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1982) Criticized the Methods of Phylogenetic (Cladis- Thyridae" (= Protorothyrididae

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1982) Criticized the Methods of Phylogenetic (Cladis- Thyridae Journal of Vertebrate Paleontology 4(4):604-608, December 1984 PHYLOGENETIC METHODS AND THE EARLY HISTORY OF AMNIOTES In a recent review of early amniote evolution, Carroll groups must have arisen within Carroll's "Protoro- (1982) criticized the methods of phylogenetic (cladis- thyridae" (= Protorothyrididae). Carroll, however, is tic, Hennigian) systematics. Specifically, he concluded concerned with entirely different characters, specifi- that several of the most important assumptions of this cally, those "in which the most primitive known mem- methodology were contradicted by the evidence from bers of each of the derived groups are more advanced early amniotes and that these contradictions resulted than the protowthyrids" (Carroll, 1982:100, italics from the evolutionary process of adaptive radiation in added). But if all the other groups of early amniotes this group. Our opinions concerning the utility of the are derived independently from "protorothyrids," then phylogenetic method contrast strongly with those of any such derived characters shared by two or more of Dr. Carroll. We feel that the problems he encountered these groups must by definition be convergent. There- result from his misunderstanding of the phylogenetic fore, we do not find it suprising that Carroll (1982:102) method rather than from inadequacies of this method finds no obvious congruence among these characters. itself Dr. Carroll is one of the most knowledgable stu- Critics of the phylogenetic method (Mayr, 1981; dents of early amniote evolution; here we present nei- Carroll, 1982) argue that evolutionary parallelism and ther a detailed reanalysis of his characters nor a precise convergence pose particular problems for this method. alternative to his phylogeny of this group. Instead, we Platnick (1977) demonstrates clearly that the opposite argue that Carroll's own procedure was formulated such is true, namely that analysis of shared apomorphies that his negative conclusions about the assumptions provides an unambiguous method of identifying cases of the phylogenetic method were inevitable. We con- involving convergence. Such a determination is made tend that many of Carroll's objections to the phylo- by considering the distribution of the feature in ques- genetic method rest on faulty reasoning. tion in relation to those of other derived features among Carroll's most severe criticism of the phylogenetic the taxa concerned. For a given character distribution, method seems to be its ". assumption that the ma- parallelism/convergence can be shown to be either nec- jority of derived features possessed in common by re- essary or unnecessary to account for the distribution lated groups are the result of inheritance from a com- of particular derived characters among the terminal mon ancestor, rather than from convergence" (Carroll, taxa (Platnick, 1977). 1982:104; but see Farris, 1983). For the early amniotes, Convergence and parallelism refer to similarities de- Carroll (1982:102) finds that the distribution of de- rived independently rather than those inherited from rived features forms "an almost random mosaic" a common ancestor. By definition then, an hypothesis among his basic taxa. This leads him to conclude (Car- of relationship must be established in order to support roll, 1982:104) that "most of the derived character a claim of convergence. As Gaffney (1979) aptly states states recognized in two or more of the derived groups the case, "An argument for parallelism or convergence must be attributed to convergence." Thus, an impor- is simply an argument (often unstated) in favor of one tant assumption of the phylogenetic method seems to phylogeny over an alternative one." Carroll (1982) pro- be violated. vides no support for his alternative hypothesis of re- We certainly agree that rampant convergence can lationships. He presents no characters indicating that lead the systematist to incorrect phylogenetic conclu- any particular "protorothyrid" shared a more recent sions. However, if Carroll's scenario about early am- common ancestor with one of his "derived" groups niote evolution is correct and his prior assumptions than with other "protorothyrids." Such evidence would about basic taxa are accepted, then he has chosen to be necessary to support his claim that 1) these "de- discuss only those characters that must be convergent rived" groups were actually "derived from protoro- (see Fig. 1). In Carroll's scenario (p. 106), the time span thyrids," and 2) that any particular derived characters from the Early Pennsylvanian to the Early Permian shared by two or more of these "derived" groups (but saw the appearance of "a succession of derived groups not by "protorothyrids") are therefore convergent. that may all be traced to the protorothyrid stock." Given that Carroll's hypothesis about early amniote Therefore, any true synapomorphies evidencing a close phylogeny is true, then all of the derived characters relationship among two or more of these "derived" shared by two or more of his derivative (non-proto- 604 1984 by The Society of Vertebrate Paleontology rothyrid) taxa are necessarily convergent. It is also pos- sible, however, that Carroll's phylogeny is incorrect and that at least some of the derived characters shared by two or more of the other amniote taxa are true synapomorphies, characters shared because they were inherited from a common ancestor with a similar con- dition. Carroll dismisses this possibility, presumably because he doubts the significance of similar derived features shared by groups with different adaptive pat- terns in demonstrating a close common ancestry (see below). Such a superficial dismissal of characters seems to us to be ill-founded. Possible relationships suggested by shared, derived features are often inappropriately discounted when the derived features in question are thought to be adaptive for a particular lifestyle. It is argued that such features are not phylogenetically meaningful because they are rotorothyridae" predisposed to convergence. We do not deny the pos- sibility of convergence; however, it is not logical to conclude summarily that such shared, adaptive fea- tures are convergent and, therefore, that the suggested FIGURE 1. Phylogenetic diagram depicting derivation of relationships do not exist. Because a phylogeny is re- various "derived" amniote groups (shaded bubbles A-E) from quired to demonstrate convergence in a particular an "ancestral Protorothyridae" (unshaded bubble) as hy- character, the synapomorphic nature of such a shared, pothesized by Carroll (1982). Only derived characters that derived feature and the relationships suggested by it arose within "Protorothyridae" (bars with asterisks) can pro- cannot be dismissed until other derived features are vide evidence for a close relationship between two or more found to evidence different relationships, that is until of the "derived" lineages. Nevertheless, Carroll (1982) dis- an alternative phylogeny is proposed and supported. cusses only derived characters not present in any protoro- Many of Carroll's arguments are strongly tied to the thyrid (bars without asterisks). Given these relationships, any concept of adaptation, particularly to the concepts of such characters shared by two or more of the groups A-E the adaptive zone and of adaptive radiation (see also are necessarily convergent. Van Valen, 1971, 1978). For example, Carroll (1982: 104-105, italics in original) states: ory of evolution predicts that living things form a hi- "A distinction should be made in the relative sig- erarchy of groups within groups. Because evolutionary nificance of similar derived character states within descent is accompanied by the divergence of lineages, groups that share a common adaptive and structural the more inclusive the group, the less likely that its pattern, and between groups that have adapted to members will be very similar in adaptation. This does different ways of life. not, however, preclude the possibility of recognizing The presence of shared derived characters can be more inclusive groups. Carroll certainly recognizes the useful in recognizing members oí particular groups. group Amniota, despite the very different adaptive . On the other hand, similar derived characters zones occupied by its component subgroups. We pre- present in groups with fundamentally different adap- dict that groups less inclusive than Amniota, but con- tive patterns may be of less significance in demon- taining two or more of Carroll's basic taxa, will even- strating a close common ancestry." tually be recognized. Some progress is already being made along these lines (GafFney and McKenna, 1979; We find this approach undesirable because it is so Reisz, 1981). dependent on subjective initial decisions of what con- Another perplexing aspect of the passage quoted stitutes an adaptively unified group. For example, the above is Carroll's seeming implication that one can "pelycosaurs" are said to have contained both large discover little more about relationships than the mem- and small carnivores and two major groups of herbi- bership of groups already assumed to exist. Carroll's vores (Carroll, 1982:93)•groups seemingly very dif- statement that shared, derived characters are useful for ferent in adaptation. Had Carroll followed his own recognizing the members of particular groups is logi- reasoning and recognized these adaptive types as his cal•provided that the groups are already character- basic taxa, he would never have been able to conclude ized. Carroll's examples show
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