Neornithine 'Big Bang'
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Were you looking for the astronomical Big Bang?
Through the Phylogenetic Looking Glass: Reexamination of The Neornithine 'Big Bang'
“”A rapid, explosive Tertiary radiation best explains why resolving phylogenetic relationships of modern orders remains intractable.
“”Molecular, morphological and fossil data all indicate that the early history of modern birds began in the Cretaceous and did not involve 'transitional shorebirds'.
|—van Tuinen et al., Trends in Ecology and Evolution, 2003|
Solve the ancestry of birds, and you have cracked the many riddles of avian phylogenetics: a pleasant fiction. Outside the derivation of Aves, no topic is debated with greater acrimony in the field of paleornithology than the origin and early evolution of Neornithes. Increasingly, the debate has become polarized—the paleontologists on one side and the molecular systematists on the other, reflecting a fundamental schism in interpretations of avian phylogeny. The matter may be so contentious because it arguably reflects a more alarming discordance between paleontologic data and that obtained by molecular analysis.
Recently, a series of publications have presented new data on this dispute. Hope (2002) has reviewed the Mesozoic record of Neornithes, and Feduccia (2003) has offered new observations on the solution of this seemingly intractable problem. In light of this a critical review of the principal bones of contention, and arguments previously advanced in defense of the bradytelyic model of neornithine evolution is presented, demonstrating the increasingly tenuous status of this model.
- 1 Historical review
- 2 Neornithes across the ages: the fossil record of crown clade Aves
- 3 Stratigraphy at the KT Boundary
- 4 Finding a place for the paleognathous assemblage
- 5 Gondwana — zoogeographic wellspring of neornithes?
- 6 Molecules, bones, and controversy: explaining the difficulty in resolving neornithine phylogeny
- 7 Conclusion: a revised scenario
- 8 Sources
- 9 See also
- 10 References
The classical view of neornithine evolution has been an orderly march through the ages, with a gradual, phyletic progression from the archaic ornithurines to crown clade Aves. In the past quarter century, as part of the vast amount of work contributed to paleornithology, various revisions of this classical hypothesis have been put forward by numerous researchers.
Early pioneer of cladistic analysis on Aves, Joel Cracraft, revived Huxley's 1867 hypothesis that Ratitae is a holophyletic assemblage, which is today relictual and proposed a vicariant biogeographical model to account for the origin and evolution of these birds, in 1973 and 1974. Subsequently, Cracraft expanded on this biogeographical principle and asserted a Cretaceous, Gondwanan origin for most if not all Neornithes, followed by wide-scale survivorship across the KT Boundary. Cracraft's defense of this viewpoint has been unwavering, and his publications to that extent, prolific.
Cracraft's work has been concordant with data gleaned from molecular research, specifically involving the use of strict genetic distance and molecular coalescence dating to determine the age at which the common ancestor for any given taxa lived (e.g., Sibley & Ahlquist 1990, Hedges et al. 1996). Systematists employing such tools have consistently placed the origin of major Neornithine clades at within 120-100 million years ago, and have vigorously asserted that: a) Neornithes sailed through the KT Boundary relatively unscathed, and b) vicariance biogeography has played a significant role in the evolution of the neornithine birds.
Paleornithologists have largely regarded this model as suspect, in that the fossil record of Neornithes is conspicuously contradictory to the gradualistic scenario envisioned by Cracraft and his colleagues. Ironically, molecular biology bolstered the view that the initial adaptive radiation of Neornithes took place in a short period of time, as evidenced by the lack of significant molecular differences among crown clade Aves.
The view that the ancestral stock of Neornithes may well have constituted a morphological bottleneck was presented in Martin's 1983 review of the avian fossil record, and it was reiterated in Olson's magisterial 1985 analysis. In 1995, Alan Feduccia formulated the most explicit argument for an "explosive"; origin and initial adaptive radiation of Neornithes post-KT Boundary, directly facilitated by the catastrophic events, which occurred 65 million years ago. Feduccia argued that but for a solitary lineage of "transitional shorebirds", represented by a group of isolated tarsal and humeral fragments from the Jersey greensands, the Mesozoic fossil record of Neornithes was non-existent. Feduccia went on to implicate this basal charadriiform assemblage as the phylogenetic wellspring from which Neornithes were derived, a model, on which he elaborated further in subsequent publications. Comparisons were made with the explosive phyletic evolution of mammals after the KT Boundary and the "transitional shorebird" morphotype was further compared to the mammalian insectivores.
For its dismissal of much of the cherished record of putative Mesozoic Neornithes, as well as its drastic revision of popular theories accounting for the phylogeny of crown clade Aves, Feduccia's so-called "big bang" model met with significant criticism in the paleontological community.
Neornithes across the ages: the fossil record of crown clade Aves
Central to this debate, are the collected scraps of bone from the Cretaceous which have at one time or another been referred to neornithes, either by default, or via alleged synapomorphies linking these specimens to various orders of extant birds. Avian fossils of any sort dazzle their students, and consequently, the systematic treatment of some has far exceeded their merit. And thus we have a fossil record for Aves cluttered with prolific taxa erected on the slightest of evidence, in some cases nothing more than pieces of bone whose very identity is questionable.
While comprehensive reviews of the avian fossil record executed in the last two decades have largely helped sort out this quagmire, recently some researchers have resurrected the significance of these supposed Cretaceous neornithines, asserting that these specimens substantiate the Lower Cretaceous origin of crown clade orders. In particular, Cooper & Penney (1997) considered these fragmentary fossils to be the linchpin for their calibration of the avian "molecular clock", from which these researchers estimated that no fewer than 21 neornithine taxa emerged from the KT Boundary unscathed!
The fundamental problem in this spate of diagnoses is twofold. The evolution of flight has severely constrained avian morphology within a stringent set of biophysical parameters, and as such, birds are perhaps the most morphologically uniform of all vertebrates. As a consequence the problems, which are legion in using fragmentary material for higher classifications, are compounded when examining the avian fossil record.
A more significant obstacle to the accurate phylogenetic interpretation of such fragmentary material is found in the nature of the fossils at the base of neornithine orders. Mosaicism is widespread amongst the basal-neornithines, as evidenced by such forms as Presbyornis pervetus, Juncitarsus merkeli, Rhynchaeites messelensis, Salmila robusta, and perhaps most strikingly by the osteology of Limnofregata azygosternon. If such basal forms present a mosaic of apomorphic and plesiomorphic characters, it stands to reason that preserved either unarticulated or disassociated, the constituent material of these taxa could be mistakenly attributed to orders to which they do not belong. Indeed, in a discussion of Limnofregata, a basal pelecaniform from the Lower Eocene, Olson noted that:
“”The humerus, radius, ulna, carpometacarpus, coracoid, furcula, scapula, tibiotarsus and probably femur, if found in isolate, either whole or in part, would have stood very little chance of being correctly identified as once having formed part of a frigatebird, and almost any one of them might have been said to constitute a new family… The distal end of the tarsometatarsus might well have been referred to the Sulidae, whereas the proximal end of the tarsometatarsus would have proved impossible to assign to a modern family. The inadvisability of basing higher taxonomic categories of Paleogene birds on fragmentary limb elements, particularly when these bear no close resemblance to modern taxa, thus becomes evident.
|—Olson (1987, p. 31)|
As Feduccia (1996) has observed, this cautionary note applies equally if not more so to the fragmentary Cretaceous fossils alleged to be neornithes. Given this situation, even the presence of apomorphic characters within this material, cannot be regarded as prima facie evidence for affinity with any given neornithine order.
Such difficulties notwithstanding, the osteology of these Cretaceous fossils must be subjected to critical analysis. For a thorough examination of these fossils, refer to Hope (2002). It should be noted that no discussion of the Cretaceous avifauna could be considered complete without reference to the assemblage, which Olson (1985a) subsumed under the title "transitional shorebirds", and which Olson and Parris (1987) later placed within form-family Graculavidae. As the neornithine affinity of these remains cannot be credibly denied, the author shall not offer any detailed review of these specimens in this essay. Instead, the reader is referred to the primary documents, in which a far more satisfactory analysis is to be found.
As it was the first fossil bird from the Cretaceous attributed to neornithes, Lambrecht's Gallornis straeleni is a fitting place to initiate this review of the fossils in question. The Gallornis material is extremely fragmentary, in an overall poor state of preservation, and consists of a fragment of the proximal femur and an indeterminate portion of the humerus. Quite simply, the material is insufficient for a diagnosis.
Kurochkin (1995) referred Horezmavis eocretacea from the Albian Khodzhakul Formation to Gruiformes on the basis of the morphology of the tubercle for M. tibialis cranialis and what he interpreted as evidence indicating the presence of a hypotarsus. First and foremost, the ossified ridges and canals of the hypotarsus (if present), which Kurochkin cited as evidence of neornithine affinity for Horezmavis are in fact plesiomorphic within Neornithes, as they are also present in that peculiar flightless enantiornithine Patagopteryx. In her review of the material, Hope (2002) found no data to support the neornithine status of this material.
The pitiful scraps of bone referred to Paleocursornis corneti and Eurolimnornis corneti by Kessler & Jurcsak (1984, 1986), are wholly insufficient for any phylogenetic diagnosis. Undaunted by this, Kessler & Jurcsak inexplicably implicated these taxa as the earliest known ratites! In his 1995 review of the avian fossil record, Kurochkin balked at referring either taxon to the ratite assemblage, and instead considered Eurolimnornis as Neornithes incertae sedis, arguing that the presence of a tendinal sulcus on the carpometacarpus was evidence of neornithine affinity. However, a tendinal sulcus is also present in Ichthyornis spp., rendering its validity as a neornithine synapomorphy extremely doubtful.
The material, which Brodkorb placed within a new genus, Palintropus, in 1970, presents a more ambiguous situation to the avian systematist. Confusion is legion in the taxonomic history of these fossils. Marsh described fragments of the pectoral girdle of at least two birds as two new species of Cimolopteryx in 1889, and 1892, describing one as C. retusa and leaving the other unnamed. After a long series of revised diagnoses offered by later authorities, Brodkorb placed C. retusa within Apatornis as a new species (1963), and still later, referred Apatornis retusa to an entirely new genus and species, Palintropus retusus, within Cimolopterygidae, apparently considering the specimen to be a charadriiform.
Hope argued that Palintropus is allied with Galliformes, and on the basis of a partial coracoid recovered from the Campanian of Alberta, referred the taxon to Mourer-Chauvire's family Quercymegapodiidae, at the base of the gallinaceous lineage. The loss of the procoracoid and presence of a distinct scar within the sulcus M. supracoracoideus suggested to Hope that Palintropus represents a basal galliform. While the newly referred material does bear a resemblance to Galliformes, both the type and paratype display a prominent pneumatic foramen for N. supracoracoideus, a character lost in even the most basal galliforms. In light of this discrepancy, and the generally plesiomorphic level of organization observed in the Palintropus material, the author concurs with the assessment that this taxon is a basal charadriiform-like bird.
In 1995, Noriega and Tambussi referred poorly preserved postcrania from the Maastrichtian unit of the Lopez de Bertodano Formation on Vega Island, Antarctica, to Presbyornithidae. These researchers subsequently concluded that the anseriform lineage was rooted sometime within the medial Cretaceous, a theme taken to fantastical heights by Stidham (1998b, c, 1999) in the argument that modern ducks and allies were present at the close of the Cretaceous.
The postcranial osteology of Presbyornithidae is strikingly plesiomorphic, and essentially that of a basal charadriiform bird. Indeed, the similarity between the referred postcrania of Graculavidae and Presbyornithidae was noted by Olson & Parris (1987) who listed an extensive catalogue of shared traits in the morphology of the proximal humerus in these taxa including:
- lanceolate scar for M. coracobrachialis cranialis absent,
- lack of excavated pneumotricipital fossa,
- presence of distinct tumescence in proximoventral portion of the pneumotricipital fossa,
- scar for M. scapulohumeralis caudalis and M. scapulohumeralis cranialis distinct,
- attachment for M. latissimus dorsi cranialis characterized by a well defined, raised protuberance dorsal to the median shaft of the humeral ridge, and,
- tuberculum dorsale well defined.
These data convincingly demonstrate the basal postcranial morphology of Presbyornithidae, and confronted with this, one must question the accuracy of using isolated postcrania to deduce the presence of this taxon. As Feduccia (1996, 1999, 2003) has pointed out, there is simply no way that the material referred to Presbyornithidae by Noriega & Tambussi can be considered sufficient to merit this diagnosis.
The misery of Noriega & Tambussi's argument is dramatically underscored in the principal character these researchers marshaled in support of the anseriform affinity of their finds: exaggerated distal excavation of the impression for M. brachialis anticus. This character is in fact charadriiform. Even the neornithine status of the material recovered by Noriega & Tambussi must be questioned, as their assessment was based primarily on what was perceived to be a minor differentiation between the scapular and humeral facets of the coracoid.
There are at least three other collections, which have been referred to Presbyornithidae by Kurochkin (1995) and by Hope (2002). This material includes part of a sternum, and scapulae, all from Lancian beds in Converse County, Wyoming. As in the case of Noriega & Tambussi's alleged "presbyornithid", we find symplesiomorphies and nebulous characters being advanced as diagnostic traits permitting referral to Anseriformes.
Anatalavis rex, from the Hornerstown layer of the Jersey greensands, was referred by Olson & Parris (1987) to Graculavidae and is represented by two distal humeral fragments. Olson (1999) noted similarities between A. rex and material from the Eocene London Clays referred to as Anatalavis oxfordi which he argued established the anseriform identity of the genus. As is the case in Presbyornis, Anatalavis is demonstrably a shorebird/anseriform mosaic and Olson noted extensive similarities between the pelves of the new Anatalavis material and that of Plataleidae and other Ciconiiformes. Olson (1999) concluded that the Anseriformes originated in the very latest Cretaceous or in the early Paleocene, apparently favoring the latter interpretation, from shorebird stock.
One of the more interesting finds referred to Neornithes, is the holotype of Volgavis marina (ZISP-PO 3638), from the latest Maastrichtian of the Volga Basin in Russia, described by Nessov & Jarkov (1989). The specimen consists of a partial mandible. As noted in the initial description, and reiterated by Hope, the referral of Volgavis to Neornithes is merited by the evident fusion of the mandibular symphysis, but exactly where within Neornithes this specimen is allied, remains contentious. Nessov considered Volgavis a basal charadriiform, a conclusion with which Hope appears to have agreed. The resemblance of the specimen to Stercorariidae, which Nessov noted is probably convergently acquired, but it does underscore the charadriiform-like nature of the specimen, and the author concurs with tentatively referring Volgavis to the diffuse group of primitive, "transitional shorebirds".
Although the lion's share of the record for Cretaceous neornithines comes from the greensands of the paleo-Atlantic offshore deposits in New Jersey, the chalk beds deposited by the ancient interior seaway of the western United States have yielded some remarkable avian fossils, including the archaic ornithurines referred to Hesperornithiformes and Ichthyornithiformes. In 1873, a badly damaged synsacrum recovered from the same region in which the type material of Ichthyornis dispar and Ichthyornis victor were recovered, made its way into O. C. Marsh's hands. Persuaded by the stratigraphic proximity and shared presence of biconcavity in the vertebral centra of the caudal series, Marsh described the new specimen (YPM 1451) as a new species, Ichthyornis celer that same year. Yet only months later, Marsh inexplicably reevaluated this material and transferred it to a new genus, Apatornis, where it has rested since.
In his influential classification of birds, Furbinger elevated Apatornis to the familial level and placed Apatornithidae besides Ichthyornis in Ichthyornithiformes. This assessment became a matter of convention, repeated by Brodkorb in 1967, Martin (1983) and Feduccia (1996). Hildegarde Howard had favored an anseriform relationship for Apatornis, and in 1985, Storrs Olson commented that Apatornis was similar to the mélange of "transitional shorebirds". Martin apparently revised his position on the phylogenetic affinities of Apatornis in 1987, in which he argued that the taxon might be closely related to Ambiortus dementjevi, a bird of doubtful neornithine status.
This confusing state of affairs is primarily the result of the quality of the material referred to Apatornis, and the ambiguity concerning the association of the referred elements into a single taxon. Various postcranial elements have been attributed to Apatornis, but as Elzanowski (1995) and Hope (2002) noted, there is no compelling evidence to view Apatornis as anything but a fauna, and not a species. In light of this situation, the taxonomic history and morphology of these important finds is in significant need of revision.
Nevertheless, we can draw limited conclusions as regards the relationships of Apatornis, be it avifaunal or an autonomous species. Apatornis is unquestionably neornithine as evidenced by the wide degree of separation between the scapular and humeral facets of the coracoid and the craniolateral orientation of the humeral facet. Olson argued that Apatornis be subsumed under the title "transitional shorebird". As an aside, the stratigraphic occurrence of this specimen in deposits of Coniacian/Santonian age is interesting in that Apatornis appears to be a sufficiently basal neornithine that it may indeed lie near the base of crown clade Aves.
Famed collector John Bell Hatcher, more known for the recovery of spectacular Triceratops material from the legendary "Ceratops beds" of Converse County, Wyoming, found in those same rocks, the bones of a most remarkable bird, which O. C. Marsh would later refer to a new genus, Cimolopteryx. Marsh first made mention of the taxon in 1889, but did not formally describe it until 1892. The taxonomic history of Cimolopteryx is confused, as is often the case with these ancient birds, but Brodkorb (1963) placed Cimolopteryx rara in Cimolopterygidae besides two new species of his description, C. maxima and C. minima and additional species of Cimolopteryx originally described by Marsh were transferred to genus Palintropus by Brodkorb (1970). Hope included the most recently described species, C. petra from the Lance Creek beds of Converse County in her review.
Cimolopteryx is the most abundant and morphologically diverse of the archaic neornithine species groups, and was a polytypic genus of birds ranging in size from that of a very small, to large gull. Hope asserts that Cimolopterygidae is represented in the medial Campanian of Alberta by an undiagnosed fragment of a proximal coracoid (RTMP 93.116.1), but this specimen is simply too fragmentary to permit confident referral to Cimolopterygidae and in the opinion of the author is best diagnosed as Neornithes incertae sedis.
In his description of Cimolopterygidae, Brodkorb (1963) found a favorable comparison in the postcranial anatomy of Cimolopteryx to the modern charadriiforms Recurvirostridae and Burhinidae. While Brodkorb's assessment may overemphasize the diagnostic features of the material referred to the genus, Cimolopteryx is near to the origin of Charadriiformes, if it is not in fact a member of this order. Cimolopterygidae is differentiated from Charadriiformes principally in that it lacks a ventromedial expansion of the acrocoracoidal clavicular facet, a trait observed in all extant charadriiforms including the basal thick-knees (albeit in reduced form in these birds). In other characters, particularly of the proximal coracoid, Cimolopteryx is very much congruent in osteology with Charadriiformes.
Procellariiformes and Gaviiformes, both basal neornithine clades undoubtedly related to Charadriiformes, have long been the recipients of various putative Cretaceous members. Brodkorb (1963) described a new genus of alleged gaviiform from the late Maastrichtian Lance Creek beds of Wyoming, Lonchodytes, and referred two specimens to this taxon, as the holotypes for the new species L. pterygius (UCMP 53961) and L. estesi (UCMP 53954). The former specimen consists of a distal carpometacarpus and part of the shaft of a tarsometatarsus. Olson & Feduccia (1980a) demonstrated that L. pterygius cannot be considered a gaviiform, and beyond that, cannot be diagnosed any more thoroughly for want of better material. It is forthwith referred to Neornithes incertae sedis.
Hope argued against the gaviiform affinity of L. estesi and instead referred the specimen (a right distal tarsometatarsus) to Procellariiformes on the basis of the following apomorphies: a) distal foramen proximal to the intertrochlear incisure between mt III and mt IV, b) distal foramen expanded, c) trochlear grooves well-defined, d) trochlea of mt II short, and retraced in plantar aspect, e) trochleae of mt III and mt IV nearly coplanar due to expansion of trochlea for mt IV, and lastly, f) trochleae of mt III and IV tightly appressed. While this is an impressive list of traits suggesting a possible relationship between Lonchodytes and Procellariiformes, the viability of this hypothesis is questionable, as all of the characters cited by Hope are homoplastic and appear in a wide range of unrelated diving birds. Considering these difficulties with Hope's diagnosis, the most parsimonious conclusion is to refer Lonchodytes estesi to the "transitional shorebird" assemblage or relegate the specimen to Neornithes incertae sedis.
Aside from Lonchodytes, the only material of substance, which has been referred to Procellariiformes is Tyttostonyx glauconiticus, which Olson & Parris described as a new species from the Main Fossiliferous Layer of the Jersey greensands in 1987, including the specimen within the monotypic family Tyttostonychidae. A single humerus, missing both the tuberculum ventrale and portions of the pectoral crest, is the holotype of Tyttostonyx (NJSM 11341), and it presents a confusing mosaic of seemingly contradictory character states. Olson & Parris noted in their description that it seemed distinct from Charadriiformes in the following traits: a) low, narrow humeral head, b) pectoral crest long, and expanded, c) capital incisure greatly reduced, d) excavation for M. coracobrachialis cranialis reduced, and e) tricipital grooves shallow and indistinctly preserved.
Considering the damage to the humeral head and the pectoral crests, and having viewed the stereo photographs presented in the initial description, I feel the emphasis placed on the morphology of the deltopectoral crest by Olson & Parris unwarranted, and discount it as a diagnostic feature. On the other hand, the humeral head is quite reduced comparative to Charadriiformes. Olson & Parris went on to compare their specimen to Procellariiformes, on the basis of the pectoral crest morphology, but as seen, this is a dubious argument. Further comparisons were made with Procellariiformes in the lack of distinct tricipital grooves. Due to the incipient status of the ectepicondylar spur and the presence of a deep brachial depression, these researchers opined that Tyttostonyx might lie close to the origin of Fregatidae and thus Pelecaniformes, with which Feduccia (1996) later agreed. While intuitively appealing, the author opts for a more conservative interpretation and considers Tyttostonyx, as tantalizing a specimen as it is, best referred to Neornithes incertae sedis.
The final and most doubtful record of Cretaceous procellariiforms comes from the Maastrichtian rocks of the Nemegt Basin in Mongolia, from which Kurochkin (1995a, b) reported, on the basis of a mere clavicular splinter, the presence of Diomedeidae in the terminal Cretaceous! Such a completely unwarranted assessment is inexplicable, as is Hope's opinion that the specimen was "consistent with this determination". Needless to say, there is no evidence for the presence of anything like albatrosses in the Maastrichtian or elsewhere within the Cretaceous.
In recent years a renewed case has been made for the Cretaceous derivation of Gaviiformes, which due to the generally "primitive" appearance of these birds, is perhaps intuitively facile. While Lonchodytes was once considered gaviiform, as discussed above, this has since proven mistaken and at the present, there are two collections upon which the case for the presence of loons in the Cretaceous rests: Neogaeornis wetzeli and Chatterjee's "Polarornis".
Hungarian paleornithologist Kalman Lambrecht described Neogaeornis wetzeli in 1929 and referred it to Gaviiformes on the basis of the tarsometatarsus—the sole element preserved in the holotype. Subsequent examination by Martin & Tate (1976) in their extensive review of the osteology and phylogeny of Hesperornithiformes placed the taxon within Baptornithidae, an opinion reiterated by Martin in his 1983 review of the avian fossil record. Martin & Tate argued that the compression of the tarsometatarsus and the greater distal expansion of the outer trochlea suggested the hesperornithid affinity of Neogaeornis, a conclusion with which Feduccia concurred (1996, 1999).
Olson (1992) restudied the type of Neogaeornis and identified the presence of two proximal metatarsal foramina, and the remnants of ossified hypotarsal ridges as definitive evidence of the neornithine status of this bird. While Martin continues to assert that Neogaeornis is a hesperornithiform, as does his colleague Alan Feduccia, the structures reported by Olson make it clear that Neogaeornis is at least referable to Neornithes incertae sedis. Less certain is the conclusion, made by Olson on the basis of the structure of the trochleae, that Neogaeornis is a primitive gaviiform. Given the fragmentary nature of the specimen and the difficulty in discerning genuine synapomorphies from homoplastic traits in the tarsometatarsus of diving forms, it seems best to consider Neogaeornis a basal neornithine of uncertain classification.
More dubious still is the fragmentary material from the Lopez de Bertodano Formation of Seymour Island, Antarctica, which Sankar Chatterjee of Texas Tech University has casually labeled "Polarornis" and yet never formally described. As is the case with his controversial "Protoavis texensis", the paleornithological community is presented with a set of precise line drawings and reconstructions, while photographic presentation of the fossils falls by the wayside.
While Chatterjee's specimen does resemble modern gaviiforms in some features, particularly Gavia stellata, without formal description, conclusions are difficult to make. The edentulous jaws of "Polarornis" would suggest that we are looking at a neornithine, but as Martin (1998) has pointed out, the possibility that the "Polarornis" material represents a toothless hesperornithiform-like bird, or another heretofore unknown taxon of archaic ornithurine, cannot be dismissed. A further problem inherent in Chatterjee's assessment is that the strata of the Lopez de Bertodano Formation are of uncertain age in some locales, and indeed do extend through the KT Boundary, raising the question of the proper age of his "Polarornis" specimen.
Among the basal neornithine assemblage, Pelecaniformes is perhaps the most spectacular of orders. Brodkorb (1963a) referred a damaged distal humerus (UCMP 53958) to Phoenicopteri (=Phoenicopteriformes), describing it as a new genus and species of basal flamingo, Torotix clemensi. Subsequent review by Olson & Feduccia (1980) and Olson (1985) demonstrated that Torotix was almost certainly not a flamingo, as evidenced by the structure of the brachial depression, and alternatively suggested that Brodkorb's specimen was a basal charadriiform-like bird. Hope referred Torotix to Pelecaniformes on the basis of the following apomorphies: a) brachial depression bladelike in ventral aspect, b) attachment for the ventral collateral ligament of humerus reduced, c) flexor process blunt, broad and shorter than ventral condyle, d) attachments for Mm. Flexor carpi ulnaris and pronator profundus ventrally oriented, e) ventral epicondyle extended ventrally slightly, and f) brachial depression of humerus deeply excavated and dorsally expanded to the cranial wall of the humerus, and ventrally to the condyles.
As is so often the case, we seen in the Torotix type a confusing mixture of traits, masking the phylogenetic affinity of this specimen, a problem compounded by the state of preservation in UCMP 53958. In characters b-e, Torotix is not exclusively allied with Pelecaniformes, and thus the utility of advancing these characters as a link with this order, is questionable. Hope's argument primarily rests on the morphology of brachial depression, but the specimen is simply too poorly preserved to make definitive conclusions in this regard. In the deep excavation of the brachial depression, with dorsally and ventrally expanded margins, Torotix disagrees with Pelecaniformes and is closer to charadriiforms or procellariiforms, but again, the material is too badly damaged to make definitive assessments. The author feels that this interesting specimen must unfortunately be referred to Neornithes incertae sedis, until such time as more complete material comes to light.
An equally enigmatic specimen assigned to Pelecaniformes, and specifically to Phalacrocoracidae, is AMNH 25272, a right femur recovered from the Maastrichtian Lance Creek beds of Converse County, Wyoming. Hope referred AMNH 25272 to Phalacrocoracidae due to the presence of prominent striations for the attachment of Mm. Femorotibiales, an autapomorphic character of cormorants. However, the specimen displays an angularly truncated trochanteric crest, which is incongruous with Phalacrocoracidae, or at least extant Phalacrocoracidae. Thus, AMNH 25272 presents a maddening contradiction of character states to the avian paleontologist.
Considering that rugose scars for the attachment of femoral musculature would presumably be homoplastic traits in diving birds, the author is reluctant to place too much emphasis on the Mm. Femorotibiales striations observed in AMNH 25272, and instead refers this specimen to Neornithes incertae sedis. More complete and diagnostic material is required before we can with confidence say that cormorants were diving in the epicontinental seaways of the Cretaceous.
Stidham reported what must be certainly the most farcical specimen attributed to an extant order in 1998 with the referral of exceptionally fragmentary jaw elements (UCMP 143274) recovered from the Lancian beds of Converse County to Psittaciformes. The specimen is poorly preserved, and moreover, bears little if any resemblance to the mandibular elements of the most basal psittaciform birds, e.g., Psittacopes. It is difficult to reconcile this morphological disparity with Stidham's claim that UCMP 143274 is a basal parrot, and given the presence of morphologically well defined parrots from the Eocene clearly basal to the order Psittaciformes, such minimal elements are not satisfactory for pushing back the origin of this taxon by some 17 million years.
The only taxa which can confidently be referred to Neornithes from the Cretaceous/Paleocene boundary are all of charadriiform affinity, either at the base of this order or referable thereto, as basal forms within this assemblage. Graculavidae, Cimolopterygidae, Volgavis and Palintropus and probably Lonchodytes are collectively representative of this larger grouping. Following Olson (1985) and Olson & Paris (1987) these fossils can be grouped on the basis of similarity in form, and do not present them as "transitional shorebirds" in a strict phylogenetic sense. We simply do not have the material needed to resolve the ingroup-phylogeny of these birds, and though they are most likely distantly related and referable to several distinct taxa, such reexamination must await the recovery of more complete material.
In the tantalizing material referred herein to Neornithes incertae sedis, we have remnants of the wellspring from which crown clade Aves is derived, sufficiently ravaged by the vagaries of geology to be beyond diagnosis at this time. They underscore in the morphological and ecological uniformity they display, the presence of a bottleneck through which the derived ornithurines passed, and today's birds descended from.
In spite of this evidence, proponents of the deep-Cretaceous origin of Neornithes continue to insist that multiple extant orders had already experienced widespread adaptive radiations prior to the KT Boundary. Cracraft (1974) asserted that the paleognathous assemblage had originated no later than the late Cretaceous, a view he has consistently supported. Hope argued that the paleognathous/neognathous dichotomy had occurred no later than the Coniacian/Santonian interval. Yet there are no fossils, which can confidently be referred to the paleognathous assemblage from the Cretaceous, and certainly there are no "ratite" remains from any known Cretaceous formations. Considering the favorable preservational bias the large, heavy bones of such birds enjoy, their utter lack from the Cretaceous record is astoundingly incongruous with the theory that they originated within this time frame and represents a failed prediction of the austral/Cretaceous origin hypothesis.
In acknowledgement of this problem, Hope argues that as Apatornis celer is referable to the neognathous lineage, it therefore follows that Paleognathae must have diverged some time before the appearance of this taxon. This argument is severely flawed in that it assumes not only the holophyly of Paleognathae but also the accurate referral of Apatornis to Neognathae. In reality neither assumption is warranted.
Given our knowledge of the extant paleognaths it seems likely that all of Paleognathae may be referred to Neognathae (Harlid & Arnason 1999), and indeed the paleognathous palate is neotenic in Aves. If today's paleognaths and the paraphyletic taxon from which they are independently derived, Lithornithiformes, are in fact secondarily paleognathous, as the evidence would indicate, then Hope's argument is baseless. Furthermore, the material attributed to Apatornis is wholly insufficient for any diagnosis beyond the level of Neornithes incertae sedis and thus speculation on the palatal configuration of this bird, out of the question.
In a similar vein, the assertions that modern Anseriformes were coeval with the archaic ornithurines and charadriiform-like birds of the Cretaceous, which Stidham (1998, 1999) has argued so vociferously for, and Hope has so unwaveringly reiterated, is nothing short of astonishing. This argument is entirely dependent on the alleged presbyornithid affinity of plesiomorphic postcrania, from which it is assumed that if Presbyornithidae were present in the Cretaceous, Anatidae, the extant sister clade to the presbyornithids, must also have been present. And yet the scraps of bone upon which this bold assessment is contingent are far too meager to be accurately referred to Presbyornithidae, to say the least of referral to Anseriformes as a whole.
The fantasia on a theme continues in the conclusion reached by Stidham (1998) and supported by Hope in her recent review that Psittaciformes was not only present by the Campanian/Maastrichtian interval, but had witnessed a robust adaptive radiation prior to the KT Boundary. Again we find a drastic revision of the supported phylogeny of these birds based on a mere fragment of bone, in this case, one of the more useless fragments ever to clutter the literature.
Similar problems abound with the diagnosis offered by Kurochkin (1995) and Hope in their review of the procellariiform record. While it is possible that Procellariiformes may have been present in basal form during the latest Maastrichtian, the current evidence is not sufficient to support this conclusion. Equally doubtful is the presence of pelecaniform birds during this period, for which the only real evidence is the un-described femoral fragment AMNH 25272, discussed earlier.
Gaviiformes, the Cretaceous origin of which Olson (1992a), Chatterjee (1989, 1997) and Hope have argued for, must also be viewed as a taxon which arose only after the KT-Boundary, especially considering the presence of Colymboides anglicus in the Lower Eocene London Clay (Feduccia 1996). The only well preserved material, which might be referable to Gaviiformes, Chatterjee's as yet undiagnosed "Polarornis" from Seymour Island, does not provide particularly overwhelming evidence for the Cretaceous origin of these birds. And as detailed earlier, the tarsometatarsus of Neogaeornis wetzeli, is too fragmentary a specimen to permit accurate referral to Gaviiformes.
We are, in the end, left precisely where we started — the basal charadriiform-like birds of the Jersey greensands and the latest Cretaceous deposits of the interior seaway that once split North America in two. They are the foremost candidates for the wellspring from which the neornithine radiation is derived. In these morphologically and ecologically restricted birds, we see the indelible mark of the ancestry of extant Aves.
Stratigraphy at the KT Boundary
It has been proposed that the medial Cretaceous origin of several basal neornithine orders was postulated, with a concomitant albeit limited adaptive radiation of these orders. This model, though rejected herein, does present an issue, which needs to be discussed in that it was based largely on the observation that at least some charadriiform-like birds were already extant in the Campanian/Maastrichtian interval. Based on this seemingly incongruous data, it would seem that Feduccia's "big bang" model is invalid (see avian phylogeny and origins).
Olson (1994) and Feduccia (1996, 1999) argued that Graculavidae and allied taxa are strictly Paleocene in age, while Chiappe (1995) felt this material was entirely Cretaceous. The difficulties arise largely from the ambiguous stratigraphy of the Jersey greensands from which the majority of the Mesozoic neornithines have been recovered.
The basal Navesink Formation is clearly of Maastrichtian age, and the overlying Hornerstown of Paleocene. However the demarcation between the two is nebulous, as the 0.3 meter thick Main Fossiliferous Layer of the greensands, presents a confusing mélange of fossils referable to both the Cretaceous and the Paleocene. Compounding these problems is the regrettable fact that the exact locality data for the various specimens recovered from these deposits have largely been lost to history.
In light of this situation, the most defensible conclusion must be that the age of the greensand fossils is indeterminate. We can only confidently date those specimens, which have been unquestionably traced to either the Navesink or Hornerstown formations, others we can only regard as either latest Cretaceous or early Paleocene. Assertions to the contrary must be viewed with skepticism.
The interesting assemblage of neornithine material from the chalks of the western United States, however, is undoubtedly of late Cretaceous age. Considering that we have a diverse collection of "transitional shorebirds" from these deposits, including the polytypic Cimolopteryx and a new species of Graculavus (Hope 1999), we seem to thus have support for Chiappe's statement that crown clade Aves were extant prior to the KT Boundary. At first examination, this stratigraphic picture apparently contradicts a "big bang" for Neornithes, as formerly concluded by the author.
However, as Feduccia has demonstrated, the terminal Cretaceous occurrence of basal, charadriiform-like birds is hardly the same as the assertion that multiple neornithine orders had already radiated prior to the KT Boundary. If anything, the occurrence of these "transitional shorebirds" near the KT turnover indicates that in the late Cretaceous we are witnessing the "phylogenetic fuse" from which the explosive diversification of Neornithes resulted.
We may postulate that the decline of the Hesperornithiformes and Ichthyornithiformes, sympatric with the basal neornithines, was the principal catalyst for the limited terminal Cretaceous radiation of these birds. However, it was only upon the extinction of the dominant component of the Mesozoic aviary—Enantiornithes—that the wide-scale diversification of Neornithes occurred.
Finding a place for the paleognathous assemblage
No single problem of neornithine phylogeny, outside the origin of this assemblage, has been as acrimoniously debated as the proper place of the paleognathous birds. Despite a century and a half of work, a solution is not readily apparent, though there is mounting evidence to support paleognath holophyly. Advocates of a deep-Cretaceous origin of Neornithes generally make several conclusions concerning the paleognathous assemblage, as witnessed by Cracraft's 1974 work discussed earlier. According to this school of thought, ratites and other paleognathous birds represent a holophyletic taxon and are of ancient, Gondwanan origin.
The origin and phylogeny of the paleognathous assemblage is of crucial importance to our understanding of the general phylogeny of crown clade Aves. Houde & Olson (1981) and Houde (1988) shed considerable light on this maddening dilemma with the description of a group of amazing, volant paleognaths from the Paleocene and Eocene of North America and Europe, which Houde (1988) consolidated within a new order, Lithornithiformes.
Lithornithiformes exhibit a remarkably plesiomorphic level of postcranial organization, which at least superficially, more closely resembles that of neognaths than it does other members of the paleognathous assemblage. The weight which should be ascribed to this fact is not easy to determine. These similarities may simply be a function of the volant faculties of these Paleogene birds, instead of an actual phylogenetic affinity. The skull is similarly plesiomorphic in its level of organization, retaining a reptilian splenial, distinct sutures between elements, and a frontoparietal joint.
Houde (1988) convincingly argued that Lithornithiformes is a paraphyletic grade of basal neornithines, perhaps near to the origin of Neornithes, and considered the lithornithids and modern tinamous closely related. Houde argued that the ratites are derived from lithornithid ancestors, though he did note that an alternative phylogeny in which the various ratite birds descended from different lithornithids was tenable (1988).
The lithornithid affinity of Tinamiformes remains questionable. Tinamous and galliforms are possibly allied on the basis of their unique sternal morphology, in which the caudal margin of the sternum is deeply incised. In contrast, the sternum is lithornithids is truncated in caudal aspect, and lacks lateral incisions. Hope (2002) suggested that far lateral extension of the coracoidal tubercle of the scapula shared by both Tinamiformes]] and Galliformes is compelling evidence for a tinamiform/galliform nexus. However, the lateral extension of the coracoidal tubercle is apparently a symplesiomorphic trait within Neornithes, as evidenced by its presence in such basal charadriiform-like birds as Palintropus, and thus it may not be synapomorphic character linking tinamous with Galliformes.
Feduccia (1996, 1999) concurred with the relationship of Tinamiformes and Lithornithiformes, and has suggested that the apparent similarities between the tinamiform and galliform sterna are of independent acquisition. The author considers an alternative scenario in which the similarities between tinamous and lithornithids are relegated to the morphological uniformity imposed upon birds by the loss of flight, as at least equally valid.
The heart of this matter, however, is what, if any, phylogenetic significance ought to be attributed to the paleognathous palate. Classically considered a synapomorphic character, the paleognathous palate is in fact neotenic in birds, as established by de Beer's magisterial 1956 study and further confirmed by Jollie (1958). Wetmore (1951, 1960) argued that Paleognathae; is paraphyletic, and without basis in phylogenetic reality, suggesting that it be subsumed within Neognathae. Houde (1988) suggested this is a possible alternative phylogenetic inference to that presented in his monographic treatment of the Lithornithiformes, though he argued against it. Feduccia (1996) concurred with this alternative assessment. There simply is no compelling evidence to indicate that the paleognathous palate represents a synapomorphic character with which to unite those taxa that display such a configuration.
Excluding the paleognathous palate, as most researchers appear to agree is necessary, what evidence is there to argue for the monophyly of Paleognathae? Various data have been invoked in support of this assessment. In 1984 and 1985, McGowan argued that, on the basis of tarsal homologies, paleognaths could not possibly be derived from neognaths. Bledsoe (1988) refined Cracraft's 1974 phylogeny of ratites and reached similar conclusions. Sibley & Ahlquist (1990) argued for the unity of Ratitae, as did Ligon (1993) and Starck (1995). In 2001, Cracraft & Clarke unveiled a cladogram asserting that Paleognathae is united by five "unambiguous" synapomorphies. Hope (2002) concurred with these arguments. Most recently, in their extensive analysis of neornithine phylogeny, Mayr & Clarke (2003) defended the holophyly of Paleognathae and Ratitae (defined in this study as Rheidae + Apterygidae).
Time and again one sees characters of doubtful validity or those, which are demonstrably plesiomorphic advanced in defense of a holophyletic paleognathous assemblage. McGowan (1984, 1985) argued that the retention of an ascending process of the astragalus in ratites is irreconcilable with the pre-tibial ossification of the tarsus observed in other carinates, and thusly concluded that ratites cannot possibly be secondarily paleognathous. However, McGowan's work is seriously called into question by multiple lines of evidence. As Martin & Stewart (1985) noted, McGowan's use of ratite embryos near to hatching and his failure to fully section those embryos studied, renders his conclusions about tarsal homologies doubtful. Embryos of Struthio camelus, still in an early stage of ontogeny, clearly display a pre-tibial, lateral ossification and not an ascending astragalar process as supposed by McGowan. And lastly, even McGowan's comparison of the ratite condition with that observed in theropods is fallacious in that theropods also display a pre-tibial ossification, which appears to have fused to the astragalus later in ontogeny (Welles 1984).
It can thus be concluded that there is nothing in the tarsal morphology of paleognaths, which precludes their secondary derivation from neognathous ancestors. Further evidence marshaled in support of the unity of Paleognathae; includes the pattern formed by the rhamphothecal grooves (Parkes & Clark 1966), which Houde (1988) demonstrated to in fact be symplesiomorphic.
Most recently, Cracraft & Clarke (2001) have presented the following characters as synapomorphic of Paleognathae and as usual, argued for the holophyly thereof: a) forked caudal dentary, b) alaparasphenoid inflated, c) braincase caudal to quadrate heavily pneumatized, d) caudal maxillary sinus absent, and e) exclusive male parental care. This follows a familiar trend in systematic ornithology, in which the search for a holophyletic Paleognathae and or Ratitae has become a sacred goal. With cladistic and molecular data marshaled in defense of this hypothesis, it has become the attitude of many in the ornithological community that the debate is settled and the unity of Paleognathae established beyond question, with Cracraft & Clarke's analysis the current working definition of the clade. Yet Cracraft's cladistic analysis, like many of his prior studies (most notably his dubious 1982 phylogeny of diving birds), is not so definitive as has been maintained.
Cracraft and Clarke's first character, the bifurcation of the dentary in caudal aspect is of questionable validity. First and foremost, its presence in the lithornithids is ambiguous, and secondly, that it is observed in a host of archaic birds from the Cretaceous not united by this trait, seems to argue that it has been convergently acquired throughout early avian history. The inflated alaparasphenoid is a potentially valid synapomorphy of Paleognathae. Character (c) is not precisely delineated, and the author refrains from decisive comment on it until such time as Cracraft and his colleagues more stringently define this trait. Character (d) is readily explicable within a neotenic context, as it represents a reversal to a primitive state, a process we know occurs during neoteny and has likewise occurred in all paleognaths, whether you feel the assemblage holophyletic or paraphyletic. Last and perhaps most interesting, is character (e), which is based on Ligon's (1993) work. However, the presence or absence of exclusive male parental care is not applicable to the extinct lithornithids, and thus it cannot be used to delineate Paleognathae itself, and is of use only in attempting to validate the holophyly of crown group paleognaths. We are thus left with at most two, and realistically one character (the inflated alaparasphenoid) with which to define Paleognathae as a clade. Considering the pervasive evidence of wide-scale neoteny in the derivation of all paleognaths, reliance on such minor characters as remain, which simultaneously are explicable within a paedomorphic context, cannot merit the sweeping phylogenetic conclusions Cracraft & Clarke offer. Quite simply the conclusion reached by some cladists that we have solved the problem of paleognath phylogeny, is unwarranted. With the current evidence available, it cannot be determined with certainty if Paleognathae is holophyletic or polyphyletic. The author feels that the predonderance of evidence lies with the polyphyletic interpretation, not the least of the reasons being that tinamous are apparently more closely related to Galliformes than to any other paleognathous taxa, but additional work and the discovery of new fossil paleognaths may prove otherwise.
The phylogeny offered by Mayr & Clarke (2003) united Paleognathae on the basis of the following ostensible synapomorphies, a) upper beak with furrow rostral to the naris, b) mesethmoid extends rostrally beyond naso-frontal hinge, c) mandible displays grooves on ventral symphyseal surface, d) dorsal surface of mandibular symphysis essentially flat, e) sternum possesses 3-4 costal processes, and f) hallux reduced. It is interesting to note the vastly disparate characters advanced to defend the holophyly of Paleognathae, and as in the case of Cracraft & Clarke (2001), there are significant problems with Mayr & Clarke's (2003) phylogeny. Character (e) is a symplesiomorphy, and reduction of the sternum and its costal processes is a known correlate of neoteny. Character (f) is convergently acquired in all flightless birds, and thus of no use in determining the evolutionary relationships of the paleognaths. Character (a) is fairly trivial and may be explicable within the context of neoteny. Characters (b), (c) and (d) are the only traits listed by Mayr & Clarke (2003) which are potential synapomorphies of the paleognathous assemblage, and thus as is the case with other analyses, we are left with a hypothesis far less definitive than its adherents would maintain.
There is compelling molecular evidence gained from analysis of nuclear DNA sequences for the holophyly of Paleognathae, which has been reviewed in Sibley & Ahlquist (1990) and van Tuinen et al. (2000). These conflicting data argued that though holophyly of Paleognathae is not yet beyond question, further data will most likely continue to bolster the reality of this group.
Resolving the relationships of the paleognathous birds to each other is still problematic, however. Lithornithiformes and Tinamiformes, if related, are clearly near to the base of the neornithine radiation, and can both be considered basal, secondarily paleognathous taxa (Feduccia 1996). The author places both in a position at the base of Neornithes, and considers the relationship between the two orders contentious. Lithornithiformes and Tinamiformes may be sister taxa, as suggested by Houde (1988), or, tinamous may be closely allied to Galliformes and only superficially similar to Lithornithiformes. Given various characters linking the former taxa (e.g., the pectoral girdle). Consensus has been reached that crown paleognaths, the Ratitae are holophyletic, allied by molecular and morphological traits (Sibley & Ahlquist 1990).
Aside from the phylogeny of Paleognathae and Ratitae, there remains another question of fundamental importance — how many times did the ratites lose flight? Cracraft, as reviewed, argued that Ratitae originated in Gondwana, with today's ratites the descendants of unwittingly vicariant passengers on the drifting continents. An attendant corollary of this hypothesis is that ratites lost flight only once. There are several principles behind this assertion that require critical examination. The fundamental assumption underwriting this vicariant model is that the present distribution of avian taxa is a reliable indicator of the historical distribution thereof. Predictions of this model, as advanced by Cracraft and his colleagues include the austral distribution of ratites in the past and a consistent similarity between present day distribution and historical distribution in any given avian taxon.
These assertions can be tested against the available evidence. Unfortunately, the assumption that present day distribution of birds mirrors past distribution utterly fails to take into account the population biology of birds, which is incredibly plastic. The migratory capacity of birds and their intrinsic motility render any reliance on modern patterns of distribution to infer historical distribution highly suspect, a point time and again confirmed by the fossil record of birds (see next section). One can then go on to examine where the earliest ratite fossils have been recovered. Herein we find an explicit prediction of the vicariant model contradicted by the available evidence. The earliest known unequivocal ratite, Palaeotis weigelti, is from the Eocene Messel oil shales of Germany, and thus can scarcely be considered coincident with the vicariant model espoused by Cracraft and his colleagues. Historically considered to be part of Otididae, Houde & Haubold (1987) demonstrated that Palaeotis is a flightless paleognath closely allied with Lithornis and went on to suggest that Palaeotis was closer to ostriches than any other ratites. Peters (1992b), Storch (1993) and Feduccia (1996) all concurred with the paleognath status of this bird, and the former two authors argued that Palaeotis was most closely allied with Rhea, as opposed to Struthio.
The example of Palaeotis illustrates several critical points. Houde & Haubold (1987) offered compelling evidence allying this bird with ostriches and Houde (1988) hypothesized that Palaeotis and thus ostriches were derived from within the Lithornis grade of lithornithids. The earliest known modern ostrich, Struthio orlovi (Kurochkin & Lungo 1970) is from Miocene deposits in the Moldavian region, again, European in distribution as opposed to austral. These data are in stark contrast with the hypothesized paleodistribution of ostriches under the vicariant biogeographical model, and offer strong evidence that ostriches arose in Laurasia and lost flight independently of other ratites.
Objections that Palaeotis is either too fragmentary to be diagnosed or is not a paleognath are entirely unfounded. Alvarenga (1983) and Martin (1992) argued that Diogenornis fragilis, from the Paleocene of South America, is allied with Rhea and thus represents the oldest known ratite bird, with which Feduccia (1996) concurred. There are difficulties in assigning the fragmentary material associated with Diogenornis confidently to Ratitae, and as Houde (1988) has noted, the temporal occurrence of this form hardly constitutes evidence for the austral origin hypothesis. As noted above, even if Diogenornis is allied with Rhea, there is still abundant evidence to suggest that ostriches at least, lost flight independent of rheas and thus the vicariant model is rendered falsified.
A further problem requiring attention is the conspicuous lack of any ratite fossils from the Cretaceous of Gondwana (Feduccia 1996, 2003). This objection, heavily stressed by some authors (e.g., Feduccia 2003) has been derided as "negative evidence" and philosophically bankrupt. Let us examine this assertion however. In part, this point does represent negative evidence but at the same time a Cretaceous fossil record for Ratitae is an expected prediction of the vicariant model, and the lack of any fossil material to meet that prediction is thus a telling blow to the vicariant model. Various special pleading has been invoked by numerous authors to the effect that the fossil record is biased, but we have an excellent fossil record of birds from the Cretaceous, and ratite bones, being heavy and abundantly preserved in the fossil record at large, should be present, were ratites extant in the Cretaceous. Consider an analogous situation. One could argue that the absence of coelurosaurian dinosaurs from the Permian scarcely constitutes evidence that coelurosaurs were not extant at the time because this would be negative evidence, and from there go on to insist that Coelurosauria was indeed present in the Permian!
In isolate none of these data are sufficient to cast much doubt on the vicariant model, but combined, they shift the preponderance of evidence from the vicariant model and its singular loss of flight to an alternative explanation in which flight has been lost multiple times among ratites. Objections that this is an unparsimonious, ad hoc explanation invoking special pleading are utterly unfounded. It both fails to take into account Occam's elaboration of the meaning or parsimony — that entities should not be duplicated without cause — and fails to note that with the preponderance of evidence against the vicariant model, the more parsimonious conclusion is that flight has been lost multiple times in ratites.
Objections are to be expected. Some researchers have attempted to conflate the issue of ratite holophyly and the number of times flight has been lost in ratites, effectively arguing that if one wishes to maintain that Ratitae is holophyletic, one must argue that flight as been lost only once in ratites. This argument is logically fallacious as it equates two distinct issues upon which neither is in isolate contingent. Ratites may be holophyletic and have lost flight more than once. It has been further argued that only negative evidence can be marshalled against the vicariant model, but as reviewed above, this viewpoint is so wanting of realistic grounding that it can be effectively dismissed without further consideration. In actuality there are at least two examples of phylogenetic analyses of extant and extinct ratite birds which have added support for the argument that these birds have lost flight at least twice, if not more times. The already reviewed case of Palaeotis and the earliest known ostriches is one such example, while another fascinating piece of evidence comes from molecular analyses of mitochondrial RNA in extant and recently extinct ratites. Cooper et al. (1991, 1992) sequenced approximately 400 base pairs of mtRNA from a kiwi (Apteryx), four species of moa (Dinornithidae) and seven other extant ratites plus a tinamou and concluded from the resultant data that kiwis were more closely related to Rhea and Struthio than to Dinornithidae. Cooper and his colleagues argued upon the basis of these data that New Zealand was colonized twice by paleognathous birds, which confirms Houde's (1988) assessment of the ratite fauna of this amazing island. A later analysis of extinct Aepyornis ("elephant bird") DNA of Madagascar showed that they were kiwis' closest relative. The analysis also argued strongly for both flightlessness and gigantism evolving multiple times among the ratites.
Another major effort to bolster the vicariant model has been the use of molecular coalescence dating derived divergence times for the ratite lineage, but as reviewed below, there are serious problems with such data.
In light of this information, the most reasonable and by far the most parsimonious conclusion, is that ratites have lost flight more than once.
Gondwana — zoogeographic wellspring of neornithes?
Cracraft (e.g., 2001a) has placed great emphasis on the austral origin of most if not all neornithine orders. As discussed above, this conclusion is clearly baseless as regards the paleognathous assemblage. The question thus remains, is it valid for other neornithine taxa?
As is the case with most of Cracraft's work, we find this hypothesis wanting of any substantiation. The former distribution of neornithine orders repeatedly corroborates the conclusion reached by Feduccia (1996, 1999) that the migratory and volant capabilities of birds renders any phylogenetic inferences based on contemporary distribution, highly suspect. Various neornithine taxa, from Coraciiformes such as the todies, to Trogoniformes and Psittaciformes, are consistently represented in the fossil record by species with zoogeographic distribution wholly inconsistent with the distribution of their extant relatives.
For instance, todies, long regarded as exemplar of an endemic West Indian avifauna, are in fact merely relictual: in the medial Tertiary these birds were widespread throughout North America, as evidenced by the discovery of Palaeotodus emryi from the Oligocene of Wyoming. More spectacular confirmation of this distributional discrepancy comes from the Eocene Phosphorites du Quercy, from which Cecile Mourer-Chauvire described two new species of Palaeotodus in 1985.
Further evidence, albeit negative, is found in the composition of avifaunal remains from strata within the range of the former Gondwanaland. We have diverse avian assemblages from the South American Lecho Formation, for instance, and yet none of the birds recovered from this locality are attributable to Neornithes. The only South American specimen referable to Neornithes is Neogaeornis wetzeli, but this enigmatic find is likely near to the base of Neornithes, and hardly constitutes evidence for the deep-Cretaceous origin and diversification of extant orders. A similar story is found in the faunal composition of avian remains from Africa, Australia, and elsewhere within the southern hemisphere.
If the fossil data indicates a post-Cretaceous origin and Palearctic distribution of neornithine orders supposedly derived from southerly ancestors, then the austral hypothesis favored by Cracraft and some molecular systematists (e.g., Ericson et al. 2001) is rendered indefensible.
Molecules, bones, and controversy: explaining the difficulty in resolving neornithine phylogeny
The intractable mire, in which the neornithine systematist finds himself, has been the norm in this field of ornithological study for at least a century, and it has long been lamented. Famed contributor to the Modern Synthesis, George Gaylord Simpson, commented on the astounding morphological unity of Tertiary birds, in 1946, as did the former dean of American Ornithology, Alexander Wetmore. To these researchers, such uniformity was explained only in part by the restrictions imposed upon the avian bauplan by the biophysical demands of flight. Streseman found much the same in his own work, and yet he was unable to envisage why it should be so absurdly difficult to trace the ordinal phylogenies of crown clade Aves. The Berkeley biochemist Allan Wilson noted that there was an amazingly small degree of molecular distance between extant birds, and commented often as to his inability to understand why this should be so. Indeed, in her review of the Mesozoic neornithines, Hope (2002) notes:
“”It is as though some aspect of the evolution of neornithine birds is distorting analyses of relationships, but only at the most basal levels.
|—Hope (2002, 341)|
We see other curious patterns in the history of Neornithes. Mosaicism is rampant at the base of multiple neornithine orders, and at the base of Neornithes itself. Such mosaicism, represented by taxa like Presbyornis and Limnofregata (discussed earlier) is only in part due to the general trend of evolutionary change to be differential. Equally important, if not more so, in explaining this mosaicism is the rapid divergence of the basal neornithine orders from their respective phyletic nodes. It is a simple observation that if divergence of derivative taxa from their parental stock is occurring over a short period of time, the daughter taxa and their antecedents will remain little changed. In other words, multiple traits of the ancestral taxa from which this progeny has descended, will be retained therein, and via this mechanism, we arrive at precisely the sort of mosaicism seen at the base of Neornithes.
The only explanation advanced thus far which can accommodate the lack of significant molecular distance between crown clade ornithurines and the widespread mosaicism and morphological uniformity of extant birds, is an extremely rapid primary adaptive radiation in the wake of the KT extinction event. With the basal orders of Neornithes diverging from their respective phyletic nodes in such a short period of time, the pattern of character distribution and molecular distance currently observed is precisely what is expected, and this may well be the most profound of observations supporting the "big bang" model.
The natural question, then, is just what can molecular data, so eagerly advanced as the phylogenetic panacea for Aves (see especially the glowing review offered in Proctor & Lynch 1993), actually do to resolve the evolutionary relationships of extant birds? Perhaps the single most pervasive problem is the fact that there is absolutely no evidence for the constancy of molecular evolution over time, and yet this is the basis upon which molecular coalescence rests. In the wake of mass-extinction and wide-scale environmental peturbation, followed by explosive adaptive radiation, it is almost inconceivable that the molecular clock would remain at a consistent pace as that observed prior to and after the extinction event (Benton 1999). Further degrading the confidence in the molecular clock studies is the demonstrable variation in the rate at which this clock runs in addition to a host of other factors influencing rate (reviewed in Benton 1999). The molecular clock-calculators have consistently pleaded that the paleontological data must be flawed as the molecular clock simply cannot be wrong. Various arguments have been presented, from the deplorable deficiencies of the fossil record, to the general superiority of molecular evidence because it is somehow "cleaner." In reality, extensive review of all these objections (Benton 1999) has not found one iota of data to support these objections. xxx
Conclusion: a revised scenario
In light of the evaluation offered within this essay, the author submits a revised scenario to that previously advanced in avian phylogeny and origins.
Originating sometime in the latest Cretaceous, within the Campanian/Maastrichtian interval, the basal neornithines, perhaps derived from forms similar to Apatornis celer and represented by the graculavids and other "transitional shorebirds," experienced a very modest adaptive radiation within a severely restricted niche, namely, the near-shore environment. The decline of the Hesperornithiformes and Ichthyornithiformes during the later Cretaceous may have facilitated this modest diversification of the "transitional shorebird" lineage, or alternatively, may have been caused by it.
With the events at the KT Boundary (whatever those were), the dominant aviary of the Mesozoic, represented by the spectacularly diverse Enantiornithes and the scattered lineages of archaic ornithurines, were extinguished, leaving the incipient neornithine dynasty to undergo an explosive primary adaptive radiation. Within a period of some ten million years, the base of the neornithine lineage diversified so extensively that almost all extant orders were present by the Lower Eocene, as witnessed by the avifauna of such deposits as the London Clays and Green River Formation of Wyoming. With such a rapid branching of clades from their respective nodes, the ordinal relationships of Neornithes are largely unknowable, and thus we have a mï¿½lange of taxa of uncertain affinity.
The base of Neornithes is something of a hub and spokes, with unresolved branching patterns for multiple orders in what is a thoroughly unpleasant poltyomy. Somewhere early within this confusing pattern of diversification and cladogenesis, at least one but possibly more lineages of secondarily paleognathous taxa split off from other neognaths, and subsequently gave rise independently to the ratite assemblage. In this scheme, the principal dichotomy among Neornithes is the basal neornithine assemblage, and the land bird assemblage, with the former represented by such orders as: Charadriiformes, Anseriformes, Procellariiformes, Gaviiformes, Pelecaniformes, Galliformes, Lithornithiformes, Tinamiformes, and Ciconiiformes, etc. The land bird assemblage, considered more or less by convention as the higher level of morphological organization amongst Neornithes, is represented by such orders as: Cuculiformes, Strigiformes, Caprimulgiformes, Apodiformes, and particularly, Coraciiformes, Piciformes, and Passeriformes. Ordinal relationships within both of these groupings are considered largely unresolved, although some consensus is possible:
- Charadriiform-like "transitional shorebirds" are the ancestral morphotype to all other neornithines
- Cuculiformes represents the base of the land bird assemblage
- Coraciiformes and Alcediniformes are at the base of the perching bird assemblage, culminating in Passeriformes
With the available evidence, this is the best conclusion that can currently be reached.
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