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The idea that ecological niches remain stable during periods of rapid climate change has long been central to methods used to assess extinction risk. However, evidence to test this assumption, particularly beyond recent timescales, remains scarce. Here we examine how a terrestrial mammal responded to rapid climate warming during the Latest Danian Event (LDE; ~62.3 Ma) in the early Paleocene. Tetraclaenodon puercensis is an archaic ungulate that exhibits a size reduction during the LDE in the San Juan Basin of New Mexico, USA. The drivers of this phenomenon – hyperthermal dwarfism – remain poorly resolved and are often linked to biogeographic range shifts rather than in situ ecological responses. Using a novel multi-comparator approach to dental microwear texture analysis, we show that T. puercensis shifted from frugivorous to folivorous diets during the LDE. Such a shift is often observed among extant forest mammals during times of food scarcity and moisture stress, which are likely during Palaeogene hyperthermals. Our results provide the first robust evidence for mammalian ecological responses and adaptation to lower quality resources during a Palaeogene hyperthermal. Dietary niche shifts therefore provide a means of dealing with rapid warming without requiring broad changes in biogeographic ranges. 

Early Paleocene floral communities were substantially restructured as a result of the Cretaceous-Paleogene (K-Pg) mass extinction approximately 66.0 Ma. While events immediately adjacent to the K-Pg boundary have been extensively studied, comparatively little research has looked at long-term terrestrial ecosystem recovery during the early Paleocene. The San Juan Basin (SJB), located in northwestern New Mexico, preserves an exceptional, large, and well-dated early Paleocene plant record making it an ideal location to study long-term recovery of early Paleocene terrestrial ecosystems. Here we investigate early Paleocene terrestrial ecosystem change using a coupled high-resolution plant and δ13C record from the SJB. Plant macrofossils were collected from the lower Paleocene Ojo Alamo Sandstone and lower Nacimiento Formation in the SJB spanning the initial ~1.5 myr of the Paleocene. Macrofloral extinction, origination, and net diversification rates were simultaneously estimated using the Pradel capture-mark-recapture (CMR) model from 66.0 – 64.5 Ma with 100 Kyr time-steps. Two intervals of decreasing floral diversity were identified: a short interval at ~65.5 Ma and a prolonged interval from ~65.2 – 64.7 Ma. Two short intervals of rapidly increasing floral diversity were also identified: the first at ~65.3 Ma and the second at ~64.6 Ma. The onset of both intervals of decreasing floral diversity are coeval with a -1.5 to -2.5 ‰ bulk organic δ13C excursion. We also applied the Pradel CMR model to contemporaneous macrofloras from the Denver Basin (DB), Colorado and the Williston Basin (WB), North Dakota and Montana. The floral diversity patterns estimated from the DB and WB indicate intervals of increasing and decreasing floral diversity that are coeval with the same intervals identified in the SJB. This suggests a regional driver in patterns of floral diversity change during the early Paleocene in western North America, which reflects prolonged terrestrial ecosystem instability following the K-Pg mass extinction.