New Early Cretaceous zalambdalestid stem placental mammal from Mongolia and evolution of Zalambdalestidae

<i>Prozalambdalestes cratodus</i> gen. et sp. nov. is based on a fragmentary dentary from the Lower Cretaceous (Aptian–Albian) Khovoor locality in Gobi Desert, Mongolia. This is the largest eutherian in the Khovoor mammal assemblage. It shows dental specializations of Zalambdalestidae such as procumbent, ever-growing gliriform first lower incisor with restricted enamel and reaching posteriorly p5, and mesiodistally compressed lower molar trigonids. The enamel is restricted to the ventrolabial and ventrolingual sides of the mesial half of the incisor and the enamel band is higher on the labial side. The new taxon retains five lower premolars, with p1 smaller than p2 and p3 smaller than adjacent premolars, and the Meckelian groove. It also differs from other zalambdalestids by position of mental foramina (under p3 and p5), p1–3 obliquely set in dentary, and lower molar talonids wider than trigonids. Phylogenetic analysis places <i>Prozalambdalestes</i> gen. nov. in a polytomy with Albian <i>Zhangolestes</i>, Turonian <i>Kulbeckia</i>, and the clade of Campanian zalambdalestids from Mongolia and Kazakhstan. CT data in <i>Zalambdalestes</i> and comparison with <i>Prozalambdalestes</i> gen. nov. show that the four lower premolars in <i>Zalambdalestes</i> are p2, p3 (variable present), p4, and p5. Consequently, the three lower premolars of <i>Barunlestes</i> and <i>Zofialestes</i> are p2, p4, and p5. The other diagnostic characters of Zalambdalestidae are reviewed.