Transitional Fossils
The real test for evolution rests in the fossil record (R. Cuffey, 1984, 1999; Glenister & Witzke, 1983). Because evolution is descent with modification, the rock record should contain fossils that illustrate how the morphology was modified. Moreover, these fossils should be found in a geochronologic succession. Such fossils that are morphologically and geochronologically intermediate between two taxa are termed transitional fossils (R. Cuffey, 1984, p. 256, 1999).
Again, consider the fossil horses; Hyracotherium lived during the Eocene whereas Equus lived during the Pleistocene and Recent. In intervening rocks, we find a geochronologic succession (Norell & Novacek, 1992) of horse species and genera that exhibit gradual, progressive morphologic change from four toes (digits II - V) to one toe (digit III) on the front feet (Simpson, 1951; Carroll, 1988, p. 533-536; MacFadden, 1992). First, digit V (pinkie) was reduced to a splint and disappeared resulting in horses with three functional toes. Subsequently, digits II and IV (index and ring finger, respectively; flank the middle toe/finger on each side) were reduced in size simultaneously, resulting first in non-functional toes (they did not contact the ground) and ultimately in tiny splints as in the modern horses (Simpson, 1951; Carroll, 1988, p. 533-536; MacFadden, 1992). Significant changes in size and shape of the skull and body, along with teeth morphology, also happened in parallel (Simpson, 1951; Carroll, 1988, p. 533-536; MacFadden, 1992). That is descent with modification, and provides very strong, incontrovertible evidence for evolution.
The United States National Museum of Natural History at the Smithsonian Institution has an excellent display of fossil horses, including feet, teeth, and skulls, that illustrate these changes. The evolutionary history of the horse was classically illustrated as a single straight line (Dott & Prothero, 1994, p. 62). It is now known that this was an oversimplification; the evolutionary history is more properly illustrated as a many-branched bush (Simpson, 1951; Carroll, 1988, p. 53 4; MacFadden, 1992; Dott & Prothero, 1994, p. 63). However, this does not negate a steady progression from Hyracotherium to Equus. Just like a branching tree, there still is a direct connection from trunk to tips of branches; no matter how many branches there are, one can still trace the pattern (MacFadden, 1992).
Consider the brachiopod Eocoelia from the Lower Silurian of Great Britain (Ziegler, 1966). We find two species both classified as Eocoelia based on the details of internal morphology. However, the shells of the older species are coarsely ribbed whereas the shells of the younger species are smooth (Ziegler, 1966). If we examine samples collected from geochronologically intermediate positions, we find a succession of Eocoelia that progressively reduced and ultimately lost the ribs (Ziegler, 1966). This morphologic progression can be illustrated both qualitatively with specimen illustrations and quantitatively by measuring rib strength and plotting the data as a series of histograms in stratigraphic order (Ziegler, 1966). Such sequences are the preserved remains of temporally successive populations of organisms that morphologically changed from one species into another. All of these intermediate forms thus qualifies as transitional fossils. The only logical conclusion is that such successive populations were produced by normal reproductive processes. That is descent with modification (Cuffey, 1984, p. 266-269).
Examination and collection of the rock and fossil record (either outcrops or subsurface cores) naturally produces many such stratigraphically, superpositionally, and hence geochronologically successive samples that show gradual and continuous morphologic change from older species into younger species (Cuffey, 1984). Numerous examples of such transitional individuals, consisting of sample by sample intermediate forms, completely documenting morphologic change between species (in some cases connecting more than one higher taxon) exist among protists, several invertebrate phyla, and vertebrates, especially mammals including hominids (Cuffey, 1984, p. 258, 259). Additional research has provided many other examples of transitional individuals in protists (Lazarus, 1983, 1986; Malmgren, Berggren, & Lohmann, 1983, 1984; Arnold, 1983), bryozoans (Cuffey, 1999), brachiopods (Hurst, 1975), conodonts (Barnett, 1972), mammals (Rose & Bown, 1984; Bookstein, Gingerich, & Kluge, 1978; Gingerich & Simons , 1977; Gingerich & Gunnell, 1979; Chaline & Laurin, 1986; Clyde & Gingerich, 1994; Gingerich, 1974, 1976a, 1980, 1985), and hominids (Cronin et al., 1981; Wolpoff, 1984).
Based upon these data, we can conclude that new species arise by descent with modification that occurs through successive generations, each produced by normal reproductive processes. Furthermore, the rates of morphologic change are highly variable. Some change is essentially constant and unidirectional, slow or fast; this is classic phyletic gradualism (Gingerich, 1974, 1976a). Other change is irregular consisting of intervals of slow change or stasis interrupted by intervals of very rapid change. This rapid change may be clearly resolvable but compressed into a narrow interval (termed punctuated gradualism; Malmgren, Berggren, & Lohmann, 1983) or so rapid that, on a geologic time frame, the transitional samples are not resolvable (termed punctuated equilibrium; Cheetham, 1986). Moreover, such change can proceed in a straight line linking a succession of several species (termed anagenesis or phyletic transformation; Ziegler, 1966; Gingerich, 1974, 1976a, 1985; Sheldon, 1990) or one species can produce two or more species in a branching pattern (termed speciation or cladogenesis; Gingerich, 1974, 1976a, 1985; Lazarus, 1986; Sheldon, 1990). These empirical observations are direct evidence of descent with modification, and lead to the inescapable conclusion that evolution has occurred.
The existence of transitional forms between species has been questioned not just by “scientific creationists,” but also by some paleontologists. For example, Gould (1977, p. 14) stated that, “The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology...The history of most fossil species includes two features particularly inconsistent with gradualism...Most species exhibit no directional change during their tenure on earth...In any local area, a species does not arise gradually by the steady transformation of its ancestors; it appears all at once and ‘fully formed.’’’ However, Gould’s position is not universally accepted within the paleontologic community (Gingerich, 1974, 1976a, 1985; Cuffey, 1984). Hoffman (1989, p. 109) stated that, “...the strong version of punctuated equilibrium—the claim that gradual phenotypic evolution of species occurs at best exceptionally, orders of magnitude less commonly than punctuated evolution—is blatantly false. There is no empirical data to support it, and a considerable body of data to contradict it.” I concur; the references cited above provide examples that contradict Gould’s claim.
In other cases, geochronologic successions of species or genera (in some cases families) exist that document the morphologic change between an older taxon and a younger taxon in several invertebrate phyla and vertebrates (Cuffey, 198 4, p. 259-262). Some good examples can be found among brachiopods (McNamara, 1984), molluscs (Newell, 1942; Erben, 1966; Hallam, 1968, 1982; Spinosa, Furnish, & Glenister, 1975; Ward & Blackwelder, 1975), trilobites (Palmer, 1965; Lesperance, 1975 ), conodonts (Behnken, 1975), and mammals (Gingerich, 1976b). The morphologic differences between the successive species and genera are no greater than that bridged by transitional individuals in more completely studied successions. The logical extrapolation is that we would find a continuous succession of transitional individuals between these successive species or genera, if we had enough specimens to do such a study.
Research has provided many examples of successive species and genera (and in some cases families) linking major higher taxa of order or class rank (Cuffey, 1984, p. 266). For example, within Phylum Mollusca, transitional fossils have been found between [1] Class Monoplacophora and Subclass Nautiloidea (Pojeta, 1980; Runnegar & Pojeta, 1974), [2] Class Monoplacophora and Class Rostroconchia (Pojeta, 1980; Runnegar & Pojeta, 1974; Pojeta & Runnegar, 1976; Runnegar, 1978), [ 3] Class Rostroconchia and Class Pelecypoda (Pojeta, 1980; Runnegar & Pojeta, 1974; Pojeta & Runnegar, 1976; Pojeta, 1978), [4] Class Rostroconchia and Class Scaphopoda (Pojeta, 1980; Runnegar & Pojeta, 1974; Pojeta & Runnegar, 1976, 1979) , [5] Subclass Bactritoidea and Subclass Ammonoidea (Erben, 1966).
Among the vertebrates, transitional fossils have been found linking [1] rhipidistian fish and amphibians (Carroll, 1988, p. 136-166, 1997; Ahlberg & Milner, 1994; Coates & Clack, 1990, 1991; Marshall, Astin, & Clack, 1999 ), [2] reptiles (theropod dinosaurs) and birds (Ostrom, 1991, 1994; Dodson, 1985, 1998; Charig et al., 1986; Carroll, 1988, p. 338-344, 1997; Norman, 1990; Osborn, 1916), [3] synapsid reptiles and mammals (see later; Broom, 1932; Kemp, 1982, 1985; Sloan, 1983; Carroll, 1988, p. 361-449; Hopson, 1969, 1970, 1987, 1994; Hotton et al., 1986; Crompton & Jenkins, 1973; Hopson & Crompton, 1969; Sidor & Hopson, 1998; Lilleg raven et al., 1979), and [4] mesonychid ungulates and whales (Gingerich & Russell, 1981; Gingerich , 1983; Kumar & Sahni, 1986; Gingerich, Smith, & Simons, 1990; Thewissen & Hussain, 1993; Gingerich et al., 1994; Thewissen, Hussain, & Arif, 1994; Berta, 1994; Thewissen et al., 1996; Thewissen & Fish, 1997).
As our knowledge of the fossil record increases, so do the number of transitional fossils and the completeness of our knowledge of particular transitions. Two poignant examples illustrate this point. First, until less than 10 years ago, the gap between reptiles (theropod dinosaurs) and birds was linked by Archaeopteryx (only 7 specimens known; Dodson, 1998) from the Upper Jurassic Solnhofen Limestone (Ostrom, 1991, 1994). Though an isolated intermediate (Cuffey, 1984, p. 262, 263), Archaeopteryx provided spectacular evidence linking these two taxa (Ostrom, 1991, 1994; Dodson, 1985, 1998; Charig et al., 1986; Carroll, 1988, p. 338-344, 1997; Norman, 1990; Osborn, 1916). During the 1990’s several well preserved birds (especially from the Lower Cretaceous of China) have been found that form a geochronologic and morphologic link between Archaeopteryx and younger Cretaceous birds (Carroll, 1997). It is also worth pointing out here that Linnean taxonomy requires us to place each species into one or another higher taxon, not floating between. Thus, we have a “bird bin” and a “reptile bin.” Whereas the skeleton of Archaeopteryx is transitional, with some theropod features (e.g., long bony tail; teeth; Carro ll, 1988, p. 338-344; Ostrom, 1991, 1994), some bird features (e.g., feathers; Carroll, 1988, p. 338-344; Ostrom, 1991, 1994), and some intermediate features (e.g., wings still with three fingers; pelvis; Carroll, 1988, p. 338-344; Ostrom, 1 991, 1994), we put it in the “bird bin” because it has feathers (Carroll, 1988, Ostrom, 1991, 1994). But recognize that this terminology obscures the true transitional nature of this and other intermediate forms (Cuffey, 1984).
As a second example, until 20 years ago, a significant gap separated whales from their inferred ancestors, the mesonychid ungulates. Mesonychids were wolf-like, carnivorous, hoofed mammals (Carroll, 1988, p. 520, 521). Subsequently, about a half-dozen genera have been described, mostly from the Tethys, that form a geochronologic and morphologic link between mesonychids and whales (Gingerich & Russell, 1981; Gingerich et al., 1983; Kumar & Sahni, 1986; Gingerich, Smith, & Simons, 1990; Thewissen & Hussain, 1993; Gingerich et al., 1994; Thewissen, Hussain, & Arif, 1994; Berta, 1994; Thewissen et al., 1996; Thewissen & Fish, 1997). These specimens document the morphologic changes in the skull , ear (Gingerich & Russell, 1981; Gingerich et al., 1983; Thewissen & Hussain, 1993), and loss of functional walking legs (Gingerich, Smith, Simons, 1990; Thewissen, Hussain, & Arif, 1994; Thewissen & Fish, 1997). |
