762 ShortCommunications andCommentaries [Auk,Vol. 112 The Auk 112(3):762-767, 1995 Avian Premaxilla and Tarsometatarsusfrom the Uppermost Cretaceous of Montana ANDRZEJ ELZANOWSKIa AND MICHAEL K. BRETT-$URMAN2 •Departmentof VertebrateZoology, National Museum of NaturalHistory, SmithsonianInstitution, Washington, D.C. 20560, USA; and 2Departmentof Geology,George Washington University, Washington, D.C. 20052,USA Despitea variety of fragmentary,apparently neog- is rounded and smooth,and the sidesare very steep. nathousavian fossilsknown from the uppermostCre- The largest among the neurovascularforamina scat- taceousdeposits (Brodkorb 1963, Olson 1985, Olson tered on each side are two elongatedorsal foramina: and Parris 1987),we still lack even an approximate the vessel from the rostral one coursed rostrad, whereas idea of how many neognathouslineages survived be- the vesselfrom the caudalone apparentlybifurcated yond the Cretaceous/Tertiaryboundary. Most of the into a smaller rostral and a larger caudal branch. In Maastrichtian avian bones reveal a charadriiform or addition, a number of smaller openingsperforates transitional charadriiform-gruiform morphology, eachside of the symphysis. which may be plesiomorphicfor most (Olson 1985) The ventral surfaceof the premaxillarysymphysis but probably not all of the neognaths(Elzanowski is strongly concave(Fig. lc, d). There are no distinct 1995). Other than that, there is some fossil evidence neurovascularforamina on the ventral (palatal) sur- for the existence of loons in the Cretaceous(Olson face,with the possibleexception of one small opening 1992)and mostly indirect evidencefor the pre-Ter- on the left side. The palatal shelvesof the premaxilla tiary origins of the relict pelecaniforms(Phaethon- begin from the symphysialtip and graduallybroaden tidae and Fregatidae) and procellariiforms (Elza- caudally where each of them occupiesone-third of nowski and Gaiton 1991). The last of these are pos- the symphysialwidth. Theyare flat and slightly sloped sibly representedin the basalportion of the Early dotsad.The tomial edgesare blunt. Betweenthe pal- PaleoceneHomerstown Formation in New Jersey atal shelvesis a deep, 1.1-mm-wide median groove (Olson and Parris 1987). whosebottom merges with the roof of the symphysis. In this paper, we describetwo fragmentaryLate The groove is imperceptibly expanded in the mid- Cretaceousavian fossils:a premaxilla that fits in the length of the symphysis.A pair of neurovascularca- charadriiform-gruiformassemblage and maypossibly nals is enclosedby the shelvesventrally. representone of the alreadyestablished genera; and The preservedfragment of the dorsalbar is com- a distalfragment of the right tarsometatarsusthat may posedexclusively of the frontal processof the pre- represent the earliest record of the anseriform lin- maxilla. It cannot be determined whether the pre- eage. maxillary processof the nasal had extendedto the Both specimenscome from the locality known as symphysisand then fell off (which would be likely Bug Creek West, Montana, Hell Creek Formation, in a juvenilebird) or terminatedmore caudally. The which is of Late Cretaceous(Maastrichtian) age; the frontal processis 3.4 mm wide and 1.8 mm thick, specimensare part of the collectionsof the Museum including a strong median ridge on the ventral sur- of Paleontologyof the University of California at facebehind the symphysis(Fig. ld, e). Berkeley(UCMP). The premaxillaUCMP 117598was In the primitive condition retained by most birds, collected in 1974 by J. D. Archibald and the tarso- the premaxillarysymphysis is openventrally (i.e. ex- metatarsusUCMP 117599in 1975 by D. Lawler. cavatedup to its roof) at least along the midline. In Premaxilla.--We have made detailed comparisons the opposite,derived condition, the palatal shelves of this specimenwith the charadriiforms,gruiforms, are fusedin the midline, enclosinga medianchamber. podicipediforms,and Gavia(i.e. with thoseextant taxa Hesperornis(Elzanowski 1991:figs. I and 3) and most having a premaxilla that showsan overall similarity of the neognathousbirds with an open premaxillary to the fossilirrespective of size).The surveycovered symphysishave the palatal surfaceof the symphysis all generaof thesegroups represented in the avian perforatedby a pair of majorneurovascular foramina osteologicalcollection (Wood and Schnell 1986) of at the rostralend, a conditionpossibly plesiomorphic the National Museum of Natural History (USNM). for the Neornithes (Elzanowski 1995). The lack of The fragmentincludes the premaxillarysymphysis these foramina seems, therefore, to be a derived con- and the rostral fragment of the dorsal bar (Fig. 1). dition that evolved in the Scolopacidae,Rallidae, He- The symphysisis 17 mm long, slightly decurvedand liornithidae, Spheniscidae,and Gayla. approximatelyas high (4.3 mm) as it is wide at its In Hesperornis(Elzanowski 1991:figs.1 and 3) and midpoint;it is slightly broadercaudally (4.8 ram).The the neognathswith an openpremaxillary symphysis, rostralend is somewhatsquared, which may or may theventral (palatal) concavity of the symphysisshows not be due to postmortemdamage. The dorsalsurface a tripartite subdivisioninto two lateral elevationsen- July1995] ShortCommunications andCommentaries 763 c d Fig. 1. PremaxillaUCMP 117598in (a) left lateral, Or)dorsal, (c) ventral, (d) ventrocaudaloblique, and (e) right ventro-obliqueviews showing neurovascularcanals. White arrows in panel a point to frontal process and in panelb to ventralgroove in premaxillarysymphysis and palatalshelf of premaxilla.Scale bar equals 10 min. 764 ShortCommunications and Commentaries [Auk, Vol. 112 closingthe neurovascularcanals and a mediantrough. view. Trochlea II has a tongue-shapedwing and no In most of these neognaths,the two lateral elevations distinct ligamental pit. Trochlea III has the lateral take the form of well-defined palatal shelvesand the margin slightly sigmoid and the ventral part of the median trough is representedby a well-definedme- articular surfacepointed and strongly asymmetrical dian groove.In the majorityof cases,the shelvestaper at the end, which is embayedmedially. Trochlea IV rostrally, whereby the tripartite shelf-and-groove hasthe lateral lip descendinggradually onto the shaft configurationdoes not extendto the symphysialtip. and bears a distinct ligamental pit. The attachment In contrast,the grooveextends to the symphysialtip scarof the sesamoidligament is distinctand slightly in the BugCreek fossil. Among extant birds, the groove elevated distally. The distal foramen is large, elon- extendsto the symphysialtip in one of the two species gate, and separatedby a very narrow bridge (which of the Rostratulidae(the American Painted Snipe; itself is much shorter than the foramen) from the Nycticryphessemicollaris), and the majority of Scolo- lateral intertrochlear incisure. pacidae,the exceptionsbeing the Broad-billedSand- The overall configurationof the trochleae agrees piper (Limicolafalcinellus), the Jack Snipe (Lymno- with: Anseriformes,including Presbyornis(Wetmore cryptesminimus), the Red Phalarope(Phalaropus fuli- 1926:figs.10-12, Olson and Parris 1987:fig.7); small carius),and the turnstones(Arenaria). In the Red-necked Phasianidae, especially Odontophorinae; Gruidae, Phalarope(P. lobatus)the rounded tomial margins Aramus,and Rallidae, which are probably closelyre- could be regardedas vestigialshelves. lated within the gruiforms; and most of the Char- What makesthe fossilpremaxilla dramatically dif- adriiformes except for the Burhinidae, Jacanidae, ferent from all rhynchokinetic charadriiformsis the Haematopodidae,Rhynchopidae, and Telmatornis(Ol- presenceof a prominent ventral ridge or bulge on son and Parris 1987:fig.7). the ventral side of the frontal processnext to the A prominent character that is shared exclusively premaxillary symphysis(Fig. ld, e). Such a structure with the anseriforms is the extremely distal position makes the dorsal bar rigid at the rostral end. The of the distal foramen in ventral view, with only a significanceof this morphologyfor cranial kinesisis narrow bridge separatingit from the lateral intertro- well illustratedby its diversityamong the Rallidae. chlear fissure(Fig. 2e). This characterhas not been In those rails (e.g. Rallus,Capellirallus) that are am- found in any other terrestrialor aquaticnonpasser- phikinetic (Zusi 1984:tableI) and, thus, have a distal ines. Although the distal foramenis relatively large bendingzone (seeOlson 1975:fig.3), the ventral sur- in the grebesand loons, it is separatedfrom the in- faceof the dorsalbar is flat, concave,or at mostslight- tertrochlearspace by a broadbony bridge.In addition, ly convex.By contrast,all prokinetic rails--including the fossil agreesspecifically with the anatids in the Atlantisia,Diaphorapteryx, Fulica, Gallinula, Porzana (see compactpositioning of the trochleae, which is prob- Olson 1977), Tribonyx(see Olson 1975:fig.1), Himan- ably a swimming adaptation,and with Presbyornisin tornis,Porphyrula, Porphyriops, Poliolimnas, and Pardir- the extraordinarysize of the distal foramen,which is allus--have the ventral surface of the dorsal bar mark- even larger than in the anatids.Since the anatidshave edly convexand/or with a distinctmedian ridge. This legsadapted for swimmingand Presbyorniswas a wad- evidencesuggests that the BugCreek bird had a pro- er, the large size and extreme distal position of the kinetic rather than rhynchokinetic or amphikinetic distal foramendo not seemto be correlatedwith any skull. particularlocomotory function.