|Wesley R. Elsberry
Joined: May 2002
I think Roger Cuffey's discussion of transitional fossils is a good starting place. He has an online version of his 1974 paper (scanned, apparently), and this is the section that gives four classes of transitional sequences.
Although the broad patterns and many details in the history of life are well known, many other details remain to be learned. Because of the unevenness of our knowledge, therefore, we can conveniently distinguish several different types of transitional-fossil situations. Let us consider these now, starting with that situation where our knowledge is most complete, and proceeding through situations in which knowledge is progressively less complete.
First, some groups have been so thoroughly studied that we know sequences of transitional fossils which grade continuously from one species to another without break (Table 1), sometimes linking several successive species which cross from one higher taxon into another (Table 2). We can say that situations of this kind display transitional individuals. Among the many available examples of transitional individuals, some particularly convincing examples can be noted.
corals (Carruthers, 1910, p. 529, 538; Easton, 1960, p. 175; Moore, Lalicker, & Fischer, 1952, p. 140; Weller, 1969, p. 123),
gastropods (Fisher, Rodda, & Dietrieh, 1964),
pelecypods (Kauffman, 1967; Kauffman, 1969, p. N198-200; Kauffman, 1970, p. 633),
echinoids (Beerbower, 1968, p. 136, 138; Kermack, 1954; Nichols, 19S9a, 1959h; Olson, 1965, p. 98; Rowe, 1899).
Second, other fossil groups have been well enough studied that we know sequences of transitional fossils comprising a series of chronologically successive species grading from an early form to a later form (Table 3), again sometimes crossing boundaries separating different higher taxa (Table 4). This type of situation can be termed successive species. Published descriptions of successive species lack explicit discussion of individuals transitional between the species, although frequently such exist in the author's collection but are not discussed because they are not directly pertinent to his purposes. Again, some especially persuasive examples of successive species can he seen, among:
foraminiferons (Wilde, 1971, p. 376),
brachiopods (Greiner, 1957; Raup & Stanley, 1971, p. 124),
pelecypods (llastoo, 1960, p. 348; Kay & Colbert, 1965, p. 327; Moore, Lalicker, & Fischer, 1952, p. 447; Newell, 1942, p. 21, 42, 47-48, 51-52, 60, 63, 65; Olson, 1965, p. 97; Stenzel, 1949; Stenzel, 1971, p. N1079-1080; Weller, 1969, p. 209),
ammonoids (Cobhan, 1961, is. 740-741).
In many fossil groups, our understanding is relatively less complete, thus giving rise to a third type of situation which we can label successive higher taxa. Here, we may not have complete series of transitional individuals or successive species, but the genera (or other higher taxa) represented in our collections form a continuous series grading from an earlier to a later form, sometimes crossing from one higher-rank taxon into another (Table 5). Because genera are relatively restricted in scope, many series of successive genera have been published. However, families and higherrank higher taxa are so broad in concept that they are not usually used to construct transitional-fossil sequences, although occasionally they are (Bulman, 1970, p. V103-104; Easton, 1960, p. 436; Flower & Kummel, 1950, p. 607).
Finally, in some fossil groups, our knowledge is quite fragmentary and sparse. We then may know of particular fossils which are strikingly intermediate between two relatively high-rank higher taxa, but which are not yet connected to either by a more continuous series of successive species or transitional individuals. We can refer to these as isolated intermediates, a fourth type of situation involving transitional fossils, a type which represents our least-complete state of knowledge.
Isolated intermediates include some of the most famous and spectacular transitional fossils known, such as Archaeopteryx (Colbert, 1969, p. 186-189; Romer, 1966, p. 166-167). This form is almost exactly intermediate between the classes Reptilia and Ayes (Cuffey, 1971a, p. 159; Cuffey, 1972, p. 36), so much so that "the question of whether Archaeopteryx is a bird or a reptile is unimportant. Both viewpoints can be defended with equal justification" (Brouwes, 1967, p. 161). The fossil onychophorans (Moore, 1959, p. 019; Olson, 1965, p. 190) and the fossil monoplacophorans (Knight & Yochelson, 1960, p. 177-83; Raup & Stanley, 1971, p. 308-309) have been regarded as annelidarthropod and annelid-mollusk inter-phylum intcrsnediates, respectively. Moreover, although invertebrate phylum origins tend to be obscure for several reasons (Olson, 1965, p. 209-211), recently discovered, Late Precambrian, soft-bodied invertebrate fossils may well alter that situation, particularly after certain peculiar forms are studied and compared with Early Cambrian forms (Kay & Colbert, 1965, p. 99, 103; Weller, 1969, p. 247).
Mention of this last prompts me to point out parenthetically that the appearance of shelled invertebrates at the beginning of the Cambrian has been widely misunderstood. The assertion is frequently made that all the major types of animals appeared suddenly and in abundance then. In actual fact, collecting in successive strata representing continuous sedimentation from Late Precambrian into Early Cambrian time reveals a progressive increase upward in abundance of individuals. Moreover, the various higher taxa-particularly the various classes and orders reflecting adaptation to different modes of life-appear at different times spread over the long interval between the Early Cambrian and the Middle Ordovician.
Finally, because of widespread interest in questions of man's origins, it is well worth emphasizing that a rather complete series of transitional fossils links modern man continuously and gradationally hack to midCenozoic, generalized pongids (see references in Table 2).
In spite of statements to the contrary . . . , the fossil record of the Hominnidea, the superfamily containing man and the apes, is quite well known, and it is therefore possible to outline a tentative evolutionary scheme for this group (Uzrcll & Pilbeam, 1971, p. 615).
(Cuffey and Moore on ASA)
So the fossils which I utilize in my TFEC fall into the first category that Cuffey mentions, that of "transitional individuals". There are several morphospecies between the G. trilobus parent species and the O. universa daughter species.
"You can't teach an old dogma new tricks." - Dorothy Parker