Paleobiology, 27(1), 2001, pp. 1–6
Snake origins and the need for scientific agreement on vernacular names Michael S. Y. Lee Department of Zoology, University of Queensland, Saint Lucia, Brisbane, QLD 4072, Australia. E-mail: firstname.lastname@example.org Accepted:
31 August 2000
When transitional fossils blur previously clear boundaries between major groups, drawing that line can be problematic. While the need to rigorously define formal taxon names is widely acknowledged (e.g., de Queiroz and Gauthier 1992; Cantino et al. 1997), the need for similar precision in the scientific use of vernacular terms is not widely appreciated. The lack of accepted usages for common names has engendered endless arguments about when a fish becomes (also?) a tetrapod (e.g., Clack 1997), when a dinosaur becomes (also?) a bird (e.g., Padian and Chiappe 1998), and when a mammal-like reptile becomes (also?) a mammal (e.g., Rowe and Gauthier 1992). Recent descriptions of fossil snakes with well-developed hindlimbs have raised similar questions about where to draw the line between lizards and snakes and initiated a lively debate over the origin of snakes. However, lack of a precise definition of the vernacular term ‘‘snake,’’ as well as lack of a consensus on what constitutes a higher taxon’s ‘‘origins,’’ has seriously hindered discussion of ‘‘snake origins.’’ Here, precise definitions of both terms are proposed and justified and their paleobiological implications discussed. The origin of higher taxa remains one of the most intriguing macroevolutionary problems, but scientists risk arguing at cross-purposes unless they agree on the exact boundaries of vernacular groups, and the exact meaning of the term ‘‘origin.’’ Some recent descriptions of Cretaceous limbed marine snakes (‘‘pachyophiids’’) considered them the sister group to all other (modern) snakes, and thus transitional beq 2001 The Paleontological Society. All rights reserved.
tween lizards and snakes. The difficulties defining snakes and thus snake origins (Lee and Caldwell 1998; Greene and Cundall 2000) have been debated in the context of this phylogeny, even though the latter authors are sympathetic to the alternative view that pachyophiids are not transitional forms but are derived snakes that have re-evolved limbs (Tchernov et al. 2000). However, even if this is the case, the same conceptual problems will arise if Dinilysia and madtsoiids (Scanlon and Lee 2000) or any other fossils are proposed to occupy this basal position. Pachyophiids were considered transitional between lizards and snakes (Lee and Caldwell 1998) because they exhibited most of the advanced features of snakes (such as highly kinetic skull, enclosed braincase, loss of external ear, elongate and almost limbless body) but retained a largely complete (albeit tiny) pelvis and hindlimb. Therefore, the vernacular term ‘‘snake’’ might be applied in two places (Fig. 1). There is almost universal agreement that the short-bodied, four-limbed mosasauroids and Adriosaurus are ‘‘lizards,’’ and that the elongated and nearly (or totally) limbless scolecophidians and alethinophidians are true ‘‘snakes.’’ To remain consistent with accepted usage, the vernacular term ‘‘snakes’’ should be applied to some group that includes scolecophidians and alethinophidians but excludes mosasauroids and Adriosaurus. This leaves two places in the tree where the term can be applied—for the clade including only scolecophidians and alethinophidians or for the slightly more inclusive clade that also includes pachyophiids. 0094-8373/01/2701-0001/$1.00
MICHAEL S. Y. LEE
FIGURE 1. A proposed phylogeny of snakes and their nearest lizard relatives, based on Lee and Caldwell (2000). ‘‘1 ’’ denotes extinct taxa. The two possible places to apply the vernacular term ‘‘snakes’’ are shown. Representative synapomorphies of each clade are as follows; note that the most significant and numerous changes occur along branch ‘‘C.’’ A1, Mobile intramandibular and symphysial joints. B1, Supratemporal positioned dorsal to parietal. C1, Enclosed braincase. C2, Loss of external ear. C3, Mobile maxillary-premaxillary joint. C4, Elongated body with over 120 presacral vertebrae. C5, Loss of shoulder girdle and forelimb. C6, Reduction of pelvic girdle and hindlimb. D1, Loss of jugal. D2, Loss of pes.
The latter, more inclusive definition was adopted (Lee and Caldwell 1998), since pachyophiids already exhibit almost all of the features traditionally considered ‘‘diagnostic’’ of snakes. A phylogenetic analysis revealed that most of the features separating snakes from ‘‘lizards’’ evolved at the base of the pachyophiid-scolecophidian-alethinophidian clade, and only a few additional ones evolved at the base of the scolecophidian-alethinophidian clade (Fig. 1). Earlier studies were also instructive in this respect. Estes and colleagues (Estes et al. 1988) presented a detailed list of diagnostic characters of ‘‘snakes’’ in 1988 and thus summarized the traits ‘‘traditionally’’ considered to separate snakes from lizards immediately before the problematic intermediate forms were identified. Of the 50 ‘‘snake’’ characters, pachyophiids can be scored for 24: they exhibit the snake condition in 21 but the lizard condition in only 3. Similar results are also obtained if less-scientific character lists are consulted: for example, standard detailed dictionaries (e.g., Gove 1976) list as snake characters traits such as a highly kinetic skull, enclosed braincase, loss of external ear, elongate almost limbless body—all of which are found in pachyophiids. Thus, the great major-
ity of the characters traditionally used to separate lizards and snakes, in both the scientific and popular literature, placed pachyophiids on the snake side of the divide. We further confirmed this conclusion by showing pictures of varanids, mosasauroids, pachyophiids, scolecophidians, and alethinophidians to a cross-section of scientists and laypeople, and asking them what forms they considered ‘‘lizards’’ and what they considered ‘‘snakes.’’ All identified varanids and mosasauroids as lizards, and pachyophiids, scolecophidians, and alethinophidians as snakes. The understanding of ‘‘snake’’ held by most people (‘‘traditional usage’’) therefore appears to include pachyophiids. Thus, a phylogenetic analysis demonstrated that pachyophiids had evolved nearly all the features usually considered diagnostic of snakes. Also, an inspection of ‘‘snake’’ character lists revealed that pachyophiids possessed the majority of these traits, and most people intuitively identified pachyophiids as snakes. Pachyophiids were thus clearly more than halfway along the branch leading from ‘‘lizards’’ to ‘‘snakes,’’ and should be considered primitive snakes rather than advanced (snakelike) lizards. Accordingly, the more in-
clusive definition of ‘‘snake’’ was adopted. Despite this detailed justification (see Lee and Caldwell 1998), Greene and Cundall (2000: p. 1940) state that we ‘‘dodge the crucial question of the point on the phylogenetic tree stem connecting mosasaurs and Pachyrhachis where a fossil becomes more snake-like than lizardlike,’’ and furthermore that we ‘‘arbitrarily define ‘‘snake’’ so that Pachyrhachis must be one.’’ The above, I hope, shows that our preferred definition of ‘‘snake’’ was not quite so capricious. In contrast to the above interpretation of the vernacular term ‘‘snakes,’’ Greene and Cundall (2000) adopt a crown-group definition (Gauthier 1986; Rowe 1988; de Queiroz and Gauthier 1992; Bryant 1994), i.e., restricting it to the most recent common ancestor of living snakes and all its descendants. This would attach ‘‘snakes’’ to the scolecophidian-alethinophidian clade, termed ‘‘modern snakes’’ by Lee and Caldwell (1998; see also Scanlon and Lee 2000). However, even for formal taxon names like Tetrapoda, Mammalia, and Aves, the use of such restricted crown-group definitions is questionable. The contents of crowngroup taxa are often much more restricted than traditional concepts. Also, employing living forms as landmarks (reference taxa) for drawing taxon boundaries does not circumvent problematic fossils, which still might fall inside or outside the crown group (e.g., Lee 1996; Padian and Chiappe 1998; Sereno 1999). Indeed, in the current example, a crown-clade definition of Ophidia (snakes) is more unstable than the more traditional definition advocated here. Ophidia (crown-group) might or might not include pachyophiids, depending on which phylogenetic position is accepted (basal or macrostomatan), whereas Ophidia (traditional) always includes pachyophiids, regardless of which of these two positions is adopted. For vernacular terms, such restricted crown-group definitions are clearly inappropriate—for instance, applying crown-group interpretations would mean that Ichthyostega is not a tetrapod, Morganucodon is not a mammal, and Archaeopteryx is not a bird. All these extinct forms have evolved most of the diagnostic features of their respective groups (e.g.,
dactylous limbs, dentary-squamosal jaw joint, and powered flight, respectively) yet lie outside the diversity of living forms. Greene (1997) advocates such a crown-group definition of ‘‘snakes’’ in a magnificent book intended for a general audience, suggesting that he interprets and/or wishes this to be the popular or traditional definition. Greene and Cundall (2000) indeed state this (p. 1940), though obliquely: ‘‘the traditional version [of snake origins] is based on a narrower definition of the word snake.’’ However, this proposal is not substantiated in that paper or elsewhere. In contrast, the above discussion shows that traditional definitions of snakes implicitly included pachyophiids—most of the characters diagnostic of snakes are present in pachyophiids, and most people identify them as snakes. Greene and Cundall (2000) further argue the ‘‘survey’’ approach is flawed, by mentioning (correctly) that many laypeople would also (mis)identify as ‘‘snakes’’ anguids, pygopodids, amphisbaenians, caecilians, eels, and other assorted elongate limb-reduced vertebrates. However, this objection rather misses the point. Surely, the question must be, if presented with the relevant taxa (those that constitute the snake stem lineage), where would most people draw the line? Showing people confusing convergent taxa (which no modern biologist would advocate including in ‘‘snakes’’ anyway) would have represented an irrelevant distraction. By analogy, if one is interested in whether the term ‘‘birds’’ is more widely understood to apply to a group including or excluding Archaeopteryx, one would presumably show people an assortment of (non-avian) theropod dinosaurs, Archaeopteryx, and an assortment of modern birds. If most people identify Archaeopteryx as well as modern birds as ‘‘birds,’’ this would demonstrate the term is usually understood to apply to this more inclusive group. To minimize confusion, therefore, one should thus advocate that the vernacular term ‘‘birds’’ be used for such a group. The possibility that some laypeople might also identify bats or pterosaurs as ‘‘birds’’ is not particularly relevant. The general public, as well as scientists, will use vernacular terms, and the general public out-
MICHAEL S. Y. LEE
FIGURE 2. Illustration of how characters on the single branch leading to modern (crown-group) snakes might have evolved in a marine ancestor, even though the latest common ancestor of modern snakes was undoubtedly terrestrial. ‘‘1’’ denotes extinct taxa. Black lines indicate marine lineages, stippled lines indicate terrestrial lineages.
numbers scientists by several orders of magnitude. Scientists should attempt to make their definition and usage of vernacular names as similar as possible to those adopted by the majority of users, while still maintaining the necessary scientific rigor (e.g., for taxonomic groups, monophyly). Thus, ‘‘snakes’’ should include modern (crown-group) snakes and also those forms along the snake stem lineage that the majority of people identify as snakes, but to maintain monophyly should exclude amphisbaenians, caecilians, and other unrelated limbless forms regardless of public opinion. Similarly, ‘‘birds’’ should include crowngroup birds and forms along the bird stem lineage that the majority of people identify as birds, but should exclude pterosaurs, bats, and other flying forms. Many vernacular names are usually considered synonymous with particular formal names—for instance, ‘‘birds’’ is equated with Aves, ‘‘mammals’’ with Mammalia, and ‘‘snakes’’ with Ophidia or Serpentes. If the above arguments for defining vernacular names are accepted, then to minimize confusion any tightly associated formal names should also be defined to refer to the same clade. Alternative solutions are likely to be very confusing. Redefining both the vernac-
ular and formal names to refer to the crown group can mean that both terms are given new and unfamiliar meanings, while continuing to use the vernacular name to refer to the ‘‘traditional’’ clade and but redefining the formal name to refer to the crown group would mean that previously synonymous terms are no longer equivalent (e.g., Archaeopteryx would be a ‘‘bird’’ but not belong to Aves). The suggestion of a marine origin for snakes was based on the inference (based on parsimony) that the snake stem lineage underwent a marine phase (Fig. 2), during which it acquired most of the features usually interpreted as ‘‘diagnostic’’ of snakes, such as a long limb-reduced body and highly flexible skull (Scanlon et al. 1999; Lee and Caldwell 2000) (Fig. 1). In contrast, a new definition of snake origins proposed by Greene and Cundall considers this aquatic phase irrelevant. These authors first restrict the term ‘‘snakes’’ to modern (crown-group) snakes. Then, they suggest that the origin of ‘‘snakes’’ consists of the (few) changes that occurred at the single branch leading to that clade (branch D in Fig. 1). As the latest common ancestor of the crown-clade was terrestrial, they further suggest, snakes can be considered to have had exclusively terrestrial origins—regardless of any
earlier marine interlude during which far more substantial changes toward a snake morphology occurred. However, this view appears problematic for several reasons. First, the origin of the snake bodyplan, like that of any higher taxon, cannot be attached to any single branch—these changes (and thus snake origins) must have occurred gradually and cumulatively over multiple branches on the snake stem lineage (e.g., Greene 1983; Cundall 1995; Kardong et al. 1997). If some of these branches were aquatic, this would by definition imply an aquatic phase in snake origins. Focusing only on a single node, and ignoring all previous and subsequent changes, amounts to a semantic sleight of hand. For instance, it would be analogous to defining ‘‘birds’’ to refer to modern (crown-group) birds, and ‘‘bird origins’’ to refer only to the changes separating crown-group birds from Archaeopteryx and similar taxa. Since feathers and flight evolved earlier on the cladogram, it could then be argued that feathers and flight had nothing whatsoever to do with bird origins. However, Greene and Cundall are justified in suggesting that there must be a ‘‘lower limit’’ on where one considers the ‘‘origin’’ of a taxon to begin. For instance, given that the features diagnostic of modern snakes accumulated gradually from the lineage leading from the first living organism to the ancestor of crown-group snakes, where should we consider ‘‘snake origins’’ to begin—and end for that matter? My proposal is this: The origin of an extant taxon should be considered to be the evolutionary changes that accumulated in the stem lineage of a group between the time it diverged from its nearest extant sister group and the time crown-group forms radiated. Thus, under this definition, ‘‘snake origins’’ consists of the changes occurring along branches A to D of Figure 1, i.e., the changes that occurred between the common ancestor of thecoglossans (varanid lizards and snakes) and the latest ancestor of crown-group snakes. This definition implicates in snake origins traits that evolved slightly before, coincident with, and slightly after the precise taxonomic boundary between lizards and snakes (defined above to correspond with traditional views). However, the definition excludes traits
which occurred earlier or later along the snake stem lineage from being considered part of snake origins (depending on their exact position along the stem lineage such changes would instead be implicated in, for example, thecoglossan origins, or alethinophidian origins). It thus strikes a balance between ‘‘telescoping’’ snake origins deep down into the reptile tree (which Greene and Cundall rightly criticize) and restricting it to changes occurring along a single branch (the overly narrow interpretation they advocate). Similarly, under the definition here proposed, ‘‘mammal origins’’ would consist of all the changes which occurred between the latest ancestor of amniotes (mammals plus reptiles) and the ancestor of crown-group mammals, and ‘‘bird origins’’ would consists of all the changes which occurred between the latest ancestor of archosaurs (birds plus crocodilians) and the ancestor of crown-group birds. This coincides quite closely with the changes discussed in many reviews of mammal and bird origins (e.g., Kemp 1982; Padian and Chiappe 1998) and thus previous implicit ideas about what constitutes ‘‘origins.’’ The second problem with Greene and Cundall’s interpretation of ‘‘snake origins’’ is that even if one chooses to identify a single branch as representing ‘‘the’’ origin of snakes, this should (arguably) be the branch during which the most numerous and significant changes from ‘‘lizardness’’ toward ‘‘snakeness’’ occurred. This, by most objective criteria (Fig. 1 caption), would be the branch leading to the pachyophiid-modern snake clade, rather than the branch Greene and Cundall prefer (the one leading to modern snakes alone). Finally, even if one accepts the suggestion that ‘‘snake origins’’ be considered synonymous with a particular branch, and further accepts that it should be the branch leading to modern snakes, these changes still might have occurred in a marine context. If one accepts the scenario that marine habits were primitive for the mososaur-snake clade and that terrestriality re-evolved in the branch leading to modern snakes (Lee and Caldwell 2000)—which Greene and Cundall acknowledge is possible—then a reversion to terrestriality occurred somewhere along this branch. The corollary is
MICHAEL S. Y. LEE
that at least a portion of this lineage—the branch considered synonymous with ‘‘snake origins’’—must have been marine. If the morphological changes occurred mostly before the ecological reversion to terrestriality, then most of ‘‘snake origins’’ would have occurred in a marine context (Fig. 2). Thus, even if one accepts Greene and Cundall’s restricted (crown-group) definition of the term ‘‘snakes,’’ and that snake origins be restricted to a single branch, and that this branch should be the one leading to crown-group snakes, snakes might have had marine origins In conclusion, lack of precise definitions of vernacular names can severely hinder debate on the origins of major groups. However, there is no clear consensus on the precise meanings of many, if not most, vernacular terms (e.g., tetrapod, amphibian, reptile, bird, or mammal). Various solutions are possible. One is to avoid the use of such terms when there is any possible ambiguity, such as when transitional fossils are being discussed. The alternative and perhaps more desirable solution is to try and reach some agreement, by adopting a scientifically acceptable (monophyletic) definition that is as similar as possible to the existing interpretation used by the majority of people (both scientists and laypeople). However, there will be cases where such an agreement cannot be reached, and in such cases the intended meaning should be explicitly noted every time these terms are used. In the current example, there are clear reasons to apply the vernacular term ‘‘snakes’’ to embrace not just modern snakes, but also recently described fossil limbed forms (pachyophiids). Also, there is no generally accepted definition of what constitutes a group’s ‘‘origins,’’ and very broad and very narrow interpretations are possible. An explicitly phylogenetic definition of ‘‘origins’’ is proposed here: The origin of a group consists of the changes that accumulated along its stem lineage between its divergence from its nearest living sister group and the radiation of extant (crown-group) forms. Acknowledgments I thank H. Greene and D. Cundall for discussion and encouragement despite our dis-
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