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. 2018 May 16;8(1):7640.
doi: 10.1038/s41598-018-25909-4.

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

Yan Wu  1   2 , Tao Deng  1   2   3 , Yaowu Hu  1   4 , Jiao Ma  1   4 , Xinying Zhou  1   2 , Limi Mao  5 , Hanwen Zhang  6   7 , Jie Ye  1 , Shi-Qi Wang  8   9   10
Affiliations

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

Yan Wu et al. Sci Rep. .

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Geography, geology, and phylogeny in relation to the study material. (A) The location of the study area (black star), details are shown in Fig. S1. The map was generated by GTOPO309 (http://www1.gsi.go.jp/geowww/globalmap-gsi/gtopo30/gtopo30.html) using Globalmapper (v10) (http://www.bluemarblegeo.com/products/global-mapper.php). (B) Stratigraphic column and polarity with palaeomagnetic age (redrawn after refs,), also denoting the horizon of study material in the strata (in light yellow). (C) The 50% majority consensus tree from 29 maximum parsimonious trees showing the phylogenetic position of the Gomphotherium species and Tetralophodon longirostris (tree length = 81, CI = 0.617; RI = 0.744, data set in Table S1), the number at each node representing the support value calculated by majority rules (percentages of supported MPTs in the total MPTs, which are always larger than 50%) and the orange frame indicating the sister-taxon relationship of G. steinheimense and T. longirostris. (D) G. steinheimense, IVPP V23283, right m3. (E) G. connexum, IVPP V8576, left M3.
Figure 2
Figure 2
Phytolith and sponge spicules, as well as their statistics in Gomphotherium dental calculus. (A–L) Morphology of various phytolith, including bilobates short cell (V8573) (A), bilobates short cell (V23283) (B), bulliform (V8575) (C), bulliform (V23283) (D), rondel (V23283) (E), rectangular (V8576) (F), reed-type bulliform (V23283) (G), long cell (V8575) (H), hair cell (V8576) (I), irregular multifaceted (V8573) (J), elongate (V8575) (K) sponge spicules (V8576) (L). (M) Percentages of grass phytoliths, eudicots phytoliths, unclassed phytoliths and sponge spicules occurring in Gomphotherium dental calculus. (N) Example of dental calculus sampling (the tooth is V8576).
Figure 3
Figure 3
Bivariate plot of the average scratch versus average pit counts in enamel microwear of G. connexum, G. steinheimense, and extant elephants (redrawn after ref.). Oval outlines = Gaussian confidence ellipses (p = 0.95) on the centroid of the comparative extant grazer and browser samples adjusted by sample size. The insert panels represent photomicrographs of enamel surfaces of G. connexum (A–C) and G. steinheimense (D).
Figure 4
Figure 4
Scatter plot of carbon and oxygen isotope values of the Gomphotherium samples.
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