John R. Wible

The earliest known eutherian mammal

The skeleton of a eutherian (placental) mammal has been discovered from the Lower Cretaceous Yixian Formation of northeastern China. We estimate its age to be about 125 million years (Myr), extending the date of the oldest eutherian records with skull and skeleton by about 40–50 Myr. Our analyses place the new fossil at the root of the eutherian tree and among the four other known Early Cretaceous eutherians, and suggest an earlier and greater diversification of stem eutherians that occurred well before the molecular estimate for the diversification of extant placental superorders (104–64 Myr). The new eutherian has limb and foot features that are known only from scansorial (climbing) and arboreal (tree-living) extant mammals, in contrast to the terrestrial or cursorial (running) features of other Cretaceous eutherians. This suggests that the earliest eutherian lineages developed different locomotory adaptations, facilitating their spread to diverse niches in the Cretaceous.

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.

Petrosals of Late Cretaceous marsupials from North America, and a cladistic analysis of the petrosal in therian mammals

ABSTRACT Ten isolated petrosals of Late Cretaceous marsupials belonging to three types—A, B, and C—are described from the Lance Formation of Wyoming and the Bug Creek Anthills of Montana. These are compared with ear regions of other Mesozoic and Recent mammals in a cladistic analysis of 24 petrosal characters among ten taxa. A clade comprising marsupials, eutherians, and Vincelestes from the Early Cretaceous of Argentina is supported by five synapomorphies of the ear region—coiled cochlea, perilymphatic duct enclosed in an osseous canal, post-promontorial tympanic sinus present, caudal tympanic process of the petrosal present behind stapedius fossa, and lateral wall of fossa incudis formed by squamosal. Marsupialia is diagnosed by four synapomorphies of the ear region—stapedial artery absent in adult, prootic canal reduced and with intramural opening, prootic sinus within deep sulcus between petrosal and squamosal, and sphenoparietal emissary vein present. Eutheria is also diagnosed by four synapomorphies...

The Eutherian Mammal Maelestes gobiensis from the Late Cretaceous of Mongolia and the phylogeny of cretaceous eutheria

Abstract Maelestes gobiensis Wible et al., 2007, is the second new eutherian mammal to be named from the rich Mongolian Late Cretaceous locality of Ukhaa Tolgod, Ukhaatherium nessovi Novacek et al., 1997, being the first. Maelestes is only the seventh Late Cretaceous eutherian known from the skull and the upper and lower dentitions, and the fifth known from some postcranial elements. The type and only known specimen, PSS-MAE 607, is described and illustrated in detail. The type is amended to include: an incomplete skull, left dentary, atlas, axis, last cervical and first 11 thoracic vertebrae, 11 partial ribs, incomplete scapula, clavicle, humerus, and proximal radius and ulna. An astragalus on a separate block was referred to Maelestes by Wible et al. (2007), but it is too large to belong to this taxon and is removed from the isotype. Several corrections and updates are made to the phylogenetic analysis of Wible et al. (2007). The original analysis and the one in this report include 408 morphological characters (127 dental, 212 cranial, and 69 postcranial) in Maelestes along with 68 other taxa (four stem therians, three metatherians, 31 Cretaceous eutherians, 20 extinct Tertiary placentals, and 11 extant placentals). Maelestes is identified as a member of Cimolestidae sensu Kielan-Jaworowska et al. (2004) along with the slightly younger and poorer known North American taxa Batodon Marsh, 1892, and Cimolestes Marsh, 1889. Cimolestidae, in turn, is grouped with Asioryctitheria sensu Archibald and Averianov (2006), which includes monophyletic Mongolian and Uzbekistani clades. The other principal Late Cretaceous clades are: a Laurasian Zhelestidae; Paranyctoides Fox, 1979 (North American and Uzbekistan) + Eozhelestes Nessov, 1997 (Uzbekistan); and an Asian Zalambdalestidae. In contrast to some previous analyses, but in common with Wible et al. (2007), no Cretaceous eutherians are identified as members of any placental group.

Reconstruction of the cranial vessels in the Early Cretaceous mammal Vincelestes neuquenianus: implications for the evolution of the mammalian cranial vascular system

ABSTRACT Vincelestes neuquenianus from the Early Cretaceous of Argentina is the earliest therian mammal known from nearly complete crania. Described here is the structure of the petrosal bone and other cranial elements inferred to be associated with the vascular system. From comparisons with Recent amniotes, the major basicranial arteries and veins that we reconstruct for Vincelestes represent a composite of the vascular patterns in monotremes and some placentals (e.g., lipotyphlan insectivorans). Vincelestes had a well-developed stapedial system that was supplied principally via the arteria diploetica magna, a transpromontorial internal carotid artery, and a well-developed lateral head vein that drained the prootic sinus via a prootic canal. As in monotremes and extinct “non-therian” mammals, Vincelestes had an enlarged anterior lamina of the petrosal (=lamina obturans), through the base of which (lateral flange) ran the ramus superior of the stapedial artery. Reconstruction of the major basicranial arte...

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.

ORIGIN OF MAMMALIA: THE CRANIODENTAL EVIDENCE REEXAMINED

ABSTRACT In Rowe (1988), 158 characters of the skull and postcranial skeleton distributed among Placentalia, Marsupialia, Monotremata, Multituberculata, Morganucodontidae, Tritylodontidae, and Exaeretodon were analyzed with PAUP. Mammalia, defined by Rowe as comprising the most recent common ancestor of living monotremes, marsupials, and placentals, was distinguished from its nearest extinct relatives by 37 osteological synapomorphies, including 24 characters of the cranium and dentition. Within Mammalia, Multituberculata was identified as the sister taxon of Marsupialia plus Placentalia. The 91 craniodental characters employed by Rowe (1988) are reevaluated here and placed in one of five categories: characters needing no alteration (11); characters with a derived state occurring in an outgroup not considered by Rowe (8); characters for which the distribution given by Rowe is altered (16); characters for which the description given by Rowe is modified (30); and characters that are excluded from further an...

Basicranial Evidence for Early Mammal Phylogeny

The distribution of thirty-eight basicranial characters is considered among monotremes, marsupials, placentals, and the following extinct taxa—Tritheledontidae, Tritylodontidae, Sinoconodon, Morganucodontidae, Haldanodon, Triconodontidae, Multituberculata, and Vincelestes. PAUP analysis of the ensuing data matrix supports the following conclusions: 1. Marsupialia and Placentalia form a clade supported by an anterior lamina of the petrosal that is greatly reduced or absent, a cavum epiptericum floored primarily by the alisphenoid, major basicranial drainage via the postglenoid foramen, and a squamosal contributing broadly to the cranial wall. 2. A clade with Vincelestes from the Early Cretaceous of Argentina as the sister taxon to Marsupialia plus Placentalia is supported by a caudal tympanic process of the petrosal, a post-promontorial tympanic sinus, a true cochlear aqueduct, and a cochlear duct coiled through at least 270 degrees. 3. A clade comprising the taxa in (2) along with Multituberculata plus Monotremata is supported by loss of support for the ventromedial part of incus on the cranium and a greatly reduced quadrate ramus of the alisphenoid. A clade with Multituberculata and Monotremata is supported by a common tympanic aperture for the prootic canal and pterygoparoccipital foramen. 4. A clade including Triconodontidae and the taxa in (3) is supported by loss of the vascular foramen in the periotic lateral flange and the suspension of the postdentary bones from the cranium. 5. A clade with Haldanodon and the taxa in (4) is based on the loss of the quadratojugal notch in the squamosal. 6. A clade including the taxa in (5) plus Sinoconodon and Morganucodontidae is supported by an anterior lamina of the petrosal expanded forward dorsal to the exit of the maxillary and mandibular nerves, a cavum epiptericum partially floored by the petrosal, facial ganglion floored by petrosal, stapes length less than 5.5% of skull length, ossified base of the pila antotica absent, major basicranial drainage via a large prootic canal that opens endocranially, a paroccipital process with a distinct ventrally directed projection for muscle attachment, a petrosal promontorium, and a well-developed dentary-squamosal contact. 7. Tritylodontidae shares support of the ventromedial part of the quadrate on the cranium via a convex surface on the crista parotica of the petrosal and a fossa for the stapedius muscle on the petrosal with the taxa in (6).

Earliest Eutherian Ear Region: A Petrosal Referred to Prokennalestes from the Early Cretaceous of Mongolia

Abstract A right petrosal from the ?Aptian or Albian Khoobur locality is referred on the basis of size and morphology to Prokennalestes trofimovi, the earliest eutherian previously known only from dentigerous elements. The petrosal shows a mosaic of primitive and derived features, bearing on the purported therian and eutherian morphotypes. Among the primitive features shared with the Early Cretaceous prototribosphenidan Vincelestes and other more basal taxa that are modified in later eutherians and metatherians are the pattern of basicranial arterial and venous circulation, including a prootic canal and an intrapetrosal inferior petrosal sinus; a vertical paroccipital process; and a fenestra semilunaris, an incomplete wall between the cavum epiptericum and cavum supracochleare. Among the derived features shared with therians is a cochlea coiled through a minimum of 360°, with Prokennalestes extending the range of the oldest occurrence of such a coiled cochlea by at least 10 million years. Shared with Late Cretaceous eutherians is a shallow internal acoustic meatus with a thin prefacial commissure. The petrosal referred to Prokennalestes is intermediate in having a reduced anterior lamina and lateral flange, both of which are well developed in Vincelestes and essentially lacking in later eutherians and metatherians. Features previously held to be part of the therian and eutherian morphotypes, such as the absence of the anterior lamina and lateral flange, may have been lost independently in metatherians and in post-Prokennalestes eutherians.

Reexamination of the morphological evidence for the cohort Epitheria (Mammalia, Eutheria)

Novacek and co-workers recognized a monophyletic clade Epitheria, comprising all eutherians except edentates and the extinct palaeoryctoids, on the basis of two synapomorphies: a stirrupshaped stapes and a foramen ovale enclosed within the alisphenoid. To evaluate this phylogenetic hypothesis, we reexamined the distributions of stapedial morphologies and positions of the foramen ovale across Recent and extinct mammals and nonmammalian cynodonts. The states and distributions of the stapes and forament ovale characters used by Novacek and coworkers were modified by recognizing two stapedial characters (one relating to shape of the crura, the other to the nature of the foramen) and a single, multistate foramen ovale character (within, behind, and lateral to the alisphenoid). The taxon-character matrix used by Novacek (1989, 1992b), substituting our amended stapedial and foramen ovale characters and adding several previously unscored extinct taxa and three new characters, was subjected to a series of PAUP manipulations. Identified among the most parsimonious trees were three major topologies for the base of Eutheria: (1) a polytomy including an Edentata/Ungulata clade, (2) a polytomy with Edentata and Ungulata as separate clades, and (3) Edentata and (when included) Palaeoryctoidea as the successive outgroups to a monophyletic Epitheria. We conclude that topology 2 best reflects the current state of knowledge. An edentate/ungulate clade is supported by three characters (from the mastoid region and subarcuate fossa); however, other morphological studies require modification of the distributions of these characters in xenarthrans and bassal ungulates, thereby eliminating support for this clade. In nearly all manipulations, obtaining a monophyletic Epitheria required that one or two steps be added to the most parsimonious trees. When a monophyletic Epitheria was obtained, it was supported by a triangular stapes and, in some trees, the reappearance of a stapedial artery (lost earlier at the level of Recent therians) and a transpromontorial internal carotid artery. In the most parsimonious trees, a foramen ovale within the alisphenoid was an equivocal synapomorphy of Recent therians or cutherians, and a stapes with strongly convex crura (our state closest to the stirrup-shaped state of Novacek and co-workers) appeared independently within various eutherian lineages. The reduction or loss of the stapedial foramen was identified as an independent event in monotremes and within marsupials and various eutherian lineages.