Mesozoic Mammals Research Articles

Two new mammal localities within the Lower Cretaceous Ilek Formation of West Siberia, Russia

Two new mammal localities have been discovered in the Lower Cretaceous Ilek Formation of West Siberia, Russia. The Smolenskii Yar locality in the Chebula District of Kemerovo Province produced an upper molariform tooth (M2) of a Gobiconodontidae indet. The Ust’-Kolba locality in the Tisul’ District of Kemerovo Province yielded a lower molar (m2) of the zhangheotherian Kiyatherium sp. These are the ninth and tenth Mesozoic mammal localities for Russia. The Kiyatherium-bearing vertebrate assemblage from the Shestakovo 3 and Ust’-Kolba localities is likely to be the youngest within the Ilek Formation, reflecting the time after the extinction of the Tritylodontidae.

New specimens of the multituberculate mammalian Sphenopsalis from the Paleocene of Inner Mongolia, China: implications for phylogeny and biology of taeniolabidoid multituberculates

Multituberculates are the most diverse and best known group of Mesozoic mammals; they also persisted into the Paleogene and became extinct in the Eocene, possibly outcompeted by rodents that have similar morphological and presumably ecological adaptations. Among the Paleogene multituberculates, those that have the largest body sizes belong to taeniolabidoids, which contain several derived species from North America and Asia and some species with uncertain taxonomic positions. Of the known taeniolabidoids, the poorest known taxon is Sphenopsalis nobilis from Mongolia and Inner Mongolia, China, represented previously by a few isolated teeth. Its relationship with other multituberculates thus has remained unclear. Here we report new specimens of Sphenopsalis nobilis collected from the upper Paleocene of the Erlian Basin, Inner Mongolia, China, during a multi-year field effort beginning in 2000. These new specimens document substantial parts of the dental, partial cranial and postcranial morphologies of Sphen...

First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism.

Previously known only from isolated teeth and lower jaw fragments recovered from the Cretaceous and Palaeogene of the Southern Hemisphere, the Gondwanatheria constitute the most poorly known of all major mammaliaform radiations. Here we report the discovery of the first skull material of a gondwanatherian, a complete and well-preserved cranium from Upper Cretaceous strata in Madagascar that we assign to a new genus and species. Phylogenetic analysis strongly supports its placement within Gondwanatheria, which are recognized as monophyletic and closely related to multituberculates, an evolutionarily successful clade of Mesozoic mammals known almost exclusively from the Northern Hemisphere. The new taxon is the largest known mammaliaform from the Mesozoic of Gondwana. Its craniofacial anatomy reveals that it was herbivorous, large-eyed and agile, with well-developed high-frequency hearing and a keen sense of smell. The cranium exhibits a mosaic of primitive and derived features, the disparity of which is extreme and probably reflective of a long evolutionary history in geographic isolation.

Mesozoic mammals of China: implications for phylogeny and early evolution of mammals

AbstractAll Mesozoic mammaliaforms reported from China are briefly documented herein. These forms can be divided into at least five major assemblages: Lufeng, Yanliao (Daohugou), Jehol, Fuxin and Bayan Mandahu, ranging from the Early Jurassic to the Late Cretaceous periods. Although the temporal and geographic distributions of these mammaliaforms are not dense, the records do reveal a pattern that is generally consistent with patterns that have been recognized globally. The initial stage of mammalian evolution was represented by stem mammaliaforms or primitive ‘triconodonts’ from the Lufeng. This was followed by the Middle-Late Jurassic Yanliao episode that showed a high diversity and disparity of mammaliaforms in which terrestrial, swimming, arboreal and gliding species were present. The disparity, at least in molar morphology and types of locomotion, decreased but the diversity persisted into the Cretaceous, a period that was dominated by eutriconodontans, multituberculates and trechnotherians. The superb specimens from nearly all major groups of Mesozoic mammals in China provided a great amount of information that contributed to our understanding on some major issues in phylogeny and the early evolution of mammals, such as divergences of mammals and the evolution of the mammalian middle ear. A hypothesis on the transformation of the allotherian tooth pattern is proposed as an example to illustrate the potential for future studies of mammalian evolution.

A deltatheroidan mammal from the Upper Cretaceous Baynshiree Formation, eastern Mongolia

Deltatheroidans are a specialized group of basal metatherians with a long fossil history and a broad geographic distribution across Asia and North America. The group was among the first Mesozoic mammals known by relatively complete cranial material, recovered from the Gobi Desert by the American Museum expeditions in the 1920s. Deltatheroidans exhibit trends in their dentition towards apparent specializations for carnivory, including strongly developed postmetacrista, pronounced postvallum-prevallid shear, and a reduction of the ultimate molar. While the oldest undisputed deltatheroidans hail from the Early Cretaceous of North America, Upper Cretaceous rocks in central Asia have yielded the best preserved specimens and the bulk of the diversity. Here we describe a new deltatheroidan, Tsagandelta dashzevegi, gen. et sp. nov., from the Upper Cretaceous Baynshiree Formation of eastern Mongolia. This taxon adds to the diversity of the group in central Asia and provides further evidence of uniformity among the Late Cretaceous vertebrate faunas across the Gobi Desert.

Dietary specializations and diversity in feeding ecology of the earliest stem mammals.

The origin and radiation of mammals are key events in the history of life, with fossils placing the origin at 220 million years ago, in the Late Triassic period. The earliest mammals, representing the first 50 million years of their evolution and including the most basal taxa, are widely considered to be generalized insectivores. This implies that the first phase of the mammalian radiation--associated with the appearance in the fossil record of important innovations such as heterodont dentition, diphyodonty and the dentary-squamosal jaw joint--was decoupled from ecomorphological diversification. Finds of exceptionally complete specimens of later Mesozoic mammals have revealed greater ecomorphological diversity than previously suspected, including adaptations for swimming, burrowing, digging and even gliding, but such well-preserved fossils of earlier mammals do not exist, and robust analysis of their ecomorphological diversity has previously been lacking. Here we present the results of an integrated analysis, using synchrotron X-ray tomography and analyses of biomechanics, finite element models and tooth microwear textures. We find significant differences in function and dietary ecology between two of the earliest mammaliaform taxa, Morganucodon and Kuehneotherium--taxa that are central to the debate on mammalian evolution. Morganucodon possessed comparatively more forceful and robust jaws and consumed 'harder' prey, comparable to extant small-bodied mammals that eat considerable amounts of coleopterans. Kuehneotherium ingested a diet comparable to extant mixed feeders and specialists on 'soft' prey such as lepidopterans. Our results reveal previously hidden trophic specialization at the base of the mammalian radiation; hence even the earliest mammaliaforms were beginning to diversify--morphologically, functionally and ecologically. In contrast to the prevailing view, this pattern suggests that lineage splitting during the earliest stages of mammalian evolution was associated with ecomorphological specialization and niche partitioning.

Mammal disparity decreases during the Cretaceous angiosperm radiation

Fossil discoveries over the past 30 years have radically transformed traditional views of Mesozoic mammal evolution. In addition, recent research provides a more detailed account of the Cretaceous diversification of flowering plants. Here, we examine patterns of morphological disparity and functional morphology associated with diet in early mammals. Two analyses were performed: (i) an examination of diversity based on functional dental type rather than higher-level taxonomy, and (ii) a morphometric analysis of jaws, which made use of modern analogues, to assess changes in mammalian morphological and dietary disparity. Results demonstrate a decline in diversity of molar types during the mid-Cretaceous as abundances of triconodonts, symmetrodonts, docodonts and eupantotherians diminished. Multituberculates experience a turnover in functional molar types during the mid-Cretaceous and a shift towards plant-dominated diets during the late Late Cretaceous. Although therians undergo a taxonomic expansion coinciding with the angiosperm radiation, they display small body sizes and a low level of morphological disparity, suggesting an evolutionary shift favouring small insectivores. It is concluded that during the mid-Cretaceous, the period of rapid angiosperm radiation, mammals experienced both a decrease in morphological disparity and a functional shift in dietary morphology that were probably related to changing ecosystems.

In pursuit of early mammals

Preface 1. Introduction 2. Methods: Collecting Materials and Establishing Relationships 3. Paleontological Exploration of Mongolia by American, Japanese, Soviet and Russian Expeditions 4. Polish-Mongolian Expeditions, and the Nomadic Expedition, 2002 5. Origins of Mammals and the Earliest Representatives of Mammaliaforms and Mammals 6. Haramiyidans and Probable Related Forms 7. Docodontans 8. Eutriconodontans 9. Monotremes and the Issue of Australosphenida and Boreosphenida 10. Multituberculates and Gondwanatherians 11. Symmetrodontans 12. Eupantotherians and the Origin of Pseudotribosphenic Molars 13. Tribotheria, Matatheria, and the Issue of Deltatheroida 14. Eutherian and Placental Mammals 15. Paleobiology of Mammaliaforms and Early Mammals 16. Diversification of Mammaliaforms and Mesozoic Mammals: A Summary Bibliography Index

Jurassic fossils and mammalian antiquity

Two new Jurassic fossils yield conflicting reconstructions of the mammalian tree. These divergent genealogies have profoundly different implications for the origin and early diversification of mammals. See Article p.163 & Letter p.199 Independent reports of two newly discovered fossils from the Jurassic of China (around 160–165 million years old) produce conflicting reconstructions of the origins of mammals. Haramiyids were Mesozoic mammals with strange, highly derived rodent-like teeth. Because of this they have been allied with multituberculates, a larger and highly successful group of rodent-like mammals that lived until the Eocene. The problem with haramiyids is that they were until recently known only from teeth. A report by Jin Meng and colleagues reveals a much more complete creature whose features ally it with multituberculates, confirming earlier views — but also implying that the roots of extant mammals lie well back in the Triassic. By contrast, the haramiyid described by Zhe-Xi Luo and colleagues is startlingly primitive in many features of the jaw and ankle, implying that the haramiyids go way back in the mammalian scheme of things and are not related to multituberculates at all. This dichotomy is a reminder of just how little we know about fossils whose interpretation is crucial to the early evolution of mammals.

A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period

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.

Testing the inhibitory cascade model in Mesozoic and Cenozoic mammaliaforms

BackgroundMuch of the current research in the growing field of evolutionary development concerns relating developmental pathways to large-scale patterns of morphological evolution, with developmental constraints on variation, and hence diversity, a field of particular interest. Tooth morphology offers an excellent model system for such ‘evo-devo’ studies, because teeth are well preserved in the fossil record, and are commonly used in phylogenetic analyses and as ecological proxies. Moreover, tooth development is relatively well studied, and has provided several testable hypotheses of developmental influences on macroevolutionary patterns. The recently-described Inhibitory Cascade (IC) Model provides just such a hypothesis for mammalian lower molar evolution. Derived from experimental data, the IC Model suggests that a balance between mesenchymal activators and molar-derived inhibitors determines the size of the immediately posterior molar, predicting firstly that molars either decrease in size along the tooth row, or increase in size, or are all of equal size, and secondly that the second lower molar should occupy one third of lower molar area. Here, we tested the IC Model in a large selection of taxa from diverse extant and fossil mammalian groups, ranging from the Middle Jurassic (~176 to 161 Ma) to the Recent.ResultsResults show that most taxa (~65%) fell within the predicted areas of the Inhibitory Cascade Model. However, members of several extinct groups fell into the regions where m2 was largest, or rarely, smallest, including the majority of the polyphyletic “condylarths”. Most Mesozoic mammals fell near the centre of the space with equality of size in all three molars. The distribution of taxa was significantly clustered by diet and by phylogenetic group.ConclusionsOverall, the IC Model was supported as a plesiomorphic developmental system for Mammalia, suggesting that mammal tooth size has been subjected to this developmental constraint at least since the divergence of australosphenidans and boreosphenidans approximately 180 Ma. Although exceptions exist, including many ‘condylarths’, these are most likely to be secondarily derived states, rather than alternative ancestral developmental models for Mammalia.

A new rhynchocephalian from the late jurassic of Germany with a dentition that is unique amongst tetrapods.

BackgroundRhynchocephalians, the sister group of squamates (lizards and snakes), are only represented by the single genus Sphenodon today. This taxon is often considered to represent a very conservative lineage. However, rhynchocephalians were common during the late Triassic to latest Jurassic periods, but rapidly declined afterwards, which is generally attributed to their supposedly adaptive inferiority to squamates and/or Mesozoic mammals, which radiated at that time. New finds of Mesozoic rhynchocephalians can thus provide important new information on the evolutionary history of the group.Principle FindingsA new fossil relative of Sphenodon from the latest Jurassic of southern Germany, Oenosaurus muehlheimensis gen. et sp. nov., presents a dentition that is unique amongst tetrapods. The dentition of this taxon consists of massive, continuously growing tooth plates, probably indicating a crushing dentition, thus representing a previously unknown trophic adaptation in rhynchocephalians.Conclusions/SignificanceThe evolution of the extraordinary dentition of Oenosaurus from the already highly specialized Zahnanlage generally present in derived rhynchocephalians demonstrates an unexpected evolutionary plasticity of these animals. Together with other lines of evidence, this seriously casts doubts on the assumption that rhynchocephalians are a conservative and adaptively inferior lineage. Furthermore, the new taxon underlines the high morphological and ecological diversity of rhynchocephalians in the latest Jurassic of Europe, just before the decline of this lineage on this continent. Thus, selection pressure by radiating squamates or Mesozoic mammals alone might not be sufficient to explain the demise of the clade in the Late Mesozoic, and climate change in the course of the fragmentation of the supercontinent of Pangaea might have played a major role.

Mallee model for mammal communities of the early Cenozoic and Mesozoic

Mallee is an endemic Australian woodland and shrubland of semi-arid, summer–dry regions between dry woodland and desert shrubland. In other parts of the world, such as Africa and the Americas, such climatic regions support grassland ecosystems. Using Australian and African climofunctions and models gives very different reconstructions of paleoclimate (subhumid versus perhumid) and paleovegetation (woodland versus rainforest) for North American fossil mammal faunas before Cenozoic evolution of grassland ecosystems. Modern mammal faunas of Africa and Australia have different ecological spectra of taxonomic units, body size, feeding, and locomotion of species of mammals on precipitation gradients. Gradients in proportions of such categories yield transfer functions for mean annual precipitation from percent species of Artiodactyla or Macropodidae, percent species of moderately large animals (45–180kg), percent species of arboreal mammals, and percent species of grazers. These transfer functions can be applied to fossil mammal faunas to estimate paleoprecipitation in Africa, Australia and North America. Modern transfer functions match well paleoprecipitation estimates based on depth to calcic horizons in paleosols at the same localities in Kenya and inland Australia back through the Miocene. For fossil mammal faunas of the Rocky Mountain region of North America, African transfer functions fail, but Australian transfer functions predict paleoprecipitation back to the Cretaceous–Tertiary boundary (66Ma). Furthermore, modern mallee soils investigated in this study closely match Cretaceous to Eocene paleosols of the Rocky Mountains. Extinct mallee-like vegetation, such as pori woodlands of Kenya and cunhaka woodlands (newly defined) of the Rocky Mountains better explains the dominance of small, nocturnal, insectivorous, arboreal mammals of Paleogene and Mesozoic mammal faunas, than comparisons with African grassland or rainforest faunas.

Postcranial Skeleton of the Cretaceous Mammal Akidolestes cifellii and Its Locomotor Adaptations

Spalacotheroid “symmetrodontans” are a group of extinct Mesozoic mammals. They are basal taxa in the trechnotherian clade that includes modern marsupials and placentals. Therefore, fossils of spalacotheroids can provide information on the ancestral condition from which marsupials and placentals likely have evolved. Here, we describe the postcranial skeleton of Akidolestes cifellii, a spalacotheroid species from the Lower Cretaceous Yixian Formation of northeastern China. Our comparison of the skeletal features of Akidolestes and the closely related Zhangheotherium and Maotherium indicates some major morphological and functional differences in the postcranium among these spalacotheroid mammals. Akidolestes shows characters for terrestrial habitat preference. Overall it appears to be a generalized terrestrial mammal. Akidolestes differs from Zhangheotherium and Maotherium in some characteristics of the scapula, the pelvis, and the hind limb, some of which can be directly correlated with different locomotor capabilities, and possibly also habitat preferences. This suggests that a greater ecomorphological differentiation occurred in these stem therian mammals than previously thought and that ecological differentiation is a major pattern in early therian mammal evolution.

Adaptive radiation of multituberculate mammals before the extinction of dinosaurs

The Cretaceous-Paleogene mass extinction approximately 66 million years ago is conventionally thought to have been a turning point in mammalian evolution. Prior to that event and for the first two-thirds of their evolutionary history, mammals were mostly confined to roles as generalized, small-bodied, nocturnal insectivores, presumably under selection pressures from dinosaurs. Release from these pressures, by extinction of non-avian dinosaurs at the Cretaceous-Paleogene boundary, triggered ecological diversification of mammals. Although recent individual fossil discoveries have shown that some mammalian lineages diversified ecologically during the Mesozoic era, comprehensive ecological analyses of mammalian groups crossing the Cretaceous-Paleogene boundary are lacking. Such analyses are needed because diversification analyses of living taxa allow only indirect inferences of past ecosystems. Here we show that in arguably the most evolutionarily successful clade of Mesozoic mammals, the Multituberculata, an adaptive radiation began at least 20 million years before the extinction of non-avian dinosaurs and continued across the Cretaceous-Paleogene boundary. Disparity in dental complexity, which relates to the range of diets, rose sharply in step with generic richness and disparity in body size. Moreover, maximum dental complexity and body size demonstrate an adaptive shift towards increased herbivory. This dietary expansion tracked the ecological rise of angiosperms and suggests that the resources that were available to multituberculates were relatively unaffected by the Cretaceous-Paleogene mass extinction. Taken together, our results indicate that mammals were able to take advantage of new ecological opportunities in the Mesozoic and that at least some of these opportunities persisted through the Cretaceous-Paleogene mass extinction. Similar broad-scale ecomorphological inventories of other radiations may help to constrain the possible causes of mass extinctions.

A new eutherian mammal from the Late Cretaceous of Kazakhstan

A dentary fragment containing the last two molars (m2–3) from the Late Cretaceous (Santonian—?Campanian) Bostobe Formation exposed at the locality of Shakh Shakh, northeast Aral Sea region, Kazakhstan, is attributed to a new taxon of Zhelestidae, Zhalmouzia bazhanovi Averianov and Archibald gen. et sp. nov. This specimen is only the second mammal described from Shakh Shakh, the unidentifiable eutherian Beleutinus orlovi Bazhanov, 1972, being the first, and it is only the fifth Mesozoic mammal named from Kazakhstan. Zhalmouzia Averianov and Archibald gen. nov. belongs to the endemic clade of Middle Asian zhelestids (Zhelestinae), better known from the Turonian of Uzbekistan.

A dating success story: genomes and fossils converge on placental mammal origins

The timing of the placental mammal radiation has been a source of contention for decades. The fossil record of mammals extends over 200 million years, but no confirmed placental mammal fossils are known prior to 64 million years ago, which is approximately 1.5 million years after the Cretaceous-Paleogene (K-Pg) mass extinction that saw the end of non-avian dinosaurs. Thus, it came as a great surprise when the first published molecular clock studies suggested that placental mammals originated instead far back in the Cretaceous, in some cases doubling divergence estimates based on fossils. In the last few decades, more than a hundred new genera of Mesozoic mammals have been discovered, and molecular divergence studies have grown from simple clock-like models applied to a few genes to sophisticated analyses of entire genomes. Yet, molecular and fossil-based divergence estimates for placental mammal origins have remained remote, with knock-on effects for macro-scale reconstructions of mammal evolution. A few recent molecular studies have begun to converge with fossil-based estimates, and a new phylogenomic study in particular shows that the palaeontological record was mostly correct; most placental mammal orders diversified after the K-Pg mass extinction. While a small gap still remains for Late Cretaceous supraordinal divergences, this study has significantly improved the congruence between molecular and palaeontological data and heralds a broader integration of these fields of evolutionary science.

Highly specialized mammalian skulls from the Late Cretaceous of South America

Dryolestoids are an extinct mammalian group belonging to the lineage leading to modern marsupials and placentals. Dryolestoids are known by teeth and jaws from the Jurassic period of North America and Europe, but they thrived in South America up to the end of the Mesozoic era and survived to the beginnings of the Cenozoic. Isolated teeth and jaws from the latest Cretaceous of South America provide mounting evidence that, at least in western Gondwana, dryolestoids developed into strongly endemic groups by the Late Cretaceous. However, the lack of pre-Late Cretaceous dryolestoid remains made study of their origin and early diversification intractable. Here we describe the first mammalian remains from the early Late Cretaceous of South America, including two partial skulls and jaws of a derived dryolestoid showing dental and cranial features unknown among any other group of Mesozoic mammals, such as single-rooted molars preceded by double-rooted premolars, combined with a very long muzzle, exceedingly long canines and evidence of highly specialized masticatory musculature. On one hand, the new mammal shares derived features of dryolestoids with forms from the Jurassic of Laurasia, whereas on the other hand, it is very specialized and highlights the endemic, diverse dryolestoid fauna from the Cretaceous of South America. Our specimens include only the second mammalian skull known for the Cretaceous of Gondwana, bridging a previous 60-million-year gap in the fossil record, and document the whole cranial morphology of a dryolestoid, revealing an unsuspected morphological and ecological diversity for non-tribosphenic mammals.

Possible persistence of the morganucodontans in the Lower Cretaceous Purbeck Limestone Group (Dorset, England)

‘Triconodonts’ are relatively rare in the new Purbeck Limestone Group mammal fauna, especially in comparison with the finds made in the nineteenth century in the same beds. However, besides some remains of the already known Triconodontidae, a new possible morganucodontan of uncertain familial affinities, Purbeckodon batei gen. et sp. nov., has been discovered, extending considerably the chronological range of the order while constituting another testimony to the vitality and diversity of Mesozoic mammals.

Trace fossils on dinosaur bones from Upper Cretaceous eolian deposits in Mongolia: Taphonomic interpretation of paleoecosystems in ancient desert environments

The formation processes of trace fossils – including shallow to deep pits, notches, borings (tunnels), and channels, particularly at the limb joints – observed on the surfaces of Velociraptor, Protoceratops, ankylosaurid, and Bagaceratops skeletons from Upper Cretaceous eolian deposits in the Gobi desert, Mongolia were investigated. The median diameters of these structures ranged from 5.25 to 7.68 mm. These structures were likely created by insects scavenging on dinosaur carcasses. This interpretation is corroborated by the presence of burrows of a size similar to the trace fossils observed on the dinosaur bone surfaces at the same locality. Broad borings (about 32 mm in diameter) created by small Mesozoic mammals have also been discovered on the ribs and scapulae of a Protoceratops skeleton. Dinosaur skeletons found at two localities, Tugrikin Shireh and Khermeen Tsav, and two formations, the Djadokhta Formation and Barun Goyot Formation, exhibited the same type of damage to the limb joints. The high frequency of trace fossils at the limb joints suggests that small animals targeted the collagen in the joint cartilage of dried dinosaur carcasses as a source of nitrogen, which was relatively scarce in the eolian environments of the Gobi desert during the Late Cretaceous.