Late Miocene fossils, climate, & ecology

Understanding the role of mosaic habitats for the last common ancestor between chimpanzees and humans has been a central goal of paleoanthropology. The Lemudong’o Formation contains important paleontological evidence from the latest Miocene (5.8-6 Ma), when the earliest hominins appeared in the fossil record. The ongoing field research on the paleontology, geology, and paleoecology of Lemudong’o has recovered mammalian fauna, tephras, sedimentary samples, and plant fossil remains. We will use a multi-proxy approach and stable isotope analyses to provide robust environmental reconstructions. The reconstructions can be compared to other late Miocene and early Pliocene paleontological sites in eastern Africa, to provide a regional perspective in climate, vegetation, and faunal composition of this critical time interval.  Our project is supported by NSF and the Leakey Foundation. 

Project website: https://sites.psu.edu/skrp/goals/

Related presentations:

• Dietary niches of large mammals from the late Miocene Lemudong'o Formation, SW Kenya. AABA 2025,  Download PDF

• Variation in faunal composition among phases of fossil collection in the Lemudong'o Formation, SW Kenya. AABA 2024, Download PDF

Sr isotopes and elephant movement

87Sr/86Sr ratios have been widely used in studies of movement/migration patterns of humans and animals. While geospatial tools such as isocapes have improved the utility of 87Sr/86Sr, enamel presents a serious challenge for the paleontology and archaeology communities due to various processes that may attenuate the 87Sr/86Sr signal. To address this challenge, we applied high-resolution laser ablation mapping and conventional sampling methods on the enamel and dentine of a zoo elephant with a known movement history. Our comparisons between the 87Sr/86Sr data series allowed us to evaluate the relative influence of three sources of signal attenuation for the first time: strontium turnover, enamel maturation, and sampling. Our study suggests that laser ablation of the inner enamel followed by inverse modeling of 87Sr/86Sr is a best-practice workflow to study past movement/migration. The workflow addresses key challenges in obtaining intra-tooth 87Sr/86Sr data and enables quantitative estimations of 87Sr/86Sr intake history, which can help answer a variety of research questions about animal movement/migration across multiple disciplines.

Related publications:

• Yang et al., 2025, Strontium isotope mapping of elephant enamel supports an integrated microsampling-modeling workflow to reconstruct herbivore migrations. Communications Biology. 

• Yang et al., 2023, BITS: A Bayesian framework for Strontium isotope turnover in Proboscideans and its utility in movement ecology. Methods in Ecology and Evolution. Active code is available on GitHub; Archived code ver. 0.9.4 on Zenodo;

Lake Turkana, climate and evolution

The Lake Turkana Basin preserves some of the most important fossils and stone tool artifacts in the study of human evolution. Understanding the ancient environments in which our ancestors and distant relatives evolved relies on a detailed understanding of both the sediments that preserve such records and our modern-day environments and their relationships with climate, water balance, ecology, and human activities. The project goal is to use the same geochemical tools that we have in the fossil record to monitor the climate and environment of the Lake Turkana region under climate change, and to inform our understanding of the climatic and environmental changes of the past. This project also involves the application of similar geochemical tools of the fauna that are found in the greater region of eastern Africa today to inform the ecology and evolution of fossil fauna and humans in the past. 

Related publications:

• Saslaw, Yang, et al., 2024, An isotope mass balance analysis of evaporative loss from Lake Turkana, Kenya using δ18O and δD of natural waters. Water Resources Research. Open Access link: https://doi.org/10.1029/2023WR036076

• Yang et al. 2022, Why the long teeth? Morphometric analysis suggests different selective pressures on functional occlusal traits in Plio-Pleistocene African suids. Paleobiology,  Open Access link: https://doi.org/10.1017/pab.2022.11 

Vegetation reconstruction from waxes

Plant wax lipids and their stable isotopes have been widely used as paleoclimatic indicators.  However, the multivariate nature of plant wax hydrocarbon chains makes it difficult to interpret the full record of extracted lipids. In most studies, isotope data from a single hydrocarbon chain are interpreted, albeit data from multiple chains have been analyzed. This project aims to build statistically robust process models that integrate information from multiple plant wax lipid hydrocarbon chains.

Related publications:

• Yang and Bowen, 2022, Integrating plant wax abundance and isotopes for paleovegetation and paleoclimate reconstructions: A multi-source mixing model using a Bayesian framework. Climate of the Past. 18, 2181–2210, 2022. Active code is available on GitHub; Archived code ver. 1.0.5 (Bok choy) on Zenodo;

• Zeng, Huang, Yang, et al., 2024, Quantifying relative contribution of submerged macrophytes to sedimentary organic matter using concentrations and δ13C of n-alkanes with the Bayesian multi-source mixing model: a case study from the Yangtze floodplain. JGR-Biogeosciences. 

Suid (pigs) ecology and evolution

Suids (pigs and their ancestors) are abundant in the Neogene and Quaternary fossil record. They are adapted to a wide range of habitats with flexible diets, which has the potential of tracking paleoenvironmental change through time. To this end, I have studied both extant and fossil suids for their dietary adaptations and behaviors. Our goals are 1) to better understand extant suid ecology with stable isotopes, and 2) to better understand suid paleoecology in the context of their evolutionary history. 

Related publications:

• Yang et al. 2024, Intra-tooth stable isotope analysis reveals seasonal dietary variability and niche partitioning among bushpigs/red river hogs and warthogs. Current Zoology, Open Access link: https://doi.org/10.1093/cz/zoae007

• Yang et al. 2022, Why the long teeth? Morphometric analysis suggests different selective pressures on functional occlusal traits in Plio-Pleistocene African suids. Paleobiology,  Open Access link: https://doi.org/10.1017/pab.2022.11 

• Yang et al. 2020, Intra-tooth stable isotope variations in warthog canines and third molars: Implications for paleoenvironmental reconstruction. Chemical Geology. Download PDF

Enamel microstructure and function

Mammalian enamel is extremely resilient against fractures. This is owing to the crisscrossing pattern of enamel bundles that can stop cracks to propagate within enamel. The pattern is visible as alternating light and dark bands in a tooth section, which are known as Hunter-Schreger Bands (HSBs). In human molars, the so called 'guiding' side of the cusps fractures more frequently than the occuding or 'functional' cusps. Intriguingly, lower molars also fracture  more frequently than upper molars. Are the differences in fracture frequency linked to HSB patterns? We found that in human molars, indeed, the 'functional' cusps display a higher density of HSBs than 'guiding' cusps, and that upper molars display a higher density of HSBs than lower molars. 

Related publication:

• Yang et al. 2022, Hunter-Schreger Band configuration in human molars reveals more decussation in the lateral enamel of 'functional' cusps than 'guiding' cusps . Archives of Oral Biology, Download PDF

Holocene climate of the Great Salt Lake 

Paleoclimatic and paleoenvironmental reconstructions often rely on the interpretation of multiple proxies. However, different proxies are often interpreted and compared in a stepwise fashion, due to the challenges associated with the disparate environmental information involved in each. This project aims to build a proxy system model that integrates the interactions between multiple proxies for the Great Salt Lake.

Related publications:

• Bowen et al. 2019, Multi-Substrate Radiocarbon Data Constrain Detrital and Reservoir Effects in Holocene Sediments of the Great Salt Lake, Utah. Radiocarbon, 61, 905 - 926.