Turtles From an Arkadelphia Formation—Midway Group Lag Deposit (Maastrichtian—Paleocene), Hot Spring County, Arkansas, USA

Abstract

The Arkadelphia Formation-Midway Group (Maastrichtian-Paleocene) contact near Malvern, Arkansas preserves a K-Pg boundary assemblage of turtle species consisting of skull, shell, and non-shell postcranial skeletal elements. The Malvern turtles are preserved within a coquina lag deposit that comprises the basalmost Midway Group and also contains an abundance of other reptiles, as well as chondrichthyans, osteichthyans, and invertebrates. This coquina lag deposit records a complex taphonomic history of exhumation and reburial of vertebrate skeletal elements along a dynamic ancestral shoreline in southwestern Arkansas during the late Cretaceous-early Paleocene. Based on stratigraphic occurrence, the Malvern turtle assemblage indicates that these marine reptiles were living at or near the time of the K-Pg mass extinction and represent some of the latest Cretaceous turtles yet recovered from the Gulf Coastal Plain of the United States.

A new species ofHypolophites(Chondrichthyes, Myliobatiformes) from the Lower Clayton Limestone Unit of the Midway Group (Paleocene), near Malvern, Arkansas, USA

“…Although, the precise age of the LCLU and Hypolophites beckeri n. sp. requires further investigation, the regional K/Pg stratigraphy and taphonomy of LCLU chondrichthyan remains support an upper Danian–Selandian age assignment (McFarland, 2004; Becker et al, 2011, 2016; Maisch et al, in press).…”

“…Preliminary studies on the Midway Group near Malvern, Arkansas, have identified Danian sediments in the lowermost LCLU (Dastas et al, 2010; Stringer and Sloan, 2018). However, temporal gaps are represented by the disconformity and lag deposit at the contact between the Arkadelphia Formation and Midway Group (i.e., K/Pg boundary) and the three additional lag deposits stratigraphically higher within the LCLU (e.g., Becker et al, 2010, 2013, 2016; Maisch et al, in press). Although, the precise age of the LCLU and Hypolophites beckeri n. sp.…”

“…In southwestern Arkansas, Upper Cretaceous and Paleocene formations are infrequently exposed due to dense vegetation overgrowth (Haley et al, 1993, 2009; McFarland, 1998, 2004). Near the town of Malvern, Arkansas, highway stabilization, commercial development, and erosion in the Ouachita River has discontinuously exposed the: (1) Maastrichtian Arkadelphia Formation that consists of dark, micaceous clays interbedded with fossiliferous sandy coquina lenses; (2) Arkadelphia Formation-Midway Group Contact (K/Pg boundary) that occurs as a coquina lag deposit that includes phosphate pebbles and distinctly Cretaceous macrofossils, including chondrichthyans, osteichthyans, plesiosaurs, turtles, and ammonites (see Becker et al, 2006, 2010, 2013, 2016; Maisch et al, in press); (3) lower Midway Group (Paleocene) that is composed of gray clays and sandy limestones; and (4) upper Midway Group (Paleocene) that occurs mainly as orange/tan clays and limestones.…”

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Figure 1.Location of the Lower Clayton Limestone Unit of the Paleocene Midway Group near Malvern, Arkansas, USA. ( 1 ) Reconstructed Late Cretaceous–Maastrichtian (a) and Paleocene–Danian (b) shorelines across the Atlantic and Gulf Coastal Plains and Western Interior Seaway indicated by arrows (modified from Scotese, 2014; Becker et al, 2016). The location of Hypolophites beckeri n. sp.

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