Tiktaalik roseae is 382 million years old and clarifies the evolutionary development from fish to animals with four limbs (tetrapods). Prior to this the earliest tetrapods were dated to about 376 million years ago. Coelacanths are Sarcopterygii or “lobe-finned” fish thought to be ancestral to Tiktaalik roseae, which is also classified as fossil Sarcopterygian fish.
In the Coelacanth, the skull is in two parts with an intracranial joint which allows up and down movement between them. A strong pair of muscles beneath the skull-base lowers the front half of the skull, giving the Coelacanth a powerful bite . Until the Tiktaalik roseae finding, the Coelacanth was the only living animal found with this type of structure. The eyes and olfactory organs are in the front part of the skull, and tiny brain and inner ear are in the rear. In the middle of the snout is a large cavity filled with a jelly-like sac which opens to the outside through three pores and may be used to detect weak electric currents and help the coelacanth to find prey. Most of the skeleton is made of cartilage. In place of the vertebral column, a large notochord extends from the skull to the tip of the caudal fin and serves as a backbone.
http://sacoast.uwc.ac.za/education/resources/fishyfacts/coelacanth.htm
Fins on freshwater fish first began transforming into limbs some 380 million years ago and this was the evolutionary step that opened the way for vertebrates—animals with backbones—to emerge from the water. Tiktaalik roseae’s wide head and sharp teeth suggest it hunted much like a crocodile and that it also breathed air. The side of the snout has a big pair of external nostrils. The creature's long snout seems to be adapted for snapping at prey and hunting with its head above water like a crocodile.
Tiktaalik and the formerly discovered Elpistostege is placed between Panderichthys and tetrapods on the phylogenetic tree The following paper provides the data used to consider Tiktaalik as a new species: location found, taxonomy, nomenclature, and description of the fossil. The head was remarkably well preserved, and three specimens were found. Coelacanth has long been considered a transitional form because of its bony fins, but when discovered alive, did not use them for walking or raising itself up in any way. The second paper discusses the pectoral fin of Tiktaalik, which is “morphologically and functionally transitional between a fin and a limb.” The front fins allowed the creature to hoist itself up and drag its tail behind. Wrist bones containing five digits extend distally and are new features of this fossil:
“Here we report the discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose. Although the body scales, fin rays, lower jaw and palate are comparable to those in more primitive sarcopterygians, the new species also has a shortened skull roof, a modified ear region, a mobile neck, a functional wrist joint, and other features that presage tetrapod conditions. The morphological features and geological setting of this new animal are suggestive of life in shallow-water, marginal and subaerial habitats… During the origin of tetrapods in the Late Devonian (385–359 million years ago), the proportions of the skull were remodelled, the series of bones connecting the head and shoulder was lost, and the region that was to become the middle ear was modified. At the same time, robust limbs with digits evolved, the shoulder girdle and pelvis were altered, the ribs expanded, and bony connections between vertebrae developed…. Panderichthys possesses relatively few tetrapod synapomorphies [convergent features], and provides only partial insight into the origin of major features of the skull, limbs and axial skeleton of early tetrapods[. In view of the morphological gap between elpistostegalian fish and tetrapods, the phylogenetic framework for the immediate sister group of tetrapods has been incomplete and our understanding of major anatomical transformations at the fish–tetrapod transition has remained limited.
A phylogenetic analysis of sarcopterygian fishes and early tetrapods supports the hypothesis that Tiktaalik is the sister group of tetrapods or shares this position with Elpistostege. Tiktaalik retains primitive tetrapodomorph features such as dorsal scale cover, paired fins with lepidotrichia, a generalized lower jaw, and separated entopterygoids in the palate, but also possesses a number of derived features of the skull, pectoral girdle and fin, and ribs that are shared with stem tetrapods such as Acanthostega and Ichthyostega. Tiktaalik is similar to these forms in the possession of a wide spiracular tract and the loss of the opercular, subopercular and extrascapulars. The pectoral girdle is derived [sic] in the degree to which the scapulocoracoid is expanded dorsally and ventrally, and the extent to which the glenoid fossa is oriented laterally. The pectoral fin is apomorphic [i.e., derived, more developed] in the elaboration of the distal endoskeleton, the mobility of segmented regions of the fin, and the reduction of lepidotrichia distally.”1
“The pectoral skeleton of Tiktaalik is transitional between fish fin and tetrapod limb. Comparison of the fin with those of related fish reveals that the manus [hand] is not a de novo novelty of tetrapods; rather, it was assembled in fishes over evolutionary time to meet the diverse challenges of life in the margins of Devonian aquatic ecosystems. The concept of “missing links” has a powerful grasp on the imagination: the rare transitional fossils that apparently capture the origins of major groups of organisms are uniquely evocative. But the concept has become freighted with unfounded notions of evolutionary ‘progress’ and with a mistaken emphasis on the single intermediate fossil as the key to understanding evolutionary transitions. Much of the importance of transitional fossils actually lies in how they resemble and differ from their nearest neighbours in the phylogenetic tree, and in the picture of change that emerges from this pattern.
“The fins are adapted to flex gently upwards - as if the fin were being used to support the body. One of the interesting differences between fins and Tiktaalik limbs is that the later contain bones that comprise mobile wrist and ankles….Although the small distal bones bear some resemblance to tetrapod digits in terms of their function and range of movement, they are still very much components of a fin. There remains a large morphological gap between them and digits as seen in, for example, Acanthostega: if the digits evolved from these distal bones, the process must have involved considerable developmental repatterning. The implication is that function changed in advance of morphology.”2
“Of course, there are still major gaps in the fossil record. In particular we have almost no information about the step between Tiktaalik and the earliest tetrapods, when the anatomy underwent the most drastic changes, or about what happened in the following Early Carboniferous period, after the end of the Devonian, when tetrapods became fully terrestrial. But there are still large areas of unexplored Late Devonian and Early Carboniferous deposits in the world – the discovery of Tiktaalik gives hope of equally ground-breaking find to come.”3
1Daeschler et al., “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature 440, 757-763 (6 April 2006) | doi:10.1038/nature04639; Received 11 October 2005; ; Accepted 8 February 2006.
2Shubin et al., “The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb,” Nature 440, 764-771 (6 April 2006) | doi:10.1038/nature04637; Received 11 October 2005; ; Accepted 8 February 2006.
3Per Erik Ahlberg and Jennifer A. Clack, “Palaeontology: A firm step from water to land,” Nature 440, 747-749 (6 April 2006) | doi:10.1038/440747a.
“If the five radials of Tiktaalik are homologous to digits, then the axis of the tetrapod limb would extend from the humerus through digit three. Unfortunately, the absence of a well-defined axis in other tetrapodomorphs leaves uncertain whether a central axis is primitive for tetrapods or if it evolved separately in Tiktaalik. Testing these competing hypotheses awaits the discovery of other tetrapodomorph fins with axes that project into the distalfin. The pectoral skeleton of Tiktaalik is transitional between fish
fin and tetrapod limb.”
http://afarensis.blogsome.com/2006/04/19/tiktaalik-roseae-and-the-origins-of-tetrapods/
Taxonomy of lungfishes presents some difficulty because of their resemblances to both fish and land-dwelling vertebrates, and have been classified in a variety of ways, ranging from class Dipnoi, to infraclass Dipnomorpha, to order Dipteriformes. However, there is general agreement that there are two main subcategories, here given as orders: Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ...
A Partial Taxonomy of the Tetrapods:
Phylum Chordata
Class Dipnoi (lungfish)
Class Sarcopterygii
Subclass Coelacanthimorpha (Coelacanths)
Subclass Tetrapodomorpha
Genus Eusthenopteron
Genus Panderichthys (also had large tetrapod-like head)
Genus Tiktaalik roseae
Superclass Tetrapoda
Family Elginerpetontidae
Family Acanthostegidae
Family Ichthyostegidae
Family Whatcheeriidae
Family Crassigyrinidae
Family Loxommatidae
Family Colosteidae
Class Amphibia - Amphibians
Class Sauropsida - Reptiles
Class Aves - Birds
Class Synapsida - Mammal-like reptiles
Class Mammalia - Mammals