Therian postcranial bones from the Upper Cretaceous Bissekty Formation of Uzbekistan

Abstract

The Upper Cretaceous (upper Turonian) Bissekty Formation of Uzbekistan produces the most diverse Cretaceous therian fauna including one stem marsupial and eleven stem placental taxa known from cranial and dental elements. Some isolated postcranial elements from the Bissekty Formation can be confidently attributed to some of these taxa based on morphology and size. The humeral fragments, astragalus, and calcanei attributed to the deltatheroidan Sulestes karakshi Nessov, 1985 are similar to these bones in other stem marsupials. Postcranial bones referred to Zhelestidae Nessov, 1985 and Asioryctitheria Novacek et al., 1997 possess plesiomorphic therian morphologies, which are also present in some stem marsupials. All fragments of scapula from the Bissekty Formation show a plesiomorphic morphology with a trough-like infraspinous fossa placed medially to the supraspinous fossa. Bones attributed to the stem placental Paranyctoides quadrans (Nessov, 1982) and to the zalambdalestid Kulbeckia kulbecke Nessov, 1993 indicate arboreal and cursorial specializations, respectively. In particular, Kulbeckia Nessov, 1993 is similar to Zalambdalestes Gregory et Simpson, 1926 in having long and distally fused tibia and fibula and a long calcaneal tuber. Its distal humerus has a deep trochlea, large medial trochlear keel, and large capitular tail separated from a cylindrical capitulum by a shallow groove. The upper ankle joint of Kulbeckia has the complete separation of medial and lateral astragalotibial articulations.