Evolution of tetrapods from crossopeygian (fin lobed)
The appearance of the tetrapods was believed to have began in the late Devonian to the Carboniferous period (Boisvert, 2009; Jeffery, 2001; Daeschler et al., 1994; Daeschler & Shubin, 1995) where the vertebrate community was diversified to occupy the aquatic environment (Boisvert, 2009) mostly as fishes that exhibited vertebrate characteristics. The fossil evidence shows there are similarities between the present day tetrapods that possess the pentadctyl limb and have adapted to terrestrial and aquatic habitats and the taxa that occupied the aquatic environment during the upper Devonian period (Jarvik, 1996). The taxa of most importance would be the osteichthyans which are further divided into two groups, the actinopterygians (ray-finned fishes) (Boisvert, 2009) and sarcopterygians (fleshy-finned fishes) (Edwards, 1989). The sarcopterygian being the most common ancestor divided into the lungfishes and the rhipidistians that have lobed fins (Edwards, 1989) and are termed crossopterygians (Jarvik, 1996) inclusive of the genus Eusthenopteron.
Figure 1: The Australian lungfish (Neoceratods forsteri) (Norman & Greenwood, 1963).
Figure 2: The Eusthenopteron. (Olson, 1971)
The question of interest in this theory would be how did the pentadactyl limb evolve from the lobed-fin of their fish ancestors? Fossil records imply that the change from aquatic fish to terrestrial tetrapods was not immediate but rather a slow, gradual shift as the Devonian tetrapods were morphologically varied and had a large distribution on the geographical plane by the end of the Farmennian period (Daeschler & Shubin, 1995). This hints that the sarcopterygians were evolving and occuring in different ecological niches and these events were induced global climatic, tectonic and atmospheric changes (Coates et al., 2008). The Eusthenopteron is one of the early fishes closely related to the tetrapods possessing lungs, gills, the internal skeleton resembling the one of tetrapods. The internal skeleton of their fins possessed skeletal equivalents of the humerus, radius and ulna of tetrapod forelimbs as well as femur, tibia and fibula of the hindlimbs (Boisvert, 2009).
Fin and limb skeletons. (a) Sauripterus, a rhizodont (b) Barameda, a rhizodont (c) Tiktaalik, an elpistostegalid (d) Eusthenopteron, a tristichopterid (e) Gogonasus, an osteolepidid (f) Sterropterygion, a megalichthyid. (g) Rhizodopsis, a megalichthyid (h) Acanthostega, a limb-bearing stem tetrapod (i) Tulerpeton, a limb-bearing stem tetrapod (j) Greererpeton, a limb-bearing stem tetrapod(k) Westlothiana, a stem amniote. Dermal fin skeleton, comprising fin rays and scales, are shown in light gray for Sterropterygion (f); similarly elaborate dermal skeletons are present, but not illustrated, in all taxa in the top two rows. These features are absent from the digit-bearing taxa in the bottom row. All skeletons are shown with leading edge to right of Figure; all are in.
Figure 3: The skeletal similarities between the primitive fishes and the tetrapods (Coates et al., 2008).
The similarity in the skeletal development of the Eusthenopteron and the tetrapod limb led to the hypothesis that the limb may have evolved from the crossopterygian fin which is still seen in some fish today such as the Coelacanths. The skeletal properties of the limbs which were proposed to be similar to that of the fins were: an unpaired humerus or femur which supports the whole limb; paired subequal ulna-radius and fibula-tibia which are functionally joined distally; two primary joints in each appendage; diastal carpals, tarsals and digits arising on post axial side of the appendage; pectoral and pelvic appendages similar in structure; muscular lobe that can be extended well below the ventral body wall and the reduction or loss of dermal rays (Rosen et al., 1981). All the similarities in these structures were not met as some conditions found in the present day tetrapods did not correlate with their assumed finned ancestors.
The unpaired humerus or femur which supports the whole limb is found in all lungfishes and rhipidistians which leads to a step forward in supporting the theory that the tetrapod limb evolved from the fin (Edwards, 1989). The paired subequal ulna-radius and fibula-tibia which are functionally joined distally was a condition not met by most of the fishes as well as some amphibians showing that this characteristic could not be used to help ascertain this theory. Rather, the condition which coincides with the fish ancestors (lungfishes, osteolepiform and rhipidistians) and the tetrapods would be the paired nature of the subbasal elements mostlyn in the embryo; the ulan being shorter than the radius and the ulna being postaxial in position (Edwards, 1989). The presence of two primary joints in the organisms is not disuted although the pectoral girdles seem to differ in morphology since in lungfishes it is a convex joint element whilst in osteolepids and tetrapods it is concave. The mere presence of some similarities in the pectoral girdles has allowed this characteristic to be accepted (Edwards, 1989).
The most recognizable feature in the tetrapods is the presence of digits which are not readily seen in the lobed- fishes and has since been a bone of contetion in trying to prove the hypothesis, and the lack of fossil evidence being an influencing factor (Boisvert, 2009). The presence of the post axial giving rise to the preaxial radial which are similar to distal crapals and digit in tetrapods prove similarities between the two. The lungfishes and rhipidistians possess these elements although digits are not pronounced in the lobed fishes (Edwards, 1989). This may have been attributed to differences in gene expression in the fishes and tetrapods that control the distal elements appearance on the post axial side of the appendages leading to the tetrapod ancestors not possessing any digits whilst the tetrapods developed digits (Fig. 4)(Boisvert, 2009).
The pectoral and pelvic appendages similarity cannot be used to define the similarities between the lobe-finned fishes and the tetrapods as they greatly differ. Five of four segments were suggested to have been present on the pelvic grirdle and pectoral girdle whilst this is not the case as these vary in the organsims (Edwards, 1989). The rhipidistains posses four elements in their pectoral fin and two in the pelvic fin, the Eusthenopteron possessing five in the pectoral fin and four in the pelvic fin differing greatly from that of the tetrapods limited to a range of four or five. (Edwards, 1989).
Figure 4: The development of the digits from the fin to the tetrapod limb over time (Boisevert, 2009).
The description of the lobed fin and its position below the body of the fish was termed loose and subjective, not giving a clear perspective as to how much is muscular and how well below the body should the fin be extending. The characteristic therefore, was not considered useful in trying to accept the hypothesis (Edwards, 1989). Most of the vertebrate characteristics noted in the rhipidistians and the lungfishes were retained in the tetrapods except for the dermal fin rays that are absent in the tetrapods. The dermal fin rays are seen in the other later fish that were closer in relation to the tetrapod, Tiktaalik, (Clack, 2009) which possessed the dermal fin rays. In Eusthenopteron, they were only retained in the tail (Boisvert, 2009). Since evolution is a gradual process, the presence of some features which are not found in the present day tetrapod may have allowed the organism to adapt to its environment and occupy its ecological niche successfully. This attributes to some of the differences that are noted in the primitive fishes believed to have given rise to the tetrapod.
Figure 5: The gradual transition from the lobe-finned fish to the tetrapod with limbs and digits occupying a terrestrial habitat (Clack, 2009).
The differences noted in some of the proposed characteristics that link the tetrapods to the crossopterygians are attributed to limitations in morphological structures only being used. Fossil records are not able to give details on the genetic characteristics of organisms which makes it difficult to prove a relation between lobe-fined fishes and the tetrapods. The skeletal morphological characteristics noted have however showed a relationship between the crossopterygians and the tetrapods, giving considerations that the tetrapod limb may have evolved from the lobed fin of the crossopterygian.
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REFERENCES
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Clack, J. A. (2009). The fin to limb transition: new data, interpretations and hypotheses from paleontology and developmemntal biology. The annual review of the earth and planetary sciences, 37, 163-179. 265, 639-
Coates, M. I., Ruta, M., & Friedman, M. (2008). Ever since Owen: Cahnging perspectives on the early evolution of tetrapods. Annual review of Ecology, Evolution and Systematics, 39, 571-592.
Daeschler, E. B., Shubin, N. H., Thomson, K. S., & Amaral, W. W. (1994). A Devonian Tetrapod from North America. Science, 265, 639-642.
Daeschler, E. B., & Shubin, N. H. (1995). Matters of the record: tetrapod origins. Paleobiology 21(4), 404-409.
Edwards, J. L. (1989). Two perspectives on the evolution of the tetrapod limb. American zoologist, 29, 235-254.
Jarvik, E. (1996). The Devonian tetrapod Ichythyostega. Fossils and Strata, 40, 1-213.
Norman, J. R., & Greenwood, P. H. (1963). A history of fishes. New York: Hill and Wang.
Olson, E. C. (1971). Vertebrate paleozoology, New York: Wiley Interscience.a
