Miocene Locality Research Articles

Testudo cf. graeca from the new Late Miocene locality of Platania (Drama basin, N. Greece) and a reappraisal of previously published specimens

In this paper we describe a new Neogene mammalian site and one of the latest discoveries of a fossil tortoise in Greece, from Platania locality, in Drama basin, Northern Greece. The preliminary study of the numerous fossil mammal remains shows that the recovered fauna is of Late Miocene age. The tortoise material consists of a partial shell and a limb bone that belong to the same individual. They are attributed to the Testudo cf. graeca, and are members of the Testudo s.s. lineage, characterized by the presence of a hypo–xiphiplastral hinge. This new material indicates that the presence of T. graeca in Greece could expand into the Late Miocene. Comparisons with other fossils of the genus Testudo s.s. from Greece, allowed the revision of previously published specimens of T. graeca.

A new species of Opisthodactylus Ameghino, 1891 (Aves, Rheidae), from the late Miocene of northwestern Argentina, with implications for the paleobiogeography and phylogeny of rheas

ABSTRACTA new species of rheid, Opisthodactylus kirchneri, sp. nov., is erected on the basis of associated elements of both hind limbs from the late Miocene in northwestern Argentina. The new species extends the biochron of Opisthodactylus from early Miocene to late Miocene and its distribution from Patagonia to northwest Argentina. Cladistic analysis recovered an Opisthodactylus-Pterocnemia clade as sister to a Rhea americana clade. The Opisthodactylus-Pterocnemia clade would have inhabited the most southern, central, and western regions of southern South America throughout the early-middle Neogene, whereas the Rhea stock would have had a north-northeastern or Brazilian ancestral distribution in the lowlands of the continent. The similar biogeographic patterns of living and fossil rheids, cariamids, and tinamids seem to roughly reflect the environmental shift from closed to open habitats that took place at the southern end of South America during the Neogene and Pleistocene, and at least in the former two families the effects of isolation produced by the ‘Paranaense’ sea. Closed-habitat taxa of these three families are recorded at early Miocene localities in Patagonia (O. horacioperezi, O. patagonicus, Noriegavis santacrucensis, and Crypturellus reai), whereas open-habitat taxa come from late Miocene–early Pliocene sites at central (Pterocnemia sp. and Eudromia sp.), northwestern (O. kirchneri and Pterocnemia cf. mesopotamica), and northeastern (Pterocnemia mesopotamica) regions in Argentina.http://zoobank.org/urn:lsid:zoobank.org:pub:0BAD0813-31F8-4101-8742-93BDBCABE807Citation for this article: Noriega, J. I., E. A. Jordan, R. I. Vezzosi, and Juan I. Areta. 2017. A new species of Opisthodactylus Ameghino, 1891 (Aves, Rheidae), from the late Miocene of northwestern Argentina, with implications for the paleobiogeography and phylogeny of rheas. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1278005.

The southernmost fossil record of Squamates

Squamates form a substantial part of the present-day South American herpetofauna, and their fossils constitute an indispensable evidence for understanding the origin and evolution of the main taxa. Squamates are relatively common in Miocene localities of Patagonia, especially in levels of the late early Miocene Santa Cruz Formation. In this contribution, remains of the three species of the extinct iguanidErichosaurusAmeghino 1899 (E. diminutus,E. bombimaxillaandE. debilis) are redescribed, and new squamate specimens are reported for first time. The genusErichosaurusis considered invalid.Erichosaurus debilis,E. diminutusand a new specimen are recognized as indeterminate species of the extant polichrotinePristidactylus, whereasE. bombimaxillaremains as an indeterminate iguanid. Snakes are represented by an indeterminate colubrid. All these specimens, together with a tupinambine teiid previously described for the same formation, represent the southernmost fossil record of squamates in South America and indicate the occurrence of the iguanidPristidactylus, the teiidTupinambisand the colubrid snakes south to their present distribution as back as during the early Miocene.

Late Oligocene caviomorph rodents from Contamana, Peruvian Amazonia

AbstractThe Deseadan South American Land Mammal Age (late Early Oligocene – Late Oligocene) attests to a time of great diversification in the caviomorph rodent fossil record. Nevertheless, Deseadan rodent‐bearing localities in Neotropical lowlands are few and poorly known. Here we describe the rodent assemblages from two Late Oligocene localities, near Contamana, Loreto, Peru. Seven taxa are new to science: Palaeosteiromys amazonensis gen. et sp. nov., Plesiosteiromys newelli gen. et sp. nov., Loretomys minutus gen. et sp. nov., Scleromys praecursor sp. nov, Ucayalimys crassidens gen. et sp. nov., Chambiramys sylvaticus gen. et sp. nov. and Chambiramys shipiborum gen. et sp. nov. These rodent faunas show that caviomorphs were relatively diverse in Peruvian Amazonia during the Late Oligocene, with the co‐occurrence of at least three extant superfamilies: Erethizontoidea, Octodontoidea and Chinchilloidea. Additionally, they mark the earliest known occurrences of Scleromys, of a small erethizontid closely related to Microsteiromys and of an adelphomyine closely reminiscent of Ricardomys (all taxa previously restricted to Miocene localities thus far). They also document a form potentially related to Eosallamys (previously known from around the Eocene–Oligocene transition at Santa Rosa in Peruvian Amazonia). Finally, the geographical range of Adelphomyinae and of Deseadomys is widely expanded to the lower latitudes of South America for the Deseadan interval. The latter elements, in addition to the record of a very primitive species of Scleromys, suggest the absence of palaeogeographical and palaeoenvironmental barriers within the southern cone of South America before the Oligocene–Miocene transition.

NEW RECORDS AND ICHNOSPECIES OF LINEAR LEAF MINES FROM THE LATE MIOCENE-PLIOCENE FROM ARGENTINA AND THE ESTABLISHMENT OF LEAF-MINING ICHNOTAXOBASES

The ichnospecies Cuniculonomus parallelus Givulescu 1984 and Stigmellites serpentina Kozlov 1988 are recorded for the first time in Argentina and the new ichnospecies Cuniculonomus saltensis isp. nov., Stigmellites vitatus isp. nov. and Stigmellites pervenae isp. nov are defined from two Miocene localities: Penas Blancas and Quebrada del Horno, from northwestern Argentina. A set of characters to identify fossil linear mines is provided as well as five ichnotaxobases to differentiate the ichnogenera Cuniculonomus , Stigmellites and Phytomyzites. Two ichnospecies are now revised and included in Phytomyzites: P. arcuata (Krassilov 2008a) comb. nov. ( Ophionoma ) and P. crucitracta (Krassilov 2008a) comb. nov. ( Troponoma ). Finally, a review of fossil record of linear mines is provided.

Palaeoecological interpretation of the late Miocene landscapes and vegetation of northern Greece: A comment to Merceron et al., 2016 (Geobios 49, 135–146)

Recent works on feeding habits of ungulates, isotopic composition of equid tooth enamel, and phytoliths from late Miocene localities of northern Greece suggested the presence of savannah and excluded dense forests. Furthermore, Mediterranean-like climates were advocated for the late Miocene of Greece. Here, I compare palaeoenvironments inferred for two mammal localities from Chalkidiki and the Axios Valley (Nikiti, upper Vallesian and lower Turolian; Dytiko, upper Turolian) with evidence from contemporaneous plant assemblages from adjacent areas in Greece and Bulgaria. I use vegetation units inferred from pollen and spore, fruit and seed, and leaf assemblages and compare them with the vegetation inferred from mammal and phytolith data. Open vegetation as inferred from mammal and phytolith data is also part of the range of vegetation units discovered from the pollen and spore, seed and fruit, and leaf record (here called the palaeobotanical record). Poaceae are consistently present in late Vallesian to late Turolian pollen records of northern Greece. Further, a number of vegetation units are indicative of forest vegetation ranging from lowland to upland forests dominated by woody angiosperms and mixed coniferous-angiosperm forests. The presence of such forests is not in conflict with the results from mammal and phytolith studies, but it broadens the view on landscapes present in the late Miocene of northern Greece. In addition to a generalized vegetation type commonly inferred by mammal studies, the palaeobotanical record demonstrates the presence of various complementary vegetation types. A comprehensive view on late Miocene landscapes in northern Greece shows that there is no conflict in inferring open herb dominated landscapes and light to dense forests and provides new opportunities for the ecological interpretation of late Miocene ungulates.

The huge-sized deinothere Deinotherium proavum (Proboscidea, Mammalia) from the Late Miocene localities Pikermi and Halmyropotamos (Greece)

Presented here are the deinotheriid dental and postcranial remains from the Late Miocene localities Pikermi and Halmyropotamos (Greece). The study and comparison of the available juvenile dental material from Pikermi with other relevant specimens from Europe showed that it belongs to the huge-sized Deinotherium proavum (= D. gigantissimum). Additionally, several postcranial specimens from Pikermi, as well as from Halmyropotamos, present deinotheriid features, which distinguish them from elephantoids, and permit their attribution also to D. proavum. This species is known from the Turolian and represents the terminal evolutionary stage of the European deinotheres. Its presence in Pikermi and Halmyropotamos is compatible with the middle Turolian (MN 12) age of the localities. Additionally, the taxonomy of European deinotheres is discussed, as well as the biostratigraphical and geographical distribution of D. proavum.

Another piece of the puzzle: the first report on the Early Miocene lizard fauna from Austria (Ottnangian, MN 4; Oberdorf locality)

The lizard fauna from the Early Miocene of Austria, namely from Oberdorf in the Styrian basin, is described here for the first time. The Oberdorf is a particularly interesting locality, because its paleoenvironment is slightly different from that of other Central European MN 4 localities, such as Dolnice in the Cheb basin. Oberdorf has rare forest and pond-like biotopes in a swampy environment near a river system. Although the fossil material is only fragmentarily preserved, it supplies important information about palaeobiodiversity and taxa spatial distribution in the Central Paratethys area during the Early Miocene. Six major lizard clades are present in Oberdorf: Gekkota, ? Chamaeleonidae, Lacertidae, ? Scincidae, Cordylidae and Anguidae. Many of these are first described from Austria. The Oberdorf lizard fauna composition is comparable to that in Lower Miocene localities including Petersbuch 2 in Germany and also Dolnice in the Czech Republic. The Oberdorf palaeobiodiversity is decidedly lower. The maxilla material designated Gekkota indet. resembles that in Euleptes; however, the limited preservation of the find precludes precise conclusions. Chamaeleonids are rare at Oberdorf, but the Scincoidea clade is abundantly represented. This includes the cordyloid material and the material tentatively assigned to Scincidae. Two types of lacertids are identified: Lacerta cf. poncenatensis and Lacerta sp. The first forms the youngest known record of this species. Among anguids, Ophisaurus cf. spinari occurs in this locality, indicating that this taxon habited a wider range of environmental conditions, for example, than O. fejfari.

Muridae from the late Miocene site of Venta del Moro (Eastern Spain)

Murids are usually the dominant faunas of the late Miocene and early Pliocene micromammal assemblages. The present work deals with the murid faunas of the well-known late Miocene locality of Venta del Moro, comprising over 2700 molars ascribed to the taxa Apodemus gorafensis, Occitanomys alcalai, Paraethomys meini and Stephanomys dubari. This list differs from those of previous studies by the presence of A. gorafensis instead of A. gudrunae. The presence of A. gudrunae in younger localities than Venta del Moro implies that both species, considered ancestor and descendant, coexisted for some time. In addition, the analysis of such an extensive collection has allowed us to check the variability of the studied species in great detail, such as the development of the longitudinal connections of O. alcalai, to a greater extent than previously described. Finally, we discuss the biometrical changes of the lineages of Paraethomys in Southern Europe from MN13 to MN15, and hypothesize about the mechanisms behind these variations in size.

The extinct river shark Glyphis pagoda from the Miocene of Myanmar anda review of the fossil record of the genus Glyphis (Carcharhiniformes: Carcharhinidae).

We redescribe an extinct river shark, Glyphis pagoda (Noetling), on the basis of 20 teeth newly collected from three different Miocene localities in Myanmar. One locality is a nearshore marine deposit (Obogon Formation) whereas the other two localities represent terrestrial freshwater deposits (Irrawaddy sediments), suggesting that G. pagoda from the Irrawaddy sediments was capable of tolerating low salinity like the extant Glyphis. Glyphis pagoda likely reached up to at least 185 cm in total body length and was probably piscivorous. The fossil species occurs in rocks of Myanmar and eastern and western India and stratigraphically ranges at least from the Lower Miocene (Aquitanian) to thelower Upper Miocene (mid-Tortonian). Ithas been classified under at least eight other genera to date, along with numerous taxonomic synonyms largely stemming from the lack of understanding of the heterodonty in extant Glyphis in the original description. Our literature review suggests that known Miocene shark faunas, particularly those in India, are manifested with unreliable taxonomic identifications and outdated classifications that warrant the need for a comprehensive taxonomic review in order to evaluate the evolutionary history and diversity pattern of Miocene shark faunas. The genus Glyphis has a roughly 23-million-year-long history, and its success may be related to the evolution of its low salinity tolerance. While extant Glyphisspp. are considered to be particularly vulnerable to habitat degradation and overfishing, the fossil record of G. pagoda provides renewed perspective on the natural history of the genus that can be taken into further consideration for conservation biology of the extant forms.

Revision of non-Passeriform birds from Polgárdi (Hungary, Late Miocene): 1. Anseriformes

Polgardi is a series of Late Miocene localities in central Hungary, which have yielded one of the richest Miocene avifaunas. The taxonomic composition of the waterfowl (Anseriformes) from Polgardi is revised. As a result of revision, Anas denesi Kessler, 2013 is transferred to the genus Aythya. This species has a combination of primitive and advanced morphological characters, which do not come in conflict with the concept of gradual formation of morphological features of the modern type in various lineages of Aythya. Another small duck from Polgardi referred to as Anas albae Janossy, 1979 is only possible to be identified as Mergini gen. indet.

A Dorcatherium (Mammalia, Ruminantia, Middle Miocene) petrosal bone and the tragulid ear region

ABSTRACTA petrosal bone from the French Middle Miocene locality Sansan is described. It belongs to the tragulid species Dorcatherium crassum. Using high-resolution X-ray computed tomography, we scanned this petrosal bone and segmented its embedded bony labyrinth. We compared the morphological data with those of three living Tragulidae: Tragulus javanicus, Hyemoschus aquaticus, and Moschiola meminna. We ran a phylogenetic analysis where fossil and extant pecoran ruminants were included to understand the distribution of characters of the petrosal bone and bony labyrinth within ruminants. We propose a set of new synapomorphies for the Tragulidae among the Ruminantia, including the absence of the basicapsular groove, a knob-shaped mastoid region, a knob anterior to the subarcuate fossa on the petrosal bone, a high number of cochlea turns, the dorsal insertion of the posterior limb of the lateral semicircular canal, the extension of the lateral semicircular canal beyond the plane of the posterior semicircular canal in dorsal view, and a pouch-like endolymphatic sac on the bony labyrinth. The clade constituted by Tragulus and Moschiola is confirmed by new synapomorphies such as a double convexity of the promontorium and a large pouch-like endolymphatic sac that covers the common crus. We demonstrate the potential of characters of the bony labyrinth for the phylogeny of ruminants.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Mennecart, B., and L. Costeur. 2016. A Dorcatherium (Mammalia, Ruminantia, Middle Miocene) petrosal bone and the tragulid ear region. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1211665.

A new Miocene vertebrate assemblage from the Río Yuca Formation (Venezuela) and the northernmost record of typical Miocene mammals of high latitude (Patagonian) affinities in South America

Geological explorations of the basal beds of the Río Yuca Formation (Tucupido region, Portuguesa State, western Venezuela) resulted in the recognition of a new vertebrate assemblage that includes eight taxa: the toxodont cf. Adinotherium, a Peltephilidae armadillo, the freshwaters fishes Platysilurus and Phractocephalus, the caiman Purussaurus, an indeterminate dolphin, turtles, and the previously recognized sloth Pseudoprepotherium venezuelanum. When compared with the higher latitudes faunas of Argentina and Chile, the presence of cf. Adinotherium and peltephilids in the Rio Yuca Formation is consistent, but not conclusive, with a Santacrucian to Frisian SALMA age. The associated fauna, as well recent apatite fission track analysis, indicates that the Río Yuca assemblage is more likely younger in age, specifically Middle to Late Miocene. So far, the Miocene localities of the northern part of South America have provided a less prolific fossil record compared to the southern part of the continent (e.g., Santacrucian and Friasian faunas of Patagonia), but the present work documents the surprising occurrence of two taxa (Peltephilidae and Nesodontinae) common in southern high latitude faunas of South America, implying the persistence of the Santacrucian-Friasian genus Adinotherium in younger strata from northern South America, and that peltephilids were much more widespread during the Miocene than previously recognized. The presence of these common Patagonian taxa (Peltephilidae and Nesodontinae) in Río Yuca also supports the hypothesis of prior researchers for the existence of biogeographical connections between the northern and southern portions of South America during the Late Oligocene or Early Miocene, which facilitated faunal interchange between the two regions. Finally, the biogeographical affinities of the freshwater fishes and the giant caiman (Purussaurus) indicate close relationships of the Tucupido region with the ancestral distribution of the Orinocoan–Amazonian drainage system.

The earliest member of Neocricetodon (Rodentia: Cricetidae): a redescription of N. moldavicus from Eastern Europe, and its bearing on the evolution of the genus

AbstractThe genus Neocricetodon represents one of the most diverse radiations of extinct cricetines. Its early evolution remains unclear, mainly owing to the incomplete fossil record. One of the earliest and least known species of this genus is N. moldavicus described from the early late Miocene localities Bujor 1 and Calfa, Moldova. We reexamined the type material of this species and compared it with other species of Neocricetodon. Despite its old geological age, N. moldavicus demonstrates rather advanced dental morphology including the anterocone of M1 being deeply split; mesolophs and mesolophids partially reduced; and the labial spur of the anterolophule, ectomesolophids, and lingual anterolophid of m2 lacking. The phylogenetic analysis of 15 Neocricetodon species based on 22 dental characters revealed three synapomorphies for the genus: presence of the M1 labial anterolophule, the four-rooted M2, and presence of the labial anterolophulid of m1. “Kowalskia cf. schaubi” from Rudabánya and “Kowalskia sp.” from Vösendorf, formerly considered as the earliest finds of Neocricetodon, do not possess these characters and rather represent the genus Democricetodon. Our study also suggests that Kowalskia complicidens is a member of Sinocricetus, and that N. neimengensis and N. zhengi are junior synonyms of N. polonicus. The recognition of N. moldavicus as the earliest member of the genus within the clade containing the species of Neocricetodon from France and Spain supports the hypothesis of the Eastern European origin of the Western European Neocricetodon species.

The Maragheh hipparions, late Miocene of Azarbaijan, Iran

Morphological analyses of hipparionine astragali, calcaneum, third phalanx and metapodials from the late Miocene locality of Maragheh, Iran, was carried out using bivariate plots and log10 ratio diagrams. The results, together with previous studies on cranial and dental material, have allowed us to characterise and define the following equid species in the Maragheh assemblage: Hipparion gettyi, aff. Hippotherium brachypus, Cremohipparion aff. moldavicum, Cremohipparion matthewi and Hipparion campbelli. These species are arrayed in three successive biostratigraphic intervals: H. gettyi, late Vallesian Lower Maragheh horizon; aff. H. brachypus and Cr. aff. moldavicum, early Turolian Middle Maragheh levels; and H. campbelli, late early Turolian Upper Maragheh interval. The small hipparion, Cr. matthewi, would appear to range throughout all biostratigraphic intervals of Maragheh. The proposed taxonomic and biostratigraphic resolution here is slightly different from previous studies especially in the recognition of aff. H. brachypus instead of Hipparion prostylum. The Maragheh hipparion assemblages are well correlated to those from Turkey, Greece, the Balkans and Black Sea region and are clearly different from those of Central and Western Europe, central Asia, China and Africa. Maragheh Cremohipparion is related to species of Cremohipparion from Greece, China and the Siwaliks. A clear niche differentiation based on palaeodiet studies has been revealed in Maragheh hipparions, indicative of different environmental adaptations amongst these species.

New Material of the Hominoid Ouranopithecus macedoniensis from the Late Miocene of the Axios Valley (Macedonia, Greece) with Some Remarks on Its Sexual Dimorphism

New hominoid teeth from the late Miocene locality Ravin de la Pluie (RPl) of the Axios Valley (Macedonia, Greece) are studied in this article. Their morphology, dimensions and proportions are similar to the hominoid Ouranopithecus macedoniensis, allowing their attribution to this taxon. The studied material provides some new morphological characters for the female P³ (small asymmetry, small mesiobuccal crown projection, paracone higher than protocone) and the lower canine (mesial groove: large in the male and small in the female, distobuccal fovea: large in the female and small in the male). The new material enriches the collection of O. macedoniensis. The estimated degree of sexual dimorphism of the RPl O. macedoniensis, calculated by the multivariate size dimorphism method, is compared with those of extant hominoids (Gorilla, Pan, Pongo) and of the late Miocene Lufengpithecus lufengensis from China, which is considered as more dimorphic than any living hominoid. The results suggest that Ouranopithecus multivariate size dimorphism for the premolar, molar and post-canine row is similar to those of Pongo and Lufengpithecus, slightly higher than that of Gorilla and clearly higher than that of Pan. Therefore, O. macedoniensis is apparently one of the most sexually dimorphic hominoids and the RPl assemblage is monospecific.

The first hominoid from the Maragheh Formation, Iran

Miocene hominoid fossils are known from Africa and Eurasia, in the latter ranging widely from western Europe to Anatolia and from South Asia to Southeast/East Asia. Iran is located between the known western and eastern Eurasian hominoid distributions and is potentially important in understanding Miocene hominoid dispersal patterns. Maragheh is a late Miocene fossil locality in northwestern Iran, well known since the nineteenth century for its abundant mammalian fossils. However, until now, the only primate fossils reported from Maragheh or Iran were the Old World monkey Mesopithecus pentelicus. Recent field research at Maragheh has changed this situation by the discovery of the first hominoid fossil from Iran, a maxillary fragment with well-preserved second and third molars. Here, we provide a detailed description of this new specimen, comparing it with other similarly large-sized Eurasian late Miocene hominoids, Ouranopithecus, Ankarapithecus, Sivapithecus, and Indopithecus. Molar morphology of the Maragheh hominoid is similar to that of these Eurasian Miocene genera, with only minor differences in morphology and wear pattern. Based on the presently available materials, we tentatively prefer the interpretation that the Maragheh hominoid may be related more closely to either Ankarapithecus or Sivapithecus rather than to Ouranopithecus, but the fragmentary nature of the fossil makes evaluations difficult. Future discoveries of this Iranian hominoid are needed to determine its phylogenetic position with more certainty.

Anurans of the Tagay Locality (Baikal Lake, Russia; Miocene): Bombinatoridae, Hylidae, and Ranidae

The Miocene (MN 5) locality of Tagay (Baikal Lake, Russia) yielded isolated bones of five anuran taxa: Bombina sp. (Bombinatoridae), Hyla sp. (Hylidae), Pelophylax sp. and Rana sp. (Ranidae), Bufo mirus (Bufonidae), the first four of which are described here. In Tagay anuran assemblage, Ranidae are dominant, whereas Hylidae and Bombinatoridae are extremely rare. The Tagay Bombina and Hyla represent the oldest records of these genera in Asia. The Tagay Bombina possibly belongs to an ancestral Bombina lineage, which originated in Asia by the Oligocene-Miocene boundary. The Tagay Hyla could be an early member of the East Palearctic species-group of Hylidae that originated during the early Miocene in Asia. Pelophylax sp. and Rana sp. are among the earliest records of the lineages of green and brown frogs, respectively, in Asia.

Nelumbo fossils from the Miocene deposits of Dobrzyń nad Wisłą, Central Poland

The classification of the Zingiberaceae infructescence of Amomum europaeum Kownas from a classic Miocene locality in Dobrzyń nad Wisłą (Central Poland) is revised. Superficially, A. europaeum resembles recent Amomum and Etlingera infructescences in shape and size. However, no other features link A. europaeum to Zingiberaceae. A re-examination of all visible structures reveals that these A. europaeum remains represent the horizontally-flattened receptacle (‘gynobase’) of Nelumbo (Nelumbonaceae). Miocene paleoclimatic and paleoecological conditions in Dobrzyń were similar to those of modern Nelumbo habitats, which supports the new interpretation.

Paleoecology of Miocene crocodylians in Florida: Insights from stable isotope analysis

Fossils from late Miocene localities in north Florida offer new information about the paleoecological relationships between sympatric alligatorine and tomistomine crocodylians. Large, robust tomistomines (traditionally referred to Gavialosuchus americanus) outnumber Alligator fossils at localities representing coastal/estuarine depositional environments. Stable isotope analysis of carbon (δ13C) and oxygen (δ18O) values in fossil crocodylian tooth enamel carbonate (CO3) provides evidence for niche partitioning between these two taxa, which may have co-inhabited estuarine paleoenvironments. Our isotopic results imply that alligators procured prey from primarily freshwater or terrestrial sources, whereas tomistomines fed mainly on marine-based prey items. As indicators of saltwater paleoenvironments, the fossil tomistomines may therefore be informative for tracking Miocene sea level change.