A new Paleogene fossil and a new dataset for waterfowl (Aves: Anseriformes) clarify phylogeny, ecological evolution, and avian evolution at the K-Pg Boundary

Despite making up one of the most ecologically diverse groups of living birds, comprising soaring, diving and giant flightless taxa, the evolutionary relationships and ecological evolution of Anseriformes (waterfowl) remain unresolved. Although Anseriformes have a comparatively rich, global Cretaceous and Paleogene fossil record, morphological datasets for this group that include extinct taxa report conflicting relationships for all known extinct taxa. Correct placement of extinct taxa is necessary to understand whether ancestral anseriform feeding ecology was more terrestrial or one of a set of diverse aquatic ecologies and to better understand avian evolution around the K-T boundary. Here, we present a new morphological dataset for Anseriformes that includes more extant and extinct taxa than any previous anseriform-focused dataset and describe a new anseriform species from the early Eocene Green River Formation of North America. The new taxon has a mediolaterally narrow bill which is not known in any previously described anseriform fossils other than portions of the pseudotoothed Pelagornithidae. The matrix created to assess the placement of this taxon comprises 41 taxa and 719 discrete morphological characters describing skeletal morphology, musculature, syringeal morphology, ecology, and behavior. We additionally combine the morphological dataset with published sequences using Bayesian methods and perform ancestral state reconstruction for ecological and behavioral characters. We recover the new Eocene taxon as a stem anseranatid across all analyses, and find that the new taxon represents a novel ecology within known Anseriformes and the Green River taxa. Results indicate that Anseriformes were likely ancestrally aquatic herbivores with rhamphothecal lamellae and provide insight into avian evolution during and following the K-Pg mass extinction.

192 the skull at the parocciptal processes and, in this feature, is more like that of Anhimidae ( Figures   193 1-3). The  213 Paakniwatavis has a deeper fossa infratrochlearis of the carpometacarpus than Anatalavis. The 214 craniocaudal lengths of manual digits II and III at the synostosis are subequal in Paakniwatavis, 215 whereas II is greater than III in Anatalavis. The epicondylus medialis of the tibiotarsus is more 216 pronounced in Paakniwatavis than in Telmabates or Presbyornis. The epicondylus medialis is 217 less pronounced than those of Presbyornis or Chaunoides antiquus Alvarenga 1999. 218 Paakniwatavis   Anseriformes. 285 A tuberculum subcapitulare is present as in all Galloanserines. A prominentia submeatica 286 appears to be present as in all Anseriformes. 287 Axial Skeleton.

288
The atlas, axis and the third cervical vertebra are poorly preserved in ventrolateral aspect 289 near the furcula, with two additional cervical vertebrae preserved caudal to the third cervical 290 vertebra in ventral aspect. The dorsal spines are rounded and not dorsoventrally prominent. This 291 condition is more similar to that of Anseranatidae rather than that of Anhimidae. The third 292 cervical vertebra appears to be more elongate and to have a less prominent dorsal spine, 293 suggesting that the cervical series elongates caudally.

294
The thoracic and pelvic areas of the specimen are poorly preserved. The bone appears to have 295 eroded due to taphonomic processes, possibly due to bacterial erosion. The symphysis and ventral clavicles of the furcula are preserved in caudal aspect. The 310 furcula is more robust than those of most Anatidae but more gracile and thin than those of 311 Anhimidae or Anseranatidae ( Figure 3). A processus interclavicularis dorsalis is absent, and an 312 apophysis is absent.

313
Both coracoids are preserved in ventral aspect. The distal bases and shafts of the coracoids 314 can be seen. The acrocoracoid process is robust and hooked. CT data reveals a procoracoid 315 process and supracoracoid nerve foramen to be present. Portions of the scapulae are present near 316 to or overlapping the coracoids, but most of their morphology is indiscernible.

318
The right humerus is preserved in cranio-medial aspect. The sulcus ligamentosus transversus 319 is extremely deep. The crista deltopectoralis is prominent, rounded and flares cranio-laterally like 320 those of Anseriformes. The humeral head is bulbous and prominent. Also as in Anas, the 321 impression of the coracobrachialis is relatively deep. The condylus dorsalis is small and hamate.
322 The tuberculum supracondylare ventral is large and bulbous like that of Anas. The epicondylus 323 dorsalis appears to be distally extensive, and most similar to that of Anas. CT data reveals the 324 morphology of the caudal humerus; the crista proximally edging the fossa pneumotricipitalis 325 appears to have been domed slightly, like that of Anatalavis.

326
The right radius and ulna are preserved in ventral aspect. Most features of these bones cannot 327 be seen or were not preserved. The ulna body is thick and robust, but shorter than the humerus. 328 The olecranon is much shorter and more rounded than that of Anas, and is most similar in size 329 and shape to that of Chauna. The impression brachialis appears to be proximally deep and ovoid, 330 similar those of Anhimidae. Much of the distal portion of the ulna is obscured by the skull. There 331 appears to be small pneumatic foramen just under the cotyla humeralis of the radius. The left 332 ulna and radius are present but not as well preserved as those of the right. They are preserved in 333 dorsal aspect.

334
The right carpometacarpus overlaps the mandible and has broken onto the skull. It is 335 preserved in ventral aspect. The processus pisiformis is elongate, rounded and caudally oriented; 336 it is very similar to that of Anseranas. The rim of the dorsal trochlea is prominent and strongly 337 angled, which is also similar to the condition of Anseranas. The processus extensorius is 338 identical in size and shape to that of Anseranas as it is triangular in shape with a rounded point.  Data. 466 Results

467
Paakniwatavis grandei is recovered across all analyses as the sister taxon of

684
The question of how anseriform beak morphology and filter feeding evolved remains an open 685 one. Either a narrower, "goose-like" beak was ancestral for Anseriformes, or this narrower beak 686 evolved several times within Anseriformes (Olsen, 2017). The "goose-like" beak is associated 687 with increased leaf consumption, decreased invertebrate consumption, and an increase in the 688 mechanical advantage of the beak that allows for more effective cropping of plants (Olsen, 689 2017), whereas "duck-like" beaks are associated with increased filter feeding and consumption 690 of invertebrates. We take this classification of a "goose-like" beak a step further in the broader 691 context of both stem and crown Anseriformes: We first identify whether the rostrum is 692 mediolaterally wider than the width of the paroccipital processes (indicating an anseranatid-like 693 beak; character 3) and, if so, whether it is anteriorly tapered and narrowed further (a "goose-like" 831 1423.0g, respectively). This is consistent with recent evidence that correlation between herbivory 832 and body mass is not significant when accounting for phylogeny (Olsen, 2015).  Tables  858 Table 1. Specimen numbers of skeletal specimens used for comparison during fossil description 859 and phylogenetic analyses.