A grazing Gomphotherium in Middle Miocene Central Asia, 10 million years prior to the origin of the Elephantidae

Yan Wu,Tao Deng,Shi-Qi Wang,Xinying Zhou,Hanwen Zhang,Jie Ye,Yaowu Hu,Jiao Ma,Limi Mao

doi:10.1038/s41598-018-25909-4

Abstract

Feeding preference of fossil herbivorous mammals, concerning the coevolution of mammalian and floral ecosystems, has become of key research interest. In this paper, phytoliths in dental calculus from two gomphotheriid proboscideans of the middle Miocene Junggar Basin, Central Asia, have been identified, suggesting that Gomphotherium connexum was a mixed feeder, while the phytoliths from G. steinheimense indicates grazing preference. This is the earliest-known proboscidean with a predominantly grazing habit. These results are further confirmed by microwear and isotope analyses. Pollen record reveals an open steppic environment with few trees, indicating an early aridity phase in the Asian interior during the Mid-Miocene Climate Optimum, which might urge a diet remodeling of G. steinheimense. Morphological and cladistic analyses show that G. steinheimense comprises the sister taxon of tetralophodont gomphotheres, which were believed to be the general ancestral stock of derived “true elephantids”; whereas G. connexum represents a more conservative lineage in both feeding behavior and tooth morphology, which subsequently became completely extinct. Therefore, grazing by G. steinheimense may have acted as a behavior preadaptive for aridity, and allowing its lineage evolving new morphological features for surviving later in time. This study displays an interesting example of behavioral adaptation prior to morphological modification.

Highlights

Dental calculus is the calcium phosphate deposited on teeth, and captures a large number of food particles that provide crucial information about the food of ancient animals[7,8]
We examine the feeding ecology of two species of gomphotheriid proboscideans, Gomphotherium connexum and G. steinheimense, based on phytoliths from their dental calculus, complemented by dental stereomicrowear and stable isotopic data
Previous studies and our pollen data have shown that the Halamagai Formation covers the Mid-Miocene Climate Optimum (MMCO)[18] and records the major palaeoenvironmental perturbation crises from a more humid Middle Miocene environment to the comparatively more arid Late Miocene ecosystem[19,20] (SI 1.1)

Summary

Introduction

Dental calculus is the calcium phosphate deposited on teeth, and captures a large number of food particles that provide crucial information about the food of ancient animals[7,8]. Previous studies and our pollen data have shown that the Halamagai Formation covers the Mid-Miocene Climate Optimum (MMCO)[18] and records the major palaeoenvironmental perturbation crises from a more humid Middle Miocene environment to the comparatively more arid Late Miocene ecosystem[19,20] (SI 1.1). This palaeoenvironmental transition had a profound impact on the evolution of terrestrial floras and faunas in Central Asia and elsewhere. Our research is of importance in understanding palaeoenvironmental changes in the middle-latitude zone of inland Asia during the Middle to Late Miocene, and holds broader significance in understanding the coevolution of plant communities and mammals in terms of habitats and feeding ecology

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Conclusion