Mesozoic Mammals Research Articles

Developmental process of the modern house shrew's molars: implications for the evolution of the tribosphenic molar in Mesozoic mammals.

Phylogenetically, the tribosphenic molars-prototypes of multi-cusped cheek teeth in marsupial and placental mammals-are derived from the single-cusped conical teeth of reptiles through the addition of cusps. Ontogenetically, mammalian molars are formed through the interface between the dental epithelium and mesenchyme (future enamel-dentin junction), becoming geometrically complex by adding epithelial signaling centers, called enamel knots, which determine future cusp positions. To reevaluate cusp homologies in Mesozoic mammals from an ontogenetic perspective, this study tracked molar development in a living placental mammal species, the house shrew (Suncus murinus), whose molars are morphologically the least derived from tribosphenic prototypes. The development of shrew molars proceeded as if it replayed the evolutionary process of tribosphenic molars. The first formed enamel knots gave rise to the evolutionarily oldest cusps-upper paracone and lower protoconid. The order of formation of other enamel knots and their location in development seemed to trace the order of cusp appearance in evolution. The parallel relationship between ontogeny and phylogeny of mammalian molars, if any, suggests that a change in the timing between developmental events rather than a change in the morphogenetic mechanism itself, should have been a major causal factor for the evolutionary transformation of tooth morphology.

The First Known Mesozoic Mammal in the Democratic People’s Republic of Korea

The First Known Mesozoic Mammal in the Democratic People’s Republic of Korea

An extraordinary fossil captures the struggle for existence during the Mesozoic

Dinosaurs and mammals have coexisted for the last ~ 230 million years. Both groups arose during the Late Triassic and diversified throughout the Mesozoic and into the Cenozoic (the latter in the form of birds). Although they undoubtedly interacted in many ways, direct fossil evidence for their interaction is rare. Here we report a new fossil find from the Lujiatun Member of the Lower Cretaceous Yixian Formation of China, showing a gobiconodontid mammal and psittacosaurid dinosaur locked in mortal combat. We entertain various hypothesized explanations for this association, but the balance of the evidence suggests that it represents a predation attempt on the part of the smaller mammal, suddenly interrupted by, and preserved within, a lahar-type volcanic debris flow. Mesozoic mammals are usually depicted as having lived in the shadows of their larger dinosaurian contemporaries, but this new fossil convincingly demonstrates that mammals could pose a threat even to near fully-grown dinosaurs. The Yixian Formation—and the Chinese fossil Jehol Biota more broadly—have played a particularly important role in revealing the diversity of small-bodied dinosaurs and other fauna. We anticipate that the volcanically derived obrution deposits specific to the Lujiatun Member will likewise continue to yield evidence for biotic interactions otherwise unknown from the rest of the fossil record.

A new euharamiyidan, Mirusodens caii (Mammalia: Euharamiyida), from the Jurassic Yanliao Biota and evolution of allotherian mammals

Abstract 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.

First postcranial remains of the Late Cretaceous gondwanatherian mammal Vintana sertichi

Postcranial remains of gondwanatherian mammals were previously known by only a single specimen, an essentially complete skull and postcranial skeleton constituting the holotype and only known specimen of Adalatherium hui (UA 9001), from the uppermost Cretaceous (Maastrichtian) Anembalemba Member of the Maevarano Formation, Mahajanga Basin, northwestern Madagascar. Here we report the discovery of a caudal vertebra (DMNH EPV.141354) that we provisionally refer to a much larger gondwanatherian, Vintana sertichi, also from the Maevarano Formation. V. sertichi, the largest known mammal from the Mesozoic of Gondwana, was previously represented only by a cranium from the Lac Kinkony Member, which represents a nearshore, peritidal paleoenvironment, but the caudal vertebra was recovered from the underlying Anembalemba Member, which represents an earlier, more fully terrestrial fluvial/floodplain paleoenvironment. The presence of V. sertichi in the latter rock unit therefore expands the age (making it more fully contemporaneous with A. hui) and the known paleoenvironmental range for the species. Comparing various metrics of vertebral body proportions suggests that (1) gondwanatherians are unique among Mesozoic mammals in possessing mid-caudal vertebral centra that are anteroposteriorly short; and (2) the tail of V. sertichi was relatively wider and shorter than that of A. hui.

THE MAIN DIRECTIONS OF MAMMALIAN EVOLUTION

Modern views on the origin and early evolution of mammals are presented. The work uses the material accumulated by the author on the morphology of modern and fossil monotremes, marsupials and placentals. Data on Mesozoic mammals, including those obtained in recent years, are summarized. A model of the mechanism of morphogenetic transformations during the evolutionary development of Mammalia is proposed. An overview of the main directions of the formation of mammals from the Late Triassic to the Cenozoic is given.

Revisiting the southernmost occurrence of Brasilichnium elusivum Leonardi, 1981 with comments on the tetrapod track record of eolian environments

Mammalian lineage is deep rooted in Mesozoic, with several taxa and ichnotaxa described worldwide. One of the most prolific mammaliaform ichnospecies is Brasilichnium elusivum, which is extremely abundant in the Lower Cretaceous Botucatu Formation of the Brazilian Paraná Basin, as well as in other Mesozoic ichnofaunas of North America and Africa. In this contribution, we revisited the proposed southernmost occurrence of B. elusivum using both classical and photogrammetric (3D digitization) approaches. The flagstone containing the studied material was found ex situ, in a sidewalk at Santa Cruz do Sul Municipality (the Rio Grande do Sul State, Brazil), and the results of the performed analysis showed that the tracks cannot be assigned to any ichnotaxon. Lastly, some significant issues related to ichnological research of ex situ track-bearing flagstones and large eolian deposits are discussed. Keywords: Early Cretaceous, Botucatu Formation, mesozoic mammals, South America, vertebrate ichnology.

A new Cretaceous fossil mammal locality from the Bass Coast of southeastern Australia

Mesozoic mammals from the polar regions of Australian Gondwana are exceptionally rare. The recovery of a partial jaw attributable to the australosphenid Ausktribosphenos nyktos from a new locality along the Bass Coast of Victoria is, therefore, significant because it comes from an uppermost Barremian to lowermost Aptian grit with abundant plant material that differs lithologically from other previously productive laminated sandstone deposits. We interpret this as evidence for a floodplain habitat that was distant from local water bodies. The identification of a new Cretaceous mammal locality in Australia highlights the exciting prospects for future fossil discoveries. Thomas H. Rich [trich@museum.vic.gov.au] Museums Victoria, Melbourne, Victoria, 3001, PO Box 666,Australia Melissa L. Lowery [melissalouiselowery@gmail.com] 82 Williams Street, Inverloch, Victoria, 3996, Australia Michael Hall [mike.hall@monash.edu] School of Earth, Atmosphere and Environment, Monash University, Victoria, 3800, Australia Lesley Kool [koollesley@gmail.com] Museums Victoria, Melbourne, Victoria, 3001, Australia PO Box 666; School of Earth, Atmosphere and Environment, Monash University, Victoria, 3800, Australia Joseph Bevitt [joseph.bevitt@ansto.gov.au] Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, Lucas Heights, Nsw, 2234, Australia Matt White [fossilised@hotmail.com] University of New England, Armidale, Nsw, 2350, Australia Patricia Vickers-Rich [pat.rich@monash.edu, prich@swin.edu.au] School of Earth, Atmosphere and Environment, Monash University, Victoria, 3800, Australia; Department of Chemistry and Biotechnology, Swinburne University of Science and Technology, Hawthorn, Victoria, 3122, Australia.

Walter Georg Kühne (1911-1991) and the discovery of the first semi-articulated titanosaurian sauropod from Europe

ABSTRACT In 1954 Walter Georg Kühne (1911–1991), the renowned discoverer of Mesozoic mammals, explored the south-Pyrenean basins of Catalonia with the aim of finding Cretaceous mammal-bearing sites. As a result, the German palaeontologist discovered one of the most important dinosaur fossils in the history of dinosaur research in Spain. In the Tremp Basin, near Orcau village, he found the remains of a large sauropod dinosaur, but a series of incomprehensible vicissitudes made the find forgotten and its extraction incomplete for over 60 years. Unprecedented research into the history of this discovery, including unpublished documents, images, and field notes, has revealed details on the costs of the campaigns, the daily logistics, the field techniques, and the fossils unearthed in the 1954 and 1955 excavations. More importantly, this new research ascertains that Kühne had already determined that the dinosaur skeleton was articulated from the pelvic girdle onwards. Even without being published, that was a historically significant achievement as, back then, the new specimen became the first and semi-articulated titanosaurian sauropod found in Europe. Currently, the sauropod fossils found by Kühne in Orcau, together with additional appendicular and axial bones, are the holotype of Abditosaurus kuehnei.

Spatial and Temporal Distribution of the Island-Dwelling Kogaionidae (Mammalia, Multituberculata) in the Uppermost Cretaceous of Transylvania (Western Romania)

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—“Haţeg Island”—and their fossils are found in the uppermost Campanian to upper Maastrichtian deposits of the Haţeg, 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 Haţeg 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 Haţeg 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 better-drained 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.

A review of monotreme (Monotremata) evolution

Advances in dating and systematics have prompted a revision of monotreme evolution to refine the timing of adaptative trends affecting body size and craniodental morphology. The oldest known monotreme, Teinolophos trusleri, is restricted to uppermost Barremian deposits of the Strzelecki Group in Victoria, Australia. Its body mass is estimated at ∼40 g, making it the smallest known monotreme. Teinolophos trusleri likely possessed an electro-sensitive and/or mechano-sensitive ‘bill’ or ‘beak’, which we suggest evolved for insectivory in seasonally dark Early Cretaceous polar forests. During the early Albian–mid-Cenomanian, monotremes diversified in Australia and evolved body masses greater than 4 kg, becoming amongst the largest Mesozoic mammals. A gap of 35 million years subsequently separates the youngest Mesozoic monotremes from the oldest Cenozoic monotreme, Monotrematum sudamericanum, which is a Paleocene stem ornithorhynchid from southern South America. We also hypothesize that tachyglossids originated in Melanesia, perhaps on the emergent Vogelkop landmass, and then dispersed to Australia during the Pliocene-Pleistocene. Finally, we present a classification of Monotremata to include five families—Teinolophidae fam. nov., Kollikodontidae, Steropodontidae, Ornithorhynchidae, and Tachyglossidae. We also propose a new genus, Murrayglossus gen. nov. for a gigantic Pleistocene echidna from southwestern Western Australia. http://zoobank.org/urn:lsid:zoobank.org:pub:E39E2644-DADD-49F4-B2B7-47A25C072B07 Timothy F. Flannery [tim.flannery@textpublishing.com.au], Kristofer M. Helgen [Kris.Helgen@Australian.Museum], Australian Museum, 1 William St Sydney 2000, Australia; Thomas H. Rich [trich@museum.vic.gov.au], Tim Ziegler [tziegler@museum.vic.gov.au], Museums Victoria, PO Box 666, Melbourne, Victoria 3001, Australia; Patricia Vickers-Rich [pat.rich@monash.edu; prich@swin.edu.au], School of Earth, Atmosphere and Environment, Monash University, Victoria 3800, Australia; Swinburne University of Science and Technology, Department of Chemistry and Biotechnology, Hawthorn, Victoria 3122, Australia; Elizabeth Grace Veatch [elizabeth.veatch@gmail.com], National Museum of Natural History, Smithsonian Institution, Washington, DC 20013, USA.

Jaw shape and mechanical advantage are indicative of diet in Mesozoic mammals

Jaw morphology is closely linked to both diet and biomechanical performance, and jaws are one of the most common Mesozoic mammal fossil elements. Knowledge of the dietary and functional diversity of early mammals informs on the ecological structure of palaeocommunities throughout the longest era of mammalian evolution: the Mesozoic. Here, we analyse how jaw shape and mechanical advantage of the masseter (MAM) and temporalis (MAT) muscles relate to diet in 70 extant and 45 extinct mammals spanning the Late Triassic-Late Cretaceous. In extant mammals, jaw shape discriminates well between dietary groups: insectivores have long jaws, carnivores intermediate to short jaws, and herbivores have short jaws. Insectivores have low MAM and MAT, carnivores have low MAM and high MAT, and herbivores have high MAM and MAT. These traits are also informative of diet among Mesozoic mammals (based on previous independent determinations of diet) and set the basis for future ecomorphological studies.

New species of mammaliaform and the cranium ofBorealestes(Mammaliformes: Docodonta) from the Middle Jurassic of the British Isles

AbstractDocodonta are one of the earliest diverging groups of mammaliaforms, and their morphology provides key information on the transition between non-mammalian cynodonts and Mammalia. We describe the partial skulls of two docodontans Borealestes serendipitus and Borealestes cuillinensis sp. nov. from the Kilmaluag Formation (Middle Jurassic: Bathonian), Isle of Skye, Scotland. We visualize their cranial anatomy using laboratory and synchrotron X-ray micro-CT. The skulls belong to two partial skeletons, currently comprising the most complete Mesozoic mammal fossils reported from the British Isles. The associated upper and lower dentitions show that the lower dentition of Borealestes is not diagnostic to species level. We establish, B. cuillinensis, based on upper molar characters, and re-identify upper molars previously assigned to ‘Borealestes’ mussettae as belonging to B. cuillinensis. ‘Borealestes’ mussettae, based on distinctive lower molars, is found to be morphologically and phylogenetically distinct from Borealestes, necessitating assignment to a new genus, Dobunnodon gen. nov. The skulls of Borealestes retain many plesiomorphic features seen in Morganucodon but absent in more crownward mammaliaforms. Our study highlights that generic and species taxonomy of docodontans are more reliable when based on both upper and lower teeth, while lower molar morphology may underrepresent the true diversity of Mesozoic mammaliaforms.

A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan.

Among extant vertebrates, mammals are distinguished by having a chain of three auditory ossicles (the malleus, incus and stapes) that transduce sound waves and promote an increased range of audible-especially high-frequencies1. By contrast, the homologous bones in early fossil mammals and relatives also functioned in chewing through their bony attachments to the lower jaw2. Recent discoveries of well-preserved Mesozoic mammals have provided glimpses into the transition from the dual (masticatory and auditory) to the single auditory function for the ossicles, which is now widely accepted to have occurred at least three times in mammal evolution3-6. Here we report a skull and postcranium that we refer to the haramiyidan Vilevolodon diplomylos (dating to the Middle Jurassic epoch (160million years ago)) and that shows excellent preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane). After comparing this fossil with other Mesozoic and extant mammals, we propose that the overlapping incudomallear articulation found in this and other Mesozoic fossils, in extant monotremes and in early ontogeny in extant marsupials and placentals is a morphology that evolved in several groups of mammals in the transition from the dual to the single function for the ossicles.

Early mammalian social behaviour revealed by multituberculates from a dinosaur nesting site.

When sociality evolved and in which groups remain open questions in mammalian evolution, largely due to the fragmentary Mesozoic mammal fossil record. Nevertheless, exceptionally preserved fossils collected in well-constrained geologic and spatial frameworks can provide glimpses into these more fleeting aspects of early mammalian behaviour. Here we report on exceptional specimens of a multituberculate, Filikomys primaevus gen. nov., from the Late Cretaceous of Montana, primarily occurring as multi-individual, monospecific aggregates of semi-articulated skulls and skeletons within a narrow stratigraphic (~9 cm thick) and geographic (<32 m2) interval. Taphonomic and geologic evidence indicates that F. primaevus engaged in multigenerational, group-nesting and burrowing behaviour, representing the first example of social behaviour in a Mesozoic mammal. That F. primaevus was a digger is further supported by functional morphological and morphometric analyses of its postcranium. The social behaviour of F. primaevus suggests that the capacity for mammals to form social groups extends back to the Mesozoic and is not restricted to therians. Sociality is probably an evolutionarily labile trait that has arisen numerous times during mammalian evolution.

Postcranial elements of small mammals as indicators of locomotion and habitat.

Many studies have shown a correlation between postcranial anatomy and locomotor behavior in mammals, but the postcrania of small mammals (<5 kg) is often considered to be uninformative of their mode of locomotion due to their more generalized overall anatomy. Such small body size was true of all mammals during the Mesozoic. Anatomical correlates of locomotor behavior are easier to determine in larger mammals, but useful information can be obtained from the smaller ones. Limb bone proportions (e.g., brachial index) can be useful locomotor indicators; but complete skeletons, or even complete long bones, are rare for Mesozoic mammals, although isolated articular surfaces are often preserved. Here we examine the correlation of the morphology of long bone joint anatomy (specifically articular surfaces) and locomotor behavior in extant small mammals and demonstrate that such anatomy may be useful for determining the locomotor mode of Mesozoic mammals, at least for the therian mammals.

Exploring ancestral phenotypes and evolutionary development of the mammalian middle ear based on Early Cretaceous Jehol mammals.

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.

Modern Data on the Origin and Early Radiation of Mammals

Modern fossil evidence shows extensive ecomorphological radiation of Mesozoic mammals. By the mid-Cretaceous, mammals had evolved all the main adaptations of modern small-sized Mammalia except for true powered flight. In addition, new paleontological and embryological data on the morphogenesis of the auditory ossicles demonstrate parallel developments of the definitive mammal middle ear in the placentals, marsupials, and monotremes, as well as the independent origins of a number of early groups: Allotheria (Haramiyida and Multituberculata) within Mammaliamorpha (irrespective of the lineage including eucynodonts Brasilodontidae and the mammaliaform Apotheria, Prototheria, and Acrotheria) and Prototheria (including Monotremata) within the Mammaliaformes (parallel to the Acrotheria lineage).

Two new localities of Mesozoic mammals in Russia (Krasnoyarsk territory, Lower Cretaceous)

Two new localities of Mesozoic mammals have been discovered: Bol’shoi Ilek and Berezovaya River (Russia, Krasnoyarsk Territory, Lower Cretaceous, Ilek Formation). Bol’shoi Ilek locality yields an edentulous fragment of the maxillary of Docodonta indet. A fragment of dentary without teeth attributed to Mammalia indet. (presumably eutriconodontan or symmetrodontan) is presented in the Berezovaya River locality. New localities fill the geographical gap between previously known mammalian localities of Ilek Formation in the basins of Kiya and Bol’shoi Kemchug rivers.

Two New Localities of Mesozoic Mammals in Russia (Krasnoyarsk Territory, Lower Cretaceous)

Two new localities of Mesozoic mammals have been discovered: Bol’shoi Ilek and Berezovaya River (Russia, Krasnoyarsk Territory, Lower Cretaceous, Ilek Formation). Bol’shoi Ilek locality yields an edentulous fragment of the maxillary of Docodonta indet. A fragment of dentary without teeth attributed to Mammalia indet. (presumably eutriconodontan or symmetrodontan) is presented in the Berezovaya River locality. New localities fill the geographical gap between previously known mammalian localities of Ilek Formation in the basins of Kiya and Bol’shoi Kemchug rivers.