Although empirical testing of Cope's rule, the tendency for size to increase over time, has received significant attention in the last few decades, there is no consensus about the applicability of this rule across taxonomic levels. In the present study, we investigate the distribution of body size of Trigoniida bivalves, at order-, family-, genus- and species-level, through the Middle Jurassic and Early Cretaceous of the Kutch region in India. Our data suggest that the body size of Trigoniida bivalves did not vary significantly in the Middle–Late Jurassic, followed by an increase after the Jurassic–Cretaceous mass extinction boundary and a reduction in the late Early Cretaceous. Changes in relative sea-level and associated sedimentary facies composition generally exhibit poor correlation with the overall stasis, or no net body size change, displayed by Trigoniida bivalves. Body-size analysis across taxonomic hierarchy reveals that order-level trends are not a simple aggregation of trends at lower taxon levels. An important observation of our study is the body-size increase immediately in the aftermath of the Jurassic– Cretaceous mass extinction, a deviation from the general observation that size reduction occurs in post-extinction communities. We argue that this increase may be result of both ecological competition and evolutionary faunal turnover.

The Hell Creek region of northeastern Montana is an excellent study system to explore the rise to dominance of mammalian faunas after the Cretaceous–Paleogene (K–Pg) mass extinction. The Tullock Member of the Fort Union Formation exposed in that region was deposited during the first 1.2 Ma after the Chicxulub bolide impact. Some aspects of post-K–Pg mammalian succession remain obscure, however, due to a lack of finer stratigraphic resolution between vertebrate fossil localities. Here, we present a new stratigraphic model for the lower and middle Tullock and identify a stratigraphic succession of five mammal-bearing sedimentary units that span the first ∼ 900 ka of the Paleocene. Most notably, we find that middle Tullock fossil localities, which were previously thought to be deposited by a single, large fluvial channel complex, are derived from two temporally and lithologically distinct sedimentary units: the Biscuit Springs unit (BS) and the Garbani channel (GC). The top of the GC is stratigraphically above the top of the BS, but in some places cuts through the entirety of the BS, a relationship that previously complicated interpretations of their relative age. This cross-cutting relationship reveals that the BS is older than the GC. Thus, the BS local fauna represents a potential intermediate between the older local faunas from the post-K–Pg ‘disaster’ interval and the younger, more taxonomically/ecologically diverse GC local fauna. This new stratigraphic framework sets the stage for future studies focused on the pattern and timing of biotic recovery in the aftermath of the K–Pg mass extinction.