New multituberculate mammal from the Early Cretaceous of eastern North America

Abstract

Multituberculates, though among the most commonly encountered mammalian fossils of the Mesozoic, are poorly known from the North American Early Cretaceous, with only one taxon named to date. Herein we describe Argillomys marylandensis, gen. et sp. nov., from the Early Cretaceous of Maryland, based on an isolated M2. Argillomys represents the second mammal known from the Arundel Clay facies of the Patuxent Formation (Lower Cretaceous: Aptian). Though distinctive in its combination of characters (e.g., enamel ornamentation consisting of ribs and grooves only, cusp formula 2:4, presence of distinct cusp on anterobuccal ridge, enlargement of second cusp on buccal row, central position of ultimate cusp in lingual row, great relative length), the broader affinities of Argillomys cannot be established because of non-representation of the antemolar dentition. Based on lack of apomorphies commonly seen among Cimolodonta (e.g., three or more cusps present in buccal row, fusion of cusps in lingual row, cusps strongly pyramidal and separated by narrow grooves), we provisionally regard Argillomys as a multituberculate of “plagiaulacidan” grade. Intriguingly, it is comparable in certain respects to some unnamed Paulchoffatiidae, a family otherwise known from the Late Jurassic – Early Cretaceous of the Iberian Peninsula.

Faunal composition and paleoenvironment of the Arundel Clay (Potomac Formation; Early Cretaceous), Maryland, USA

“…USNM locality 41614 has yielded a diverse vertebrate fauna (e.g., Lipka, 1998;Kranz, 1998a). To date, a lungfish (Frederickson et al, 2016), a turtle (Lipka et al, 2006), two mammals (Cifelli et al, 1999;Rose et al, 2001;Cifelli et al, 2013), and multiple dinosaurs (Gilmore, 1921;Kranz, 1998a;Brownstein, 2017) have been formally described.…”

“…Arundelconodon is rather similar to other alticonodontines from the late Early and early Late Cretaceous (Patterson, 1951;Turnbull and Cifelli, 1999;Cifelli and Madsen, 1998;, such as Astroconodon (Aptian-Albian, Texas, and Oklahoma; Albian-Cenomanian, Utah), Corviconodon (Albian-Cenomanian, Utah, Montana) and Jugulator (Albian-Cenomanian, Utah), differing in the retention of a few plesiomorphies (such as the slight development of lower molar cusp d). MULTITUBERCULATA Cope, 1884"PLAGIAULACIDA" Ameghino, 1889 Family uncertain Genus ARGILLOMYS Cifelli, Gordon, and Lipka, 2013 Argillomys marylandensis Cifelli, Gordon, and Lipka, 2013 Material. Only the holotype, USNM 497739, an isolated left M2, described by Cifelli et al (2013).…”

“…MULTITUBERCULATA Cope, 1884"PLAGIAULACIDA" Ameghino, 1889 Family uncertain Genus ARGILLOMYS Cifelli, Gordon, and Lipka, 2013 Argillomys marylandensis Cifelli, Gordon, and Lipka, 2013 Material. Only the holotype, USNM 497739, an isolated left M2, described by Cifelli et al (2013). Description and remarks.…”

“…Affinities of Argillomys are uncertain owing to its retention of many plesiomorphic states. Cifelli et al (2013) recognized it as a late-surviving relative of one of several "plagiaulacidans" groups typical of the Late Jurassic or earliest Cretaceous. Intriguingly, closest similarity is to an unnamed taxon of Paulchoffatiidae, a group known mainly from the Kimmeridgian-age Guimarota beds of Portugal.…”

“…* indicates species not observed here, but published elsewhere. Species data from Marsh (1888), Gilmore (1921), Chinnery et al (1998), Harris (1998), Kranz (1998a,b), Lipka (1998), , Lipka et al (2006), Cifelli et al (2013), Frederickson et al (2016), and Brownstein (2017). contradictory changes to explain the Potomac sequence, including simultaneous elevation of deposits-to account for the shift from nearshore marine deposition in the Patuxent, to terrigenous in the Arundel-coupled with "landward depression of the continental border" to explain the character of Arundel sediments.)…”