1 Late Cretaceous neornithine from Europe illuminates crown bird
2 origins
3
4 Daniel J. Field1*, Juan Benito1,2, Albert Chen1,2, John W.M. Jagt3, Daniel T. Ksepka4
5
6 1Department of Earth Sciences, University of Cambridge, Cambridge, UK. 2Department of
7 Biology & Biochemistry, Milner Centre for Evolution, University of Bath, Bath, UK.
8 3Natuurhistorisch Museum Maastricht, Maastricht, The Netherlands. 4Bruce Museum,
9 Greenwich, CT, USA. *e-mail: [email protected]
10
11 Our understanding of the earliest stages of crown bird evolution is hindered by an
12 exceedingly sparse Mesozoic fossil record. The most ancient phylogenetic divergences
13 among crown birds are known to have occurred in the Cretaceous1-3, but stem lineage
14 representatives of the deepest crown bird subclades—Palaeognathae (ostriches and
15 kin), Galloanserae (landfowl and waterfowl), and Neoaves (all other extant birds)—are
16 entirely unknown from the Mesozoic. As a result, key questions related to ancestral
17 crown bird ecology4,5, biogeography3,6,7, and divergence times1,8-10 remain unanswered.
18 Here, we report a new Mesozoic fossil that occupies a position close to the last common
19 ancestor of Galloanserae, filling a key phylogenetic gap early in crown bird
20 evolutionary history10,11. Asteriornis maastrichtensis, gen. et sp. nov., from the
21 Maastrichtian of Belgium, is represented by a nearly complete, three-dimensionally
22 preserved skull and associated postcranial elements. The fossil represents one of the
23 only well-supported crown birds from the Mesozoic Era12, and is the first Mesozoic
24 crown bird with well represented cranial remains. A. maastrichtensis exhibits a
25 heretofore undocumented combination of galliform (landfowl)-like and anseriform 26 (waterfowl)-like features, and, along with a previously reported Ichthyornis-like taxon
27 from the same locality13, provides the first direct evidence of co-occurring crown birds
28 and avialan stem birds. Its occurrence in the northern hemisphere challenges
29 biogeographic hypotheses of a Gondwanan origin of crown birds3, and its relatively
30 small size and possible littoral ecology may corroborate recently proposed ecological
31 filters4,5,9 influencing crown bird persistence through the end-Cretaceous mass
32 extinction.
33
34 By any measure, crown birds (Neornithes) are among the most diverse and
35 conspicuous extant tetrapods, yet their early evolutionary history is scarcely understood.
36 Apart from birds, all major extant tetrapod groups—lissamphibians14, squamates15, turtles16,
37 mammals17, and crocodylians18—are well-known from pre-Cenozoic crown group fossils. By
38 contrast, the Mesozoic record of well-supported crown birds is restricted to a single latest
39 Maastrichtian taxon, Vegavis iaai12. Several fragmentary Mesozoic fossils have at times been
40 referred to Neornithes19, although justifications for such assignments are questionable and
41 these purported records have not unambiguously withstood re-evaluation20,21.
42 We report a remarkable new crown bird from the Late Cretaceous of Belgium. The
43 fossil is between 66.8 and 66.7 million years old—making it the oldest unambiguous crown
44 bird fossil yet discovered—and provides important insight into the extent of Mesozoic
45 neornithine diversification prior to the end-Cretaceous mass extinction event, 66.02 MYA22.
46 Uniquely among crown birds from the Mesozoic and earliest Paleocene, the new fossil
47 includes a nearly complete, three-dimensionally preserved skull, yielding the first direct
48 insights into the nature of the crown bird skull early in neornithine evolutionary history. The
49 specimen exhibits a previously unseen combination of features diagnostic of Galliformes and
50 Anseriformes, which together form the crown clade Galloanserae, one of the most deeply- 51 diverging clades of crown birds and the sister group to the hyperdiverse extant clade
52 Neoaves1-3. Our most parsimonious phylogenetic hypothesis suggests that the fossil sits on
53 the stem lineage of Galloanserae, which would make it the only stem galloanseran yet known
54 and fill one of the most conspicuous phylogenetic gaps in the neornithine fossil record.
55
56 Systematic Palaeontology
57 Avialae Gauthier, 1986
58 Neornithes Gadow, 1892
59 Neognathae Pycraft, 1900
60 Pangalloanserae Gauthier and de Queiroz, 2001
61 Asteriornis maastrichtensis gen. et sp. nov.
62
63 Remarks. We use Avialae to refer to theropods crownward of Dromaeosauridae and
64 Troodontidae. Neornithes is equivalent to the bird crown group (Aves sensu23).
65 Pangalloanserae defines the most inclusive clade including Anser anser and Gallus gallus but
66 not Passer domesticus (i.e. the galloanseran total group). Further phylogenetic definitions are
67 presented in the Supplementary Information.
68
69 Etymology. Asteriornis from the name of the Titan goddess Asteria and the Greek ornis for
70 bird. In Greek mythology Asteria is the goddess of falling stars and transforms herself into a
71 quail, attributes reflected by both the impending K-Pg asteroid impact and the galloanseran
72 affinities of Asteriornis. The specific epithet maastrichtensis reflects the holotype’s
73 provenance from the Maastricht Formation (the type locality of the Late Cretaceous
74 Maastrichtian Stage).
75 76 Holotype. Natuurhistorisch Museum Maastricht (NHMM) 2013 008, a nearly complete,
77 articulated skull including mandibles and associated postcranial remains preserved in four
78 blocks (Fig. 1, Extended Data Figs. 1-7; see Supplementary Information for videos, character
79 information, measurements, and additional description and discussion). Preserved elements
80 include the premaxillae, maxillae, nasals, frontals, laterosphenoid, basisphenoid, mesethmoid,
81 left quadrate, left jugal, right palatine, and lower jaws. Associated postcranial elements
82 include incomplete femora, tibiotarsi, tarsometatarsus, and radius.
83
84 Locality and age. CBR-Romontbos Quarry, Eben Emael, Province of Liège, Belgium.
85 Valkenburg Member (66.8-66.7 Ma24), Maastricht Formation, Late Maastrichtian,
86 Cretaceous. Additional details regarding the locality and stratigraphic setting are presented in
87 the Supplementary Information.
88
89 Diagnosis. Asteriornis is unique among known taxa in exhibiting caudally pointed nasals
90 overlying the frontals and meeting at the skull’s midline, and a slightly rounded, unhooked
91 tip of the premaxilla. Additional unique character combinations from phylogenetic analyses
92 that differentiate Asteriornis are presented in the Supplementary Information.
93
94 Description
95 Asteriornis maastrichtensis is a small pan-galloanseran, with a mean body mass estimate of
96 394 g from hindlimb scaling regressions (~21st percentile among extant Galloanserae,
97 Extended Data Fig. 8)25. Complete measurements of NHMM 2013 008 are provided in the
98 Supplementary Information.
99 Most major cranial components are largely in their original anatomical positions. The
100 general appearance of the premaxillary beak resembles that of extant Galliformes, 101 particularly in its gently downcurved tip and delicate construction, lacking ossified joints
102 among the rostral components26. The contralateral frontal processes of the premaxillae are
103 unfused along their length, and the premaxillae and nasals are unfused at both their tomial
104 and narial contacts. The beak tip is unhooked, distinguishing Asteriornis from most
105 Galloanserae except certain Anatidae and Presbyornithidae.
106 The skull lacks a distinct nasofrontal hinge. As such, the architecture of this region
107 more closely resembles that of extant Galliformes than Anseriformes. At the midpoint of the
108 orbits the frontals are constricted, yielding an hourglass-shaped cranial roof with wider rostral
109 and caudal extremities. In dorsolateral view, the right postorbital process sweeps strongly
110 ventrally before deflecting rostrally to define part of the ventral margin of the orbit.
111 The left quadrate is well preserved in three dimensions and is generally similar to the
112 quadrate of the fossil pan-anseriform Presbyornis27 (Fig. 2, Extended Data Fig. 4). The
113 prootic and squamosal heads are divided by a well-developed incisure as in almost all
114 Neognathae, contrasting with the condition in crownward stem birds like Ichthyornis where
115 the division between these condyles is poorly marked, and Palaeognathae where the condyles
116 merge into a single head. Two pneumatic foramina pierce the quadrate: the foramen
117 pneumaticum rostromediale and the foramen pneumaticum basiorbitale. A tuberculum
118 subcapitulare is moderately developed on the lateral face of the otic process. This character
119 has been considered a derived feature of Galloanserae27; however, it is present in many
120 Neoaves, and given its absence in Palaeognathae and Ichthyornis may instead represent a
121 synapomorphy of Neognathae. The cotyla jugalis of the quadrate is fairly deep, with a
122 complete, un-notched rim. The pterygoid articulation is more widely separated dorsally from
123 the mandibular condyle than in most extant Galloanserae, and is very similar to that of
124 Presbyornis. 125 As in all known Galloanserae, the mandible of Asteriornis exhibits two cotylae for
126 articulation with the bicondylar quadrate and a blade-like retroarticular process. This process
127 is strongly hooked, and in its shape and proportions bears a strong resemblance to the
128 condition in the pan-anseriform Anatalavis oxfordi (Fig. 1d; Extended Data Fig. 5). An
129 elongate, slightly dorsally oriented processus medialis is preserved on the right jaw, as in all
130 Galloanserae.
131 The left distal femur is well preserved, exhibiting a medial condyle with a subangular
132 profile between the condyle’s articular surface and its cranial margin. Detailed anatomical
133 description of the skull and postcranium is provided in the Supplementary Information.
134
135 Phylogenetic Analysis
136 We investigated the phylogenetic position of Asteriornis under alternative optimality criteria,
137 with and without molecular scaffolds1,2, using a matrix of 39 taxa and 297 characters
138 modified from previous studies28,29 (Fig. 3, Extended Data Fig. 9; see Methods and
139 Supplementary Information). We added recently described taxa such as Protodontopteryx30 ,
140 and revised scorings for several taxa including Anatalavis, Presbyornis, and Conflicto. Our
141 analyses were uniformly consistent with a position of Asteriornis near the last common
142 ancestor of Galloanserae. Under parsimony we recovered a single most parsimonious tree
143 with Asteriornis as the sister taxon to crown-Galloanserae, while analyses under a tip-dated
144 Bayesian framework recovered Asteriornis as the stemward-most pan-galliform. We are
145 cautious in the interpretation of this result because few unambiguous synapomorphies can be
146 optimised in support of an Asteriornis + Galliformes clade to the exclusion of Anseriformes.
147 The few potential synapomorphies for such a clade that are observable in Asteriornis also
148 occur in early pan-anseriforms such as Conflicto (see Methods and Supplementary
149 Information). Likewise, only 1-2 steps are required to move Asteriornis to an alternate 150 placement as a stem-anseriform or stem-galliform in the parsimony analysis. The difficulty in
151 resolving the precise placement of the fossil, as well as its striking combination of
152 anseriform- and galliform-like anatomical features, is consistent with Asteriornis occupying a
153 short phylogenetic branch proximal to the most recent common ancestor of Galloanserae, one
154 of the deepest nodes in the neornithine tree of life31.
155
156 Discussion
157 The latest Cretaceous fossil record of crown birds is extremely sparse. Other than Asteriornis,
158 Vegavis iaai (~66.5 Ma32) is the only Cretaceous neornithine known from a partial skeleton
159 to have been thoroughly examined12. However, its phylogenetic position is debated33—
160 recent studies have suggested variable positions within Neognathae12,28,33-35 and even outside
161 Neornithes36— emphasising the importance of obtaining new information on Vegavis and
162 Mesozoic crown birds in general. Under parsimony, we recovered Vegavis as the sister taxon
163 to Neornithes (i.e. the crownward-most stem bird), while under a Bayesian framework
164 Vegavis was recovered in an unresolved position at the base of Neognathae.
165
166 Galloanseran cranial anatomy.
167 All discernible character evidence is consistent with neornithine (crown bird) affinities for
168 Asteriornis, and support its hierarchical placement within Neornithes (local synapomorphies
169 such as a toothless beak; no coronoid bone; bony mandibular symphysis), crown Neognathae
170 (e.g., palatine-premaxilla contact; incisure between prootic and squamosal cotylae of
171 quadrate), and total group Galloanserae (e.g., maxillary process of premaxilla dorsoventrally
172 deep and lateromedially compressed; palatines long, thin, and widely separated rostrally;
173 bicondylar mandibular process of the quadrate; long, dorsally oriented internal articular
174 process of mandible; retroarticular process long, curving, and strongly compressed 175 lateromedially)37-39 (Fig. 1). Given the scarcity of three-dimensional crown bird skulls from
176 the earliest stages of the crown bird radiation, Asteriornis provides a key reference point for
177 understanding how the impressive variability of the crown bird skull came to be, and will
178 inform estimates of early neornithine cranial disparity and rates of phenotypic evolution40,41.
179 With the exception of the autapomorphic morphology of the posterior nasals, virtually
180 none of the discernible anatomy of Asteriornis falls outside the range of variation exhibited
181 by extant galloanserans, despite the widely differing cranial morphology of extant
182 Galliformes and Anseriformes26. Although galloanseran monophyly is now almost
183 universally accepted (and was first suggested on the basis of basicranial morphology as early
184 as the mid-19th Century42), the validity of this clade has been questioned26,43, with some
185 authors enumerating lengthy lists of anatomical dissimilarities between these groups26.
186 Strikingly, Asteriornis reveals a previously undocumented combination of ‘galliform’
187 features such as weakly fused rostral elements and rostrally forked nasals, with ‘anseriform’
188 features such as a rostrally-projecting postorbital process and a tall and strongly hooked
189 retroarticular process (Fig. 1, Extended Data Figs. 1-6), illuminating the plesiomorphic
190 condition of the galloanseran skull. The fact that such distinctive features of extant
191 galloanseran anatomy are observable in this ~66.7 million-year-old fossil corroborates the
192 hypothesis of beak shape canalization arising early in, or predating, crown bird evolutionary
193 history44,45, emphasising the modular nature of the skull and bill of crown birds40.
194
195 Ancestral neornithine biogeography and the Mesozoic radiation of Neornithes.
196 Much of the species-level and higher clade diversity of extant birds is confined to vestiges of
197 Gondwana, suggesting a southern hemisphere origin of crown birds3. This hypothesis has
198 seemingly been bolstered by reports of Mesozoic crown birds in Antarctica12,34. However, the
199 Paleogene fossil record illustrates numerous examples of crown bird fossils well outside the 200 ranges of their closest extant relatives6,21,46,47, casting some doubt on the hypothesis of a
201 Gondwanan cradle for crown bird evolution and suggesting that factors such as Cenozoic
202 climate shifts may have overprinted ancestral neornithine biogeographic patterns7,48. As the
203 oldest and among the most deeply diverging crown birds yet identified, Asteriornis provides
204 the first conclusive Mesozoic evidence of Neornithes in the northern hemisphere,
205 emphasising that convincingly identifying the neornithine Centre of Origin must await the
206 discovery of further Mesozoic fossils.
207 Asteriornis provides a firm calibration point for the minimum divergence of the major
208 bird clades Galloanserae and Neoaves. We recommend a minimum age of 66.7 Ma for this
209 pivotal neornithine node in future divergence time studies, reflecting the youngest possible
210 age of the Asteriornis holotype including geochronological uncertainty. The paucity of crown
211 bird remains from the Mesozoic is such that our current understanding of the deepest stages
212 of the neornithine evolutionary timescale is dependent on the outcome of molecular
213 divergence time analyses9. However, molecular clock estimates for the deepest crown bird
214 divergences differ markedly depending on factors such as node calibration decisions and
215 alternative parameterisations of statistical probability distributions8,10. Furthermore, such
216 estimates may be affected by the interplay of nucleotide substitution rates and deep-time
217 directional selectivity on life history variables and body size, leading to dissent regarding the
218 extent of neornithine survivorship across the end-Cretaceous mass extinction event8-10. We
219 suggest that the terminal Cretaceous age and rootward phylogenetic position of Asteriornis
220 are consistent with a limited, Late Cretaceous diversification of crown birds, and restricted
221 survivorship of crown birds across the end-Cretaceous mass extinction event. This
222 interpretation is in line with developing consensus from recent molecular divergence time
223 studies1,2,8-10, as well as evidence for a mass extinction of avialans at the K–Pg boundary20.
224 Furthermore, the oldest credible occurrences of crown birds in both the northern (this study) 225 and southern hemispheres12 derive from terminal Mesozoic sediments, which may
226 substantiate the recently articulated null expectation of a rapid evolutionary transition
227 between stem birds and crown birds31. The occurrence of a large Ichthyornis-like avialan13
228 from the same horizon and quarry as the Asteriornis holotype (with temporal separation
229 amounting to no more than tens of thousands of years—see Supplementary Information)
230 provides the best evidence to date of crown birds and avialan stem birds occurring in the
231 same environment in the immediate lead up to the K-Pg mass extinction.
232 Considerable discussion has focussed on hypothetical scenarios of neornithine
233 survivorship across the end-Cretaceous mass extinction4. A consensus hypothesis is
234 emerging, whereby survivors most likely exhibited a suite of features that would have proven
235 selectively beneficial through the extinction event, including: relatively small body size to
236 limit total metabolic requirements9, flying ability and a non-arboreal ecology4, an advanced
237 digestive system49, and dietary flexibility to capitalise on sparse resources which may have
238 included insects and seeds5. However, due to the scarcity of latest Cretaceous and earliest
239 Paleocene neornithine fossils, little direct fossil evidence can be brought to bear on this
240 question. Asteriornis may therefore provide the best direct insight into the probable biology
241 of neornithine K–Pg survivors, and, notably, no discernible aspects of its palaeobiology are
242 inconsistent with these hypothetical expectations. Indeed, its relatively small size (less than
243 50% of the median body size of known latest Maastrichtian avialans9), narrow and elongate
244 hindlimbs (Supplementary Information), and provenance from nearshore marine sediments
245 may indicate a littoral ecology, perhaps validating an ecological—though not phylogenetic—
246 prediction of the now-unfashionable hypothesis of ‘shorebird’-like origins for much of crown
247 bird diversity50.
248 Fossils bearing directly on the Mesozoic origins of the extant bird radiation are among
249 the rarest and most sought-after palaeontological discoveries. Asteriornis helps fill a critical 250 phylogenetic and stratigraphic gap between the most crownward-known stem birds and the
251 rich Cenozoic neornithine fossil record. This first record of a Mesozoic neornithine from
252 Europe reveals long-sought insights into the origin of extant bird diversity, and illustrates that
253 future discoveries of even earlier Cretaceous neornithines may be as likely to derive from the
254 northern hemisphere as the southern hemisphere.
255
256 References
257 1 Prum, R. O. et al. A comprehensive phylogeny of birds (Aves) using targeted next-
258 generation DNA sequencing. Nature 526, 569-573, doi:10.1038/nature15697 (2015).
259 2 Jarvis, E. D. et al. Whole-genome analyses resolve early branches in the tree of life of
260 modern birds. Science 346, 1320-1331, doi:10.1126/science.1253451 (2014).
261 3 Claramunt, S. & Cracraft, J. A new time tree reveals Earth history’s imprint on the
262 evolution of modern birds. Science Advances 1, doi:10.1126/sciadv.1501005 (2015).
263 4 Field, D. J. et al. Early Evolution of Modern Birds Structured by Global Forest
264 Collapse at the End-Cretaceous Mass Extinction. Current Biology 28, 1825-
265 1831.e1822, doi:10.1016/j.cub.2018.04.062 (2018).
266 5 Larson, D. W., Brown, C. M. & Evans, D. C. Dental disparity and ecological stability
267 in bird-like dinosaurs prior to the end-Cretaceous mass extinction. Current Biology
268 26, 1325-1333 (2016).
269 6 Mayr, G. Avian higher level biogeography: Southern Hemispheric origins or Southern
270 Hemispheric relicts? Journal of Biogeography 44, 956-958 (2017).
271 7 Saupe, E. E. et al. Climatic shifts drove major contractions in avian latitudinal
272 distributions throughout the Cenozoic. Proceedings of the National Academy of
273 Sciences 116, 12895-12900, doi:10.1073/pnas.1903866116 (2019). 274 8 Ksepka, D. T. & Phillips, M. J. Avian diversification patterns across the K-Pg
275 boundary: influence of calibrations, datasets, and model misspecification. Annals of
276 the Missouri Botanical Garden 100, 300-328, doi:10.3417/2014032 (2015).
277 9 Berv, J. S. & Field, D. J. Genomic Signature of an Avian Lilliput Effect across the K-
278 Pg Extinction. Systematic Biology 67, 1-13, doi:10.1093/sysbio/syx064 (2018).
279 10 Field, D. J. et al. Timing the extant avian radiation: The rise of modern birds, and the
280 importance of modeling molecular rate variation. PeerJ Preprints 7, e27521v27521,
281 doi:10.7287/peerj.preprints.27521v1 (2019).
282 11 Mayr, G. Avian Evolution (John Wiley & Sons, 2016).
283 12 Clarke, J. A., Tambussi, C. P., Noriega, J. I., Erickson, G. M. & Ketcham, R. A.
284 Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433,
285 305-308 (2005).
286 13 Dyke, G. J. et al. Europe's last Mesozoic bird. Naturwissenschaften 89, 408-411,
287 doi:10.1007/s00114-002-0352-9 (2002).
288 14 Xing, L., Stanley, E. L., Bai, M. & Blackburn, D. C. The earliest direct evidence of
289 frogs in wet tropical forests from Cretaceous Burmese amber. Scientific Reports 8,
290 8770, doi:10.1038/s41598-018-26848-w (2018).
291 15 Simões, T. R. et al. The origin of squamates revealed by a Middle Triassic lizard from
292 the Italian Alps. Nature 557, 706-709, doi:10.1038/s41586-018-0093-3 (2018).
293 16 Evers, S. W., Barrett, P. M. & Benson, R. B. J. Anatomy of Rhinochelys pulchriceps
294 (Protostegidae) and marine adaptation during the early evolution of chelonioids.
295 PeerJ 7, e6811, doi:10.7717/peerj.6811 (2019).
296 17 Bi, S. et al. An Early Cretaceous eutherian and the placental–marsupial dichotomy.
297 Nature 558, 390-395, doi:10.1038/s41586-018-0210-3 (2018). 298 18 Lee, M. S. Y. & Yates, A. M. Tip-dating and homoplasy: reconciling the shallow
299 molecular divergences of modern gharials with their long fossil record. Proceedings
300 of the Royal Society B: Biological Sciences 285, 20181071,
301 doi:10.1098/rspb.2018.1071 (2018).
302 19 Hope, S. The Mesozoic radiation of Neornithes. Mesozoic Birds: Above the Heads of
303 Dinosaurs, 339-388 (2002).
304 20 Longrich, N. R., Tokaryk, T. & Field, D. J. Mass extinction of birds at the
305 Cretaceous-Paleogene (K-Pg) boundary. Proceedings of the National Academy of
306 Sciences 108, 15253-15257 (2011).
307 21 Mayr, G. Paleogene Fossil Birds. (Springer, 2009).
308 22 Clyde, W. C., Ramezani, J., Johnson, K. R., Bowring, S. A. & Jones, M. M. Direct
309 high-precision U–Pb geochronology of the end-Cretaceous extinction and calibration
310 of Paleocene astronomical timescales. Earth and Planetary Science Letters 452, 272-
311 280 (2016).
312 23 Gauthier, J. A. & de Queiroz, K. in New Perspectives on the Origin and Early
313 Evolution of Birds: Proceedings of the International Symposium in Honor of John H.
314 Ostrom (eds Jacques A. Gauthier & Larry F. Gall) (Peabody Museum of Natural
315 History, Yale University, 2001).
316 24 Keutgen, N. A bioclast-based astronomical timescale for the Maastrichtian in the type
317 area (southeast Netherlands, northeast Belgium) and stratigraphic implications: the
318 legacy of PJ Felder. Netherlands Journal of Geosciences 97, 229-260 (2018).
319 25 Field, D. J., Lynner, C., Brown, C. & Darroch, S. A. F. Skeletal correlates for body
320 mass estimation in modern and fossil flying birds. PLoS ONE 8, e82000 (2013).
321 26 Olson, S. L. & Feduccia, A. Presbyornis and the origin of the Anseriformes (Aves:
322 Charadriomorphae). Smithsonian Contributions to Zoology 323, 1-24 (1980). 323 27 Elzanowski, A. & Stidham, T. A. Morphology of the quadrate in the Eocene
324 anseriform Presbyornis and extant galloanserine birds. Journal of Morphology 271,
325 305-323, doi:10.1002/jmor.10799 (2010).
326 28 Worthy, T. H., Degrange, F. J., Handley, W. D. & Lee, M. S. Y. The evolution of
327 giant flightless birds and novel phylogenetic relationships for extinct fowl (Aves,
328 Galloanseres). Royal Society Open Science 4, doi:10.1098/rsos.170975 (2017).
329 29 Tambussi, C. P., Degrange, F. J., De Mendoza, R. S., Sferco, E. & Santillana, S. A
330 stem anseriform from the early Palaeocene of Antarctica provides new key evidence
331 in the early evolution of waterfowl. Zoological Journal of the Linnean Society 186,
332 673-700, doi:10.1093/zoolinnean/zly085 (2019).
333 30 Mayr, G., De Pietri, V. L., Love, L., Mannering, A. & Scofield, R. P. Oldest, smallest
334 and phylogenetically most basal pelagornithid, from the early Paleocene of New
335 Zealand, sheds light on the evolutionary history of the largest flying birds. Papers in
336 Palaeontology n/a, doi:10.1002/spp2.1284 (2019).
337 31 Budd, G. E. & Mann, R. P. The dynamics of stem and crown groups. bioRxiv,
338 633008, doi:10.1101/633008 (2019).
339 32 Ksepka, D. T. & Clarke, J. Phylogenetically vetted and stratigraphically constrained
340 fossil calibrations within Aves. Palaeontologia Electronica 18.1.3FC, 1-25,
341 doi:palaeo-electronica.org/content/fc-3 (2015).
342 33 Mayr, G., De Pietri, V. L., Scofield, R. P. & Worthy, T. H. J. C. R. On the taxonomic
343 composition and phylogenetic affinities of the recently proposed clade Vegaviidae
344 Agnolín et al., 2017‒neornithine birds from the Upper Cretaceous of the Southern
345 Hemisphere. 86, 178-185 (2018).
346 34 Clarke, J. A. et al. Fossil evidence of the avian vocal organ from the Mesozoic.
347 Nature 538, 502, doi:10.1038/nature19852 (2016). 348 35 Agnolín, F. L., Egli, F. B., Chatterjee, S., Marsà, J. A. G. & Novas, F. E. Vegaviidae,
349 a new clade of southern diving birds that survived the K/T boundary. The Science of
350 Nature 104, 87 (2017).
351 36 O'Connor, J. K., Chiappe, L. M. & Bell, A. in Living Dinosaurs: The evolution of
352 modern birds (eds G. Dyke & G. Kaiser) 39-114 (Wiley-Blackwell, 2011).
353 37 Cracraft, J. The major clades of birds. The Phylogeny and Classification of the
354 Tetrapods 1, 339-361 (1988).
355 38 Livezey, B. C. A phylogenetic analysis of basal Anseriformes, the fossil Presbyornis,
356 and the interordinal relationships of waterfowl. Zoological Journal of the Linnean
357 Society 121, 361-428 (1997).
358 39 Cracraft, J. & Clarke, J. The basal clades of modern birds. New Perspectives on the
359 Origin and Early Evolution of Birds: Proceedings of the International Symposium in
360 Honor of John H. Ostrom (J. Gauthier and LF Gall, Eds.). Peabody Museum of
361 Natural History, Yale University, New Haven, Connecticut, 143–156 (2001).
362 40 Felice, R. N. & Goswami, A. Developmental origins of mosaic evolution in the avian
363 cranium. Proceedings of the National Academy of Sciences 115, 555-560,
364 doi:10.1073/pnas.1716437115 (2018).
365 41 Field, D. J. Endless skulls most beautiful. Proceedings of the National Academy of
366 Sciences 115, 448-450, doi:10.1073/pnas.1721208115 (2018).
367 42 Huxley, T. H. On the classification of birds; and on the taxonomic value of the
368 modifications of certain of the cranial bones observable in that class. Proceedings of
369 the Zoological Society of London 1867, 415-472 (1867).
370 43 Ericson, P. G. P. Systematic relationships of the Palaeogene family Presbyornithidae
371 (Aves: Anseriformes). Zoological Journal of the Linnean Society 121, 429-483
372 (1997). 373 44 Cooney, C. R. et al. Mega-evolutionary dynamics of the adaptive radiation of birds.
374 Nature 542, 344-347, doi:10.1038/nature21074 (2017).
375 45 Bright, J. A., Marugán-Lobón, J., Rayfield, E. J. & Cobb, S. N. The multifactorial
376 nature of beak and skull shape evolution in parrots and cockatoos (Psittaciformes).
377 BMC Evolutionary Biology 19, 104, doi:10.1186/s12862-019-1432-1 (2019).
378 46 Field, D. J. & Hsiang, A. Y. A North American stem turaco, and the complex
379 biogeographic history of modern birds. BMC Evolutionary Biology 18, 102,
380 doi:10.1186/s12862-018-1212-3 (2018).
381 47 Mourer-Chauviré, C. Les oiseaux fossiles des phosphorites du Quercy (Éocène
382 supérieur a Oligocène supérieur): Implications paléobiogéographiques. Geobios 15,
383 Supplement 1, 413-426, doi:http://dx.doi.org/10.1016/S0016-6995(82)80130-7
384 (1982).
385 48 Mayr, G. Two-phase extinction of "Southern Hemispheric" birds in the Cenozoic of
386 Europe and the origin of the Neotropic avifauna. Palaeobiodiversity and
387 Palaeoenvironments 91, 325-333, doi:10.1007/s12549-011-0062-4 (2011).
388 49 O'Connor, J. K. & Zhou, Z. The evolution of the modern avian digestive system:
389 insights from paravian fossils from the Yanliao and Jehol biotas. Palaeontology 63,
390 13-27, doi:10.1111/pala.12453 (2020).
391 50 Feduccia, A. Explosive evolution in Tertiary birds and mammals. Science 267, 637-
392 638 (1995).
393
394
395
396
397 398 Figure Legends
399
400 Fig. 1 | Digitally segmented skull of Asteriornis maastrichtensis. a, left lateral view, b,
401 right lateral view, c, dorsal view. d, Comparison of retroarticular processes in lateral view of
402 Anatalavis oxfordi (right side) and Asteriornis (left side, reflected). Selected synapomorphies
403 are denoted with numbers: red for Neornithes and green for Galloanserae37-39. 1, bony
404 mandibular symphysis; 2, toothless beak; 3, no coronoid bone; 4, bicondylar mandibular
405 process of quadrate; 5, retroarticular process long, curving, strongly compressed
406 mediolaterally; 6, dorsally oriented internal articular process of mandible; 7, maxillary
407 process of premaxilla dorsoventrally deep and lateromedially compressed.
408
409 Fig. 2 | Comparative quadrate and skull morphology of selected total group
410 Galloanserae. Left quadrates (and reflected right quadrate of Presbyornis) in medial view
411 (left) and caudal view (middle). Skulls in right lateral view except Alectura which is in
412 reflected left lateral view. Skull of Presbyornis USNM 299846 shown. Scale bars for
413 quadrates are 5 mm, and for skulls are 1 cm. See Extended Data Fig. 4 for additional high-
414 resolution imagery of these taxa and other extant and fossil Galloanserae.
415
416 Fig. 3 | Relationships of Asteriornis maastrichtensis and stratigraphic provenance of
417 holotype. a, Cladogram showing the phylogenetic position of Asteriornis and Vegavis
418 inferred under parsimony (solid coloured lines) and tip-dated Bayesian analyses (dashed
419 lines). A position for Vegavis allied with Anseriformes, as previously found12,28,34, is also
420 shown (curved branch). Numbers within clades denote extant species richness. Based on a
421 newly modified dataset; see Extended Data Fig. 9 and Supplementary Information for full
422 phylogenetic results. b, simplified stratigraphy of the Maastricht Formation exposed in the 423 vicinity of the holotype locality and surrounding areas24. Asteroid icon denotes K–Pg
424 boundary. c, Location of the CBR-Romontbos Quarry near Eben-Emael, Belgium.
425
426 Methods
427 Preparation and imaging of specimens
428 Due to fragility of the Asteriornis holotype, only minor mechanical preparation was
429 performed to reduce the volume of rock matrix for improved CT scans. Scans of NHMM
430 2013 008, UW Presbyornis specimens, and all extant taxa were performed at the Cambridge
431 Biotomography Centre (CBC). Presbyornis UMNH.VP. 29030 and UMNH.VP.29031 were
432 scanned at the University of Utah Health Science Cores small animal imaging core research
433 facilities. Anatalavis oxfordi NHMUK PV A5922 was scanned at the Natural History
434 Museum London. All scanned material was digitally segmented and rendered using
435 VGStudio Max 3.3.0. Full CT scanning details are provided in the Supplementary
436 Information.
437
438 Phylogenetic analyses
439 To assess the phylogenetic position of Asteriornis, we scored it into a considerably modified
440 version of a recent phylogenetic matrix targeting deep neornithine divergences and focussing
441 on Galloanserae29, which in turn was modified from a prior study28. We additionally added
442 the crownward stem-bird Ichthyornis dispar as an outgroup taxon to improve inference of
443 character polarity at the base of crown-birds, based on direct examination of specimens and
444 previously published literature51,52. Through direct examination and reference to published
445 descriptions we also scored the stem-galliform Gallinuloides wyomingensis53,54, and scored
446 two pelagornithids based on published literature: Protodontopteryx ruthae30 and Pelagornis
447 chilensis55. The phylogenetic position of Pelagornithidae is debated, and they have been 448 posited as early-diverging pan-neognaths or pan-galloanserans by previous studies56,57. A
449 right humerus and ulna associated with the Protodontopteryx holotype are of questionable
450 assignment to the taxon30 and were therefore disregarded for the purposes of scoring.
451 Additional details of matrix modifications and taxon sampling are presented in the
452 Supplementary Information.
453 Maximum parsimony analyses were conducted in TNT 1.558. After increasing the
454 maximum number of trees to 10,000, a new technology search was run in which a minimum
455 length tree was found in 10 replicates and default parameters were set for sectorial search,
456 ratchet, tree drift, and tree fusion. Following this, the maximum number of trees was set to
457 100 and a traditional search with default parameters was run on the trees in RAM to explore
458 treespace more extensively. Absolute bootstrap frequencies were obtained from 1,000
459 replicates under a traditional search with default parameters. The tree length induced by
460 alternative phylogenetic topologies was evaluated by unchecking the “lock trees” option in
461 TNT and manually moving branches to alternative phylogenetic positions.
462 Bayesian phylogenetic analyses were conducted in MrBayes59 using the CIPRES
463 Science Gateway60. Data were analysed under the Mkv model61, taking into account the
464 absence of invariant characters in our dataset. To allow for variation in evolutionary rate
465 across different characters, gamma-distributed rate variation was assumed. Analyses were
466 conducted using four chains and two independent runs, with a tree sampled every 4,000
467 generations and a burn-in of 25%. We performed Bayesian tip-dating under the fossilized
468 birth-death model62, implemented in MrBayes following established procedures63. An
469 Independent Gamma Rates (IGR) clock model was used. The clock rate prior was set at a
470 diffuse, uninformative rate, whereas the IGR variation, speciation, extinction, and
471 fossilization priors were set at the MrBayes default distributions: exp(10) for the former two
472 parameters and beta(1,1) for the latter two. These settings follow other recent tip-dating 473 studies64,65. Given that our extant taxon sample did not reflect the relative diversity of extant
474 bird groups—focussing primarily on galloanserans (which represent 72% of our extant taxon
475 sample, contrasting with ~4% of extant bird diversity based on recent species counts66)—we
476 assumed a random instead of diversified sampling strategy. Taxon sampling probability was
477 assigned a value of 0.0023, based on the fact that our extant taxon sample covers roughly
478 0.23% of extant bird diversity66.
479 To account for ongoing controversies regarding the timing of crown-bird
480 diversification, two tip-dating analyses were run, each with different prior age probability
481 distributions. In one analysis, a soft maximum upper bound was set at 86.5 Ma for crown-
482 birds, following recent justifications1. This represents the age of the Niobrara Formation, a
483 well-sampled Upper Cretaceous fossil-bearing formation from which no crown-bird fossils
484 have been recovered despite an abundance of small vertebrates and crownward stem-bird
485 fossils52,67. Berv and Field9 found evidence for elevated substitution rates at the base of
486 Neornithes. These authors suggested that this phenomenon may be partially responsible for
487 the older ages estimated by previous studies68,69 and outlined criteria under which the use of
488 “appropriately conservative” age priors are justified, concluding that crown-birds, like
489 placental mammals70, are one clade for which such conservative treatment may be
490 warranted9. As such, a more restrictive soft maximum of 72.72 Ma was used for the second
491 tip-dating analysis, based on the age of crown-birds estimated by Prum et al.1. For both tip-
492 dating analyses, a soft maximum of 120 Ma was assigned to the root (Ichthyornis +
493 Neornithes) reflecting the minimum age of the Jiufotang Formation, the youngest phase of
494 the Jehol Biota71. This assemblage preserves an abundance of Lower Cretaceous avialan
495 fossils, but none so far that have been recovered phylogenetically crownward of
496 Ichthyornis72. Given that no fossil crown-neoavians were included in our dataset, a prior age
497 probability distribution was assigned to the neoavian clade Gruiformes, with a minimum age 498 of 53.95 Ma (the age of Pellornis mikkelseni, the oldest well-established crown gruiform73)
499 and a soft maximum of 66 Ma (the age of the Cretaceous-Paleogene mass extinction, prior to
500 which no well-corroborated crown-neoavian fossils have been identified74). An exponential
501 distribution was used for all of these age priors, allowing for a 5% prior probability of each
502 clade having originated earlier than its specified soft maximum.
503 The ages of fossil taxa were fixed at the minimum age constraints for the specimens
504 used to score our morphological dataset, in line with best practices75 and the results of
505 experimentally testing alternative calibrations on an empirical dataset76. Asteriornis was
506 accordingly assigned a minimum age of 66.7 Ma. Most minimum age calibrations for taxa
507 previously included in our dataset follow published ages28. Following the recommended ages
508 of relevant fossil-bearing localities, Gallinuloides from the Green River Formation was
509 assigned an age of 51.57 Ma and Anatalavis from the London Clay Formation was assigned
510 an age of 53.5 Ma32. Despite being known from younger strata, Presbyornis was also
511 assigned an age of 51.57 Ma because it had been scored into the dataset based on specimens
512 held at the National Museum of Natural History in Washington, D.C.77, which had primarily
513 been collected from the Green River Formation. Ichthyornis was assigned an age of 86.5
514 Ma78, Conflicto was assigned an age of 61 Ma29, Pelagornis was assigned an age of 4.8 Ma55,
515 and Protodontopteryx was assigned an age of 61.5 Ma30.
516 Undated Bayesian analyses were run for 30,000,000 generations, whereas tip-dated
517 analyses were run for 60,000,000 generations. Analytical convergence was assessed using
518 standard diagnostics provided in MrBayes (average standard deviation of split frequencies <
519 0.02, potential scale reduction factors = 1, effective sample sizes > 200). Results of
520 independent runs of the same analyses were summarized using the sump and sumt commands
521 in MrBayes. Morphological synapomorphies were optimized under parsimony onto resulting 522 tree topologies using TNT. Molecular scaffolds followed consensus from recent
523 phylogenomic studies1,2,79-81.
524
525 Body size estimation
526 Minimum mediolateral cross-sectional diameter of the tarsometatarsus shaft (3.32 mm) was
527 taken from high-resolution CT scans. This measurement was incorporated into a published
528 scaling equation for estimating volant bird body masses25 (R2 = 0.93) and compared with a
529 published compilation of extant galloanseran body sizes82.
530
531 Data availability
532 The holotype specimen of Asteriornis maastrichtensis is deposited in the permanent
533 collection of the Natuurhistorisch Museum Maastricht under collection number NHMM 2013
534 008. Digital models of the A. maastrichtensis skull and postcranial elements, .tre files from
535 phylogenetic analyses, and CT scans of the A maastrichtensis holotype are available at
536 Zenodo.org (doi: 10.5281/zenodo.3610226). The Life Science Identifier for Asteriornis is
537 urn:lsid:zoobank.org:pub:32192A46-4A43-48CE-8F17-447900FCC6DF.
538
539 Acknowledgments We thank M. van Dinther for collecting and donating the specimen, J.
540 Vellekoop for advice on geochronology, K. Smithson, T. Thompson and V. Fernandez for
541 scanning support, M. Brooke, M. Lowe, M. Clementz, L. Vietti, J. Cooper, J. White, C.
542 Levitt, R. Irmis, K. MacKenzie and J. Sertich for collections assistance, B. Creisler for
543 etymological information, and T. Worthy for anatomical advice. We are grateful to L.
544 Witmer and F. Degrange for sharing 3D models of Presbyornis and Conflicto, respectively,
545 and to P. Krzeminski for his artwork. D.J.F. acknowledges support from UK Research and
546 Innovation Future Leaders Fellowship MR/S032177/1, Royal Society Research Grant 547 RGS/R2/192390, a Systematics Association Research Grant, and the Isaac Newton Trust.
548 J.B. acknowledges the Hesse Award from the American Ornithological Society, and grants
549 from the Jurassic Foundation, Geological Association, and Paleontological Society. D.T.K.
550 acknowledges support from NSF award DEB 1655736.
551
552 Author contributions J.W.M.G. provided the material and stratigraphic data. D.J.F.
553 prepared the specimens. D.J.F. and J.B. acquired CT scans and discovered the skull. D.J.F.,
554 J.B. and A.C. performed digital segmentation of the material and created figures. D.J.F., J.B.,
555 A.C. and D.T.K. performed anatomical comparisons and J.B. and A.C. performed the
556 phylogenetic analyses. D.J.F. wrote the paper, with contributions from all authors.
557
558 Competing interests The authors declare no competing interests.
559
560 Additional Information
561 Supplementary information is available for this paper.
562 Correspondence and requests for materials should be addressed to D.J.F.
563 Reprints and permissions information is available at www.nature.com/reprints
564
565
566
567
568
569
570
571 572
573 Methods References
574 51 Clarke, J. A. Morphology, phylogenetic taxonomy, and systematics of Ichthyornis and
575 Apatornis (Avialae: Ornithurae). Bulletin of the American Museum of Natural History
576 286, 1-179 (2004).
577 52 Field, D. J. et al. Complete Ichthyornis skull illuminates mosaic assembly of the avian
578 head. Nature 557, 96-100, doi:10.1038/s41586-018-0053-y (2018).
579 53 Mayr, G. & Weidig, I. The early Eocene bird Gallinuloides wyomingensis – a stem
580 group representative of Galliformes. Acta Palaeontologica Polonica 49, 211-217
581 (2004).
582 54 Ksepka, D. T. Broken gears in the avian molecular clock: new phylogenetic analyses
583 support stem galliform status for Gallinuloides wyomingensis and rallid affinities for
584 Amitabha urbsinterdictensis. Cladistics 25, 173-197 (2009).
585 55 Mayr, G. & Rubilar-Rogers, D. Osteology of a new giant bony-toothed bird from the
586 Miocene of Chile, with a revision of the taxonomy of Neogene Pelagornithidae.
587 Journal of Vertebrate Paleontology 30, 1313-1330 (2010).
588 56 Bourdon, E. Osteological evidence for sister group relationship between pseudo-
589 toothed birds (Aves: Odontopterygiformes) and waterfowls (Anseriformes).
590 Naturwissenschaften 92, 586-591, doi:10.1007/s00114-005-0047-0 (2005).
591 57 Mayr, G. Cenozoic mystery birds - on the phylogenetic affinities of bony-toothed
592 birds (Pelagornithidae). Zoologica Scripta 40, 448-467, doi:10.1111/j.1463-
593 6409.2011.00484.x (2011).
594 58 Goloboff, P. A. & Catalano, S. A. TNT version 1.5, including a full implementation
595 of phylogenetic morphometrics. Cladistics 32, 221-238 (2016). 596 59 Ronquist, F. et al. MrBayes 3.2: Efficient Bayesian phylogenetic inference and model
597 choice across a large model space. Syst Biol 61, 539 - 542 (2012).
598 60 Miller, M. A., Pfeiffer, W. & Schwartz, T. in Gateway Computing Environments
599 Workshop (GCE), 2010. 1-8 (Ieee).
600 61 Lewis, P. O. A likelihood approach to estimating phylogeny from discrete
601 morphological character data. Systematic Biology 50, 913-925 (2001).
602 62 Heath, T. A., Huelsenbeck, J. P. & Stadler, T. The fossilized birth-death process for
603 coherent calibration of divergence-time estimates. Proceedings of the National
604 Academy of Sciences 111, E2957-E2966 (2014).
605 63 Zhang, C., Stadler, T., Klopfstein, S., Heath, T. A. & Ronquist, F. Total-Evidence
606 Dating under the Fossilized Birth–Death Process. Systematic biology, syv080 (2016).
607 64 Kealy, S. & Beck, R. Total evidence phylogeny and evolutionary timescale for
608 Australian faunivorous marsupials (Dasyuromorphia). BMC Evolutionary Biology 17,
609 240, doi:10.1186/s12862-017-1090-0 (2017).
610 65 Vinther, J., Parry, L., Briggs, D. E. & Van Roy, P. Ancestral morphology of crown-
611 group molluscs revealed by a new Ordovician stem aculiferan. Nature 542, 471
612 (2017).
613 66 Gill, F. & Donsker, D. (2019).
614 67 Field, D. J., LeBlanc, A., Gau, A. & Behlke, A. D. B. Pelagic neonatal fossils support
615 viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology, 401-
616 407, doi:10.1111/pala.12165 (2015).
617 68 Ericson, P. G. P. et al. Diversification of Neoaves: integration of molecular sequence
618 data and fossils. Biology Letters 2, 543-547, doi:doi: 10.1098/rsbl.2006.0523 (2006). 619 69 Jetz, W., Thomas, G. H., Joy, J. B., Hartmann, K. & Mooers, A. O. The global
620 diversity of birds in space and time. Nature 491, 444-448,
621 doi:doi:10.1038/nature11631 (2012).
622 70 Phillips, M. J. Geomolecular Dating and the Origin of Placental Mammals. Systematic
623 biology, syv115 (2015).
624 71 He, H. Y. et al. Timing of the Jiufotang Formation (Jehol Group) in Liaoning,
625 northeastern China, and its implications. Geophysical Research Letters 31,
626 doi:10.1029/2004GL019790 (2004).
627 72 Wang, X. et al. The earliest evidence for a supraorbital salt gland in dinosaurs in new
628 Early Cretaceous ornithurines. Scientific Reports 8, 3969, doi:10.1038/s41598-018-
629 22412-8 (2018).
630 73 Musser, G., Ksepka, D. T. & Field, D. J. New Material of Paleocene-Eocene Pellornis
631 (Aves: Gruiformes) Clarifies the Pattern and Timing of the Extant Gruiform
632 Radiation. Diversity 11, 102 (2019).
633 74 Ksepka, D. T., Stidham, T. & Williamson, T. Early Paleocene landbird supports rapid
634 phylogenetic and morphological diversification of crown birds after the K-Pg mass
635 extinction. Proceedings of the National Academy of Sciences 114, 8047-8052 (2017).
636 75 Parham, J. F. et al. Best practices for justifying fossil calibrations. Systematic Biology
637 61, 346 - 359 (2012).
638 76 Püschel, H. P., O'Reilly, J. E., Pisani, D. & Donoghue, P. C. J. The impact of fossil
639 stratigraphic ranges on tip-calibration, and the accuracy and precision of divergence
640 time estimates. Palaeontology n/a, doi:10.1111/pala.12443 (2019).
641 77 Worthy, T. H. et al. Osteology Supports a Stem-Galliform Affinity for the Giant
642 Extinct Flightless Bird
643 Galloanseres). PLoS ONE 11, e0150871, doi:10.1371/journal.pone.0150871 (2016). 644 78 Benton, M. J. & Donoghue, P. C. J. Paleontological evidence to date the tree of life.
645 Molecular Biology and Evolution 24, 26 (2007).
646 79 Reddy, S. et al. Why do phylogenomic data sets yield conflicting trees? Data type
647 influences the avian tree of life more than taxon sampling. Systematic biology 66,
648 857-879 (2017).
649 80 Hackett, S. J. et al. A phylogenomic study of birds reveals their evolutionary history.
650 Science 320, 1763-1768 (2008).
651 81 Kimball, R. T. et al. A Phylogenomic Supertree of Birds. Diversity 11, 109 (2019).
652 82 Dunning, J. B. in CRC Handbook of Avian Body Masses, Second Edition 3-554 (CRC
653 Press, 2007).
654
655 Extended Data
656 Extended Data Fig. 1 | Detailed cranial and mandibular anatomy of Asteriornis
657 maastrichtensis (NHMM 2013 008). Views of cranium are similar to those illustrated in Fig.
658 1, but with jaws removed to illustrate the ventral portion of the skull.
659
660 Extended Data Fig. 2 | Higher resolution cranial and mandibular anatomy of Asteriornis
661 maastrichtensis (NHMM 2013 008). Labels have been removed and images enlarged to
662 show details.
663
664 Extended Data Fig. 3 | Morphology of individually segmented skull elements from the
665 Asteriornis maastrichtensis holotype (NHMM 2013 008). Dorsal, ventral, and rostral views
666 of the frontals show the nasals separated from their in-situ position to illustrate morphology
667 of the nasofrontal contact. Scale bars are 1 cm.
668 669 Extended Data Fig. 4 | Detailed comparisons of galloanseran quadrate morphology.
670 Skulls and quadrates of extant Galliformes and total group Anseriformes. Skull of
671 Presbyornis USNM 299846 shown. Scale bar is 5 mm for quadrates, and 1 cm for skulls.
672 Skulls in left lateral view except Presbyornis which is in reflected right lateral view. FPR,
673 foramen pneumaticum rostromediale; FPB, foramen pneumaticum basiorbitale.
674
675 Extended Data Fig. 5 | Detailed comparisons of galloanseran retroarticular morphology.
676 Retroarticular regions of left mandibles in lateral (left), medial (middle), and dorsal (right)
677 views. Both the left and right mandibles of Asteriornis are shown in dorsal view, as the
678 retroarticular process is only preserved on the left mandible and the medial process is only
679 preserved on the right mandible. Images of Anatalavis are mirrored. Scale bars are 1 cm.
680
681 Extended Data Fig. 6 | Postcranial morphology of Asteriornis maastrichtensis holotype
682 NHMM 2013 008. Left distal femur of Presbyornis pervetus (UW 27596) illustrated for
683 comparison.
684
685 Extended Data Fig. 7 | Internal composition of NHMM 2013 008 blocks. a, block
686 containing the left femur, left tibiotarsus, and main portion of skull, viewed from the side
687 with the femur exposed. b, same block as in a, viewed from the side exposing the tibiotarsus.
688 c, block containing the right femur and tarsometatarsus. d, block containing the right distal
689 radius, several unidentified bone fragments, and a portion of the cranial roof near the fronto-
690 parietal suture. e, block containing the right tibiotarsus. Numerous fossil echinoderm and
691 mollusk fragments are visible within the blocks. All scale bars 1 cm.
692 693 Extended Data Fig. 8 | Relative body size of Asteriornis maastrichtensis. Mean body size
694 estimate of Asteriornis holotype25 compared with extant Galloanserae82, ranked on the x-axis
695 from smallest to largest. The mean body size estimate for Asteriornis (394 g) is closest to that
696 of male Anas crecca (392 g; 7.8th percentile among Anseriformes), and female Perdix perdix
697 (393 g; 33rd percentile among Galliformes).
698
699 Extended Data Fig. 9 | Expanded phylogenetic results. a, results of parsimony analysis.
700 Asteriornis (pink) resolves as the sister taxon to crown-Galloanserae. b, results of tip-dated
701 Bayesian analysis with a soft-maximum neornithine root age of 86.5 Ma. Estimated timescale
702 shown on x-axis, though see notes on divergence time caveats in Supplementary Information.
703 Asteriornis (pink) resolves as the stemward-most member of Pangalliformes. Colours match
704 those from Fig. 3. Extinct taxa denoted with daggers. See Supplementary Information for full
705 details of phylogenetic analyses, character list, synapomorphies of key clades, support values
706 and tree files.