Rahonavis
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Extinct (fossil)
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| Rahonavis ostromi (Forster et al., 1998a) |
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Rahonavis (Etymology: approximately "menace from the clouds": Rahonavis, from Malagasy rahona (RA-hoo-na, "cloud" or "menace") + Latin avis "bird". Specific name R. ostromi, dedicated to John Ostrom) is a genus of dinosaur from the Late Cretaceous (Campanian?, 70.6 - 83.5 mya) of what is now northwestern Madagascar. It is known from a partial skeleton (UA 8656) found in Maevarano Formation rocks at a quarry near Berivotra, Mahajanga Province.
It is the subject of some controversy as to its proper taxonomic position--whether it is a member of the clade Aves or a closely-related dromaeosaurid. The presence of what have been interpreted to be quill knobs on its ulna led initially to its inclusion among the birds; however, the rest of the skeleton is rather typically dromaeosaurid in its attributes. Given the extremely close affinities between primitive birds and their dromaeosaurid cousins, along with the possibility that flight may have developed and been lost multiple times among these groups, it may be impossible to place Rahonavis firmly among or outside the birds. At this point, the controversies regarding the relative relationships among birds, dromaeosaurids, and troodontids are numerous and change with each new feathered fossil discovery. Makovicky et al. (2005) considered Rahonavis to be closely related to the South American dromaeosaurids Unenlagia and Buitreraptor, and thus a member of the subfamily Unenlagiinae. Norell et al. (2006) also found Rahonavis to lie within the Unenlagiinae, as the sister taxon to Unenlagia itself. Alternately, Rahonavis could be closer to Archaeopteryx, as originally suggested by the describers, and thus a member of the clade Aves, but while the pelvis shows adaptations to flight similar in function to those of Archaeopteryx, they seem to be independently derived (Geist & Feduccia, 2000).
What is clear from the remains is that the living Rahonavis was a small predator, about the size of Archaeopteryx, with the typical Velociraptor-like raised sickle claw on the second toe. Its discoverers (Forster et al., 1998a) initially named it Rahona but changed the name after discovering that the name Rahona was already assigned to a genus of lymantriid moths (Forster et al., 1998b). Although numerous artists' reconstructions of Rahonavis show it in flight, it is not clear that it could fly; there has even been some doubt that the forearm material, which includes the quill knobs, belongs with the rest of the skeleton. Its discoverers are convinced that it does, while others would rather consider Rahona as described a nomen dubium (Geist & Feduccia, 2000). The nearby discovery of the primitive bird Vorona berivotrensis at least shows that the possibility of a mix-up cannot be fully excluded. Indeed, the "raptor" claw of Rahonavis is not infrequently ascribed to Vorona in error.
The lack of well-documented relatives of this species nonwithstanding, a single thoracic vertebra (NMC 50852) most similar to those of R. ostromi was found in mid-Cretaceous sediments (Albian/Cenomanian, c. 99-100 mya) of the Kem Kem region, Morocco. Lacking the pleurocoels found in Rahonavis and having a larger neural canal, it appears to belong to a different genus. Although former character can vary in species of the same genus, in individual vertebrae of the same animal, and ontogenetically, the distance in space and time suggests that whatever this specimen may be, it does not belong into Rahonavis (Riff et al. 2002).
[edit] References
- Makovicky, Peter J.; Apesteguía, Sebastián & Agnolín, Federico L. (2005): The earliest dromaeosaurid theropod from South America. Nature 437: 1007-1011. DOI:10.1038/nature03996 (HTML abstract) Supplementary information
- Forster, Catherine A.; Sampson, Scott D.; Chiappe, Luis M. & Krause, David W. (1998a): The Theropod Ancestry of Birds: New Evidence from the Late Cretaceous of Madagascar. Science 279(5358): 1915-1919. DOI:10.1126/science.279.5358.1915 (HTML abstract)
- Forster, Catherine A.; Sampson, Scott D.; Chiappe, Luis M. & Krause, David W. (1998b): Genus Correction. Science 280(5361): 179.
- Forster, Catherine A. & O'Conner (2000): The avifauna of the Upper Cretaceous Maevarano Formation, Madagascar. J. Vertebr. Paleontol. 20(3): 41A-42A.
- Geist, Nicholas R. & Feduccia, Alan (2000): Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 40: 664–675. PDF fulltext
- Norell, M.A., Clark, J.M., Turner, A.H., Makovicky, P.J., Barsbold, R., and Rowe, T. (2006). "A new dromaeosaurid theropod from Ukhaa Tolgod (Omnogov, Mongolia)." American Museum Novitates, 3545: 1-51.
- Riff, Douglas; Kellner, Alexander W. A.; Mader, Bryn & Russell, Dale (2002): On the occurrence of an avian vertebra in Cretaceous strata of Morocco, Africa. Anais da Academia Brasileira de Ciências 74(2): 367-368. DOI:10.1590/S0001-37652002000200023 PDF fulltext
- Schweitzer, Mary H.; Watt, John A.; Avci, Recep; Forster, Catherine A.; Krause, David W.; Knapp, Loren; Rogers, Raymond R.; Beech, Iwona & Marshall, Mark (1999): Keratin immunoreactivity in the Late Cretaceous bird Rahonavis ostromi. J. Vertebr. Paleontol. 19(4), 712-722. HTML abstract
