Morphology of the narial region and mandible of three gomphothere families compared with an extant elephant, and the elephantiformes phylogeny. (A) Phylogenetic reconstruction of major longirostrine elephantiforms at the species level based on the Bayesian tip-dating method. The node support (the number at each node) is the posterior probability, and the bars represent chronologic ranges of each taxon. (B–D) Representative cranium and mandible specimens of the three gomphothere families, including IVPP V22780, cranium, and IVPP V22781, mandible, of Gomphotherium tassyi [B], “Gomphotheriidae”, from Heijiagou Fauna, Tongxin region; HMV 0930, cranium and associated mandible of Platybelodon grangeri [C], from Zengjia Fauna, Linxia Basin; and IVPP V23457, cranium and associated mandible of Choerolophodon chioticus [D], from Middle Miaoerling Fauna, Linxia Basin. (E–H) Narial morphology of bunodont elephantiforms and elephantids in dorsal view, including IVPP OV733, Elephas maximus [E], a living elephantid; HMV 0930, Platybelodon grangeri [F]; IVPP V22780, Gomphotherium tassyi [G]; and IVPP V23457, Choerolophodon chioticus [H]. Mandibular morphology of bunodont elephantiforms. (I–J) Mandibular symphysis and tusks of HMV 0930, Platybelodon grangeri, in dorsal [I] and distal [J] views. (K) Mandibular symphysis and tusks of IVPP V22781, Gomphotherium tassyi, in dorsal view. (L) Mandibular symphysis of IVPP V25397, Choerolophodon chioticus, showing the deep slits at both sides of the distal alveolar crests in dorsal view. (M) Reconstruction of keratinous cutting plates in the slits, in dorsolateral view. Anatomic abbreviations: ce, cutting edge of the distal mandibular tusk in Platybelodon; kcp, reconstructed keratinous cutting plates in Choerolophodon; nb, nasal process of nasal bone; mc, slit or groove for mesethmoid cartilage insertion (white in color); pf, perinasal fossa; ps, prenasal slope in Platybelodon; s, slit for holding kcp in Choerolophodon.

Relative abundance, tooth enamel δ13C, and stratigraphic ranges of the three gomphothere families in the Shanwangian and Tunggurian stages (∼20–11.5 Ma) of northern China. (A) Relative abundance of the three gomphothere families, including Choerolophodontidae, only represented by Choerolophodon (pink); Amebelodontidae, represented by Protanancus, Aphanobelodon (dark blue), and Platybelodon (light blue); and “Gomphotheriidae”, only represented by Gomphotherium (green). Horizontal bars indicate the average ages of the fossil assemblages, which are shown in C–F. The ages were determined by paleomagnetism (Table S1). The red curve shows the global reference benthic foraminifer oxygen isotope curve, which represents the global temperature [after (Westerhold et al., 2020)]. (B) Tooth enamel stable carbon isotopic compositions of various gomphothere taxa. Each circle represents the bulk enamel δ13C values of a single tooth. The data of LX4 and TG2,3 are from previous publications (Wang and Deng, 2005; Zhang et al., 2009). Solid and dashed lines represent the mean and maximum enamel δ13C values for C3 diets that have been corrected for Miocene atmospheric CO2 δ13C [after (Tipple et al, 2010)]. (C–F) Synthetic stratigraphic columns of typical fossil bearing regions of northern China during ∼19–11.5 Ma, which incorporated different fossil assemblages with different ages, from the Linxia Basin [C], Tongxin region [D], Junggar Basin [E], and Tunggur region [F]. Vertical bars represent the temporal ranges of different gomphothere taxa. For fossil assemblage abbreviations, please see Table S1. Abbreviations for gomphothere taxa: Apzh, Aphanobelodon zhaoi; Chch, Choerolophodon chioticus; Chco, Choerolophodon connexus; Chgu, Choerolophodon guangheensis; Goan, Gomphotherium cf. angustidens; Goco, Gomphotherium cooperi; Goin, Gomphotherium inopinatum; Gost, Gomphotherium steinheimense; Gota, Gomphotherium tassyi; Plda, Platybelodon dangheensis; Plgr, Platybelodon grangeri; Plte, Platybelodon tetralophus; Plto, Platybelodon tongxinensis; Prbr, Protanancus brevirostris; Prsp, Protanancus sp.; Prwi, Protanancus wimani.

Finite element analyses of feeding behaviors among three longirostrine gomphothere families and reconstruction of their feeding ecology. (A–C) von Mises stress color maps of Choerolophodon, Gomphotherium, and Platybelodon models, with the full muscle forces exerted, and an additional 5000 N external vertical force applied on the distal end of the mandibular symphysis. (D) Strain energy curves of the three mandibles under the following three steps: 1, muscle forces linearly exerted; 2, a 5000 N external vertical force suddenly applied on the distal end; and 3, the 5000 N external force gradually changed from vertical to horizontal. (E) Sum of equivalent plastic strain from total elements (SEPS) of twigs cut by mandible models in three different directions (i.e., twig horizontal, 45° oblique, and vertical). Larger SEPS values indicate higher efficiency of twig cutting. (F) Scenery reconstruction of feeding behaviors of the three longirostrine gomphothere families (by X. Guo), represented by Choerolophodon (Choerolophodontidae), feeding in a closed forest, Gomphotherium (“Gomphotheriidae”), feeding at the margin between the closed forest and open grassland, and Platybelodon (Amebelodontidae), feeding on open grassland. (G) Detailed 3D reconstruction of Platybelodon feeding by grasping the grass-blades using their flexible trunk and cutting the grass blades using the distal edge of their mandibular tusks.

Evolutionary levels of narial region (A) and of characters in relation to horizontal cutting (B). Value in A was PC1 of characters 54–57 (see Supplementary Appendix S1, Figure S3C, and Data S1); and that in B was PC2 of characters 5, 9, 11, 72, and 77 (see Figure S3D). Note that the clade of Platybelodon (marked by an asterisk) shows high evolutionary levels and that of Choerolophodontidae (marked by a circle) shows low evolutionary levels in both character-combines, strongly suggesting the co-evolution of narial morphology and horizontal cutting behavior.