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Los Alvarezsauridae (Dinosauria, Theropoda, Coelurosauria) de América del Sur: Anatomía y relaciones filogenéticas.
Abstract
This Doctoral Thesis presents an exhaustive review of the Patagonian alvarezsaurids (Dinosauria, Theropoda). It includes a detailed osteological description of specimens of Patagonykus puertai (Holotype, MCF-PVPH-37), cf. Patagonykus puertai (MCF-PVPH-38), Patagonykinae indet. (MCF-PVPH-102), Alvarezsaurus calvoi (Holotype, MUCPv-54), Achillesaurus manazzonei (Holotype, MACN-PV-RN 1116), Bonapartenykus ultimus (Holotype, MPCA 1290), and cf. Bonapartenykus ultimus (MPCN-PV 738). A phylogenetic analysis and a discussion about the taxonomic validity of the recognized species and the taxonomic assignment of the materials MCF-PVPH-38, MCF-PVPH-102 and MPCN-PV 738 are presented. Different evolutionary and paleobiological studies were carried out in order to elucidate functional and behavioral aspects.
Alvarezsaurus calvoi (MUCPv-54), Achillesaurus manazzonei (MACN-PV-RN 1116), Patagonykus puertai (MCF-PVPH-37) and Bonapartenykus ultimus (MPCA 1290) are valid species due to the presence of many autapomorphies. In this sense, the hypothesis proposed by P. Makovicky and collaborators that Achillesaurus manazzonei is a junior synonym of Alvarezsaurus calvoi is rejected. Likewise, certain morphological evidence allows hypothesizing that Alvarezsaurus calvoi represents a growth stage earlier than skeletal maturity. Specimen MCF-PVPH-38 is referable as cf. Patagonykus puertai, while MCF-PVPH-102 is considered an indeterminate Patagonykinae. In turn, MPCN-PV 738 is assigned as cf. Bonapartenykus ultimus based on the little overlapping material with the Bonapartenykus ultimus holotype.
The results obtained from the mineralogical characterization through the X-ray diffraction method of specimens MPCN-PV 738 and the holotype of Bonapartenykus ultimus (MPCA 1290), allow to suggest that both specimens come from the same geographical area and stratigraphic level.
The phylogenetic analysis, which is based upon the matrix of Gianechini and collaborators of 2018 with the inclusion of proper characters, and the database of Xu and collaborators of 2018, recovered the South American members of Alvarezsauria, such as Alnashetri cerropoliciensis (Candeleros Formation; Cenomanian), Patagonykus puertai (Portezuelo Formation, Turonian-Coniacian), Alvarezsaurus calvoi and Achillesaurus manazzonei (Bajo de La Carpa Formation, Coniacian-Santonian), and Bonapartenykus ultimus (Allen Formation, Campanian-Maastrichtian), nesting within the family Alvarezsauridae. In this sense, the forms that come from the Bajo de La Carpa Formation (Coniacian-Santonian) are recovered at the base of the Alvarezsauridae clade, while Alnashetri cerropoliciensis nests as a non-Patagonykinae alvarezsaurid. Regarding the type specimens of Patagonykus puertai and Bonapartenykus ultimus, they are recovered as members of the Patagonykinae subclade, a group that is recovered as a sister taxon of Parvicursorinae, both nested within the Alvarezsauridae. In addition, the topology obtained allows discerning the pattern, rhythm and time of evolution of the highly strange and derived alvarezsaurian skeleton, concluding in a gradual evolution. The Bremer and Bootstrap supports of the nodes (Haplocheirus + Aorun), [Bannykus + (Tugulusaurus + Xiyunykus)], and Patagonykinae, show indices that represent very robust values for these nodes. Likewise, these values suggest that two endemic clades originated early in Asia, while one endemic clade is observed in Patagonia, i.e., Patagonykinae.
The analysis of the directional trends of the Alvarezsauria clade, tested by means of a own database on body masses based on the Christiansen and Fariña method, subsequently calibrated with the group's phylogeny using the R software, shows two independent miniaturization events in the alvarezsaurid evolution, namely the former originating from the base of the Alvarezsauridae (sustained by Alvarezsaurus), and the latter within the Parvicursorinae. Analysis of the Alvarezsauria dentition reveals possible dental synapomorphies for the Alvarezsauria clade that should be tested in an integrative phylogenetic analysis. The general characterization of the forelimb and a partial reconstruction of the myology of alvarezsaurs demonstrate different configurations for Patagonykinae and Parvicursorinae. The multivariate analyzes carried out from the databases of Elissamburu and Vizcaíno, plus that of Cau and collaborators, show that the Patagonykinae would have had ranges of movements greater than those observed in Parvicursorinae, although the latter would have had a greater capacity to carry out more strenuous jobs. The morphometric analysis of the hindlimb and the use of the Snively and collaborators equations, show that the configuration of this element in Alvarezsauria is indicative of a highly cursorial lifestyle, as well as possible particular strategies for more efficient locomotion. The topology obtained in the phylogenetic analysis that was carried out in this Doctoral Thesis, allowed clarifying the ontogenetic changes observed in the ontogenetic series of the manual ungueal element II-2 within the clade Alvarezsauridae. In addition, the multivariate analysis carried out from the manual phalanx II-2 allows us to infer that alvarezsaurs could have performed functions such as hook-and-pull and piercing, where the arm would function as a single unit. The anatomy and myology of the alvarezsaurian tail show that the caudal vertebrae of alvarezsaurians exhibit a combination of derived osteological features that suggests functions unique among theropods, such as considerable dorsal and lateral movements, as well as exceptional abilities to support distal loading of their long tail without compromising stability and/or mobility.
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Parvicursor remotus Karhu & Rautian, 1996 Karhu, A. A. & Rautian, A. S. 1996. A new family of Maniraptora (Dinosauria: Saurischia) from the Late Cretaceous of Mongolia. Paleontological Journal, 30, 583–592. [Google Scholar], based on a fragmentary skeleton from the Campanian Barungoyot Formation at Khulsan, in the Gobi Desert of Mongolia, is re-described in detail. Contrary to the original description, we find no evidence that the last dorsal vertebra is opisthocoelous. A biconvex last dorsal vertebra could be present in Parvicursor as in other parvicursorines. The transversely compressed sacral centrum with a sharp ventral keel is the last sacral, not the first sacral as originally alleged. The pelvic bones are not fused at the acetabulum. The holotype of P. remotus, the smallest known alvarezsaurid specimen, is a juvenile individual far from skeletal maturity. New morphological details of Parvicursor that are revealed include a ventral ridge on the posterior dorsal vertebrae; a horizontal supraacetabular crest of the ilium whose anterior end is placed anterior to the most ventrally projecting part of the pubic peduncle; the possible presence of a pubic obturator foramen; an entocondylar tuber on the femur; internal flexor flanges on metatarsals II and IV; a lateral flexor ridge on metatarsal IV; and a proximoventral notch on pedal phalanx IV-1. The Tugriken Shireh alvarezsaurid is not referable to Parvicursor because of its much larger ungual phalanx on the second pedal digit. Kol ghuva is excluded from Alzarezsauridae. The two successive sister taxa for Parvicursor are Ceratonykus and Linhenykus. Ceratonykus differs little from Parvicursor and might be a synonym of the latter. Linhenykus is very similar to Parvicursor in all overlapping skeletal elements. The Parvicursorinae includes all Asian Late Cretaceous alvarezsaurids except Qiupanykus and Nemegtonykus.
Khulsanurus magnificus gen. et sp. nov., is described based on a partial skeleton, including cervical and caudal vertebrae, scapulocoracoids, humerus, and pubis from the Late Cretaceous (Campanian) Barungoyot Formation at Khulsan locality in Gobi Desert, Mongolia. The new taxon differs from other alvarezsaurids by a combination of characters that include cervicals lacking pleurocoels, carotid processes, and epipophyses, dorsoventrally thick and subtriangular in cross-section transverse process of the anterior caudals, prominent infrapostzygapophyseal fossa on the transverse process of anterior caudals, short and mostly anteriorly directed prezygapophyses of anterior caudals, neural arch of anterior caudal lacking the interzygapophyseal ridges and having a prominent dorsal depression around the anterior end of the neural spine. The new taxon retains a number of plesiomorphic traits: slightly convex posterior centrum condyle of anterior caudals, prominent anterior, and posterior bumps at the neurocentral junction in anterior caudals, neural arch of anterior caudals extending for the entire length of the centrum, and pubic foot and apron. Khulsanurus shares with Shuvuuia the deltopectoral crest of humerus that is continuous with the humeral head. The phylogenetic analysis placed Khulsanurus in the Parvicursorinae in a polytomy with Mononykus, Shuvuuia, Albinykus, and Xixianykus.
In this chapter we discuss methods for analyzing continuous traits, with an emphasis on those approaches that rely on explicit statistical models of evolution and incorporate genealogical information (ancestor–descendant or phylogenetic relationships). After discussing the roles of models and genealogy in evolutionary inference, we summarize the properties of commonly used models including random walks (Brownian motion), directional evolution, and stasis. These models can be used to devise null-hypothesis tests about evolutionary patterns, but it is often better to fit and compare models equally using information criteria and related approaches. We apply these methods to a published data set of dental measurements in a sequence of ancestor–descendant populations in the early primate Cantius , with the particular goal of determining the best-supported mode of evolutionary change in this lineage. We also assess a series of questions about the evolution of femoral dimensions in several clades of dinosaurs, including testing for a trend of increasing body size (Cope's Rule), testing for correlations among characters, and reconstructing ancestral states. Finally, we list briefly some additional models, approaches, and issues that arise in genealogically informed analyses of phenotypic evolution.
The Coelurosauria are a group of mostly feathered theropods that gave rise to birds, the only dinosaurians that survived the Cretaceous-Paleogene extinction event and are still found today. Between their first appearance in the Middle Jurassic up to the end Cretaceous, coelurosaurians were party to dramatic geographic changes on the Earth’s surface, including the breakup of the supercontinent Pangaea, and the formation of the Atlantic Ocean. These plate tectonic events are thought to have caused vicariance or dispersal of coelurosaurian faunas, influencing their evolution. Unfortunately, few coelurosaurian biogeographic hypotheses have been supported by quantitative evidence. Here, we report the first, broadly sampled quantitative analysis of coelurosaurian biogeography using the likelihood-based package BioGeoBEARS. Mesozoic geographic configurations and changes are reconstructed and employed as constraints in this analysis, including their associated uncertainties. We use a comprehensive time-calibrated coelurosaurian evolutionary tree produced from the Theropod Working Group phylogenetic data matrix. Six biogeographic models in the BioGeoBEARS package with different assumptions about the evolution of spatial distributions are tested against geographic constraints. Our results statistically favor the DIVALIKE+J and DEC+J models, which allow vicariance and founder events, supporting continental vicariance as an important factor in coelurosaurian evolution. Ancestral range estimation indicates frequent dispersal events via the Apulian route (connecting Europe and Africa during the Early Cretaceous) and the Bering land bridge (connecting North America and Asia during the Late Cretaceous). These quantitative results are consistent with commonly inferred Mesozoic dinosaurian dispersals and continental-fragmentationinduced vicariance events. In addition, we recognize the importance of Europe as a dispersal center and gateway in the Early Cretaceous, as well as other vicariance events such as those triggered by the disappearance of land bridges.
Insect eusociality is characterized by cooperative brood care, reproductive division of labour and multiple generations of adults within a colony. The morphological specializations of the different termite castes from Burmese amber were recently reported, indicating the termites possessed advanced sociality in the mid-Cretaceous. Unfortunately, all the reported Cretaceous termites are individually preserved, which does not cover the behaviours of the cooperative brood care and multiple generations of adults in the nests of the Cretaceous termites. Herein, we report three eusocial aggregations from colonies of the oldest known Stolotermitidae, Cosmotermes gen. nov., in 100 Ma mid-Cretaceous Burmese amber. One large aggregation, comprising 8 soldiers, 56 workers/pseudergates and 25 immatures of different instars, additionally presents the behaviours of cooperative brood care and overlapping generations. Furthermore, taphonomic evidence indicates Cosmotermes most probably dwelled in damp/rotting wood, which provides a broader horizon of the early societies and ecology of the eusocial Cosmotermes.
Dinosaur locomotion and biomechanics, especially of their pelvic girdles and hindlimbs, have been analyzed in numerous studies. However, detailed volumetric musculoskeletal models of their tails are rarely developed. Here, we present the first detailed three-dimensional volumetric reconstruction of the caudal epaxial and hypaxial musculature of the Late Jurassic sauropod Giraffatitan brancai, and highlight the importance and necessity of 3D modeling in musculoskeletal reconstructions. The tail of this basal macronarian is relatively short compared to diplodocids and other coexisting macronarians. The center of mass lies well in front of the hindlimbs, which support only ca. half the body weight. Still, our reconstruction suggests a total weight for the entire tail of ca. 2500 kg. We conclude that the hypaxial and tail-related hindlimb muscles (most specifically the M. caudofemoralis longus and its counterpart the M. ilioischiocaudalis) in Giraffatitan were well developed and robustly built, compensating for the shorter length of the M. caufodemoralis longus, the main hindlimb retractor muscle, in comparison with other sauropods. Our methodology allows a better-constrained reconstruction of muscle volumes and masses in extinct taxa, and thus force and weight distributions throughout the tail, than non-volumetric approaches.
Limb length, cursoriality and speed have long been areas of significant interest in theropod paleobiology, since locomotory capacity, especially running ability, is critical in the pursuit of prey and to avoid becoming prey. The impact of allometry on running ability, and the limiting effect of large body size, are aspects that are traditionally overlooked. Since several different non-avian theropod lineages have each independently evolved body sizes greater than any known terrestrial carnivorous mammal, ~1000kg or more, the effect that such large mass has on movement ability and energetics is an area with significant implications for Mesozoic paleoecology. Here, using expansive datasets that incorporate several different metrics to estimate body size, limb length and running speed, we calculate the effects of allometry on running ability. We test traditional metrics used to evaluate cursoriality in non-avian theropods such as distal limb length, relative hindlimb length, and compare the energetic cost savings of relative hindlimb elongation between members of the Tyrannosauridae and more basal megacarnivores such as Allosauroidea or Ceratosauridae. We find that once the limiting effects of body size increase is incorporated there is no significant correlation to top speed between any of the commonly used metrics, including the newly suggested distal limb index (Tibia + Metatarsus/ Femur length). The data also shows a significant split between large and small bodied theropods in terms of maximizing running potential suggesting two distinct strategies for promoting limb elongation based on the organisms’ size. For small and medium sized theropods increased leg length seems to correlate with a desire to increase top speed while amongst larger taxa it corresponds more closely to energetic efficiency and reducing foraging costs. We also find, using 3D volumetric mass estimates, that the Tyrannosauridae show significant cost of transport savings compared to more basal clades, indicating reduced energy expenditures during foraging and likely reduced need for hunting forays. This suggests that amongst theropods, hindlimb evolution was not dictated by one particular strategy. Amongst smaller bodied taxa the competing pressures of being both a predator and a prey item dominant while larger ones, freed from predation pressure, seek to maximize foraging ability. We also discuss the implications both for interactions amongst specific clades and Mesozoic paleobiology and paleoecological reconstructions as a whole.
In recent decades, intensive research on non-avian dinosaurs has strongly suggested that these animals were restricted to terrestrial environments1. Historical proposals that some groups, such as sauropods and hadrosaurs, lived in aquatic environments2,3 were abandoned decades ago4–6. It has recently been argued that at least some of the spinosaurids—an unusual group of large-bodied theropods of the Cretaceous era—were semi-aquatic7,8, but this idea has been challenged on anatomical, biomechanical and taphonomic grounds, and remains controversial9–11. Here we present unambiguous evidence for an aquatic propulsive structure in a dinosaur, the giant theropod Spinosaurus aegyptiacus7,12. This dinosaur has a tail with an unexpected and unique shape that consists of extremely tall neural spines and elongate chevrons, which forms a large, flexible fin-like organ capable of extensive lateral excursion. Using a robotic flapping apparatus to measure undulatory forces in physical models of different tail shapes, we show that the tail shape of Spinosaurus produces greater thrust and efficiency in water than the tail shapes of terrestrial dinosaurs and that these measures of performance are more comparable to those of extant aquatic vertebrates that use vertically expanded tails to generate forward propulsion while swimming. These results are consistent with the suite of adaptations for an aquatic lifestyle and piscivorous diet that have previously been documented for Spinosaurus7,13,14. Although developed to a lesser degree, aquatic adaptations are also found in other members of the spinosaurid clade15,16, which had a near-global distribution and a stratigraphic range of more than 50 million years14, pointing to a substantial invasion of aquatic environments by dinosaurs. Discovery that the giant theropod dinosaur Spinosaurus has a large flexible tail indicates that it was primarily aquatic and swam in a similar manner to extant tail-propelled aquatic vertebrates.
Xenarthra, one of the major clades of placentals, comprises two different lineages (sloths and anteaters, and armadillos) with extant representatives showing strongly different morphologies and life habits. Sloths are arboreal herbivores, anteaters are insectivores with digging/climbing abilities, and armadillos are terrestrial diggers with varied diets. The ulna is one of the forelimb elements that exhibits distinct morphological specializations for different abilities (e.g., digging and climbing). A sample of xenarthrans was analyzed in this work from a functional and ecological perspective, using 2-D geometric morphometry. The analyses performed were a Principal Components Analysis (PCA), a regression of the shape on the centroid size, and a PCA with the residuals from the regression. The first PCA shows that the morphospace is strongly influenced by differences in length of the olecranon with respect to the shaft between the three clades. Allometry was detected for the whole sample. In the residual PCA, the allometry-free morphospace allows the differentiation between the ecological categories of substrate preference: armadillos and giant anteaters (terrestrial) are located towards the negative side of PC1, while sloths and silky anteaters (arboreal) are located near the positive end, with collared anteaters (semiarboreal) placed near the center. The terrestrial taxa present a more robust diaphysis, and a comparatively long, diaphysis-aligned olecranon, while the arboreal taxa show a relatively long ulna with an anteriorly curved shaft and an anteriorly deflected carpal facet. The ulnar curvature has biomechanical implications in relation to the bone response to different loadings produced in the context of posture and locomotion in each substrate.
The gross dental morphology of the holotype of the theropod Sinraptor dongi from the Jurassic Shishugou Formation of China is comprehensively described. We highlight a combination of dental features that appear to be restricted to Sinraptor: i) crowns with denticulated mesial and distal carinae extending from the root, and an irregular surface texture on the enamel; ii) D- to salinon-shaped cross-sectional outline at the crown-base in mesialmost teeth; iii) mesial crowns with mesial carinae spiraling mesiolingually and lingually positioned longitudinal groove adjacent to the mesial carina; and iv) labiolingually compressed lateral teeth with weakly labially deflected distal carinae, flat to concave basocentral surfaces of the labial margins of the crowns, and horizontally elongated distal denticles showing short to well-developed interdenticular sulci. Using cladistic, multivariate, discriminant and cluster analyses, we demonstrate that the dentition of Sinraptor is relatively similar to that of ceratosaurids, megalosauroids and other allosauroids, and particularly close to that of Allosaurus. The dental anatomy of Sinraptor and Allosaurus, which mainly differs in the labiolingual compression of the lateral crowns and the number of premaxillary teeth, shows adaptations towards a predatory lifestyle, including premaxillary teeth capable of enduring bone-to-bone contact, and crowns with widely separated mesial and distal carinae capable of inflicting widely open wounds.
Daspletosaurus is a large tyrannosaurine found in upper Campanian deposits of Alberta and Montana. Although several large subadult and adult individuals of this taxon are known, only one juvenile individual, TMP 1994.143.1, has been identified. This specimen has played a key role in the idea that juvenile tyrannosaurid individuals are difficult to differentiate among species. Here the taxonomic affinity of TMP 1994.143.1 is reassessed in light of a juvenile tyrannosaurine postorbital recently discovered in the Dinosaur Park Formation of Alberta. Anatomical comparisons and phylogenetic analyses reveal that TMP 1994.143.1 is referable to the albertosaurine Gorgosaurus libratus, whereas the new postorbital belongs to a small juvenile Daspletosaurus. This taxonomic reassignment of TMP 1994.143.1 results in the juvenile ontogenetic stage of Daspletosaurus being known only from two isolated cranial elements. The new postorbital provides insights into early Daspletosaurus ontogeny, revealing that the cornual process developed earlier or faster than in other tyrannosaurids. Although some ontogenetic changes in the postorbital are found to be unique to Daspletosaurus, overall changes are most consistent with those of other large tyrannosaurines. Our results also show that diagnostic features develop early in ontogeny, such that juveniles of different tyrannosaurid species are easier to differentiate than previously thought.
Isolated theropod teeth are some of the most common fossils in the dinosaur fossil record and are continually reported in the literature. Recently developed quantitative methods have improved our ability to test the affinities of isolated teeth in a repeatable framework. But in most studies, teeth are diagnosed on qualitative characters. This can be problematic because the distribution of theropod dental characters is still poorly documented, and often restricted to one lineage. To help in the identification of isolated theropod teeth, and to more rigorously evaluate their taxonomic and phylogenetic potential, we evaluated dental features in two ways. We first analyzed the distribution of 34 qualitative dental characters in a broad sample of taxa. Functional properties for each dental feature were included to assess how functional similarity generates homoplasy. We then compiled a quantitative data matrix of 145 dental characters for 97 saurischian taxa. The latter was used to assess the degree of homoplasy of qualitative dental characters, address longstanding questions on the taxonomic and biostratigraphic value of theropod teeth, and explore the major evolutionary trends in the theropod dentition.
In smaller phylogenetic datasets for Theropoda, dental characters exhibit higher levels of homoplasy than non-dental characters, yet they still provide useful grouping information and optimize as local synapomorphies of smaller clades. In broader phylogenetic datasets, the degree of homoplasy displayed by dental and non-dental characters is not significantly different. Dental features on crown ornamentations, enamel texture, and tooth microstructure have significantly less homoplasy than other dental features and can be used to identify many theropod taxa to ‘family’ or ’sub-family’ level, and some taxa to genus or species. These features should, therefore, be a priority for investigations seeking to classify isolated teeth.
Our observations improve the taxonomic utility of theropod teeth and in some cases can help make isolated teeth useful as biostratigraphic markers. This proposed list of dental features in theropods should, therefore, facilitate future studies on the systematic paleontology of isolated teeth.
Alvarezsaurid diversity has been markedly increased by recent discoveries from China. However, the number of alvarezsaurid specimens in the Nemegt Formation of Mongolia remained low since the initial report on Mononykus olecranus in 1993. Here we report three new alvarezsaurid specimens from this formation, which were associated with each other and also with multiple oviraptorid skeletons in a small multi-species assemblage. Two of the alvarezsaurid specimens represent a new taxon, Nemegtonykus citus gen. et sp. nov., which is mainly distinguished from other alvarezsaurids by the first sacral vertebra with a subtrapezoidal lamina, the second sacral centrum which is directly co-ossified with ilium, the posterodorsally oriented postacetabular process of ilium, and partial co-ossification between metatarsals II and IV. The other specimen is very similar to M. olecranus in morphology and referred to cf. Mononykus sp. Our phylogenetic analysis recovered Nemegtonykus as a parvicursorine forming a polytomy with several other taxa from the Gobi Desert. The presence of three alvarezsaurid individuals in the same locality indicates that the abundance of alvarezsaurids have been greatly underestimated in the Nemegt dinosaur faunas.
Alvarezsaurian dinosaurs, a group of bizarre theropods with greatly shortened and modified forelimbs, are known mostly from the Cretaceous of Asia and South America. Here we report a new alvarezsaurian, Shishugounykus inexpectus gen. et sp. nov, based on a specimen recovered from the Middle–Upper Jurassic Shishugou Formation of the Junggar Basin, western China. Together with two other alvarezsaurians from this formation, i.e., Haplocheirus sollers and Aorun zhaoi, these Shishugou forms represent the only known Jurassic alvarezsaurians worldwide. Similar to the two other Shishugou alvarezsaurians, this new alvarezsaurian displays early stages in the development of the highly modified alvarezsaurian forelimb, but it possesses a number of manual features closer to the typical coelurosaurian theropod condition. Combining morphological and histological features, our analysis indicates that the earliest known alvarezsaurians are variable in size and other important morphological features, and in particular display a mosaic distribution of forelimb features.
The last two decades have seen a remarkable increase in the known diversity of basal avialans and their paravian relatives. The lack of resolution in the relationships of these groups combined with attributing the behavior of specialized taxa to the base of Paraves has clouded interpretations of the origin of avialan flight. Here, we describe Hesperornithoides miessleri gen. et sp. nov., a new paravian theropod from the Morrison Formation (Late Jurassic) of Wyoming, USA, represented by a single adult or subadult specimen comprising a partial, well-preserved skull and postcranial skeleton. Limb proportions firmly establish Hesperornithoides as occupying a terrestrial, non-volant lifestyle. Our phylogenetic analysis emphasizes extensive taxonomic sampling and robust character construction, recovering the new taxon most parsimoniously as a troodontid close to Daliansaurus, Xixiasaurus, and Sinusonasus. Multiple alternative paravian topologies have similar degrees of support, but proposals of basal paravian archaeopterygids, avialan microraptorians, and Rahonavis being closer to Pygostylia than archaeopterygids or unenlagiines are strongly rejected. All parsimonious results support the hypothesis that each early paravian clade was plesiomorphically flightless, raising the possibility that avian flight originated as late as the Late Jurassic or Early Cretaceous.
The Middle Jurassic is a largely mysterious interval in dinosaur evolution, as few fossils of this age are known worldwide. In recent years, the Isle of Skye has yielded a substantial record of trackways, and a more limited inventory of body fossils, that indicate a diverse fauna of Middle Jurassic dinosaurs living in and around lagoons and deltas. Comparatively little is known about the predators in these faunas (particularly theropod dinosaurs), as their fossils are among the rarest discoveries. We here report two new isolated theropod teeth, from the Valtos Sandstone Formation and Lealt Shale Formation of Skye, which we visualized and measured using high-resolution x-ray computed microtomographic scanning (µCT) and identified via statistical and phylogenetic analyses of a large comparative dental dataset. We argue that these teeth most likely represent at least two theropod species – one small-bodied and the other large-bodied – which likely belonged to one or several clades of basal avetheropods (ceratosaurs, megalosauroids, or allosauroids). These groups, which were diversifying during the Middle Jurassic and would become dominant in the Late Jurassic, filled various niches in the food chain of Skye, probably both on land and in the lagoons.
Synopsis
Tyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods.
Methods
To compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a ( p ) to stringently test the null hypothesis of equal agility.
Results
Tyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration).
Implications
The capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.
Ongoing studies of a multiple track-bearing horizons from massive excavations in the Jinju Formation (Lower Cretaceous) of South Korea have yielded a remarkable diversity of avian, nonavian dinosaur, pterosaur, crocodilian and mammal tracks, many very small and well preserved. Here we report diminutive, didactyl tracks (~1.0 cm long) assigned to a new dromaeosaurid ichnogenus Dromaeosauriformipes, which resembles the larger, but still quite small, ichnogenus Dromaeosauripus, also from the same formation only 30 km away. These diminutive tracks are consistent with the foot size of smaller dromaeosaurid taxa like Early Cretaceous Microraptor from China, and may represent diminutive species or juveniles of larger species. The association of tracks with lakeshore sediments is consistent with the evidence that Microraptor was a fish eater. Two trackways and isolated tracks indicate variable trackmaker gaits and speeds. If oviparous, as assumed for most non-avian dinosaur neonates, the trackmakers must have hatched from tiny eggs. Previous studies of the Korean Cretaceous indicate the presence of other diminutive (~1.0 cm long) theropod tracks (Minisauripus). Such occurrences strongly suggest that small tracks attributed to juveniles, or very small tetrapod species, are more common than previously supposed especially where suitable preservation conditions prevailed.
Since its original description as a feather belonging to a basal bird, the phylogenetic position of Praeornis
sharovi was debated. It was considered as belonging to a bird, a cycad leaf, or as a ‘transitional’ integumentary
structure between reptile scales and bird feathers. Recently, a basal enantiornithine bird was collected in
Early Cretaceous beds of Brazil. This specimen shows very well-preserved rachis-dominated tail feathers
with a very thick rachis and thick and rigid barbules. These features are present in Praeornis, suggesting
that this fossil may be interpreted as the tail feather of a basal bird. In this way, Praeornis constitutes one of
the oldest records of rachis-dominated feathers in the world. Rachis-dominated tail feathers, including that
of Praeornis appear to be rigid paired structures not performed for aerodynamical purposes, suggesting
that may be important in body balance.
Maniraptora includes birds and their closest relatives among theropod dinosaurs. During the Cretaceous period, several maniraptoran lineages diverged from the ancestral coelurosaurian bauplan and evolved novel ecomorphologies, including active flight, gigantism, cursoriality and herbivory. Propagation X-ray phase-contrast synchrotron microtomography of a well-preserved maniraptoran from Mongolia, still partially embedded in the rock matrix, revealed a mosaic of features, most of them absent among non-avian maniraptorans but shared by reptilian and avian groups with aquatic or semiaquatic ecologies. This new theropod, Halszkaraptor escuilliei gen. et sp. nov., is related to other enigmatic Late Cretaceous maniraptorans from Mongolia in a novel clade at the root of Dromaeosauridae. This lineage adds an amphibious ecomorphology to those evolved by maniraptorans: it acquired a predatory mode that relied mainly on neck hyperelongation for food procurement, it coupled the obligatory bipedalism of theropods with forelimb proportions that may support a swimming function, and it developed postural adaptations convergent with short-tailed birds.
Procoelous caudal vertebrae, a carpometacarpus with a hypertrophied metacarpal II, and robust proximal and ungual phalanges of manual digit II of a small theropod dinosaur from the Upper Cretaceous (Turonian) Bissekty Formation at Dzharakuduk, Uzbekistan, show unequivocal synapomorphies of the clade Alvarezsauridae and thus are referred to it. The caudal vertebrae have a unique longitudinal canal within the neural arch. The carpometacarpus, with metacarpal III occupying about one third of the width of the carpometacarpus, shows the most plesiomorphic stage of the evolution of the forelimb among known alvarezsaurids. The proximal phalanx of manual digit II differs from the corresponding bone in Parvicursorinae in having a less asymmetrical proximal articular surface without a dorsal process and short ventral ridges. The ungual phalanx of manual digit II has laterally open ventral foramina. The Bissekty alvarezsaurid possibly represents a basal parvicursorine and is the stratigraphically oldest known alvarezsaurid in Asia known to date.
ABSTRACT A new azhdarchoid pterosaur from the Upper Cretaceous of Patagonia is described. The material consists of an incomplete edentulous lower jaw that was collected from the upper portion of the Portezuelo Formation (Turonian-Early Coniacian) at the Futalognko site, northwest of Neuquén city, Argentina. The overall morphology of Argentinadraco barrealensis gen. et sp. nov. indicates that it belongs to the Azhdarchoidea and probable represents an azhdarchid species. The occlusal surface of the anterior portion is laterally compressed and shows blunt lateral margins with a medial sulcus that are followed by two well-developed mandibular ridges, which in turn are bordered laterally by a sulcus. The posterior end of the symphysis is deeper than in any other azhdarchoid. This unique construction of the lower jaw suggests the existence of an elaborate interlocking mechanism with the upper jaw. Furthermore, although speculative, it is advocated here that Argentinadraco barrealensis might have used the lower jaw to obtain its prey by cutting or ploughing through unconsolidated sediment in shallow waters, a feeding behavior not previously proposed for pterosaurs.
Speed is the fundamental constraint on animal movement, yet there is no general consensus on the determinants of maximum speed itself. Here, we provide a general scaling model of maximum speed with body mass, which holds across locomotion modes, ecosystem types and taxonomic groups. In contrast to traditional power-law scaling, we predict a hump-shaped relationship resulting from a finite acceleration time for animals, which explains why the largest animals are not the fastest. This model is strongly supported by extensive empirical data (474 species, with body masses ranging from 30 μg to 100 tonnes) from terrestrial as well as aquatic ecosystems. Our approach unravels a fundamental constraint on the upper limit of animal movement, thus enabling a better understanding of realized movement patterns in nature and their multifold ecological consequences. Maximum speed could scale with body mass, but the largest animals are not actually the fastest. A general scaling law explains this with a hump-shaped relationship due to a finite limit on acceleration time.
Animal species differ in the variability of their clutch sizes, as well as in mean clutch sizes. This phenomenon is particularly obvious in lizards, where virtually invariant clutch sizes have evolved independently in at least 23 lineages in seven families. Reduced variance in clutch size may arise either as an adaptation (because females with less variable clutch sizes have higher fitness) or as an indirect by-product of selection on other life-history characteristics. Comparative data on Australian scincid lizards indicate that variance in clutch sizes is lowest among species with low mean clutch sizes, small body sizes and a low variance in body sizes of adult females. Phylogenetic analysis shows that evolutionary decreases in the variance of clutch size have accompanied decreases in mean clutch sizes and decreases in the variance of adult female body sizes. Tropical lizards may also exhibit lower variance in clutch size. Most of these characteristics are correlated in occurrence, and may be allometrically tied to small body size. Hence, low variance in clutch size may be a consequence of allometric effects on a correlated suite of life-history characteristics. Exceptions to the general patterns noted above-especially, lizard species with invariant clutch sizes but large body sizes-may be due to loss of genetic variance for clutch sizes in lineages that have passed through a "bottleneck" of small body sizes and hence, low variance in clutch sizes.
Four new specimens of Anchiornis huxleyi (PKUP V1068, BMNHC PH804, BMNHC PH822, and BMNHC PH823) were recently recovered from the Late Jurassic fossil beds of the Tiaojishan Formation in northeastern China. These new specimens are almost completely preserved with cranial and postcranial skeletons. Morphological features of Anchiornis huxleyi have implications for paravian character evolution and provide insights into the relationships of major paravian lineages. Anchiornis huxleyi shares derived features with avialans, such as a straight nasal process of the premaxilla and the absence of an external mandibular fenestra in lateral view. However, Anchiornis huxleyi lacks several derived deinonychosaurian features, including a laterally exposed splenial and a specialized raptorial pedal digit II. Morphological comparisons strongly suggest Anchiornis is more closely related to avialans than to deinonychosaurians or troodontids. Anchiornis huxleyi exhibits many conservative paravian features, and closely resembles Archaeopteryx and other Jurassic paravians from Jianchang County, such as Xiaotingia and Eosinopteryx. The other Jianchang paravian, Aurornis xui, is likely a junior synonym of Anchiornis huxleyi.
A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism generating species diversity amongst dinosaurs, and perhaps beyond. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis. Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. However, given their close phylogenetic relationship, geographic proximity, and temporal succession, where D. torosus (~76.7?75.2 Ma) precedes the younger new species (~75.1?74.4 Ma), we argue that the two forms most likely represent a single anagenetic lineage. Daspletosaurus was an important apex predator in the late Campanian dinosaur faunas of Laramidia; its absence from later units indicates it was extinct before Tyrannosaurus rex dispersed into Laramidia from Asia. In addition to its evolutionary implications, the texture of the facial bones of the new taxon, and other derived tyrannosauroids, indicates a scaly integument with high tactile sensitivity. Most significantly, the lower jaw shows evidence for neurovasculature that is also seen in birds.
Se presentan nuevos restos provenientes de estratos campaniano-maastrichtianos de la Formación Alien, en Salitral Ojo de Agua (Río Negro, Argentina), los cuales son asignables a Alvarezsauridae indet. Este ciado de pequeños terópodos celurosaurios es conocido de estratos de edad similar en Mongolia y de sedimentitas más antiguas (de edad turoniana y coniaciana) en Argentina. De este modo, el material que aquí se informa extiende el registro temporal de los alvarezsáuridos del Hemisferio Sur hasta el Cretácico Tardío alto. Los huesos reportados estaban asociados a cascaras de huevo de la oofamilia Elongatoolithidae, los cuales son actualmente atribuidos a Theropoda.We present new remains from the Cam-panian-Maastrichtian beds of the Alien Formation, in Salitral Ojo de Agua (Río Negro, Argentina), which are assignable to Alvarezsauridae indet. This clade of small coelurosaurian theropods is known from strata of similar age in Mongolia, and from older sediments (Turonian and Coniacian in age) in Argentina. Thus, the material presented here extends the temporal record of the alvarezsaurids from the Southern Hemisphere up to the Latest Cretaceous. The fossil bones reported were associated to eggshells of the oofamily Elongatoolithidae, which are actually attributed to Theropoda.
Se estudiaron 13 dientes aislados de dinosaurios terópodos provenientes de dos yacimientos cercanos ubicados en la localidad de Paso Córdoba (General Roca, Rio Negro); en ambos casos, los dientes se encontraban en asociación con restos esqueletarios de saurópodos titanosaurios. Estos yacimientos fosilíferos corresponden a la Formación Allen (Campaniano-Maastrichtiano), concretamente a facies de interdunas secas. Mediante caracteres cualitativos y cuantitativos, se han identificado cuatro Morfotipos dentales, el primero asignado al clado Abelisauridae, el segundo a Tetanurae indeterminado, el tercero a Megaraptora, y el último a Theropoda indeterminado. Se efectuó un análisis de componentes principales y un análisis discriminante de los dientes con el objetivo de buscar una organización de los datos a través de las medidas efectuadas, y reducir las dimensiones o número de variables. Los resultados obtenidos fueron consistentes con las asignaciones sistemáticas realizadas a partir de parámetros tradicionales, y coherentes con los análisis filogenéticos vigentes. El Morfotipo 1 presenta los siguientes caracteres compartidos con Abelisauridae: A, margen distal recto en vista lateral; B, dentículos mesiales en forma de gancho; C, dentículos centrales de la carena distal tan altos como anchos en vista lateral; D, dentículos apicales de la carena mesial orientados apicalmente; E, carena mesial extendida hasta el cuello dentario; F, dentículos mesiales centrales tan altos como anchos en vista lateral; y G, dentículos centrales de la carena distal orientados apicalmente. El Morfotipo 2, no permite una asignación clara, pero podemos establecer que estos materiales carecen completamente de caracteres que permitan asignarlos al clado Ceratosauria. Los caracteres que unen al Morfotipo 3 con el clado Megaraptora son: A, contorno basal de la sección transversal de la corona en forma de D o de J; B, superficie cóncava en la cara lingual y adyacente a la carena mesial; y C, carena mesial esplazada mesio-labialmente o labialmente.
Locomotion has influenced the ecology, evolution, and extinction of species throughout history, yet studying locomotion in the fossil record is challenging. Computational biomechanics can provide novel insight by mechanistically relating observed anatomy to whole-animal function and behavior. Here, we leverage optimal control methods to generate the first fully predictive, three-dimensional, muscle-driven simulations of locomotion in an extinct terrestrial vertebrate, the bipedal non-avian theropod dinosaur Coelophysis. Unexpectedly, our simulations involved pronounced lateroflexion movements of the tail. Rather than just being a static counterbalance, simulations indicate that the tail played a crucial dynamic role, with lateroflexion acting as a passive, physics-based mechanism for regulating angular momentum and improving locomotor economy, analogous to the swinging arms of humans. We infer this mechanism to have existed in many other bipedal non-avian dinosaurs as well, and our methodology provides new avenues for exploring the functional diversity of dinosaur tails in the future.
Sustained miniaturization, here defined as a drop in body size of at least two orders of magnitude from ancestors to descendants, is a widespread and important phenomenon in animals, but among dinosaurs,
miniaturization occurred only rarely, once in the lineage leading to birds and once in the Alvarezsauroidea, one of the most bizarre theropod groups. Miniaturization and powered flight are intimately linked
in avialan theropods, but the causes and patterns of body size reduction are less clear in the nonvolant Alvarezsauroidea. Here, we present results from analyses on a comprehensive dataset, which
not only includes new data from early-branching alvarezsauroids but also considers the ontogenetic effect
based on histological data. Our analyses show that alvarezsauroid body mass underwent rapid miniaturization from around 110 to 85 mya and that there was a phylogenetic radiation of small-sized alvarezsauroids in
the Late Cretaceous. Our analyses also indicate that growth strategies were highly variable among alvarezsauroids, with significant differences among extremely small taxa. The suggested alvarezsauroid miniaturization and associated phylogenetic radiation are coincident with the emergence of ants and termites, and
combining previous functional morphological data, our study suggests that alvarezsauroid miniaturization
might have been driven by ecological changes during the Cretaceous Terrestrial Revolution, more specifically by a shift to the myrmecophagous ecological niche.
The discovery of theropod shed teeth associated with sauropod remains is relatively common in Cretaceous deposits of Patagonia. However, only a handful of studies have thoroughly explored the phylogenetic affinities of the theropod dental material. Here, we describe and identify twelve theropod shed teeth associated with a partially complete skeleton of a titanosaur sauropod from the Allen Formation (middle Campanian–lower Maastrichtian; Upper Cretaceous) of Paso Córdoba, Río Negro, Argentina. Using three methods, namely a cladistic analysis performed on a dentition-based data matrix, and a discriminant and cluster analyses conducted on a large dataset of theropod teeth measurements, we identify three dental morphotypes which are confidently referred to abelisaurid theropods. Whether the morphotypes represent different abelisaurid subclades or different positional entities within the jaw of the same abelisaurid species, is unknown. Such an identification, nevertheless, provides additional evidence of abelisaurids feeding on sauropod carcasses. This study highlights the importance of using combined qualitative and quantitative methodologies to identify isolated theropod teeth, especially those that can provide direct information on feeding ecology.
Revealing behavioral secrets in extinct species
Extinct species had complex behaviors, just like modern species, but fossils generally reveal little of these details. New approaches that allow for the study of structures that relate directly to behavior are greatly improving our understanding of the lifestyles of extinct animals (see the Perspective by Witmer). Hanson et al. looked at three-dimensional scans of archosauromorph inner ears and found clear patterns relating these bones to complex movement, including flight. Choiniere et al. looked at inner ears and scleral eye rings and found a clear emergence of patterns relating to nocturnality in early theropod evolution. Together, these papers reveal behavioral complexity and evolutionary patterns in these groups.
Science , this issue p. 601 , p. 610 ; see also p. 575
The deposits corresponding to the Upper Cretaceous Neuquén and San Jorge Gulf basins from northern and central Patagonia have provided two of the most complete sequences of terrestrial vertebrate faunas of all Gondwanan landmasses. Among the carnivorous components, the carcharodontosaurid theropods appeared as common elements during the Early Cretaceous and the earliest Late Cretaceous in northern and central Patagonia. Although recorded mostly in the lower Turonian, isolated teeth suggest their presence in younger strata in northern and central Patagonia, reaching the clade in the region as late as the early Maastrichtian. Here, we verify the assignment of such isolated teeth previously identified as belonging to Carcharodontosauridae from the Upper Cretaceous strata of northern and central Patagonia. Using three different methods, namely a cladistic analysis performed on a dentition-based data matrix, and discriminant and cluster analyses conducted on a large dataset of theropod crown measurements, we assign a tooth from Candeleros Formation to carcharodontosaurid theropods and teeth from Cerro Lisandro, Bajo Barreal, Portezuelo, Plottier and Allen formations to abelisaurid theropods. These new reappraisals provide additional evidence about the extinction of Carcharodontosauridae in South America at about the late Turonian–earliest Coniacian as part of a general faunistic turnover event, with the last clear evidence of this lineage in Patagonia coming from the early–middle Turonian.
The Alvarezsauridae comprise a clade of small-bodied theropod dinosaurs with highly-specialized skeletal adaptations, primarily reduced but proportionately robust forelimbs which have been hypothesized as relating to an insectivorous ecology. Alvarezsaurids are known predominantly from Mongolia, China, and South America, with a comparatively sparse North American record, where alvarezsaurid remains are among the most rarely recovered vertebrate fossils from the Upper Cretaceous Western Interior. North American alvarezsaurids include Albertonykus borealis from the lower Maastrichtian Horseshoe Canyon Formation of Alberta, Canada, known from a partial skeleton and additional isolated remains, and the recently named Trierarchuncus prairiensis from the upper Maastrichtian Hell Creek Formation of eastern Montana, USA, known previously from isolated specimens, including three manual D-I unguals, a partial radius, and a partial metatarsal III. Here we describe two new manual D-I unguals referable to the alvarezsaurid Trierarchuncus from the Hell Creek Formation of eastern Montana, USA. These new unguals contribute additional detail to the currently known ontogenetic series of Trierarchuncus manual D-I unguals and indicate the sequence and degree of change that occurred through growth within these alvarezsaurid hand claws. These changes appear to be modifications and reinforcement related to stresses induced from the hypothesized ecology of alvarezsaurids as insectivores which would tear apart substrates in search of insect prey, and as such can be characterized as ecologically-driven ontogenetic changes. Additionally, we demonstrate morphological indicators for determining whether isolated alvarezsaurid manual D-I unguals pertain to the right or left manus.
Gregarious behaviour among dinosaurs has been inferred from several lines of evidence (monospecific bonebeds, skeletal morphology, phylogenetic inferences, comparison with modern ecosystems and parallel trackways with particular characteristics), but is relatively poorly documented for non-avian theropods. Here, we report five parallel theropod trackways of large track size (average length of 28 cm) in the Cretaceous (Cenomanian) Candeleros Formation from northwestern Patagonia, Argentina. The tracks are provisionally assigned to aff. Asianopodus pulvinicalx and considering the autopod morphology of the theropod taxa documented in the Candeleros Formation, abelisaurid theropods are suggested as trackmakers. The trackways possess similar stride, speed estimation, direction and preservational features, and track show uniform depth and do not overlap. Physical barriers (i.e., large fluvial channels or perennial lake) that could influence the direction of the theropod trackmaker gaits were not recognized in the section. Taking into account these considerations, a gregarious behaviour for the abelisaurid theropod trackmakers is proposed. The tracks are preserved as shallow undertracks in a medium-grained sandstone bed deposited in a floodplain setting. The tracking surface is interpreted as the overlaying layer of muddy-siltstone. The occurrence of swelling clays and microbial mats in the track-bearing level could have contributed to the substrate stabilization and their role in the preservation is discussed.
New postcranial bones (cervical and caudal vertebrae, chevron, fibula) of the proceratosaurid tyrannosauroid Kileskus aristotocus from the Middle Jurassic (Bathonian) Itat Formation at Berezovsk coal mine, Krasnoyarsk Territory, Western Siberia, Russia, reveal three possible autapomorphies of Kileskus: a deep anterior recess between the neural canal and prezygapophysis; a distinct pit at the base of the neural spine on the middle cervical vertebrae; and a ventral groove on the middle caudal vertebrae. Phylogenetic analysis recovered a nearly horizontal posterior centrodiapophyseal lamina in the anterior-middle cervicals with the infrapostzygapophyseal fossa located primarily dorsally to the lamina as a new synapomorphy for the Proceratosauridae. The deep oval fossa on the medial surface of the fibula with well-defined margins is a tyrannosauroid synapomorphy of Kileskus. In other Proceratosauridae the fibula is either absent or not described in detail. The longitudinal and reticular vascularization of the primary cortex in the Kileskus tibia suggests a slower growth rate compared with large-bodied tyrannosaurids.
Phylogenetic relationships of megaraptorid theropods are under intense debate. Some authors interpret them as archaic allosauroids that survived up to Late Cretaceous, whereas others consider megaraptorids as basal tyrannosauroids. The recently described Patagonian taxon Murusraptor barrosaensis offers novel information on skull, axial and hind limb anatomy, all of which may help in elucidating the phylogenetic affinities of megaraptorids as a whole. Murusraptor is particularly similar to juvenile specimens of tyrannosaurids; both share: 1) lacrimal with a long anterior process; 2) corneal process and; 3) lateral pneumatic fenestra; 4) square and dorsoventrally low frontals; 5) parietals with well-developed sagittal and nuchal crests, among other features. The current study lends further support to the hypothesis that megaraptorans are basal members of Coelurosauria (supported by 20 synapomophies), with strongest affiliation with Tyrannosauroidea (supported by >20 synapomorphies).
Graphical Abstract Highlights d Two new alvarezsaurian dinosaurs are described from Northwest China d They are intermediate between Late Jurassic and Late Cretaceous alvarezsaurians d They showcase the evolution of highly specialized alvarezsaurian forelimb d Specialized alvarezsaurian forelimb morphology evolved slowly, in a mosaic fashion In Brief Xu et al. report two new Early Cretaceous alvarezsaurian theropods representing transitional stages in alvarezsaurian evolution. The analyses indicate that the evolutionary transition from a typical theropod forelimb configuration to a highly specialized one was slow and occurred in a mosaic fashion during the Cretaceous.
Functional reconstructions of extinct animals represent a crucial step towards understanding palaeocological interactions, selective pressures and macroevolutionary patterns in the fossil record. In recent years, computational approaches have revolutionised the field of 'evolutionary biomechanics' and have, in general, resulted in convergence of quantitative estimates of performance on increasingly narrow ranges for well studied taxa. Studies of body mass and locomotor performance of Tyrannosaurus rex - arguably the most intensively studied extinct animal - typify this pattern, with numerous independent studies predicting similar body masses and maximum locomotor speeds for this animal. In stark contrast to this trend, recent estimates of maximum bite force in T. rex vary considerably (> 50%) despite use of similar quantitative methodologies. Herein we demonstrate that the mechanistic causes of these disparate predictions are indicative of important and underappreciated limiting factors in biomechanical reconstructions of extinct organisms. Detailed comparison of previous models of T. rex bite force reveals that estimations of muscle fibre lengths and architecture are the principal source of disagreement between studies, and therefore that these parameters represents the greatest source of uncertainty in these reconstructions, and potentially therefore extinct animals generally. To address the issue of fibre length and architecture estimation in extinct animals we present data tabulated from the literature of muscle architecture from over 1100 muscles measured in extant terrestrial animals. Application of this dataset in a reanalysis of T. rex bite force emphasises the need for more data on jaw musculature from living carnivorous animals, alongside increased sophistication of modelling approaches. In the latter respect we predict that implementing limits on skeletal loading into musculoskeletal models will narrow predictions for T. rex bite force by excluding higher-end estimates.
An alvarezsaurid dinosaur skeleton was discovered from the Late Cretaceous Qiupa Formation of Luanchuan, Henan Province of central China. It represents a new alvarezsaurid dinosaur Qiupanykus zhangi gen. et sp. nov. A phylogenetic analysis recovers Qiupanykus nested within the unresolved clade, which includes Asian and north American taxa. The skeleton of the new specimen is preserved in association with eggshells. The eggshell morphologies show that these eggs belong to oviraptorid eggs, skeletal remains of which were discovered from the same area. The alvarezsaurid skeleton associated with eggshell fragments may indicate that these eggs were broken by the strong thumb-claws of the former and that alvarezsaurid dinosaurs may be egg-eaters.
A spectacular pair of Sinosauropteryx skeletons from Jurassic-Cretaceous strata of Liaoning in northeastern China attracted worldwide notoriety in 1996 as the first dinosaurs covered with feather-like structures. Sinosauropteryx prima is important not only because of its integument, but also because it is a basal coelurosaur and represents an important stage in theropod evolution that is poorly understood. Coelurosauria, which includes (but is not limited to) dromaeosaurids, ornithomimosaurs, oviraptorosaurs, troodontids, and tyrannosaurids, formed the most important radiation of Cretaceous carnivorous dinosaurs in the Northern Hemisphere. It also includes Aves. Sinosauropteryx prima has a number of characters that were poorly preserved in known specimens of the closely related Compsognathus longipes from Europe. These include the longest tail known for any theropod and a three-fingered hand dominated by the first digit, which is longer and thicker than either of the bones of the forearm. Both specimens have a thick coat of feather-like structures, which seem to be simple branching structures. The claim that one skeleton of Sinosauropteryx has preserved the shape of the liver is unsupportable, if only because the fossil had collapsed into a single plane, which would have distorted any soft, internal organs.
We describe Tratayenia rosalesi gen. et sp. nov., a new megaraptoran theropod dinosaur from the Upper Cretaceous of Patagonia, Argentina. The holotype consists of a well-preserved, mostly articulated series of dorsal and sacral vertebrae, two partial dorsal ribs, much of the right ilium, and pubis and ischium fragments. It was found in a horizon of the Upper Cretaceous (Santonian) Bajo de la Carpa Formation of the Neuquén Group in the Neuquén Basin exposed near the town of Añelo in Neuquén Province of northwestern Patagonia. Phylogenetic analysis recovers Tratayenia within the Gondwanan megaraptoran subclade Megaraptoridae. The new taxon exhibits similarities to other megaraptorids such as Aerosteon riocoloradensis, Megaraptor namunhuaiquii, and Murusraptor barrosaensis, but also presents differences in the architecture of the dorsal and sacral vertebrae and the morphology of the ilium. Tratayenia is the first megaraptoran that unequivocally preserves the complete sequence of sacral vertebrae, thereby increasing knowledge of the osteology of the clade. Moreover, depending on the chronostratigraphic ages of the stratigraphically controversial megaraptorids Aerosteon and Orkoraptor burkei, as well as the phylogenetic affinities of several fragmentary specimens, the new theropod may be the geologically youngest megaraptorid or megaraptoran yet discovered. Tratayenia is also the largest-bodied carnivorous tetrapod named from the Bajo de la Carpa Formation, reinforcing the hypothesis that megaraptorids were apex predators in southern South America from the Turonian through the Santonian or early Campanian, following the extinction of carcharodontosaurids.
Birds evolved from and are phylogenetically recognized as members of the theropod dinosaurs; their first known member is the Late Jurassic Archaeopteryx, now represented by seven skeletons and a feather, and their closest known non-avian relatives are the dromaeosaurid theropods such as Deinonychus. Bird flight is widely thought to have evolved from the trees down, but Archaeopteryx and its outgroups show no obvious arboreal or tree-climbing characters, and its wing planform and wing loading do not resemble those of gliders. The ancestors of birds were bipedal, terrestrial, agile, cursorial and carnivorous or omnivorous. Apart from a perching foot and some skeletal fusions, a great many characters that are usually considered ‘avian’ (e.g. the furcula, the elongated forearm, the laterally flexing wrist and apparently feathers) evolved in non-avian theropods for reasons unrelated to birds or to flight. Soon after Archaeopteryx, avian features such as the pygostyle, fusion of the carpometacarpus, and elongated curved pedal claws with a reversed, fully descended and opposable hallux, indicate improved flying ability and arboreal habits. In the further evolution of birds, characters related to the flight apparatus phylogenetically preceded those related to the rest of the skeleton and skull. Mesozoic birds are more diverse and numerous than thought previously and the most diverse known group of Cretaceous birds, the Enantiornithes, was not even recognized until 1981. The vast majority of Mesozoic bird groups have no Tertiary records: Enantiornithes, Hesperornithiformes, Ichthyornithiformes and several other lineages disappeared by the end of the Cretaceous. By that time, a few Linnean ‘Orders’ of extant birds had appeared, but none of these taxa belongs to extant ‘families’, and it is not until the Paleocene or (in most cases) the Eocene that the majority of extant bird ‘Orders’ are known in the fossil record. There is no evidence for a major or mass extinction of birds at the end of the Cretaceous, nor for a sudden ‘bottleneck’ in diversity that fostered the early Tertiary origination of living bird ‘Orders’.
Neotheropoda includes the vast majority of the predatory dinosaurs and their oldest members are Late Triassic in age. The Triassic neotheropod record is restricted to North America and Europe with the exception of a few specimens from South America, which includes Zupaysaurus rougieri and Lucianovenator bonoi. Here, the South American record of the group is enriched with the description of the new genus and species Powellvenator podocitus from the middle Norian Los Colorados Formation (Ischigualasto-Villa Unión Basin) of northwestern Argentina. The new taxa are represented by previously undescribed partial hindlimbs collected by J. F. Bonaparte and associated to the hypodigm of the pseudosuchian Riojasuchus tenuisceps. In addition, a specimen originally interpreted by Bonaparte in 1972 as an indeterminate coelurosaur is here referred to the new species. Powellvenator podocitus differs from other basal dinosaurs in character-states that include an astragalus with a distinctly sigmoid posterodorsal margin and a rounded dorsal expansion on the anteromedial portion of the astragalar body in anterior view, calcaneum with a laterally projected flange, and strongly reduced shaft of metatarsal II. The phylogenetic relationships of the new species were tested in a comprehensive analyses focused on early neotheropods, which recovered Powellvenator podocitus within Coelophysoidea and as the sister-taxon of Procompsognathus triassicus and Coelophysinae. Powellvenator podocitus represents the first coelophysoid from South America together with Lucianovenator bonoi from the Marayes-El Carrizal Basin. © 2017 Asociacion Paleontologica Argentina. All Rights Reserved.
The largest known dinosaurs weighed at least 20 million times as much as the smallest, indicating exceptional phenotypic divergence. Previous studies have focused on extreme giant sizes, tests of Cope's rule, and miniaturization on the line leading to birds. We use non-uniform macroevolutionary models based on Ornstein–Uhlenbeck and trend processes to unify these observations, asking: what patterns of evolutionary rates, directionality and constraint explain the diversification of dinosaur body mass? We find that dinosaur evolution is constrained by attraction to discrete body size optima that undergo rare, but abrupt, evolutionary shifts. This model explains both the rarity of multi-lineage directional trends, and the occurrence of abrupt directional excursions during the origins of groups such as tiny pygostylian birds and giant sauropods. Most expansion of trait space results from rare, constraint-breaking innovations in just a small number of lineages. These lineages shifted rapidly into novel regions of trait space, occasionally to small sizes, but most often to large or giant sizes. As with Cenozoic mammals, intermediate body sizes were typically attained only transiently by lineages on a trajectory from small to large size. This demonstrates that bimodality in the macroevolutionary adaptive landscape for land vertebrates has existed for more than 200 million years.
Tail anatomy of unenlagiids is nearly unknown. The aim of the present contribution is to describe in detail the tail of the unenlagiid Buitreraptor gonzalezorum, and to compare it with Archaeopteryx and other paravians. A detailed specimen overview yielded a large number of similarities between Archaeopteryx and Buitreraptor, including long postzygapophyses and lateral laminae and concavities at mid-vertebral centra. The differences in caudal vertebrae morphology along the tail indicate three differentiable functional regions. This contrasts with the presence of only two in other basal paravians, including dromaeosaurids, such as Deinonychus. This contribution sheds light on paravian tail evolution, and provides new data to on the changes in tail morphology that occurred along the theropod line to modern birds.
Forelimb reduction occurred independently in multiple lineages of theropod dinosaurs. Although tyrannosaurs are renowned for their tiny, two-fingered forelimbs, the degree of their reduction in length is surpassed by abelisaurids, which possess an unusual morphology distinct from that of other theropods. The forelimbs of abelisaurids are short but robust and exhibit numerous crests, tubercles, and scars that allow for inferences of muscle attachment sites. Phylogenetically based reconstructions of the musculature were used in combination with close examination of the osteology in the Malagasy abelisaurid Majungasaurus to create detailed muscle maps of the forelimbs, and patterns of the muscular and bony morphology were compared with those of extant tetrapods with reduced or vestigial limbs. The lever arms of muscles crossing the glenohumeral joint are shortened relative to the basal condition, reducing the torque of these muscles but increasing the excursion of the humerus. Fusion of the antebrachial muscles into a set of flexors and extensors is common in other tetrapods and occurred to some extent in Majungasaurus. However, the presence of tubercles on the antebrachial and manual elements of abelisaurids indicates that many of the individual distal muscles acting on the wrist and digits were retained. Majungasaurus shows some signs of the advanced stages of forelimb reduction preceding limb loss, while also exhibiting features suggesting that the forelimb was not completely functionless. The conformation of abelisaurid forelimb musculature was unique among theropods and further emphasizes the unusual morphology of the forelimbs in this clade.
Fossils and casts of forelimb bones of the dromaeosaurids Deinonychus antirrhopus and Bambiraptor feinbergi were manually manipulated to determine range of motion and to test functional hypotheses. Shoulder motion in Bambiraptor resembles that found by a previous study on Deinonychus. The humerus can be retracted and elevated to subhorizontal positions and protracted somewhat beyond the vertical. In both taxa, the elbow can be strongly flexed but cannot be fully extended. Supination and pronation cannot occur by movement of the radius, which is immobile relative to the ulna. The palms therefore face medially except during wrist extension, which causes obligatory supination. The fingers of Deinonychus remain spread during flexion. In contrast, torsion of the distal articular surface of metacarpal I and the long axis of phalanx III-3 cause the first and third digits of Bambiraptor to approach each other during flexion, the first known instance of opposable fingers in a dinosaur. The morphology and range of motion in the forelimbs of Deinonychus and Bambiraptor enable two-handed prehension with the wrist flexed, one-handed clutching of objects to the chest, use of the hand as a hook, arm-swinging or -raising displays, and use of the forelimbs to maintain balance. Feathered wings, if present, precluded manual apprehension of objects on the ground, two-handed clutching of objects to the chest, and use of digit II to probe crevices. The forelimbs could not be used to dig. Opposability of the fingers of Bambiraptor enabled one-handed prehension, whereas Deinonychus required both hands to hold objects.