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Alvarezsauroids are an enigmatic clade of predominantly small-bodied theropod dinosaurs that are known mainly from the Jurassic to Cretaceous periods of Asia and South America1–3. Late Cretaceous alvarezsauroids possess specialized forelimbs adapted for digging4,5, minute supernumerary teeth and heightened sensory capacities6, and are interpreted as myrmecophagous. They are hypothesized to exhibit evolutionary miniaturization coupled to their dietary specialization2. Fragmentary South American taxa are traditionally arrayed as a paraphyletic grade with respect to the Late Cretaceous Asian subclade Parvicursorinae2,3, invoking dispersal to explain their disjunct distributions. Here we describe a skeleton of the alvarezsauroid Alnashetri cerropoliciensis7 representing to our knowledge the most complete and smallest South American taxon to date. We also recognize two alvarezsauroids among historic taxa from the Northern Hemisphere. Phylogenetic analysis recovers Alnashetri among basal non-alvarezsaurids rendering South American taxa polyphyletic. Combined with the new taxa recognized here, our biogeographical analyses infer a Pangaean ancestral distribution for Alvarezsauroidea, with vicariance dominating the early history of the clade. The early branching position of Alnashetri among larger-bodied relatives revises best-fit models of body size evolution in alvarezsauroids—we find no support for evolutionary miniaturization but, rather, find support for repeated evolution within a narrow body size range. 

Abstract Feathers are complex structures exhibiting structural/functional disparity across species and plumage. Flight was lost in >30 extant lineages from ~79.58 Ma–15 Ka. Effects of flight loss on senses, neuroanatomy, and skeletomusculature are known. To study how flightlessness affects feathers, we measured 11 feather metrics across the plumage of 30 flightless taxa and their phylogenetically closest volant taxa, with broader sampling of primaries across all orders of crown birds. Our sample includes 27 independent flight losses, representing nearly half of extant flightless species. Feather asymmetry measured by barb angle differences between trailing and leading vanes decreases in flightless lineages, most prominently in flight feathers and weakest in contour feathers. Greatest changes in feather anatomy occur in older flightless lineages (penguins, ratites). Comparative methods show that many microscopic feather traits are not dramatically modified after flightlessness compared to body mass increase and relative wing and tail fan reduction. Changes involved with greater vane symmetry show stronger shifts, however. Relaxing selection for flight does not rapidly modify feather flight adaptations, apart from asymmetry. Developmental constraints and relaxed selection for novel feather morphologies may explain some observed changes. Macroscopic changes to flight apparati (skeletomusculature, airfoil size) are more evident in recently flightless taxa and could more reliably detect flightlessness in fossils, with increased feather symmetry as a potential microscopic signal. We observed apical modification in later stages of feather development (asymmetric displacement of barb loci), while morphologies arising during early developmental stages are only altered after millions of years of flightlessness. 

The first fossil eggshell from the Cenomanian-age Mussentuchit Member of the Cedar Mountain Formation was described over fifty years ago. In the half-century since, oodiversity of this rock unit has been limited to a single, taxonomically unstable ootaxon, currently formulated asMacroelongatoolithus carlylei. Recently, there has been a renewed effort to recover and describe the macrofauna of the Mussentuchit; however, these advances are limited to the body fossil record. Here, we examine the range of eggshells present in the Mussentuchit Member and assess the preserved biodiversity they represent. Gross morphological and microstructural inspection reveals a greater diversity of eggshells than previously described. We identify six ootaxa: three Elongatoolithidae oogenera (Macroelongatoolithus,Undulatoolithus,Continuoolithus), eggs laid by oviraptorosaur dinosaurs; two oospecies ofSpheroolithuslaid by ornithopod dinosaurs; andMycomorphoolithus kohringi, laid by a crocodylomorph. The diversity of Elongatoolithidae in the Mussentuchit requires a co-occurrence of at least three putative oviraptorosaurs, the oldest such phenomenon in North America. The occurrence of the crocodylomorph oogenusMycomorphoolithusis the first recognized occurrence outside of Europe, and the youngest yet documented. This new ooassemblage is more representative of the known paleobiodiversity of Cenomanian-age strata of Western North America and complements the body fossil record in improving our understanding of this crucial—yet poorly documented—timeslice within the broader evolution of the Cretaceous Western Interior Basin. 

Abstract Thescelosaurines are a group of early diverging, ornithischian dinosaurs notable for their conservative bauplans and mosaic of primitive features. Although abundant within the latest Cretaceous ecosystems of North America, their record is poor to absent in earlier assemblages, leaving a large gap in our understanding of their evolution, origins, and ecological roles. Here we report a new small bodied thescelosaurine—Fona herzogaegen. et sp. nov.—from the Mussentuchit Member of the Cedar Mountain Formation, Utah, USA.Fona herzogaeis represented by multiple individuals, representing one of the most comprehensive skeletal assemblages of a small bodied, early diverging ornithischian described from North America to date. Phylogenetic analysis recoversFonaas the earliest member of Thescelosaurinae, minimally containingOryctodromeus, and all three species ofThescelosaurus, revealing the clade was well‐established in North America by as early as the Cenomanian, and distinct from, yet continental cohabitants with, their sister clade, Orodrominae. To date, orodromines and thescelosaurines have not been found together within a single North American ecosystem, suggesting different habitat preferences or competitive exclusion. Osteological observations reveal extensive intraspecific variation across cranial and postcranial elements, and a number of anatomical similarities withOryctodromeus, suggesting a shared semi‐fossorial lifestyle. 

Taphonomic processes create bias in the fossil record, and understanding these processes is integral to interpreting the record of extinct life worldwide. Bones preserved in fluvial environments make up a substantial part of the vertebrate fossil record. These bones have often been transported varying distances from the location of death before becoming buried. Experiments in flumes and natural settings have explored the fluvial taphonomy of mammal skeletons, but the taphonomy of other terrestrial vertebrates, especially extinct clades, has only been sparingly studied directly. Hadrosauroids are a dinosaur clade known from extensive remain throughout the Cretaceous and across the globe, making them an ideal group for taphonomic study. Previous examinations regarding the fluvial taphonomy of their skeletons have often applied bone transport groups derived from classic studies on mammals. Some researchers have raised concerns that the morphologies of non-mammalian bones would not exhibit the same hydraulic properties as mammals, producing different transport patterns. Here, we investigate hadrosauroid bone transport under various flow conditions through actualistic flume experiments using 3d printed models with comparable densities to real bone. We aimed to characterize the timing of transport of different elements (Voorhies Groups), orientation of bones relative to flow direction, and bone surface abrasion patterns. Some elements behave similarly to those described in mammals. As would be expected from previous work, relatively heavy bones such as the femur tend to move last, acting as lag elements. Lighter elements such as the scapula and radius tended to begin moving at much lower flow speeds. Because dinosaur pelvic bones are not fused as in mammals, we observed that the isolated pubis is often among the first elements to commence movement, often rotating or sliding along the bed. Cylindrical limb bones tend to roll or slide along the bed, orienting to be parallel to flow faster or slower depending on element size and flow velocity. Bones with more complex shapes, such as the curved and concave blade of the scapula, moved in less straightforward and unique ways, even vaulting over other bones. We also found that burial by fine silt and mud could be achieved relatively quickly even at slower flow speeds, and burial by sand played an important part in inhibiting transport in higher flow regimes.