A series of 33 Late Cretaceous (earliest Campanian through Maastrichtian) paleoshoreline maps was developed to document the migrational evolution of the western edge of the North American Western Interior Seaway. The maps represent a geologic span of roughly 18 million years, and portray the estimated positions of the strandline for each standard Western Interior ammonite zone, beginning with the Clioscaphites choteauensis zone and continuing to the end of the Mesozoic. We attempted correlation of all significant mammal-bearing localities known from the Western Interior with the ammonite-based marine zonation. First approximations of correspondence between ammonite zones and North American Land-Mammal “Ages” (NALMAs) include: Lancian ( Sphenodiscus through “Triceratops” zones); “Edmontonian” (a name not yet faunally defined; Didymoceras cheyennense through Baculites clinolobatus zones); Judithian (the smooth, late form of Baculites sp. through Exiteloceras jenneyi zones); and Aquilan ( Scaphites hippocrepis through Baculites asperiformis zones). Correlations emphasize use of provincial biostratigraphic terminology designed specifically for use in the Western Interior. On the basis of temporal constraints suggested herein, known mammalian fossils from the upper Fruitland and/or lower Kirtland Formations of New Mexico probably are of “Edmontonian,” not Judithian age. Although considerable latitudinally based taxonomic diversification of Judithian mammals is now recognizable across the Western Interior, comparative data are inadequate to defend a similar statement for the remaining Late Cretaceous NALMAs. Quantitative evaluation of geographic patterns of shoreline change suggests occurrence of a general, regional regression of the sea during the entire geologic interval represented in the study. We favor explanation by a slow sea-level depression resulting from topographic evolution of the world’s mid-oceanic system of ridges and volcanic plateaus. Local and subregional asynchronous episodes of shoreline transgressions, stillstands, and regressions are superimposed upon the general regressive trend, and probably represent influences of local tectonism, not eustatic changes in sea level. Strandline evolution of the epeiric sea during the last 20 million years of the Cretaceous in the North American Western Interior is inconsistent with: (1) existence of geologically brief (1 to 10 m.y.) global fluctuations in sea level; and (2) the concept that the late Campanian was represented by an unusually high global sea level.

Abstract Multituberculates are an extinct mammalian order that lived in Mesozoic and early Cenozoic eras. Lambdopsalis, a Paleocene multituberculate recovered in China, preserves cranial remains that allow in this study: (1) a description of its skull morphology; (2) a reconstruction of its nonfossilized structures such as the cranial nerve system and major cranial vasculature; (3) an analysis of functional adaptation of its auditory system; and (4) an interpretation of phylogenetic relationships within multituberculates themselves and among other major mammalian groups. Character analysis reveals that a number of previously used cranial features in reconstructions of mammalian phylogeny are unreliable. These include premaxillo-frontal contact, exclusion of septomaxilla from face, number of infraorbital foramina, extent of orbital exposure of palatine, presence versus absence of jugal, lacrimal, and parasphenoid, extent of cranial process of squamosal, and reduction of alisphenoid. The bulla-like structure of Lambdopsalis is the expanded vestibular apparatus, not an enlarged tympanic bulla. The expanded vestibular apparatus, flat incudomalleal joint, and absence of a well defined fossa muscularis minor in Lambdopsalis suggest that Lambdopsalis (possibly a burrower) adapted to low-frequency perception. Lambdopsalis possesses a large alisphenoid (perforated by the trigeminal foramina) and a slender “anterior lamina of the petrosal.” The discovery supports Presley’s (1981) argument of close affinity between “nontherian” and “therian” mammals, and invalidates the hypothesis of fundamental nontherian/therian dichotomy. Contrary to general consensus, available paleontological evidence does not indicate the existence of a uniform structural pattern of the braincase for nontherian groups. Cranial characters coupled with dental features document monophyly for nonharamiyid multituberculates. The skull morphology of Lambdopsalis shows phylogenetic unity with taeniolabidids, and invalidity of Lambdopsalidae Chow and Qi, 1978. Assuming monophyly of Mammalia, the class is divisible into a crown group and a stem group. The crown group includes all living mammals plus the fossil therians that shared the latest common ancestor with monotremes. The stem group consists of all remaining extinct mammals. Multituberculates belong to the paraphyletic stem group, and diverged from the main lineage leading to living mammals prior to emergence of the latest common ancestor of modern mammals. More intimate relations among members of the stem group remain uncertain, but are limited to but a few reasonable alternatives.

Abstract Multituberculates are an extinct mammalian order that lived in Mesozoic and early Cenozoic eras. Lambdopsalis , a Paleocene multituberculate recovered in China, preserves cranial remains that allow in this study: (1) a description of its skull morphology; (2) a reconstruction of its nonfossilized structures such as the cranial nerve system and major cranial vasculature; (3) an analysis of functional adaptation of its auditory system; and (4) an interpretation of phylogenetic relationships within multituberculates themselves and among other major mammalian groups. Character analysis reveals that a number of previously used cranial features in reconstructions of mammalian phylogeny are unreliable. These include premaxillo-frontal contact, exclusion of septomaxilla from face, number of infraorbital foramina, extent of orbital exposure of palatine, presence versus absence of jugal, lacrimal, and parasphe— noid, extent of cranial process of squamosal, and reduction of alisphenoid. The bulla-like structure of Lambdopsalis is the expanded vestibular apparatus, not an enlarged tympanic bulla. The expanded vestibular apparatus, flat incudomalleal joint, and absence of a well defined fossa muscularis minor in Lambdopsalis suggest that Lambdopsalis (possibly a burrower) adapted to low-frequency perception. Lambdopsalis possesses a large alisphenoid (perforated by the trigeminal foramina) and a slender “anterior lamina of the petrosal.” The discovery supports Presley's (1981) argument of close affinity between “nontherian” and “therian” mammals, and invalidates the hypothesis of fundamental nontherian/therian dichotomy. Contrary to general con— sensus, available paleontological evidence does not indicate the existence of a uniform structural pattern of the brain— case for nontherian groups. Cranial characters coupled with dental features document monophyly for nonharamiyid multituberculates. The skull morphology of Lambdopsalis shows phylogenetic unity with taeniolabidids, and invalidity of Lambdopsalidae Chow and Qi, 1978. Assuming monophyly of Mammalia, the class is divisible into a crown group and a stem group. The crown group includes all living mammals plus the fossil therians that shared the latest common ancestor with monotremes. The stem group consists of all remaining extinct mammals. Multituberculates belong to the paraphyletic stem group, and diverged from the main lineage leading to living mammals prior to emergence of the latest common ancestor of modern mammals. More intimate relations among members of the stem group remain uncertain, but are limited to but a few reasonable alternatives.