Dental enamel hypoplasia and intra-tooth isotope records as indicators of physiological stress and environmental variability in Early Pleistocene hippopotamids from Melka Kunture (Upper Awash, Ethiopia)

Enamel hypoplasia results from temporary disruptions of enamel deposition during tooth development. It is widely used as an indicator of physiological stress in archaeological and paleontological contexts, yet its relationship with environmental conditions during tooth formation remains poorly constrained. Here, we combine macroscopic identification of Linear Enamel Hypoplasia (LEH) with intra-tooth stable carbon (δ13C) and oxygen (δ18O) isotope records from 100 sequential enamel samples to investigate short-term environmental variability and physiological stress in Early Pleistocene hippopotamids from Melka Kunture (Upper Awash Valley, Ethiopia). The two continuously growing teeth (specimen IDs: MK 18 SIMB III 128 and MK 75 GOM II 672), respectively dated to ~1.4 and ~1.0 Ma, were selected because they were the only specimens in the examined assemblages in which LEH was identified. Although both teeth exhibit LEH, they show different distributions of the enamel defects and contrasting intra-tooth isotopic patterns. MK 18 SIMB III 128 (~1.4 Ma) displays broad intra-tooth variation and strong positive cross-correlation between its δ13C and δ18O profiles. Enamel within the LEH-affected interval has higher δ13C and δ18O values than unaffected apical enamel, and samples positioned directly on visible LEH differ from adjacent enamel in both isotope systems. These patterns are consistent with discrete stress episodes associated with changing hydrological conditions, although dietary and physiological contributions to the carbon isotope record cannot be separated. MK 75 GOM II 672 (~1.0 Ma) displays narrower isotopic variation and LEH across most of the preserved tooth surface. Its profile is compatible with prolonged or recurrent stress under relatively stable, water-limited hydrological conditions, but the localized isotopic differences do not identify a single causal mechanism. The differences between the specimens may reflect both their distinct developmental histories and local variation in fluvial dynamics and water-source stability. Because enamel mineralization and sampling integrate isotopic information over extended developmental intervals, the observed associations should be interpreted at the scale of broader developmental phases and not as evidence of exact synchrony between LEH formation and individual isotope values. The integration of LEH and intra-tooth isotope analysis provides a promising framework for investigating relationships among individual life-histories, physiological stress, and environmental variability in fossil mammals.