Discussion of Maryanska et al. (2002)

From EvoWiki

Jump to: navigation, search

Discussion of Maryanska et al. (2002)

Contents

Introduction

In a recent analysis of oviraptorosaur phylogeny, Maryanska et al. (2002) have retiterated the sporadically voiced hypothesis that the oviraptorosaurs constitute a clade of secondarily flightless forms either more closely related to urvogel than other theropods, or more closely related to Neornithes than Archaeopteryx (e.g., Paul 1988). Much to the credit of this study, it has made a careful analysis of 195 skeletal characters (though auto-correlation seems to be very much lacking) subjected to algorithmic analysis in an attempt to sort out the phylogeny of Oviraptorosauria vis-a-vis Aves (the authors prefer to use Gauthier's taxon "Avialae"). This study reaches several conclusions:

  1. Oviraptorosauria is the sister clade of Confuciusornithidae, thus, oviraptorosaurs are the earliest known flightless birds.
  2. Avimimus portentosus is the sister taxon to all other oviraptorosaurs
  3. Caudipteryx zoui is the sister taxon to Oviraptoroidea

While the last conclusion listed is broadly congruent with multiple independent cladistic analyses of caudipterygian phylogeny, the second and particularly first conclusions, are contentious. The assertion that Oviraptorosauria shares a more recent common ancestry with the archaic bird Confuciusornis constitutes a drastic revision of widely accepted phylogenetic hypotheses, and thus must be critically examined. Similarly, the authors of this study have grouped Avimimus conclusively with Oviraptorosauria, when this enigmatic taxon has traditionally floated about in phylogenetic limbo. The merits of this conclusion need to be discussed as well.

Avian Status for Oviraptorosauria?

As noted by Maryanska et al. (2002), it has been a general matter of consensus amongst dinosaur paleontologists and indeed paleornithologists, that Oviraptorosauria, though paravian, are nonetheless theropods without exception. Paul (1988) was among the first to argue for the secondarily flightless, post-urvogel status of the oviraptorosaurs, which he reiterated in his 2002 opus. While Paul (1988, 2002) offered no explicit cladistic analysis for oviraptorosaur relationships, he placed Oviraptorosauria deeply within Aves in 1988, and nearer Archaeopteryx in his 2002 study. Olshevsky argued much the same (1991) and reiterated this in his controversial 1994 "birds came first" theory. Polish paleontologist Andrzej Elzanowski (1999) favored avian or paravian status for the Oviraptorosauria based upon palatal and other cranial characters. Thus, the central conclusion of Maryanska et al. (2002) is not without precedent, however, Maryanska et al. (2002) marks the first attempt to subject the avian hypothesis to a rigorous cladistic analysis, and is thus noteworthy.

A single most parsimonious tree recovered from cladistic analysis of Maryanska et al.'s data matrix grouped Confuciusornithidae and Oviraptorosauria as sister clades, within Pygostylia (implicit), at a node labelled "Clade B." This assessment differs sharply from most others in that it not only considers oviraptorosaurs secondarily flightless birds, but places them at a fairly derived level within Aves (note that the authors of this paper use "Avialae" and thus restrict Aves to crown-clade definition, which generally causes confusion). Five characters are advanced as synapomorphic of this clade:

  1. Quadrate possesses lateral cotyla for quadratojugal
  2. Manidubular symphysis tightly sutured
  3. Maxillary teeth lost
  4. Dentary teeth lost
  5. Parietal equal to or larger than frontal in size

Characters three and four are conceivably one character, not two, and thus we have in fact four potential synapomorphic characters of Confuciusornis + Oviraptorosauria, all of which are problematic. Character one, the presence of a lateral cotyla for reception of the quadratojugal, is a bone fide avian character, and the author concurs with Maryanska et al. (2002) in advancing it as a potential synapomorphy. Character two is rather ambiguous, and Maryanska et al. (2002) never clarify precisely what is meant by "tightly sutured." Their data matrix keys this character as absent in Archaeopteryx and present in Confuciusornis and yet there is no readily apparent justification for this disparity in character coding and thus it seems somewhat dubious. Character three and four, the edentulous jaws, are highly homplastic traits also seen in Ornithomimosauria, and indeed, it would appear to be parallelism in Oviraptorosauria as well (see below). Character five is the most perplexing, in that it is arguably the prima facie evidence offered linking Confuciusornis and Oviraptorosauria, and yet even cursory examination of the parietal/frontal in Confuciusornithidae reveals that contra Maryanska et al., the parietal is not longer than or otherwise enlarged at the expense of the frontal, and a review of the pertinent literature concerning this archaic bird (e.g., Hou et al. 1995b, c, Hou 1997, Martin et al. 1998, Feduccia 1999, Zhou and Hou 2002) corroborates this observation. Among major studies of the Confuciusornithidae, only Chiappe et al.'s (1999) reconstruction of the confuciusornithid skull concurs with that of Maryanska et al. (2002). Given the significant defects in Chiappe et al.'s (1999) treatment of confuciusornithid osteology and paleobiology (Olson 2000), there is little to lend credence to this unusual interpretation of confuciusornithid cranial morphology. Thus, character five cannot be considered synapomorphic of a clade linking Confuiusornithidae and Oviraptorosauria.

The greatest blow was to come shortly after the study was published, in September 2002, with Xu et al's preliminary descripion of Incisivosaurus gauthieri, the most basal and stratigraphically oldest oviraptorosaur yet discovered. It is interesting to compare unambiguous synapomorphies of "clade B" as diagnosed by Maryanska et al with the Incisivosaurus material. Unlike Caudipteryx, Incisivosaurus has a quadrate that articulates with the quadratojugal in the normal archosaurian manner along a caudally sloping facet. Unlike other oviraptorosaurs, Incisivosaurus has a toothed maxilla and dentary. The nine maxillary teeth are markedly lanceolate and without serrations, and the eight or nine dentary teeth are similar to those of the premaxilla, the former's similarity to those of therizinosaurs further supporting a sister clade relationship between the two groups. Likewise, the parietal is no longer than the frontal. The Incisivosaurus jugal is not rodlike, and the ectopterygoid has a jugal hook. The palatine has an accessory fenestra, and it lacks the long maxillary process. Postcranial characters, including the number of cervicals, caudals, and the shape of the furcula are unknown; the holotype is only represented by a nearly complete skull and a partial cervical.

Maryanska et al. (2002) augment these synapomorphies with a list of the impressively avian characters of oviraptorosaurs, some of which are accurately observed, while others are at best ambiguous (e.g., the rodlike jugal bar, see Paul 2002) or demonstrably homoplastic (e.g., doubled otic process of the quadrate). The vast majority of these traits (e.g., robust furcula) are also observed in non-avian theropods and thus are clearly plesiomorphic at any level of organization beyond Aves itself (the postulated phylogenetic position of Oviraptorosauria in this study).

Thus, such similarities with birds must be regarded as reflective of the general trend of "ornithization" present in all Neotetanurae (Barsbold 1983). There are further complications with viewing the Oviraptorosauria as secondarily flightless, in that though they are in fact quite birdlike, they do not generally conform to the observed morphological patterns in flightless birds. For example, the furcula in such forms as Ingenia yanshini is hypertrophied. Yet in all flightless birds the furcula degenerates into vestigial clavicular splints. Why it would be retained in a flightless cursor is not readily apparent. Similar problems include the orientation of the glenoid, which in all oviraptorosaurs is directed ventrally as in most theropods, the orientation relative to the rib cage of the scapular blade, the distal expansion thereof, the structure of the pelvic girdle (particularly that of the ischia and the retention of an obturator process) and morphology of the coracoid. These character data are difficult to reconcile with a secondarily flightless status for Oviraptorosauria as they are entirey incongruent with the stereotypical morphology of flightless birds.

While Maryanska et al. (2002) present a compelling argument supported by careful research in defense of their conclusion that Oviraptorosauria are neoflightless post-urvogels, currently this assessment cannot be reliably substantiated by the data at hand.

It is interesting to note tangentially, however, that the very suggestion that Oviraptorosauria are secondarily flightless birds more derived than the urvogel, has been pounced on eagerly by opponents of the theropod origin of birds (e.g., Feduccia 2002, Olson 2002). In a recent Auk article, for example, Alan Feduccia the luminary of the so-called "thecodont" hypothesis presented a familiar argument: if oviraptorosaurs are actually birds then their provide no support for the theropod origin of birds at all. This entire standpoint is difficult to accept for two reasons, first and foremost, in accepting the results published in Maryanska et al. (2002), Feduccia and his colleagues are concurring with a cladistic analysis precisely the sort of which they have tirelessly railed against. Second and more seriously, the argument by taxonomic convention is fundamentally flawed philosophically as it denies the mosaic nature of evolution. It is in effect, anti-evolutionary.

Avimimus Finds a Home

Another major conclusion of the paper herein discussed is the basal status of Avimimus within Oviraptorosauria, significant due to the general taxonomic and phylogenetic malaise that has plagued the history of this species. Maryanska et al. (2002) present compelling character data to substantiate this placement of Avimimidae, based upon the following synapormphies of their "Clade C" + Avimimus:

  1. Foramen magnum larger than the occipital condyle
  2. Cornoid eminence of the mandible is present
  3. Hypapophyses are prominent on the cervicodorsal vertebrae
  4. Humeral ectepicondyle larger and more prominent than the entepicondyle
  5. Infratemporal fenestra large, square.

The robustness with which the oviraptorosaur affinities of the avimimids has been upheld should effectively end debate as to where within Maniraptora this enigmatic form belongs. (See Vickers-Rich et al. 2002 for a succinct discussion of the debate as to the phylogenetic status of Avimimus).

Repeating the Obvious: Caudipteryx is a Feathered Dinosaur

Substantiating multiple independent cladistic analyses, Maryanska et al. (2002) found Caudipteryx to be nested within Oviraptorosauria, and the sister taxon of their Oviraptoidea within an undesignated "Clade C." Though character support for this assemblage was not particularly robust, the relevant point is that once again an analysis of morphological characters grouped Caudipteryx not merely with oviraptorosaurs but well within this containing clade, dramatically underscoring the fact that however avian, Caudipteryx is not a "Mesozoic kiwi." Caudipteryx displays all the synapomorphies of Oviraptorosauria outlined above, and arguments for the avian identity of the taxon (Feduccia 1999, Jones et al. 2000, Geist & Feduccia 2000, Martin & Czerkas 2000, Ruben & Jones 2000, Feduccia 2002, Olson 2002) have not been borne out by further examination of the Caudipteryx material.

Conclusions

Though carefully researched with an exhaustive review of skeletal characters, the central conclusion of this recent paper on oviraptorosaur phylogeny--that these dinosaurs are in fact secondarily flightless birds--remains unsubstantiated and in need of further character evidence before it can be regarded as a robust hypothesis. Other aspects of this study, such as the placement of Avivmimus and Caudipteryx within Oviraptorosauria are corroborated by compelling data.

References

  1. Barsbold, R. 1983. Carnivorous dinosaurs from the Cretaceous of Mongolia. Transactions of the Joint Soviet-Mongolian Paleontological Expedition 8: 39-44.
  2. Chiappe, L., Ji S., Ji, Q., & Norell, M. 1999. Anatomy and systematics of the Confuciusornithidae (Aves) from the Late Mesozoic of northeastern China. Bulletins of the American Museum of Natural History 242: 1-89.
  3. Elzanowski, A. 1999. A comparison of the jaw skeleton in theropods and birds with a description of the palate in the Oviraptoridae. In: Olson, S., (ed.), Avian Paleontology at the Close of the 20th Century: Proceedings of the 4th International Meeting of the Society of Avian Paleontology and Evolution, 311-323.
  4. Feduccia, A. 1999. The Origin and Evolution of Birds, Second Edition. Yale University Press, New Haven.
  5. Feduccia, A. 2002. Birds are dinosaurs: Simple answer to a complex problem. The Auk 119(4): 1187-1201.
  6. Geist, N. R. & Feduccia, A. 2000. Gravity defying behaviors: identifying models for protoaves. American Zoologist 40: 664-675.
  7. Hou, L. 1997. Mesozoic Birds of China. Translated courtesy of the Society of Avian Paleontology and Evolution.
  8. Hou, L., Zhou, Z., Gu, Y., & Sun, Y. 1995b. Confuciusornis sanctus, a new Late Jurassic sauriurine bird from China. Chinese Science Bulletin 40(18): 1545-1551.
  9. Hou, L., Martin, L., Zhou, Z., & Feduccia, A. 1995c. A beaked bird from the Jurassic of China. Nature 277: 616-618.
  10. Jones, T., Farlow, J., Ruben, J., Henderson, D., & Hillenius, W. J. 2000. Cursoriality in bipedal archosaurs. Nature 406: 716-718.
  11. Martin, L., Zhou, Z., & Feduccia, A. 1998. Confuciusornis compared to Archaeopteryx. Naturewissenschaften 85: 286-289.
  12. Martin, L. & Czerkas, S. 2000. The fossil record of feather evolution in the Mesozoic. American Zoologist 40: 687-694.
  13. Maryanska, T., Osmolska, H., & Wolsam, M. 2002. Avialian status for Oviraptorosauria. Acta Palaeontologica Polonica 47(1): 97-116.
  14. Olshevsky, G. 1991. A revision of the parainfraclass Archosauria Cope, 1869, excluding the advanced Crocodylia. Mesozoic Meanderings 2: 1-196.
  15. Olshevsky, G. 1994. The birds first? A theory to fit the facts. Omni 16: 34-38, 40-43, 80-84.
  16. Olson, S. L. 2000. Review: Anatomy and systematics of the Confuciusornithidae (Theropoda: Aves) from the Late Mesozoic of Northeastern China. The Auk 117(3): 836-839.
  17. Olson, S. L. 2002. Review: New Perspectives on the Origin and Early Evolution of Birds. Proceedings of the International Symposium in Honor of John H. Ostrom. The Auk 119(4): 1202-1205.
  18. Paul, G. 1988. Predatory Dinosaurs of the World. Simon & Schuster, New York.
  19. Paul, G. 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Johns Hopkins University Press, Baltimore.
  20. Ruben, J. & Jones, T. 2000. Selective factors associated with the origin of fur and feathers. American Zoologist 40: 585-596.
  21. Vickers-Rich, P., Chiappe, L., & Kurzanov, S. 2002. The enigmatic birdlike dinosaur Avimimus portentosus. In: Chiappe, L. & Witmer, L., (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 65-86.
  22. Xu, X. et al. 2002. An unusal oviraptorosaurian dinosaur from China. Nature 419: 291-293.
  23. Zhou, Z. & Hou, L. 2002. The discovery and study of Mesozoic birds in China. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 160-183.

Acknowledgements

JGK/GFA

Retrieved from "http://wiki.cotch.net/index.php/Discussion_of_Maryanska_et_al._%282002%29"
Personal tools