Luis M. Chiappe

The origin and early evolution of birds

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’.

Sauropod dinosaur embryos from the Late Cretaceous of Patagonia

Definitive non-avian dinosaur embryos, those contained inside fossil eggs, are rare,. Here we describe the first known unequivocal embryonic remains of sauropod dinosaurs—the only known non-avian dinosaur embryos from Gondwana—from a nesting ground in the Upper Cretaceous stage of Patagonia, Argentina. At this new site, Auca Mahuevo (Fig. 1), thousands of eggs are distributed over an area greater than 1 km2. The proportion of eggs containing embryonic remains is high: over a dozen in situ eggs and nearly 40 egg fragments encasing embryonic material were recovered. In addition to bone, these specimens contain large patches of fossil skin casts, the first definitive portions of integument ever reported for a non-avian dinosaur embryo. As morphology of the eggs enclosing these osseous and integumentary remains is identical, we propose that these specimens belong to the same sauropod species. This discovery allows the confident association of the megaloolithid type of dinosaur eggshell with sauropod dinosaurs.

A Theropod Dinosaur Embryo and the Affinities of the Flaming Cliffs Dinosaur Eggs

An embryonic skeleton of a nonavian theropod dinosaur was found preserved in an egg from Upper Cretaceous rocks in the Gobi Desert of Mongolia. Cranial features identify the embryo as a member of Oviraptoridae. Two embryo-sized skulls of dromaeosaurids, similar to that of Velociraptor, were also recovered in the nest. The eggshell microstructure is similar to that of ratite birds and is of a type common in the Djadokhta Formation at the Flaming Cliffs (Bayn Dzak). Discovery of a nest of such eggs at the Flaming Cliffs in 1923, beneath the Oviraptor philoceratops holotype, suggests that this dinosaur may have been a brooding adult.

A Nearly Modern Amphibious Bird from the Early Cretaceous of Northwestern China

Three-dimensional specimens of the volant fossil bird Gansus yumenensis from the Early Cretaceous Xiagou Formation of northwestern China demonstrate that this taxon possesses advanced anatomical features previously known only in Late Cretaceous and Cenozoic ornithuran birds. Phylogenetic analysis recovers Gansus within the Ornithurae, making it the oldest known member of the clade. The Xiagou Formation preserves the oldest known ornithuromorph-dominated avian assemblage. The anatomy of Gansus, like that of other non-neornithean (nonmodern) ornithuran birds, indicates specialization for an amphibious life-style, supporting the hypothesis that modern birds originated in aquatic or littoral niches.

Neuquenornis volans, a new Late Cretaceous bird (Enantiornithes: Avisauridae) from Patagonia, Argentina

ABSTRACT A new genus and species of Late Cretaceous (Coniancian–Santonian) birds, Neuquenornis volans, is described. This new taxon is known from a fairly complete, articulated specimen from the Rio Colorado Formation of northwestern Patagonia. Synapomorphies of the humerus, radius, ulna, coracoid, femur, tibiotarsus, tarsometatarsus, and thoracic vertebrae, support its allocation to the Enantiornithes. The tarsometatarsal structure of Neuquenornis refers it to the Late Cretaceous family Avisauridae. The structure of the wing and pectoral girdle of Neuquenornis indicates that it was an active flyer, an aptitude also inferred for the El Brete enantiornithines from the Upper Cretaceous Lecho Formation of northwestern Argentina. The pedal structure of Neuquenornis and Soroavisaurus from El Brete, indicates that avisaurids were capable of perching in trees.

A new close relative of Carnotaurus sastrei Bonaparte 1985 (Theropoda: Abelisauridae) from the Late Cretaceous of Patagonia

Infoquest Foundation, 160 Cabrini Boulevard #48, New York, New York 10033, U.S.AINTRODUCTIONAbelisaur dinosaurs, theropods with unusual horned skullssuch as Majungatholus (Sampson et al., 1998) and Carnotaurus(Bonaparte, 1985), are the most abundant land predators fromthe Late Cretaceous of Gondwana (Bonaparte, 1996; Novas,1997; Sampson et al., 1998; Coria and Salgado, 1998). Al-though a number of abelisaur species are known from Argentina(Bonaparte, 1996), as well as India and Madagascar (Sampsonet al., 1998), much of their anatomy and evolutionary relation-ships remains poorly understood.In March of 1999, a joint expedition from the Museo Mu-nicipal Carmen Funes and the Natural History Museum of LosAngeles County recovered a nearly complete and articulatedskeleton of a large theropod from Auca Mahuevo, a PatagonianLate Cretaceous site famous for its abundant sauropod eggs andembryos (Chiappe et al., 1998, 2000). In spite of being 30%smaller, the Auca Mahuevo theropod is remarkably similar toCarnotaurus sastrei (Bonaparte, 1985). Nonetheless, a uniquecombination of characters distinguishes the new skeleton fromCarnotaurus sastrei. Thus, following the predicates of the phy-logenetic species concept (Nixon and Wheeler, 1992), the AucaMahuevo skeleton forms the basis for a new abelisaur species,Aucasaurus garridoi.GEOLOGICAL SETTINGThe 150 m-thick section at Auca Mahuevo containing Au-casaurus garridoi runs transitionally from the Anacleto Mem-ber of the Ri´o Colorado Formation up through the Allen For-mation (Fig. 1A). The lower 85 m of the section that comprisesthe Anacleto portion is composed predominantly of pale reddishbrown, fine-grained sandstone, siltstone and mudstone, punc-tuated with occasional beds of greenish gray to light bluishgray, micaceous sandstone. These units appear to represent flu-vial channels and adjacent overbank deposits laid down acrossa low-gradient floodplain (Chiappe et al., 1998; Dingus et al.,2000). The lowest 45 m contain four levels of mudstone andsiltstone that produce fossilized eggs, embryos and embryonicskin of neosauropods (Chiappe et al., 2000). The lowest 20 moccur in a reversed magnetozone tentatively correlated withC33r in the early to middle Campanian portion of the Geomag-netic Polarity Timescale (Dingus et al., 2000). The new speci-men described here was found in an unusual laminated unit,composed by varves of unknown periodicity of greenish graymudstone and very fine-grained sandstone about 70 m abovethe base of the section near the top of the exposures of theAnacleto Member. This bed appears to represent a shallow wa-ter, lacustrine deposit based on the well-developed laminae andthe presence of freshwater arthropods.Institutional Abbreviations MCF-PVPH, Museo Munic-ipal Carmen Funes, Paleontologi´a de Vertebrados, Plaza Huin-cul, Argentina; MACN-CH, Museo Argentino de Ciencias Na-turales, Coleccio´n Chubut, Buenos Aires, Argentina.SYSTEMATIC PALEONTOLOGYTHEROPODA Marsh, 1881ABELISAURIA Novas, 1992ABELISAUROIDEA Novas, 1989ABELISAURIDAE Bonaparte and Novas, 1985CARNOTAURINAE Sereno, 1998CARNOTAURINI new taxonPhylogenetic Definition Carnotaurini is phylogeneticallydefined as the common ancestor of Carnotaurus sastrei andAucasaurus garridoi plus all the descendants of this ancestor.Diagnosis Two unambiguous synapomorphies support themonophyly of carnotaurs (term used to refer to members of theCarnotaurini): the presence of hyposphene–hypantrum articu-lations in the proximal and middle sections of the caudal series,and cranial processes in the epipophyses of the cervical verte-brae. Several other synapomorphies provide further support forthe monophyly of carnotaurs. These, however, are ambiguouslyoptimized because they are not preserved in other abelisaurs.These ambiguous synapomorphies include: a very broad cora-coid (coracoid maximum width three times the distance acrossthe scapular glenoid area), a humerus with a large and hemi-spherical head, an extremely short ulna and radius (ulna to hu-merus ratio 1:3 or less), and frontal prominences (swells orhorns) that are located laterally on the skull roof.Aucasaurus garridoi new taxonEtymology ‘‘Auca,’’ from Mapuche language, in referenceto Auca Mahuevo, the fossil locality where the specimen wasfound; ‘‘saurus,’’ from the Greek, lizard; ‘‘garridoi,’’ in hom-age to Mr. Alberto Garrido, who discovered the holotype.Holotype MCF-PVPH-236 was found lying on its rightside and articulated from the skull to the middle section of thetail (13

The skull of a relative of the stem-group bird Mononykus

In joint expeditions, researchers from the American Museum of Natural History and the Mongolian Academy of Sciences have recovered over 20 alvarezsaurid (Theropoda: Aves) specimens in the Late Cretaceous beds of Mongolias Gobi Desert. Here we describe a new taxon that is closely related to Mononykus,. This new taxon is represented by two exquisitely preserved skulls — the first known for Alvarezsauridae — details of which support the theory that the group is related to birds,. This theory was first put forward on the basis of primarily postcranial evidence,, including the presence of avian characteristics such as the absence of a contact between the jugal and postorbital, and between the quadratojugal and squamosal, articulations. Other earlier evidence that suggested that the alvarezsaurids were birds included the presence of a movable joint between the quadratojugal and quadrate, separate squamosal and braincase articulations of the quadrate, confluence between the caudal tympanic recess and columellar recess, a triradiate palatine, an unusually large foramen magnum, and the loss of a coronoid bone. The configuration of the temporal region of the skull and its articulation with the rostrum indicate the capability for prokinetic movement in which flexing occurs at the junction of the upper jaw and neurocranium, and support the idea that prokinesis preceded other types of avian intracranial kinesis.

Mesozoic avian bone microstructure: Physiological implications

We report on the bone microstructure of the Late Cretaceous birds Patagopteryx deferrariisi and members of the Enantiornithes. These birds represent the most primitive birds ever studied histologically. The occurrence of growth rings indicating alternating periods of slowed and fast growth suggests that these basal birds had slower growth rates, and differed physiologically from their modern relatives. Our findings also call into question previous ideas suggesting that nonavian theropods developed a full avian degree of homeothermic endothermy, which was later inherited by birds. On the contrary, our findings suggest that birds developed classic endothermy relatively late in their phylogenetic history.

The wing of Archaeopteryx as a primary thrust generator

Since the late 1800s, the debate on the origin of flight in birds has centred around two antagonistic theories: the arboreal (take-off from trees) and cursorial (take-off from running) models. Despite broad acceptance of the idea that birds evolved from bipedal and predominantly terrestrial maniraptoriform dinosaurs,, the cursorial model of flight origins has been less successful than the arboreal model. Three issues have contributed to this lack of success: the gap between the estimated maximum running speed of Archaeopteryx (2 metres per second) and its estimated minimum flying speed (6 metres per second); the high energy demands of evolving flight against gravity,; and the problem of explaining the origin of the ‘flight’ stroke in an earthbound organism,. Here we analyse the take-off run of Archaeopteryx through lift-off from an aerodynamic perspective, and emphasize the importance of combining functional and aerodynamic considerations with those of phylogeny,,. Our calculations provide a solution to the ‘velocity gap’ problem and shed light on how a running Archaeopteryx (or its cursorial maniraptoriform ancestors) could have achieved the velocity necessary to become airborne by flapping feathered wings.

Nest Structure for Sauropods: Sedimentary Criteria for Recognition of Dinosaur Nesting Traces

Abstract Six egg-filled depressions discovered in the Upper Cretaceous Anacleto Formation (Campanian) of Patagonia, Argentina, and interpreted as dinosaur nests, provide the only known evidence of titanosaurid sauropod nest construction. These nest trace fossils show truncation of sedimentary structures as well as differences in texture between the host substrate and in-filling sediment. Titanosaurid sauropods excavated and laid eggs in open nests rather than burying clutches in sediment. In addition, this paper establishes criteria for definitive recognition of excavated nests in the stratigraphic record.