Elastic symmetries of defective crystals

I construct discrete and continuous crystal structures that are compatible with a given choice of dislocation density tensor, and (following Mal’cev) provide a canonical form for these discrete structures. The symmetries of the discrete structures extend uniquely to symmetries of corresponding continuous structures—I calculate these symmetries explicitly for a particular choice of dislocation density tensor and deduce corresponding constraints on energy functions which model defective crystals

A classification of the symmetries of uniform discrete defective crystals

Crystals which have a uniform distribution of defects are endowed with a Lie group description which allows one to construct an associated discrete structure. These structures are in fact the discrete subgroups of the ambient Lie group. The geometrical symmetries of these structures can be computed in terms of the changes of generators of the discrete subgroup which preserve the discrete set of points. Here a classification of the symmetries for the discrete subgroups of a particular class of three-dimensional solvable Lie group is presented. It is a fact that there are only three mathematically distinct types of Lie groups which model uniform defective crystals, and the calculations given here complete the discussion of the symmetries of the corresponding discrete structures. We show that those symmetries corresponding to automorphisms of the discrete subgroups extend uniquely to symmetries of the ambient Lie group and we regard these symmetries as (restrictions of) elastic deformations of the continuous defective crystal. Other symmetries of the discrete structures are classified as ‘inelastic’ symmetries

Complete Ichthyornis skull illuminates mosaic assembly of the avian head

The skull of living birds is greatly modified from the condition found in their dinosaurian antecedents. Bird skulls have an enlarged, toothless premaxillary beak and an intricate kinetic system that includes a mobile palate and jaw suspensorium. The expanded avian neurocranium protects an enlarged brain and is flanked by reduced jaw adductor muscles. However, the order of appearance of these features and the nature of their earliest manifestations remain unknown. The Late Cretaceous toothed bird Ichthyornis dispar sits in a pivotal phylogenetic position outside living groups: it is close to the extant avian radiation but retains numerous ancestral characters 1-3. Although its evolutionary importance continues to be affirmed 3-8, no substantial new cranial material of I. dispar has been described beyond incomplete remains recovered in the 1870s. Jurassic and Cretaceous Lagerstatten have yielded important avialan fossils, but their skulls are typically crushed and distorted 9. Here we report four three-dimensionally preserved specimens of I. dispar- including an unusually complete skull-as well as two previously overlooked elements from the Yale Peabody Museum holotype, YPM 1450. We used these specimens to generate a nearly complete three-dimensional reconstruction of the I. dispar skull using highresolution computed tomography. Our study reveals that I. dispar had a transitional beak-small, lacking a palatal shelf and restricted to the tips of the jaws-coupled with a kinetic system similar to that of living birds. The feeding apparatus of extant birds therefore evolved earlier than previously thought and its components were functionally and developmentally coordinated. The brain was relatively modern, but the temporal region was unexpectedly dinosaurian: it retained a large adductor chamber bounded dorsally by substantial bony remnants of the ancestral reptilian upper temporal fenestra. This combination of features documents that important attributes of the avian brain and palate evolved before the reduction of jaw musculature and the full transformation of the beak. </p

A new large-bodied oviraptorosaurian theropod dinosaur from the Latest Cretaceous of Western North America

The oviraptorosaurian theropod dinosaur clade Caenagnathidae has long been enigmatic due to the incomplete nature of nearly all described fossils. Here we describe Anzu wyliei gen. et sp. nov., a new taxon of large-bodied caenagnathid based primarily on three well-preserved partial skeletons. The specimens were recovered from the uppermost Cretaceous (upper Maastrichtian) Hell Creek Formation of North and South Dakota, and are therefore among the stratigraphically youngest known oviraptorosaurian remains. Collectively, the fossils include elements from most regions of the skeleton, providing a wealth of information on the osteology and evolutionary relationships of Caenagnathidae. Phylogenetic analysis reaffirms caenagnathid monophyly, and indicates that Anzu is most closely related to Caenagnathus collinsi, a taxon that is definitively known only from a mandible from the Campanian Dinosaur Park Formation of Alberta. The problematic oviraptorosaurs Microvenator and Gigantoraptor are recovered as basal caenagnathids, as has previously been suggested. Anzu and other caenagnathids may have favored well-watered floodplain settings over channel margins, and were probably ecological generalists that fed upon vegetation, small animals, and perhaps eggs

Birds in Cretaceous Ecosystems

At least three ecological types are distinguished among the known Cretaceous birds: piscivores, shore birds and terrestrial birds. Striking rarity of terrestrial birds is considered as a special case of the rarity of smaller (1—10 kg) among the medium-sized vertebrate specimens in the Cretaceous record. This is probably caused, inter alia, by reptilian scavengers which swallow food items as large as possible, and decalcify bones completely or nearly so. Therefore, the smallest chance for preservation would be for those animals which are small enough to be swallowed whole and large enough to be well detectable and/or accessible. The great abundance of large and flightless piscivorous birds (Hesperornithes) in the warm seas of Western Interior contrasts with the lack of comparable forms in the Cenozoic warm seas. The extinction of toothed birds may have been caused by the explosive radiation of acanthopterygian fishes

A new genus and species for the largest specimen of Archaeopteryx

The Solnhofen (Sixth) specimen of Archaeopteryx is assigned to Wellnhoferia grandis gen. et sp. n. on the basis of qualitative, size-independent autapomorphies. Wellnhoferia differs from Archaeopteryx in a short tail with the estimated number of 16-17 caudals; a nearly symmetric pattern of pedal rays II-IV with metatarsals II and IV of equal length and digit IV substantially shorter than in Archaeopteryx; and the number of four (instead of five) phalanges of pedal digit IV, which most probably results from a phylogenetic reduction rather than individual variation. A combination of large size and details of the pelvic limb suggests a locomotor specialization different from that of Archaeopteryx.Solnhofeński, czyli szósty okaz praptaków przeniesiony zostaje z Archaeopteryx lithographica von Meyer, 1861 do Wellnhoferia grandis gen. et sp. n. na podstawie autapomorfii niezależnych od wielkości ciała, w tym cech jakościowych, merystycznych i proporcji. Okaz ten różni się od okazów Archaeopteryx krótszym ogonem o szacunkowej liczbie kręgów 16-17, bardziej symetryczną budową stopy z kośćmi sródstopia II i IV jednakowej długości i palcem IV zbliżonym długością do palca II, a więc znacznie krótszym niż u Archaeopteryx, i składającym się z 4 zamiast 5 paliczków. Taka budowa stopy przy stosunkowo dużych rozmiarach ciała sugeruje również nieco inną niż u Archaeopteryx, prawdopodobnie bardziej kursorialną, specjalizację lokomototyczną