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8-bit TIFF image stack of microCT scans through the premaxilla of Omorhamphus (YPM 13106; Lower Gray Bull Fm, Eocene, Big Horn Basin, Wyoming). Scan taken at University of Texas High-Resolution X-ray CT Facility by Matthew Colbert. Scan parameters: NSI scanner. Feinfocus source, high power, 200 kV, 0.14 mA, 1 aluminum filter, Perkin Elmer detector, 0.25 pF gain, 1 fps (999.911 ms integration time), no binning, no flip, source to object 218.58 mm, source to detector 1316.527 mm, continuous CT scan, no frames averaged, 0 skip frames, 3600 projections, 6 gain calibrations, 5 mm calibration phantom, data range [-10, 100] (grayscale range adjusted from NSI defaults), beam-hardening correction = 0.3. Voxel size = 0.0302 mm. Total slices = 1813., {"references":["Louchart A, Bhullar B-A, Riamon S and Field DJ (2021) The True Identity of Putative Tooth Alveoli in a Cenozoic Crown Bird, the Gastornithid Omorhamphus. Front. Earth Sci. 9:661699. doi: 10.3389/feart.2021.661699"]}

1. Introduction Europe has a comparatively rich and well-studied Eocene avian fossil record, but bird remains from the Paleocene period are very scant (Mayr, 2009, 2017a, b; Mayr et al., 2019a). The exact age of some localities is furthermore controversially discussed, which impedes well-founded biogeographic inferences. The Paleocene is divided into three stages, the Danian (66.0‒61.6 million years ago [Ma]), Selandian (61.6‒59.2 Ma), and Thanetian (59.2‒56.0 Ma). With the exception of Scaniornis lundgreni from the Danian of Sweden ‒ a bird of uncertain phylogenetic affinities that is only known from the poorly preserved wing bones of the holotype (Dames, 1890) ‒, all bird fossils from the Paleocene of Europe stem from the Selandian and Thanetian stages. Already in the 19th century, fossils of the large flightless Gastornithidae were described from the late Thanetian (~57.5 Ma) localities Cernay-les-Reims and Mont-de-Berru in the eastern Paris Basin in France (Lemoine, 1878, 1881; Martin, 1992; Buffetaut, 1997; Mourer-Chauvire & Bourdon, 2016). Equally long known from these sites is the palaeognathous taxon Remiornis, whereas the description of other avian remains from the Mont-de-Berru area commenced much later (Mourer-Chauvire, 1994, 1995). This is also true for bird fossils from the fissure fillings of Walbeck in Germany, which were found in the first half of the 20th century, but have been described more than 70 years later (Mayr, 2002a, 2007). Even on a global scale

Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited., The extinct dromornithids, gastornithids and phorusrhacids are among the most spectacular birds to have ever lived, with some giants exceeding 500 kg. The affinities and evolution of these and other related extinct birds remain contentious, with previous phylogenetic analyses being affected by widespread convergence and limited taxon sampling. We address these problems using both parsimony and tip-dated Bayesian approaches on an expansive taxon set that includes all key extinct flightless and flighted (e.g. Vegavis and lithornithids) forms, an extensive array of extant fowl (Galloanseres), representative Neoaves and palaeognaths. The Paleogene volant Lithornithidae are recovered as stem palaeognaths in the Bayesian analyses. The Galloanseres comprise four clades inferred to have diverged in the Late Cretaceous on Gondwana. In addition to Anseriformes and Galliformes, we recognize a robust new clade (Gastornithiformes) for the giant flightless Dromornithidae (Australia) and Gastornithidae (Eurasia, North America). This clade exhibits parallels to ratite palaeognaths in that flight presumably was lost and giant size attained multiple times. A fourth clade is represented by the Cretaceous Vegavis (Antarctica), which was strongly excluded from Anseriformes; thus, a crucial molecular calibration point needs to be reconsidered. The presbyornithids Wilaru (Australia) and Presbyornis (Northern Hemisphere) are robustly found to be the sister group to Anatoidea (Anseranatidae + Anatidae), a relatively more basal position than hitherto recognized. South America's largest bird, Brontornis, is not a galloansere, but a member of Neoaves related to Cariamiformes; therefore, giant Galloanseres remain unknown from this continent. Trait analyses showed that while gigantism and flightlessness evolved repeatedly in groups, diet is constrained by phylogeny: all giant Galloanseres and palaeognaths are herbivores or mainly herbivorous, and giant neoavians are zoophagous or omnivorous.

Fossils attributable to the extinct waterfowl clade Presbyornithidae and the large flightless Gastornithidae from the early Eocene (~52–53 Ma) of Ellesmere Island, in northernmost Canada are the oldest Cenozoic avian fossils from the Arctic. Except for its slightly larger size, the Arctic presbyornithid humerus is not distinguishable from fossils of Presbyornis pervetus from the western United States and the Gastornis phalanx is within the known size range of mid-latitude individuals. The occurrence of Presbyornis above the Arctic Circle in the Eocene could be the result of annual migration like that of its living duck and geese relatives, or it may have been a year-round resident similar to some Eocene mammals on Ellesmere and some extant species of sea ducks. Gastornis, along with some of the mammalian and reptilian members of the Eocene Arctic fauna, likely over-wintered in the Arctic. Despite the milder (above freezing) Eocene climate on Ellesmere Island, prolonged periods of darkness occurred during the winter. Presence of these extinct birds at both mid and high latitudes on the northern continents provides evidence that future increases in climatic warming (closer to Eocene levels) could lead to the establishment of new migratory or resident populations within the Arctic Circle.

The Late Paleocene locality of Louvois is located about 20 km south of Reims, in the department of Marne (France). These marly sediments have yielded numerous vertebrate remains. The Louvois fauna is coeval with those of the localities of Cernay-les-Reims and Berru and is dated as reference-level MP6, late Thanetian. Here we provide a detailed description of the remains of giant flightless gastornithids that were preliminarily reported in a study of the vertebrate fauna from Louvois. These fragmentary gastornithid remains mainly include a carpometacarpus, several tarsometatarsi, and numerous pedal phalanges. These new avian fossils add to the fossil record of Gastornis, which has been reported from various Early Paleogene localities in the Northern Hemisphere. Tarsometatarsi and pedal phalanges show large differences in size, which may be interpreted as sexual size dimorphism. The specimens from Louvois are morphologically different from those of Gastornis parisiensis and G. russelli, from Cernay and Berru. The Louvois carpometacarpus is also different from that of the North American G. giganteus, and the Louvois pedal phalanges are shorter and stouter than in other species of Gastornis. Moreover, the Louvois tarsometatarsi are more similar to that of the much younger G. sarasini, from Monthelon, reference-level MP 10, late Ypresian. We refrain from assigning the Louvois specimens to a new species of Gastornis and we designate it as Gastornis sp., owing to the fragmentary nature of the material. However, the morphological features of the Louvois material are sufficiently distinct for us to propose that three different forms of Gastornis were present in the Late Paleocene of North-eastern France.

Estimating the locomotion type of fossil ground birds is necessary for a better understanding of their ecology. Until now, only one method has allowed us to estimate the locomotion of fossil ground birds, but its application is complicated in the majority of fossil cases because it requires data from the three bones from the same hindlimb of one individual. Here, we propose a new method using only the maximum length and minimum width of the tarsometatarsus to estimate the locomotion of these fossil birds. This method is easy to apply and better distinguishes the intermediate cases between graviportal (walking) and cursorial (running) birds. This study shows that the species of large ground birds of the families Gastornithidae and Brontornithidae were all graviportal, probably because of their large body mass. More unexpectedly, it also shows that some Phorusrhacidae (e.g. Paraphysornis), usually described as cursorial birds, were in fact graviportal, probably because of their considerable body mass. All these new data about locomotion provide a better understanding of the ecology of fossil ground birds.

Almost 70 years after their discovery, more than 450 isolated avian bones from the Paleocene fissure filling of Walbeck are described. Fissuravis weigelti gen. et sp. nov. is represented by a coracoid with a very large cotyla scapularis and short processus acrocoracoideus and tentatively assigned to the palaeognathous Lithornithidae. Another coracoid of a large flightless bird, misidentified as a mammalian scapula before, is assigned to the Gastornithidae and is the earliest fossil record of this taxon. Most other bird bones from Walbeck belong to Walbeckornis creber gen. et sp. nov., which is represented by all major limb elements; this species resembles charadriiform birds and the ‘gruiform’ Messelornithidae, but its plesiomorphic morphology in combination with the poorly resolved higher level phylogeny of extant birds does not allow a definitive phylogenetic assignment. Several coracoids, humeri, and tarsometatarsi are assigned to Gradiornis walbeckensis gen. et sp. nov., which resembles some ...

The pattern of the evolutionary radiation of modern birds (Neornithes) has been debated for more than 10 years. However, the early fossil record of birds from the Paleogene, in particular, the Lower Eocene, has only recently begun to be used in a phylogenetic context to address the dynamics of this major vertebrate radiation. The Cretaceous-Paleogene (K-P) extinction event dominates our understanding of early modern bird evolution, but climate change throughout the Eocene is known to have also played a major role. The Paleocene and Lower Eocene was a time of avian diversification as a result of favourable global climatic conditions. Deteriorations in climate beginning in the Middle Eocene appear to be responsible for the demise of previously widespread avian lineages like Lithornithiformes and Gastornithidae. Other groups, such as Galliformes display replacement of some lineages by others, probably related to adaptations to a drier climate. Finally, the combination of slowly deteriorating climatic conditions from the Middle Eocene onwards, appears to have slowed the evolutionary rate in Europe, as avian faunas did not differentiate markedly until the Oligocene. Taking biotic factors in tandem with the known Paleogene fossil record of Neornithes has recently begun to illuminate this evolutionary event. Well-preserved fossil taxa are required in combination with ever-improving phylogenetic hypotheses for the inter-relationships of modern birds founded on morphological characters. One key avifauna of this age, synthesised for the first time herein, is the Lower Eocene Fur Formation of Denmark. The Fur birds represent some of the best preserved (often in three dimensions and with soft tissues) known fossil records for major clades of modern birds. Clear phylogenetic assessment of these fossils will prove critical for future calibration of the neornithine evolutionary timescale. Some early diverging clades were clearly present in the Paleocene as evidenced directly by new fossil material alongside the phylogenetically constrained Lower Eocene taxa. A later Oligocene radiation of clades other than Passeriformes is not supported by available fossil data.

The stratigraphic distribution of the three main groups of large flightless birds known from the Palaeogene of Europe, Gastornithidae, Phorusrhacidae and Ratitae, is reviewed. The huge, herbivorous gastornithids, represented by the single genus Gastornis , are known from the Selandian (Middle Palaeocene) to the late Lutetian (Middle Eocene), being recorded from reference levels MP5 to MP13. The carnivorous phorusrhacids are represented by a single species, Eleutherornis cotei , from the late Lutetian (MP14, late Middle Eocene). The ratites have a patchy distribution, being represented by two species of moderate size, Remiornis heberti from the Thanetian (MP6, Late Palaeocene) and Palaeotis weigelti from the Lutetian (MP11 to MP13, Middle Eocene). The stratigraphic distributions of large eggs referred to gastornithids in the Late Palaeocene and Early Eocene of southern Europe and the occurrence of enigmatic large avian footprints in the Late Eocene of France are discussed. Whereas gastornithids and ratites co-existed in both the Palaeocene and the Middle Eocene, phorusrhacids seem to have been the only large ground birds in Europe at the end of the Middle Eocene. The palaeobiogeographical and evolutionary implications of the stratigraphic distributions of those groups of large birds in Europe are discussed. As Gastornis first appears in North America and in Asia in the Early Eocene, it is likely that gastornithids originated in Europe and later spread to other land masses during a dispersal event close to the Palaeocene–Eocene boundary. Prior to that, gastornithids evolved on the European “island continent”, where they were the largest terrestrial tetrapods during the Palaeocene. Gastornithids do not seem to have been significantly affected by the PETM. Ratitae have a more patchy record and relationships between Remiornis and Palaeotis remain unclear. Nevertheless, those European forms are among the earliest known ratites and this should not be overlooked in discussions of ratite evolution and palaeobiogeography. Phorusrhacids appear to have been present in Europe for only a short time and are interpreted as the result of dispersal from Africa followed by local extinction.