The Phylogenetic Definition of Reptilia

Naming taxa is an important endeavor in the documentation of life by systematists, whether it is conducted in the context of traditional rank-based classification or within a phylogenetic framework. Proponents of the phylogenetic approach distinguish between the diagnosis of a group and its definition (Ghiselin, 1984; Rowe, 1987, 1988), and this distinction forms the basis for a phylogenetically based method of naming taxa formerly referred to as Phylogenetic Taxonomy (de Queiroz and Gauthier, 1990, 1992, 1994) and now called Phylogenetic Nomenclature (PN; Cantino et al., 1999; Gauthier and de Queiroz, 2001; Bryant and Cantino, 2002). Emphasis in naming has been placed on ancestry using phylogenetic definitions, and the widespread adoption of node- and stem-based definitions (apomorphy-based definitions have yet to receive as widespread acceptance, but see Gauthier and de Queiroz, 2001; Anderson, 2002; Laurin and Anderson, 2004) has led to a proliferation of new names and definitions. This shift in nomenclatural practice has, unfortunately, fostered a growth in redundant names and definitions for well-known taxa (Benton, 2000). The PhyloCode (Cantino and de Queiroz, 2003) has modified the rule of priority as used in other codes (i.e., International Code of Zoological Nomenclature) to determine which of two or more possible names with equivalent definitions is valid (Brochu and Sumrall, 2001), or which of several definitions for a given name is valid (Cantino and de Queiroz, 2003). Unfortunately, it is now apparent that some of the definitions for well-known taxon names established early in the emergence of PN were not devised following conventions now widely accepted, by either defining groups in an overly restrictive manner, or via selection of reference taxa without due consideration of the ramifications of differing tree topologies (Anderson, 2002; Laurin and Anderson, 2004). It has become evident in broad-scale amniote taxonomy that the first published definition for Reptilia (Gauthier et al., 1988a), which would have priority under a binding PhyloCode, is problematic because of the dramatic controversies over the affinities of the specifier taxon Testudines (see Zardoya and Meyer, 2001 for review of hypotheses for turtle relationships). Recent morphological and molecular studies have challenged conventional hypotheses concerning the affinities of turtles, and this has led to unexpected and undocumented changes in the composition of the well-known taxon Reptilia, with additional ramifications for the nomenclature of some of its included taxa. We examine the consequences of the application of priority to the nomen Reptilia as our understanding of early amniote interrelationships has progressed over the past two decades, and offer a new definition that brings the phylogenetic concept of this taxon name into line with both currently accepted conventions of PN and historical usage. This new definition corrects an error created by the combination of the selection of a higher taxon (rather than a species) as a specifier, and an unexpected topology. We believe that now is an appropriate time to examine the definitions established when PN was in its earliest stages, and hope to correct what we consider to be a poorly formulated definition upon publication of a binding PhyloCode. To understand the motivations of some workers and their formation of phylogenetic definitions for the nomen Reptilia, it is important to review the history of this taxon. Reptilia was erected originally as a Class by Laurenti (1768) for the inclusion of tetrapods that were neither mammals nor birds. Amphibians were included in Reptilia by Owen (1854, 1859) and other authors, but following Haeckel's (1866) work that demonstrated reptiles, birds, and mammals shared a common reproductive strategy (the amniotic egg), frogs, salamanders, and caecilians were placed unanimously in a separate taxon, Linnaeus' (1758) Amphibia. Hence, reptiles were primarily distinguished from birds and mammals by poikilothermy and lack of integumentary features such as hair and feathers (e.g., Zittel, 1902), a concept that lasted well into the succeeding century. Fossil tetrapods that satisfied this diagnosis were placed within Reptilia. However, some of the first fossils assigned to Reptilia were recognized by early workers to have conspicuously mammalian characteristics (Owen, 1860, 1884; Cope, 1878, 1898). Thus, species of the groups Pelycosauria, Dinocephalia, Dicynodontia, and Theriodontia came to be known colloquially as “mammal-like reptiles,” and were eventually placed formally within their own taxon, Synapsida, which was established as a Subclass of Reptilia (Osborn, 1903). It was recognized early that synapsids were related more closely to mammals than to other fossil taxa included within Reptilia (e.g., Baur, 1887; Broom, 1914), but most early systematists had no objection to a paraphyletic Reptilia. This view became entrenched as the classic hypothesis that explained the origin of mammals from reptiles (Romer and Price, 1940; Romer, 1966). During the time that paleontologists were beginning to distinguish fossil synapsids from unequivocal reptiles, Haeckel (1866) distinguished reptiles, birds, and mammals from other tetrapods (amphibians) by placing them in a new taxon that he called Amniota, which was ignored as a taxon name (although the concept was accepted) in many subsequent published classifications (e.g., Zittel, 1902; Piveteau, 1952; Romer, 1966). A more testable picture of amniote interrelationships appeared following the adoption of phylogenetic systematics (Gaffney, 1980; Reisz, 1981; Gardiner, 1982; Heaton and Reisz, 1986; Gauthier et al., 1988a, 1988b). The emerging consensus (Gauthier et al., 1988a; Laurin and Reisz, 1995) divided amniotes into two main groups: synapsids on one hand, and all remaining amniotes on the other (Fig. 1a and b). Synapsida was defined as a stem-based group with Mammalia as its crown (Rowe, 1988), whereas Reptilia was defined as a node-based group using extant turtles, snakes, lizards, and crocodiles as reference taxa (Gauthier et al., 1988a). Gauthier et al. (1988a) recognized a sister-group relationship between turtles and captorhinids (Fig. 1a), a relationship identified formally as Anapsida, which they defined as “extant turtles and all other extinct taxa that are more closely related to them than they are to other reptiles.” In the Gauthier et al. (1988a) tree, Anapsida is the sister group of a clade formed by the Carboniferous taxon Paleothyris and the diapsid groups Araeoscelidia and Sauria (crowngroup diapsids, sensuGauthier, 1984). The crownbased concept for Reptilia by definition omitted several Permian and Triassic taxa that, since their respective discoveries during the late 19th and early-mid 20th centuries, were regarded to be basal reptiles (mesosaurids, millerettids, procolophonians, and pareiasaurs). These taxa together formed a clade (informally designated “parareptiles”) that Gauthier et al. (1988a) identified as the sister taxon of Reptilia. Cladograms showing the progression of ideas concerning amniote phylogeny and nomenclature that are in line with traditional ideas for reptile interrelationships (i.e., Diapsida does not include Testudines). (a) Gauthier et al. (1988a). (b) Laurin and Reisz (1995). (c) Modesto (1999). The nomenclature shown employs a strict priority among the phylogenetic definitions for taxon names. Arrows indicate taxon names for clades. The Gauthier et al. (1988a) phylogeny was the principal reference for amniote interrelationships and taxonomy until Laurin and Reisz (1995) identified turtles as the sister taxon of procolophonid “parareptiles.” The Laurin and Reisz (1995) results expanded the content of the reptilian crown group by incorporating “parareptiles,” and thereby produced a topology in which Reptilia was divisible into turtles and their fossil relatives on one side and diapsids and their close relatives on the other (Fig. 1b). Laurin and Reisz (1995) bestowed Olson's (1947) names Parareptilia and Eureptilia on the former and the latter groupings, respectively, as stem-based taxa. The other major departure from the work of Gauthier et al. (1988a) is that Laurin and Reisz (1995) found no support for a relationship between Mesosauridae, a group that comprises the oldest known fully aquatic amniotes, and the other “parareptiles.” Mesosauridae formed a clade with the augmented reptilian crown, a grouping that was given Huxley's (1864) rarely used nomen Sauropsida (Gauthier, 1994; Laurin and Reisz, 1995). Gauthier (1994:137), citing “Laurin and Reisz (in press),” defined Sauropsida in stem-based fashion as “reptiles plus all other amniotes more closely related to them than they are to mammals,” whereas Laurin and Reisz (1995:180) themselves defined this taxon name as a node-based group, “the last common ancestor of mesosaurs, testudines and diapsids, and all its descendents.” The Laurin and Reisz (1995) analysis supplanted that of Gauthier et al. (1988a) as the standard view of early amniote phylogeny. It was slightly modified by Modesto (1999) who, in work stimulated by new data on mesosaur anatomy, placed mesosaurs back in a clade with millerettids, pareiasaurs, and procolophonids (Fig. 1c), thus recovering the original content of Gauthier et al.'s (1988a) “parareptiles.” Reptilia and Sauropsida, sensuGauthier et al. (1988a) and Gauthier (1994), respectively, shared the exact same content (although not the same definitions), so Reptilia was recognized by Modesto (1999) as the senior synonym because it was the older, more established name. Because Parareptilia, as defined by Laurin and Reisz (1995), was equated in both definition and content with Anapsida (sensuGauthier et al., 1988a) by Modesto (1999), deBraga and Reisz's (1996) definition of Parareptilia (“the most recent common ancestor of millerettids, Acleistorhinus, lanthanosuchids, Macroleter, Procolophonia, and all its descendents”) was recognized as the valid definition (rendering Parareptilia a subclade of Anapsida). Anapsida, which in traditional rank-based classifications grouped several early amniote groups together on the basis of a plesiomorphy (the absence of temporal fenestrae), is an unfortunate name because it epitomizes a paraphyletic group to most students of early amniote phylogeny. Despite its phylogenetic “reinterpretation” by Gauthier et al. (1988a), this nomen has received little support in recent phylogenetic studies, as evidenced by the observation that the clade of mesosaurs and parareptiles (sensudeBraga and Reisz, 1996) is either left unnamed on cladograms (e.g., Berman et al., 2000: Fig. 4) or Mesosauridae is simply pruned from the amniote tree, thereby circumventing the problem of Anapsida (e.g., Zardoya and Meyer, 2001: Fig. 3). Reisz and Scott (2002) did use the term Anapsida for the clade of mesosaurs, millerettids, pareiasaurs, and procolophonids, but these authors did not include turtles in their analysis because of the ongoing controversy over turtle relationships. Reisz and Scott (2002) therefore appear to have associated the nomen Anapsida with the content of the group that was recognized by Modesto (1999), rather than with the definition of the nomen created by Gauthier et al. (1988a) insofar as the specifier taxon “Testudines” was intentionally omitted from consideration. Amniote phylogeny illustrating revised nomenclature assuming diapsid identity of Testudines as hypothesized by Zardoya and Meyer (1998), Hedges and Poling (1999), and Rieppel and Reisz (1999), employing the revised phylogenetic definition for Reptilia suggested in the text. Arrows indicate taxon names for clades. Sources for skeletal reconstructions, from top to bottom: Dimetrodon from Romer and Price (1940); Procolophon from deBraga (2003) with permission from the National Research Council of Canada; Scutosaurus modified from Lee (1997); Mesosaurus original reconstruction by S. P. Modesto; Captorhinus (= Eocaptorhinus) from Heaton and Reisz (1980) with permission from The Paleontological Society; Paleothyris from Carroll (1969) with permission from The Paleontological Society; Araeoscelis reproduced from Reisz et al. (1984) with permission from R. R. Reisz and the Society of Vertebrate Paleontology; and Hesperornis from Marsh (1880). The amniote phylogenies of Gauthier et al. (1988a) and Laurin and Reisz (1995) may be thought of as most closely reflecting historical views of amniote classification in that turtles represent a lineage distinct from that comprising other living reptiles. In traditional rank-based classifications, turtles were grouped with several early reptile groups within Anapsida (Williston, 1917; Romer, 1966; Carroll, 1988). However, recent morphological and molecular studies suggest that turtles are instead diapsid reptiles, variously identified as lepidosauromorphs (lizards, snakes, tuataras, and their fossil relatives; Rieppel and deBraga, 1996; deBraga and Rieppel, 1997), or archosauromorphs (crocodiles, birds, and their fossil relatives; Hedges and Poling, 1999; Kumazawa and Nishida, 1999; Zardoya and Meyer, 1998, 2001). We will not go into further detail about these competing hypotheses for the diapsid affinities of turtles because they are beyond the scope of the present work, and the reader is referred to Zardoya and Meyer (2001) and Lee (2001) for detailed reviews and treatments of these hypotheses. If the hypothesis that turtles are diapsid reptiles becomes the consensus view among systematists, it creates problems for the nomenclature of the reptilian side of the amniote tree. Gauthier et al. (1988a: 142) defined Reptilia as “the most recent common ancestor of extant turtles and saurians, and all its descendents.” This has been interpreted to mean Reptilia can be regarded to be a crown group (Laurin and Reisz, 1995). However, if a diapsid identity for turtles is accepted, the Gauthier et al. (1988a) definition renders Reptilia and Sauria (sensuGauthier, 1984) as competing names for the exact same clade. According to priority by date of first publication of a name, the former nomen should be recognized as the senior synonym, although the phylogenetic definition of Sauria has priority over that of Reptilia. It seems apparent to us that no systematist would regard the name “Sauria” to have priority over the older, more widely used name “Reptilia.” Of greater concern is the fact that, if turtles are saurians, Gauthier et al.'s (1988a) definition for Reptilia is redundant, because it uses Sauria and a saurian group (turtles) as specifiers. Questions of historical continuity aside, the definition for Reptilia must be emended because of its now circular construction. Unfortunately, definitions of Reptilia published subsequent to Gauthier et al. (1988a) offer no viable alternative. The definition provided by Laurin and Reisz (1995: 183, “the most common ancestor of testudines and diapsids, and all its descendents”) and deBraga and Rieppel (1997: 228, “the most common ancestor of diapsids and all its descendents”), renders Reptilia synonymous with Diapsida if turtles are nested within Diapsida. The deBraga and Rieppel (1997) definition is clearly a lapsus calami, because the content outlined in the definition is not consonant with the placement of Reptilia on their tree; their definition of Reptilia can be rejected for this reason. The definition of Laurin and Reisz (1995) suffers the problem of internested specifiers if it is applied to phylogenies in which turtles are diapsids; this includes most recent studies of reptile interrelationships (deBraga and Rieppel, 1997; Zardoya and Meyer, 1998; Hedges and Poling, 1999; Kumazawa and Nishida, 1999; Rieppel and Reisz, 1999). Lee (2001) is the only recent worker who espouses a parareptilian origin for turtles. Given the lack of consensus on the phylogenetic of turtles, a phylogenetic definition of Reptilia that is not to the of this specifier taxon is clearly A problem by a diapsid origin for turtles is that Anapsida (sensuGauthier et al., amniotes to turtles than to becomes a therefore definition or definition for Anapsida (turtles) and all other amniotes more closely related to them than they are to the same as Gauthier et al.'s (1988a) Despite its phylogenetic by Gauthier et al. (1988a), it is that Anapsida is as a name in of its as the name of a paraphyletic group of amniotes, and our observation that most workers the term with its morphological (the absence of temporal 2001: Fig. with Anapsida does not to other that have been recognized as paraphyletic in traditional classifications, such as and Reptilia, because it is only with Anapsida that a morphological is associated with the name. Amniote phylogeny illustrating revised nomenclature assuming the diapsid identity of Testudines as hypothesized by Zardoya and Meyer (1998), Hedges and Poling (1999), and Rieppel and Reisz (1999), and employing strict priority for phylogenetic definitions. The use of Reptilia the original crown group definition for the nomen (Gauthier et al., not the definition provided by Laurin and Reisz (1995). The name Reptilia is one of the oldest names in the history of and is known to both the only from the term and The phylogenetic definitions that have been devised for Reptilia (Gauthier et al., 1988a; Laurin and Reisz, deBraga and Rieppel, indicate a among systematists to it as a These phylogenetic definitions, are not for In the view that Reptilia, many established taxon should be defined as a crown group is not 1994; 1996; Lee and 1997; 1999; see Anderson, and Laurin and Anderson, for a more detailed The main for using definitions is that some workers believe such definitions are more than other phylogenetic definitions. such as Lee Lee and and have been in their to definitions for well-known names because they appear to have the on fossil taxa. several basal amniote groups pareiasaurs, millerettids, such as and recognized as reptiles, would be regarded as if one uses a definition for Reptilia in with phylogenies that turtles within extant Given that the for originally the name Reptilia to crown is not the becomes definition the historical of the taxon A Reptilia (e.g., Fig. does not to be the The of and (the “mammal-like of from Reptilia was accepted because these taxa clearly a lineage to separate from the other groups of amniotes Baur, However, it seems that the will be to an Reptilia, a of in which recognized can no be recognized as reptiles. This is to the controversy the definition for in which many workers the that is not a because it of a 1997; 2001; but see Gauthier and de Queiroz, 2001). that most workers are to a Reptilia is in recent cladograms that the interrelationships of basal amniotes and diapsid affinities for most Reptilia as the sister group of Synapsida (deBraga and Rieppel, Rieppel and Reisz, and 2001: Zardoya and Meyer, 2001: are two in which Sauropsida is used in or Reptilia. following (Benton, applied Sauropsida to the clade of Diapsida plus Sauropsida a crown taxon (Benton, In Sauropsida is a of the sister taxon to Synapsida, a clade “parareptiles”) that did not to have the phylogeny of Gauthier et al. (1988a), but used Sauropsida in to Reptilia. In Sauropsida and Synapsida as sister a that is not with the clade names in This is an because the phylogenies in both and both use Sauropsida as a crown whereas (1997) uses Sauropsida as a group, as Gauthier and Laurin and Reisz (1995). use of the name Sauropsida as the concept of this group in the 2004). Lee (2001) used the names Sauropsida and Reptilia in this to be due to the fact that Lee following deBraga and Rieppel Mesosauridae from consideration of reptilian which in the content of the stem-based nomen Sauropsida equated with that of the node-based Reptilia. Gauthier et al.'s (1988a) definition for Reptilia has due to the controversy over the phylogenetic of turtles within the sister taxon. traditional rank-based Testudines would be and it is that this in the placement of turtles with to and crocodiles would the of what is and what is not a reptile that has been by Gauthier et al.'s (1988a) outlined are not as a of the and practice of but as a of what time has to be an formed definition devised in the early of A definition for Reptilia that is to tree topologies is clearly one that the of traditional for Reptilia (e.g., does not with extinct taxa that consensus has to with and the emerging phylogenetic of reptilian nomenclature (e.g., the widespread of We offer a new stem-based definition for the most clade and Laurenti but not are used as specifiers to the of that the use of higher taxa led to the of the original following the of of the PhyloCode (Cantino and de Queiroz, 2003). We have used specifier taxa in to the application of the following the of of the PhyloCode. This definition the name Reptilia to the group that includes all of the sister group, of the interrelationships of turtles and other extant amniotes (Fig. 3). The new definition is more consonant with traditional classifications than the hypothesis for Reptilia, and with traditional taxonomy is by the PhyloCode (e.g., and Cantino and de Queiroz, 2003). new definition creates a nomenclatural with the well-known taxa Reptilia and Synapsida stem-based of a with the node-based our definition is equivalent to for Sauropsida plus all other amniotes more closely related to them than they are to mammals,” we believe that the use of the well-known Reptilia is to the more Sauropsida because it the traditional of the taxon as the concept has The of the rule of priority to phylogenetic definitions is a recent In the following the publication of Gauthier et al. (1988a) and the definitions for many taxon names found of amniote phylogeny have published their own definitions for the same and new taxa. This has led to a of definitions. Reptilia has definitions of which we are (Gauthier et al., 1988a; Gauthier, 1994; Laurin and Reisz, deBraga and Rieppel, 1997), Diapsida has (Gauthier et al., Gauthier, 1994; Laurin and Reisz, deBraga and Rieppel, 1997), and Parareptilia has (Laurin and Reisz, deBraga and Reisz, 1996; deBraga and Rieppel, In some authors to the definitions created in but to their own definitions without an as to definitions nor did authors for their definitions over the definitions. We that the adoption of the PhyloCode will this and hope it will to more definition systematists have the of PN on the that it traditional rank-based systematics and it with a that has led to of names for groups and of (Benton, 2002). are and to both and Cantino, 2002). We that the of names in PN is a problem because of the of but we if the of that is by taxon in traditional rank-based It is our that most of the of PN will be upon publication of the PhyloCode, by a standard for the and of definitions among all workers using This was in by a of to and a and Research Council of to We for on an early of the This was from the reviews of and an

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