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Diets, Habitat Preferences, and Niche Differentiation of Cenozoic Sirenians from Florida: Evidence from Stable Isotopes

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Diets, Habitat Preferences, and Niche Differentiation of Cenozoic Sirenians from Florida: Evidence from Stable Isotopes Paleobiology, 30(2), 2004, pp. 297±324 Diets, habitat preferences, and niche differentiation of Cenozoic sirenians from Florida: evidence from stable isotopes Bruce J. MacFadden, Pennilyn Higgins, Mark T. Clementz, and Douglas S. Jones Abstract.ÐCenozoic sediments of Florida contain one of the most highly fossiliferous sequences of extinct sirenians in the world. Sirenians ®rst occur in Florida during the Eocene (ca. 40 Ma), have their peak diversity during the late Oligocene±Miocene (including the widespread dugongid Me- taxytherium), and become virtually extinct by the late Miocene (ca. 8 Ma). Thereafter during the Pliocene and Pleistocene, sirenians are represented in Florida by abundant remains of fossil man- atees (Trichechus sp.). Stable isotopic analyses were performed on 100 teeth of fossil sirenians and extant Trichechus manatus from Florida in order to reconstruct diets (as determined from d13C val- ues) and habitat preferences (as determined from d18O values) and test previous hypotheses based on morphological characters and associated ¯oral and faunal remains. A small sample (n 5 6) of extant Dugong dugon from Australia was also analyzed as an extant model to interpret the ecology of fossil dugongs. A pilot study of captive manatees and their known diet revealed an isotopic enrichment («*) in d13C of 14.0½, indistinguishable from previously reported «* for extant medium to large terrestrial 18 mammalian herbivores with known diets. The variation in d OV-SMOW reported here is interpreted to indicate habitat preferences, with depleted tooth enamel values (ø25½) representing freshwater rivers and springs, whereas enriched values (ø30½) indicate coastal marine environments. Taken together, the Eocene to late Miocene sirenians (Protosirenidae and Dugongidae) differ signi®cantly in both d13Candd18O from Pleistocene and Recent manatees (Trichechidae). In general, Protosiren and the fossil dugongs from Florida have carbon isotopic values that are relatively positive (mean d13C 520.9½) ranging from 24.8½ to 5.6½, interpreted to represent a specialized diet of pre- dominantly seagrasses. The oxygen isotopic values (mean d18O 5 29.2½) are likewise relatively positive, indicating a principally marine habitat preference. These interpretations correlate well with previous hypotheses based on morphology (e.g., degree of rostral de¯ection) and the known ecology of modern Dugong dugon from the Paci®c Ocean. In contrast, the fossil and extant Trichechus teeth from Florida have relatively lower carbon isotopic values (mean d13C 527.2½) that range from 218.2½ to 1.7½, interpreted as a more generalized diet ranging from C3 plants to seagrasses. The relatively lower oxygen isotopic values (mean d18O 5 28.1½) are interpreted as a more diverse array of freshwater and marine habitat preferences than that of Protosiren and fossil dugongs. This study of Cenozoic sirenians from Florida further demonstrates that stable isotopes can test hy- potheses previously based on morphology and associated ¯oral and faunal remains. All these data sets taken together result in a more insightful approach to reconstructing the paleobiology of this interesting group of ancient aquatic mammalian herbivores. Bruce J. MacFadden, Pennilyn Higgins, and Douglas S. Jones. Florida Museum of Natural History, Uni- versity of Florida, Gainesville, Florida 32611. E-mail: bmacfadd@¯mnh.u¯.edu, E-mail: loligo@¯mnh. u¯.edu, and E-mail: dsjones@¯mnh.u¯.edu Mark T. Clementz.* Department of Earth Sciences, University of California, Santa Cruz, California 95064. Accepted: 15 August 2003 Introduction waters throughout the subtropical and tropi- Extant sirenians are fully aquatic mamma- cal Paci®c and Indian Oceans (Walker 1975; lian herbivores that constitute the Order Sir- Husar 1978a). The second family, the Tri- enia. One of the two modern sirenian families, chechidae, consists of a single genus, Triche- the Dugongidae, with an extant monotypic chus, the manatee, a mixed-feeding herbivore genus of hypergrazer, the dugong (Dugong du- with three species distributed throughout riv- gon), is widely distributed in coastal marine ers, estuaries, and coastal marine areas of the tropical and subtropical Atlantic Ocean. Of relevance to the present study, the manatee * Present address: Smithsonian Marine Station at Fort Pierce, 701 Seaway Drive, Fort Pierce, Florida 34949. native to Florida is referred to T. manatus lati- E-mail: [email protected] rostrius, the West Indian Manatee (Walker q 2004 The Paleontological Society. All rights reserved. 0094-8373/04/3002-0009/$1.00 298 BRUCE J. MACFADDEN ET AL. 1975; Husar 1978b). Despite this limited mod- Eocene of Florida (Ivany et al. 1990), demon- ern diversity and disjunct distribution, which strating very well preserved seagrass beds is further threatened by human impact, sire- (with Thalassia-like taxa similar to those of the nians have had a rich and fascinating macro- Caribbean basin today) associated with fossil evolutionary history of diversi®cation and ad- sirenian bones referable to Protosiren sp. aptation over the past 50 million years since (Domning et al. 1982; Hulbert 2001). their split from the basal Tethytheria (a clade In contrast to the specialized seagrass graz- also including proboscideans [McKenna and ing of dugongs, manatees are considerably Bell 1997]). more generalized mixed-feeders, with Triche- One of the richest and most extensive fossil chus manatus latirostrius reported to consume records of sirenians occurs in the nearshore, over 60 plant species in Florida (Ames et al. shallow marine and estuarine sediments ex- 1996). In addition to seagrasses, the diet of Tri- posed throughout Florida (Hulbert 2001), and chechus can consist of an array of freshwater this sequence is critical to understanding the and marshy vegetation, including a consider- diversi®cation of this order. The systematics, able proportion of true grasses (Poaceae) such morphology, and adaptive hypotheses con- as Distichlis, Spartina, Paspalum,andPanicum cerning these sirenians have been treated ex- (Domning 1982, 2001a). The rostral de¯ection tensively in earlier descriptions (e.g., Simpson angle of Trichechus is relatively weak (,408; 1932; Reinhart 1976) and in a series of more Fig. 1F; Domning 2001a). With the exception recent papers by Domning (e.g., 1988, 1989a,b, of the earliest trichechid Potamosiren magdalen- 1990, 1994, 1997a). sis from the middle Miocene of Colombia In addition to developing a modern system- (Domning 1997b), other manatees have rela- atic framework for the Sirenia, Domning (e.g., tively short-crowned teeth that are shed and 1981, 2001a) has developed hypotheses about continually replaced ``conveyor-belt''-style the diet and ecology of extinct dugongs and from back to front throughout ontogeny. This manatees based on those of modern sirenians, continuous source of teeth is interpreted as an morphological correlates in the fossil record, initial adaptation to feeding on abrasive grass- and associated geological and paleontological es (Domning 1982, 1997b, 2001b), although evidence. Domning (2001a) de®nes the more recently manatees have adapted to a ``aquatic megaherbivore adaptive zone'' and more diverse herbivorous diet. explains how different sirenians partition Within the past decade, stable isotopes have food resources and other aspects of individual become increasingly useful in reconstructing species niches. Modern dugongs are predom- the paleoecology and paleoenvironment of ex- inantly grazers, feeding on seagrasses (Hy- tinct vertebrates. The original literature on drocharitaceae and Potamogetonaceae), al- this subject is expanding rapidly (see, e.g., though they also eat algae when seagrasses Koch 1998 for recent review). These geochem- are limited or unavailable. These sirenians are ical studies have allowed (1) testing of previ- characterized by bunodont (bulbous-cusped) ously existing hypotheses based on more tra- cheek teeth with limited tooth replacement ditional (i.e., morphological) data, and (2) re- and a high degree of rostral de¯ection (Fig. construction of various aspects of aut- and 1E), which indicates the use of the down- synecology of vertebrate species and com- turned snout for digging up seagrasses, in- munities not previously discernable with con- cluding their rhizomes. This high degree of ventional data. Studies of stable carbon iso- rostral de¯ection (up to 708 [Domning 1977]) topes preserved in tooth enamel carbonate can be traced back into the fossil record to the have mostly concentrated on terrestrial ver- middle Eocene, and for extinct dugongs is tebrates and communities in an attempt to re- likewise interpreted as an adaptation to feed- construct ancient herbivore diets and their ing on ancient seagrasses. Paleontological ev- feeding adaptations on C3 or C4 plants. Stable idence suggesting the long-standing sea cow oxygen isotopes, which have been used exten- and seagrass plant-animal coevolutionary in- sively in ancient marine ecosystems, have terrelationship ®rst comes from the middle been more dif®cult to apply to terrestrial set- FLORIDA CENOZOIC SIRENIANS 299 FIGURE 1. Comparisons of degree of rostral de¯ection in extant and fossil sirenians. A, Primitive morphology in Prorastomus sirenoides from the middle Eocene of Jamaica (adapted from Savage et al. 1994). B, Protosiren fraasi from the middle Eocene of Egypt (adapted from Domning 2001b). C, High degree of rostral de¯ection in Metaxytherium ¯oridanum from the early Miocene Lower Bone Valley Formation, Florida (adapted from
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