Stratigraphic range extension of the turtle Boremys pulchra (Testudinata, Baenidae) through at least the uppermost Cretaceous

Abstract

New material of the derived baenid turtle Boremys pulchra from the Hell Creek Formation of Montana extends the stratigraphic range of the taxon through at minimum the latest Maastrichtian. Previously, the species was constrained to the Campanian of Montana and Alberta, so this extension constitutes at least 5 million years. Due to fossil reworking at the Bug Creek Anthills assemblage, where Maastrichtian and Paleocene deposits are mixed, a definitive extension for B. pulchra cannot currently include Paleocene strata. However, the presence of B. pulchra in latest Cretaceous strata, previous identification of Paleocene Boremys sp. and the general success of baenid taxa across the K–Pg boundary, make it quite plausible that B. pulchra survived the extinction event and that previously described Maastrichtian and Paleocene Boremys sp. material probably represents a new taxon. A stratigraphic extension beyond the Campanian indicates that B. pulchra survived the paleoenvironmental conditions of the latest Cretaceous, where adaptation to locally heterogeneous aquatic habitats and paleotemperature fluctuations may have facilitated latest Cretaceous and K–Pg survivorship. Additionally, ectoparasitic bore marks on the Boremys pulchra specimen described here can be attributed to the ichnotaxon Karethraichnus lakkos.

Updated cranial and mandibular description of the Late Cretaceous (Maastrichtian) baenid turtle Saxochelys gilberti based on micro-computed tomography scans and new information on the holotype-shell association

“…According to Joyce & Lyson ( 2015 ), the diversity of Baenidae was highest during the Maastrichtian (Late Cretaceous) and the Danian (Early Paleogene). A common topic of interest is the survival of baenid turtles across the Cretaceous/Paleogene (K/Pg) boundary (Hutchison & Archibald, 1986 ; Lyson & Joyce, 2009a , 2015 ; Lyson et al, 2011 ; Holroyd et al, 2014 ; Jasinski et al, 2018 ; Adrain, 2022 ), which is associated with an asteroid-impact driven mass extinction event (e.g., Chiarenza et al, 2020 ; Fastovsky & Sheehan, 2005 ; Hull et al, 2020 ; Raup & Sepkoski, 1982 ; Schulte et al, 2010 ). To understand survivorship, including the potential selective survivorship of ecological guilds (e.g., Evers & Joyce, 2020 ; Jasinski et al, 2018 ; Lyson & Joyce, 2009a ), the communities on either side of the extinction event should be known well.…”