Asteriornis maastrichtensis, Field & Benito & Chen & Jagt & Ksepka, 2020

Field, Daniel J., Benito, Juan, Chen, Albert, Jagt, John W. M. & Ksepka, Daniel T., 2020, Late Cretaceous neornithine from Europe illuminates the origins of crown birds, Nature 579, pp. 397-414 : 397-399

Asteriornis maastrichtensis gen.et sp.nov.

Remarks.We use Avialae to refer to theropods crownward of Dromaeosauridae and Troodontidae . Neornithes is equivalent to the bird crown group ( Aves sensu Gauthier,1986 23).Pangalloanserae defines the most inclusive clade including Anser anser and Gallus gallus but not Passer domesticus (that is,the galloanseran total group).Further phylogenetic definitions are presented in the Supplementary Information.

Etymology. Asteriornis , from the name of the Titan goddess Asteria and the Greek ornis for bird.In Greek mythology Asteria is the goddess of falling stars and transforms herself into a quail—attributes that are reflected by both the impending Cretaceous–Palaeogene(K–Pg) asteroid impact and the galloanseran affinities of Asteriornis . The specific epithet maastrichtensis reflects the provenance of the holotype—the Maastricht Formation (the type locality of the Late Cretaceous Maastrichtian stage).

Holotype. Natuurhistorisch Museum Maastricht ( NHMM) 2013 008 , a nearly complete, articulated skull including mandibles and associated postcranial remains preserved in four blocks ( Fig. 1 , Extended Data Figs.1–7 ;see Supplementary Information for videos,character information,measurements,additional description and discussion). Preserved elements include the premaxillae,maxillae,nasals,frontals, laterosphenoid,basisphenoid,mesethmoid, left quadrate,left jugal, right palatine and lower jaws.Associated postcranial elements include incomplete femora,tibiotarsi,tarsometatarsus and radius.

Locality and age. CBR-Romontbos Quarry,Eben-Emael,Liège , Belgium. Valkenburg Member(66.8–66.7million years old 24), Maastricht Formation,Late Maastrichtian,Cretaceous.Additional details regarding the locality and stratigraphic setting are presented in the Supplementary Information.

Diagnosis. Asteriornis is unique among known taxa in exhibiting caudally pointed nasals that overlie the frontals and meet at the midline of the skull,and a slightly rounded, unhooked tip of the premaxilla. Additional character combinations from phylogenetic analyses that dif- ferentiate Asteriornis are presented in the Supplementary Information.

Description

Asteriornis maastrichtensis is a small pan-galloanseran, with an estimated mean body mass from hindlimb scaling regressions of 394 g (around the 21st percentile among extant Galloanserae 25; Extended Data Fig.8 ).Complete measurements of NHMM 2013 008 are provided in the Supplementary Information.

Most major cranial components are in their original anatomical positions.The general appearance of the premaxillary beak resembles that of extant Galliformes ,particularly in its gently down-curved tip and delicate construction,with no ossified joints among the rostral components 26. The contralateral frontal processes of the premaxillae are unfused along their length,and the premaxillae and nasals are unfused at both their tomial and narial contacts. The beak tip is unhooked, which distinguishes Asteriornis from most Galloanserae except certain Anatidae and Presbyornithidae .

The skull lacks a distinct nasofrontal hinge.As such,the architecture of this region more closely resembles that of extant Galliformes than Anseriformes .At the midpoint of the orbits the frontals are constricted,yielding an hourglass-shaped cranial roof with wider rostral and caudal extremities. Indorsolateralview,therightpostorbitalprocesssweepsstronglyventrally beforedeflectingrostrallytodefinepartoftheventralmarginoftheorbit.

The left quadrate is well-preserved in three dimensions and is generally similar to the quadrate of the fossil pan-anseriform Presbyornis 27

( Fig. 2 , Extended Data Fig. 4 ). The prootic and squamosal heads are divided by a well-developed incisure,as in almost all Neognathae;this contrasts with the condition in crownward stem birds such as Ichthyornis, in which the division between these condyles is poorly marked,and Palaeognathae,in which the condyles merge into a single head.Two pneumatic foramina pierce the quadrate:the foramen pneumaticum rostromediale and the foramen pneumaticum basiorbitale.A tuberculum subcapitulare is moderately developed on the lateral face of the otic process.This characteristic has been considered a derived feature of Galloanserae 27; however,it is present in many Neoaves,and given its absence in Palaeognathae and Ichthyornis it might instead represent a synapomorphy of Neognathae.The cotyla quadratojugalis of the quadrate is fairly deep,with a complete,un-notched rim.The pterygoid articulation is more widely separated dorsally from the mandibular condyle than in most extant Galloanserae,and is very similar to that of Presbyornis .

As in all known Galloanserae,the mandible of Asteriornis exhibits two cotylae for articulation with the bicondylar quadrate and a bladelike retroarticular process.This process is strongly hooked,and in its shape and proportions bears a strong resemblance to the condition in the pan-anseriform Anatalavis oxfordi ( Fig. 1d , Extended Data Fig. 5 ). An elongate,slightly dorsally oriented processus medialis is preserved on the right jaw,as in all Galloanserae.

The left distal femur is well-preserved,exhibiting a medial condyle with a bluntly angular profile between the articular surface of the condyle and its cranial margin. A detailed anatomical description of the skull and postcranium is provided in the Supplementary Information.

Figures

Fig.1 | Digitallysegmented skull ofAsteriornis maastrichtensis. a–c,Left lateral view (a), rightlateral view (b) anddorsal view (c) of the A.maastrichtensisskull.d,Comparison of retroarticular processes inlateral view of Anatalavis oxfordi(rightside) and Asteriornis(left side,reflected). Selected synapomorphies aredenoted withnumbers (red,Neornithes;green, Galloanserae37–39):1,bony mandibular symphysis;2,toothless beak;3,no coronoid bone;4,bicondylar mandibularprocess of quadrate;5,retroarticular process long,curving,stronglycompressed mediolaterally;6,dorsally oriented internal articular process of mandible;7,maxillary process of premaxilla dorsoventrallydeep and lateromediallycompressed.Scale bars, 5 mm.

Fig.2| Comparativequadrate and skull morphology ofselected total-group Galloanserae.a–d, Left quadrates(andreflected rightquadrate of Presbyornis) inmedial view (left) andcaudal view (middle).Skulls arein right lateral view,except Alectura which is inreflected left lateral view.The skull of PresbyornisUSNM 299846 is shown.Scale bars,5mm (quadrates);1cm (skulls). See Extended DataFig.4 for additional high-resolution images of these taxa andother extant and fossil Galloanserae.

Fig.3 | Relationships ofA. maastrichtensisand stratigraphic provenanceof holotype.a, Cladogram showing the phylogeneticposition of Asteriornis and Vegavis inferred under parsimony(solid coloured lines) and tip-dated Bayesian analyses (dashedlines).A position forVegavis allied withAnseriformes,as previously found12,28,34, isalso shown (curved branch).Numbers withinclades denote extant species richness.The analysis is based ona newlymodified dataset;see Extended Data Fig.9 andSupplementary Information for full phylogenetic results.Extinct taxa aredenoted withdaggers.b, Simplified stratigraphy of the Maastricht Formation,exposed inthe vicinityof the holotype localityand surrounding areas24. c, Locationof theCBR-Romontbos Quarry near Eben-Emael,Belgium.

Extended DataFig.5 | Detailed comparisonsofgalloanseran retroarticular morphology.Retroarticularregions of left mandiblesin lateral (left),medial (middle) and dorsal (right)views.Both the left andrightmandibles of Asteriornis areshown in dorsal view,as theretroarticular process is only preserved on theleft mandible andthe medial processis only preserved onthe right mandible.Imagesof Anatalavis are mirrored.Scalebars,1 cm.

Extended DataFig.6 | Postcranial morphology of A.maastrichtensis(NHMM2013008). Theleft distal femur of Presbyornispervetus(UW 27596) is shownfor comparison.

Extended DataFig.7| Internalcomposition ofNHMM 2013008 blocks. a, Block containing the left femur,left tibiotarsusand the mainportionof the skull,viewed from theside withthe femur exposed.b, Same blockas ina, viewed from theside containing thetibiotarsus.c, Block containing theright femur andtarsometatarsus.d, Block containing therightdistal radius,several unidentified bonefragments and a portionof thecranial roofnear the frontoparietal suture.e,Block containing the righttibiotarsus.Numerous fragments of fossil echinoderms andmolluscs are visiblewithin theblocks. Scale bars,1 cm.

Extended DataFig.8 | Relativebody sizeofA.maastrichtensis. Estimateof themean body sizeof the Asteriornisholotype25 comparedwith extant Galloanserae82, rankedon the xaxis from smallest tolargest.Themean body- size estimatefor Asteriornis (394g)is closest tothat of male Anas crecca(392 g; 7.8thpercentile amongAnseriformes) andfemale Perdixperdix (393 g;33rd percentile among Galliformes).