Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary.

Estimates of the time of origin for placental mammals from DNA studies span nearly the duration of the Cretaceous period (145 to 65 million years ago), with a maximum of 129 million years ago and a minimum of 78 million years ago. Palaeontologists too are divided on the timing. Some support a deep Cretaceous origin by allying certain middle Cretaceous fossils (97-90 million years old) from Uzbekistan with modern placental lineages, whereas others support the origin of crown group Placentalia near the close of the Cretaceous. This controversy has yet to be addressed by a comprehensive phylogenetic analysis that includes all well-known Cretaceous fossils and a wide sample of morphology among Tertiary and recent placentals. Here we report the discovery of a new well-preserved mammal from the Late Cretaceous of Mongolia and a broad-scale phylogenetic analysis. Our results exclude Cretaceous fossils from Placentalia, place the origin of Placentalia near the Cretaceous/Tertiary (K/T) boundary in Laurasia rather than much earlier within the Cretaceous in the Southern Hemisphere, and place afrotherians and xenarthrans in a nested rather than a basal position within Placentalia.

Implications of Deltatheridium specimens for early marsupial history.

We describe here two new specimens of the mammal Deltatheridium pretrituberculare from the Late Cretaceous period of Mongolia. These specimens provide information on tooth replacement in basal therian mammals and on lower jaw and basicranial morphology. Deltatheroidans, known previously from isolated teeth, partial rostra and jaws from the late Cretaceous of Asia and possibly North America, have been identified variously as eutherians, as basal metatherians (the stem-based clade formed by marsupials and their extinct relatives), or as an outgroup to both eutherians and metatherians. Resolution of these conflicting hypotheses and understanding of the early evolution of the therian lineage have been hampered by a sparse fossil record for basal therians. The new evidence supports metatherian affinities for deltatheroidans and allows a comprehensive phylogenetic analysis of basal metatherians and marsupials. The presence of specialized marsupial patterns of tooth replacement and cranial vascularization in Deltatheridium and the basal phylogenetic position of this taxon indicate that these features are characteristic of Metatheria as a whole. Other morphological transformations recognized here secure the previously elusive diagnosis of Metatheria. The new specimens of Deltatheridium illustrate the effectiveness of fairly complete fossil specimens in determining the nature of early evolutionary events.

Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia.

An important transformation in the evolution of mammals was the loss of the epipubic bones. These are elements projecting anteriorly from the pelvic girdle into the abdominal region in a variety of Mesozoic mammals, related tritylodonts, marsupials and monotremes but not in living eutherian (placental) mammals. Here we describe a new eutherian from the Late Cretaceous period of Mongolia, and report the first record of epipubic bones in two distinct eutherian lineages. The presence of epipubic bones and other primitive features suggests that these groups occupy a basal position in the Eutheria. It has been argued that the epipubic bones support the pouch in living mammals, but epipubic bones have since been related to locomotion and suspension of the litter mass of several attached, lactating offspring. The loss of the epipubic bones in eutherians can be related to the evolution of prolonged gestation, which would not require prolonged external attachment of altricial young. Thus the occurrence of epipubic bones in two Cretaceous eutherians suggests that the dramatic modifications connected with typical placental reproduction may have been later events in the evolution of the Eutheria.

Development of the vomeronasal organ in Rousettus leschenaulti (Megachiroptera, Pteropodidae).

A functional vomeronasal organ (VNO) is known to be lacking in adult bats of the suborder Megachiroptera, family Pteropodidae, studied to date. However, whether the VNO every forms during ontogeny in megachiropterans has not been addressed. We report here on the development of the VNO in megachiropterans via study of 8 stages of rousette fruit bat Rousettus leschenaulti, ranging from an early limb bud embryo to a young specimen attached to the nipple. A vomeronasal primordium appears in the 4 youngest stages (7-14 mm crown-rump length), but there is no sign of any of the components of the vomeronasal system (neuroepithelial tube, nerves, sinuses, glands, or trough-like cartilage) in the septal region of the 4 oldest stages examined, or in the adult. Given the number of genera investigated to date and their taxonomic diversity, a conclusion that a VNO is entirely lacking in Megachiroptera seems reasonable. However, final confirmation awaits study of the additional 27 genera not yet reported (out of a total of 41).

Observations on the vomeronasal organ of the colugo Cynocephalus (Mammalia, Dermoptera).

This paper presents the first description of a functional vomeronasal organ in the colugo or 'flying lemur' Cynocephalus, the sole living representative of the order Dermoptera (Mammalia). The vomeronasal organ complex comprises patent nasopalatine and vomeronasal ducts, a 10-mm-long epithelial tube consisting of an 8- to 10-cell thick, 65-microns-tall neurosensory epithelium, the vomeronasal nerve, and the accessory olfactory bulb. The vomeronasal glands are sparse. Among mammals, the vomeronasal organ of the colugo is one of the relatively longest, at nearly 48% of nasal cavity length. It is exceeded among archontans (colugos, bats, primates, and tree shrews) only by that of the strepsirhine primate Microcebus murinus.

Further examination of the basicranial anatomy of the megachiroptera: a reply to A.J. King.

Four basicranial characters supporting a single origin of megachiropterans and microchiropterans have been identified by Wible and Novacek. King was not able to find two of these characters in specimens of the megachiropteran Pteropus and concluded that these characters be rejected in analyses of chiropteran phylogeny. To address King's claims, the two characters in question (regarding the tegmen tympani and ramus inferior of the stapedial artery) are documented here for Pteropus. The discrepancies between this report and King's appear to result from different criteria for identifying the tegmen tympani. Employed here are the criteria generally accepted by students of the mammalian chondrocranium.

Middle ear morphology of Lemur variegatus. Some implications for primate paleontology.

The middle ear morphology of Lemur variegatus, a strepsirhine primate, is described. Although no promontory branch of the internal carotid artery appears, there is a well-developed "promontory canal" containing a nerve trunk. This structure, which is previously undescribed in strepsirhines, is made up of the tympanic nerve and the internal carotid nerve. The implications of this discovery for paleontology, systematics, and future research are discussed.