Phytoliths indicate significant arboreal cover at Sahelanthropus type locality TM266 in northern Chad and a decrease in later sites

We analyzed phytolith and diatom remains preserved at 45 Miocene and Pliocene localities dated between 8 and 1 Ma in northern Chad (16-17°N). Some of these localities yielded cranial remains, lower jaws, and teeth of the hominin species Australopithecus bahrelghazali (∼3.6 Ma) and Sahelanthropus tchadensis (∼7 Ma). Of the 111 sediment samples analyzed, 41 yielded phytoliths, 20 yielded diatoms, and seven yielded both phytoliths and diatoms. Freshwater planktonic and tychoplanktonic diatom species, indicative of lacustrine conditions, are dominant (>91%) in the samples. The phytolith assemblages indicate an opening of the vegetation and a general trend toward an expansion of grass-dominated environments during the time spanning the two hominin occurrences in Chad. The phytoliths suggest the presence of a mosaic environment, including closed forest patches, palm groves, and mixed/grassland formations, between 7.5 and 7 Ma, the replacement by palm grove-like vegetation at approximately 6.5-5 Ma, and the presence of exclusive grass-dominated formations after 4.5 Ma. The type-locality of S. tchadensis (TM266) was likely similar to modern palm grove formations with an arboreal cover percentage ≥40%. The type locality of A. bahrelghazali (KT12) was a grass-dominated ecosystem (likely savanna) with an unrated percentage of arboreal cover. Furthermore, the grass phytolith data support the existence of a (recurrent) Sahelian-like dry climate in northern Chad since at least 8 Ma. Therefore the local closed vegetation formations in the Djurab region at 7.5-7 Ma were sustained by aquatic systems (such as lakes or related rivers, marshes) rather than by extensive annual precipitation.