower Cambrian Qiongzhusi Forma- tion near Chengjiang. These specimens give a glimpse into the first fishlike de- signs, but they bear no sign of the hard back bone and dentine structures regarded as fully vertebrate features. The Chinese fossils do highlight Figure 4. Fossil of Doliodus problematicus (left) excavated in New Brunswick, Canada, in 1997 the close connection between proto- is the oldest complete shark. It was preserved lying dorsal side up, with the head at the top, vertebrates and early sharks or fishes. and extending to the pelvic region. The map at right represents the fossil’s main features, some However, it’s still not clear at what not evident in the photograph. Pectoral fin spines are not previously known from a chondrich- point sharks can be said to exist for thyan of the early Devonian. (Author photographs courtesy of New Brunswick Museum.)
www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2005 May–June 247 with permission only. Contact [email protected] certain. When do the first sharks un- equivocally appear in the fossil record? Textbooks still parrot the convention- al thinking that no fossil sharks are found before the late Devonian, but this dogma ignores work from the last three decades. The oldest microfossils definitely attributable to sharks are scales in Silurian strata (440 mya) of Siberian and Arctic Russia. (Modern sharks’ skin is covered with minute scales, or denticles, that give the skin a sandpapery texture.) A whole new suite of forms dating from just a little later, about 430 mya, Figure 5. In a late Devonian marine community, represented here by a museum exhibit, some has been found in Cornwallis Island animals would be vaguely familiar to today’s scuba diver; others are long-extinct species that only a paleontologist would recognize. A Cladoselache shark, an early elasmobranch, swims in and other islands in the Canadian Arc- search of prey that might have included soft-bodied invertebrates. Sessile marine life included a tic. These scales bear features, such as variety of plants and invertebrates. (Photograph courtesy of Royal Tyrrell Museum, Canada.) a relatively wide and short base com- pared to the crown, that distinguish ��� them from other scales and have led ��������� to their being assigned to a new shark ��� ��������� ������������������������������� ������ family dubbed the Kannathalepididae. Comprising a series of odontodes— mineralized structures that are the basic unit of vertebrate teeth—these ��������� scales are also found in early Silurian deposits in Canada, possibly pushing ���� the date of the earliest sharks even far- �������� ther back. Early Silurian deposits in the Tarim Basin of western China have ���� �������� also yielded fin spines associated with ���� sharklike scales.
�������� �������� ������������������������������������������������������� Are these fossils true sharks? If so, ���� the lineage was apparently toothless for millions of years. The first indis- ������ putable shark teeth do not turn up un- ������������������������������������������������� til about 50 million years after the ap- ����������������������������������������������������� ���� pearance of these first putative shark ������� scales in the late Ordovician. And they ���� ������������������������������������������� ���������� are quite recognizable. ��������� ���� Among the most significant recent ��� ������������������������������������������������������� ��������� contributions to a modern classification ���� of fossil shark teeth was that of Herman ���������� ���� �������������������������������������������������� Mader, working during the 1980s at the
�������� ������� University of Göttingen in Germany. ������������ Based on his 1986 description of scales and teeth, several modern groups of ���� ������������������������������������������������� ������������������������������������������������ sharks had their origins pushed back ���������� to the very beginning of the Devonian. ���� He was also the first to present an evo- lutionary tree, or cladogram, for sharks ���������� ���������������������������������������� based on teeth. Sadly, Mader did not ����������� ���� �������������������������������������������������� continue his work for health reasons. ����������� ���� ���� Figure 6. Timeline of shark evolution begins ��������������������������������� to emerge from fossil findings. Although the ���������� ��������� New Brunswick Doliodus problematicus find pushes back the earliest intact shark fossil to ���� the early Devonian—the subperiod known as the Emsian—there are pre-Devonian fos- ����������� sils that may place the origin and subsequent diversification of sharklike fauna in earlier ���� periods, possibly even the late Ordovician.
248 American Scientist, Volume 93 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] Figure 7. Jeff McGovern of the New Brunswick Museum staff (right) points to the site at the Campbellton Formation, near Campbellton, New Brunswick, where he discovered the oldest intact shark fossil, assigned to Doliodus problematicus. A close-up of the fossil in situ in a rock outcrop is shown at left. (Photographs courtesy of Randall Miller.) The oldest shark teeth described by specimens, which provides some as- ple—and these structures, along with Mader belong to Leonodus carlsi. This surance on their taxonomic status as other features, are under study in an shark lived around 418 mya, on either chondrichthyans. Other teeth belong- attempt to elucidate the evolutionary side of the Silurian-Devonian bound- ing to putative sharks are found earlier, affinities among the various group- ary, in what is today southwestern Eu- but their kinships remain unresolved. ings. Especially with the discovery of rope. Like Doliodus, Leonodus fossils are Fossil teeth display internal as well as Doliodus’s special internal scales on the now being studied by paleontologists external structure—some have typi- gill arches, of particular interest is how in Spain and Germany as articulated cal dentine nutrient canals, for exam- sharks are related to a contempora-
Weighing Up Scales
lthough the classification of shark skin scales ap- distinctive trait helps pin things down: A small open- Apears to be a rather esoteric branch of paleontology, ing in the side of the scale identifies it as coming from it turns out to be important in untangling the knots of a chondrichthyan, or cartilaginous fish. An odontode early shark evolution. Classifying these scales is fraught has three parts: a crown formed of dentine, sometimes with traps for the unwary. Designations are based on capped with a thin layer of enameloid (hypermineral- studying the morphology of the scale and comparing it ized tissues), penetrated with internal tubules that co- to the features of later fossils and modern sharks. A typi- alesce into a pulp cavity, and a base of bony tissue for at- cal shark skin or placoid (platelike) scale is made up of a tachment. The odontode is the basis for the development single odontode; the complex scales seen in many fossil of all scales (dermal denticles) and teeth. In sharks, as in sharks have several odontodes. An early fossil placoid people, a tooth is a highly sophisticated odontode, but scale might belong to a shark or to a thelodont, a primi- shark scales may be made up of a series of tiny simple or tive but separate jawless group that had skin scales. A complex odontodes. Scales are by far the most abundant vertebrate fossils from the Paleozoic. Along with the far scarcer teeth and spine fossils, they delineate the present knowledge of early sharks from the Ordovician and Silurian periods. The first articulated shark fossils—fossils including body parts, not just isolated bits—appear in the Devonian. Consequently, considerable effort is devoted to identifying the remains and arranging the enormously varied specimens into class- es or categories. One of the most detailed and complete classification schemes was developed by Valentina Karatajute-Talimaa of the Lithuanian Geological Research Institute. She has spent many years classifying early to mid-Paleozoic shark scales based on shape and microscopic structures, and Shark skin scales show similarity to thelodont branchial scales, came up with nine basic types. Although future finds will shown in a scanning electron micrograph from Loganellia scotica, no doubt lead scientists to modify and add to these, any providing evidence for the two groups’ common evolutionary analysis of chondrichthyan phylogeny should incorporate origin. (Image courtesy of Wim van der Brugghen.) this systematization of early scale types.
www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2005 May–June 249 with permission only. Contact [email protected] of this work has been geared to gar- nering evidence to support the early realization that thelodonts and sharks share key features. A thelodont origin for sharks was put forward by early paleontologists such as Traquair, but the idea was not extensively explored until recently. Resolving the issue depends on �������� ��������� the microsquamose exoskeleton—a skin �������� ��������� made up of tiny scales—that is charac- teristic of thelodonts, and whether this is a feature of primitive vertebrates only or is also seen in advanced fishes. The nature of the scaly thelodont skin has implications for our under- standing of where thelodonts fit into the evolutionary scheme of things. The pattern of scale distribution in thelo- donts is complex, comprising varied zones and including both internal scales that line the mouth down to the ��������� �������� pharynx and specialized scales around the branchial (gill) region. Branchial �������� Figure 8. Teeth of early��������� sharks, reconstructed from fossils, display a diverse range of shapes. scales have been found in several the- Teeth classified as hybodont (top left) are elongated, with a low profile, bear sinuous ridges lodont genera, and thus might be char- and are pierced with numerous nutrient canals. The two-pronged or diplodont teeth (top right) acteristic of thelodonts in general. are among the earliest known fossil shark teeth. The cladodont tooth (lower left) comprises a Some authors claim that thelodont main central cusp and smaller lateral cusps and does not appear until after the Middle Devo- branchial scales are primitive tooth nian. Symmorid teeth (lower right) are similar to cladodonts in design, but don’t appear until whorls, putative tooth-rows as seen the late Devonian. in the typical pattern of sharks and neous group of scale-covered jawless ceding the earliest evidence of sharks other jawed fishes. Walter Gross of the fishes, the thelodonts. is exceedingly spotty: Virtually all the University of Tübingen first character- information comes from isolated scales ized scales from Loganellia scotica, but Sharks and Thelodonts from other kinds of fishes. Among the he failed to recognize their significance Since sharks must have evolved from most abundant types of these belong to as branchial scales. One of us (Turner) an earlier vertebrate, their origins thelodonts. Because of their potential anticipated that all thelodonts would might be easier to establish if this an- importance as shark ancestors, one of possess these branchial scales, along with cestor can be determined. However, us (Turner) has been investigating thelo- other types lining the upper pharynx. This the fossil record during the era pre- dont fossils for more than 30 years. Much prediction was based on comparing the features of modern sharks with those of L. scotica, from examination of a collection made by Alex Ritchie, now at the Australian Museum. It is well known that the skin of mod- ern sharks is covered with tiny tooth- like scales, but it was not until 1970 that Gary Nelson, then at the American Museum of Natural History, made an elegant study of their specialized in- ternal scales: intricate banks of scales that line the branchial bars inside the throat. The similarity of these scales to those of thelodonts alerted Turner to the possibility that thelodonts were more closely related to sharks than had been thought. In 1991 two Dutch pa- leontologists, Wim van der Brugghen and Jo Vergoossen, made beautiful scanning electron microscope images that highlight the complexity of L. scot- Figure 9. Doliodus problematicus tooth embedded in matrix shows how this fossil fits into the ica branchial scales. Similar scales were tooth morphology classification scheme above. (Photograph courtesy of the authors.) found in the Canadian Arctic in 1998
250 American Scientist, Volume 93 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] by Mark Wilson and Michael Caldwell of the University of Alberta. Another early Silurian thelodont from Wisconsin has scales that are made up of multiple odontodes. The zonation and growth ����������� pattern of thelodont branchial scales ���������� now appears more complex than pa- leontologists first thought and pushes back in time the formation of complex “pre-teeth.” These details of scale morphology and evolution together imply a need to rethink previous notions of how teeth became evolutionarily associated with jaws. The accepted model is that teeth evolved from the modification of specialized skin scales that became attached to the jaw cartilages (parts of Figure 10. Fossilized braincase of the same Doliodus problematicus specimen shown in Figure the gill branches that are believed to 4 preserved the precerebral fontanelle, an unossified region between skull bones. This feature have evolved into the lower and up- establishes the specimen as the earliest chondrichthyan cranium. (Photograph courtesy of the per jaws), and the structures evolved authors.) in tandem. Imagine a primitive jaw- very rare only 15 or 20 years ago but understanding of the early evolution less fish whose foreparts (lips) are cov- since then a bewildering diversity of sharks. ered with sharp, mineralized scales, of specimens has come to light. The Such analyses in themselves may not and these toothlike scales attaching to Devonian, the “Age of Fishes,” was a be enough. The presence of putative proto-jaws. But there is an alternative time when nature was experimenting sharks in the Ordovician—the period be- scenario. In 2000, based on their claim with multitudes of different forms, and tween the Cambrian and the Silurian—is that conodont fossils are in fact verte- species were exploring new ecologi- suggested by scales that are at least as brate teeth, Moya Smith at King’s Col- cal niches. Clearly this was a crucial complex as those of accepted sharks. lege London and Michael Coates at the time period. Certain specimens have But as mentioned, it’s uncertain whether University of Chicago hypothesized been identified as chondrichthyans, or or not sharks originated as early as the that dermal odontodes (or toothlike even neoselachians (modern sharks) Ordovician. Part of the problem is that skin scales) and teeth arose as separate such as the early Devonian Mcmurdo- modern analyses seem to work back to structures. This idea uncouples the dus whitei from western Queensland, front. Rather than looking at a fossil and synchronous evolution of early teeth the teeth of which closely resemble trying to decide if it’s a shark, we need and jaws. Fitting this hypothesis with those of the cow sharks. Other studies to look closely at the early record and the thelodont observations suggests based on Late Devonian to Carbonifer- base our definition of “shark” on those that these fish were the first to evolve a ous sharklike fishes typically called fossils. This is necessary to determine recognizable “dentition” (the complex “cladodonts” do not actually help our whether the modern and the fossil fishes branchial denticles)—but within their throat, not on the jaw cartilages. Significantly, the specialized, whorl- like branchial scales in thelodonts are different in size and shape from the buccal scales, which line the inside of the mouth. The buccal scales more closely resemble the typical external head scales and so could instead have been teeth precursors, consistent with the traditional model. The road to a full understanding of early fish evolu- tion is beset with twists and turns. It might also help to stop and consider what a shark is. As so often in paleon- tology, clearing up the various ques- tions will require more specimens. This prospect is looking increasingly likely with the spate of new discover- ies in recent years. Figure 11. Articulated fossil specimens of Loganellia scotica suggest how thelodonts might have looked. The internal features of thelodonts may be more important for current debates Fossil Frenzy in paleontology. Thelodont fossils display not only complex patterned skin scales but also Fossils from the first 50 million years branchial scales—specialized internal banks of scales in the gill region that may be the earli- of cartilaginous fish evolution were est “pre-teeth” known.
www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2005 May–June 251 with permission only. Contact [email protected] Figure 12. Distinctive fin spine on the pectoral fin of the Doliodus problematicus fossil (top right), evident in the fossil map (lower right), was the most surprising feature of the find. Isolated spines had not been thought to be associated with sharks but with acanthodians, the so-called “spiny sharks,” that survived from the Devonian into the early Triassic. A more typical isolated spine (left) is that identified as Climatius latispinosus, a type of acanthodian. (Photographs courtesy of the authors.) now called “sharks” are indeed closely schemes. Next, theorists must explore Miller, R. F., R. Cloutier and S. Turner. 2003. related. During the first 50 million years in more detail how existing speci- The oldest articulated chondrichthyan from we already seem to have several early mens might demonstrate new aspects the Early Devonian period. Nature 425:501– 504. different types of “sharks.” of relatedness between the various Smith, M. M., and M. I. Coates. 2001. The evo- This initial period of shark evolution groups of early fishes. Finally, more lution of vertebrate dentitions: phyloge- will continue to be an exciting if confus- fieldwork is needed. Climate and ter- netic pattern and developmental models. ing area of study for paleontologists. rain in the areas in which the latest In Major Events in Early Vertebrate Evolution. Most of the fossils are tiny scales and fossils have been found are demand- Palaeontology, Phylogeny, Genetics and De- velopment, Systematics Association Special teeth, found as isolated specimens out ing and even hostile, but we expect Volume Series 61, ed. P. E. Ahlberg. New of context from the original animal. that continued exploration will pay York: Taylor and Francis. Imagine you are an alien trying to fig- off handsomely. Traquair, R. H. 1893. Notes on the Devonian ure out what a human looks like from a fishes of Campbelltown and Scaumenac Bay fingernail or molar, and you get a sense Bibliography in Canada. Geological Magazine 10:145–149. of the difficulties. This is why complete Brugghen, W. van der, and J. Vergoossen. 1991. Turner, S. 1991. Monophyly and interrelation- fossils are so important. All the thou- Late Llandovery scales. Ichthyolith Issues 6:46. ship of the Thelodonti. In Early Vertebrates and Related Problems of Evolutionary Biol- Coates, M. I., and S. E. K. Sequeira. 2001. Early sands of other fragmentary fossils can ogy, ed. M.-M. Chang, Y.-H. Liu and G.-R. sharks and primitive gnathostome interrela- be put in perspective. Before the new Zhang. Beijing, Science Press, pp. 87–119. tionships. In Major Events in Early Vertebrate Doliodus fossil, only one other articulat- Evolution. Palaeontology, Phylogeny, Genetics Woodward, A. S. 1892. On the Lower Devoni- ed fossil shark from the early Devonian and Development, Systematics Association an fish-fauna of Campbellton, New Bruns- was known, Pucapampella from South Special Volume Series 61, ed. P. E. Ahlberg. wick. Geological Magazine 9:1–6. Africa. Since Doliodus, an early Devo- New York: Taylor and Francis. Young, G. C. 1982. Devonian sharks from Janvier, P. 1996. Early Vertebrates. Oxford: Ox- south-eastern Australia and Antarctica. Pal- nian Leonodus carlsi has been found. Ar- aeontology 25:817–843. ticulated fossils from the middle Devo- ford University Press. Long, J. A., and G. C. Young. 1995. Sharks from nian are more abundant. Typical species the Middle-Late Devonian Aztec Siltstone, include Pucapampella from Bolivia, Ant- southern Victoria Land, Antarctica. Records arctilamna prisca from Antarctica and of the Western Australia Museum 17:287–308. Australia and, possibly, Gladbachus ad- Mader, H. 1986. Schuppen und Zähne von entatus from Germany. What is needed Acanthodiern und Elasmobranchiern aus now is a good Silurian example. dem Unterdevon Spaniens (Pisces). Göt- tinger Arbeiten für Geologie und Paläontologie For relevant Web links, consult this The pieces are slowly but surely 28:1–58. issue of American Scientist Online: coming together. The “problematic Maisey, J. G. 1996. Discovering Fossil Fishes. New deceiver” is living up to the name as- York: Henry Holt. http://www.americanscientist.org/ signed to it more than a century ago, Maisey, J. G. 2001. A primitive chondrichthyan IssueTOC/issue/721 posing as many new questions as it braincase from the Middle Devonian of answers. The task ahead is daunting Bolivia. In Major Events in Early Vertebrate Evolution. Palaeontology, Phylogeny, Genetics but rich with potential rewards. The and Development, Systematics Association first step is to assess the new finds Special Volume Series 61, ed. P. E. Ahlberg. in terms of existing classification New York: Taylor and Francis.
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