Morphology, diet, and stable carbon isotopes: On the diet of Theropithecus and some limits of uniformitarianism in paleoecology

Abstract

Geladas were long supposed to be the only living primates feeding almost entirely on graminoids and accordingly display dramatic dental and manual adaptive traits. A recent study of Theropithecus gelada, the first in a relatively undisturbed habitat, revealed a more diverse diet, also incorporating large quantities of forbs. The peculiar adaptive traits of T. gelada are also observed in extinct Theropithecus as early as 3.7 Ma. Stable carbon isotopic data of extinct Theropithecus from eastern Africa indicate that specimens older than 3 Ma consumed a significant proportion of C plants (on average ca. 40% of total food intake) whereas specimens younger than 2 Ma consumed more C plants (on average ca. 80%). Recent paleobotanical evidence suggests that C herbaceous plants were still present in non-negligible proportions in Plio-Pleistocene lowland tropical ecosystems. Together, the shared morphological adaptive traits of extant and extinct Theropithecus and the varied diets of extant T. gelada suggest that the paleodiets of Theropithecus may have been dominated by herbaceous plants, comprising both C forbs and graminoids and C graminoids. The changes in stable carbon isotopes could correspond to a replacement of C plants by C plants within the herbaceous strata rather than a shift from C woody vegetation to C graminoids. This synthesis highlights the need for a more exhaustive knowledge of the ecology of extant species to achieve meaningful paleodietary and paleoenvironmental reconstructions. A strong selectivity for food resources that are rare in the landscapes (as in T. gelada) should also be considered when interpreting stable carbon isotopes of extinct African mammals (and notably hominids).

Calcium isotopic ecology of Turkana Basin hominins

“…2 ). Morphological evidence shows grass-eating adaptations in extinct Theropithecus 42 ; carbon isotopes confirm an early dietary adaptation to C 4 grasses as an important component of their diet 43 . Here, δ 44/42 Ca values for T .…”

“…Conducting a calcium isotope survey of such plants but also of C 4 sedges, especially in including plant parts that are known to fractionate differently 47 , would certainly add relevant data to better tackle this problem. On the other hand, we cannot exclude that both G. gorilla and P. boisei had physiological similarities that would lead to 44 Ca/ 42 Ca values that are enriched relative to other primates.…”

“…After purification, Ca samples were dissolved in ultrapure 0.05 M HNO 3 and Ca concentration was set at 2 ppm for all samples and standards. All Ca isotope compositions are expressed using the 'delta' notation defined as follows for the 44 Ca/ 42 Ca ratio: 12,24,30,32,33,[52][53][54][55] . All newly reported samples were measured as part of the same batches as the samples reported in Martin et al 29 , i.e., in June and August 2016 and in June and July 2017.…”

“…Nonetheless, other proxies are needed to infer hominin diet beyond the ultimate plant source. Here, we investigate resource use in Turkana Basin hominins from Kenya using calcium and carbon isotopes and discuss their relevance for inferring the evolution of dietary habits in the hominin lineage Seminal studies on calcium isotopes in vertebrates 22 recognized a decrease in the 44 Ca/ 42 Ca ratio with increasing trophic level. Subsequent studies supported this trophic level effect, notably in the marine realm 23,24 , but also raised awareness on individual physiological variability influencing Ca isotope variability 25,26 or on the insensitivity of Ca isotope ratios to distinguish between primary and secondary consumers 27 .…”

Buccal dental-microwear and feeding ecology of Early Pleistocene Theropithecus oswaldi from Cueva Victoria (Spain)

“…As a whole, the carbon 13 isotopic dietary proxy shows a shift from a mixed C 3 /C 4 diet to a major C 4 dietary component along the evolutionary lineage of Theropithecus (Souron, 2018 Q1 ). Before 3 Ma, the diets of the fossil Theropithecus species (T. brumpti and T. oswaldi) comprised both C 3 (up to 65%) and C 4 plant foods (Cerling et al, 2013;Levin et al, 2015;Wynn et al, 2016;Souron, 2018 (Bonnefille, 2010;Albert et al, 2015;Magill et al, 2016). Levin et al (2014) Q2…”