My favorite transitional is
Acanthostega. It's like the stereotypical "Darwin fish" on people's cars. It used its little feet to move around in the water, but they probably weren't strong enough to support it on dry land. Icthyostega is similar, and probably used its limbs to move from puddle to puddle. These are both fish-amphibian transitionals because they are not exactly lobe-finned fish but don't have all the amphibian characters either. They are a blend of the two.
Reptile-mammal transitionals include the following:
a.Sphenacodon (late Pennsylvanian to early Permian, about 270 million years ago (Mya)). Lower jaw is made of multiple bones; the jaw hinge is fully reptilian. No eardrum.
b.Biarmosuchia (late Permian). One of the earliest therapsids. Jaw hinge is more mammalian. Upper jaw is fixed. Hindlimbs are more upright.
c.Procynosuchus (latest Permian). A primitive cynodont, a group of mammal-like therapsids. Most of the lower jaw bones are grouped in a small complex near the jaw hinge.
d.Thrinaxodon (early Triassic). A more advanced cynodont. An eardrum has developed in the lower jaw, allowing it to hear airborne sound. Its quadrate and articular jaw bones could vibrate freely, allowing them to function for sound transmission while still functioning as jaw bones. All four legs are fully upright.
e.Probainognathus (mid-Triassic, about 235 Mya). It has two jaw joints: mammalian and reptilian (White 2002a).
f.Diarthrognathus (early Jurassic, 209 Mya). An advanced cynodont. It still has a double jaw joint, but the reptilian joint functions almost entirely for hearing.
g.Morganucodon (early Jurassic, about 220 Mya). It still has a remnant of the reptilian jaw joint (Kermack et al. 1981).
h.Hadrocodium (early Jurassic). Its middle ear bones have moved from the jaw to the cranium (Luo et al. 2001; White 2002b).
http://www.talkorigins.org/indexcc/CC/CC215.htmlDinosaur to bird transitionals:
•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.
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