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Number 3386, 11 pp., 3 figures, 1 table

10024 December 27, 2002

A New Presbyornithid Bird (Aves, Anseriformes) from the Late Cretaceous of Southern Mongolia

EVGENY N. KUROCHKIN,!? GARETH J. DYKE,” AND ALEXANDR A. KARHU!

ABSTRACT

We describe a new large representative of the important fossil anseriform taxon Presbyor- nithidae from the latest Cretaceous (Maastrichtian) Nemegt Formation of southern Mongolia. This new taxon, Teviornis gobiensis, n. gen. et n. sp., is known from the associated manual portion of a right wing and the distal end of a right humerus, but is clearly diagnosable with respect to all other known representatives of the fossil Presbyornithidae. It is placed within the clades Anseriformes and Presbyornithidae, respectively, on the basis of a number of de- rived characters of the carpometacarpus and digits. Importantly, description of Teviornis con- firms the presence of members of the neornithine clade Anseriformes (“‘waterfowl’’) in the Late Cretaceous, as has been suggested previously on the basis of much less diagnostic fossil material as well as from clade divergence estimates founded on molecular sequence data. The extinct Presbyornithidae thus has a worldwide distribution and ranged in age from at least the

Maastrichtian through to the uppermost Eocene.

INTRODUCTION

The fossil remains of modern birds (= Neornithes sensu Cracraft, 1988) are abun- dant and well represented in early Tertiary deposits from all over the world, but partic- ularly in Asia, Europe, and North America

(Olson, 1985; Unwin, 1993). A great deal of controversy, however, surrounds the question of the timing and extent of the diversification of these so called ‘“‘modern”’ clades by the time of the Cretaceous-Tertiary (K-T) bound- ary. Although some interpretations of the fossil record have led to the proposal that the

' Paleoherpetology and Paleornithology Laboratory, Paleontological Institute of the Russian Academy of Sciences, 123 Profsouznaya Street, Moscow GSP-7, 117997, Russia. e-mail: enkur@paleo.ru

? Chapman Research Fellow, Division of Vertebrate Zoology (Ornithology), American Museum of Natural History. Present address: Department of Zoology, University College Dublin, Belfield Dublin 4, Ireland. e-mail: gareth.

dyke @ucd.ie 3 To whom correspondence should be addressed.

ISSN 0003-0082

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bulk of the neornithine diversification oc- curred in the aftermath of the K-T event (Feduccia, 1999), recent “‘molecular clock”’ hypotheses have estimated some divergences to have been deep in the Cretaceous (e.g., Cooper and Penny, 1997; Rodionov, 1997; Van Tuinen et al., 2000; Paton et al., 2002). As a result, recent debates have focused on the question of just how deep the extant lin- eages of birds can be traced prior to the K- T boundary (Kurochkin, 1995; Cracraft, 2001) in the absence of well-dated and even partially complete fossil material from the Mesozoic (Dyke and Mayr, 1999; Dyke, 2001a).

In spite of the known diversity of birds from the Paleocene and Eocene (e.g., Brod- korb, 1964; Olson, 1985; Unwin, 1993; MIi- kovsky, 2002), thus far the well-known fossil anseriform Presbyornis Wetmore remains the most abundant described taxon of this age simply in terms of the sheer numbers of specimens that have been recorded (Olson and Feduccia, 1980; Olson, 1985; Ericson, 2000). Presbyornis, along with the other members of the clade Presbyornithidae, has been described from Tertiary localities in both northern and southern hemispheres (Wetmore, 1926; Howard, 1955; Harrison and Walker, 1978; Olson, 1985; Ericson, 1997, 2000; Dyke, 2001b) as well as puta- tively from the latest stages of the Mesozoic (Noriega and Tambussi, 1995; Ericson, 2000; Hope, 2002; Hope and Stidham, 2001). However, many Cretaceous specimens thus far considered are poorly constrained tem- porally and preserve too few anatomical characters to be informative within a phylo- genetic context (Clarke and Chiappe, 2001; Chiappe and Dyke, 2002). The recognition of well-dated Cretaceous fossil material that can be demonstrated to have certain affinities within Neornithes is imperative to further ad- dress questions relating to both the timing and extent of the evolutionary radiation of modern birds before, and after, the K-T boundary (Dyke, 2001a; Kurochkin, 2001).

We describe a fossil presbyornithid bird that was collected in 1987 from the Late Cre- taceous of southern Mongolia by an expedi- tionary team of the Joint Russian-Mongolian

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Paleontological Expedition*. This new taxon is similar in size to both the known speci- mens of Presbyornis isoni Olson (from the Upper Paleocene of Maryland, USA; Olson, 1994) and to the less well-known Headon- ornis hantoniensis Harrison and Walker (from the Upper Eocene of England; see also Dyke, 2001b). However, comparisons with both these larger taxa as well as the smaller Presbyornis pervetus Wetmore, Telmabates antiquus Howard, and large numbers of ad- ditional bones referred to Presbyornithidae from the Mongolian Lower Eocene locality of Tsagaan Khushuu? present a number of osteological features that allow us to diag- nose an additional taxon.

The new specimen (PIN 4499-1) is from the locality of Gurilyn Tsav in the central part of the Umnogobi Aimak, east of the Al- tan Ula Ridge, but close to the well-known fossil site of Bugeen Tsav (fig. 1). Sediments at Gurilyn Tsav form the lower portions of the Nemegt Horizon, at the base of the the Nemegt Formation, and have been dated Campanian to Early Maastrichtian in age (Martinson, 1982; Barsbold, 1983; Shuvalov, 2000; Khand et al., 2000). Fossil vertebrates are relatively common from Gurilyn Tsav; taxa that have been collected (some of which remain undescribed) include a variety of the- ropods and hadrosaurian dinosaurs, as well as varanid lizards, turtles, fish, and the large enantiornithine bird Gurilynia (see Kuroch- kin, 1999).

For many years, Presbyornis and its kin were considered intermediate in their mor- phology, apparently combining characters

4 Although PIN 4499-1 was collected in 1987, its identification as presbyornithid was not seriously con- sidered at first because of both its Cretaceous age and the fact that all other records for these birds were from the Paleocene and Eocene. However, at a round table discussion at the Fourth Meeting of the Society of Avian Paleontology and Evolution in Washington, D.C., in June 1996 (Olson et al., 1999), data were presented that documented the presence of presbyornithids from the Cretaceous(?) of Antarctica (Noriega and Tambussi, 1995). Notes from members of the original 1987 PIN field team indicated that more remains of this single specimen were initially collected from this locality. This material was subsequently damaged and lost.

> Olson (1985: 171) and Ericson (2000: 2) have re- ferred to our data regarding the presence of Presbyornis sp. in the Paleocene of Mongolia (Tsagaan Khushuu lo- cality).

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seen in both extant ducks and flamingos. Thus members of Presbyornithidae were widely cited as providing evidence for an evolutionary link between a number of the traditional avian orders by the time of the earliest Eocene (Olson and Feduccia, 1980; Feduccia, 1995, 1999). However, the recent inclusion of Presbyornis within numerical analyses of Recent waterfowl have demon- strated that this taxon can be placed well within order Anseriformes (commonly re- ferred to as “‘waterfowl’’; also comprising the extant screamers, ducks, and geese), closely related to the true ducks (Anatidae; see Ericson, 1997, 2000; Livezey, 1997, 1998; see also Paton et al., 2002).

MATERIALS AND METHODS

Our use of anatomical terminology fol- lows Baumel and Witmer (1993), with some modifications to English after Howard (1929). For discussion of diagnosis and char- acter distributions within Anseriformes we follow Livezey (1997, 1998) and Ericson (1997); within fossil Presbyornithidae we follow Ericson (2000). Institutional abbrevi- ations are as follows: AMNH, American Museum of Natural History, New York; BMNH PAL, The Natural History Museum, London (Palaeontology Department Collec- tions); MLP, Department of Vertebrate Pa- leontology, Museo de la Plata; PIN, Pale- ontological Institute of the Russian Academy of Sciences, Moscow; USNM, United States National Museum, Washington D.C.

SYSTEMATIC PALEONTOLOGY

AVES LINNAEUS, 1758 ANSERIFORMES (““WATERFOWL’) WAGLER, 1831 ANATOIDEA (“TRUE WATERFOWL’) SENSU LIVEZEY, 1997 PRESBYORNITHIDAE WETMORE, 1926

TYPE GENUS: Presbyornis Wetmore, 1926.

CURRENTLY INCLUDED GENERA: Presbyor- nis Wetmore, 1926; Telmabates Howard, 1955; Headonornis Harrison and Walker, 1976 (Harrison and Walker, 1979); Teviornis, n. gen. (this paper).

STRATIGRAPHIC AND GEOGRAPHIC DISTRI- BUTION: Members of Presbyornithidae are known from the Upper Cretaceous of Mon-

KUROCHKIN ET AL.: PRESBYORNITHID BIRD 3

@Ulan Bator

Mongolia

oun

Fig. 1. Map showing the Mongolian Late Cre- taceous localities of Gurilyn Tsav (1), where PIN 4499-1 was found, and Bugeen Tsav (2). Both have been dated Campanian to Early Maastrich- tian in age (see text for details).

golia (this paper) as well as putatively from Antarctica and North America (Noriega and Tambussi, 1995; Hope and Stidham, 2001; see Discussion), the Upper Paleocene—Lower Eocene of North America (Wetmore, 1926; Olson and Feduccia, 1980; see also Ericson, 2000), Argentina (Howard, 1955), and Mon- golia (Kurochkin, 1988), as well as the Mid- dle—Upper Eocene of England (Harrison and Walker, 1978; see also Dyke, 2001b). Isolat- ed elements that may also be presbyornithid have been reported from the Late Paleocene of Kazakhstan (Nessov, 1988), and the Eo- cene of Khirghizia (personal commun. cited in Ericson, 2000).

Teviornis gobiensis, new genus and species Figures 2, 3, table 1

ETYMOLOGY: For Victor Tereschenko, pa- leontologist at the PIN, who found the spec- imen; ornis (Greek), bird (masculine); Gobi, a severe desert in Central Asia.

HOLOTYPE: PIN 44991-1, partially crushed associated remains of a right forelimb, in- cluding a fragment of the distal humerus, both the radiale and ulnare, the complete car- pometacarpus, phalanx 1 of digiti majoris, and the phalanx of digiti minoris (figs. 2, 3).

TyPE LOCALITY AND Horizon: Gurilyn Tsav, Nemegt Basin (fig. 1); 15 km north of the Altan Ula ridge and 7 km east of Bugeen Tsav; north west corner of Umnogobi Aimak, Mongolia; Nemegt Formation, lower Nemegt Horizon, Upper Cretaceous (Maastrichtian).

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Fig. 2. in dorsal view; B, Right phalanx 1 of digiti majoris in dorsal view; C, Right phalanx of digiti minoris in dorsocaudal view; D, Right carpometacarpus in ventral view; E, Distal end of the right carpometa- carpus in distal view; F, Proximal end of right phalanx 1 of digiti majoris in proximal view; G, Fragment of the right humerus in dorsal view; H, in cranial view; and I, distal view. Specimens have been coated with ammonium chloride to enhance contrast. Scale bar is | cm.

KNOWN DISTRIBUTION: Upper Cretaceous (Maastrichtian) of southern Mongolia.

DIAGNOsIs: Our referral of Teviornis to Anseriformes is based, in the first place, on clear similarity with the other known mem- bers of Presbyornithidae. In addition, the pre- served carpometacarpus of this new taxon exhibits the following character listed by Livezey (1997) as diagnostic for Anserifor- mes (based on cladistic analysis to the exclu- sion of Tinamidae and Galliformes; Livezey, 1998): absence of craniocaudal curvature of corpus of carpometacarpus (os metacarpale minus) relative to os metacarpale majus. Tev- iornis 1s placed within Presbyornithidae on the basis of the following character condi- tions as listed by Ericson (2000): dorsal ridge

Teviornis gobiensis, new genus and species, holotype PIN 4499-1. A, Right carpometacarpus

of trochlea carpalis well developed and con- nected to the dorsal emargination of os me- tacarpale majus (see also Howard, 1955); presence of well-developed scars for lig. ul- nocarpometacarpale dorsale and lig. radi- ocarpometacarpale dorsale; presence of a small canalis interosseus distalis in fossa in- fratrochlearis.

DIFFERENTIAL DIAGNOSIS: Teviornis is dis- tinguished from the other known members of Presbyornithidae on the basis of the follow- ing character conditions: tuberculum supra- condylare dorsale of humerus extended far proximally; fossa infratrochlearis of carpo- metacarpus stretched markedly craniocaudal- ly; proximal portion of the branch of os me- tacarpale minus widened both dorsoventrally

2002 KUROCHKIN ET AL.: PRESBYORNITHID BIRD 5)

Fig. 3. Drawings of the preserved elements of the right wing of Teviornis gobiensis, new genus and species, holotype PIN 4499-1. Carpometacarpus in dorsal (A), ventral (D), proximal (FE), cranial (F), and distal (G) views. Phalanx | of digiti majoris in dorsal (B), ventral (J), and proximal (H) views. Phalanx of digiti minoris in dorsocaudal view (C) and in ventrocranial view (I). Ulnare in dorsal (K) and caudal (L) views. Radiale in cranial (M) and caudal (N) views. Fragment of distal end of the humerus in cranial (O), dorsal (P), and distal (Q) views. Abbreviations: ads, area distal of sulcus tendineus; adm, facies articularis digiti minoris; dmm, distal branch of os metacarpale majus; cpe, cranial end of processus extensorius; cpp, area cranial of proc. pisiformis; csp, cranial side of the phalanx 1; ea, excavated area between condylus dorsalis and tuberculum supracondylare dorsale; ed, epicondylus dorsalis; fit, fossa infratrochlearis; pp, processus pisiformis; prm, proximal area of os metacarpale minus; saf, smaller portion of facies articularis digiti majoris; ss, sulcus scapulotricipitalis; ts, tuberculum su- pracondylare dorsale; vaf, ventral metacarpal articular facet. Scale bar is 1 cm.

and craniocaudally with respect to its distal continuation; facies articularis digitis minoris on os metacarpale minus divided by a cran- iocaudal groove into two distinct facets; dis- tal extension of facies articularis digitalis ma- jor widened distally; ventral portion of facies articularis digitalis major elevated in the di- rection of the tubercle for the insertion of m. abductor digiti majoris. It should be noted, however, that although portions of the hu- merus are known for Presbyornis isoni (Ol- son, 1994), Dyke (2001b) only tentatively re- ferred this element to the Eocene genus Hea-

donornis; hence, further differentiation with respect to this taxon must await the recovery of additional fossil material. On the basis of the carpometacarpus (as far as this element is known in the genera of Presbyornithidae), we consider Teviornis to be a distinct taxon (see below).

TYPE SPECIES: Teviornis gobiensis; only currently recognized species.

MEASUREMENTS (in mm): Craniocaudal width of condylus ventralis of the humerus, 5.3; length of carpometacarpus, 65; dorso- ventral height of proximal end of carpome-

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TABLE 1 Selected Measurements of Preserved Wing Elements for Members of Presbyornithidae (from various sources)

V. gobiensis P. pervetus P. recurvirostris P. isoni T. antiquus

Carpometacarpus

Greatest length 65 44-54 63

Proximal width 15.2 9.7-11.9 13 15.8—-16.6

Distal width Pot 5.1-7.1 7.1-7.4 Phalanx 1 of major digit

Greatest length 30.8 19.8—27.6 40.6 29.7—30.7

Proximal width 8.3 4.3-5.8 9.5

tacarpus, 7.2; craniocaudal width of proximal end of carpometacarpus across the processus extensorius, 15.2; dorsoventral height of dis- tal end of carpometacarpus, 7.7; length of phalanx 1 digiti majoris, 30.8; ventral prox- imal width of phalanx digiti majoris, 8.3; length of phalanx digiti minoris, 13.9; prox- imal width of phalanx digiti minoris, 4.2 (for comparative measurements, see table 1).

DESCRIPTION AND COMPARISONS

HUMERUS (figs. 2G—I, 30—Q): The portion of the distal right humerus that is preserved as part of PIN 4499-1 is broken ventral to the condylus ventralis and across the remain- der of the fossa m. brachialis. However, the shapes of both the dorsal and ventral con- dyles are discernible; notably, a well-exca- vated depression is present between the tu- berculum supracondylare dorsale and _ the condylus dorsalis that is not evident in spec- imens referred to either Presbyornis or Tel- mabates (Olson, 1994; Ericson, 2000). Since a depression of similar depth and width is also seen in BMNH PAL A 5105, a distal end of a left humerus referred to the large Headonornis by Harrison and Walker (1979; also figured by Dyke, 2001b), but it is not evident on the holotype humerus of the sim- larly sized Presbyornis isoni (USNM 294116; Olson, 1994), this feature does not seem to be related to overall size (see Dis- cussion). Teviornis is also distinguished from Presbyornis and Telmabates because the tu- berculum supracondylare dorsale is remark- ably far extended proximally in the former. Because of the poor preservation of this el- ement, most of the morphological features

listed by Ericson (2000) as characteristic for Presbyornithidae are lacking in PIN 4499-1, with the exception of a weakly developed sulcus scapulotricipitalis (resulting from the small epicondylus dorsalis in this specimen).

ULNARE: The small but remarkably well- preserved ulnare of Teviornis (fig. 3K, L) ex- hibits two characters listed by Ericson (2000) as distinctive to Presbyornithidae, a distinct but shallow sulcus on its lateral side as well as a wide incisura metacarpalis. These char- acters, however, are based entirely on the known material of Presbyornis pervetus, the only presbyornithid other than Teviornis for which this element is known.

RADIALE: The radiale is also represented as part of PIN 4499-1 (fig. 3M, N). As noted by Ericson (2000) again for Presbyornis pervetus, this element has a mediolaterally compressed caudal end, a deep sulcus on its proximal side with a pronounced and cen- trally located tuberculum cranial to it. The incisura on the cranial edge of the radiale in Teviornis is also wide and shallow.

CARPOMETACARPUS: The best preserved and most anatomically informative element of PIN 4499-1 is an almost complete right carpometacarpus (figs. 2A—D, 3A—H). Be- cause this element is well known in the other presbyornithids with the exception of Pres- byornis isoni (Olson, 1994) and Headonornis (Dyke, 2001b), it forms the basis of our di- agnosis for Teviornis. As is the case in the known presbyornithids (Ericson, 2000), the Lower Eocene Anatalavis oxfordi Olson (Ol- son, 1999), and the basal Anseriformes An- hima, Chauna (Anhimidae), and Anseranas, the distal end of the carpometacarpus bears

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a well-developed and deep sulcus tendineus on its dorsal surface that extends at least one- third of its length. Although only the proxi- mal and distal ends of the right carpometa- carpus and the distal end of the left are pre- served as part of the holotype of Telmabates (AMNH 3170), this conformation of the sul- cus is seen in a number of the additional specimens referred to this taxon (Howard, T9553).

On the proximal end of the carpometacar- pus, the fossa infratrochlearis of PIN 4499-1 is stretched craniocaudally to a greater extent than it is in either Presbyornis pervetus or Telmabates, which both have a much more circular contour of this fossa. The angle be- tween the processus extensorius and the trochlea carpalis is almost perpendicular, as opposed to being distinctly curved, as is the case in the other presbyornithids. In this re- spect, this element of Teviornis closely re- sembles another large and as yet undescribed presbyornithid carpometacarpus from the Lower Eocene London Clay Formation of England (BMNH PAL A 6241).

The proximal portion of the os metacar- pale minus in Teviornis is distinctly widened dorsoventrally when viewed in dorsal view as opposed to being equal in width, as is the case in both Presbyornis pervetus and Tel- mabates. Just as in Telmabates, the dorsal part of the trochlea carpalis in PIN 4499-1 extends as far caudally as does the ventral part of the trochlea carpalis; in Presbyornis pervetus, the dorsal ridge of the trochlea car- palis extends far caudally with respect to the ventral ridge of the trochlea carpalis. In both Teviornis and Presbyornis the processus pis- iformis lies on the same level as os metacar- pale majus, but in Telmabates this process follows the line of extension of os metacar- pale minus. Variation in the relative degree of excavation of the area cranial to the pro- cessus pisiformis is seen between the three genera: in Victornis this area is somewhat ex- cavated, whereas it is shallow in Telmabates and very deep in Presbyornis pervetus. Fur- ther, the fossa infratrochlearis in PIN 4499-1 bears several pronounced muscle scars and excavations (as is seen in Presbyornis)—in Telmabates this fossa is almost flat (Howard, 1955).

The distal end of the carpometacarpus of

KUROCHKIN ET AL.: PRESBYORNITHID BIRD fi

PIN 4499-1 is stretched dorsoventrally whereas it is much shorter in both Telma- bates and Presbyornis pervetus (perhaps as a result of the much larger size of Teviornis). The articular facet for phalanx digiti minoris is undivided in the two smaller taxa but is grooved craniocaudally in Teviornis, and the area distal to the sulcus tendineus (in dorsal view) bears no depression (Gin both Telma- bates and Presbyornis pervetus this area is excavated to varying degrees).

Ericson (2000) listed a number of char- acters of the carpometacarpus as diagnostic for Presbyornithidae, the majority of which are seen clearly in PIN 4499-1. Of these fea- tures, the following are preserved in the ho- lotype of Teviornis: carpometacarpus straight with major and minor metacarpals parallel; caudal portion of dorsal part of trochlea car- palis well developed and connected to the edge of the major metacarpal; presence of large scars for the insertions of lig. ulnocar- pometacarpale dorsale and lig. radiocarpo- metacarpale dorsale; small vascular foramen present in the fossa infratrochlearis; marked and deep sulcus tendineus; sulcus interosseus deep, having an ossified bridge.

PHALANX | Dicit1 MAJorIs: The proximal phalanx of the major digit (figs. 2B, 3B, J) in Teviornis is long and slender and bears two deep depressions on its dorsal side. On the proximal side of this element there is a blunt and cranioventrally protruding tuber- culum, no dorsal tendinal sulcus on the pila cranialis, and no distal projection on the cau- dal margin. All three of these characters are seen in members of Presbyornithidae; this el- ement is known for both Presbyornis (Olson, 1994; Ericson, 2000) and Telmabates (AMNH 3180; Howard, 1955). The pre- served phalanx 1 of Teviornis exhibits a fur- ther two characters that allow taxonomic dif- ferentiation from other taxa: a straight cranial margin (in dorsal view) and an elevated ven- tral portion of the metacarpal articular facet (in the direction of the tubercle for the in- sertion of m. abductor digiti majoris). In oth- er presbyornithids, this phalanx is not raised ventrally and has a curved cranial margin when viewed dorsally (e.g., AMNH 3180 [Telmabates|, USNM 294117 [P. isoni], USNM 483164 [P. pervetus]).

PHALANX DIGITI MINoRIS: The small, well-

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preserved phalanx of the minor digit of Tev- iornis (fig. 2C, I) is flattened dorsoventrally and has a sharp and projected pila caudalis and a well-developed tuberculum located dis- tally with respect to the pila caudalis. These features were described for Presbyornis pervetus by Ericson (2000).

DISCUSSION

Previous studies have demonstrated that the Presbyornithidae were an extremely widespread group of birds during early Ter- tiary times. Members of this clade have been recovered from the Paleocene—Eocene of Eu- rope (Harrison and Walker, 1978, 1979; Dyke, 2001b), South America (Howard, 1955; Cracraft, 1970), and Asia. Indeed, these birds are probably the most abundantly known taxa from throughout the Tertiary of North America (see, for example, McGrew and Feduccia, 1974; Olson and Feduccia, 1980; Olson, 1994; Leggitt and Bucheim, 1997; Benson, 1999; Feduccia, 1999; Eric- son, 2000). Recent phylogenetic studies have placed Presbyornis within Anseriformes, close to the divergence of the Anatidae, the true ducks, geese, and swans (Ericson, 1997; Livezey, 1997, 1998), indicating that the di- vergence of this large extant taxon must have occurred earlier than the oldest known mem- ber of Presbyornithidae. Our description of Teviornis serves to confirm previous sugges- tions that Anseriformes did indeed occur pri- or to the K-T boundary—existing proposals that this was the case have been based either on more incomplete fossil material (i.e., sin- gle and isolated elements presenting few characters for comparisons; e.g., Hope and Stidham, 2001; Hope, 2002) or on records collected from rocks that are of uncertain Cretaceous age (e.g., Olson and Parris, 1987; Noriega and Tambussi, 1995; see contrary ci- tations in Olson, 1994; Clarke and Chiappe, 2001). Comparisons between PIN 4499-1 and other putative records of Presbyornithi- dae from the ?Cretaceous are not possible here, either because of the fragmentary pres- ervation of the available material (see illus- trations in Hope, 2002) or because of the lack of corresponding skeletal elements (as is the case in MLP 93-I-3-1 described by Noriega and Tambussi [1995] from the ?Cretaceous

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of Antarctica). It is however of note that re- cords of ‘‘Presbyornis-like’’ birds from the Mongolian Cretaceous have been listed pre- viously by some authors (Olson, 1985; Ku- rochkin, 1995) on the basis of a small, dam- aged, and isolated tarsometatarsus figured as Presbyornithidae(?) by Kurochkin (1995). This single specimen, from the Udan Sayr locality within the Baruungoyot Formation, is considered Aves incertae sedis here as it preserves no anatomical features that can be considered diagnostic of either Presbyorni- thidae or Anseriformes (Ericson, 2000).

The internal composition of Presbyorni- thidae remains controversial and complex. For more than 100 years the remains of fossil birds bearing varying degrees of resemblance to living Anseriformes have been described from the earliest Tertiary (e.g., Owen, 1846; Milne-Edwards, 1867-1871; Lydekker, 1891; Kurochkin, 1976), but only recently have clear character diagnoses for this group been presented (Livezey, 1997, 1998; Eric- son, 1999, 2000). The genus Telmabates, for example, originally described by Howard (1955) from the Eocene of Patagonia has var- iously been considered to be synonymous with Presbyornis, especially following the description of the giant Presbyornis isoni by Olson (1994). Olson (1985, 1994), for ex- ample, has considered that Presbyornithidae comprises simply a number of differently sized species within the single genus Pres- byornis. Recently, Ericson (2000: 7, 11) has shown that not only do the two genera vary significantly in the relative sizes of their hindlimb bones (those of Telmabates are also somewhat more robust; cf. AMNH 25552, 28505 [slabs containing Presbyornis perve- tus remains] and AMNH 3167, 3169, 3180, 3171 [referred specimens of Telmabates]), but that there are clear osteological differ- ences between them (e.g., linea muscularis cranialis of coracoid obsolete in Presbyornis [welldeveloped in Telmabates.| More prob- lematical with respect to these diagnoses are the less completely known taxa Headonornis hantoniensis Harrison and Walker and Pres- byornis isoni Olson. Although the holotype of Headonornis is an isolated coracoid (BMNH PAL A 30325), Dyke (2001b) showed that incomplete portions of humeri (BMNH PAL A 3686, A 5105) referred to

2002

this taxon by Harrison and Walker (1976, 1979) are in fact referrable to P. isoni (on the basis of a more recently collected and more complete humerus, BMNH PAL A 6240). It remains entirely possible that these two large presbyornithids, P. isoni and Hea- donornis, are actually the same taxon, but the discovery of additional material will be re- quired to confirm this (Dyke, 2001b). Be- cause Presbyornis isoni is currently known largely on the basis of the humerus (Olson, 1994; Benson, 1999), only a few of the dif- ferential characters listed by Ericson (2000) (with respect to Telmabates) apply (i.e., im- pressio m. pronator superficialis on distal hu- merus extends much farther than lig. colla- terale ventrale in Presbyornis). An additional specimen, BMNH PAL A 43164, an incom- plete portion of sternum described by Lydek- ker (1891) as the holotype of Proherodius owen, iS very similar in its size and overall morphology (e.g., in the shape of the cora- coidal sulci) to this element in P. pervetus (Dyke, 2001b); although tantalizingly incom- plete, it remains possible that this specimen is in fact the earliest described presbyornithid (as suggested by Harrison and Walker, 1978). Whatever the final outcome of these nig- gling taxonomic problems, it is clear that Presbyornithidae was one of the most abun- dant groups of Late Cretaceous—Tertiary neornithine birds. The abundance of these birds in deposits of this age is probably a result of their aquatic or semiaquatic ecolo- gy, resulting in their differential preservation across the K-T boundary (when compared to other groups of largely terrestrial taxa). Since a similar bias is seen in the fossil record of non-neornithine birds throughout the Creta- ceous (Padian and Chiappe, 1998; Chiappe and Dyke, 2002), it seems logical to infer that members of other clades of modern birds were also present prior to the terminal Cre- taceous extinction event; these have either yet to be found or have not so far been rec- ognized convincingly on the basis of phylo- genetically informative fossil material.

ACKNOWLEDGMENTS

We thank Sandra Chapman (BMNH), Chris Collins, Paul Sweet (AMNH), and Mark Florence (USNM) for providing access

KUROCHKIN ET AL.: PRESBYORNITHID BIRD 9

to specimens in their care, and Luis Chiappe and Gerald Mayr for their helpful comments on the manuscript. This research was sup- ported by grants from the Russian Founda- tion for Basic Research no. 00-04-49348 and No. OO0-15-97754 and the Frank. M. Chap- man Fund of the Department of Ornithology, American Museum of Natural History.

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