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Both the evolution of tooth morphology and the relationship between dental features and diet in toothed birds have long been studied. Here we quantify variation in tooth crown shape in 28 key Mesozoic bird species, and examine differences in dental morphology among birds belonging to different taxonomic groupings and inferred to have had different diets. Using geometric morphometric methods (GMM) and phylogenetic comparative methods (PCM), we found few clear differences in tooth crown shape between different taxonomic and ecological categories, and our analysis provides little support for many dietary inferences drawn in previous studies. However, the Solnhofen Archaeopteryx, Jeholornis, Protopteryx, Pengornis, Longipteryx, Tianyuornis, Mengciusornis, Ichthyornis and Hesperornis all were found to possess relatively specialized tooth crown shapes, perhaps reflecting specialized diets such as insectivory, granivory, piscivory and consumption of soft-shelled arthropods. Similarity in tooth crown shape across many Mesozoic birds may indicate the lack of dietary specialization, and the association between tooth form and diet may have been weakened in any case by ‘functional replacement’ of the dentition by a horny beak and, in many cases, gastroliths.

Taphonomic and diagenetic processes inevitably distort the original skeletal morphology of fossil vertebrate remains. Key aspects of palaeobiological datasets may be directly impacted by such morphological deformation, such as taxonomic diagnoses and phylogenetic hypotheses, interpretations of the shape and orientation of anatomical structures, and assessments of interspecific and intraspecific variation. In order to overcome these ubiquitous challenges we present a novel reconstruction workflow combining retopology and retrodeformation, allowing the original morphology of both symmetrically and asymmetrically damaged areas of fossils to be reconstructed. As case studies, we present idealised three-dimensional reconstructions of the sternum of the crownward stem-bird Ichthyornis dispar, and cervical vertebrae of the diplodocid sauropod Galeamopus pabsti. Multiple Ichthyornis sterna were combined into a single, idealised composite representation through superimposition and alignment of retopologised models, and this composite was subsequently retrodeformed. The Galeamopus vertebrae were individually retrodeformed and symmetrised. Our workflow enabled us to quantify deformation of individual specimens with respect to our reconstructions, and to characterise global and local taphonomic deformation. Our workflow can be integrated with geometric morphometric approaches to enable quantitative morphological comparisons among multiple specimens, as well as quantitative interpolation of “mediotypes” of serially homologous elements such as missing vertebrae, haemal arches, or ribs.

Ecological or sensory system shifts associated with brain shape and size change may have contributed to unique avian survivorship., Birds today are the most diverse clade of terrestrial vertebrates, and understanding why extant birds (Aves) alone among dinosaurs survived the Cretaceous-Paleogene mass extinction is crucial to reconstructing the history of life. Hypotheses proposed to explain this pattern demand identification of traits unique to Aves. However, this identification is complicated by a lack of data from non-avian birds. Here, we interrogate survivorship hypotheses using data from a new, nearly complete skull of Late Cretaceous (~70 million years) bird Ichthyornis and reassess shifts in bird body size across the Cretaceous-Paleogene boundary. Ichthyornis exhibited a wulst and segmented palate, previously proposed to have arisen within extant birds. The origin of Aves is marked by larger, reshaped brains indicating selection for relatively large telencephala and eyes but not by uniquely small body size. Sensory system differences, potentially linked to these shifts, may help explain avian survivorship relative to other dinosaurs.

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.

Ornithuromorph birds (the clade which includes modern avian radiation) first appeared in the Early Cretaceous in Asia and achieved a great diversity during the latest ages of the Late Cretaceous (Campanian and Maastrichtian). The evolutionary history of orithuromorphs during the first 17 MYAs of the Late Cretaceous (Cenomanian to Santonian ages) remains very poorly known, as the fossil record for this time interval is largely restricted to several isolated finds of the classic avian genus Ichthyornis in North America. Here we describe an isolated distal tibiotarsus of an evolutionary advanced bird, morphologically similar to Ichthyornis, from the middle Cenomanian of Saratov Province, European Russia. This is the first documentation of an Ichthyornis-like bird in the Old World. The find further constitutes only the second pre-Campanian record of the Late Cretaceous Ornithuromorpha in Eurasia, the second record of Cenomanian birds in Russia. This discovery shows that Ichthyornis-like birds enjoyed a wide geographical distribution as early as the beginning of the Late Cretaceous. Given that the earliest and the most primitive ornithuromorph birds are known from Asia, the new find supports a Eurasian origin for Ichthyornithidae.

Comparative anatomy studies of the skull of archosaurs provide insights on the mechanisms of evolution for the morphologically and functionally diverse species of crocodiles and birds. One of the key attributes of skull evolution is the anatomical changes associated with the physical arrangement of cranial bones. Here, we compare the changes in anatomical organization and modularity of the skull of extinct and extant archosaurs using an Anatomical Network Analysis approach. We show that the number of bones, their topological arrangement, and modular organization can discriminate birds from non-avian dinosaurs, and crurotarsans. We could also discriminate extant taxa from extinct species when adult birds were included. By comparing within the same framework, juveniles and adults for crown birds and alligator (Alligator mississippiensis), we find that adult and juvenile alligator skulls are topologically similar, whereas juvenile bird skulls have a morphological complexity and anisomerism more similar to those of non-avian dinosaurs and crurotarsans than of their own adult forms. Clade-specific ontogenetic differences in skull organization, such as extensive postnatal fusion of cranial bones in crown birds, can explain this pattern. The fact that juvenile and adult skulls in birds do share a similar anatomical integration suggests the presence of a specific constraint to their ontogenetic growth., BE-A has received fnancial support through the Postdoctoral Junior Leader Fellowship Programme from “la Caixa” Banking Foundation (LCF/BQ/LI18/11630002) and also thanks the Unidad de Excelencia María de Maeztu funded by the AEI (CEX2018-000792-M). HWL’s Master Tesis that inspired this project was funded by Imperial College London and Natural History Museum, London.

We describe a previously unreported left humerus of the Late Cretaceous toothed seabird, cf. Ichthyornis sp., from the basal Lincoln Limestone Member (late Cenomanian) of the Greenhorn Limestone Formation in central Kansas. This specimen represents one of the oldest bird specimens in North America, and together with previously described ichthyornithid specimens, it also provides new insights into the size distribution of the fossil taxon. Our analysis indicates that Ichthyornis body size remained relatively consistent throughout its temporal range from the Cenomanian through the Campanian. The present fossil record suggests that Ichthyornis measured up to about 0.5 m in skeletal wingspan, 0.2 m in height, and 0.3 m in skeletal length.

Birds are dinosaurs, linked to their extinct relatives by feathers and other aspects of their anatomy. But birds9 beaks—splendidly versatile adaptations that allow their owners to grasp, pry, preen, and tear—are nothing like stiff dinosaurian snouts, and how they evolved has been a mystery. Now, 3D computer scans of new fossils of an iconic ancient bird capture the beak just as it took form. Scientists have scanned four new fossil specimens of Ichthyornis , an ancient seabird from about 90 million years ago, which has long been famous for having a body like a modern bird, but a snout lined with teeth like a dinosaur. By bringing details from multiple specimens together, the scans allow researchers to glimpse the beginnings of key features of bird heads, including a large brain, a pointed beak, and a movable upper jaw that helps make beaks so nimble.

A well-preserved right humerus collected from the upper Austin Group at Ejido Piedritas in Coahuila, Mexico is referable to the carinate bird Ichthyornis. This occurrence extends the known distribution of Ichthyornis to the southwest, and is among a younger (Coniacian-Campanian) group of specimens, recovered generally to the south of older (Cenomanian-Turonian) Ichthyornis specimens. The southward shift in occurrence of Ichthyornis may accord with the withdrawal of subtropical marine biota from the Western Interior Seaway of North America during the Late Cretaceous.

Ichthyornis dispar is a key extinct bird species from when birds were shedding characteristics of their dinosaur ancestors and evolving their current features. A reconstructed skull of I. dispar now illuminates this transition. Ichthyornis dispar is a key extinct bird species from when birds were shedding characteristics of their dinosaur ancestors and evolving their current features. A reconstructed skull of I. dispar now illuminates this transition.

Background The dentitions of extinct organisms can provide pivotal information regarding their phylogenetic position, as well as paleobiology, diet, development, and growth. Extant birds are edentulous (toothless), but their closest relatives among stem birds, the Cretaceous Hesperornithiformes and Ichthyornithiformes, retained teeth. Despite their significant phylogenetic position immediately outside the avian crown group, the dentitions of these taxa have never been studied in detail. To obtain new insight into the biology of these ‘last’ toothed birds, we use cutting-edge visualisation techniques to describe their dentitions at unprecedented levels of detail, in particular propagation phase contrast x-ray synchrotron microtomography at high-resolution. Results Among other characteristics of tooth shape, growth, attachment, implantation, replacement, and dental tissue microstructures, revealed by these analyses, we find that tooth morphology and ornamentation differ greatly between the Hesperornithiformes and Ichthyornithiformes. We also highlight the first Old World, and youngest record of the major Mesozoic clade Ichthyornithiformes. Both taxa exhibit extremely thin and simple enamel. The extension rate of Hesperornis tooth dentine appears relatively high compared to non-avian dinosaurs. Root attachment is found for the first time to be fully thecodont via gomphosis in both taxa, but in Hesperornis secondary evolution led to teeth implantation in a groove, at least locally without a periodontal ligament. Dental replacement is shown to be lingual via a resorption pit in the root, in both taxa. Conclusions Our results allow comparison with other archosaurs and also mammals, with implications regarding dental character evolution across amniotes. Some dental features of the ‘last’ toothed birds can be interpreted as functional adaptations related to diet and mode of predation, while others appear to be products of their peculiar phylogenetic heritage. The autapomorphic Hesperornis groove might have favoured firmer root attachment. These observations highlight complexity in the evolutionary history of tooth reduction in the avian lineage and also clarify alleged avian dental characteristics in the frame of a long-standing debate on bird origins. Finally, new hypotheses emerge that will possibly be tested by further analyses of avian teeth, for instance regarding dental replacement rates, or simplification and thinning of enamel throughout the course of early avian evolution. Electronic supplementary material The online version of this article (doi:10.1186/s12862-016-0753-6) contains supplementary material, which is available to authorized users.

Our analysis of vertebrate remains collected from a Cenomanian microsite in the Lincoln Limestone, the basal-most member of the Greenhorn Formation in Russell County, Kansas, identifies ornithurine (sensu Chiappe, 1996) avian fossils. The specimens presented here are slightly younger than the oldest avian remains known from North America (from the Woodbine Formation, Texas) and of a similar age as the avians from the Asheville Formation in Saskatchewan, Canada. This find thus connects the extensive geographic range of the earliest North American birds from Saskatchewan south through central Kansas and on to Texas. The specimens discussed here are fragmentary, yet show features definitive of ornithurine birds. One specimen is attributable to Ichthyornis, whereas another preserves teeth characteristic of ornithurine birds. In addition to bird bones, the microsite yielded numerous bony fish remains, shark teeth, coniasaur vertebrae, and other lizard bones.

We show that the topology of separate elements in avian compound rhamphothecae are strongly similar among different clades, not only in external appearance but in how these elements conform to underlying structures. We conducted a morphological survey of 81 extant bird species and tested superficial similarities in external beak morphology for substantive similarity in associated skeletal structures and nerve courses. A revised set of morphological characters for compound rhamphothecae was optimized onto three recent phylogenetic trees to assess the relationships of homology and homoplasy in rhamphothecal morphology. Osteological correlates of rhamphothecae from the extinct basal ornithurine birds Hesperornis and Ichthyornis show that compound rhamphothecae are the primitive state for the clade including extant birds (Neornithes). Simple rhamphothecae are the result of the loss of softer keratinous grooves between rhamphothecal components, and there are many examples of transitional forms betwee...

FHSM VP-2139 is the proximal end of a right carpometacarpus of the Late Cretaceous toothed seabird, Ichthyornis sp. (Aves: Ichthyornithiformes), housed in the Sternberg Museum of Natural History, Hays Kansas. The specimen was found near the contact between the Carlile and Greenhorn Formations in southern Ellis County, Kansas. The specimen marks the oldest record of Ichthyornis in Kansas, and this report represents the first detailed account of this specimen. The estimated total “skeletal length” (from the beak tip to the pygostyle tip) and “skeletal wingspan” (between the right and left phalangeal tips) of the bird individual are 24 cm and 43 cm, respectively. The bird specimen is paleoecologically intriguing, because it occurred in an offshore deposit which formed during the maximum transgressive phase of the Greenhorn Cyclothem of the Late Cretaceous Western Interior Sea.

Testing models of macroevolution, and especially the sufficiency of microevolutionary processes, requires good collaboration between molecular biologists and paleontologists. We report such a test for events around the Late Cretaceous by describing the earliest penguin fossils, analyzing complete mitochondrial genomes from an albatross, a petrel, and a loon, and describe the gradual decline of pterosaurs at the same time modern birds radiate. The penguin fossils comprise four naturally associated skeletons from the New Zealand Waipara Greensand, a Paleocene (early Tertiary) formation just above a well-known Cretaceous/Tertiary boundary site. The fossils, in a new genus (Waimanu), provide a lower estimate of 61-62 Ma for the divergence between penguins and other birds and thus establish a reliable calibration point for avian evolution. Combining fossil calibration points, DNA sequences, maximum likelihood, and Bayesian analysis, the penguin calibrations imply a radiation of modern (crown group) birds in the Late Cretaceous. This includes a conservative estimate that modern sea and shorebird lineages diverged at least by the Late Cretaceous about 74 +/- 3 Ma (Campanian). It is clear that modern birds from at least the latest Cretaceous lived at the same time as archaic birds including Hesperornis, Ichthyornis, and the diverse Enantiornithiformes. Pterosaurs, which also coexisted with early crown birds, show notable changes through the Late Cretaceous. There was a decrease in taxonomic diversity, and small- to medium-sized species disappeared well before the end of the Cretaceous. A simple reading of the fossil record might suggest competitive interactions with birds, but much more needs to be understood about pterosaur life histories. Additional fossils and molecular data are still required to help understand the role of biotic interactions in the evolution of Late Cretaceous birds and thus to test that the mechanisms of microevolution are sufficient to explain macroevolution.

A new bird from the Late Cretaceous of Patagonia (Argentina), known from associated wing elements, is described and its phylogenetic position evaluated. Fossil taxa as well as representatives of species of extant birds sampled from lineages considered to be basal within the crown clade were included in a cladistic analysis of 72 characters primarily from the thoracic limb. Based on the results of the phylogenetic analysis and identification of autapomorphies in the specimen, we name a new taxon Limenavis patagonica. Limenavis patagonica is identified as closer to the crown clade than Enantiornithes by the presence of three unambiguous synapomorphies: a fossa (sometimes with two distinguishable subparts) on the dorsal, distalmost extremity of the humerus; distal fusion of metacarpals II and III; and an extensor process on metacarpal I. It is placed closer to the crown clade than Ichthyornis, and, thus, unambiguously as a carinate (see Methods for terminology), by two further synapomorphies: the ab...

We report on the bone microstructure of the Cretaceous birdsHesperornis regalisandIchthyornis victor. Thin sections of representative elements of both these ornithurine birds show a rapid, sustained bone deposition without any pauses or interruptions in bone formation. This growth pattern contrasts sharply with the cyclical pattern of bone deposition previously reported for the Cretaceous non-ornithurine birdsPatagopteryxand representatives of the enantiornithines. These findings suggest physiological advancement in Cretaceous ornithurine birds. The bone microstructure of the divingHesperornisshows similarities to the bone structure of modern penguins, and to that of a loon from the Cretaceous of Antarctica.

Five taxa of fossil birds comprising at least 17 individuals have been discovered in a Cenomanian (early Late Cretaceous) nearshore marine deposit along the Carrot River, near the Pasquia Hills of Saskatchewan, Canada. More than a hundred fossils, all from a single locality, constitute the oldest and most diverse avifauna from the Cretaceous of North America. Four species represent the earliest known North American Ornithurae, two of them new species of the baptornithid Pasquiaornis, n. gen. (Hesperornithiformes), and the other two referred to the ichthyornithid Ichthyornis (Ichthyornithiformes). Pasquiaornis is more primitive than Baptornis, and its humerus and femur show resemblances to those of flying birds. A presumed enantiornithine is also present.

Birds are characterized by pneumatization of their skeletons by epithelial diverticula from larger, air—filled cavities. The diverticula—or ‘air sacs’—that invade the postcranium result from outgrowths of the lungs; postcranial pneumaticity has been very well studied. Much more poorly understood are the air sacs that pneumatize the skull. Study of craniofacial pneumaticity in modern birds (Neornithes) indicates the presence of two separate systems: nasal pneumaticity and tympanic pneumaticity. The lacrimal and maxillary bones are pneumatized by diverticula of the main paranasal cavity, the antorbital sinus. There are five tympanic diverticula in neornithines that pneumatic the quadrate, articulare and the bones of the braincase. The pneumatic features of the following six genera of Mesozoic birds are examined: Archaeopteryx, Enaliornis, Baptornis, Parahesperornis, Hesperornis and Ichthyornis. Despite the ‘archaic’ aspect of most of these birds, many of the pneumatic features of neornithines are found in Mesozoic birds and are considered primitive for Aves. The phylogenetic levels at which most of the avian pneumatic features arose within Archosauria are uncertain. Until the phylogenetic levels at which homologous pneumatic features arose are determined, it is unwise to use most pneumatic characters in the discussion of avian origins. Within avian phylogeny, Ornithurae and Neornithes are well–supported by pneumatic synapomorphies. There is a trend towards reduction of craniofacial pneumaticity within Hesperornithiformes. Within Neornithes, four derived pneumatic characters suggest that the Palaeognathae (ratites and tinamous) is monophyletic.

Charles Darwin commented that Ichthyornis, as one of the “toothed birds” from the Late Cretaceous of Kansas, offered some of “the best support to the theory of evolution” (in litt., C. Darwin to O.C. Marsh, August 31, 1880). Ichthyornis figures no less prominently today. It is one of the closest outgroups to crown clade Aves, and remains one of the only Mesozoic avialans known from more than a handful of specimens. As such, Ichthyornis is an essential taxon for analyses of deep divergences within Aves because of its influence in determining the morphologies ancestral to the crown clade. Ichthyornis, however, has languished in need of new anatomical description and taxonomic revision. Many of the best Ichthyornis specimens were largely inaccessible, plastered into Yale Peabody Museum (YPM) exhibit mounts for nearly a century. The focus of this study was the entire YPM Ichthyornis collection, the largest at any institution. The elements removed from the mounts were identified to the specimens with ...

Birds known from more than isolated skeletal elements are rare in the fossil record, especially from the European Mesozoic. This paucity has hindered interpretations of avian evolution immediately prior to, and in the aftermath of, the Cretaceous-Tertiary (K-T) extinction event. We report on a specimen of a large ornithurine bird (closely related to Ichthyornis) from the uppermost Cretaceous (Maastricht Formation) of Belgium. This is the first record of a bird from these historic strata and the only phylogenetically informative ornithurine to be recovered from the Mesozoic of Europe. Because this new specimen was collected from 40 m below the K-T boundary (approximate age of 65.8 Ma), it is also the youngest non-neornithine (=non-modern) bird known from anywhere in the world.

Benjamin Franklin Mudge (1817-79), originally from Massachusetts, was appointed State Geologist and Director of the First Geological Survey of Kansas in 1864. After failing to be reappointed in 1865 he became Professor of Natural Science at Kansas State Agricultural College, Manhattan, whose president, Joseph Denison, was an old friend and fellow Methodist. Mudge taught courses in all areas of science and spent his summers geologizing in western Kansas. An avid collector, he sent fossil specimens to Edward Cope, O. C. Marsh, and others. In the summer of 1872 he discovered a Cretaceous bird, Ichthyornis dispar, described by Marsh at Yale as the first fossil bird known to have teeth. In 1873 the KSAC regents replaced Denison by John Anderson, who dismissed Mudge and two others in February 1874 after they complained to members of the legislature about misuse of college funds and tried unsuccessfully to defeat legislative confirmation of some of the regents. Mudge then was employed by Marsh to collect fossil vertebrates (1874-77). Assisted by Samuel Williston and other former students, he sent to Marsh a large number of specimens of marine reptiles, pterodactyls, and birds from the Cretaceous beds of western Kansas. In 1877 he was sent to Colorado, where he supervised the quarrying of dinosaur bones at Cañon City. Strongly religious and a staunch opponent of slavery and alcohol, Mudge was regarded highly as a teacher and collector. He published in 1875 a description of the geology of Kansas which contained the first geological map of the State. He also was cofounder and first president of the Kansas Academy of Science.

An isolated left humerus from the Cretaceous toothed bird Ichthyornis is described. The fossil comes from the Vimy Member of the Kaskapau Formation at Watino, Alberta, and is early Turonian in age. The Watino specimen is the oldest well authenticated record of Ichthyornis and the oldest Cretaceous skeletal fossil of a bird from the Western Hemisphere. The wide geographic distribution of Ichthyornis within the Western Interior of North America is in keeping with nomadic habits, as inferred from strongly flying modern marine birds.

In 1872, O. C. Marsh described a new Cretaceous bird Ichthyornis dispar based on a posteranial skeleton (Marsh 1872a). Later in the same year, he described a new reptilian genus Colonosaurus for two toothed jaws found associated with the type specimen of Ichthyornis dispar (Marsh 1872b). In the following year, Marsh referred the toothed "Colonosaurus" jaws to Ichthyornis dispar stating that: "On subsequently removing the surrounding shale, the skull and additional portions of both jaws were brought to light, so that there cannot now be a reasonable doubt that all are parts of the same bird" (Marsh 1873). This assignment was accepted until 1952 when J. T. Gregory compared the jaws of Icthyornis dispar to those of mosasaurs. Gregory (1952) concluded that the toothed jaws assigned to Ichthyornis by Marsh were actually the jaws of a small mosasaur which had accidently been deposited together with the skeleton of a bird. The recent classifications of fossil birds by Storer (1960), Romer (1966, 1968), and Brodkorb (1967) all follow Gregory and removed Ichthyornis from the toothed birds. After careful study of the toothed jaws associated with the type specimen of Ichthyornis dispar (Yale Peabody Museum [YPM] 1450) M. V. Walker (1967) recently stated his opinion that until further proof is available the toothed jaws should be considered to belong to Ichthyornis. Fortunately, Walker has since found a second partial skeleton of Icthyornis, again with associated toothed jaws. The jaws associated with Walker's partial skeleton consist only of portions of the toothed dentaries; the quadrate articulations of both jaws are unfortunately missing (Walker, unpubl.). I recently discovered an additional mandibular fragment of Ichthyornis (I. cf. dispar) which was catalogued together with bones of a specimen of the much larger Cretaceous bird Hesperornis (YPM 1478) in the Yale Peabody Museum collection. This new mandibular fragment of Ichthyornis is the only known

Both Hesperornis and Ichthyornis are toothed birds; they and Archaeopteryx share broad flattened teeth with highly expanded roots. The teeth of Ichthyornis have been reported to have been set in sockets as were those of Archaeopteryx , but new specimens of Ichthyornis show the teeth set in a groove as in Hesperornis . These new fossils are from an adult but not old bird; thus, the presence or absence of tooth sockets in birds may be dependent on age.

In contrast to the vast majority of reptiles, the skulls of adult crown birds are characterized by a high degree of integration due to bone fusion, e.g., an ontogenetic event generating a net reduction in the number of bones. To understand this process in an evolutionary context, we investigate postnatal ontogenetic changes in the skulls of crown bird and non-avian theropods using anatomical network analysis (AnNA). Due to the greater number of bones and bone contacts, early juvenile crown birds have less integrated skulls, resembling their non-avian theropod ancestors, including Archaeopteryx lithographica and Ichthyornis dispars. Phylogenetic comparisons indicate that skull bone fusion and the resulting modular integration represent a peramorphosis (developmental exaggeration of the ancestral adult trait) that evolved late during avialan evolution, at the origin of crown-birds. Succeeding the general paedomorphic shape trend, the occurrence of an additional peramorphosis reflects the mosaic complexity of the avian skull evolution.

A striking example of convergence, the evolution of similar structures from different antecedent body plans by unrelated organisms, is provided by the comparison of the lower jaws of the Cretaceous toothed bird, Hesperornis, and those of the giant marine lizards known as mosasaurs, whose skeletons are entombed within the same strata. 0. C. Marsh, the discoverer of Hesperornis, himself briefly noted the presence of an intramandibular joint in its jaw (1880, p. 11), but his interest in the retention of teeth and the supposedly primitive brain (cf., Edinger, 1951) of this bird led him to neglect further consideration of this feature. Elsewhere among tetrapods a splenio-angular articulation is known only in the mosasaurs in which it has been described in detail by Williston (1898, pp. 130-134). The structural similarities of the mandibles of mosasaurs and Hesperornis are so great that it is hard to realize that they have arisen independently from rather dissimilar types of reptilian jaws. Yet such is clearly the case, for Hesperornis was a bird, derived from the reptilian Subclass Archosauria, whereas mosasaurs (fig. 6) are modified lizards [Subclass Lepidosauria] close to the living Varanus in their basic structure and undoubtedly derived either from the early Varanidae or their immediate precursors. Besides the distinctive mandibular joint the mosasaurs and Hesperornis show similarities in several other features, all of which may be related to their aquatic habitat and presumably piscivorous diet. Their convergence is thus adaptive and typical of that phenomenon. This discussion is a by-product of an investigation of the jaws which Marsh attributed to another Cretaceous bird, Ichthyornis. Evidence is presented elsewhere which shows that these jaws probably did not belong with that bird skeleton but are actually young mosasaur jaws which were washed into the, same deposit with bird bones of similar size. Routine comparison with Hesperornis, the only other Cretaceous bird sufficiently well known for comparison, brought the convergent condition mentioned above to light. The present report is based upon detailed study of the lower jaws of Hesperornis regalis Marsh, Yale Peabody Museum No. 1206, and of Hesperornis gracilis' Marsh, Kansas University Museum of Natural History No. 2287, which to the best of my knowledge are the only extant specimens of these birds in which skull and mandible are preserved. In the course of this study I have received valued advice and much encouragement from many colleagues, particularly Dr. Tilly Edinger, Dr. Hildegarde Howard, Dr. S. Dillon Ripley, and Dr. Alexander Wetmore. The illustrations were prepared by Miss Shirley Glaser.

IN that portion of the article “Birds,” which I have lately written for the “Encyclopaedia Britannica,” I said (page 729, column 2) that Odontopteryx had “jaws armed with true teeth,” and in this respect resembled Ichthyornis. The mistake has just been pointed out to me, and I shall be greatly obliged by being allowed to correct it, as far as is possible, in NATURE. The sentence should run thus: “jaws armed with tooth-like processes, and in this respect differing from Professor Marsh's Ichthyornis.”

AMONG some very useful and important papers recently issued by the Museum of Zoology of the University College, Dundee, appears one by Prof. D'Arcy Thompson, “On the Systematic Position of Hesperornis.” In this excellent scientific brochure, Prof. Thompson has critically compared in detail the skeleton of a diver (Colymbus septentrionalis) with the skeleton of Hesperornis, as presented us by Marsh; and the outcome of this investigation has fully convinced the author of the work in question of the kinship, which undoubtedly exists, between the extinct Hesperornis and the existing pogopodine birds, as the divers, loons, and grebes. We have long been satisfied of these affinities, and firmly believed that the Colymbidae were the descendants, perhaps direct descendants, of the toothed birds of the genus Hesperornis. It required but such comparison as has been so ably instituted by Prof. Thompson to make it quite clear to any thoughtful anatomist; and, as he hints, a similar comparison will probably go to show the fact that another extinct toothed bird-form, Ichthyornis, lies in the line of descent of the terns and their allies.