Descriptions of three new specimens of cimolodontans and a phylogenetic study of the postcranial anatomy of Multituberculata (Mammalia, Synapsida)

The position of Multituberculata within Mammalia and the phylogenetic relationships among multituberculates remain controversial. Previous comparative studies on the postcranial skeletons of multituberculates have suggested that this part of the skeleton is highly conserved and provides little phylogenetic information for resolving relationships within Multituberculata; however, detailed examination of published specimens and anatomical descriptions of three new partial multituberculate skeletons demonstrate that this is not the case.;Three new skeletons are described (1) a partial skeleton of Ectypodus sp. from the newly discovered Bangtail-2 locality (Early-Late Paleocene, Montana, USA), (2) partially articulated skeleton of an undescribed taxon referred to as the Udan Multi, which is closely related to Tombaatar sabuli, from the Udan Sayr locality (Late Cretaceous, Mongolia), and (3) a nearly complete and articulated skeleton of Kryptobaatar dashzevegi from the locality of Ukhaa Tolgod (Late Cretaceous, Mongolia). The skeleton of Kryptobaatar includes the only multituberculate manus preserved in 3 dimensions, and it displays a mosaic of features; large centrale, contact of metacarpal V and cuneiform as in monotremes, enlarged trapezium and cranial displacement of metacarpal I as in trechnotherians (symmetrodonts and therian mammals), a fairly minor degree of humeral torsion, and fusion of cervical ribs to form transverse foramina of the cervical vertebrae. The new specimen of Ectypodus has a surprisingly derived ankle; its lateral astragalotibial facet on the astragalus is pulley-shaped. The calcaneus of the Udan Multi has a well developed calcaneal crest, reminiscent of the morphology seen in early mammaliaforms.;Previous phylogenetic hypotheses for the relationships within Multituberculata were tested with a data matrix of 35 taxa coded for 95 postcranial characters, of which 77 are new. Noteworthy findings include: (1) postcranial evidence for a clade of Late Cretaceous Mongolian multituberculates excluding Djadochtatherium, (2) morphology of the tibial articular surface on the astragalus supports a close phylogenetic relationship between Ectypodus and Stygimys, which previously had been placed in separate families, (2) discovery of eight new postcranial autapomorphies for Multituberculata, and (3) strong support for a close relationship between two Mongolian multituberculates, the Udan Multi and Catopsbaatar .