INTRODUCTION

5, Sixty-six million years (Ma) ago, one of the largest and most transformative cataclysms of the Phanerozoic occurred, when a ~10-km-wide extraterrestrial bolide struck the Yucatán Peninsula, in the Gulf of Mexico ( 1 4 ). This impact caused a panoply of ecological and environmental catastrophes, including tsunamis and wildfires, as well as global darkness (i.e., an “impact winter”) due to the injection of sunlight-blocking debris and climate-forcing gases into the atmosphere ( 1 6 ). These upheavals destabilized all trophic levels and triggered severe extinctions that spread devastation worldwide ( 2 ). In terrestrial environments, all non-avian dinosaurs, “archaic” birds, and pterosaurs vanished following the impact ( 7 9 ), while other groups, such as mammals ( 10 11 ) and squamates ( 12 ), suffered considerable losses. On the other hand, groups such as freshwater salamanders, turtles, and crocodylians seemingly survived nearly unscathed ( 13 ). In the aftermath of this Cretaceous/Paleogene (K/Pg) mass extinction, surviving lineages recovered relatively rapidly ( 13 16 ), accompanied by concomitant ecospace shifts that favored their expansion into vacated niches and large-scale explosive radiations in placental mammals ( 17 18 ), neornithine birds ( 8 19 ), and squamates ( 20 ), laying the foundations for the diverse range of faunas that we share the planet with today.

25– It is widely postulated that the extinction of the non-avian dinosaurs resulted in empty niches and novel ecological opportunities for surviving organisms. This paradigm is based almost exclusively on taxonomic ( 11 21 ), morphological ( 10 22 ), and phylogenetic ( 14 23 ) evidence. Mass extinctions, however, act on the structure and function of ecosystems. Much less understood is how the ecology of dinosaurs, mammals, and other terrestrial animals changed in the lead-up to and aftermath of the extinction event. Focusing on functional and trophic ecology, rather than on the classic trends in biostratigraphic ranges ( 24 ), can help disentangle the potential ecological drivers of survivorship and recovery. Understanding the ecological dynamics of the latest Cretaceous faunal components is central to answering two long-standing questions. First, were non-avian dinosaurs in long-term decline before their end-Cretaceous demise ( 9 31 )? Second, why were some members of the terrestrial and freshwater biota (e.g., mammals, lizards, neornithine birds, and crocodylians) able to survive the mass extinction but not others?

37–29, 33, These questions can be addressed by modeling long-term patterns in food webs ( 31 32 ) and ecospace occupancy dynamics (i.e., the multidimensional combination of paleoenvironmental conditions under which species developed) ( 29 33 ). Direct fossil evidence of trophic interactions is still limited ( 32 34 ), but methodological advances in network theory ( 35 ) and ecological niche partitioning ( 36 ) might hold the key to long-standing questions about food web stability and the functional roles of species in ancient ecosystems, both of which are at the core of modern evolutionary and ecological research ( 34 39 ). Although the application of these emerging approaches is not yet commonplace in paleontology, a few studies have demonstrated that they are ideal tools for revealing species habitat distributions in deep time ( 6 40 ) and the tempo and mode of ecological reorganization after mass extinctions ( 37 39 ).

Here, we quantify the magnitude of ecological change before and after the K/Pg boundary, from the Campanian stage of the Late Cretaceous to the Danian stage of the early Paleogene (83.6 to 61.6 Ma ago). Our analyses are based on a spatiotemporally and taxonomically standardized presence-only dataset ( Fig. 1A ), comprising more than 1600 fossil occurrences representing more than 470 genera of cartilaginous and bony fish, salamanders, frogs, albanerpetontids, lizards, snakes, champsosaurs, turtles, crocodylians, dinosaurs (including birds), and mammals (e.g., table S1) across the best sampled region representing this interval ( 9 ), the Western Interior of North America [sensu Gardner and DeMar ( 41 )]. This region includes the highly fossiliferous western subcontinent, Laramidia, which, during much of the Late Cretaceous, was separated from its eastern counterpart, Appalachia, by an epicontinental seaway stretching from present-day Alaska to Mexico. Using a spatially explicit Markov network approach ( Fig. 1, B and C ) ( 42 43 ) and state-of-the-art Earth System models ( Fig. 1D ) ( 44 46 ), in combination with multivariate niche-modeling techniques ( Fig. 1E ) ( 47 ), we simulate how inferred trophic dynamics and niche occupancy patterns shaped the trajectories of North American continental ecosystems across the latest Cretaceous and during the recovery from the mass extinction. By doing so, we explicitly test whether (i) shifts in food web architecture underwent major restructuring before and after the K/Pg extinction, including whether some trophic guilds were more prone to these shifts than others; and (ii) any of these changes were associated with fluctuations in the realized niche space, helping to explain why some groups survived and others went extinct across the K/Pg boundary.