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Shuotheriids are Jurassic mammaliaforms that possess pseudotribosphenic teeth in which a pseudotalonid is anterior to the trigonid in the lower molar, contrasting with the tribosphenic pattern of therian mammals (placentals, marsupials and kin) in which the talonid is posterior to the trigonid 1-4 . The origin of the pseudotribosphenic teeth remains unclear, obscuring our perception of shuotheriid affinities and the early evolution of mammaliaforms 1,5-9 . Here we report a new Jurassic shuotheriid represented by two skeletal specimens. Their complete pseudotribosphenic dentitions allow reidentification of dental structures using serial homology and the tooth occlusal relationship. Contrary to the conventional view 1,2,6,10,11 , our findings show that dental structures of shuotheriids can be homologized to those of docodontans and partly support homologous statements for some dental structures between docodontans and other mammaliaforms 6,12 . The phylogenetic analysis based on new evidence removes shuotheriids from the tribosphenic ausktribosphenids (including monotremes) and clusters them with docodontans to form a new clade, Docodontiformes, that is characterized by pseudotribosphenic features. In the phylogeny, docodontiforms and 'holotherians' (Kuehneotherium, monotremes and therians) 13 evolve independently from a Morganucodon-like ancestor with triconodont molars by labio-lingual widening their posterior teeth for more efficient food processing. The pseudotribosphenic pattern passed a cusp semitriangulation stage 9 , whereas the tribosphenic pattern and its precursor went through a stage of cusp triangulation. The two different processes resulted in complex tooth structures and occlusal patterns that elucidate the earliest diversification of mammaliaforms., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The dual jaw joint of Morganucodon 1,2 consists of the dentary-squamosal joint laterally and the articular-quadrate one medially. The articular-quadrate joint and its associated post-dentary bones constitute the precursor of the mammalian middle ear. Fossils documenting the transition from such a precursor to the mammalian middle ear are poor, resulting in inconsistent interpretations of this hallmark apparatus in the earliest stage of mammaliaform evolution 1-5 . Here we report mandibular middle ears from two Jurassic mammaliaforms: a new morganucodontan-like species and a pseudotribosphenic shuotheriid species 6 . The morganucodontan-like species shows many previously unknown post-dentary bone morphologies 1,2 and exhibits features that suggest a loss of load-bearing function in its articular-quadrate joint. The middle ear of the shuotheriid approaches the mammalian condition in that it has features that are suitable for an exclusively auditory function, although the post-dentary bones are still attached to the dentary. With size reduction of the jaw-joint bones, the quadrate shifts medially at different degrees in relation to the articular in the two mammaliaforms. These changes provide evidence of a gradual loss of load-bearing function in the articular-quadrate jaw joint-a prerequisite for the detachment of the post-dentary bones from the dentary 7-12 and the eventual breakdown of the Meckel's cartilage 13-15 during the evolution of mammaliaforms., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Mammaliamorpha comprises the last common ancestor of Tritylodontidae and Mammalia plus all its descendants 1 . Tritylodontids are nonmammaliaform herbivorous cynodonts that originated in the Late Triassic epoch, diversified in the Jurassic period 2-5 and survived into the Early Cretaceous epoch 6,7 . Eutriconodontans have generally been considered to be an extinct mammalian group, although different views exist 8 . Here we report a newly discovered tritylodontid and eutriconodontan from the Early Cretaceous Jehol Biota of China. Eutriconodontans are common in this biota 9 , but it was not previously known to contain tritylodontids. The two distantly related species show convergent features that are adapted for fossorial life, and are the first 'scratch-diggers' known from this biota. Both species also show an increased number of presacral vertebrae, relative to the ancestral state in synapsids or mammals 10,11 , that display meristic and homeotic changes. These fossils shed light on the evolutionary development of the axial skeleton in mammaliamorphs, which has been the focus of numerous studies in vertebrate evolution 12-17 and developmental biology 18-28 . The phenotypes recorded by these fossils indicate that developmental plasticity in somitogenesis and HOX gene expression in the axial skeleton-similar to that observed in extant mammals-was already in place in stem mammaliamorphs. The interaction of these developmental mechanisms with natural selection may have underpinned the diverse phenotypes of body plan that evolved independently in various clades of mammaliamorph.

Rhinoceroses have been considered to have originated from tapiroids in the middle Eocene; however, the transition remains controversial, and the first unequivocal rhinocerotoids appeared about 4 Ma later than the earliest tapiroids of the Early Eocene. Here we describe 5 genera and 6 new species of rhinoceroses recently discovered from the early Eocene to the early middle Eocene deposits of the Erlian Basin of Inner Mongolia, China. These new materials represent the earliest members of rhinocerotoids, forstercooperiids, and/or hyrachyids, and bridge the evolutionary gap between the early Eocene ceratomorphs and middle Eocene rhinocerotoids. The phylogenetic analyses using parsimony and Bayesian inference methods support their affinities with rhinocerotoids, and also illuminate the phylogenetic relationships and biogeography of Ceratomorpha, although some discrepancies are present between the two criteria. The nearly contemporary occurrence of various rhinocerotoids indicates that the divergence of different rhinocerotoid groups occurred no later than the late early Eocene, which is soon after the split between the rhinocerotoids and the tapiroids in the early early Eocene. However, the Bayesian tip-dating estimate suggests that the divergence of different ceratomorph groups occurred in the middle Paleocene.

Evolution of the definitive mammalian middle ear (DMME) as a textbook example in vertebrate evolution has been extensively studied during the last 200 years. Fossils provide the direct evidence on evolutionary stages of the DMME, but because of delicacy of the miniscule ossicles, unequivocal evidence about them has always been rare. Recent work on a stem therian mammal (124 million years old) shows presence of the surangular bone in the basal mammals as a primitive feature and potentially retained in the embryonic stage of some extant mammals. The work also proposed that the DMME and mammalian jaw evolved in a modular fashion. It started as a highly integrated complex in structures and functions, the two modules were regulated by similar developmental genetic mechanisms and eventually decoupled under natural selection so that the physical constraint the two modules imposed on each other was removed, allowing future improvement of each module for better function., (Copyright © 2020 Elsevier GmbH. All rights reserved.)

On the basis of multiple skeletal specimens from Liaoning, China, we report a new genus and species of Cretaceous stem therian mammal that displays decoupling of hearing and chewing apparatuses and functions. The auditory bones, including the surangular, have no bone contact with the ossified Meckel's cartilage; the latter is loosely lodged on the medial rear of the dentary. This configuration probably represents the initial morphological stage of the definitive mammalian middle ear. Evidence shows that hearing and chewing apparatuses have evolved in a modular fashion. Starting as an integrated complex in non-mammaliaform cynodonts, the two modules, regulated by similar developmental and genetic mechanisms, eventually decoupled during the evolution of mammals, allowing further improvement for more efficient hearing and mastication., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The holotypes of euharamiyidan Arboroharamiya allinhopsoni and Arboroharamiya jenkinsi preserve the auditory and hyoid bones, respectively. With additional structures revealed by micro-computerized tomography (CT) and X-ray micro-computed laminography (CL), we provide a detailed description of these minuscule bones. The stapes in the two species of Arboroharamiya are similar in having a strong process for insertion of the stapedius muscle. The incus is similar in having an almond-shaped body and a slim short process, in addition to a robust stapedial process with a short lenticular process preserved in A. allinhopsoni. The plate-like ectotympanic in the two species of Arboroharamiya is similar and comparable to that of Qishou jizantang. The surangular in the two species has a fan-shaped body and a needle-shaped anterior process. The malleus, ectotympanic, and surangular are fully detached from the dentary and should have functioned exclusively for hearing. All the auditory bones of Arboroharamiya display unique features unknown in other mammaliaforms. Moreover, hyoid elements are found in the two species of Arboroharamiya and co-exist with the five auditory bones in the holotype of A. allinhopsoni. The element interpreted as the stylohyal is similar to the bone identified as the ectotympanic in Vilevolodon. We reconstruct the auditory apparatus of Arboroharamiya and compare it with that of Vilevolodon as well as those in extant mammals and basal mammaliaforms. The comparison shows diverse morphological patterns of the auditory region in mammaliaforms. In particular, those of Vilevolodon and Arboroharamiya differ significantly: the former has a mandibular middle ear, whereas the latter possesses a definitive mammalian middle ear. It is puzzling that the two sympatric and dentally similar taxa have such different auditory apparatuses. In light of the available evidence, we argue that the mandibular middle ear reconstructed in Vilevolodon encounters many problems, and the so-called ectotympanic in Vilevolodon may be interpreted as a stylohyal; thus, the dilemma can be resolved., (© 2019 Anatomical Society.)

The evolution of the mammalian middle ear is thought to provide an example of 'recapitulation'-the theory that the present embryological development of a species reflects its evolutionary history. Accumulating data from both developmental biology and palaeontology have suggested that the transformation of post-dentary jaw elements into cranial ear bones occurred several times in mammals 1,2 . In addition, well-preserved fossils have revealed transitional stages in the evolution of the mammalian middle ear 1,3,4 . But questions remain concerning middle-ear evolution, such as how and why the post-dentary unit became completely detached from the dentary bone in different clades of mammaliaforms. Here we report a definitive mammalian middle ear preserved in an eobaatarid multituberculate mammal, with complete post-dentary elements that are well-preserved and detached from the dentary bones. The specimen reveals the transformation of the surangular jaw bone from an independent element into part of the malleus of the middle ear, and the presence of a restricted contact between the columelliform stapes and the flat incus. We propose that the malleus-incus joint is dichotomic in mammaliaforms, with the two bones connecting in either an abutting or an interlocking arrangement, reflecting the evolutionary divergence of the dentary-squamosal joint 4 . In our phylogenetic analysis, acquisition of the definitive mammalian middle ear in allotherians such as this specimen was independent of that in monotremes and therians. Our findings suggest that the co-evolution of the primary and secondary jaw joints in allotherians was an evolutionary adaptation allowing feeding with unique palinal (longitudinal and backwards) chewing. Thus, the evolution of the allotherian auditory apparatus was probably triggered by the functional requirements of the feeding apparatus.

The island effect is a well-known evolutionary phenomenon, in which island-dwelling species isolated in a resource-limited environment often modify their size, anatomy, and behaviors compared with mainland relatives. This has been well documented in modern and Cenozoic mammals, but it remains unclear whether older, more primitive Mesozoic mammals responded in similar ways to island habitats. We describe a reasonably complete and well-preserved skeleton of a kogaionid, an enigmatic radiation of Cretaceous island-dwelling multituberculate mammals previously represented by fragmentary fossils. This skeleton, from the latest Cretaceous of Romania, belongs to a previously unreported genus and species that possesses several aberrant features, including an autapomorphically domed skull and one of the smallest brains relative to body size of any advanced mammaliaform, which nonetheless retains enlarged olfactory bulbs and paraflocculi for sensory processing. Drawing on parallels with more recent island mammals, we interpret these unusual neurosensory features as related to the island effect. This indicates that the ability to adapt to insular environments developed early in mammalian history, before the advent of therian mammals, and mammals with insular-related modifications were key components of well-known dwarfed dinosaur faunas. Furthermore, the specimen suggests that brain size reduction, in association with heightened sensory acuity but without marked body size change, is a novel expression of the island effect in mammals., Competing Interests: The authors declare no conflict of interest.

The middle ear bones of Mesozoic mammals are rarely preserved as fossils and the morphology of these ossicles in the earliest mammals remains poorly known. Here, we report the stapes and incus of the euharamiyidan Arboroharamiya from the lower Upper Jurassic (∼160 Ma) of northern China, which represent the earliest known mammalian middle ear ossicles. Both bones are miniscule in relation to those in non-mammalian cynodonts. The skull length/stapedial footplate diameter ratio is estimated as 51.74 and the stapes length as the percentage of the skull length is 4%; both numbers fall into the stapes size ranges of mammals. The stapes is "rod-like" and has a large stapedial foramen. It is unique among mammaliaforms in having a distinct posterior process that is interpreted as for insertion of the stapedius muscle and homologized to the ossified proximal (stapedial) end of the interhyal, on which the stapedius muscle attached. The incus differs from the quadrate of non-mammalian cynodonts such as morganucodontids in having small size and a slim short process. Along with lack of the postdentary trough and Meckelian groove on the medial surface of the dentary, the ossicles suggest development of the definitive mammalian middle ear (DMME) in Arboroharamiya. Among various higher-level phylogenetic hypotheses of mammals, the one we preferred places "haramiyidans" within Mammalia. Given this phylogeny, development of the DMME took place once in the allotherian clade containing euharamiyidans and multituberculates, probably independent to those of monotremes and therians. Thus, the DMME has evolved at least three times independently in mammals. Alternative hypothesis that placed "haramiyidans" outside of Mammalia would require independent acquisition of the DMME in multituberculates and euharamiyidans as well as parallel evolution of numerous derived similarities in the dentition, occlusion pattern, mandibles, cranium, and postcranium between the two groups and between "haramiyidans" and other mammals. J. Morphol. 279:441-457, 2018. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

Gliding is a distinctive locomotion type that has been identified in only three mammal species from the Mesozoic era. Here we describe another Jurassic glider that belongs to the euharamiyidan mammals and shows hair details on its gliding membrane that are highly similar to those of extant gliding mammals. This species possesses a five-boned auditory apparatus consisting of the stapes, incus, malleus, ectotympanic and surangular, representing, to our knowledge, the earliest known definitive mammalian middle ear. The surangular has not been previously identified in any mammalian middle ear, and the morphology of each auditory bone differs from those of known mammals and their kin. We conclude that gliding locomotion was probably common in euharamiyidans, which lends support to idea that there was a major adaptive radiation of mammals in the mid-Jurassic period. The acquisition of the auditory bones in euharamiyidans was related to the formation of the dentary-squamosal jaw joint, which allows a posterior chewing movement, and must have evolved independently from the middle ear structures of monotremes and therian mammals.

Deltatheroidans are primitive metatherian mammals (relatives of marsupials), previously thought to have become extinct during the Cretaceous mass extinction. Here, we report a tiny new deltatheroidan mammal (Gurbanodelta kara gen. et sp. nov.) discovered at the South Gobi locality in China (Xinjiang Province) that is the first Cenozoic record of this clade and renders Deltatheroida a Lazarus taxon (with a new record 10 million years younger than their supposed extinction). The vertebrate fauna associated with Gurbanodelta is most similar to that from the slightly older late Paleocene Subeng locality in Inner Mongolia. The upper molars of Gurbanodelta exhibit a broad stylar shelf with one prominent cusp (stylocone), and a paracone that is sharp and significantly taller than the metacone. The lower molar tentatively assigned to Gurbanodelta has a very small talonid without an entoconid. This combination of these features is known only in deltatheroidans. Phylogenetic analysis places Gurbanodelta as the sister taxon of the North American latest Cretaceous Nanocuris. Gurbanodelta is the smallest-known deltatheroidan, and roughly the same size as the smallest living marsupial. It is likely that the Gurbanodelta lineage dispersed between Asia and North America as part of known intercontinental mammalian dispersals in the late Paleocene, or possibly earlier.

We report the discovery of Anebodon luoi, a new genus and species of zhangheotheriid symmetrodont mammal from the Lujiatun site of the Lower Cretaceous Yixian Formation, China. The fossil is represented by an associated partial skull and dentaries with a nearly complete dentition, and with a dental formula of I4/3 C1/1 P5/4 M3/4. This new taxon lacks the high molar count typical of derived symmetrodonts, differing from the well-represented zhangheotheriids Zhangheotherium and Maotherium in having a postcanine dental formula that resembles more primitive tinodontid symmetrodonts on the one hand, and sister taxa to therians such as Peramus on the other. Upper and lower distal premolars are strongly molariform and are captured undergoing replacement, clarifying positional homology among related taxa. We also describe the rostrum and, for the first time in a symmetrodont, much of the orbital mosaic. Importantly, our new taxon occupies a basal position within the Zhangheotheriidae and permits discussion of trechnotherian character evolution, ultimately shedding additional light on the evolution of therians.

A new multituberculate, Yubaartar zhongyuanensis gen. and sp. nov., is reported from the Upper Cretaceous of Luanchuan County, Henan Province, China. The holotype of the new taxon is a partial skeleton with nearly complete cranium and associated lower jaws with in situ dentitions. The new species is the southern-most record of a Late Cretaceous multituberculate from outside of the Mongolian Plateau in Asia and represents the largest known Mesozoic multituberculate from Eurasia. The new specimen displays some intriguing features previously unknown in multituberculates, such as the first evidence of replacement of the ultimate upper premolar and a unique paleopathological case in Mesozoic mammals in which the animal with a severely broken right tibia could heal and survive in natural condition. The phylogenetic analysis based on craniodental characters places Yubaartar as the immediate outgroup of Taeniolabidoidea, a group consisting of a North American clade and an Asian clade. This relationship indicates at least a faunal interchange of multituberculates before the K-Pg transition. The new evidence further supports the hypothesis that disparity in dental complexity, which relates to animal diets, increased with generic richness and disparity in body size, and that an adaptive shift towards increased herbivory across the K-Pg transitional interval.

Tooth enamel microstructure is a reliable and widely used indicator of dietary interpretations and data for phylogenetic reconstruction, if all levels of variability are investigated. It is usually difficult to have a thorough examination at all levels of enamel structures for any mammals, especially for the early mammals, which are commonly represented by sparse specimens. Because of the random preservation of specimens, enamel microstructures from different teeth in various species are often compared. There are few examples that convincingly show intraspecific variation of tooth enamel microstructure in full dentition of a species, including multituberculates. Here we present a systematic survey of tooth enamel microstructures of Lambdopsalis bulla, a taeniolabidoid multituberculate from the Late Paleocene Nomogen Formation, Inner Mongolia. We examined enamel structures at all hierarchical levels. The samples are treated differently in section orientations and acid preparation and examined using different imaging methods. The results show that, except for preparation artifacts, the crystallites, enamel types, Schmelzmuster and dentition types of Lambdopsalis are relatively consistent in all permanent teeth, but the prism type, including the prism shape, size and density, may vary in different portions of a single tooth or among different teeth of an individual animal. The most common Schmelzmuster of the permanent teeth in Lambdopsalis is a combination of radial enamel in the inner and middle layers, aprismatic enamel in the outer layer, and irregular decussations in tooth crown area with great curvature. The prism seam is another comparably stable characteristic that may be a useful feature for multituberculate taxonomy. The systematic documentation of enamel structures in Lambdopsalis may be generalized for the enamel microstructure study, and thus for taxonomy and phylogenetic reconstruction, of multituberculates and even informative for the enamel study of other early mammals.

Mimotonids share their closest affinity with lagomorphs and were a rare and endemic faunal element of Paleogene mammal assemblages of central Asia. Here we describe a new species, Mimolagus aurorae from the Middle Eocene of Nei Mongol (China). This species belongs to one of the most enigmatic genera of fossil Glires, previously known only from the type and only specimen from the early Oligocene of Gansu (China). Our finding extends the earliest occurrence of the genus by at least 10 million years in the Paleogene of Asia, which closes the gap between Mimolagus and other mimotonids that are known thus far from middle Eocene or older deposits. The new species is one of the largest known pre-Oligocene Glires. As regards duplicidentates, Mimolagus is comparable with the largest Neogene continental leporids, namely hares of the genus Lepus. Our results suggest that ecomorphology of this species was convergent on that of small perissodactyls that dominated faunas of the Mongolian Plateau in the Eocene, and probably a result of competitive pressure from other Glires, including a co-occurring mimotonid, Gomphos.

Background: Two recent studies published in the same issue of Nature reached conflicting conclusions regarding the phylogeny of early mammals: One places the clade containing haramiyidans and multituberculates within the Mammalia and the other separates haramiyidans from multituberculates and places the former outside of the Mammalia. These two contrasting results require that the minimally oldest divergence time of the Mammalia was within the Late Triassic or the Middle Jurassic, respectively. Morphological descriptions of the species named in the two papers were brief, and no comparisons between the newly named species were possible., Principal Findings: Here we present a detailed description of the dentary bone, teeth, occlusal and wear patterns of the haramiyidan Arboroharamiya and compare it with other haramiyidans and Megaconus. Using this new information, we suggest that tooth identifications and orientations of several previously described haramiyidan species are incorrect, and that previous interpretations of haramiyidan occlusal pattern are problematic. We propose that the published upper tooth orientation of Megaconus was problematic and question the number of upper molars, the length of dentition and mandible, and presence of the mandibular middle ear in Megaconus., Conclusions: The additional morphological descriptions and comparisons presented here further support the view that Arboroharamiya, as a derived haramiyidan, shows similarity to multituberculates in tooth and mandible morphologies. Our comparison also suggests that Megaconus lacks many diagnostic features for the family Eleutherodontidae and that its close affinity with multituberculates cannot be ruled out. The detailed morphological data demonstrate that haramiyidans are more similar to multituberculates than to any other mammaliaforms.

The phylogeny of Allotheria, including Multituberculata and Haramiyida, remains unsolved and has generated contentious views on the origin and earliest evolution of mammals. Here we report three new species of a new clade, Euharamiyida, based on six well-preserved fossils from the Jurassic period of China. These fossils reveal many craniodental and postcranial features of euharamiyidans and clarify several ambiguous structures that are currently the topic of debate. Our phylogenetic analyses recognize Euharamiyida as the sister group of Multituberculata, and place Allotheria within the Mammalia. The phylogeny suggests that allotherian mammals evolved from a Late Triassic (approximately 208 million years ago) Haramiyavia-like ancestor and diversified into euharamiyidans and multituberculates with a cosmopolitan distribution, implying homologous acquisition of many craniodental and postcranial features in the two groups. Our findings also favour a Late Triassic origin of mammals in Laurasia and two independent detachment events of the middle ear bones during mammalian evolution.

Tree-building with diverse data maximizes explanatory power. Application of molecular clock models to ancient speciation events risks a bias against detection of fast radiations subsequent to the Cretaceous-Paleogene (K-Pg) event. Contrary to Springer et al., post-K-Pg placental diversification does not require "virus-like" substitution rates. Even constraining clade ages to their model, the explosive model best explains placental evolution.

A major unsolved problem in mammalian evolution is the origin of Allotheria, including Multituberculata and Haramiyida. Multituberculates are the most diverse and best known Mesozoic era mammals and ecologically resemble rodents, but haramiyids are known mainly from isolated teeth, hampering our search for their phylogenetic relationships. Here we report a new haramiyid from the Jurassic period of China, which is, to our knowledge the largest reported so far. It has a novel dentition, a mandible resembling advanced multituberculates and postcranial features adapted for arboreal life. Our phylogenetic analysis places Haramiyida within crown Mammalia, suggesting the origin of crown Mammalia in the Late Triassic period and diversification in the Jurassic, which contrasts other estimated divergence times of crown Mammalia. The new haramiyid reveals additional mammalian features of the group, helps to identify other haramiyids represented by isolated teeth, and shows again that, regardless of various phylogenetic scenarios, a complex pattern of evolution involving many convergences and/or reversals existed in Mesozoic mammals.

To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria.

The transference of post-dentary jaw elements to the cranium of mammals as auditory ossicles is one of the central topics in evolutionary biology of vertebrates. Homologies of these bones among jawed vertebrates have long been demonstrated by developmental studies; but fossils illuminating this critical transference are sparse and often ambiguous. Here we report the first unambiguous ectotympanic (angular), malleus (articular and prearticular) and incus (quadrate) of an Early Cretaceous eutriconodont mammal from the Jehol Biota, Liaoning, China. The ectotympanic and malleus have lost their direct contact with the dentary bone but still connect the ossified Meckel's cartilage (OMC); we hypothesize that the OMC serves as a stabilizing mechanism bridging the dentary and the detached ossicles during mammalian evolution. This transitional mammalian middle ear narrows the morphological gap between the mandibular middle ear in basal mammaliaforms and the definitive mammalian middle ear (DMME) of extant mammals; it reveals complex changes contributing to the detachment of ear ossicles during mammalian evolution.

Background: The cusp homology of Lagomorpha has long been problematic largely because their teeth are highly derived relative to their more typically tribosphenic ancestors. Within this context, the lagomorph central cusp has been particularly difficult to homologize with other tribosphenic cusps; authors have previously considered it the paracone, protocone, metacone, amphicone, or an entirely new cusp., Methodology/principal Findings: Here we present newly described fossil duplicidentates (Lagomorpha and Mimotonidae) in the context of a well-constrained phylogeny to establish a nomenclatural system for cusps based on the tribosphenic pattern. We show that the central cusp of lagomorphs is homologous with the metaconule of other mammals. We also show that the buccal acquisition of a second cusp on the premolars (molarization) within duplicidentates is atypical with respect to other mammalian lineages; within the earliest lagomorphs, a second buccal cusp is added mesially to an isolated buccal cusp., Conclusions/significance: The distal shift of the 'ancestral' paracone within early duplicidentates amounts to the changing of a paracone into a metacone in these lineages. For this reason, we support a strictly topological approach to cusp names, and suggest a discontinuity in nomenclature to capture the complexity of the interplay between evolutionary history and the developmental process that have produced cusp patterns in duplicidentates.

A new genus and species of eutherian mammal, Acristatherium yanensis gen. et sp. nov., is described from the Early Cretaceous Jehol biota, China. The new taxon is based on a partial skull that is preserved in three dimensions from the Lujiatun bed of the Yixian Formation and dated 123.2+/-1.0 Ma. Its right upper and lower dentitions are nearly complete and it has a dental formula 4.1.5.3/3.1.5.3. The new mammal reveals several craniodental characteristics of Early Cretaceous eutherians previously unknown in fossil records of therians, such as a possible vestige of the septomaxilla. The craniodental features of the new taxon are compared with those of relevant Early Cretaceous eutherians and therians. Phylogenetic analyses based on a data matrix containing 70 taxa and 408 characters place A. yanensis as the most basal eutherian in the selected group. The morphological differences between Acristatherium and Eomaia indicate that eutherians already had a significant degree of generic diversification ca 125 Ma.

Gliding flight has independently evolved many times in vertebrates. Direct evidence of gliding is rare in fossil records and is unknown in mammals from the Mesozoic era. Here we report a new Mesozoic mammal from Inner Mongolia, China, that represents a previously unknown group characterized by a highly specialized insectivorous dentition and a sizable patagium (flying membrane) for gliding flight. The patagium is covered with dense hair and supported by an elongated tail and limbs; the latter also bear many features adapted for arboreal life. This discovery extends the earliest record of gliding flight for mammals to at least 70 million years earlier in geological history, and demonstrates that early mammals were diverse in their locomotor strategies and lifestyles; they had experimented with an aerial habit at about the same time as, if not earlier than, when birds endeavoured to exploit the sky.

We describe several fossils referable to Gomphos elkema from deposits close to the Paleocene-Eocene boundary at Tsagan Khushu, Mongolia. Gomphos shares a suite of cranioskeletal characters with extant rabbits, hares, and pikas but retains a primitive dentition and jaw compared to its modern relatives. Phylogenetic analysis supports the position of Gomphos as a stem lagomorph and excludes Cretaceous taxa from the crown radiation of placental mammals. Our results support the hypothesis that rodents and lagomorphs radiated during the Cenozoic and diverged from other placental mammals close to the Cretaceous-Tertiary boundary.

Mesozoic mammals are commonly portrayed as shrew- or rat-sized animals that were mainly insectivorous, probably nocturnal and lived in the shadow of dinosaurs. The largest known Mesozoic mammal represented by substantially complete remains is Repenomamus robustus, a triconodont mammal from the Lower Cretaceous of Liaoning, China. An adult individual of R. robustus was the size of a Virginia opossum. Here we report a new species of the genus, represented by a skeleton with most of the skull and postcranium preserved in articulation. The new species is 50% larger than R. robustus in skull length. In addition, stomach contents associated with a skeleton of R. robustus reveal remains of a juvenile Psittacosaurus, a ceratopsian dinosaur. Our discoveries constitute the first direct evidence that some triconodont mammals were carnivorous and fed on small vertebrates, including young dinosaurs, and also show that Mesozoic mammals had a much greater range of body sizes than previously known. We suggest that Mesozoic mammals occupied diverse niches and that some large mammals probably competed with dinosaurs for food and territory.

After successfully diversifying during the Paleocene, the descendants of the first wave of mammals that survived the end‐Cretaceous mass extinction waned throughout the Eocene. Competition with modern crown clades and intense climate fluctuations may have been part of the factors leading to the extinction of these archaic groups. Why these taxa went extinct has rarely been studied from the perspective of the nervous system. Here, we describe the first virtual endocasts for the archaic order Tillodontia. Three species from the middle Eocene of North America were analyzed: Trogosus hillsii, Trogosus grangeri, and Trogosus castoridens. We made morphological comparisons with the plaster endocast of another tillodont, Tillodon fodiens, as well as groups potentially related to Tillodontia: Pantodonta, Arctocyonidae, and Cimolesta. Trogosus shows very little inter‐specific variation with the only potential difference being related to the fusion of the optic canal and sphenorbital fissure. Many ancestral features are displayed by Trogosus, including an exposed midbrain, small neocortex, orbitotemporal canal ventral to rhinal fissure, and a broad circular fissure. Potential characteristics that could unite Tillodontia with Pantodonta, and Arctocyonidae are the posterior position of cranial nerve V3 exit in relation to the cerebrum and the low degree of development of the subarcuate fossa. The presence of large olfactory bulbs and a relatively small neocortex are consistent with a terrestrial lifestyle. A relatively small neocortex may have put Trogosus at risk when competing with artiodactyls for potentially similar resources and avoiding predation from archaic carnivorans, both of which are known to have had larger relative brain and neocortex sizes in the Eocene. These factors may have possibly exacerbated the extinction of Tillodontia, which showed highly specialized morphologies despite the increase in climate fluctuations throughout the Eocene, before disappearing during the middle Eocene. [ABSTRACT FROM AUTHOR]

Allotheria are an extinct group of mammaliaforms that originally comprised multituberculates, to which 'haramiyidans' and gondwanatherians were later added. Phylogenetic relationships of allotherians have remained controversial since the first reports of the allotherian fossils in the 1800s. Here we report a new euharamiyidan based on a skeletal specimen from the Jurassic Daohugou phase of the Yanliao Biota, China. The superb specimen preserves impressions of differentiated hairs. The pes with impressions of toe skin and claw sheath represents the first such evidence in Mesozoic mammaliamorphs. The remarkable tooth morphologies surpass in complexity any Mesozoic mammals previously known. Phylogenetic analyses place the new species within euharamiyidans from Eurasia and support euharamiyidans, multituberculates, and gondwanatherians as a tripartite monophyletic Allotheria within mammals. This clade is supported not only by their similar molar pattern, but also by many shared derived features of the dental system recognized from discoveries during the last two decades. Allotherians and therians represent the two most successful groups of mammals that are characterized by distinct dental systems. They originated at the latest in the Late Triassic and became diversified in the Middle Jurassic, representing two directions of ecomorphological adaptation during early evolution of mammals. [ABSTRACT FROM AUTHOR]

Early Cenozoic "insectivorans" possess some of the most primitive morphologies among eutherian mammals. Studies of these archaic mammals offer insights into the early diversifications of basal eutherians. Despite such importance, early fossil "insectivorans" from Asia are poorly known due to a scarcity of fossil remains, which often consist only of fragmentary jaws and teeth. Discoveries of remarkably well-preserved fossil "insectivorans", including complete skulls and articulated postcranial skeletons, from the early Eocene Hengyang Basin in south-central Hunan Province, China, offer a rare opportunity to thoroughly study two taxa belonging to different families. Fine-grained red beds from Hengyang Basin preserve extraordinary fossils with morphological structures rarely seen elsewhere. Thin sections of a skull of Hsiangolestes youngiZheng and Huang, 1984, for example, reveal the extremely delicate nasal and maxillary turbinates, which, as far as we are aware, are the first known from fossils of this age. We thus take this opportunity to document in detail the cranial and dental morphology, as well as postcranial skeletons, of the Hengyang "insectivorans.". In this monograph, we describe several complete skulls and serial sections of a skull, as well as many partial skulls, mandibles, and postcranial skeletons of Hsiangolestes youngi, an Asian early Eocene insectivoran-grade mammal. We also report a new species of NaraniusRussell and Dashzeveg, 1986—N. hengdongensis—an Asian early Eocene cimolestid and describe its well-preserved skulls and mandibles. Hsiangolestes is endemic to Asia. It is currently known only from the earliest Eocene Lingcha Formation, Hengyang Basin, Hunan Province, China. Naranius closely resembles CimolestesMarsh, 1889, the type genus of the family Cimolestidae. It is mainly distributed in Asia and known from the earliest Eocene deposits in the Bumban Member of the Naran Bulak Formation, Nemegt Basin, of Mongolia, and the Lingcha Formation, Hengyang Basin, Hunan Province, China. The only record of Naranius reported outside of Asia is N. americanus from the early Wasatchian Red Hot Local Fauna, Mississippi, United States. Using PAUP and TNT search algorithms, we place these Hengyang taxa within phylogenetic context of other fossil "insectivorans" from the Mesozoic and early Cenozoic of Asia together with some well-known Holarctic taxa. A phylogenetic analysis of 290 cranial and dental characters from 36 fossil and modern insectivoran-grade taxa is presented, focusing on new materials of Hsiangolestes youngi and Naranius hengdongensis. Based on the results of our phylogenetic analyses, we propose that (1) Hsiangolestes, Prosarcodon, Sarcodon, and Sinosinopa, form a monophyletic group, for which we propose the family name Sarcodontidae; (2) the family Cimolestidae should be restricted to Naranius and Cimolestes, which are sister taxa; (3) the systematic position of Naranius americanus is uncertain; and (4) the family Micropternodontidae should be restricted to Micropternodus and its allies in North America. [ABSTRACT FROM AUTHOR]

The latest Cretaceous kogaionid multituberculates from Transylvania (western Romania) were part of an endemic European clade of mammals that underwent an insular radiation at the end of the Cretaceous and then survived the end-Cretaceous mass extinction that extinguished many groups of contemporary therians. Transylvanian kogaionids lived on what was an island during the latest Cretaceous-"Hațeg Island"-and their fossils are found in the uppermost Campanian to upper Maastrichtian deposits of the Hațeg, Rusca Montană, and southwestern Transylvanian basins. This fossil record has improved dramatically over the past several decades, in part resulting from our decade-long joint Romanian-American-Scottish fieldwork, and comprises one of the most impressive and complete archives of Mesozoic mammals, including not only jaws and teeth but several incomplete skulls and partial skeletons. We here review the fossil record of kogaionids from Transylvania. We report four new occurrences from the Hațeg Basin, update information on previously described ones, and use our database to reassess the chronostratigraphical and geographical distribution of kogaionids and their evolutionary patterns. Although it was previously suggested that large and small kogaionids had largely mutually exclusive spatial distributions, we recognize the cooccurrence of small and large taxa in various units, suggesting a sympatric distribution across their entire chronostratigraphic range. We also identify a novel pattern: small kogaionids appear somewhat earlier than their larger relatives in all well-sampled sedimentary successions, suggesting that kogaionid colonizations of Hațeg Island and component regions took place at small body size and that body size increased only later through local evolution. We find correlations between body size, preservation style, and sedimentary context, which give insight into kogaionid paleobiology and diversity. Larger kogaionids are represented more often by partial skulls and occasionally skeletons compared with small kogaionids, which are usually represented only by isolated teeth, regardless of provenance. Larger kogaionids currently have a higher recognized local taxic diversity than their smaller relatives. We hypothesize that this may be in part a consequence of preservational bias related to body size, as more complete specimens may be more easily diagnosed as distinct taxa than those that are represented by more fragmentary and/or incomplete fossils. If true, the taxic diversity of smaller kogaionids may currently be underestimated. Finally, we identify correspondence between sedimentary facies and preservation style. Red-colored fine-grained rocks, suggestive of well-drained, oxidized floodplain paleoenvironments, yield more complete specimens than drab, greenish or grayish sediments deposited in more poorly drained parts of the floodplain. This pattern may suggest habitat preferences for betterdrained floodplain environments and a semifossorial lifestyle for some taxa. As the kogaionid fossil record improves, we can further test the hypotheses and patterns outlined above. The pace of new kogaionid discoveries by our team and others indicates that a more complete picture of kogaionid distribution, paleobiology, and evolution will emerge in the coming years, contributing to a more profound understanding of this peculiar group of island-dwelling Mesozoic mammals. [ABSTRACT FROM AUTHOR]

Schlosseria magister, Breviodon? minutus and Lophialetes expeditus are dominant species in the family of Lophialetidae, an extinct group of tapiroids widely distributed in the Eocene sediments of Asia. In this study, we provide insight into the paleodiet and palaeoecology of these Eocene lophialetids via microwear analysis of fossil teeth (N = 132) from Huheboerhe area, Erlian Basin, China. The results suggest that L. expeditus was likely a mixed feeder consuming substantial grit and a small amount of fruit during feeding, while Breviodon? minutus was a browser with fruit and/or seeds incorporated into its dietary regime. The dietary preference of S. magister shifted towards more abrasive diets over time (from browsers to mixed feeders). This dietary shift is probably correlated to the decreasing trend in global temperatures following the Early Eocene Climatic Optimum which likely led to the changes of terrestrial vegetation and environment in the Huheboerhe area during that time interval. Combining the microwear analysis and other evidence (mesowear, stable carbon isotope and the fossil record), the landscape of the early-middle Eocene Erlian Basin was probably becoming more arid and/or open over time and a mixed habitat, which included forest, woodland and shrubland, was most likely present in the middle. Eocene Erlian Basin. [ABSTRACT FROM AUTHOR]

We report a new Cretaceous multituberculate mammal with 3D auditory bones preserved. Along with other fossil and extant mammals, the unequivocal auditory bones display features potentially representing ancestral phenotypes of the mammalian middle ear. These phenotypes show that the ectotympanic and the malleus-incus complex changed notably during their retreating from the dentary at various evolutionary stages and suggest convergent evolution of some features to extant mammals. In contrast, the incudomalleolar joint was conservative in having a braced hinge configuration, which narrows the morphological gap between the quadroarticular jaw joint of non-mammalian cynodonts and the incudomalleolar articulations of extant mammals. The saddle-shaped and abutting malleus-incus complexes in therians and monotremes, respectively, could have evolved from the braced hinge joint independently. The evolutionary changes recorded in the Mesozoic mammals are largely consistent with the middle ear morphogenesis during the ontogeny of extant mammals, supporting the relation between evolution and development. [ABSTRACT FROM AUTHOR]

Lophialetidae is an extinct group of endemic Asiatic tapiroids that are widely distributed in the Eocene sediments of Asia. Schlosseria magister and Lophialetes expeditus are the most abundant species in this family. However, their dietary and ecological characteristics are largely unknown. For the first time, we reconstruct the palaeodiet and habitat of these two lophialetids using a combination of mesowear and stable carbon isotope analysis of fossil teeth excavated from the Erlian Basin, China. Mesowear analysis (n = 141) suggests that the dietary structure of S. magister and L. expeditus shifted from less to more abrasive diets from ~52 to ~42 Ma. Stable carbon isotope analysis (n = 137) suggests that the habitats of S. magister and L. expeditus became drier and/or more open through time. The dietary shifts of the two lophialetids are consistent with evident changes in habitat. The changes in the diet and habitat were probably related to global climate change during that time period. The gradual drop in global temperatures during the early–middle Eocene led to a drier and more open terrestrial ecosystem in the Erlian Basin, probably resulting in changes in floral composition of the environment inhabited by S. magister and L. expeditus. Hence, herbivores highly susceptible to vegetation modification had to develop new resource exploitation strategies to adapt to these changes. Schlosseria magister, considered to be the sister‐group of L. expeditus and with a low level of ecological flexibility, was unable to adapt to the habitat changes finally becoming extinct at ~45 Ma. [ABSTRACT FROM AUTHOR]

'Haramiyidans' are extinct mammaliaforms often clustered with Multituberculata as Allotheria, and with a fossil record extending from the Upper Triassic to possibly the Upper Cretaceous. For many decades, 'haramiyidans' were known only from isolated teeth, and their relationships to other mammaliaforms remain unclear. With the discovery of several euharamiyidans represented by skeletal specimens from the Jurassic Yanliao Biota, north‐western China, our knowledge of this group has significantly advanced. Nonetheless, much morphology, including dentition and occlusal patterns, has only been briefly described for most taxa, and phylogenetic reconstruction and interpretation of the biology of the group still remain unclear or controversial. Here we provide systematic descriptions of the dental wear of shenshouids (Qishou and Shenshou) from the Yanliao Biota. We reconstruct the occlusal mode of shenshouids and compare it with those of other taxa based on our observation of specimens of most known 'haramiyidan' taxa. Several occlusal patterns are recognized. In particular, that of Thomasia, Haramiyavia and Maiopatagium is similar to the multituberculate M1/m1 occlusal relationship and probably represents the primitive occlusal condition in 'haramiyidans'. The occlusal mode of shenshouids resembles the multituberculate M2/m2 occlusion. The double‐engaged mode in Arboroharamiya and Vilevolodon, as well as that of Eleutherodon, represents a more advanced occlusal pattern. The diverse dental occlusions suggest diverse diet adaptations and resource partitioning from co‐existing taxa in the Jurassic forests; they also reflect mosaic evolutionary transformations and complicated phylogenetic relationships within the group. Some aspects of the dental evolution remain challenging and require further vigorous testing. [ABSTRACT FROM AUTHOR]

The Paleogene is the first period after the Mesozoic Mass Extinction. Mammals become the dominant group in the terrestrial ecosystem with a rapid radiation, and Asia has been considered to be the origin place of several mammalian groups. The Paleogene System consists mostly of terrestrial deposits in Asia, especially in East Asia. A well-established regional chronostratigraphic framework is the foundation for understanding both the Paleogene geologic history and evolutionary history of Asia and their relationships. The Paleogene is subdivided into the Paleocene, Eocene and Oligocene in the International Chronostratigraphic Chart. Based on the land mammal ages, the Chinese terrestrial Paleogene can be subdivided into 11 stages: the Shanghuan, Nongshanian and Bayanulanian stages of the Paleocene, the Lingchan, Arshantan, Irdinmanhan, Sharamurunian, Ulangochuian and Baiyinian stages of the Eocene, and the Ulantatalian and Tabenbulukian stages of the Oligocene. These stages have distinctive paleontological characters, with special significance of fossil mammals, which provide a reliable practical basis. The bases of the Shanghuan, Lingchan, and Ulantatalian stages are coincident respectively with those of the Paleocene, Eocene and Oligocene. The ages for their bases are determined as 66.0, 56.0 and 33.9 Ma, respectively, following that for the corresponding series in the International Chronostratigraphic Chart. For other stages, estimated ages are provided based on available paleomagnetic results. [ABSTRACT FROM AUTHOR]