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A Toothy Tale of Evolution

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A Toothy Tale of Evolution The Sedimentary Record INTRODUCTION The idea of faunal niche replacement has been discussed for well over a century. Darwin (1859) first proposed the idea by using A Toothy Tale the analogy of “wedges hammered into a log.” Classic paleonto- logical examples include the replacement of non-avian dinosaurs by mammals (Benton, 1990, 1996a,b), the replacement of bra- chiopods by pelecypods as the dominant post-Paleozoic benthic of Evolution: bivalved fauna (Gould and Calloway, 1980), and even the replacement of whole monophyletic groups of articulated bra- Convergence in Tooth Morphology chiopods by other monophyletic articulated brachiopod groups (Ciampaglio, 2004). Less attention has been paid to morphologi- among Marine Mesozoic – Cenozoic cal convergence as a direct consequence of faunal niche replace- Sharks, Reptiles, and Mammals ment. Although the phenomenon has been addressed, particular- ly within reef complexes throughout the Phanerozoic Charles N. Ciampaglio*, Gregory A. Wray†, and Bruce H. Corliss‡ (McKerrow, 1978; Wood, 1999), studies involving quantitative analyses are rare in the paleontological literature. *Wright State University, Lake Campus, 7600 State Route 703, Celina, OH Morphological convergence among Mesozoic marine reptiles 45822 [email protected] and Cenozoic marine mammals is well known (Figure 1). When †Duke University, Department of Biology, Box 90338, Durham, North the fossilized remains of Eocene whales were first uncovered in Carolina 27708 Louisiana and Alabama they were originally identified as belong- ‡Duke University, Earth and Ocean Sciences, Box 90338, Durham, North ing to a “plesiosaur-like” reptile and named Basilosaurus by Carolina 27708 Harlan (1834, 1835). It was nearly five years later that Owen (1839) corrected Harlan’s taxonomic mistake and properly iden- ABSTRACT tified the fossilized animals as marine mammals. Recently, many Although mechanisms of niche replacement are discussed thoroughly studies analyzing the biomechanics of swimming and feeding in the evolutionary paleontological literature (i.e., extinctions, competi- have used the modern cetaceans as suitable proxies for Mesozoic tion, evolution of new adaptive morphologies), actual studies involving marine reptiles (Taylor, 1987; Massare, 1987, 1997; Budney, quantitative analyses are not common. In this study, morphological fea- 2002). In addition to converging upon similar body plan, tures of dentition in Late Cretaceous and Cenozoic marine vertebrate marine mammals have also converged upon the small number of predators were analyzed.The analysis included species of Late tooth morphologies exhibited by Mesozoic marine reptiles Cretaceous and Cenozoic sharks, Late Cretaceous marine reptiles, and (Massare, 1997). Cenozoic marine mammals. Dental characters used in the study were In this study, morphological features of dentition of Late both discrete and continuous. Species included in the analysis were Cretaceous and Cenozoic marine vertebrate predators were ana- originally collected from Late Cretaceous and Cenozoic rocks from lyzed using multivariate statistics. While Mesozoic marine rep- the south-central, southeastern, and the mid-Atlantic regions of the tiles and Cenozoic marine mammals show a surprising amount United States, as well as Europe and the Pacific Rim. of morphological convergence, elasmobranchs (sharks and rays) A morphometric “tooth space” was constructed using the eigenvec- display a wide, divergent array of morphological tooth types tors generated from Principal Component Analysis of the dental char- (Kent, 1994; Capetta, 1987). Although morphological tooth acter data.The results of the analysis show that Mesozoic marine rep- diversity among elasmobranchs is high during the Late tiles occupied a small, discrete region of the tooth morphospace, Cretaceous, the greatest number of tooth designs are present whereas Cretaceous sharks occupied a much larger, diffuse region of throughout the Eocene. the morphospace. During the Paleogene a profusion of shark tooth Since tooth morphologies reflect dietary preferences, conver- morphologies occurred and then expanded into new areas of tooth gence among functionally homologous tooth types most likely morphospace.Yet,no overlap with the morphospace previously occu- indicates dietary convergences as well. Thus, by analyzing the pied by Mesozoic marine reptiles occurred.A large number of novel species content of the occupied regions of the tooth morpho- tooth morphologies evolved with the evolution of marine mammals space we propose that it is possible to unravel guild structures, during the Cenozoic. Remarkably, many of the tooth forms converged investigate cases of tooth convergence, and determine the overall on the Mesozoic marine reptile designs, and hence a major overlap of expansion or contraction of feeding strategies among the marine marine mammal tooth morphospace with the previously occupied vertebrate predators from the Late Cretaceous Period through Mesozoic marine reptile morphospace occurred.Additionally, the shift the Cenozoic Era. from heterodonty (teeth of different types) to homodonty (teeth of similar types) occurred in several members of both the Mesozoic DENTITION IN MARINE VERTEBRATES marine reptiles and the Cenozoic marine mammals. Late Cretaceous marine reptiles – Marine reptiles were the domi- Based on dental morphology, this study indicates that following the nant predators of the Jurassic and Cretaceous seas. Large, mobile, extinction of the Mesozoic marine reptiles during the Late Cretaceous, fully pelagic, predatory groups include the orders Ichthyosauria, Cenozoic sharks failed to occupy the vacated niches, yet Cenozoic Sauropterygia (plesiosaurids and pliosaurids), and the family marine mammal dentition converged on the previous Mesozoic marine Mosasauridae. Although the basic dental crown design is a sim- reptile tooth designs.Apparently, Cenozoic marine mammals occupied ple cone, many complex morphologies have arisen, including the vacated Mesozoic marine reptile dietary niches. modified designs for piercing, crushing, cutting, and tearing (Peyer, 1968; Pollard, 1968; Massare, 1987). Most Mesozoic 4 | December 2005 The Sedimentary Record and marine mammals (Peyer, 1968; Hillson, 1986; Massare, 1987). Since this study involves all three vertebrate groups, a compre- hensive classification scheme that accommo- dates each major tooth type was required. What follows is a hybrid tooth scheme based on the previous classifications referenced above (Figure 2). Clutching – Teeth are generally small, with a low profile crown, typically surrounded by small cusplets. The enamaloid is frequently folded or ridged longitudinally, increasing the strength of the crown. Clutching dentition is used to restrain prey, as well as crush weakly armored prey. Crushing/Grinding – Teeth generally have a high crown with a polygonal outline, or are robust with a bulging crown that is transverse- Figure 1: Selected skulls and dentitions of Late Cretaceous marine reptiles and Cenozoic marine ly streamlined. Typically individual teeth form mammals. Clockwise from upper-left: Ichthyosaurus; Killer Whale; False Killer Whale; Pacific White Sided Dolphin; the plesiosaur Thalassomedon; the mosasaur Tylosaurus. a dental plate with a nearly planar surface, or functional rows. This form of dentition is used to fragment or smash open hard-shelled prey marine reptiles have a homodont dentition Elasmobranchs (sharks and rays) – While the that typically inhabit benthic or near bottom (teeth of similar types), but in many cases, fossil record of sharks extends back into the settings. e.g., ichthyosaurs, homodonty is a derived Late Silurian, it was during the Late Devonian Grasp/Crush/Chop – Teeth are usually coni- character (Massare and Calloway, 1990). that a burst of evolutionary diversification cal and robust. Sometimes teeth bear fine lon- Cetaceans and pinnipeds (Cenozoic marine occurred (Zangerl, 1981; Carroll, 1988). gitudinal ridges. This type of dentition is used mammals) – The predominant Cenozoic verte- Paleozoic tooth forms are generally sharply to puncture and restrain prey, as well as to brate predators belong to the mammalian pointed, with one or more cusps. This form of smash and chop bones. orders Cetacea (whales, dolphins, and porpois- dentition provides an effective means for Pierce/Gouge – Teeth are smooth and point- es) and Pinnipedia (seals, sea-lions, and wal- grasping and holding prey. During the later ed. Sometimes teeth are recurved. Teeth may ruses). The first known cetaceans date to the part of the Mesozoic and early Cenozoic, be delicate and slender, or relatively robust. early Eocene (Gingerich et al., 1983). Early neoselachian (modern sharks, rays, and skates) This dentition is used to pierce and grasp prey, whales, the protocetids and basilosaurids, had squaliform, lamniform, and carchariniform or used to wrench out pieces of flesh. a heterodont dentition (teeth of different shark groups developed a highly effective jaw Slicing/Gouge – Teeth are generally flattened types). Molars are generally multi-cusped and mechanism that allowed these sharks to gouge labio-lingually or ellipsoid in cross-section. have a more complicated form than pre- and wrench large pieces from prey. The crown may be serrated, or have cutting molars. The early cetaceans could use their Additionally, many neoselachians evolved edges. Teeth function to slice and/or gouge the dentition to shear and grind, as well as pierce sharp, blade-like teeth, often with serrated
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