A large therian mammal from the Late Cretaceous of South America.

Theria represent an extant clade that comprises placental and marsupial mammals. Here we report on the discovery of a new Late Cretaceous mammal from southern Patagonia, Patagomaia chainko gen. et sp. nov., represented by hindlimb and pelvic elements with unambiguous therian features. We estimate Patagomaia chainko attained a body mass of 14 kg, which is considerably greater than the 5 kg maximum body mass of coeval Laurasian therians. This new discovery demonstrates that Gondwanan therian mammals acquired large body size by the Late Cretaceous, preceding their Laurasian relatives, which remained small-bodied until the beginning of the Cenozoic. Patagomaia supports the view that the Southern Hemisphere was a cradle for the evolution of modern mammalian clades, alongside non-therian extinct groups such as meridiolestidans, gondwanatherians and monotremes.

Evolutionary process toward avian-like cephalic thermoregulation system in Theropoda elucidated based on nasal structures.

It has long been discussed whether non-avian dinosaurs were physiologically closer to ectotherms or endotherms, with the internal nasal structure called the respiratory turbinate present in extant endotherms having been regarded as an important clue for this conundrum. However, the physiological function and relevance of this structure for dinosaur physiology are still controversial. Here, we found that the size of the nasal cavity relative to the head size of extant endotherms is larger than those of extant ectotherms, with that of the dromaeosaurid Velociraptor being below the extant endotherms level. The result suggests that a large nasal cavity accommodating a well-developed respiratory turbinate is primarily important as a thermoregulation apparatus for large brains characteristic of endothermic birds and mammals, and the nasal cavity of Velociraptor was apparently not large enough to carry out this role required for an endothermic-sized brain. In addition, a hypothesis that the enlargement of the nasal cavity for brain cooling has been associated with the skull modification in the theropod lineage toward modern birds is proposed herein. In particular, the reduction of the maxilla in derived avialans may have coincided with acquisition of the avian-like cephalic thermoregulation system.

First monotreme from the Late Cretaceous of South America.

Monotremata is a clade of egg-lying mammals, represented by the living platypus and echidnas, which is endemic to Australia, and adjacent islands. Occurrence of basal monotremes in the Early Cretaceous of Australia has led to the consensus that this clade originated on that continent, arriving later to South America. Here we report on the discovery of a Late Cretaceous monotreme from southern Argentina, demonstrating that monotremes were present in circumpolar regions by the end of the Mesozoic, and that their distinctive anatomical features were probably present in these ancient forms as well.

The relationship between hard and soft tissue structures of the eye in extant lizards.

The sizes of the eye structures, such as the lens diameter and the axial length, are important factors for the visual performance and are considered to be related to the mode of life. Although the size of these soft structures cannot be directly observed in fossil taxa, such information may be obtained from measuring size and morphology of the bony scleral ossicle ring, which is present in the eyes of extant saurospids, excluding crocodiles and snakes, and is variously preserved in fossil taxa. However, there have been only a few studies investigating the relationships between the size, the scleral ossicle ring, and soft structures of the eye. We investigated such relationships among the eye structures in extant Squamata, to establish the basis for inferring the size of the soft structures in the eye in fossil squamates. Three-dimensional morphological data on the eye and head region of 59 lizard species covering most major clades were collected using micro-computed tomography scanners. Strong correlations were found between the internal and external diameters of the scleral ossicle ring and soft structures. The tight correlations found here will allow reliable estimations of the sizes of soft structures and inferences on the visual performance and mode of life in fossil squamates, based on the diameters of their preserved scleral ossicle rings. Furthermore, the comparison of the allometric relationships between structures in squamates eyes with those in avian eyes suggest the possibility that the similarities of these structures closely reflect the mechanism of accommodation.

A large Megaraptoridae (Theropoda: Coelurosauria) from Upper Cretaceous (Maastrichtian) of Patagonia, Argentina.

Megaraptora is a theropod clade known from former Gondwana landmasses and Asia. Most members of the clade are known from the Early to Late Cretaceous (Barremian-Santonian), with Maastrichtian megaraptorans known only from isolated and poorly informative remains. The aim of the present contribution is to describe a partial skeleton of a megaraptorid from Maastrichtian beds in Santa Cruz Province, Argentina. This new specimen is the most informative megaraptoran known from Maastrichtian age, and is herein described as a new taxon. Phylogenetic analysis nested the new taxon together with other South American megaraptorans in a monophyletic clade, whereas Australian and Asian members constitute successive stem groups. South American forms differ from more basal megaraptorans in several anatomical features and in being much larger and more robustly built.

Homology and osteological correlates of pedal muscles among extant sauropsids.

Archosaurs displayed an evolutionary trend toward increasing bipedalism in their evolutionary history, that is, forelimbs tend to be reduced in contrast to the development of hindlimbs becoming major weight-bearing and locomotor appendages. The archosaurian locomotion has been extensively discussed based on their limb morphology because the latter reflects their locomotor modes very well. However, despite some attempts of reconstructing the hindlimb musculature in Archosauria, that of the most distal portion, the pes, has often been neglected. In order to rectify this trend, detailed homologies of pedal muscles among sauropsids were established based on dissections and literature reviews of adult conditions. As a result, homologies of some pedal muscles between non-avian sauropsids and avians were revised, challenging classical hypotheses. The present new hypothesis postulates that the avian m. tibialis cranialis and non-avian m. extensor digitorum longus, as well as the avian m. extensor digitorum longus and non-avian m. tibialis anterior, are homologous with each other, respectively. This is more plausible because it requires no drastical change in the attachment sites between the avian and non-avian homologues unlike the classical hypothesis. Many interosseous muscles in non-archosaurian sauropsids that have long been regarded as a part of short digital extensors or flexors are also divided into multiple distinct muscles so that they can be homologized with short pedal muscles among all sauropsids. In addition, osteological correlates of attachments are identified for most of the pedal muscles, contributing to future attempts of reconstruction of this muscle system in fossil archosaurs.

New record of a Mesozoic gondwanatherian mammaliaform from Southern Patagonia.

Gondwanatheria is an enigmatic mammaliaform clade distributed in the Cretaceous and Paleogene of South America, Africa, Madagascar, India and Antarctica. The Mesozoic record in South America is restricted to the Latest Cretaceous of Río Negro and Chubut provinces, Argentina and Magallanes Region of southern Chile. The aim of the present contribution is to describe a new specimen of gondwanatherian mammaliaforms from beds belonging to the Maastrichtian Chorrillo Formation, cropping out 30 km SW of El Calafate, Santa Cruz Province, Argentina. It is represented by a single molariform referable to the species Magallanodon baikashkenke with which it shares a unique combination of characters. Analysis of the unique combination of characters exhibited by Magallanodon shed doubts on the monophyly of Ferugliotheriidae and suggest that South American taxa may be closely related to each other. The wide geographical distribution and occurrence of gondwanatherians on geological units of diverse origins suggest that they were capable of facing disparate environmental conditions.

The phylogeny of desmostylians revisited: proposal of new clades based on robust phylogenetic hypotheses.

BACKGROUND: Desmostylia is a clade of extinct aquatic mammals with no living members. Today, this clade is considered belonging to either Afrotheria or Perissodactyla. In the currently-accepted taxonomic scheme, Desmostylia includes two families, 10 to 12 genera, and 13-14 species. There have been relatively few phylogenetic analyses published on desmostylian interrelationship compared to other vertebrate taxa, and two main, alternative phylogenetic hypotheses have been proposed in previous studies. One major problem with those previous studies is that the numbers of characters and OTUs were small. METHODS: In this study, we analyzed the phylogenetic interrelationship of Desmostylia based on a new data matrix that includes larger numbers of characters and taxa than in any previous studies. The new data matrix was compiled mainly based on data matrices of previous studies and included three outgroups and 13 desmostylian ingroup taxa. Analyses were carried out using five kinds of parsimonious methods. RESULTS: Strict consensus trees of the most parsimonious topologies obtained in all analyses supported the monophyly of Desmostylidae and paraphyly of traditional Paleoparadoxiidae. Based on these results, we propose phylogenetic definitions of the clades Desmostylidae and Paleoparadoxiidae based on common ancestry.

Description of Tooth Ontogeny and Replacement Patterns in a Juvenile Tarbosaurus bataar (Dinosauria: Theropoda) Using CT-Scan Data.

Teeth are continually replaced in most of non-mammalian gnathostomes to maintain their functional dentitions. To clarify the tooth replacement patterns in tyrannosaurid theropod dinosaurs, we examined well-preserved dentitions (both premaxillae, left maxilla, partial right maxilla, and both dentaries) of a juvenile Tarbosaurus bataar (MPC-D 107/7) using X-ray computed tomographic (CT) imaging. Three-dimensional (3D) rendering of the dentitions and staging of replacement teeth allowed quantitative analyses of the tooth ontogeny and replacement patterns in this specimen. These strategies were validated by comparing the results between MPC-D 107/7 and extant crocodilians, which are taxa that have previously been studied using non-CT methods. 3D-rendered dentitions of MPC-D 107/7 showed alternate replacement patterns between odd- and even-numbered alveoli. Such patterns were discontinuous at the premaxilla-maxilla junctions, suggesting the division of replacement patterns between the two dentitions possessing morpho-functionally different features. The replacement process in the odd-numbered alveoli of the left maxilla sequentially proceeded from distal alveoli. Meanwhile, in the both dentaries, there were simple alternate patterns in which functional teeth would be simultaneously shed out in every second alveoli. Such a simple alternation had never been reported in the adult tyrannosaurid dentaries. Under this pattern, the half of functional teeth in a single dentition would be shed at the same time, which may hamper foraging functions. We conclude that the simple alternate patterns found in the dentary dentitions of MPC-D 107/7 represent transient condition in juvenile tyrannosaurids, suggesting ontogenetic changes in tooth replacement patterns in the tyrannosaurid dentary. Anat Rec, 302:1210-1225, 2019. © 2018 Wiley Periodicals, Inc.

An exquisitely preserved troodontid theropod with new information on the palatal structure from the Upper Cretaceous of Mongolia.

Troodontidae is a clade of small-bodied theropod dinosaurs. A new troodontid, Gobivenator mongoliensis gen. et sp. nov., is described based on the most complete skeleton of a Late Cretaceous member of this clade presently known, from the Campanian Djadokhta Formation in the central Gobi Desert. G. mongoliensis is different from other troodontids in possessing a pointed anterior end of the fused parietal and a fossa on the surangular in front of the posterior surangular foramen. The skull was superbly preserved in the specimen and provides detailed information of the entire configuration of the palate in Troodontidae. Overall morphology of the palate in Gobivenator resembles those of dromaeosaurids and Archaeopteryx, showing an apparent trend of elongation of the pterygoid process of the palatine and reduction of the pterygopalatine suture toward the basal Avialae. The palatal configuration suggests that the skull of Gobivenator would have been akinetic but had already acquired prerequisites for later evolution of cranial kinesis in birds, such as the loss of the epipterygoid and reduction in contact areas among bones.

Evolutionary and developmental aspects of avian-specific traits in limb skeletal pattern.

The two sets of paired appendages, called limbs, are locomotory organs in tetrapods that are used for various functions (e.g., walking, running, crawling, digging, climbing, diving, swimming, and flying). Unlike such organs as the eye, which contain specialized tissues such as the lens and photoreceptor, the limb does not have any specialized cells or tissues, but consists of common tissues, such as bone, cartilage, muscle, blood vessels, and dermis. However, limb morphology is highly specialized and varies to provide species-specific modes of locomotion. As do the vertebrae and skull, the limb skeleton varies in morphology among species. The diversity of limb skeletal morphology provides examples of material for studies on morphogenesis. Avian forelimbs have evolved into wings for flight. The skeletal pattern in the avian limb has many traits that are unique among extant species of vertebrates; some of such traits are avian-specific, others are shared with more basal members of Theropoda, to which Aves belongs. Since such avian traits generally form during ontogenic development, determining when and how they appear in the developing embryonic limbs or limb buds provides important insights into the mechanisms underlying the generation of vertebrate morphological diversity. Here, we present an overview of several features of the skeletal pattern in the avian limb and discuss the developmental mechanisms responsible for their unique and lineage-specific traits.

Finding the neck-trunk boundary in snakes: anteroposterior dissociation of myological characteristics in snakes and its implications for their neck and trunk body regionalization.

The neck and trunk regionalization of the presacral musculoskeletal system in snakes and other limb-reduced squamates was assessed based on observations on craniovertebral and body wall muscles. It was confirmed that myological features characterizing the neck in quadrupedal squamates (i.e., squamates with well-developed limbs) are retained in all examined snakes, contradicting the complete lack of the neck in snakes hypothesized in previous studies. However, the posterior-most origins of the craniovertebral muscles and the anterior-most bony attachments of the body wall muscles that are located at around the neck-trunk boundary in quadrupedal squamates were found to be dissociated anteroposteriorly in snakes. Together with results of a recent study that the anterior expression boundaries of Hox genes coinciding with the neck-trunk boundary in quadrupedal amniotes were dissociated anteroposteriorly in a colubrid snake, these observations support the hypothesis that structures usually associated with the neck-trunk boundary in quadrupedal squamates are displaced relative to one another in snakes. Whereas certain craniovertebral muscles are elongated in some snakes, results of optimization on an ophidian cladogram show that the most recent common ancestor of extant snakes would have had the longest craniovertebral muscle, M. rectus capitis anterior, that is elongated only by several segments compared with that of quadrupedal squamates. Therefore, even such a posteriorly displaced "cervical" characteristic plesiomorphically lies fairly anteriorly in the greatly elongated precloacal region of snakes, suggesting that the trunk, not the neck, would have contributed most to the elongation of the snake precloacal region. A similar dissociation of structures usually associated with the neck-trunk boundary in quadrupedal squamates is observed in limb-reduced squamates, suggesting that these forms and snakes may share a developmental mechanism producing modifications in the anterior-posterior patterning associated with body elongation.

Role of paraxial mesoderm in limb/flank regionalization of the trunk lateral plate.

To understand the developmental mechanism that determines limb size and the consequent limb-to-trunk proportions in the tetrapod body, we investigated the role of the paraxial mesoderm in the specification of the limb and flank fields in the chick embryo. We found that the paraxial mesoderm subjacent to the limb field can affect the size of the limb bud along the anterior-posterior and proximal-distal axes. We also found that the paraxial mesoderm subjacent to the flank plays roles in suppressing the emergence and growth of the limb bud and in promoting flank-specific apoptosis in the lateral plate mesoderm. Our results suggest that signals from the paraxial mesoderm specify the limb and flank fields in the competent lateral plate mesoderm.

Reconstructions of the axial muscle insertions in the occipital region of dinosaurs: evaluations of past hypotheses on marginocephalia and tyrannosauridae using the extant phylogenetic bracket approach.

The insertions of the cervical axial musculature on the occiput in marginocephalian and tyrannosaurid dinosaurs have been reconstructed in several studies with a view to their functional implications. Most of the past reconstructions on marginocephalians, however, relied on the anatomy of just one clade of reptiles, Lepidosauria, and lack phylogenetic justification. In this study, these past reconstructions were evaluated using the Extant Phylogenetic Bracket approach based on the anatomy of various extant diapsids. Many muscle insertions reconstructed in this study were substantially different from those in the past studies, demonstrating the importance of phylogenetically justified inferences based on the conditions of Aves and Crocodylia for reconstructing the anatomy of non-avian dinosaurs. The present reconstructions show that axial muscle insertions were generally enlarged in derived marginocephalians, apparently correlated with expansion of their parietosquamosal shelf/frill. Several muscle insertions on the occiput in tyrannosaurids reconstructed in this study using the Extant Phylogenetic Bracket approach were also rather different from recent reconstructions based on the same, phylogenetic and parsimony-based method. Such differences are mainly due to differences in initial identifications of muscle insertion areas or different hypotheses on muscle homologies in extant diapsids. This result emphasizes the importance of accurate and detailed observations on the anatomy of extant animals as the basis for paleobiological inferences such as anatomical reconstructions and functional analyses.

Homologies of the longissimus, iliocostalis, and hypaxial muscles in the anterior presacral region of extant diapsida.

Homologies of muscles of the m. longissimus and m. iliocostalis groups in the dorsal and cervical regions, as well as those of the subvertebral muscles and mm. intercostales externi that continue from the dorsal into the cervical regions, in extant Diapsida are proposed based on detailed dissections and published accounts of lepidosaurs, crocodylians, and birds. The morphology of tendons and innervation patterns suggest that the avian "m. iliocostalis" in the dorsal region include the homologs of both m. longissimus and m. iliocostalis in non-avian diapsids. The conserved nature of the morphology of tendons in palaeognath birds also revealed that the avian mm. intertransversarii in the cervical region consist of muscles of the both m. longissimus and m. iliocostalis groups despite having been treated as a single series of muscles, and thus are not homologous with muscles of the same name in Lepidosauria or Crocodylia. The avian mm. inclusi that lie medial to mm. intertransversarii are homologous with mm. intercostales externi in Lepidosauria and mm. intercostales externi and m. scalenus combined in Crocodylia. Innervation patterns suggest that a muscle ("m. iliocostalis capitis") connecting the atlas rib and occiput in Crocodylia includes contributions from the subvertebral layer and m. cucullaris complex, and possibly m. iliocostalis as well. The present findings may serve as a basis for revising the currently used avian nomenclature so that it will reflect homologies of muscles with their non-avian counterparts.

Homologies of the transversospinalis muscles in the anterior presacral region of Sauria (crown Diapsida).

Homologies of muscles of the m. transversospinalis group in the dorsal and cervical regions in Sauria are established based on detailed dissections and published accounts of lepidosaurs, crocodylians, and birds. Attachments and directions of tendons comprising this muscle group are fairly conserved among the saurian clades, enabling rather robust inferences on muscle homologies. The innervation pattern indicates that mm. ascendentes are the most lateral muscles of the m. transversospinalis group in Aves, and are inferred to be homologous with the crocodylian m. tendinoarticularis based on their topological similarities. It is suggested here that the lepidosaurian articulo-parietalis part of m. longissimus cervico-capitis actually belongs to the m. transversospinalis group because its tendons of origin are shared with those of m. semispinalis. The avian m. complexus and the lateral part of the crocodylian m. transversospinalis capitis have origins and insertions similar to this lepidosaurian muscle, and are proposed to be homologous with the latter. In some birds, m. longus colli dorsalis, pars profunda continues directly into the anterior cervical region as m. splenius accessorius, suggesting a serially homologous relationship. Similarly, m. splenius anticus continues anteriorly from m. longus colli dorsalis, pars cranialis, and both of these muscles lie dorsal to m. splenius accessorius. Therefore, the currently used nomenclature that regards m. splenius accessorius as a part of m. longus colli dorsalis, pars cranialis and that regards m. splenius anticus as a part of the former muscle does not accurately reflect the serial homologies among these muscles and may not be justified.