J. David Archibald

Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals

Extant eutherian mammals and their most recent common ancestor constitute the crown group Placentalia. This taxon, plus all extinct taxa that share a more recent common ancestor with placentals than they do with Metatheria (including marsupials), constitute Eutheria. The oldest well documented eutherian-dominated fauna in the world is Dzharakuduk, Uzbekistan. Among eutherians that it yields is Kulbeckia, an 85–90-Myr-old member of Zalambdalestidae (a family of Late Cretaceous Asian eutherians). This extends Zalambdalestidae back by some 10 million years from sites in the Gobi Desert, Mongolia. A phylogenetic analysis of well described Late Cretaceous eutherians strongly supports Zalambdalestidae, less strongly supports ‘Zhelestidae’ (a Late Cretaceous clade related to Tertiary ungulates), but does not support Asioryctitheria (a group of Late Cretaceous Asian eutherians). A second analysis incorporating placentals from clades that include rodents (Tribosphenomys), lagomorphs (Mimotona) and archaic ungulates (Protungulatum and Oxyprimus) strongly supports Zalambdalestidae in a clade with Glires (rabbits, rodents and extinct relatives) and less strongly ‘Zhelestidae’ within a clade that includes archaic ungulates (‘condylarths’). This argues that some Late Cretaceous eutherians belong within the crown group Placentalia. The ages of these taxa are in line with molecularly based estimates of 64–104 Myr ago (median 84 Myr ago) for the superordinal diversification of some placentals, but provide no support for a Late Cretaceous diversification of extant placental orders.

Quantitative Analysis of the Timing of the Origin and Diversification of Extant Placental Orders

Fossil evidence is consistent with origination and diversification of extant placental orders in the early Tertiary (Explosive Model), and with the possibility of some orders having stem taxa extending into the Cretaceous (Long Fuse Model). Fossil evidence that 15 of 18 extant placental orders appeared and began diversification in the first 16 m.y. of the Cenozoic is, however, at odds with molecular studies arguing some orders diversified up to 40 m.y. earlier in the Early Cretaceous (Short Fuse Model). The quality of the fossil record was assessed by tabulating localities of all mammals in the last 105 m.y. Global locality data (except Africa) for 105 m.y. of eutherian evolution indicate discernible biogeographic patterns by the last 15 m.y. of the Cretaceous. Eutherian genera increase from 11 in latest Cretaceous to 139 in earliest Tertiary, although both are represented by about 50 localities. Yet even in the Late Cretaceous of North America and Asia where eutherians are abundant, none of the 18 extant orders are definitely known. A series of Monte Carlo simulations test whether the rapid appearance of most mammalian orders is statistically significant, and if so, whether it is a radiation event or an artifact of a limited fossil record. Monte Carlo tests affirm that the clustering of appearances in the early Cenozoic is statistically significant. Quantitative analysis of the locality data suggests that the number of genera described is a function of the number of localities sampled. In contrast, the number of orders is not a simple function of localities and thus does not appear to be limited by localities. A second set of Monte Carlo simulations confirms that the increase in orders cannot be explained by the limited number of localities sampled. Even for best-fit simulations, the observed pattern of ordinal appearances is steeper than expected under a variety of null models. These quantitative analyses of the fossil record demonstrate that the rapid ordinal appearances cannot be ascribed to limited Late Cretaceous sample sizes; thus, early Tertiary ordinal diversification is real. Although the fossil record is incomplete, it appears adequate to reject the hypothesis that orders of placentals began to diversify before the K/T boundary.

Cretaceous Extinctions: Multiple Causes

![Figure][1] Deccan plateau basalts. Lava from Deccan volcanism formed distinct layering. CREDIT: GSFC/NASA In the Review “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene boundary” (P. Schulte et al. , 5 March, p. [1214][2]), the terminal Cretaceous

Upper Cretaceous–Paleocene biostratigraphy and magnetostratigraphy, Hell Creek and Tullock Formations, northeastern Montana

Fossils from the Hell Creek and Tullock Formations in northeastern Montana provide detailed documentation of terrestrial faunal and floral evolution during latest Cretaceous (Lancian) and early Paleocene (Puercan) time. Here the replacement of Lancian faunas by those of Puercan age, most obviously signaled by the extinction of dinosaurs, and the changes in pollen floras sometimes used to mark the Cretaceous-Tertiary boundary occurred during a period of reversed magnetic polarity. Paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the Red Deer Valley area, Alberta, during the same period of reversed polarity. Furthermore, also on the basis of paleontological correlations, the extinction of dinosaurs in the San Juan Basin, New Mexico, appears to have occurred either during the same period of reversed polarity or, possibly, during the preceding period of normal polarity.

Out with a whimper not a bang

The recurring debate over the causes of the massive extinctions of such groups as the dinosaurs, ammonites, and calcareous nannoplankton, and the crash of the distinctive Aquilapollenites pollen province, used by biostratigraphers to mark the close of the Cretaceous, has been considerably enlivened with the resurgence of hypotheses suggesting catastrophic events. The impact of an asteroid, explosion of a supernova, sudden changes in oceanic circulation and composition, or atmospheric perturbations, to mention a few, have been put forward as malefactors. But, just how sound is the evidence of a biotic catastrophe?

New material of the Late Cretaceous deltatheroidan mammal Sulestes from Uzbekistan and phylogenetic reassessment of the metatherian-eutherian dichotomy

Sulestes karakshi Nessov, 1985b (= Deltatheroides kizylkumensis Nessov, 1993 = Marsasia aenigma Nessov, 1997) from the Late Cretaceous (Turonian) Bissekty local fauna, Kyzylkum Desert, Uzbekistan, is revised based on additional material from the type locality. It is characterized by an absence of palatal vacuities, double-rooted P1, an asymmetrical M3 with reduced metastylar lobe, an unreduced M4 and m4, obliquely oriented p1, anterior wall of the upper canine alveolus formed by premaxilla, and Meckelian groove on the dentary. PAUP analyses using a data matrix modified from Rougier et al. (1998, 2004) places Sulestes within Deltatheridiidae in an unresolved trichotomy with the Mongolian Campanian Deltatheridium and Deltatheroides. Oklatheridium from the Early Cretaceous of North America is sister taxon to these Late Cretaceous Asiatic deltatheridiidans. Deltatheridiidae is the sister group to other Metatheria including the crown clade Marsupialia. A Deltatheroides-like taxon from the Maastrichtian at Guriliin Tsav, Mongolia, is not related to the Stagodontidae but is sister taxon to other Boreometatheria. The North American Early Cretaceous Atokatheridium, Pappotherium, and Montanalestes are stem tribosphenic mammals, while Holoclemensia is at the base of the eutherian lineage.

Mammals from the Upper Cretaceous Aitym Formation, Kyzylkum Desert, Uzbekistan

The Upper Cretaceous (?Santonian) Aitym Formation in the central Kyzylkum Desert, Uzbekistan, produced remains of a cimolodontan multituberculate (Uzbekbaatar wardi), a spalacotheriid symmetrodont (cf. Shalbaatar sp.), a deltatheroidan (cf. Deltatherus sp.), possibly the asioryctithere aff. Daulestes sp., possibly two zalambdalestids (Kulbeckia sp. cf. K. kulbecke and aff. Kulbeckia sp.), two supposed lipotyphlans (Paranyctoides sp. cf. P. aralensis and Paranyctoides sp.), and zhelestid ungulatomorphs (cf. Aspanlestes sp., Parazhelestes sp. aff. P. minor, Parazhelestes sp. cf. P. robustus, and Eoungulatum sp. cf. E. kudukensis). The Aitym mammal fauna is most similar to the more diverse mammal fauna of the underlying Bissekty Formation (upper Turonian–Coniacian). Uzbekbaatar and Shalbaatar were most probably derived from North American immigrants. Paranyctoides and ‘Zhelestidae’ are of Middle Asian (a commonly and long used Soviet geographic region approximately extending from the Caspian Sea on the west to the Chinese border on the east, and from the Iranian and Afghan borders on the south to southern Kazakhstan on the north) origin and migrated to North America during the late Santonian–early Campanian. A dispersal route was present in Beringia during the entire Late Cretaceous, which may have worked as a cold filter, allowing intercontinental dispersals between Asia and western North America in both directions, especially during shorter, warm climatic phases. 2003 Published by Elsevier Ltd.

The importance of phylogenetic analysis for the assessment of species turnover: a case history of Paleocene mammals in North America

During the latest Cretaceous and the Paleocene in western North America, disappearance rates for mammalian genera track appearance rates, both reaching their peak in the early Paleocene (Puercan) following the extinction of non-avian dinosaurs. Some of the disappearances during this time were pseudoextinctions that resulted when ancestral species disappeared during speciation. Species-level cladistic analyses and a well-constrained biostratigraphic framework are required to study this form of pseudoextinction. Cladistic analyses show that monophyly cannot be estab- lished or rejected for some species because these species lack autapomorphies (uniquely derived character states) that unite their constituent members. Such taxa, termed metaspecies, are potential ancestors to species and higher clades with which they share a node in the cladogram. A hypothetical species-level cladistic analysis coupled with three different hypothetical biostra- tigraphies shows how different models of speciation (bifurcation, budding, or anagenesis) result in very different patterns of true versus pseudoextinction. Depending on the speciation model, true extinction can be overestimated by as much as a factor of four, raising the specter of mass extinction. Species-level studies for three early Tertiary mammalian taxa-taeniodont eutherians, taeniolabidid multituberculates, and periptychid ungulates-use the same procedures. They show that almost 25% of disappearances during the early Paleocene (Puercan) for species in the analysis were pseu- doextinctions of metaspecies. Budding and anagenetic-like peripatric speciation, but not bifurcation, are seen in the three examples. Equating disappearance to true extinction can profoundly affect interpretations of faunal turnover, especially during mass extinctions or major faunal reorganizations. Some authors use pseudoex- tinction to describe the taxonomic rather than evolutionary disappearance of nonmonophyletic groups. Pseudoextinction, as used here refers only to the evolutionary disappearance of metaspecies via speciation. Both usages seem appropriate but should not be confounded.

Mammalian Faunal Succession in the Cretaceous of the Kyzylkum Desert

Both metatherians and eutherians are known from the Early Cretaceous (Barremian, 125 mya; million years ago) of China, while eutherian-dominated mammalian faunas appeared in Asia at least by the earliest Late Cretaceous (Cenomanian, 95 mya). The approximately 99–93 my old (Cenomanian) Sheikhdzheili l.f. from western Uzbekistan is a small sample of only eutherians, including three zhelestids and a possible zalambdalestoid. The much better-known 90 my old (Turonian) Bissekty l.f. at Dzharakuduk in the central Uzbekistan includes 15 named and unnamed species, based on ongoing analyses. Of these, 12 are eutherians represented by at least the three groups—asioryctitheres, zalambdalestids, and zhelestids—plus an eutherian of uncertain position—Paranyctoides. Zalambdalestids and zhelestids have been argued to be related to the origin of the placental gliriforms (Euarchontoglires) and ferungulates (Laurasiatheria), respectively. Although there are four previously recognized metatherians, we believe three are referable to the deltatheroid Sulestes karakshi and the fourth, Sailestes quadrans, may belong to Paranyctoides. There is one multituberculate and one symmetrodont in the Bissekty l.f. While comparably aged (Turonian) localities in North America have somewhat similar non-therians, they have more metatherians and no eutherians. The next younger localities (early Campanian, ∼80 mya) in North America have both a zhelestid and Paranyctoides, suggesting dispersal of eutherians from Asia. At Dzharakuduk, the approximately 85 my old (late Turonian/Coniacian) Aitym l.f. is much less well known than the Bissekty l.f., but yields nearly identical taxa, with two non-therians, one metatherian, and six eutherians.

The Late Cretaceous placental mammal Kulbeckia

Abstract Kulbeckia, a placental mammal from the late Turonian–Coniacian (Late Cretaceous) of Uzbekistan, was originally placed in the monotypic Kulbeckiidae. Important new material indicates that Kulbeckia is the basal most member of “Zalambdalestidae”, which also includes Zalambdalestes, Barunlestes, and the poorly known Alymlestes, all from the Late Cretaceous of Asia. Kulbeckia shares with other zalambdalestids: a narrow, somewhat elongated snout; procumbent, enlarged, and open-rooted medial lower incisor with enamel restricted to the more ventrolabial surface; and anteroposteriorly compressed and centrally pinched molar trigonids. Commensurate with its 10-million-year earlier age relative to other zalambdalestids, it is notable in its smaller size, probable retention of four lower incisors, bifurcated or two-rooted lower canine, relatively smaller or absent diastemata between anterior teeth, more lingually placed cristid obliqua, less reduced M3 and m3, and more dorsal and posterior placement of the angular process in adults. Kulbeckia kansaica (Tadjikistan) and Kulbeckia rara (Uzbekistan) are regarded as synonyms of Kulbeckia kulbecke, the only recognized species of Kulbeckia.