FIGURE 1. Map locality, 1) state of Baja California Sur, Mexico; 2) Oligocene outcrops of San Juan Member, El Cien Formation; 3) El Saladito and mesa El Tosoro fossilferous localities, north to San Juan de la Costa village.

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FIGURE 2. General stratigraphy and observations at mesa El Tesoro; 1) view of the strata over the mined phosphatic Humboldt bed; 2-3) position of the gray phosphatic sandstone over the Humboldt bed; 4) fossils Tlaxcallicetus cf. guaycurae in situ in the gray phosphatic sandstone and 5) ventral view, left ear region of Tlaxcallicetus sp., field observation at mesa El Tesoro.

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FIGURE 3. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, dorsal view.

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FIGURE 4. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, ventral view.

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FIGURE 5. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, frontal view.

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FIGURE 6. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, posterior view.

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FIGURE 7. Tlaxcallicetus guaycurae (MU EcSj5/06/31), cranium, lateral view.

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FIGURE 8. Tlaxcallicetus guaycurae (MU EcSj5/06/31), ear region (1) with the left periotic bone (2-4), ventral view. Pars cochlearis (3), posteromedial view.

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FIGURE 9. Detailed features of Tlaxcallicetus guaycurae (MU EcSj5/06/31), (1) pitted edge in the glenoid fossa that suggest a fibrocartilaginous joint; (2) inflated and rounded squamosal prominence; (3) external occipital sulcus on the posterior middle part of the supraoccipital.

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FIGURE 10. Ear region and left periotic comparison between Tlaxcallicetus species, ventral view. Tlaxcallicetus sp. (MU EcSj5/18/95) (1-2), Tlaxcallicetus guaycurae (MU EcSj5/06/31) (3-4), and specimen found at mesa El Tesoro, Tlaxcallicetus sp., field observation (5-6).

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FIGURE 11. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, dorsal view.

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FIGURE 12. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, ventral view.

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FIGURE 13. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, frontal view.

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FIGURE 14. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, posterior view.

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FIGURE 15. Tlaxcallicetus sp. (MU EcSj5/18/95), cranium, lateral view.

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FIGURE 16. Left periotic with anatomical terms, Tlaxcallicetus sp. (MU EcSj5/18/95), medial (1), lateral (2), ventral (3), dorsal (4), posterior (5), and anterior (6) views.

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FIGURE 17. Left periotic, Tlaxcallicetus sp. (MU EcSj5/18/95), medial (1), lateral (2), ventral (3), dorsal (4), posterior (5), and anterior (6) views.

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FIGURE 18. Left periotic, Tlaxcallicetus sp. (MU EcSj5/18/95), closer view of internal acoustic meatus, dorsal view.

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FIGURE 19. Left bulla with anatomical terms, Tlaxcallicetus sp. (MU EcSj5/18/95), ventral (1), dorsal (2), lateral (3), medial (4), posterior (5), and anterior (6) views.

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FIGURE 20. Left bulla, Tlaxcallicetus sp. (MU EcSj5/18/95), ventral (1), dorsal (2), lateral (3), medial (4), posterior (5), and anterior (6) views.

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FIGURE 21. Left thyrohyoid, Tlaxcallicetus sp. (MU EcSj5/18/95), dorsal (1), anterior (2), and posterior (3) views.

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FIGURE 22. Philogenetic position of Tlaxcallicetus. 1) Under equal weights, consensus tree recovered from 260 parsimonious trees (1327 steps, consistency index CI= 0.246, retention index RI= 0.686, rescaled consistency index RCI= 0.168756, tree total length 1474). 2) Under implied weights: k = 6, consensus tree from 3 parsimonious trees (85.63773 steps, CI= 0.271, RI= 0.724, RCI= 0.196204, tree total length 1338. Black squares mark unequivocal characters that support the different groups (see legend). Support branch belongs only to those clades with 50% or more consistency. Dashed lines suggest a probable inclusion of taxa in clade.

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