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Abstract
The origin and spread of grass-dominated biomes in North America have traditionally been inferred from indirect lines of evidence, most importantly the evolution of ungulates with high-crowned (hypsodont) teeth during the early Miocene. In contrast, direct paleobotanical evidence for this change has been scant, due to taphonomic biases in Tertiary deposits in the continental interior.
However, plant silica (phytoliths) offers a new fossil record of vascular plants that is particularly useful for reconstructing the spread of grass-dominated habitats. Phytoliths are preserved in association with mammal fossils and can therefore provide paleoecological information. Also, phytolith assemblages record information primarily about plant community structure (e.g., forest vs. grassland), which is not always captured in palynomorphs or macrofossil floras.
The main objective of this research was to provide a direct, paleobotanical record for the evolution of grasslands based on phytolith assemblages, and to compare it to other lines of evidence. Since phytolith analysis has previously mainly been used in archaeology and pre-Pleistocene paleoecology, a secondary goal was to adapt this approach, including analytical and laboratory methods, for use in the Tertiary fossil record.
Eocene-Miocene sediment samples from Montana, Nebraska, Colorado, and New Mexico, forming a north-to-south transect through the continental interior, were extracted for phytoliths. To aid in analysis, a comprehensive reference collection of modern plants was described and analyzed quantitatively. The phytolith analysis showed that open, grass-dominated habitats spread in the earliest Miocene (≥22 Ma) in the central Great Plains and somewhat later in the northern Great Plains. Prior to this time, vegetation consisted primarily of closed forests or woodlands with an understory of bambusoid/basal grasses (Central Great Plains) or other potentially closed-habitat grasses (Northern Great Plains). This timing for the spread of grass-dominated habitats differs from that inferred from both faunal (late early Miocene), paleopedological (early Oligocene), and available paleobotanical (middle Miocene) data. The new phytolith record further allowed the hypothesis that hypsodonty in equids was an adaptation to grasslands to be tested in a phylogenetic context. The analysis supported the adaptive explanation for hypsodonty, but suggested that the trait evolved slowly due to weak selection and phylogenetic inertia.
