Mark T. Clementz

Whales originated from aquatic artiodactyls in the Eocene epoch of India

Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.

Fluvial response to abrupt global warming at the Palaeocene/Eocene boundary

Climate strongly affects the production of sediment from mountain catchments as well as its transport and deposition within adjacent sedimentary basins. However, identifying climatic influences on basin stratigraphy is complicated by nonlinearities, feedback loops, lag times, buffering and convergence among processes within the sediment routeing system. The Palaeocene/Eocene thermal maximum (PETM) arguably represents the most abrupt and dramatic instance of global warming in the Cenozoic era and has been proposed to be a geologic analogue for anthropogenic climate change. Here we evaluate the fluvial response in western Colorado to the PETM. Concomitant with the carbon isotope excursion marking the PETM we document a basin-wide shift to thick, multistoried, sheets of sandstone characterized by variable channel dimensions, dominance of upper flow regime sedimentary structures, and prevalent crevasse splay deposits. This progradation of coarse-grained lithofacies matches model predictions for rapid increases in sediment flux and discharge, instigated by regional vegetation overturn and enhanced monsoon precipitation. Yet the change in fluvial deposition persisted long after the approximately 200,000-year-long PETM with its increased carbon dioxide levels in the atmosphere, emphasizing the strong role the protracted transmission of catchment responses to distant depositional systems has in constructing large-scale basin stratigraphy. Our results, combined with evidence for increased dissolved loads and terrestrial clay export to world oceans, indicate that the transient hyper-greenhouse climate of the PETM may represent a major geomorphic ‘system-clearing event’, involving a global mobilization of dissolved and solid sediment loads on Earth’s surface.

Isotopic records from early whales and sea cows: contrasting patterns of ecological transition

Abstract Recent fossil discoveries of early cetaceans and sirenians document the functional transitions that occurred as each group adapted to a completely aquatic existence, but the timing and path of their ecological transition remain uncertain. We analyzed the stable-isotope composition of tooth enamel from several early members of each group to reconstruct the dietary, foraging, and habitat preferences of basal taxa. Carbon isotope (δ13C) values provided evidence of foraging within freshwater, terrestrial, and marine environments, including seagrass beds, whereas oxygen isotope (δ18O) variation was used as a measure of commitment to aquatic habitats. Enamel samples were collected from four regions (south Asia, north and west Africa, and southern Europe) spanning the late early Eocene (ca. 53.5 Ma) to the late Eocene (ca. 36 Ma). Sirenian and cetacean taxa included species that were likely capable of some terrestrial locomotion, and more specialized forms that were morphologically fully aquatic. Cetacean δ13C and δ18O values indicate that some early members of this group (some pakicetids) inhabited freshwater environments, but later members (e.g., remingtonocetids, protocetids, and basilosaurids) moved quickly into estuarine and marine environments. Low δ18O variation confirms that all of these early forms were primarily aquatic, but δ13C and δ18O values for early sirenians indicate an early transition to a marine seagrass diet without any evidence of an intermediate connection to freshwater habitats.

Early Eocene warming events and the timing of terrestrial faunal exchange between India and Asia

The timing of initiation of continent-continent collision between Asia and India is controversial, but this major tectonic event is generally thought to have occurred in the Early Eocene, ca. 50 Ma. New and independent data from strontium isotopes, stable carbon isotopes, microfossil biostratigraphy, and mammal fossils from an Early Eocene marginal marine sequence (Cambay Shale) at the Vastan Lignite Mine of western India indicate that terrestrial faunal exchanges, and therefore continental collision, between Asia and the Indian subcontinent took place before 53.7 Ma. This age coincides with the second Eocene Thermal Maximum (ETM2), a short-lived warming episode that followed the Paleocene-Eocene Thermal Maximum (PETM) ca. 55.5 Ma. Our data also document, for the first time, a clear record of the ETM2 in terrestrial organic material from a low-latitude site, which is represented by a 3‰−4‰ carbon isotope excursion (CIE) in lignite and dispersed organic carbon δ13C values. The magnitude of the CIE at this location closely matches that observed in marine cores from the Arctic Ocean, which supports an interpretation that this hyperthermal event, though of lower magnitude, was similar in character to that of the PETM, being a global phenomenon that affected both terrestrial and marine ecosystems.

Identifying Aquatic Habits Of Herbivorous Mammals Through Stable Isotope Analysis

Abstract Large-bodied, semiaquatic herbivorous mammals have been a recurring component of most continental ecosystems throughout the Cenozoic. Identification of these species in the fossil record has largely been based on the morphological similarities with present-day hippopotamids, leading to the designation of this pairing of body type and ecological niche as the hippo ecomorph. These morphological characters, however, may not always be diagnostic of aquatic habits. Here, enamel δ13C and δ18O values from living hippopotamuses were examined to define an isotopic signature unique to the hippo ecomorph. Although δ13C values do not support unique foraging habits for this ecomorph, living and fossil hippopotamids typically have low mean δ18O values relative to associated ungulates that fit a linear regression (δ18Ohippopotamids = 0.96 ± 0.09·δ18Ofauna − 1.67 ± 2.97; r2 = 0.886, p < 0.001). Modeling of oxygen fluxes in large mammals suggests that high water-turnover rates or increased water loss through feces and urine may explain this relationship. This relationship was then used to assess the aquatic adaptation of four purported hippo ecomorphs from the fossil record: Coryphodon (early Eocene), Moeritherium and Bothriogenys (early Oligocene), and Teleoceras (middle–late Miocene). Only fossil specimens of Moeritherium, Bothriogenys, and large species of Coryphodon had δ18O values expected for hippo ecomorphs; δ18O values for Teleoceras and a small species of Coryphodon were not significantly different from those of the associated fauna. These results show that the mean δ18O value of fossil specimens is an effective tool for assessing the aquatic habits of extinct species.

New insight from old bones: stable isotope analysis of fossil mammals

Abstract Stable isotope analysis of fossil materials has become an increasingly important method for gathering dietary and environmental information from extinct species in terrestrial and aquatic ecosystems. The benefits of these analyses stem from the geochemical fingerprint that an animals environment leaves in its bones, teeth, and tissues. Ongoing study of living mammals has found the stable isotopic composition of several light (hydrogen, carbon, nitrogen, oxygen, and sulfur) and even a few heavy (calcium and strontium) elements to be useful tracers of ecological and physiological information; many of these can be similarly applied to the study of fossil mammals. For instance, the carbon isotopic composition of an animals tissues tracks that of the food it eats, whereas the oxygen isotopic compositions of the carbonate and phosphate in an animals bones and teeth are primarily controlled by that of the surface water it drinks or the water in the food it ingests. These stable isotope proxies for diet and habitat information are independent of inferences based on morphological characters and thus provide a means of testing ecological interpretations drawn from the fossil record. As such, when well-preserved specimens are available, any dietary study of fossil species should seriously consider including this approach. To illustrate the potential benefits associated with applying these methods to paleontological research, a review of current work on the ecological and evolutionary history of fossil mammals through geochemical analysis is presented. After a brief introduction to issues associated with the preservation of stable isotopic information in soft and mineralized tissues, a series of case studies involving the application of stable isotope analysis to fossil mammal research is discussed. These studies were selected to highlight the versatility of this analytical method to paleontological research and are complemented by a discussion of new techniques and instrumentation in stable isotope analysis (e.g., laser ablation and compound-specific isotope ratio mass spectrometry, and calcium and clumped isotopes), which represent the latest advances in the extension of these geochemical tools to the paleontology of fossil mammals.

Anthracobunids from the middle eocene of India and pakistan are stem perissodactyls.

Anthracobunidae is an Eocene family of large mammals from south Asia that is commonly considered to be part of the radiation that gave rise to elephants (proboscideans) and sea cows (sirenians). We describe a new collection of anthracobunid fossils from Middle Eocene rocks of Indo-Pakistan that more than doubles the number of known anthracobunid fossils and challenges their putative relationships, instead implying that they are stem perissodactyls. Cranial, dental, and postcranial elements allow a revision of species and the recognition of a new anthracobunid genus. Analyses of stable isotopes and long bone geometry together suggest that most anthracobunids fed on land, but spent a considerable amount of time near water. This new evidence expands our understanding of stem perissodactyl diversity and sheds new light on perissodactyl origins.

Diets, habitat preferences, and niche differentiation of Cenozoic sirenians from Florida: evidence from stable isotopes

Abstract Cenozoic sediments of Florida contain one of the most highly fossiliferous sequences of extinct sirenians in the world. Sirenians first occur in Florida during the Eocene (ca. 40 Ma), have their peak diversity during the late Oligocene–Miocene (including the widespread dugongid Metaxytherium), 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 manatees (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 δ13C values) and habitat preferences (as determined from δ18O values) and test previous hypotheses based on morphological characters and associated floral and faunal remains. A small sample (n = 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 (ϵ*) in δ13C of 14.0‰, indistinguishable from previously reported ϵ* for extant medium to large terrestrial mammalian herbivores with known diets. The variation in δ18OV-SMOW reported here is interpreted to indicate habitat preferences, with depleted tooth enamel values (≈25‰) representing freshwater rivers and springs, whereas enriched values (≈30‰) indicate coastal marine environments. Taken together, the Eocene to late Miocene sirenians (Protosirenidae and Dugongidae) differ significantly in both δ13C and δ18O from Pleistocene and Recent manatees (Trichechidae). In general, Protosiren and the fossil dugongs from Florida have carbon isotopic values that are relatively positive (mean δ13C = −0.9‰) ranging from −4.8‰ to 5.6‰, interpreted to represent a specialized diet of predominantly seagrasses. The oxygen isotopic values (mean δ18O = 29.2‰) 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 deflection) and the known ecology of modern Dugong dugon from the Pacific Ocean. In contrast, the fossil and extant Trichechus teeth from Florida have relatively lower carbon isotopic values (mean δ13C = −7.2‰) that range from −18.2‰ to 1.7‰, interpreted as a more generalized diet ranging from C3 plants to seagrasses. The relatively lower oxygen isotopic values (mean δ18O = 28.1‰) 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 hypotheses previously based on morphology and associated floral 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.

Effect of late Cenozoic aridification on sedimentation in the Eastern Cordillera of northwest Argentina (Angastaco basin)

This study evaluates the effect of climate on facies, grain size, and sedimentation rates using sedimentology, geochronology, and stable isotope geochemistry for Miocene–Pliocene deposits in the Angastaco basin (Eastern Cordillera, northwest Argentina). U-Pb zircon data from ash layers constrain the transition between the finer grained fluvial-lacustrine Palo Pintado Formation and the coarser grained fluvial-alluvial San Felipe Formation to ca. 5.2 Ma and the first deposition of sediment derived from the present-day orographic barrier to ca. 4 Ma. δ 13 C values from pedogenic carbonate nodules range from −15.4‰ to −10.2‰ for the Palo Pintado Formation and from −9.5‰ to −8.2‰ for the San Felipe Formation; this can be best explained by increased, sustained aridity since ca. 5 Ma. The δ 18 O values range from −9.6‰ to −5.9‰ for the Palo Pintado Formation and from −6.1‰ to −5.2‰ for the San Felipe Formation, corroborating this interpretation. The shift toward more arid conditions correlates with a significant increase in grain size but no significant change in sedimentation rate. Because aridity precedes the development of an orographic effect, we interpret the grain size increase in the Angastaco basin since ca. 5 Ma to be a response of the sedimentary system to aridification resulting from regional climate change.

Early Cenozoic uplift of the Puna Plateau, Central Andes, based on stable isotope paleoaltimetry of hydrated volcanic glass

Uplift of the Central Andes is largely thought to have occurred during the past 10 m.y. based on paleoaltimetry studies from the Altiplano of Bolivia. However, the spatio-temporal uplift history may not be uniform across the Central Andes. We present new stable isotopic results from the Salar de Antofalla, Salina del Fraile, and Arizaro Basin on the Puna Plateau (24°–26°S) of northwestern Argentina. Samples of volcanic glass give δD paleowater values and modeled paleoelevations that indicate an elevated (∼4 km) Puna Plateau since ca. 36 Ma with limited (