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Velociraptor a Mesozoic kiwi? A look at the neoflightless hypothesis
Reversals, Neoflightlessness, and Other Assorted Weirdness: Comments on The Acquisition and Loss of Flight in Aves, and the Strange Case of the Mesozoic “kiwis”
As the resolution with which we can glimpse the phylogenetic base of Aves increases, it is paradoxical that we should be no closer to unraveling the many riddles we find there. A century and a half of work, and we are still faced with a fundamental problem: how do we define what birds actually are. It is easy enough to do this today, from our comfortable Quaternary vantage point, but as we move backwards in time, to the distant Tithonian of 145 million years ago, problems are legion.
The past two decades have witnessed the collapse of every single classical autapomorphy (or “holodiagnostic” to use Charig’s term) of Aves, from furculae, to feathers. Accordingly, distinguishing a par-avian theropod from a bona fide bird, is increasingly a matter of subjectivity. Add into this mess the argument that taxa traditionally classified as lying outside Aves are in fact neoflightless forms closer to Neornithes than is Archaeopteryx, and you have enough to drive the prospective student of avian phylogenetics to despair.
This proposal is not in and of itself new, but it was most recently and most extensively presented in Greg Paul’s 2002 opus, Dinosaurs of the Air: The Evolution and the Loss of Flight in Dinosaurs and Birds (but also see Paul 1988, 1996, for a general treatment of this central theme in his research). The astounding theropod remains from the Yixian lagerstatten of China, are to Paul and others, prima facie evidence for this argument. Such a drastic recalculation of the current theropod and avian phylogeny, however, must bear a considerable burden of proof. Thus, the question naturally becomes, what data supports this argument?
While these suggestions are intriguing, it must be concluded from the data currently available that they remain to be demonstrated unequivocally. Further research will either confirm or refute this neoflightless scenario herein discussed.
As indicated, the view that some dinosaurs are either a) neoflightless, b) closer to modern birds than is the urvogel, or c) both, is not a recent heresy in orthodox thought on avian origins and evolution. There have been various proposals that Archaeopteryx is a theropod without any close affinity to Aves, and that other theropods lie closer to the origin of birds, the longtime advocate of which was R. A. Thulborn. In a series of papers culminating in a 1984 article published in the Zoological Journal of the Linnean Society, Thulborn argued that tyrannosaurs, ornithomimids/troodonts and Avimimus portentosus were respectively, closer to Aves than was Archaeopterygidae (Thulborn 1975, Thulborn & Hamley 1982, Thulborn 1984). In his work on Avimimus, Kurzanov (1981, 1985, 1987) largely agreed. In his popular 1988 review of Theropoda, Paul offered a similar hypothesis, in which he placed dromaeosaurs, ornithomimids, troodonts, oviraptorosaurs, and Avimimus as nearer Ornithurae than all other taxa, including Archaeopteryx.
The landmark Eichstatt Conference largely dispelled the idea that Archaeopteryx is phylogenetically more distant from other birds than any given theropod, and it is a matter of almost universal agreement that the urvogel is indeed the most basal known bird. Indeed, the preponderance of evidence suggests as much, and though it is a slim margin, which delineates Archaeopterygiformes from other Eumaniraptora, the two are not synonymous.
In light of this, most researchers have abandoned the view that some theropods might be neoflightless and closer to the origin of Aves. Paul’s work is the shining beacon of dissent, in that his stance on this matter has been unwavering for over a decade. While some researchers (e.g., Feduccia 1999, Jones et al 2000b) have argued that various taxa attributed to Theropoda, namely the contentious Yixian forms Caudipteryx and Protarchaeopteryx, are neoflightless, besides Paul, the theory that multiple derived theropod lineages are neoflightless and nearer Neornithes than is the urvogel, has been championed by few.
A notable twist on this argument was that presented by Olshevsky in 1994, in which all dinosaurs are hypothetically descended from birds, as neoflightless, secondarily cursorial forms. This outlandish scenario is so devoid of evidence as to be rejected without further consideration in this treatment, and the reader is referred to Witmer (2002) for discussion of the Olshevsky hypothesis.
The Indelible Stamp of Flight Lost
Ultimately, any argument for the neoflightless status of any taxon must rest on the character distribution we seen in that taxon. There are certain traits, which are positive correlates with the loss of flight in birds.
As it is the center of the flight apparatus, the pectoral girdle will be the region most drastically affected by the loss of flight. The furcula is lost, the corpora degenerating into vestigila splints. The coracoid is de-retroverted, becoming procumbent and thus the scapulocoracoidal angle assumes an obtuse degree. The coracoid and scapula ossify into a single unit, the scapulocoracoid. Tubercles such as the acrocoracoid and procoracoid are reduced with the loss and reduction of their associated musculature. The carina of the sternum, and often the sternum as a whole, are lost. The forearms are much reduced. The feathers lose their aerodynamic asymmetry, assuming near-perfect symmetry due to the breakdown of the pennaceous vane.
Concomitant with the reduction of the pectoral architecture is the reduction of the pectoral musculature. Both M. Pectoralis major and M. supracoracoideus are reduced, but as noted by Diamond (1981) and Feduccia (1996), the entire musculature associated with wing supination and pronation, as well as the flexion and extension of the forearm, is reduced, and thus the list of muscles which undergo alteration upon the loss of flight is vast, including (after George & Berger 1966, Van Tyne & Berger 1976, Ostrom 1976):
- M. Coracobrachialis cranialis
- M. Deltoideus major cranialis
- M. Deltoideus major caudalis
- M. Deltoideus minor
- M. Latissimus dorsi cranialis
- M. Latissimus dorsi caudalis
- M. Scapulohumeralis cranialis
- M. Scapulohumeralis caudalis
- M. Subcoracoideus
- M. Subscapularis
- M. Biceps brachii
- M. Brachialis
- M. Flexor carpi ulnaris
- M. Flexor digitorum sublimis
- M. Flexor digitorum profundum
- M. Supinator
- M. Triceps brachii
- M. Extensor metacarpi radialis
- M. Extensor digitorum communis
- M. Serratus superficialis metapatagialis
- M. Pectoralis propatagialis longus
- M. Pectoralis propatagialis brevis
- M. Cucullaris propatagialis
- M. Propatagialis longus
- M. Propatagialis brevis
- M. Expansor secundariorum
- M. Pronator sublimis
- M. Pronator profundus
- M. Entepicondylo-ulnaris
- M. Ulnimetacarpalis ventralis
The reduction or loss of these muscles, which in total account for nearly a quarter of the bird’s body weight results in considerably energy savings, subsequently exploited for more advantageous ends. Further metabolic changes have been noted in flightless birds, correlated with loss of high-energy musculature such as that listed above and as a result, the resting metabolic rate of most secondarily flightless forms is lower than the avian average (McNab 1994).
The Search for Neoflightless Characters in the Mesozoic Menagerie
Paul (2002) has presented the seminal account of the neoflightless hypothesis, and has argued two points, which to him equate to immutable evidence of this hypothesis. If the presence of characters indicative of flighted ancestry, and the presence of characters indicative of the loss of flight can be enumerated within any one taxon, it becomes likely that this taxon is indeed a neoflightless bird. This is the cornerstone of the neoflightless hypothesis, whether Gregory Paul or others elucidate it. Much to his credit, Paul has presented a fairly thorough review of character distribution in potentially neoflightless Theropoda, including Archaeopteryx. From this matrix Paul has scored his sampled taxa and assigned on the basis of their character states, grades indicative of their likelihood of being neoflightless.
While the method is not to be criticized, the data with which Paul made his conclusions must be examined critically. Paul has presented a list of characters, which, he maintains, suggest flighted ancestry in those taxa possessed of them. This list includes:
- Furcula U-shaped
- Robust pectoral girdle
- Horizontal scapular blade
- Scapula tapering caudally
- Acromion process well developed, at least as well as that seen in Archaeopteryx
- Coracoid strutlike, retroverted.
- Ossified sternum
- Broad sternal plates
- Sternal ribs ossified
- Robust deltopectoral crest
- Forelimbs lengthened
- Radial linkage with humerus and metacarpus, acted upon by the humeral extensors and flexors permitting extreme lateral flexion and hyperextension of the manus
- Fusion of the carpals and metacarpals to form a carpometacarpus
- Metacarpal IV posterolaterally bowed
- Posterolateral flange of proximal phalanx of central manal digit present
- Manal digit III as robust as digit II
- Manal digit IV reduced in mobility and size
- Symmetrical remiges
- Ossified uncinate processes present
- Caudal count reduced
- Expansion of the sacrum at the expense of the caudal thoracic series and cranial caudal series.
- Pygostyle present
- Brain enlarged above reptilian maximum (using EQ as the index)
- Olfactory lobe reduced
- Stereoscopic vision
Though suggestive of flighted ancestry, these characters might also represent a general trend of “ornithization” so noted by Barsbold (1983). They may well represent either exaptations, or homoplasies, the latter perhaps being more concordant with Barsbold's concept. It remains a conspicuous fact that the vast majority of Theropoda do not display the characters undeniably associated with the loss of flight. The standard rebuttal to the interpretation of this list of characters as indicative of an “ornithization” trend in theropod phylogeny, is that they cannot possibly have arisen for anything other than flight-function. Paul (1988, 2002) has particularly stressed this point.
Yet I would challenge the adherents of the neoflightless hypothesis to provide the data, which would preclude any of these characters from arising as exaptations/parallelisms, co-opted for flight only later. Paul and his acolytes have with much voracity and little veracity, failed to do this. While an extensive review of the original roles of these characters is beyond the scope of this paper, the author will nonetheless examine a few of the more promising characters from this list.
The presence of a carpometacarpus is difficult to envisage arising for any reason other than flight-function. However, there is no known theropod outside of Aves which displays a fused carpometacarpus, and the most archaic birds indeed lack this trait altogether. Therefore, it is equally difficult to explain why Paul included it in his character matrix.
Paul (2002) cites the presence of an ossified carpometacarpus in Alvarezsauridae as evidence to the contrary, and yet these enigmatic creatures are of uncertain phylogenetic affinity and thus extreme caution is warranted in any sweeping phylogenetic assessments based upon these forms. The status of the distal end of the carpus in Avimimus portentosus is indeterminate. Kurzanov (1981) figured what appear to be the co-ossified bases of the metacarpals, and Norman (1990) concurred with this assessment. Even given the review of the material offered by Vickers-Rich et al (2002), it remains difficult to interpret this structure accurately. Given that the presence of a carpometacarpus cannot be unequivocally demonstrated in Avimimus, the author feels Paul’s assertion to the contrary unwarranted.
There is a suite of traits of the manus, including the posterolateral bowing of metacarpal IV, the posterolateral flange of the proximal phalanx of the central manal digit (assumed herein to be III), and the reduction of manal digit IV, which are unquestionably avian. Paul has argued that these characters cannot be exaptations/parallelisms in that no clear role can be imagined for them outside of supporting flight function. The distribution of these characters amongst derived theropods is interesting, in that we see in the most avian manal osteology amongst Maniraptora, the posterolateral bowing of the fourth metacarpal, and the presence of a posterolateral flange on the proximal phalanx of digit III. The reduction of digit IV, while avian, was also independently derived in such famously didigitate theropods as the tyrannosaurids, and thus it seems most logical to attribute this character to functional, and not phylogenetic stimuli. Given this distribution of characters, it is not readily evident why we are not in fact witnessing the trend of “ornithization” amongst the derived Maniraptora, and indeed no credible reason to suggest otherwise has yet been suggested.
As they are the only traits which are reliably referred to the various par-avian "proto-dromaeosaurs," the function and evolution of the posterolateral bowing of metacarpal IV and the development of a flange at the base of the proximal phalanx of digit III, will be analyzed so as to shed light on this subject. The posterolateral flange of the proximal phalanx of digit III, as well as the bowing of metacarpal IV are associated with the support of feathers arising from the manus, namely the primary remiges.
Given this role, it seems plausible to suggest that the presence of these characters indicates modification of the manus to support feathery integument, filamentous as in Sinornithosaurus, or otherwise. Although Paul (2002, pg. 231) insists the condition serves to protect the remiges from bending during flight, we are left to wonder why this is necessarly the case; that is, why it is used for feathers in flight, and not simply feathers per se, and arrived at convergently. If these characters were in fact directly related to flight, it by default follows that feathers arose through aerodynamic pressures alone, and yet this viewpoint though championed by some (e.g. Feduccia 1996), is untenable. This is not to say, however, that entirely convincing exaptation scenarios have been advanced, and it is these characters that count most strongly in Paul's favor.
It is interesting to note that certain traits of the manus, which should be present if these taxa were in fact more derived than Archaeopteryx, are lacking. For instance, the distal carpal elements have not fused to form the trochlea carpalis, to increase the range of motion available to the manus. The lack of a well developed extensor process on the medial surface of the manus, is also interesting, in that an incipient extensor process is observed in Archaeopteryx. While intriguing, it must on grounds of objectivity be noted that neither of these are disproofs of a neoflightless hypothesis, but they do warrant explanation within the context of that theory.
The radial linkage with humerus and metacarpus, acted upon by the humeral extensors and flexors permitting extreme lateral flexion and hyperextension of the manus, is of interest in that it permits a characteristically avian folding of the arms. Hyperextension of the manus is restricted in all Neornithes due to the expansion of the extensor process on the medial surface of the carpometacarpus, adjacent to the trochlea carpalis. The degree to which any Neotetanurae were able to laterally flex and hyperextend the manus is indeterminate, but it seems logical that only in those form in which we see a robust semilunate distal carpal, would either have been possible. Combined with the radial linkage between the humerus and metacarpus, it is likely that some theropods could fold their arms in the avian fashion, to some degree. However, this very system is seen in incipient form at the base of Maniraptoriformes, and thus its validity as a character indicative of flighted ancestry must be questioned.
In his defense of this hypothesis, Paul (2002) has argued that the presence of a pygostyle in both Caudipteryx spp. and other Oviraptorosauria is prima facie evidence for the flighted ancestry of these forms. Feduccia (1999) and Jones et al. (2000b) reached a similar conclusion, albeit within a different phylogenetic context. The distal caudal series in Caudipteryx spp., however, are in fact not fused, as reported by Ji et al. (1998) in their initial description of the taxon, and confirmed by Zhou et al. (2000) and Witmer (2002). Barsbold et al. (2000a, b) reported a pygostyle from the oviraptorosaur Nomingia, but the homologies of this structure with the avian pygostyle are questioned (Witmer 2002), and open to further interpretation. As is the case with the alleged carpometacarpus of Kurzanov’s Avimimus, we need more material before conclusions can be made. In the meantime, it seems best to err on the side of caution and regard the pygostyle as a trait as of yet restricted to Aves.
It is interesting to note that the modifications of the pectoral girdle, and the thoracic region, so stressed by adherents of this neoflightless hypothesis, are accounted for within the context of par-avian respiratory capacity and an ecological shift towards arboreal habitat (Chatterjee 1997).
The argument that the possession of any of these traits indicates flighted ancestry must be regarded as deficient until such time as an immutable correlate between the presence of these characters and a flighted ancestor can be demonstrated. It is doubtful, in the opinion of the author, that such a task is feasible, inasmuch as the vast majority if not all of the characters enumerated above, are in fact exaptations in extant birds or parallelisms arising from functional constraints.
If any theropods are neoflightless, then it is only logical that they should display those characters, which accompany the loss of flight. And yet in the vast osteological repertoire of these predatory dinosaurs, we find not a single taxon, which presents the requisite character state. Paul (2002) argues that as such forms lie near to the origin of Aves, many classic traits associated with neoflightlessness will not be observed, owing to selective pressure to retain them.
However, flight is not lost via a series of gradual alterations, but rather it is during embryogenesis that the appropriate morphological changes occur. The primary mechanism by which flight is lost is neoteny, and this is the very reason for the uniformity we seen in flightless forms. Paul seems to envision an early and still awkward flier, clambering about the trees of the latest Jurassic, losing flight via a steady series of incremental modifications towards a more cursorial lifestyle. And yet this is not, to our knowledge, how the process works. If flight is to be lost the developmental arrest of the pectoral architecture and resultant paedomorphosis would produce a “big chick” looking little like Paul’s putative neoflightless dromaeosaurs, lying closer to Neornithes than Archaeopteryx.
Disputing the role of neoteny in the loss of flight is a logical objection to this scenario. While a defense of the integral function of neoteny in the origin of flightlessness is beyond the scope of this paper, I will introduce the following points as evidence that differential development and "derived undervelopment" are crucial aspects of the skeleto-muscular changes which occur when flight is lost amongst birds:
- Characters indicative of flight-loss are also neotenic.
- There is a positive correlate between delayed developmantal schedules and alticial chicks, and taxa which are prone to produce flightless derivatives (e.g., Gruiformes).
- There is a positive correlate between rapid developmental schedules and subsequently precocial chicks, and taxa which rarely if ever produce flightless derivatives (e.g., Galliformes).
The question then becomes, what was the role of neoteny in the phylogenetic history of Aves, and was it as integrally linked with the loss of flight at the base of the avian lineage? Although logic would dictate a biological application of uniformitarian thought, let us grant that neoteny is not the essentialistic driving force behind flight-loss, and there may exist other correlates, especially as one moves towards the origin of higher avian groups. There are two taxa, which can serve as a litmus test, to determine the answer to this pressing question: the possibly neoflightless Alvarezsauria, and the peculiar South American archaic bird, Patagopteryx.
As has been noted elsewhere, the phylogenetic status of Alvarezsauria presents one of the most intractable problems to dinosaur systematists and students of paleornithology, in that these bizarre taxa display a mosaic of apomorphic and plesiomorphic characters defying accurate phylogenetic treatment. For the sake of clarity, I will reproduce much of the text to be found under the entry for Alvarezsauria below, to spare the reader endless cross-referencing:
- Initial work by Ostrom (1994), Wellnhofer (1994), Kurochkin (1995), Zhou (1995), Feduccia (1996) and Martin (1997) in which the presence of avian characters were discounted in Alvarezsauria are no longer congruent with subsequent material attributed to this taxon. The most recent phylogenetic analyses of the alvarezsaur material (Karhu & Rautian 1996, Chiappe et al 1996, 1998, Novas 1997, Padian & Chiappe 1998, Longrich 2000, Chiappe et al. 2002) have demonstrated the presence of multiple derived avian characters, particularly in the skull.
- The skull has been modified to permit prokinesis, and the orbits are confluent with the ventral aspect of the post-temporal fenestra, as the postorbital bar is incomplete medially. The jugal and quadratojugal are fused into a jugal bar, and the articulation with the quadrate is highly kinetic. The proximal condyle of the quadrate is doubled, and articulates with the braincase, and the mobility of the quadrate is furthered by the lack of descending process of the squamosal. Lastly, the ectopterygoid is absent (Chiappe et al 1996, 1998, Padian & Chiappe 1998, Paul 2002). These are derived avian characters, reflecting a higher level of organization than the Archaeopteryx node as least as regards the cranial osteology.
- Further similarities between Aves and Alvarezsauria are found in the pelvic girdle and the femur, which especially in proximal aspect, is astonishingly avian. A robust antitrochanter is present on the acetabular wall and the pubic symphysis is limited to the distal portion of the pubes. The pubes are stronly opisthopubic in orientation, forming a 45 degree angle relative to the postacetabular process of the ilium. A pubic boot is lacking. The femora display confluence of the lesser and greater trochanters. The fibuula is reduced to a splint. A secondary cnemial crest is present in Mononykus, but absent in Patagonykus and Parvicursor. The distal aspect of the crus is completely coosified in all but Alvarezsaurus (Chiappe et al 1996, 1998, Padian & Chiappe 1998, Chiappe et al. 2002).
- Paradoxically, however, the alvarezsaurs also display a number of plesiomorphic traits which are difficult to reconcile with the presence of numerous derived, avian characters in both the postcrania and crania. For instance, the presence of some derived avian traits, such as the secondary cnemial crest, is greatly restricted within Alvarezsauria. The tarsometatarsus is arctometatarsalian, which as Karkhu & Rautian (1996) noted is not readily associated with the tarsal architecture observed in Aves. Sereno (1997, 1999) and Martin (1997) argued that this condition allied Alvarezsauria with Ornithomimosauria. Novas & Pol (2002) found that alvarezsaurs lack a list of characters synapomorphic of a higher level of organization within Maniraptora, which they labelled "node X", including: Therizinosauria, Oviraptorosauria, Deinonychosauria, and Aves. Such characters missing from alvarezsaurs include:
- Cervical centra with a kidney-shaped articular surface in cranial aspect
- Exaggerated proximodorsal lip on the manal unguals
- Supracetabular crest absent
- Iliac postacetabular process is subvertical in orientation
- Medial flange of ilium reduced
- Pedal unguals of digits III and IV are dorsoventrally deep, and display enlarged flexor tubercles.
- Novas & pol (2002) concluded from these data that Alvarezsauria lies outside Aves and thus argued that previously supposed similarities to birds are in fact homoplastic traits. Considering the lack of of other avian synapomorphies in the postcrania and crania of alvarezsaurs, the author finds this revised hypothesis certainly plausible. However, it must be cautioned that the avian relationship hypothesis, and those posed by Novas & Pol (2002) and other authors, in which alvarezsaurs are considered a group of aberrant theropods, are still equally viable. Further data is needed before the phylogenetic status of these extremely peculiar dinosaurs can be resolved.
- Chiappe et al. (1996, 1998, 2002) listed the following characters as synapomoprhic of Alvarezsauria:
- Synsacrals laterally compressed
- Cranial and caudal articular facets of the centra are concave and convex, respectively
- Coracoid short, wider than tall, biceps tubercle absent
- Sternum hypertrophied
- Humerus bears a prominent tubercle in ventral aspect
- Olecranon process of ulna hypertrophied
- Metacarpal I hypertrophied, depressed strongly
- Manal digit I robust, ungual with paired proximoventral foramina
- Ischiadic and pubic fusion distally not observed
It has been argued that the alvarezsaurs are at least united in their lack of traits which might readily be ascribed to neoteny, should they in fact be neoflightless birds, either at or below the Archaeopteryx node. I would suggest that the great reduction of the forelimb, including the morphometrics of the associated elements, suggest neoteny, but in light of stronger evidence this conclusion must be regarded as speculative. Considering that the phylogenetic status of Alvarezsauria cannot accurately be determined at this time, and thus neither the avian nor non-avian hypotheses can be ruled out, we must be cautious in making bold phylogenetic assessments based on the osteology of this clade. Until such time as further material and analyses of greater resolution can be presented, one is forced to conclude that Alvarezsauria can contribute little to our understanding of the relationship between flightlessness and neoteny within the earliest stages of avian history.
What of Patagopteryx deferrariisi? This marvelous South American bird, recovered from the Bajo de La Carpa member of the Rio Colorado Formation, and dating to the Campanian, was recovered in 1984-1985 and described by Herculano Alvarenga and famed South American paleontologist Jose Bonaparte in 1992. As celebrated as the Las Hoyas bird, Iberomesornis romerali, Patagopteryx provided a heretofore unknown opportunity to test some of the most basic ornithological principals regarding the loss and acquisition of flight, at a level of organization not far removed temporally or phylogenetically, from the urvogel.
Emphasizing the undeniable presence of characters indicating flightlessness, Alvarenga & Bonaparte (1992) allied Patagopteryx with the Ratitae. Kurochkin (1995b) concurred with this assessment and argued that Patagopteryx represented a basal paleognath. Subsequent reevaluation of the Patagopteryx material (e.g. Chiappe 1995a, b) cast doubt on the neornthine affinity of this taxon by demonstrating the plesiomorphic nature of the characters marshaled in support of this classification. Chiappe (1995a, b, 2002) argued that Patagopteryx was near to the origin of Ornithurae, although occupying a more basal node than the common ancestor of Hesperornithiformes and all other ornithurines. A principal character invoked in defense of this hypothesis is the morphology of the scapular blade, which in both Patagopteryx and ornithurines, is curved in sagittal aspect with a ventrally concave margin. The validity of this presumed synapomorphy is called into question, however, by the histological data gathered by Chinsamy et al. (1994) which conclusively demonstrated LAG (lines of arrested growth) in the femora of Patagopteryx, a trait unknown in any ornithurine. On the basis of these data, Feduccia (1996) apparently suggested that Patagopteryx is more closely allied with Enantiornithes, a view which with the author at least tentatively concurs.
Regardless of the exact phylogenetic position of Patagopteryx, it remains clear that in this marvelous bird, we are witnessing an archaic form with a very generalized level of organization, and thus it serves as an admirable albeit imperfect proxy for a form close to the origin of Aves. As Patagopteryx is clearly flightless, we might ask, does it display traits associated with this condition that can be explained within the context of neoteny, or does it not? The postcranial anatomy of Patagopteryx is congruent in most respects with the stereotypical pattern observed in flightless taxa, a pattern noted (thought over-interpreted) by Alvarenga & Bonaparte (1992) in their initial description. The sternum is reduced and lacks a carina. The coracoids articulate with the sternum via widely separated facets, and are reduced in size. The furcula is absent from all specimens, and due to the preservational quality of the Patagopteryx material, it is difficult to attribute this discrepancy to preservational vagaries. The carpometacarpus is reduced, and the morphometric data from the forelimb is congruent with that observed in a broad range of flightless taxa.
It must be noted that these characters are readily associated with neoteny, and thus in the earliest bird which without ambiguity can be considered flightless, we observe a pattern of flight-loss very much concordant with that observed in extant Aves. While not conclusive, these data do seem to favor the so-called "standard model," as opposed to the hypothesis concerning the loss of flight advanced by Paul and his adherents.
This article was the product of much discussion about this very hypothesis by the author and GFA and it is not as complete or as thorough as either of us would like. It is my hope, however, that it may precipitate a running dialogue on this interesting proposal in theropod phylogenetics, which will achieve more than the original scope of the article.
Please consult the Talk page for this article, as it contains a highly productive dialogue between myself, GFA, and a fellow student of avian phylogenetics, MWAK.
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