Rahonavis

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Rahonavis
Fossil range: Late Cretaceous

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Family: Dromaeosauridae
Genus: Rahonavis
Forster et al., 1998b
Species
  • R. ostromi (Forster et al., 1998a [originally Rahona]) (type)
Synonyms
  • Rahona Forster et al., 1998a
    preoccupied by Griveaud, 1975

Rahonavis[1] is a genus of dinosaur from the Late Cretaceous (Campanian?, 83.5 - 70.6 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.

Contents

[edit] Description

Rahonavis was a small predator, about the size of Archaeopteryx, with the typical Velociraptor-like raised sickle claw on the second toe.

[edit] Classification

Rahonavis has historically been the subject of some uncertainty as to its proper taxonomic position - whether it is a member of the clade Aves or a closely-related dromaeosaurid. The presence of quill knobs on its ulna (forearm bone) 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 has been difficult to place Rahonavis firmly among or outside the birds. Rahonavis could be a close relative 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[2]. Alternately, Makovicky and colleagues considered Rahonavis to be closely related to the South American dromaeosaurids Unenlagia and Buitreraptor, and thus a member of the subfamily Unenlagiinae[3]. Norell and colleagues (2006) also found Rahonavis to lie within the Unenlagiinae, as the sister taxon to Unenlagia itself[4]. A 2007 study by Turner and collegues again found it to be an unenlagiine dromaeosaurid, closely related to Unenlagia[5].

The discoverers of Rahonavis initially named it Rahona but changed the name after discovering that the name Rahona was already assigned to a genus of lymantriid moths[6][7].

[edit] Discovery and species

The fossilized remains of Rahonavis were first recovered in 1995 by a joint expedition of SUNY and the University of Antananarivo, near the village of Berivotra. Most geological formations in this area are covered in dense grass, making identification of fossils difficult. However, when a portion of hillside was exposed by fire, the remains of a giant titanosaur were revealed. It was during the excavation of this find that paleontologists discovered the bones of Rahonavis among the bones of the much larger dinosaur. Rahonavis is known from this single specimen, consisting of the hind limbs, trunk, portions of the tail (all of which were found articulated), as well as portions of the wing and shoulder bones. Rahonavis was one-fifth larger than the closely related Archaeopteryx, about the size of a modern raven[8].

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. 100-99 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[9].

[edit] Paleobiology

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. Some researchers have suggested that Rahonavis represents a chimera consisting of the forelimb of a bird conflated with the skeleton of a dromaeosaurid, and consider Rahona as described a nomen dubium[2].The nearby discovery of the primitive bird Vorona berivotrensis at least shows that the possibility of a mix-up cannot be fully excluded. However, many other scientists, including the original describers of Rahonavis, maintain that its remains belong to a single animal, citing the close proximity of the wing bones to the rest of the skeleton. All the bones attributed to Rahonavis were buried in an area "smaller than a letter-sized page", according to co-describer Luis M. Chiappe in his 2007 book Glorified Dinosaurs. Additionally, Chiappe argued that suggestions of a chimera by paleornithologist Larry Martin were based on Martin's misinterpretation of the wing and shoulder bones as being more advanced than they really are[8]. Chiappe maintained that Rahonavis could probably fly, noting that its ulna was large and robust compared to Archaeopteryx, and that this fact, coupled with the prominent quill knobs, suggest that Rahonavis had larger and more powerful wings than that earlier bird. Additionally, Rahonavis shoulder bones show evidence of ligament attachments allowing the independent mobility needed for flapping flight. Chiappe concluded that Rahonavis was capable of flight, though it would have been more "clumsy in the air than modern birds[8].

[edit] References

  1. ^ Etymology: approximately "cloud menace bird": Rahonavis, from Malagasy rahona (RA-hoo-na, "cloud" or "menace") + Latin avis "bird". Specific name R. ostromi, dedicated to John Ostrom.
  2. ^ a b Geist, Nicholas R.; Alan Feduccia (2000). "Gravity-defying Behaviors: Identifying Models for Protoaves". American Zoologist (40): pp. 664–675. doi:10.1668/0003-1569(2000)040[0664:GDBIMF]2.0.CO;2.  PDF fulltext.
  3. ^ Makovicky, Peter J.; Apesteguía, Sebastián & Agnolín, Federico L. (2005). The earliest dromaeosaurid theropod from South America, pp. 1007-1011.  doi:10.1038/nature03996. (HTML abstract). Supplementary information.
  4. ^ 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): pp. 1–51. doi:10.1206/0003-0082(2006)3545[1:ANDTFU]2.0.CO;2. 
  5. ^ Turner, Alan H.; Pol, Diego; Clarke, Julia A.; Erickson, Gregory M.; and Norell, Mark (2007). "A basal dromaeosaurid and size evolution preceding avian flight" (pdf). Science 317: 1378–1381. doi:10.1126/science.1144066. 
  6. ^ 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 (5358): pp. 1915–1919.  doi:10.1126/science.279.5358.1915. (HTML abstract).
  7. ^ Forster, Catherine A.; Sampson, Scott D.; Chiappe, Luis M. & Krause, David W. (1998b). "Genus Correction.". Science (5361): p. 179. 
  8. ^ a b c Chiappe, L.M.. Glorified Dinosaurs: The Origin and Early Evolution of Birds. Sydney: UNSW Press. 
  9. ^ 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): pp. 367–368.  doi:10.1590/S0001-37652002000200023. PDF fulltext.
  • Forster, Catherine A.; O'Conner (2000). "The avifauna of the Upper Cretaceous Maevarano Formation, Madagascar". Journal of Vertebrate Paleontology 3 (20): pp. 41A–42A. 
  • 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". Journal of Vertebrate Paleontology 4 (19): pp. 712–722. HTML abstract.
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