Early Cretaceous mammal from North America and the evolution of marsupial dental characters

The evolutionary origin of jaw yaw in mammals

Theria comprises all but three living mammalian genera and is one of the most ecologically pervasive clades on Earth. Yet, the origin and early history of therians and their close relatives (i.e., cladotherians) remains surprisingly enigmatic. A critical biological function that can be compared among early mammal groups is mastication. Morphometrics and modeling analyses of the jaws of Mesozoic mammals indicate that cladotherians evolved musculoskeletal anatomies that increase mechanical advantage during jaw rotation around a dorsoventrally-oriented axis (i.e., yaw) while decreasing the mechanical advantage of jaw rotation around a mediolaterally-oriented axis (i.e., pitch). These changes parallel molar transformations in early cladotherians that indicate their chewing cycles included significant transverse movement, likely produced via yaw rotation. Thus, I hypothesize that cladotherian molar morphologies and musculoskeletal jaw anatomies evolved concurrently with increased yaw rotation of the jaw during chewing cycles. The increased transverse movement resulting from yaw rotation may have been a crucial evolutionary prerequisite for the functionally versatile tribosphenic molar morphology, which underlies the molars of all therians and is retained by many extant clades.

Phylogeny explains better than ecology or body size the variation of the first lower molar in didelphid marsupials

Tribosphenic molars are considered great innovations in mammals and are related to several structures and variables that can explain adaptation. The aim of this study was to investigate the importance of body size and habitat relation, using a phylogenetic approach, in the first lower molar shape in didelphid marsupials. Geometric morphometric analyses of the lower molar’s shape were performed on 261 specimens, 130 females and 131 males, covering 14 genera and 37 species of the Didelphidae family. The molar conformation showed a larger talonid in relation to the trigonid in more arboreal genera, and narrower and longer molars in genera with a larger body size. Phylogeny was the variable with the highest explanation for both females and males (16.17% and 9.02%, respectively). The body size was significant in males, presenting an important influence on molar shape, while the body size in females was not significant when phylogenetic relationship was controlled for. In both sexes, habitat presents a strong effect of phylogeny, with no direct effect on molar shape. Didelphid molar shape is another result of its phylogenetic history and does not respond very much to environmental pressures. Male body size influences molar shape in didelphids, even in the presence of a strong phylogenetic signal.

A new Cretaceous Metatherian mammal from Henan, China

We report a new deltatheroidan mammal from the Upper Cretaceous of Henna, China. The new taxon, Lotheridium mengi, is based on a nearly complete skull and associated lower jaws with full adult dentition. Deltatheroidans are known mostly from fragmentary specimens from Asia and North America. Previous views on deltatheroidan relationships were diverse, but recent studies favored their metatherian affinity. The new specimen represents the most complete skull known for deltatheroidans and provides additional evidence that deltatheroidans already had the distinctive metatherian dental formula and replacement pattern and several other derived metatherian features, supporting the metatherian status for this clade. The new species also indicates that deltatheroidan mammals were more diverse and had broader geographical distributions than previously thought.

Local-global overlap in diversity informs mechanisms of bacterial biogeography

Spatial variation in environmental conditions and barriers to organism movement are thought to be important factors for generating endemic species, thus enhancing global diversity. Recent microbial ecology research suggested that the entire diversity of bacteria in the global oceans could be recovered at a single site, thus inferring a lack of bacterial endemism. We argue this is not the case in the global ocean, but might be in other bacterial ecosystems with higher dispersal rates and lower global diversity, like the human gut. We quantified the degree to which local and global bacterial diversity overlap in a diverse set of ecosystems. Upon comparison of observed local-global diversity overlap with predictions from a neutral biogeography model, human-associated microbiomes (gut, skin, mouth) behaved much closer to neutral expectations whereas soil, lake and marine communities deviated strongly from the neutral expectations. This is likely a result of differences in dispersal rate among 'patches', global diversity of these systems, and local densities of bacterial cells. It appears that overlap of local and global bacterial diversity is surprisingly large (but likely not one-hundred percent), and most importantly this overlap appears to be predictable based upon traditional biogeographic parameters like community size, global diversity, inter-patch environmental heterogeneity and patch connectivity.

The origin and early evolution of metatherian mammals: the Cretaceous record

Metatherians, which comprise marsupials and their closest fossil relatives, were one of the most dominant clades of mammals during the Cretaceous and are the most diverse clade of living mammals after Placentalia. Our understanding of this group has increased greatly over the past 20 years, with the discovery of new specimens and the application of new analytical tools. Here we provide a review of the phylogenetic relationships of metatherians with respect to other mammals, discuss the taxonomic definition and diagnosis of Metatheria, outline the Cretaceous history of major metatherian clades, describe the paleobiology, biogeography, and macroevolution of Cretaceous metatherians, and provide a physical and climatic background of Cretaceous metatherian faunas. Metatherians are a clade of boreosphendian mammals that must have originated by the Late Jurassic, but the first unequivocal metatherian fossil is from the Early Cretaceous of Asia. Metatherians have the distinctive tightly interlocking occlusal molar pattern of tribosphenic mammals, but differ from Eutheria in their dental formula and tooth replacement pattern, which may be related to the metatherian reproductive process which includes an extended period of lactation followed by birth of extremely altricial young. Metatherians were widespread over Laurasia during the Cretaceous, with members present in Asia, Europe, and North America by the early Late Cretaceous. In particular, they were taxonomically and morphologically diverse and relatively abundant in the Late Cretaceous of western North America, where they have been used to examine patterns of biogeography, macroevolution, diversification, and extinction through the Late Cretaceous and across the Cretaceous-Paleogene (K-Pg) boundary. Metatherian diversification patterns suggest that they were not strongly affected by a Cretaceous Terrestrial Revolution, but they clearly underwent a severe extinction across the K-Pg boundary.

The oldest modern therian mammal from Europe and its bearing on stem marsupial paleobiogeography

We report the discovery of mammalian tribosphenic teeth from the basal Cenomanian of southwestern France that we refer to a new primitive marsupial-like form identified as a basal taxon of Marsupialiformes, a new clade recognized here to include the crown group Marsupialia and primitive stem lineages more closely related to Marsupialia than to Deltatheroida. Arcantiodelphys marchandi gen et sp nov. shares several significant marsupial-like features (s.l.) with marsupialiform taxa known from the North American Mid-Cretaceous. Among marsupialiforms, it shows a closer resemblance to Dakotadens. This resemblance, which is plesiomorphic within "tribotherians," makes Arcantiodelphys one of the most archaic known Marsupialiformes. Moreover, Arcantiodelphys is characterized by an original and precocious crushing specialization. Both the plesiomorphic and autapomorphic characteristics of Arcantiodelphys among Marsupialiformes might be explained by an Eastern origin from Asian stem metatherians, with some in situ European evolution. In addition, the presence of a mammal with North American affinities in western Europe during the early Late Cretaceous provides further evidence of a large Euramerican biogeographical province at this age or slightly before. Concerning the paleobiogeographical history of the first stem marsupialiforms during the Albian-Cenomanian interval, 2 possible dispersal routes from an Asian metatherian ancestry can be proposed: Asia to Europe via North America and Asia to North America via Europe. The main significance of the Archingeay-Les Nouillers mammal discovery is that it indicates that the beginning of the stem marsupialiforms history involved not only North America but also Europe, and that this early history in Europe remains virtually unknown.

Dental morphology and variation in theropod dinosaurs: implications for the taxonomic identification of isolated teeth

Isolated theropod teeth are common Mesozoic fossils and would be an important data source for paleoecology biogeography if they could be reliably identified as having come from particular taxa. However, obtaining identifications is confounded by a paucity of easily identifiable characters. Here we discuss a quantitative methodology designed to provide defensible identifications of isolated teeth using Tyrannosaurus as a comparison taxon. We created a standard data set based as much as possible on teeth of known taxonomic affinity against which to compare isolated crowns. Tooth morphology was described using measured variables describing crown length, base length and width, and derived variables related to basal shape, squatness, mesial curve shape, apex location with respect to base, and denticle size. Crown curves were described by fitting the power function Y = a + bX(0.5) to coordinate data collected from lateral-view images of mesial curve profiles. The b value from these analyses provides a measure of curvature. Discriminant analyses compared isolated teeth of various taxonomic affinities against the standard. The analyses classified known Tyrannosaurus teeth with Tyrannosaurus and separated most teeth known not to be Tyrannosaurus from Tyrannosaurus. They had trouble correctly classifying teeth that were very similar to Tyrannosaurus and for which there were few data in the standard. However, the results indicate that expanding the standard should facilitate the identification of numerous types of isolated theropod teeth.

New DNA data from a transthyretin nuclear intron suggest an Oligocene to Miocene diversification of living South America opossums (Marsupialia: Didelphidae)

Phylogenetic relationships of 19 species of didelphid marsupials were studied using two nuclear markers, the non-coding transthyretin intron 1 (TTR) and the coding interphotoreceptor retinoid binding protein exon 1 (IRBP), and two mitochondrial genes, the protein-coding cytochrome b (cyt-b) and the structural 12S ribosomal DNA (12S rDNA). Evolutionary dynamics of these four markers were compared to each other, revealing the appropriate properties presented by TTR intron 1 together with its well supported and resolved phylogenetic signal. Nuclear markers supported the monophyly of medium and large-sized opossums Metachirus+(Chironectes, Lutreolina, Didelphis, Philander), and the paraphyly of mouse-sized opossums, with the genera Gracilinanus, Thylamys, and Marmosops as a sister group to medium and large-sized didelphids. Conflicting branching patterns between mitochondrial and nuclear data involved the phylogenetic position of Marmosa-Micoureus-Monodelphis relative to other mouse-sized opossums. Nuclear phylogenetic inferences among genera were confirmed by the presence of synapomorphic indels observed in TTR intron 1. A Bayesian relaxed molecular clock dating of didelphid evolution using nuclear markers estimated their origin in the Middle Eocene (39.8 million years ago), with subsequent diversification during the Oligocene (Deseadan) and Miocene.

Review of the monotreme fossil record and comparison of palaeontological and molecular data

Monotremes have traditionally been considered a remnant group of mammals descended from archaic Mesozoic stock, surviving to the present day on the relatively isolated Australian continent. Challenges to this orthodoxy have been spurred by discoveries of 'advanced' Cretaceous monotremes (Steropodon galmani, Archer, M., et al., 1985. First Mesozoic mammal from Australia-an Early Cretaceous monotreme, Nature. 318, 363-366) as well as by results from molecular data linking monotremes to therian mammals (specifically to marsupials in some studies). This paper reviews the monotreme fossil record and briefly discusses significant new information from additional Cretaceous Australian material. Mesozoic monotremes (including S. galmani) were a diverse group as evidenced by new material from the Early Cretaceous of New South Wales and Victoria currently under study. Although most of these new finds are edentulous jaws (limiting dental comparisons and determination of dietary niches), a range of sizes and forms has been determined. Some of these Cretaceous jaws exhibit archaic features-in particular evidence for the presence of a splenial bone in S. galmani-not seen in therian mammals or in post-Mesozoic (Tertiary and Quaternary) monotreme taxa. Tertiary monotremes were either archaic ornithorhynchids (toothed platypuses in the genera Monotrematum and Obdurodon) or tachyglossids (large echidnas in the genera Megalibgwilia and Zaglossus). Quaternary ornithorhynchid material is referable to the sole living platypus species Ornithorhynchus anatinus. Quaternary echidnas, however, were moderately diverse and several forms are known (Megalibgwilia species; 'Zaglossus' hacketti; Zaglossus species and Tachyglossus aculeatus).

Timing and biogeography of the eutherian radiation: fossils and molecules compared

Theria includes Eutheria and its sister taxon Metatheria. Placentalia includes extant eutherians plus their most recent common ancestor. The oldest eutherian is from 125mya (million years ago). Molecular studies place this origin at about 130-185mya. Older dates cannot be refuted based on fossil evidence as earliest eutherian remains are scarce. Earliest superordinal clades (hence Placentalia) range from 64-104mya (median 84mya) based on molecules, similar to 85-90mya based on fossils. Superordinal clades Archonta, Ferungulata, Glires, and Paenungulata based on fossils are similar to molecularly based clades, except Afrotheria was not predicted by fossils. Both fossils and molecules recognize 16 of 18 extant placental orders. Fossils place the origins of orders around 65mya as do some molecular studies, but others suggest ordinal diversification as old as 100mya. Fossil evidence supports a Laurasian origin for Eutheria (and Metatheria) and Placentalia, although some molecular studies suggest a Gondwanan origin for both taxa.

Molecular estimates of primate divergences and new hypotheses for primate dispersal and the origin of modern humans

The concept of recent hominoid divergences has been a mainstay in molecular primatology since the 1970's. However, the ages allocated to the calibration points used to establish these divergence times and the estimates resulting from their application, notably the commonly accepted divergence between Pan (chimpanzees) and Homo 5 million years before present (MYBP), are now palaeontologically refutable. Here we estimate the ages of various primate divergences using three references with a more detailed fossil record than any of the traditional primate calibration points. Our findings suggest that the latter yield datings that are too recent by a factor of about two. For example, our estimates place the divergence between Pan and Homo 10.5-13 MYBP. The revised estimates of primate divergence times suggest a new hypothesis for primate evolution and dispersal: that the divergence between strepsirhines (lorises, lemurs) and anthropoids was contemporary with the break-up of Southern continents about 90 MYBP, with strepsirhines becoming isolated on Madagascar and later dispersing to Africa (and Asia) and anthropoids evolving in South America and subsequently colonizing Africa (and Asia), or possibly North America. In addition we present a new hypothesis, which accommodates the strikingly similar coalescence times for human mitochondrial DNA and the Y-chromosome. This hypothesis posits a common mitochondrial and Y-chromosome bottleneck about 400,000 years ago, associated with the origination of the human 2n = 46 karyotype, obstructing genetic exchange with the 2n = 48 Homo contemporaries.

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.

The origin of the dog-like borhyaenoid marsupials of South America

Dog-like marsupials (superfamily Borhyaenoidea) were the largest predacious mammals during the Tertiary period in South America. They are critical to our understanding of marsupial origin, phylogeny and palaeobiogeography because they have been related to various marsupial lineages of several continents: didelphoids (mainly New World, but also Europe, Asia and Africa), pediomyid, stagodontids (North America), dasyuroids (Australia) and deltatheroidans (predominantly Asian). These relationships, based mainly on dental morphology, have been discussed and rejected several times. Here we report the discovery of exceptionally well preserved skulls and skeletons, referrable to the didelphoid Andinodelphys, which shed new light on the phylogenetic and palaeobiogeographic origin of dog-like marsupials. The skulls of Mayulestes (boryhyaenoid), Andinodelphys and Pucadelphys (didelphoids) from the early Palaeocene epoch of Bolivia are the oldest known for American marsupials. Comparison of their basicranial anatomy suggests that dog-like marsupials are closely related to an early didelphimorphian radiation in South America, rather than to Asiatic (deltatheroidan), North American (stagodontid), or Australian (dasyuroid) lineages.

New Cretaceous marsupial from Mongolia and the early radiation of Metatheria

A marsupial mammal from Mongolia representing a previously unreported group, Asiadelphia, offers unequivocal evidence that metatherians were represented on the continent of Asia during the Late Cretaceous (Campanian), even in the northern arid interior. Asiatherium is known by skull, left and right mandibles, and most of its postcranial skeleton. Comparisons of the dentition, ear region, and aspects of the postcranium not only allow its diagnosis as an Asian group of metatherians but also permit a diagnosis of the Metatheria based on the unequivocally apomorphous condition of the dental formula and probable replacement pattern of the protometatherian, in contrast to the more primitive therian dental formula, represented by the few known Aegialodontidae and the first dental eutherians. It appears an inescapable conclusion that the first metatherians had a more derived dental formula (and probably replacement pattern) than the earliest dental eutherians, regardless of what specific derivation from a therian ancestry is contemplated. Such a diagnosis also supports the metatherian status of other Cretaceous Asian and American taxa such as the Deltatheroida and Kokopellia. The hypothesis that Metatheria originated in North America is largely dependent on the preponderance of Cretaceous forms from North America and negative evidence from Asia (i.e., on the lack of lower latitude Early Cretaceous tropical faunas). Conversely, the relative Cretaceous paucity of placentals from North America and their greater abundance in Asia suggests the earliest flowering of the Eutheria in an unspecified region of the Old World. The concept of a holophyletic Theria (= Tribosphenida) employed here, based on the biologically significant apomorphy of the molar form-function of the first therian, entails only the tribosphenic mammals (infraclasses Tribotheria, Metatheria, and Eutheria) but not the sundry quasi-triangular-toothed mammals and their derivatives in the Mesozoic (Eupantotheria, Peramura, Monotremata, etc.). It is suggested that the Asiadelphia and Deltatheroida, the protospecies of both, with the apomorphic therian dental formula, are ancient lineages of Metatheria independent from their North American relatives probably since the early part of the Late Cretaceous.