majstro hat geschrieben: Es gibt keine Tierart, die eine Zwischenform zwischen Reptil und Vogel ist.
Zwischenformen:
* Sinosauropteryx prima. A dinosaur covered with primitive feathers, but structurally similar to unfeathered dinosaurs Ornitholestes and Compsognathus (Chen et al. 1998; Currie and Chen 2001).
* Ornithomimosaurs, therizinosaurs, and oviraptorosaurs. The oviraptorosaur Caudipteryx had a body covering of tufted feathers and had feathers with a central rachis on its wings and tail (Ji et al. 1998). Feathers are also known from the therizinosaur Beipiaosaurus (Xu et al. 1999a). Several other birdlike characters appear in these dinosaurs, including unserrated teeth, highly pneumatized skulls and vertebrae, and elongated wings. Oviraptorids also had birdlike eggs and brooding habits (Clark et al. 1999).
* Deinonychosaurs (troodontids and dromaeosaurs). These are the closest known dinosaurs to birds. Sinovenator, the most primitive troodontid, is especially similar to Archaeopteryx (Xu et al. 2002). Byronosaurus, another troodontid, had teeth nearly identical to primitive birds (Makovicky et al. 2003). Microraptor, the most primitive dromaeosaur, is also the most birdlike; specimens have been found with undisputed feathers on their wings, legs, and tail (Hwang et al. 2002; Xu et al. 2003). Sinornithosaurus also was covered with a variety of feathers and had a skull more birdlike than later dromaeosaurs (Xu, Wang, and Wu 1999; Xu and Wu 2001; Xu et al. 2001).
* Protarchaeopteryx, alvarezsaurids, Yixianosaurus and Avimimus. These are birdlike dinosaurs of uncertain placement, each potentially closer to birds than deinonychosaurs are. Protarchaeopteryx has tail feathers, uncompressed teeth, and an elongated manus (hand/wing) (Ji et al. 1998). Yixianosaurus has an indistinctly preserved feathery covering and hand/wing proportions close to birds (Xu and Wang 2003). Alvarezsaurids (Chiappe et al. 2002) and Avimimus (Vickers-Rich et al. 2002) have other birdlike features.
* Archaeopteryx. This famous fossil is defined to be a bird, but it is actually less birdlike in some ways than some genera mentioned above (Paul 2002; Maryanska et al. 2002).
* Shenzhouraptor (Zhou and Zhang 2002), Rahonavis (Forster et al. 1998), Yandangornis and Jixiangornis. All of these birds were slightly more advanced than Archaeopteryx, especially in characters of the vertebrae, sternum, and wing bones.
* Sapeornis (Zhou and Zhang 2003), Omnivoropteryx, and confuciusornithids (e.g., Confuciusornis and Changchengornis; Chiappe et al. 1999). These were the first birds to possess large pygostyles (bone formed from fused tail vertebrae). Other new birdlike characters include seven sacral vertebrae, a sternum with a keel (some species), and a reversed hallux (hind toe).
* Enantiornithines, including at least nineteen species of primitive birds, such as Sinornis (Sereno and Rao 1992; Sereno et al. 2002), Gobipteryx (Chiappe et al. 2001), and Protopteryx (Zhang and Zhou 2000). Several birdlike features appeared in enantiornithines, including twelve or fewer dorsal vertebrae, a narrow V-shaped furcula (wishbone), and reduction in wing digit bones.
* Patagopteryx, Apsaravis, and yanornithids (Chiappe 2002; Clarke and Norell 2002). More birdlike features appeared in this group, including changes to vertebrae and development of the sternal keel.
* Hesperornis, Ichthyornis, Gansus, and Limenavis. These birds are almost as advanced as modern species. New features included the loss of most teeth and changes to leg bones.
* Modern birds.
References:
1. Chen, P., Z. Dong and S. Zhen, 1998. An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China. Nature 391: 147-152.
2. Chiappe, L. M., 2002. Osteology of the flightless Patagopteryx deferrariisi from the Late Cretaceous of Patagonia (Argentina). In Chiappe and Witmer, pp. 281-316.
3. Chiappe, L. M. and L. M. Witmer (eds.), Mesozoic Birds: Above the Heads of Dinosaurs. Berkeley: Univ. of California Press.
4. Chiappe, L. M., M. A. Norell and J. M. Clark, 2001. A new skull of Gobipteryx minuta (Aves: Enantiornithes) from the Cretaceous of the Gobi Desert. American Museum Novitates 3346: 1-15.
http://diglib1.amnh.org/novitates/i0003 ... 1-0001.pdf
5. Chiappe, L. M., M. A. Norell and J. M. Clark, 2002. The Cretaceous, short-armed Alvarezsauridae. In: Chiappe and Witmer, pp. 87-120.
6. Chiappe, L. M., S. Ji, Q. Ji and M. A. Norell, 1999. Anatomy and systematics of the Confuciusornithidae (Theropoda: Aves) from the Late Mesozoic of northeastern China. Bulletin of the American Museum of Natural History 242: 1-89.
http://diglib1.amnh.org/bulletins/i0003 ... 1-0001.pdf
7. Clark, J. M., M. A. Norell and L. M. Chiappe, 1999. An oviraptorid skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest. American Museum Novitates 3265: 1-36.
8. Clarke, J. A. and M. A. Norell, 2002. The morphology and phylogenetic position of Apsaravis ukhaana from the late Cretaceous of Mongolia. American Museum Novitates 3387: 1-46.
http://diglib1.amnh.org/novitates/i0003 ... 1-0001.pdf
9. Currie, P. J. and P. Chen, 2001. Anatomy of Sinosauropteryx prima from Liaoning, northeastern China. Canadian Journal of Earth Sciences 38: 1705-1727.
10. Forster, C. A., S. D. Sampson, L. M. Chiappe and D. W. Krause, 1998. The theropod ancestry of birds: New evidence from the Late Cretaceous of Madagascar. Science 279: 1915-1919.
11. Hwang, S. H., M. A. Norell, Ji Q. and Gao K., 2002. New specimens of Microraptor zhaoianus (Theropoda: Dromaeosauridae) from northeastern China. American Museum Novitates 3381: 1-44.
http://research.amnh.org/users/sunny/hw ... l.2002.pdf
12. Ji, Q., P. Currie, M. A. Norell and S-A. Ji, 1998. Two feathered dinosaurs from northeastern China. Nature 393: 753-761.
13. Makovicky, P. J., M. A. Norell, J. M. Clark and T. Rowe, 2003. Osteology and relationships of Byronosaurus jaffei (Theropoda: Troodontidae). American Museum Novitates 3402, 1-32.
http://diglib1.amnh.org/novitates/i0003 ... 1-0001.pdf
14. Maryanska, T., H. Osmólska and M. Wolsan, 2002. Avialan status for oviraptorosauria. Acta Palaeontologica Polonica 47(1): 97-116.
http://app.pan.pl/acta47/app47-097.pdf
15. Paul, Gregory S., 2002. Dinosaurs of the Air. Baltimore: Johns Hopkins University Press.
16. Sereno, P. C. and C. Rao, 1992. Early evolution of avian flight and perching: New evidence from the Lower Creates of China. Science 255: 845-848.
17. Sereno, P. C., C. Rao and J. Li, 2002. Sinornis santensis (Aves: Enantiornithes) from the Early Cretaceous of Northeastern China. In: Chiappe and Witmer, pp. 184-208.
18. Vickers-Rich, P., L. M. Chiappe and S. Kurzanov, 2002. The enigmatic birdlike dinosaur Avimimus portentosus. In: Chiappe and Witmer, pp. 65-86.
19. Xu, X. and X. Wang, 2003. A new maniraptorian dinosaur from the Early Cretaceous Yixian Formation of Western Liaoning. Vertebrate Palasiatica 41(3): 195-202.
20. Xu, X. and X-C. Wu, 2001. Cranial morphology of Sinornithosaurus millenii Xu et al. 1999 (Dinosauria: Theropoda: Dromaeosauridae) from the Yixian Formation of Liaoning, China. Canadian Journal of Earth Sciences 38: 1739-1752.
21. Xu, X., Z. Tang and X. Wang, 1999a. A therizinosaurid dinosaur with integumentary structures from China. Nature 399: 350-354.
22. Xu, X., X-L. Wang and X-C. Wu, 1999b. A dromaeosaur dinosaur with a filamentous integument from the Yixian Formation of China. Nature 401: 262-266.
23. Xu, X., Z. Zhou and R. O. Prum, 2001. Branched integumental structures in Sinornithosaurus and the origin of feathers. Nature 410: 200-204.
24. Xu, X., M. A. Norell, X. Wang, P. J. Makovicky and X. Wu, 2002. A basal troodontid from the Early Cretaceous of China. Nature 415: 780-784.
25. Xu, X., Z. Zhou, X. Wang, X. Kuang, F. Zhang and X. Du, 2003. Four-winged dinosaurs from China. Nature 421: 335-340.
http://www.cals.ncsu.edu/course/zo501/4WingedDino.pdf
26. Zhang, F. and Z. Zhou, 2000. A primitive enantiornithine bird and the origin of feathers. Science 290: 1955-1959.
27. Zhou, Z. and F. Zhang, 2002. A long-tailed, seed-eating bird from the Early Cretaceous of China. Nature 418: 405-409.
28. Zhou, Z. and F. Zhang, 2003. Anatomy of the primitive bird Sapeornis chaoyangensis from the Early Cretaceous of Liaoning, China. Canadian Journal of Earth Sciences 40: 731-747.
(aus Talk.Origins)
Hühner haben immer noch das genetische Material zur Ausbildung von Zähnen.
Modern birds do not have teeth. Rather, they develop a specialized keratinized structure, called the rhamphotheca, that covers the mandible, maxillae, and premaxillae. Although recombination studies have shown that the avian epidermis can respond to tooth-inductive cues from mouse or lizard oral mesenchyme and participate in tooth formation, attempts to initiate tooth development de novo in birds have failed. Here, we describe the formation of teeth in the talpid2 chicken mutant, including the developmental processes and early molecular changes associated with the formation of teeth. Additionally, we show recapitulation of the early events seen in talpid2 after in vivo activation of beta-catenin in wild-type embryos. We compare the formation of teeth in the talpid2 mutant with that in the alligator and show the formation of decidedly archosaurian (crocodilian) first-generation teeth in an avian embryo. The formation of teeth in the mutant is coupled with alterations in the specification of the oral/aboral boundary of the jaw. We propose an epigenetic model of the developmental modification of dentition in avian evolution; in this model, changes in the relative position of a lateral signaling center over competent odontogenic mesenchyme led to loss of teeth in avians while maintaining tooth developmental potential.
Quelle
Die molekulare Analyse zeigt, dass Krokodile und Schildkröten die nächsten lebenden Verwandten der Vögel sind.
Sister Group Relationship of Turtles to the Bird-Crocodilian Clade Revealed by Nuclear DNA–Coded Proteins
Quelle