The Morphology and Systematics of

Transcription

1 The Morphology and Systematics of Po/arornis, a Cretaceous Loon (Aves: Gaviidae) from Antarctica Sankar Chatterjee Museum of Texas Tech University, Box 43191, Lubbock, Texas , USA; sankar.chatterjee@ttu.edu Abstract Polarornis gregorii, a new fossil loon from the Late Cretaceous Lopez de Bertodano Formation of Seymour Island, Antarctic Peninsula, offers critical insight into the early radiation of neognathous birds. The holotypical specimen is known from an associated partial skeleton including a skuli, four articulated cervical vertebrae, a sternal fragment, and partial hindlimbs. In size and proportion, Polarornis resembles the common loon Gavia immer. The skuli is edentulous, prokinetic, and the sutures are closed. The palate is neognathous; the quadrate has a short orbital process. Evidence of cranial pneumaticity is preserved in the maxilla, laterosphenoid, mesethmoid, and the tympanic region. The cervical vertebrae are long and heterocoelous, with ribs fused to them to enclose the transverse foramen. The femur is curved in lateral aspect with a deep patellar groove; its distal condyles are subequal, hemicylindrical, and projected caudaliy. The tibiotarsus is craniocaudaliy compressed; its cranial and lateral cnemial crests form a long, pyramidlike process, indicating that Polarornis had already acquired a foot-propelied diving mechanism. The shaft of the tibiotarsus shows pathological lesions resulting from hypertrophic osteoarthropathy. Both bone histology and large nasal passages indicate that Polarornis had acquired endothermy like modern birds. Numerical cladistic analysis of 52 characters preserved in Polarornis supports its inclusion within Neognathae in the family Caviidae by the presence of six unambiguous characters. It is diagnosed by ten autapomorphic characters. Polarornis has considerable temporal and biogeographic importance. Recent discovery of two other Late Cretaceous birds, a presbyornithid, and a charadriiform from nearby Vega Island indicates that Antarctica may have been an important center for the origination and dispersal of neognathous birds during the Late Cretaceous period. These Antarctic birds transcended the K-T extinction, rebounded in the Early Tertiary, and dispersed northward. Similar examples of high latitude heterchroneity of the Southern Hemisphere are known among other groups of animals and plants from Antarctica. 125

2 126 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution Introduction The recent discovery of numerous Mesozoic birds has greatly enhanced our knowledge about their origin, diversity, phylogeny, and early radiation (Chiappe, 1995a; Padian and Chiappe. 1998; Feduccia, 1996; Chatterjee, 1997; Hou, 2000). However, most of the Cretaceous birds represent strange and archaic groups such as Hesperornithiformes, Ichthyornithiformcs, and Enantiorni thes, which became exti nct at the end of the Cretaceous. They were not directly linked to the ancestry of neognathous or modern fl y ing birds. Remains of neognathous birds from the Mesozoic are few; many taxa are known from fragmentary material, often represented by a single bone, and their affinities are uncertain. One of the earliest records of neognathus is a loon from the Late Cretaceous of Antarctica (Chatterjee, 1989). From this southern polar origin, loons migrated northward and radiated throughout the Cenozoic period. Loons are regarded as an ancient lineage; their fossil record extends as far back as the Late Cretaceous of Chile and Antarctica. The Chi lean taxon Neogaeornis is fragmentary and controvers ia l, represented by an isolated tarsometatarsus (Olson, 1992). Another fossil loon, Coly mboides is known from the Late Eocene of England and Early Miocene of France (Storer, 1956). The modern genus Gavia is known widely from the Tertiary deposits of Europe and North America (Svec, 1982; Delle Cave el al., 1984). In the following account, the morphology of a new Antarctic loon, Polarornis gregorii, is described on the basis ofa partial skeleton that includes the skull, vertebrae, a sternal fragment, and hindlimbs. The material is incomplete, but beautifully preserved, and provides a wealth of anatomical detail. It was found during the austral summer of 1983 in the Late Cretaceous Lopez de Bertodano Fonnation ofseymour Island, Antarctica, along with plesiosaurs, mosasaurs, and ammonites. Because very few neognath fossils have been found with adequate and diagnostic material from Cretaceous deposits, the Antarctic loon sheds new light on the origin and early radiation of modern birds. Living loons (Gaviidae) are a small and closely related group of foot-propelled divers. They are represented by one genus and four species, and are restricted to the Northern Hemisphere. There is a consensus that specializations for foot-propelled diving have evolved independently in loons, grebes, and hesperornthiforms in different geologic periods; they are purely convergent in their hindlimb similarities and did not share a common diving ancestor (Storer, 1960; Olson, 1985; Raikow, 1985; but see Cracraft, 1982 for a contrary vie"v). Recent phylogenetic analys is, di scussed in this paper, supports this view of convergent evolution of foot-propelled divers. Stratigraphy Seymour Island, located on the northeast side of the Antarctic Peninsula, contains a thick and richly fossiliferous sequence of near-shore marine and coastavdeltaic strata of Upper Cretaceous to Eocene age, representing a transgressive-regressive cycle (Fig. I A). The Joon material was found in the upper part (Unit 9) of the loosely consolidated 1200-m-thick Lopez de Bertodano Formation (Fig. I B). Macellari (1986, 1988) identified ten informal units within the poorly indurated strata of the Lopez de Bertodano Formation; the lower six (Rotularia Units) are characterized by the annelid worm tube Rotularia, but are poor in macrofauna; the upper four (Molluscan Units) contain an abundant and diverse macrofauna including bivalves, gastropods, and ammonites. Especially fossiliferous Unit 9 is in a valley across central Seymour Island near the coast of the Lopez de Bertodano Bay and contains local concentrations of pycnodontid oysters and ammonites. The loon material was recovered from this unit during an unsuccessful attempt to extract a 'large plesiosaur skeleton from a pernlafrost bed on a high cliff on 31 December Our disappointment from leaving the mosasaur FIGURE 1. A, location and simplified geological map of the Seymour Island. Black circle indicates the Polarornis fossil locality in the Late Cretaceous Lopez de Bertodano Fonnation (simplified from Elliot et ai., 1994). B, correlation ofmeasured sections of Lopez de Bertodano Fonnation, Seymour Island. Polarornis fossil was collected from 9a Unit. The K-T boundary lies between Units 9 and 10. (Simplified from Macellari, 1988; Elliot et ai., (994).

3 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press '- Edge of shelf ice m isobath / I I I I / / 64"15'8 T N 60W c::::=j surficial deposits r:=:;:j La Meseta Fm. (Eocene) EZZ2I Wiman Fm. (Paleocene) Ill]]] Cross Valley Fm. (Paleocene) Sobral Fm. (Paleocene)- Cape E;SSj Lopez de Wiman Bertodano Fm. (Cretaceous Tertiary) "5 ~ ~~~"7.{f;'.?<>\...-<\"-./ Fossil Locallty.;:;::.>-.; :;: :~'_:.::':':: ~ ':' ~~~~~.-;':...:~~:{\~~:-::... Seymour Island A 56"45'W o 5 km 1400 meter" 1000 ~ ~ 0: '" 0 '" A B 10 b a Polaromis C D E !: a F NE B Iridium Anomaly KT Boundary E u z «0 0 r co '" " w '" w " [;:;] Pebbly sa nd D Sandstone cz::j Silty sand C3 Sand y silt o Mudstone ~ Concre tions ~ Burrows ~ Cross-bedding ~ Channe ls (2J Section not e xposed

4 128 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution skeleton in the field was quickly compensated when Bryan Small, my former student and field assistant, discovered this remarkable loon fossil, just below the dig site while descending from the cliff. Because of the importance of the material, the specimen was briefly mentioned on several occasions (Chatterjee, 1989, 1987; Ol son, 1994; Chiappe, J996a), but has never been formally named and described. The loon fossil was encased in a small nodule of calcareous sandstone; only the associated cervical vertebrae were partially exposed. Apart from invertebrates, the sitc has yielded a diverse vertebrate fauna including hexancbifonn sharks, berycifonn teleosts (Grande and Chatterjee, 1987), a chimaerid shark (Stahl and Chatterjee, 1999), plesiosaurs and mosasaurs (Chatterjee el of., 1985; Chatterjee and Small, 1989). The presence of ammonite zone fossils such as Pachydiscus ullimus from this horizon indicates Late Maastrichtian age (Macellari, 1986). A single iridium anomaly correlated with the Cretaceous-Tertiary (K T) boundary is present slightly above this horizon in a glauconite sandstone bed between Units 9 and 10 (Elliot et of., 1994). The sequence, which ends in gray to tan Paleocene siltstones (Unit 10), includes both shallow marine and non-marine coastal or deltaic facies. Recently the Lopez de Bertodano Formation has been subdivided into two formal units; the lower one is the Camp Lamb Member, the upper one is the Sandwich Bluff Member from which the loon material was recovered (Crame et of., 1991). Material and Methods The loon material was found in a hard, calcareous sandstone nodule and was prepared chemically using weak formic acid (5 percent) in a bath. The exposed bone was coated with polyvinyl butyral (Butvar B 76) and thoroughly dried before placing the nodule in the acid bath. While preparing the nodule it was realized that the material belonged to one individual. The bones were somewhat associated but jumbled together, containing a partial skull, four cervicals, a complete left femur attached to the proximal half of a tibiotarsus, the proximal part of a right femur, and a sternal fragment containing four ribs. The bones are dark gray, compact, heavy, beautifully preserved, and undistorted. In size and proportion, tbe fossil matches well with common loons (Gavia immer). The rostral tip of the beak, dorsal part of the braincase, and the lower jaw are missing from the skull. The right quadrate, left squamosal, the lateral wall of the left braincase, and the left exoccipitai were found separated from the skull. To clarify the spatial association of the cranial elements, these four separate elements were assembled to the main body of the skull and the missing parts were reconstmcted (see below) (Figs. 2-5). Because this fossil represents the holotypical specimen, the processes used in its reconstruction are reversible, to assure that any sculpted areas could be later removed from the fossil without injury to the bone. All adhesion and consolidation of bone was done using polyvinyl acetate (Vinac B-15). Major areas of reconstmction were created using Paper Pulp Epoxy (PPE). This combination of acid-free, 100 % cottonrag paper and a water-based, ph neutral, polyvinyl acetate glue is fully reversible, UV resistant, and nontoxic (McQuilkin, 1999). Delicate areas, such as the tip of the beak, jugals, and the palatine were constmcted by pressing the paper pulp into a mold that was sculpted first in clay. The braincase was prepared by shaping Styrofoam and covering it in PPE. The Styrofoam could then be dissolved in acetone, leaving a hollow endocranial cavity. This allows for examination of the medial wall of the braincase. The left quadrate was sculpted by scanning the right quadrate into Adobe Photoshop 4.0, and flipping the image horizontally. This produced a mirror image upon which to base the reconstruction of the missing element. All reconstructed areas were painted with gouache, an opaque watercolor paint. This paint is also soluble and will not inhibit the reversibility of the PPE or Vinac B-15. Systematic Paleontology Aves Linnaeus, 1758 Ornithurae Haeckel, 1866 Neornithes Gadow, 1893 Gaviidae Allen, 1897 Polarornis gregorii, gen. et sp. nov. Etymology Polar, referring to Antarctica where it was found; ornis (Latin): bird; gregorii: in honor of Professor Joseph T. Gregory for his contributions to

5 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press A 5cm B FIGURE 2. Comparison of skulls in lateral view: A, restoration of sk ull of Polarumis; B, Gavis immer; C, Hesperornis (modified from Buhler et 01.,1988). In all figures, the actual bones are shown by dark shading, and the missing parts by light gray tone.

6 130 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution ~. - "..:.:::~---",~ ~- f1 ~- -, -~ ~~ t:: C j~ A -= ~pm~~mlf or 5cm v B pm qj 5cm c pm qj FIGURE 3. Comparison of ShIlls in dorsal and ventral views. A, restoration of the skull of PoLaromis, dorsal view; B, restoration of the skull of PoLalOrnis, ventral view; C, ventral view of HespelOmis (modified from Witmer and Martin, 198 7).

7 Editors: Zhong he Zhou and Fucheng Zhang. Beijing: Science Press A B 5cm c o FIGURE 4. Comparison of skulls in dorsal and ventral views. A. Cavia immer, dorsal view; B, Pofarornis, dorsal view; C, Cavia immer, ventral view; D. Pofarornis, ventral view.

8 132 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution A 5cm B 2cm E F G FIGURE 5. A. Lateral view of the skull, Gavia immer; 8, lateral view of skull, Polarornis; C, medial view of the right quadrate of Gavia immer; 0, medial of the right quadrate of Polamrnis; E, CT scan of the cross-section of the narial region of Polarornis showing the voluminous space for respiratory turbinates; F, ventral view of the 7"' cervical of Gavia immer; G, ventral view of conjoined seventhninth cervicals; H, dorsal view of the right sterno-coracoidal process of the sternum of Polarornis.

9 Editors: Zhong he Zhou and Fucheng Zhang. Beijing: Science Press vertebrate paleontology. Holotype Museum of Texas Tech University, paleontology collection, TTU P 9265, associated skull, 4 cervical vertebrae, complete left femur, proximal half of a left tibiotarsus, proximal part of a right femur, and a stcrnal fragment (Figs. 4, 5, 10, II). Horizon and locality Sandwich Bluff Member, Lopez de Bertodano Formation (Unit 9), Late Cretaceous (Maastrichtian), central valley of Seymour Island, near the coast of the Lopez de Bertodano Bay (Fig. IA). Affinity The fused cranial bones, edentulous jaws, double-hcaded quadrate, neognathous palate, and carotid flanges on thc cervicals suggest the affinity of Polaromis with Neognathae. It is a member of the Gaviidae because it shares the following synapomorphies: medial surface of the orbital process deeply excavated with an overhanging ridge; quadrate with a caudal projection on the ventral surface; femoral shaft bighly curved in lateral aspect; proximal end of femur compressed craniomedially; cnemial crest long and fonned entirely by the tibia; and lateral cnemial crest intlated. Diagnosis It is diagnostically distinct from known species of the Gaviidae and differs as follows: e.'(lernal naris small; frontals narrow across the orbil with narrow salt gland depressions; orbital process of the quadrate thick, short and directed horizontally; rterygoid condyle of the quadrate is bordered by a dorsal groo\e; caudal tympanic recess in the same plane with the fenestra oval is and the fenestra pseudorotunda, and not bounded caudally by an o\crhanging wall: lateral wall of the rostral tympanic recess is flat, not bulbous; vomers fused and rounded rostrally; femoral neck long; distal condyles offemur are subequal and project caudally, not twisted sidewise; intercnemial sulcus perforated by a nutrient foramen. Description of Polarornis Skull The skull of Polarornis is extremely avian and modern looking, unlike thal of any other Mesozoic bird. It exhibits two derived features that are absent among Cretaceous aqualic birds: lack of teeth and the closing of sutures. In contrast, both Hesperornis and fchthyornis contain a large number of teeth and their cranial sutures remain open in the adult individuals (Marsh, 1880). The sutures of the cranial bones in Polarornis are mostly oblilerated making the demarcalion of individual elemenls 1110re difficult. Various juvenile skulls of recent birds were used to delineate the individual bones. The estimated skull length of Polammis is 18 cm (Fig. 2), about the size of a common loon (C(ll'ia illlll1a). Dermal roofing bones The skull is long and narrow wilh a tapering beak, formed entirely by the compact and fused premaxillae (Figs. 3A, 4B, 5B). The frontal process of the premaxilla is elongate and cxtends considerably backward to approach lhe frontal, where it overlaps the rostral end of the mesethmoid. The premaxilla borders the rostral margin of the external naris; its palatine process is somewhat reduced as in modern loons. Tbe external naris is small, elliptical, holorhinal, and situated far back, near the base of the beak. The maxilla contributes little to the margin of the jaw, but has a palatal component, lateral to the rostral end of the palatine. The nasal is a small peripheral bone forming a slanling bar belween tbe external naris and the antorbital fenestra. Dorsally it overlaps tbe rostral end oftl1e frontal and contacts the lacrimal. The antorbital fenestra is lriangular, bounded caudally by the sloping lacrimal, which fails to reacb tbe narrow jugal bar. Dorsally, thc lacrimal has a caudal process around the orbital rim to contact the nasal. The orbit is very large, circular, and somewhat frontally placed. The frontals are extremely narrow transverscly across the orbit as in Hesperomis, in contrast to those of modern loon species. There are well-marked depressions on the frontals along the orbital rim for housing salt glands, as in Hesperornis, loons, and penguins, indicative of their marine habits. The orbital septum is fully ossified, fenestrated by several foramina, and forms a verlical wall between the two orbits. Most of the skull roof containing tbe fronto-parietal region and occiput are missing. The jugal is a slender bar behind the maxilla; its caudal extension toward the quadratojugal is not preserved. The squamosal is reduced and forms an overhanging roof over the quadrate. Ventrally it has an articular cotyle for the lateral head of the quadrate. Behind this socket, there is an entrance for the dorsal tympanic recess. Palatal complex The palate is beautifully

10 134 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution preserved in Polarornis except for the caudal region of the palatine and the pterygoid (Figs. 3B, 4D). The palate of Mesozoic birds is poorly known except for Hesperornis. However, the palate of Hesperornis is bizarre and controversial (Fig. 3C). It has a combination of autapomorphic and plesiomorphic features and is difficult to place it into a Paleognathae/ Neognathae category (Witmer and Martin, 1987). Elzanowski (1991) interpreted the palate of Hesperornis as of neognathous type on the basis of an unusual palatine/pterygoid joint. However, the pterygoid is short and complex in Hesperornis, whereas the palatine is elongate; there is no evidence of rostral pterygoid segmentation. Moreover, Hesperornis has an unusual reverse peg and socket joint between the pterygoid and the palatine. The palate of Hesperornis lacks the typical morphology and connections ofneognathous birds (Witmer and Martin, 1987). In Polarornis, the palate is reminiscent of a neognathous bird, and does not form a complete shelf between the nasal cavity and the mouth (Figs. 4C, D). The palatine processes of the premaxillae are fused along the midline at the front of the beak. Behind the premaxillae, there is an extensive cleft down the midline, which is confluent with the choana. The cleft is bordered laterally by the palatine, which extends forward as a long, slender process to fuse with the premaxilla. The vomer is considerably reduced, fused, and occurs as a median bar between the two palatines. It is separated from the premaxilla by the median cleft. Unlike modem loons, the vomer of Polarornis shows a rounded ventral margin and lacks the vertical, bladelike process (Fig. 4B). At the base of the antorbital fenestra, the palatine is overlapped dorsally by the palatine process of the maxilla. Passing backward, the bone is inflated and vaulted, and has a hook-shaped choanal process bordering the choana caudomedially. The choana has been shifted both medially and caudally. Behind this contact, the palatine has an ascending process lateral to the parasphenoid rostrum. Medially, the palatine has a longitudinal facet for sliding with the parasphenoid rostrum. The pterygoid is not preserved in the specimen. Although the movable joint between the pterygoid and the palatine cannot be ascertained from the specimen, the preserved part ofthe palate ofpolarornis shows several hallmarks of a neognathous palate (Witmer and Martin, 1987). These include: (I) the bony palate has a cleft down most of its mid-length; (2) the palatines contact each other caudally along the midline; (3) the palatines are fused rostrally to the premaxillae; 5cm 1cm pre A B c FIGURE 6. A, Skull of Polarornis, occcipital view; Band C, lateral and medial view of the right quadrate of Polarornis.

11 Editors: Zhong he Zhou and Fucheng Zhang. Beijing: Science Press and (4) the vomer does not contact the premaxilla. It appears that the palate of Polarornis represents the earliest documentation ofa neognathous conformation in the fossi I record. The quadrate shows all the attributes ofstreptostyly (Figs. 5C, D, 68, C). It has a short orbital process directed rostrally for attachment of the protractor quadrati muscle. However, the orbital process is not as long as that of modern loons. The dorsal articular head shows two distinct condyles, which articulate movably with the squamosal and prootic respectively by two separate facets, the lateral and medial cotyles. The two heads are separated by a furrow due to penetration of the dorsal tympanic diverticulum from the middl"e ear cavity (Jollie, 1957). The lower end of the quadrate has a lateral cotyle for the quadratojugal and a medial condyle for the pterygoid. Unlike most neognaths, the pterygoid condyle is separated from the body of the quadrate dorsally by a distinct groove. The medial surface of the orbital process is deeply excavated with an overhanging rim as in modern loons. The ventral articular surface for the jaw joint is complex: it has a lateral condyle and medial and rostral facets. The quadrate is more derived than that of Hesperornis with the development of a separate, bicondylar head. Like all diving birds, the quadrate of Polarornis appears to be solid and apneumatic. Braincase The braincase lacks the roofing elements (partial frontal, parietal, and supraoccipital) and much of the basioccipital-basitemporal complex. The left side of the lateral wall of the braincase is preserved in association with the squamosal and exoccipital. The preserved part includes laterosphenoid, basisphenoid-parasphenoid complex, prootic, opisthotic and exoccipital (Figs. 5B, 7 A). The topographic positions of major cranial foramina, quadrate articulation, tympanic recesses, otic capsule, and the course of the carotid artery match well with those of primitive extant species, such as the redthroated loon (Ca via stellata), but are somewhat different from that of Hesperornis (Fig. 7). The laterosphenoid forms the rostrolateral vertical wall of the braincase. Dorsally it rises toward the skull roof to meet the frontal and the squamosal. Ventrally it joins the prootic to enclose a pair of large foramina A B 1cm 1cm FIGURE 7. Comparison of the braincase, left lateral view: A, Polarornis; B, Gavia stellata ; C, Hesp erornis (simplified from Witmer, 1990). c

12 136 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution for three branches of the tri geminal (V) nerve. Bclow the trigeminal openings, the laterosphenoid extends as a thin pl ate to envelope the dosum sella of the basisphenoid. The prootic has a constricted shaft with expanded dorsaland ventral ends. Behind the rostral tympanic recess, the prootic is pierced by the fa c ialis (VlI) foramen. The palatine branch of the faciali s would descend downward internally and ex it through a foramen (VII, pal) in the basisphenoid. The lowcr expanded surface ofthe prootic meets the basisphenoid to enclose a large rostral tympanic recess. Caudally, the shaft of the prootic borders the rostral margin of the fenestra oval is. The upper expanded surface of the prootic shows a distinct cotyle for the medial head of the quadrate. Rostral to it lies a deep foramen for the dorsal tympanic recess. The otic capsule of Polarornis is similar to that of ornithurine birds, where it contains three openings: fenestra ovalis, fenestra pseudorotunda, and caudal tympanic recess. Unlike the modem form, both the rostral and caudal wings surrounding the otic capsule are subdued. The opisthotic is highly reduced and forms a slender bar, the cris ta interfenestralis, separating the fenestra ovalis from the fenestra pseudorotunda. The crista is flat, relatively broad, and forms the rostroventral margin of the caudal tympanic recess. In occipital view, the opisthotic is intimately fused with the exoccipita1. The conjoined bone extends as a wing, the paroccipital process, on either side of the foramen magnum that is fused with the squamosal in rostral aspect. The exoccipital forms the rostral margin of the fenestra pseudorotunda. Behind this opening, the metotic strut of the exoccipital forms the floor and mos t of the cauda l wall of the recess us scalae tympani. Here the base of the metotic strut is penetrated by foramina for glossopharyngeal (IX), vag us (X), and spinal accessory (XI). In occipital aspect, the exoccipital contributes a small part to the occipital condyle and forms the lateral rim of the foramen magnum. Lateral to the foramen magnum, the exoccipital is pierced by the hypoglossal (XII) nerve. A deep, curved, vertical channel on the metotic strut, between the glossopharyngeal and vagus foramina, marks the sinuous course of the internal carotid artery. Here the artery would bifurcate into the dorsal (stapedial) and ventral (cerebral carotid) bra nches (Midtgard, 1984). The dorsal, stapedial foramen along the channel represents the entrance of the stapedial branch ofthe carotid artery into braincase medial to the prootic contact of the quadrate. The ventral, carotid foramen along the channel marks the rostra l course of the cerebral carotid artery, which would travel in the ventral portion of the middle ear cavity in a bony, parabasal canal. The vcntral basitemporal plate of the basisphenoid-parasphenoid complex is not preserved, but its mamillary process is retained. The process is somewhat abradcd and opened up revealing a cavity. Apparently it is permeated by the pneumatization of the caudal tympanic recess. Below and behind the rostral tympanic recess is the foramen for the Eustachian canal, tunneling ventrally and rostrally through the basisphenoid. Craniofacial pneumaticity The avian skull has long been noted for its lightness and pneumaticity by air-filled cavities (Bremer, 1940), but the origin and function of this pneumaticity are poorly understood. Craniofacial pneumaticity includes two separate systems: the rostrally located nasal pneumaticity and the caudally placed tympanic pneumaticity (Fig. 8A). Recently, Witmer (1990, 1997) has reviewed the distribution ofcraniofacial pneumaticity ofarchosaurs in a phylogenetic context. He offers a novel interpretation that the function of these epithelial air sacs is simply to pneumatize bones in an in vasive and opportunistic manner fostering both strength and lightness. In dried or fo ssil skulls, this pneumaticity can be recognized by several osteological correlates in distinct topographic positions. For example, the presence of nasal pneumaticity can be inferred by the sinuses preserved in the premaxilla, maxilla, lacrimal, and mesethmoid bones surrounding the antorbital fenestra. Similarly, the tympanic pneumaticity can be recognized by the presence of various cavities surrounding the otic capsule, quadrate and articular bones. As in most diving birds, cranial pneumaticity in Polarornis is well represented but appears to have been somewhat reduced because of extensive pachyostosis (Witmer, 1990). In many cases, the body of the bone itself around the recess is not pneumatized and trabeculated contrary to that of typica l neognaths.

13 Editors: Zhong he Zhou and Fucheng Zhang. Beijing: Science Press Instead, the bone shows a dense and compact texture as in diving birds. The morphology and distribution of pneumatic recess are somewhat similar to the condition of Hesperornis (Witmer, 1990), loons, grebes, and penguins. Evidence of nasal pneumaticity is preserved in lhe specimen (Fig. 8B). The triangular antorbital fenestra apparently housed a paranasal air sinus (Witmer, 1997). The dorsal surface of the palatine process of the maxilla contains a cup-shaped depression at its caudal end, indicating the presence of the caudal maxillary sinus. In living loons, this maxillary depression is more elaborate with various pneumatic foramina and is oriented almost vertically. Farther forward, the long and tapering premaxilla appears to be apneumatic as in Hesperornis and other diving birds (Witmer, 1990). The medial surface of the lacrimal contains a shallow fossa dorsally and rostrally near the junction of the frontal. This fossa may indicate the evidence of a lacrimal sinus. The mesethmoid bone shows a large pneumatic fossa and several foramina at the lateral wall near the ventral roof of the skull, which may indicate the presence of a mesethmoid sinus. The tympanic air system communicates with the external environment via the ventral openings of the Eustachian tube in the roof ofthe mouth. In Pofarornis, all three tympanic recesses, the rostral, dorsal and caudal, are present in the same topographic position as in ornithurine birds (Fig. 8C). However, the tympanic walls surrounding the rostral and caudal recesses are subdued in Pofarornis, compared to modern forms. The large entrance of the dorsal tympanic recess is located between the lateral and medial quadrate cotyles. Although the opening extends considerably mediall y, there is no evidence of contralateral communication of the dorsal tympanic recess as in other Mesozoic birds (Witmer, 1990). The position ofthis recess in relation to the medial quadrate cotyle is variable among extant birds and has been used in the past as a taxonomic character for classification of birds (Lowe, 1926). The caudal tympanic recess is best observed in the otic capsule area, dorsal and caudal to the fenestra ovalis and fenestra pseudorotunda. Unlike modern neognaths, these three cavities are not clustered and bordered caudally by the prominent, overhanging wall of the metotic strut (Figs. 7 A, B, 8C). As a result, the threshold of the crista fenestralis does not slope into the caudal tympanic recess ; it merely borders the rostral rim. Internally, the caudal tympanic recess expands considerably into the paroccipital process and extends ventrally into the metotic strut and mamillary process. The rostral tympanic recess is preserved between the prootic and the basisphenoid. Unlike modern loons, the lateral wall (alaparasphenoid) of the rostraltympanic recess is not inflated and bulbous; instead the wall is less developed and more flattened to occupy the position of the rostral rim. The rostral tympanic recess is conical, directed rostrally and medially, and may have established contralateral communication. This feature is attributed to the mechanism of sound localization in birds (Rosowski and Saunders, 1980). The quadrate is apneumatic as in diving birds, whereas the articular bone is missing in the specimen. Articular pneumaticity appears to be lacking in Hesperornis and Gavia. Cranial kinesis Like modem birds, the skull of Pofarornis shows all the morphological correlates of prokinesis (Fig. 9). The upper bill is kinematically colmected to the rest of the sku II by three types of bending zones (Bock, 1964; Btihler, 1981): one dorsally along the craniofacial (=nasal-frontal) hinge; a pair laterally along the jugal bars, and a pair ventrally in the palatal bars. These bending zones are characterized by flattening or thinning of bones, or the development of a multilayered, sandwich-like structure. Pofarornis has a streptostylic quadrate like that of ornithurine birds with the following features: (1) the bipartite head fits into a pair of cotyles to form a fulcrum with the braincase; (2) an orbital process serves as an effective lever for insertion of M. protractor quadrati and M. pseudotemporalis profundus for forward and backward movements; (3) It has minimized its articulations with the quadratojugal and the pterygoid and shifted their contacts ventrally to make the fulcrum more effective. Medially, the quadrate receives the pterygoid ventrally in a condyle, and the quadratojugal in a cotyle to fonn pin joints that allow flexibility during streptostylic movement of the quadrate (Chatterjee, 1991, 1997). Like modern prokinetic birds, the skull of Pofarornis shows four functional units and four principal joints on each side of the skull (Bock, 1964;

14 138 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution A B c dtr sinus Id cms tympanic sac ---',,----~ Is... dtd mes FIGURE 8. Craniofacial pneumaticity. A, Diagrammatic representation of the diverticula of the nasal and pneumatic sinuses of a nesting neognath (after Witmer, 1990); B, reconstruction of antorbital fossa ofpolarornis showing nasal pneumaticity; C, left lateral view of braincase of Polarornis showing tympanic pneumaticity. Bi.i hler, 1981; Alexander, 1983; Zusi, 1984; Chatterjee, 1991, in press). These functional units are: (I) the upper jaw, (2) the jugal bar plus the pterygoid-palatine bar, (3) the quadrate, and (4) the braincase. These four units form a four-bar crank chain mechanism of mobility. It has four links and four joints (Fig. 9). The quadrate is the vertical link and the main crank device. The head of the quadrate forms a fulcrum on the undersurface of the braincase. As the quadrate is swung forward, the force is transmitted to the beak by a pair of horizontal links. The jugal bar forms the lateral link, whereas the pterygoid-palatine acts as a medial link. Because the jugal bar and the pterygoid-palatine bar share a similar mechanical function as a "push-rod" between the upper bill and the quadrate, they are considered functionally as a single link to simplify the model. The forward movement of the quadrate sets in motion a chain of events leading to the protraction of the upper bill. As the quadrate foot moves forward, this force is imparted to the jugal bar laterall y and pterygoid medially, which, in tum, pushes the upper bill fof\vard. Since the upper bill is hinged flexibly at the N-F joint, the forward push of the jugal bar rotates the upper bill dorsally (Fig. 9). Vertebrae Four articulated cervical vertebrae of Polarornis are preserved in the collection, representing the mid-cervical region (Figs. 5C, G, loa, B, C, D). Comparison with Gavia immer suggests that they are the sixth, seventh, eighth and ninth cervicals. The most anterior cervicals in Gavia have pronounced hypapophyses on the ventral surface of the centra, which are atrophied in the mid- and posterior cervicals. The sixth vertebra is represented by the caudal part of the neural arch only, showing the elliptical postzygapophyseal facet, large neural canal, and a short neural spine. It was carefully removed from the rest of the series to expose the anterior view of the seventh cervical. The remaining three vertebrae are complete, and elongated with short ribs fused to them to enclose the transverse foramen for the vertebral artery. The vertebrae appear to be compact and apneumatic as in other diving birds. The neural spines are low and subdued. The zygapophyses are well developed and tilted about 45 towards each other, and extend beyond the faces of the centra. The neural canal is highl y enlarged, where its diameter corresponds well with the diameter of the centrum face, as in modem birds. This enlarged canal indicates a high demand for nerve signal traffic. The centra are heterocoelous where its rostral articular surface is concave transversely and convex dorsoventrally, whereas at the caudal face, these

15 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press outlines are reversed. These saddle-shaped joints permit a great range ofdorsoventral flexion ofthe neck but prevent rotary motion along the axis of the spine. The parapophysis lies low at the base of the centrum. Below the parapophyses, there are paired carotid processes on the rostroventral surface ofeach centrum to protect the carotid artery. These ventral flanges are reduced backward and converge near the middle of the centrum to form a median ridge, but they diverge again caudally without any ventral projections. Sternum In Polarornis, only the right side of the sterno-coracoidal process is preserved with four sternal ribs still attached (Figs. 5H, I OE). However, the medial coracoid sulcus is entirely missing. Hindlimb The left side ofthe femur and tibiotarsus were found in articulation. In addition, the proximal part of the right femur is also preserved (Figs. II A, B, 14C, D). The femur is short and stout and resembles the femur of modem loons in general features. The head is spherical, directed medially, and is supported by a constricted neck. There is a deep pit on the medial surface of the head for the insertion of the capital ligament. Laterally, the greater trochanter is large and extends proximally above the femoral head. Between the greater trochanter and the head lies a concave anti trochanteric fossa that slides against the anti trochanter of the ilium. Laterally, a prominent obturator ridge runs distally from the greater trochanter for a short distance for the attachment of obturator and iliotrochanteric muscles. The shaft of the femur is cylindrical, laterally compressed, somewhat arched in side view, and is oval in cross-section. Although the bone is compact, it shows a large medullar cavity (Fig. II C). The shaft is marked by faint cranial and caudal intermuscular lines. The distal end of the femur is expanded transversely and is divided into two condyles for articulations with the tibiotarsus and fibula. The lateral condyle is more robust, twisted laterally, and is marked by a narrow, spool-shaped, fibular groove. [n modem loons, the fibular groove is relatively shallow and wide. The medial condyle of Polarornis shows a narrow, circular crest projecting caudally. [n modem loons, this condyle is highly reduced and twisted somewhat medially. [n caudal aspect, the popliteal fossa between the two condyles is shallow. Cranially, in the opposite topographic position, the patellar groove between the two distal condyles facilitates the sliding of the cnemial crest of the tibiotarsus. The proximal part of the conjoined tibiotarsus and fibula are preserved (Figs. II D, E, 14H, I). They agree essentially with those of Gavia immer except for the size ofthe cnemial crests at the proximal end. [n cranial aspect, a pair of large cnemial crests is hallmarks of the tibiotarsus, which is similar to the condition ofother diving birds and for the attachment of extensor muscles. The elongation of the crest is linked with the shortening of the femur. The cranial cnemial crest is fairly large and elevated above the knee joint. However, this crest is relatively shorter than that ofmodem loons. The medial surface of this crest shows a facet for the Braincase (Stationary) Upper Jaw pm, +~, Prenarial Bar Jugal Bar Weight Out 1...li.nk.s,A..JDlDls FIGURE 9. Left lateral view of the skull of Polarornis showing cranial kinesis; the shaded upper jaw region showing the prokinetic movement.

16 140 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution nsp prz nsp poz A B cr 1cm prz poz ce cr c D cr 5cm E

17 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press origin of the M. gastrocnemius. The lateral cnemial crest is relatively short, and is separated from the cranial one by a deep, longitudinal intercnemial sulcus. Both the tibiotarsus and the fibula articulate with the femur at a right angle in a deep, concave articular surface. The lateral articular surface is slightly convex whereas the medial one is somewhat concave. In caudal aspect, there is a flexoris fossa just below the proximal end for the attachment of M. flexor digitorum longus. The shaft of the tibiotarsus is compressed craniocaudally and shows a series of transverse ridges (Figs. lid, E), indicating pathological defomlity (see below). The preserved part of the fibula is reduced to a slender rod and is separated from the tibia by an interosseal gap. The proximal end ofthe fibula is slightly expanded and is fused to the tibiotarsus. Pathology Macroscopic and radiological examinations of the tibial shaft reveal lesions in both external and internal surfaces (Rothschild, pers. comm.). Postmortem breaks in the shaft do not show any sign of healing but with periosteal reaction, which indicates new bone fonnation in the outer layer, the periosteum (Figs. 11 D, E). Lack ofremodelling of bone edges of the broken component suggests that the periosteal reaction was not caused by a fracture. In living birds, this would represent hy pertrophic osteoarthropathy, an avian viral disease. It is different from the intra-thoracic pathology-related phenomenon of hypertrophic osteoarthopathy in mammals. A detailed account of the pathology of Polarornis will be published elsewhere in collaboration with Rothschild. Locomotion Like modern foot-propelled divers, Polarornis probably acquired two independent and specialized methods of locomotion: flying with the wings, and diving with the hindlimbs. Because of the lack of shoulder and wing elements, the flight capability of Polarornis cannot be assessed at this stage. Modem FIGURE 10. Vertebral column, and sternal fragment of Polarornis. A, B, C, D, cranial, lateral, dorsal and ventral views of seventh and eighth cervicals; E. sternal fragment showing the sternocoracoidal process and ribs. loons are graceful in swimming and flight with their streamlined bodies, but they are almost comical in terrestrial ambulation, or during takeoff, and landing. In a typical running or walking bird, the center of mass lies over the feet. In contrast, the legs of loons are set far back on the body, towards the tail, far behind the center of mass to facilitate underwater propulsion. As a result, they cannot balance their body on two feet on the ground; they have great difficulty walking or even standing. As they walk, they move clumsily. one foot at a time, keeping their breast close to the ground, often supporting some ofthe weight (Mclntyre, 1988). They are extremely awkward and most vulnerable on land. Because of their large size and di ving adaptations, the bones are compact and heavy while the wings are small, making takeoff and landing more difficult for them. The red-throated loon (Gavid stellata) is the only extant species, which can take off from land; other species run several hundred meters across water before they gain sufficient speed and thrust to take off. Once airborne, the loons are powerful flyers and migrate great distances. The skull ofpolarornis is unusually thick and heavy with a long, tapering beak to reduce drag during swimming and diving. The skull was held in line with the laterally compressed neck during diving, while heterocoely permitted maximum extension of the neck during strikes at prey. Several skeletal features in the hindlimbs indicate that Polarornis had already acquired foot-propelled diving adaptations. These include: (I) heavy, compact, and apneumatic bones to overcome buoyancy; (2) a short, stout femur with a double-hinge articulation with the acetabulum and antitrochanter; (3) the femur extending perpendicular to the pelvic girdle; (4) a long tibiotarsus equipped with a strong and powerful cnemial crest for the attachment of the gastrocnemius, the major extensor of the foot; and (5) both femur and tibia were presumably tied to the body by muscles and fiber tissues. The locomotory evolution of foot-propelled diving birds is interesting (S torer, 1960). Some volant birds probably went to the sea to exploit the abundant food resources. Many became highly specialized footpropelled diving birds, which evolved several times independently in avian history. In the Cretaceous sea, we see the evidence of two lineages-hesperornithi

18 142 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution h gt obr gt atf elp h sh eril -----* I~: Ie Ie me A B c pag fg eail gf nf fi fi 01 o 3 o E

19 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press forms and gaviiforms; the former could not fly as is evident from their diminutive wings. To keep the body submerged, the bones in the divers became heavy with the reduction of air sacs and pneumaticity. The most striking adaptations of these birds were for propulsion by the hindlimb: the development ofa long and narrow pelvis, a short femur, a long tibiotarsus with an extensive cnemial crest, and a laterally compressed tarsometatarsus (Fig. 12). In the foot-propelled diving adaptation, there was a distal shift ofthe functional movement from the knee joint of a typical terrestrial bird to the ankle joint. In a walking or running bird, the femur is generally held horizontal in a parasagittal plane with little arc of rotation. The main movement is in the knee joint where the tibia rotates considerably during locomotion. Movements of the knee account for most of the foot displacement (Gatesy, 1991). On the other hand, in foot-propelled divers, the femur is held at right angle to the body of the axis (Fig. 12). Moreover, its femur and tibia are held stationary during aquatic locomotion and are entirely enclosed within the skin (Heilmann, 1926). The main movement is along the ankle joint, which lies on the side of the body. In surface swimming, the feet are directed to either side of the body like an oar, and move in a horizontal plane. Loons are also specialized for using the wings in underwater locomotion. With relatively small wings, they swim like feathered seals underwater, where they twist and tum to chase fish (McIntyre, 1988). Paleoecology Polarornis was a large bird about the size of a common loon, 60 Col long and weighing nearly 4 kg (Table I). It lived in the coastal water of the Antarctic Peninsula along with anseriforms, charadriiforms, fish, plesiosaurs, mosasaurs, ammonites, and mollusks. Plesiosaurs and mosasaurs were presumably ectothermic like modern aquatic reptiles and their FIGURE II. Left hindlimb of Polaornis. A, B, cranial and caudal views of the femur; C, crosssection ofthe femoral shaft showing medullar cavity; D and E, cranial and caudal views of the tibiotarsus and fibula. TABLE 1. Main measurements of Polarornis (All measurements are in mm; e, estimated length). Skull Length 180 (e) Width 41 (e) Height 37 (e) Length of antorbital fenestra 22 Quadrate Length 15 Height 16 7th Cervical Length 16 Width (cranial face) 20 Height II Left femur Length 62 Shaft diameter 7 Width, proximal end 18 Width, distal end 15 Left tibiotarsus Length 155 (e) Width, proximal end 18 Total estimated length 600 distribution was affected by latitudes, continental configurations, and environmental temperatures. Extant analogs such as crocodilians and turtles are confined to the tropics and some warm edges of the temperate zones. During the Late Cretaceous, the last fragmentation of the Pangea occurred, and the various continents were separated by shallow seas. Antarctica was still in contact with Australia and South America, and consequently there was no circum-antarctic gyre to cool the deep polar sea. The occurrence of several species of dinosaurs (Gasparini et al., 1996), plesiosaurs and mosasaurs (Chatterjee and Small, 1989) in the Antarctic Peninsula indicates that the temperature within the Antarctic Circle was equable and probably cool temperate during the Late Cretaceous period. There is abundant evidence from plant fossils that cool-temperate forests flourished in

20 144 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution Jj ;" FIGURE 12. Skeletal restoration of Polarornis in dorsal view. i'j o o 3 presbyomithids and charadriiforms, also lived in the Antarctic water (Noriega and Tambussi, 1995; Case and Tambussi, 1999). These Antarctic birds probably migrated northward shortly before the long, dark, winter season. The compact jaw of Polarornis with its sharp and pointed beak was designed for catching fish, mussels, and aquatic insects. Apparently Polarornis exploited the abundant fish fauna available in the Antarctic water (Grande and Chatterjee, 1987; Stahl and Chatterjee, 1999). The endothem1ic physiology of Polarornis can be inferred from several osteological correlates. Chinsamy et at. (1998) studied the femoral microstructure of Polarornis and noticed the pachyostotic nature of the bone wall, which consists of fibro-iamellar bone tissue, highly vascularized by both primary and secondary osteons. The bone histology indicates a rapid rate of bone formation without any periodic interruptions as seen in living endotherms such as birds and mammals. Ruben et at. (1996) observed that the respiratory turbinates are unique to modern endotherms but are conspicuously absent in living ectothenns such as reptiles. These turbinates are scrolls of mucosa covered cartilage or bone in the nasal passage which play an important role in controlling water and heat loss. They are delicate structures and rarely preserved in the fossils; however, their presence in endothem1s can be speculated from the volume of the nasal passage to accommodate the turbinates. Ruben et al. (1996) examined various extinct and extant animals and found that the nasal passages in endotherms are about four times bigger than those of similar-size reptiles. Thus the large volume of a nasal passage is evidence of turbinates in life. Cross-sectional CT scan of Polaromis indicates a large nasal passage and stmcture comparable to that of Gavia, hinting its endothermic physiology (Fig. 10E). In the polar environment, cooling ofexhaled air would substantially reduce respiratory heat loss In Polarornis. West Antarctica during that time (Dettman, 1989). Because ofhigh latitude, the animals and plants living during the Late Cretaceous on the Antarctic Peninsula would have had to cope with prolonged period of annual darkness in the winter. Several other contemporary birds, such as Comparisons and Discussion The long cnemial crest of Polarornis clearly indicates its foot-propelled diving adaptation. Although Polarornis superficially resemble grebes and hesperornithifonns in the mechanical design of the

21 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press hindlimbs, the way in which the cnemial crest is developed among different foot-propelled diving birds may be an important phylogenetic character (Heilmann, 1926). The cnemial crest of the loon is derived solely from the tibiotarsus, but the patella is lacking. In Hesperornis, the cnemial crest evolved from the development and expansion of the patella. In grebes, on the other hand, both patella and tibiotarsus contribute to the formation of the cnemial crest (Fig. 13). Ifthese diving birds were closely related (Cracraft, 1982), it is unlikely that the formation of this structure would have evolved along such a very different pathway. It is likely that loons, grebes, and hesperomithiform birds are purely convergent in their adaptations and did not share a common ancestry (Storer, 1960; Olson, 1985). Loons and grebes show many morphological and reproductive differences, such as differences in the texture of the plumage, form of tail feathers, location of the nest, and the number of eggs (Storer, 1956). Moreover, loons have webbed feet and grebes have lobed feet, and the two groups use different methods ofpropulsion. Webbing and lobation are very different ways ofaquatic special izations. Also, the structure and mechanism of the ankle and toe joints arc different between these two groups (Stolpe, 1935). It is unlikely that loons and grebes originated from a common ancestor (Raikow, 1985). Polarornis shows several derived features, which are absent in the hesperornithiforms. In the skull, cranial sutures are closed; the teeth are lost; the palate is neognathous; the quadrate is bicondylar; and bony Eustachian tube is present (Figs. 2-7). In the femur, the shaft is relatively slim; the trochanteric crest is low; the medial condyle at the distal end is small and wide (Fig. 14). The cnemial crest is formed entirely by thc tibia. Polarornis shows several distinguishing features from grebes in skull and hindlimb morphology (Storer, 1956). In Polarornis, a prominent salt gland depression on the frontals is present. This feature is absent or subdued in grebes. In grebes, the capital ligamental fossa on the femur is nearly a circular depression with a lip extending caudally. In Polarornis, the corresponding depression is elongate without any lip. [n the grebes, the trochanteric ridge shows a rounded projection distally, whereas it is nearly straight in Po larorn is. In grebes, the lateral surface of the outer cnemial crest is smooth and convexly rounded for the attachment of the patella. In Polarornis, the lateral Tibia Patella fi i Tibia c t B D I FIGURE 13. Morphological variations of cnemial crest among diving birds (left tibiotarsus and fibula, cranial view). The large cnemial crest is a hallmark for foot-propelled diving birds. A, Hesperornis ; B, Aechmophorus;C, Gavia; D, Colymboides; E, Polarornis. Tn Hesperornis and Aechmophorus (grebe), a separate patella forms part of the cnemial crest, whereas in living (C) and fossil loons CD-E), the cnemial crest is made entirely of the tibia; the patella is lacking.

22 146 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution 5cm A B c o E 5cm f l I.,\ : F G H J :, i,'i

23 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press surface of the outer crest is concave and roughened. There is no indication of an articulation with the patella. Polarorl1is shares several features with the Gaviidae, especially in the morphology of the quadrate, femur and tibia, justifying its inclusion within this family. The orbital process of the quadrate is sharply concave with an overhanging ridge; the ventral articular surface of the quadrate shows a caudal projection; the femur is highly arched in lateral aspect; its proximal end is highly compressed craniomedially; the cnemial crest is long and consists entirely of tibia, the patella lacking; the lateral cnemial crest is pronounced and flares distally. Because of fragmentary nature ofother fossil loons, it is difficult to compare Polarornis with other fossil taxa, such as Neogaeornis, Colymboides and Gavia. Neogaeornis is known from a solitary right tarsometatarsus from the Late Cretaceous Quiriquina Formation of Chile (Olson, 1992). Unfortunately, the tarsometatarsus is not known in Polarornis, thus precluding any direct comparison with Neogaeornis. Colymboides is known from the Late Eocene of England (C anglicus), Early Miocene of France (C minutus) and the Czech Republic (Storer, 1956, Delle Cave et af., 1984). C anglicus is known from a frontal, a humerus, a coracoid, and an ulna (Harrison and Walker, 1976). C. minutlls was a small loon and is known from abundant postcranial material. The cnemial crest of Colymboides is similar in size and proportion to that of Polarornis, indicating similar grade of diving adaptations (Fig. 13). However, the lateral cnemial crest is very pronounced in Polarornis, where this feature is subdued in Colymboides. The femoral shaft of Polarornis appears to be heavier than that of Colymboides. The fibular groove appears to be relatively shallow and wide in Colymboides whereas in Polarornis it is narrow and spool-shaped. Gavia is \videly known from the Paleogene and Neogene deposits of Europe and North America. A beautiful skull of Gavia is reported from the Pliocene of central FIGURE 14. Comparison of left hindlimbs, A-E, left femur; caudal view; F-J, tibiotarsus, cranial view; A, F, Hesperornis; B, G, Baptornis; C, H, Po/arornis; D, I, Cavia; E, J, Aechmophorus. Italy (Delle Cave et ai., 1984). Svec (1982) reported tvvo sympatric species from the lower Miocene deposits in Czechoslovakia, Gavia egeriana and CO~}'Inboides milllltus, thus establishing the contemporaneity of these two genera. Storer (1956) traced the ancestry of Gavia to the Early Miocene Colymboides. Discovery of Gavia from the Middle Eocene of Germany and the Lower Miocene of Czech Republic would indicate an earlier differentiation between Gavia and Colymboides than is currently recognized (Delle Cave et af., 1984; Svec, 1982). There are several unique features in Polarornis, which are absent in Gavia. These are: external naris small; frontals narrow, and across the orbit with narrow salt gland depressions; orbital process of the quadrate thick, short and directed horizontally; pterygoid condyle of the quadrate bordered by a dorsal groove; caudal tympanic recess in the same plane with the fenestra ovalis and the fenestra pseudorotunda, and not bounded caudally by an overhanging wall; lateral wall of the rostral tympanic recess flat, not bulbous; vomers fused and rounded rostrally; femoral neck long; distal condyles of femur subequal and projecting caudally, not twisted sidewise; and intercnemial sulcus perforated by a nutrient foramen. The modem loons are an extremely homogenous group, belonging to a single genus Gavia. At present the genus Gavia comprises four species: common (G. immer), yellow-billed (G. adamsi), arctic (G. arctica), and red-throated (G. stellata), all occurring in the northern parts of the Northern Hemisphere (Storer, 1978). This, with three fossil genera (Polarornis, Neogaeornis, and Colymboides), constitutes the sole famil y Gaviidae within the order Gaviiformes. Phylogenetic Relationships The phylogenetic position of Polarornis within Aves can be demonstrated in a nested set of synapomorphies. I have selected 52 unambiguous characters preserved in Polarornis, which are distributed in different hierarchical levels of avian phylogeny. These characters have already been established in literature (Cracraft, 1986, 1988; Chatterjee, 1991, 1997,1999; Chiappe, 1995, 1996b). Several derived characters suggested that Polarornis is closer to Neognathae than Paleognathae. Thus, in

24 148 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution the present cladistic analysis, Enantiornithes, Hesperornithiformes, and paleognaths are used as successive outgroups. For paleognaths, r have used Lithornis as the terminal taxon (Houde, 1988). Representatives of four extant neognath taxa, such as Galliformes (CaLLus), Gaviiformes (Cavia), Sphenisci formes (Pygoscelis) and Procellari formes (FuLmaris) which are generally considered to represent the earliest divergences in modern birds, are also included (Cracraft, 1988; Sibley and Ahlquist, 1990). The interrelationships of neognaths are highly controversial. Cracraft (1982, 1988) attempted a broad phylogenetic analysis of the basal groups, which will be followed here. Given the historically controversial phylogenetic position ofgrebes, this group is excluded from the analysis (see discussion later). The core ingroup taxa include Archaeopteryx, Enantiornithes, Hesperorni thi formes, Lithornis, Gall iformes, Polarornis, Cavia, Sphenisciformes and Procellariformes. Missing characters in PoLarornis are not used in this analysis. Only binary characters (0, primitive and I, derived) are coded. Character Analysis I. Prefrontal bone: (0) present, (\) absent. 2. Ascending process of the jugal: (0) present, (I) absent. 3. Olfactory lobes: (0) large, (I) small. 4. Nasal process of premaxilla extends caudally to contact frontal: (0) no, (I) yes. 5. Ectopterygoid bone: (0) present, (I) absent. 6. Squamosal roof to the dorsal tympanic recess: (0) no, (I) yes. 7. HeterocoeJous anterior cervicals: (0) absent, (I) present. 8. Cervical neural spine: (I) tall, (I) reduced. 9. Fusion of premaxillae in adults: (0) absent, (I) present. 10. External naris: (0) terminal, (I) shifted backward close to the rostral margin of the jugal. II. Mesethmoid ossified: (0) absent, (I) present. 12. Orbital septum ossified: (0) absent, (I) present. 13. Femur, capital ligament fossa at head: (0) absent, (I) present. 14. Femur, deep patellar groove at the distal end: (0) absent, (I) present. 15. Postorbital hone: (0) present, (I) absent. 16. Quadrate/prootic articulation: (0) absent, (I) present. 17. Caudal maxillal), sinus \-vith cup-shaped depression: (0) absent, (I) present. 18. Quadrate, orbital process: (0) absent, (1) present. 19. Quadrate, ptel),goid condyle: (0) absent, (I) present. 20. Quadrate, quadratojugal cotyle: (0) absent, (I) present. 21. Palatine/pterygoid articulation: (0) fused, (I) unfused. 22. Femur with trochanteric crest: (0) absent, (I) present. 23. Tibiotarsus, lateral cnemial crest: (0) absent, (I) present. 24. fibula: (0) long and robust, (I) slim and greatly reduced distally. 25. Nasal bones extend far caudally relative to the caudal border of the premaxilla: (0) absent, (I) present. 26. Rostral wall of the middle ear cavity thickened and cancellous: (0) absent, (l) present. 27. Eustachian tubes open laterally: (0) absent, (1) present. 28. Femur with low trochanteric crest: (0) absent, (I) present. 29. Medial condyle of the femur, small and wide: (0) absent,( I) present. 30. Zygomatic process on the squamosal: (0) short, (I) long. 31. Individual skull bones: (0) unfused, (I) fused. 32. Lacrimal/jugal contact: (0) present, (I) absent. 33. Maxilla, ascending process with sutural connection to nasal: (0) present, (I) absent. 34. Quadrate head: (0) single, (I) double. 35. Bony palate: (0) solid, (I) cleft at mid-length. 36. Palatine/premaxilla contact: (0) absent, (I) present. 37. Vomer/premaxilla contact: (0) present, (I) absent. 38. Palatines excluded from the midline: (0) present, ( I) absent. 39. Carotid flanges on the cervicals: (0) absent, (I) present. 40. Mediopalatine process: (0) unfused, (I) fused. 41. Dorsal tympanic recess extends caudal to the articular facets of the quadrate: (0) present, (I) absent. 42. Bills relatively long and sharply pointed: (0) absent, ( I) present. 43. Medial surface of the orbital process sharply

25 Editors: Zhonghe Zhou and Fucheng Zhang. Beijing: Science Press concave with an overhanging ridge: (0) absent,( I) present. 44. Mandibular articular surface has a prominent caudal projection: (0) absent, (I) present. 45. Femur arched in lateral aspect: (0) moderately, (I) highly. 46. Proximal end of femur highly compressed craniomedialy: (0) absent, ( I) present. 47. Cnemial crest is long and consists entirely of tibia; patella lacking: (0) absent, (I) present. 48. Lateral cnemial crest pronounced and flares distally: (0) absent, (I) present. 49. Supraoccipital highly inflated without any sagittal crest: (0) absent, (I) present. 50. Palatines wide distally: (0) absent, (I) present. 51. Lacrimal foramen: (0) small or absent, (l) large 52. Femoral trochanteric crest with cranial projection: (I) absent, (l) present. Results 52 cranial and postcranial characters were scored among 9 ingroup taxa (Table 2). The polarity of these synapomorphies was arranged in a morphocline using successive outgroups. A cladistic analysis was undertaken to evaluate the phylogenetic position of Polarornis, using the branch-and-bound algorithm of PAUP version 3.1 (Swofford, 1993). The analysis generated a single-most parsimonious tree (tree length = 52, consistency index = I, retention index = 1). The analysis clearly indicates that Polarornis shows close phylogenetic relationship to extant Gavia and is more deri ved than the Cretaceous, foot-propelled, and toothed bird Hesperornis (Fig. 15). Relationships Sibley and Ahlquist (1990) listed 12 characters that differentiate the loons from the grebes. Both immunological (Ho el al., 1976) and DNA hybridization (Sibley and Ahlquist, 1990) distances indicate that loons are closer to penguins than to grebes. The DNA comparisons indicate that loons are member ofa clade, which also includes the penguins (Spheniscidae), tubenosed swimmers (Procellariidae), and the frigate birds (Frigatidae). The grebes are not members ofthis cluster, and they seem to have no close living relatives. Loons and penguins share a number of similarities in their skeletal morphology. Olson (1985) described a giant penguin (?PalaeoeudypLes sp.) from the Late Eocene of Seymour Island in which the bill is long, pointed, and dagger-like, unlike that of modern penguins, but is quite similar to that of the modem loon. Also, the cervical vertebrae of PalaeoeudypLes are very different from modem penguins, but closely resemble those of loons, indicating their close relationships. Additionally, Cracraft (1982) noted several synapomorphies shared by loons and penguins such as: (I) lateral surface of the orbital process and body of the quadrate sharply excavated; (2) sternum with low keel and a pair of rounded notches at the caudoventral margin; and (3) squamosal extends laterally as a horizontal platform to form a roof over TABLE 2. Character matrix. Character-state distributions. 0 = primitive state; I = derived state. Ta xa/character Velociraptor Archaeopteryx Enantiornithes Hesperornis Lithornis Galliformes Polarornis Gavia Sphenisciformes Proce11ariformes

26 150 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution C/) V> (1) aj (/) <1> E E ~.'2 C/) ~ ~.2 c EO OJ.'2 g- U ~.~.Q 2 E E.<!! ~ (l) (l) E 2 c Q) <1l C Q. 0 ~.,!! <1> -C: u ro U) ~.c (l) ~ c :S ro <1l "t: w l: ~ <.9 &. '" a. e <.9 (j) Cl. NEOGNATHAE (31-42) PALAEOGNATHAE (25-30) ORNITHURAE (15-24) ORNITHOTHORACES (4-14) FIGURE 15. Cladogram showing the placement ofpolarornis among neognathous birds in the family Gaviidae. The single-most parsimonious hypothesis of the core ingroup taxa of Mesozoic birds resulted from PAUP analysis of 52 character-states preserved in Polarornis (tree length = 52, consistency index = I, retention index = I). Character-state distributions are listed in Table 2. the quadrate head. However, Polarornis differs from penguins as follows: (I) external naris elliptical, small, and caudally placed; (2) a gap between lacrimal and jugal bar; (3) large palatal contribution by premaxillae with a long median symphysis; (4) cervical vertebrae longer than wide; (5) absence of cranial projection of trochanteric crest on femur; (6) ligamental fossa subdued; and (7) presence of long cnemial crest on tibiotarsus. Polarornis shares the following synapomorphies with Cavia: (I) medial surface of the orbital process sharply concave with an overhanging ridge; (2) mandibular articular surface of the quadrate with a prominent caudal projection; (3) femur highly arched in lateral aspect; (4) proximal end of femur highly compressed craniomedially; (5) cnemial crest is long and consists entirely of tibiotarsus, patella lacking; and (6) lateral cnemial crest pronounced and flares distally. Polarornis shows the following autapomorphies within Gaviidae, justifying its allocation into a new genus and species. These are: (I) small external naris; (2) waist of frontal across the orbit extremely narrow; (3) salt gland depressions form narrow bands; (4) orbital process of quadrate thick, short and directed horizontally; (5) pterygoid condyle separated from the body of the quadrate by a distinct dorsal groove; (6) caudal tympanic recess in the same level with fenestra ovalis and fenestra pseudorotunda, and not bounded caudally by the overhanging wall; (7) lateral wall of the rostral tympanic recess flat, not bulbous; (8) rostral end ofvomers rounded in cross-section, not ridge-like; (9) femoral neck long; (10) distal condyles of femur subequal and projecting caudally, not twisted sidewise; and (II) intercnemial sulcus perforated by a nutrient foramen.

27 Editors: Zhong he Zhou and Fucheng Zhang. Beijing: Science Press Antarctica: Cretaceous Cradle of Modern Birds Loons are ancient birds whose antiquity has been documented in the Late Cretaceous of C hile and Antarctica. Indeed, their haunting cry sounds eeril y prehistoric when they wai I their lonely calls across a lake on a s ummer night. Loons are truly " living fossils" with little morphologic changes in their 66 million years of evolution. The earliest record of two genera of Late CretaceoLis loons, Polarornis and Neogaeornis in the high southern latitude may have broad temporal and evolutionary significance. The extant loons and their Cenozoic fossi l record are confined to the Northern Hemisphere. Apparently, loons migrated to the Northern Hemisphere during the Paleocene-Eocene probably to avoid direct confrontations with emerging penguins. With the disappearance of hesperornithiforms by the end of the Cretaceous, the niche for the foot-propelled diving bird became open in the Northern Hemisphere, and loons might have filled this vacant niche. The Tertiary loon Distribution of Fossil Loons Pleistocene o Pliocene... Miocene Eocene * Late Cretaceous FIGURE 16. Distribution of fossil loons in time and space plotted on a present-day world map showing that high-latitude region of Southern Hemisphere acted as a center of origin and dispersal of the Gaviidae in the Late CretaceoLis until they spread to Northern Hemisphere in Eocene. Similar pattern of high-latitude heterochroneity is seen among other neognath birds such as presbyornithids and charadriiforms suggesting that Antarctica might be a holding tank for new avian taxa in the Late Cretaceous until they migrated to lower latitudes in the Early Tertiary.

28 152 Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution fossils have been found from the Eocene beds of England and Germany. Since then, loons became a well-marked Holarctic group; their fossils are unknown from the Tertiary sediments of the Southern Hemisphere. The early record of loons in Antarctica and their subsequent dispersal northward fit a pattern of high latitude heterochronei ty (di fferential appearance of taxa between high and mid-to low latiludes), as seen among a wide spectrum of faunas a nd floras. Polarornis would be as much as 25 million years older than the early loon fossils known from the Eocene beds of Germany and England (Fig. 16). Zinsmeister and Feldman (1984) identified eleven genera of invertebrates in the Early Tertiary La Meseta Formation of Seymour Island, that predated their midlatitude descendants by as much as 40 million years. They concluded that the high latitude region of the Southern Hemisphere acted as a " holding tank" for new taxa until they would spread to the lower latitude. The most obvious reason for such a northwa rd dispersal was the onset ofcooler conditions in the polar regions after the development of the psychrosphere in the Cenozoic. Dettmann (1989) observed that Antarctica was the cradle of austral temperate rainforests during the Cretaceous. These plants gradually migrated northwards during the Tertiary. The Antarctic fossil record contradicts the popular view that the tropics serve as the principal center for the origin and dispersal of biota; some groups of plants and animals apparently had polar origins during the Cretaceous and Early Tertiary period. Recent discovery of two other Late Cretaceous birds, a presbyornithid (Noriega and Tambussi, 1995) and a charadrii form (Case and Tambussi, 1999) from the nearby Vega Island indicates that several groups of recent water birds such as the Gaviiformes, Anseriformes, and Chradriiformes made their first appearances in high southern latitudes, inhabiting the Antarctic seacoasts. Although land connections between Antarctica and other continents had broken in the Tertiary, flying neognaths could still cross the developing oceans with the onset ofa cooler climate, and migrate northward. New fossil evidence indicates that Antarctica may have been an important center for the origination of extant bird groups before the K-T (Cretaceous/Tertiary) extinction. These aquatic birds lived at a key time during the last days of dinosaurs in the high latitudes until harsh conditions forced their dispersal northward in the early Tertiary. The Late Cretaceous Antarctic avifauna consists entirely of neognaths and shed new Iight on their high latitude heterochroneity. In contrast, the Cretaceous birds from other Gondwana continents represent several primitive, archaic groups, dominated by Enantiornithes that disappeared at the K-T extinction (Chiappe, 1996a). Recently, Feduccia (1995) concluded that most of the Mesozoic birds such as enantiornithes, hesperornithiforms, and ichthyornithiforms disappeared suddenly at the K-T extinction along with non-avian dinosaurs, leaving no descendants; the radiation of neognaths took pi ace suddenly during the early Tertiary period from this K-T evolutionary bottleneck. However, Feduccia's assertion may not be correct with the discovery of three groups of the Antarctic neognaths. The high latitudes of Antarctica served as refu ge for many neognaths, which transcended the K-T extinction, rebounded, and radiated explosively in the Early Tertiary. More recently, Cooper and Penny (1997), using both mitochondrial and nuclear DNA, concluded that at least 22 lineages of extant birds diverged well before the K-T boundary. Both paleontological and molecular data demonstrate that a diverse group of modern birds crossed the K-T cataclysm unscathed. Acknowledgments I want to thank Bryan J. Small, Michael W. Nickell, and W. J. Zinsmeister for field assistance and logistics at Seymour Island, Antarctica; Bryan J. Small, Zhong Zheng and Kyle McQuilkin for preparation ofthe loon material at different stages; and Bill Mueller for photography. I am indebted to Kyle McQuilkin for his beautiful illustrations and computer graphics. I greatly appreciated the stimulating discussions and suggestions provided by Storrs Olson, Erich Weber, and Walter 1. Bock. The manuscript was reviewed by Kyle McQuilkin, Larry Witmer, and Joel Cracraft whose comments helped improve the paper. I thank Storrs Olson (Smithsonian Institution), John Bolt (Field Museum of Natural History), David W. Steadman (University of Florida), and R. B. Payne (Univer

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