The order Passeriformes was largely defined by 19th century anatomists on the basis of phenetic similarities. The purpose of this study was to determine whether the group is monophyletic in the strict contemporary sense. Similarities used by previous workers were reanalyzed to determine whether or not they could be shown to be derived character states corroborating the hypothesis of passeriform monophyly. Of 18 traditional taxonomic characters analyzed, none refuted the hypothesis of passeriform monophyly, 13 failed to corroborate the hypothesis, and 5 did corroborate it. These are the aegithognathous palate, the "passerine" tensor propatagialis brevis, the bundled spermatozoa with coiled head and large acrosome, the enlarged hallux, and the type VII deep plantar tendons. To this analysis is added new information from the hind limb musculature. Previous knowledge of oscine musculature is augmented by new information from representative suboscines. The division of M. pubo-ischio-femoralis into Pars cranialis and Pars caudalis is confirmed for suboscines, distinguishing the entire order Passeriformes from nonpasserine birds. In the foot, the loss of a set of intrinsic muscles of the forward digits, previously known from oscines, is also confirmed for suboscines. These limb-muscle characters supplement the analysis of traditional characters in corroborating the hypothesis that the order Passeriformes is monophyletic. A functional analysis of the passerine foot shows that it is a derived mechanism specialized for perching, but with a reduction in the subtlety and variety of certain other movements. Received 9 March 1981, accepted 3 August 1981.

Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 5260 USA I this paper I address the question of whether or not the avian order Passeriformes is monophyletic, using the concept of mono- phyly in its strict contemporary sense. I would define as monophyletic a group whose char- acteristics support most strongly the hypothe- sis that it consists of all the known descendents of a single common ancestor, that is, a group all of whose members share a more recent com- mon ancestor with each other than with any other taxa. In other words, the question is whether the order Passeriformes is a clade. There are four reasons why this question is important in the context of my research. First, passeriform monophyly has long been a tacit assumption based on a century-old definition of the assemblage as being "natural" or mono- phyletic in an old and vague sense at best. It is a phenetic cluster of groups traditionally united by various "similarities" without con- sideration of the nature of those similarities or of the sorts of information that different kinds of similarity can or cannot provide about phy- logenetic relationships. In recent years our concepts of monophyly and our methods of character analysis have been sufficiently re- fined that it is desirable now to reconsider the nature of the largest order of living birds. Eldredge and Cracraft (1980: 158) have noted that a recurrent theme in the history of classi- fication has been the elimination of nonmono- phyletic taxa. The fact that such apparently nonmonophyletic groups as "Pisces," "Reptil- ia," and "Carduelinae" have long persisted in classifications warns us not to assume that a group is monophyletic just because it has tra- ditionally been formally classified as a taxon. In our previous studies of the appendicular muscles, my students and I have questioned the monophyly of various traditional groups and have found that such hypotheses are sup- ported to different degrees. The family Drep- anididae is hardly definable as monophyletic except by a geographic argument (Raikow 1977b, 1978). Monophyly of the ploceid/estril- did complex remains uncertain, because no synapomorphies were found in the limb mus- cles to reinforce previous arguments (Bentz 1979). The shrikes (Laniidae) are only weakly defined as monophyletic (Raikow et al. 1980). Some support for monophyly of the Coracii- formes was found (Maurer and Raikow 1981), but it is not unequivocal. On the other hand, monophyly of the Piciformes is strongly cor- roborated (Swierczewski and Raikow 1981), as is that of the New World nine-primaried oscine assemblage when the Vireonidae are excluded (Raikow 1978). The evidence is likewise strong that the Atrichornithidae and Menuridae form a clade (Raikow MS). A second reason for reexamining passeri- form monophyly is that Feduccia (1975, 1977) recently proposed that the suboscines and os- cines together do not form a clade, but that the suboscines are instead part of a clade with sev- eral coraciiform families, which he termed the Alcediniformes. This conclusion followed from a study of the stapes or middle ear ossicle. Fed- uccia argued that oscines have a primitive stapes, while suboscines and Alcediniformes share a distinctive derived condition. Later Feduccia (1979) recanted this heresy and decid- ed that the suboscines and Alcediniformes do not form the clade originally postulated, but evolved their derived stapes independently. Feduccia (1979) also provided evidence from spermatozoan morphology that supports the idea of passeriform monophyly. Nevertheless, inasmuch as Feduccia did raise a serious ques- tion about passerine monophyly, and because his hypothesis and its subsequent retraction were based on a limited range of data, the mat- ter remains worthy of reconsideration. Third, the long-term goal of my research is to work out the phylogenetic relationships among passerine birds, using a cladistic anal- ysis mostly of the limb muscles as the method. Before undertaking such an analysis, I consider it prudent to determine with reasonable con- fidence that the order is monophyletic, be- cause, if it is not, then any hypothesis about its genealogy will be wrong. Fourth, there is a practical problem related to the previous point. In analyzing cladistic relationships among passerines, I expect to do outgroup comparisons between passerines and nonpasserines in order to cluster clades within the Passeriformes. The logical validity of this procedure requires that the monophyly of the order be established before attempts are made to determine the polarity of character transfor- mations within the order (see below). METHODS In order to demonstrate passeriform monophyly it is not enough just to list similarities, because not all similarities will support such a hypothesis. Mono- phyly can be shown by the possession of characters that are best interpreted as being derived at the level of the group in question. I looked for such synapo- morphies in two places: (1) traditional taxonomic characters from the literature and (2) new informa- tion from my dissections of the limb muscles. I reexamined the phenetic similarities used by pre- vious workers as evidences for grouping. Such sim- ilarities could be shared primitive characters (sym- plesiomorphies), or they could be derived states shared with nonpasserines and not demonstrable as having evolved independently in passefines. Such characters would not corroborate a hypothesis of monophyly. Alternatively, traditional characters found to be most reasonably interpreted as uniquely or independently derived by the Passeriformes would indicate monophyly. In addition, I searched for new synapomorphies in the structure of the limb muscles. Most of the literature on avian limb muscles deals either with oscines or with nonpasserines, and little work has previously been done on the muscles of suboscines. To date I have completed only a lim- ited survey of suboscine muscles, but it includes rep- resentatives of all the suborders and major families, and is adequate for the purposes of this paper. Fol- lowing the classification of Wetmore (1960), the sub- oscine species dissected include the following: Sub- order EURYLAIMh Calyptomena viridis (Eurylaimidae); Suborder TYRANNI: Dendrocolaptes certhia (Dendrocolaptidae); Certhiaxis cinnamomea (Fumariidae); Thamnophilus doliatus (Formicariidae); Acropternis orthonyx (Rhinocryptidae); Procnias nu- dicollis (Cotingidae); Pipra erythrocephala (Pipridae); Tyrannus tyrannus, Pachyramphus rufus (Tyranni- dae); Pitta guajana (Pittidae); Suborder MENURAE: Menura novaehollandiae (Menuridae); Atrichornis clamosus (Atrichomithidae). Comparative data on oscines and nonpasserines from our laboratory include numerous families as reported in the following works: Raikow 1973, 1975, 1976, 1977a, 1977b, 1978; Bentz 1976, 1979; Maurer 1977; Swierczewski 1977; Raikow et al. 1979; Raikow et al. 1980; Swierczewski and Raikow 1981; Berman and Raikow 1981; Borecky 1977, 1978; Maurer and Raikow 1981. Additional avian myological data were taken from the summaries by Hudson (1937) and George and Berger (1966). OUTGROUP COMPARISON The traditional taxon Passeriformes is a phenetic cluster. In order to demonstrate that it is monophy- letic, we must show that its members possess char- acter states that are derived within the class of birds, that is, conditions for which nonpasserine birds are primitive. This requires a method for determining the polarity (primitive-derived directionality) of the characters in question. The most widely used meth- od of character analysis is the outgroup comparison. Often in papers in which this method is used, it is cited without an adequate explanation of how it works. Perhaps the most detailed explanations of the method are given by Ross (1974: 152) who calls it "ex-group comparison," Eldredge and Cracraft (1980: 26, 63), and Watrous and Wheeler (1981). As usually explained, outgroup comparison works as follows. If a certain character shows variations among the members of a group of organisms (which will be called the ingroup), and if one of these vari- ations also occurs in other organisms (the outgroup), then the variant that occurs only within the ingroup is considered a derived state in that group, the other being primitive within the group. Here is an ex- ample. Some Hawaiian honeycreepers (Drepanidi- dae) have a particular type of tubular tongue (used for nectar-feeding), while others have a simple non- tubular tongue. Which is the derived condition in the family? If we look at the other New World nine- primaried oscines we see that such a tubular tongue is never present. Therefore, we may conclude that in the Drepanididae a nontubular tongue is primitive and a tubular tongue is derived. We may further con- clude that the species having the tubular tongue form a monophyletic subgroup of the Drepanididae (Rai- kow 1977b, 1978). There is a potential problem with this analysis, however. As given above, it contains the unstated assumption that the Hawaiian honeycreepers them- selves are a monophyletic group, being more closely related to each other genealogically than to any of the other New World nine-primaried oscines. This assumption is necessary for the outgroup compari- son to be logically valid, because character variations arise through the process of evolutionary change in an evolving lineage; there is a temporal polarity to the process. If we say that some Hawaiian honey- creepers have the primitive condition and others the derived condition, we mean that the character underwent a change in one lineage and that all of the tubular-tongued species are descended from the ancestral form that first had this changed character. All of this requires the existence of a monophyletic group of species within which this character trans- formation took place in one lineage. If we do not have reason to believe that the Hawaiian honey- creepers form a monophyletic group, then we cannot logically make a comparison between this group and an outgroup, because we do not know that some Hawaiian honeycreepers are not members of that outgroup. Explanations of the outgroup comparison do not always mention this point. For example Ross (1974: 153) illustrates the procedure by describing two vari- ations in the form of the male genitalia of the leaf- hopper genus ExitJanus: triangular with setae or circular without setae. In related genera the form is triangular with setae. Ross concludes that the trian- gular, setose condition is primitive. Here, the un- stated assumption is that Exitianus is monophyletic, that the ancestral species giving rise to the members of this genus had the primitive state, and that the circular nonsetose condition arose within a lineage of this group. It is not uncommon for a writer to assume that a familiar taxon is monophyletic, but one of the points of this paper is that such assump- tions should not be made. MONOPHYLY OF AVES In order to demonstrate passeriform mono- phyly we require derived states that define the order as a clade within some larger group, of which the practical choice is the class Aves. As discussed above, this requires that Aves be considered monophyletic, so that the appro- priate outgroup comparisons can be made. We must therefore argue for the monophyly of Aves, but to do this by an outgroup compari- son would require the prior assumption that some still larger group, e.g. the amniotes, is monophyletic. That would also have to be demonstrated, requiring yet another hypoth- esis, such as the monophyly of the tetrapods. This series of dilemmas is not infinite; even- tually one comes to a hypothesis of the mono- phyly of all life, and then there is no outgroup. The solution to this problem is to use a method other than outgroup comparison to demon- strate the monophyly of some group so that the study can begin. Gaffney (1979: 95) suggests that "... in practice one usually assumes the correctness of a higher-level (more inclusive or more general) hypothesis and makes compar- isons within it." In the present case, I believe that one can do better than this and provide an independent argument for the monophyly of Aves. Then, it will be possible to do valid out- group comparisons between Aves and non- Aves in order to test the monophyly of the Pas- seriformes within Aves, which is the real purpose of this exercise. Here is an argument, not based on an out- group comparison, that the class Aves is monophyletic. Feathers are considered to be homologous with reptilian epidermal scales based on their similar composition of keratin- ized epidermal tissues and their similar devel- opment through the interaction of the ecto- derm with a dermal papilla. Epidermal scales occur throughout the "reptilia," which predate birds in the fossil record. Epidermal scales are themselves regarded as a derived specializa- tion of the stratum corneum, a universal tet- rapod character first appearing in the "am- phibia," which appear in the fossil record even earlier. Not only are feathers clearly derived in vertebrates, but they are no doubt uniquely derived. The structural complexity of feathers, and their several diverse but often intergrading types, makes it appear highly unlikely that they evolved more than once. Similar arguments may be made regarding other avian characters, such as the specializa- tion of the forelimb as a wing. This involves a complex of losses and fusions of elements, which first appear separately in the embryo, to produce the specialized structure of the wrist and hand. Likewise derived is the avian fore- limb musculature, with its enormously en- larged wing depressor M. pectoralis and the elevator M. supracoracoideus, situated ventral to the wing but inserting dorsally via the pul- ley provided by the foramen triosseum. The evolution of this flight mechanism from the reptilian condition via an intermediate stage in Archaeopteryx has been described by Os- from (1976a). Again, the respiratory system, with its com- plicated system of air sacs and its unique or- ganization of air capillaries, is seen to be de- rived in birds through its functional correlation with flight and endothermy, which are also derived in comparison with more ancient tet- rapod groups. The preceding argument for avian mono- phyly is based on the temporal (stratigraphic) sequence of fossil vertebrates correlated with the distribution of characters associated with the evolution of adaptive specializations, some aspects also being corroborated by embryolog- ical information. Any of these features alone is convincing evidence of avian monophyly; their co-occurrence in birds reinforces the ar- gument. ANALYSIS OF TRADITIONAL CHARACTERS The characters on which the order Passeri- formes is based are mostly anatomical features discovered during the nineteenth century and summarized by Beddard (1898) and Ridgway (1901), though some were described more re- cently. The number and diversity of these characters suggests at first glance that the order Passeriformes is firmly established by a thor- ough technical diagnosis. As presented, how- ever, these are phenetic characters, similarities recorded without consideration of their nature. The purpose of the following analysis is to see which of these characters may be considered to be derived at the level of the Passeriformes and which will, therefore, corroborate the hy- pothesis of passeriform monophyly. The analysis was performed by asking a se- ries of questions about each character, as out- lined in Fig. 1. First, is the character valid or invalid; that is, was the character described correctly in the first place? If not valid, I re- jected it as possible evidence for passeriform monophyly. The validity of most characters was not checked by reexamination of speci- mens, so in general validity was assumed, and the analysis began with the second question: Is the character primitive or derived within Aves? If primitive within Aves, a character cannot be derived for passeriforms and was rejected. If the question could not be answered, then the character was also rejected, as it is preferable to reject potentially corroborative evidence than to chance accepting false evidence. If the character is derived within Aves, the next question is whether it is unique to Passeri- formes. If it is, then it strongly corroborates the hypothesis of monophyly. If it is derived with- in Aves but is not unique to Passeriformes, there are two possible explanations: it may or may not have been independently evolved in the Passeriformes. If it was not independently evolved by the Passeriformes, then it is a de- rived state shared by the passerines and some other group(s), was presumably present in the common ancestor of the passerines and those other groups, and cannot be used to argue monophyly of the Passeriformes. On the other hand, if the state is derived within Aves, is not unique to passerines, but is independently evolved in Passeriformes from its origin in other groups (convergence or parallelism), then it does support passerine monophyly. Again, if the question could not be decided, then the character was rejected so as to avoid the chance of error. The most convincing argument for mono- phyly would be given by characters that are derived within Aves and unique to Passeri- formes. Characters derived within Aves, not unique to Passeriformes, but thought to have been independently evolved by them are somewhat less certain, because they require one more decision in their analysis and hence involve one more potential source of error. In the following analysis the polarity of some traditional characters will be tested by out- CHARACTER VA LID DERIVED IN AVES UNIQUE IN PASSERIFORMES INVALID PRIMITIVE IN AVES NOT INDEPENDENTLY NOT UNIQUE IN  EVOLVED IN PASSER FORMES PASSERIFORMES INDEPENDENTLY EVOLVED IN PASSERIFORMES Fig. 1. A scheme for determining whether traditional phenetic characters are derived within Aves at the level of Passeriformes and whether they will therefore corroborate the hypothesis of passeriform monophyly. Characters that are accepted do corroborate the hypothesis; those that are rejected do not. See text for dis- cussion. group comparisons with "reptiles" on the as- sumption, discussed above, that the class Aves is monophyletic. In the case of characters for which such comparisons cannot be made be- cause they occur only among birds, other methods of analysis will be used where pos- sible. (1) Palate aegithognathous.--Huxley (1867) defined several palatal types on the arrange- ment of the bones. I examined the descriptions and illustrations of the palate in archosaurs given by Romer (1956, 1966). Often the palate is poorly known, but it appears clear that ae- githognathism is not present in the Thecodon- tia, Crocodilia, Saurischia, Ornithischia, or Pterosauria. Generally, the vomers, if known, are paired and do not show the characteristic form of aegithognathous birds. In addition, the maxillopalatines are lacking, unless McDowell (1978) is correct in his assumption that the avian maxillopalatines are homologous with the reptilian palatines, in which case they are still dissimilar. I conclude that the aegithog- nathous palate is a derived condition within Aves. It is not unique to passerines, however. Bock and McEvey (1969: 205) point out that Huxley (1867) used a complex of characters in defining his palatal types but that more recent workers often simplified the definitions, with a result- ing loss of precision. On the basis of the full set of characters, these authors determined that the palate in the Pedionomidae and Turnicidae is schizognathous and not aegithognathous, as is sometimes stated (e.g. Beddard 1898: 321). In the Capitonidae (Piciformes), the skull is "aegithognathous with a desmognathous ten- dency" (Beddard 1898: 195), meaning that the maxillopalatines may blend with the nasal sep- tum or with each other across the midline. The vomer is truncated caudal to the line of the palatines, instead of rostral to it as in passer- ines (Beddard 1898: 196). There are other dif- ferences. Collectively, these suggest that the capitonid condition is not directly comparable to the passerine condition. Also among Pici- formes, Indicator is reported as being aegith- ognathous (Beddard 1898: 197). Swierczewski and Raikow (1981) studied the limb muscula- ture of the Piciformes and found that the Cap- itonidae are a fairly derived family within the order and that the Indicatoridae are even more highly derived. Synapomorphy of the ques- tionable piciform aegithognathism with the passerine condition would involve extensive con, flicts with myological features that clearly place both barbets and honeyguides in the monophyletic order Piciformes. The swifts (Apodidae) also have an aegith- ognathous palate. Beddard (1898: 229) notes that, while in passerines the vomer is truncat- ed in front of a line joining the maxillopala- tines, in swifts this truncation is at the level of this line, though the significance of this dif- ference is obscure. At the turn of the century the close relationship of the swifts to the pas- serine swallows (Hirundinidae) was hotly de- bated, but their similarities have now been dismissed as convergence associated with ae- rial insect hawking. Beddard (1898: 224) points out many differences between swifts and pas- serines, including several in the limb muscu- lature that seem highly significant to me, as they depart greatly from the usual passerine conditions. Sibley and Ahlquist (1972: 198) re- view the problem thoroughly. Although a con- nection to the Passeriformes is possible, the question of the swifts' relationships to the Tro- chilidae, Caprimulgiformes, Coliiformes, and Trogonidae appears at least as important. I agree with Sibley and Ahlquist (1972: 206) that this problem is one of the most interesting in nonpasserine systematics. Meanwhile, how- ever, I see little evidence that the swifts are particularly close to the passerines and think it more likely that their palatal similarities are due to convergence than to immediate com- mon ancestry. In general, then, the aegithognathous palate appears to be a character of some structural complexity that is probably independently de- rived in the Passeriformes and therefore cor- roborates the hypothesis that the order is monophyletic. (2) Atlas perforated.--The atlas and axis are the first two cervical vertebrae. The atlas artic- ulates within the odontoid process of the axis. The opening in the atlas into which the odon- toid process fits may be fully enclosed (perfo- rated atlas), or it may be open dorsally (notched atlas). It is perforated in the Passeri- formes. I cannot determine the polarity of this character, but it does not matter. If primitive, it is of no value. If derived, it is far from unique in the Passeriformes, being present in various other groups, including many piciforms and coraciiforms (Beddard 1898). There is no indi- cation in the literature that the passerine con- dition is in any way distinctive; hence, this character will not corroborate the hypothesis of passeriform monophyly. (3) Only left carotid artery present.--Both Beddard (1898) and Ridgway (1901) cite the presence of only the left carotid artery as a pas- serine character. More recently, Glenny (1955) studied the patterns of avian carotid arteries in some detail. He proposed that the primitive condition in birds is the presence of both left and right arteries and that various patterns of reduction and loss could be recognized, which he identified by a coding system. "All birds, insofar as presently known, develop a com- plete aortic arch system, and this system undergoes a series of developmental (atrophic) deletions and other modifications which result in the adult arterial arrangement-patterns ..." (Glenny 1955: 609). In current terms, Glenny proposes that loss patterns are derived states, as indicated by a developmental crite- rion. Passeriforms share a derived arterial con- dition, termed B-4-s by Glenny, but this also occurs in most piciforms, in trogons, in colies, and in several other orders and families of birds. Thus, the characteristic is derived in passerines but is far from unique. There is no apparent method to determine whether it is autapomorphic for the Passeriformes or shared with other groups. Therefore, this traditional character cannot corroborate the hypothesis of passeriform monophyly. (4) Oil gland nude.--In some birds, the orifice of the uropygial gland is surrounded by a circle of feathers forming a tuft that acts like a wick; this condition is called tufted. In others, in- cluding passerines, the circlet is lacking, giv- ing the nude condition. Both conditions are so wide-spread among birds that the character cannot be shown to be derived for the Passer- iformes and therefore does not corroborate the hypothesis of passerine monophyly. (5) Wing eutaxic.--In many birds there is a gap in the row of secondary remiges, the fifth secondary appearing to be absent while its covert remains in place. The condition in which the gap occurs is called diastataxy, while that in which the gap is absent is called eutaxy. Passeriformes are eutaxic. I cannot determine the polarity of this character by outgroup com- parison, because "reptiles" lack feathers, so both possibilities must be considered. If it is primitive within Aves, then it is not derived for the Passeriformes. If it is derived, however, it is not unique to Passeriformes; on the con- trary, there are many groups that show each condition, and occasionally both conditions occur in a single order or family. Because of this widespread occurrence of both conditions, I can see no basis for suggesting that the eu- taxic condition in passerines, even if derived, was derived independently from its origin in any other eutaxic groups. Therefore, the con- dition of eutaxic featbering cannot be used as evidence for monophyly of the order Passeri- formes. (6) Intestinal ceca srnall.--Beddard (1898) and Ridgway (1901) both report that the intestinal ceca are small in passerines. This character ap- pears to be of no value; "The caeca are among the most variable organs of birds" (Beddard 1898: 30). Van Tyne and Berger (1976: 576) also discuss this character. The ceca are often ves- tigial as in the Passeriformes and may vary considerably in closely related birds with dif- ferent feeding habits. Small.ceca are not unique to the Passeriformes, nor is there a ba- sis for determining the polarity of this char- acter; hence, it cannot be cited as evidence of passeriform monophyly. (7) Expansor secundariorurn lacking.--Both Beddard (1898) and Ridgway (1901) state that this small forelimb muscle is absent in passer- ine birds. Berger (1956) reported it in 23 fam- ilies of both oscine and suboscine passerines, however, including the Eurylaimidae, Furna- riidae, Formicariidae, Cotingidae, and Tyran- nidae among the latter. Berger also refered to earlier papers reporting its presence in passer- ines. We have also found this muscle to be present in passerine bids, including the Me- nuridae and Atrichornithidae (Raikow MS). Clearly, the early anatomists overlooked this muscle in the passerines that they dissected, probably because it is small and requires stain- ing to be clearly visible. This character is there- fore invalid and does not support the hypoth- esis of passeriform monophyly. (8) Biceps slip lacking.--Passeriform birds are characterized by 'the absence of the biceps slip, a muscular branch that arises from the belly of the biceps brachii and passes through the pa- tagium to join the tendon of the propatagialis pars Ionga. This structure occurs in many groups of birds. It is also absent in the Corac- iiformes, however (neither Beddard 1898 nor Maurer 1977 mentions it). It is also lacking in at least some piciforms and in hummingbirds, swifts, owls, parrots, cuckoos, turacos, and some caprimulgiformes (Beddard 1898). It is therefore unnecessary to worry about the po- larity of this character; the absence of the bi- ceps slip is so widespread that it cannot be proposed as a uniquely derived state of the Passeriformes. (9) Tensor propatagialis brevis tendon "passer- ine".--This muscle arises in the shoulder and sends its tendon through the patagium to an insertion on the surface of the extensor meta- carpi radialis, a forearm muscle. The tendon does not end there, however. After making its attachment, it turns proximad and passes to an insertion on the humerus. Garrod (1876) con- sidered this to be a good character for defining the passerine birds. The condition as described does appear to be characteristic of passerines generally, including suboscines, though Gar- rod did note some minor variations, as I have also. These seem insignificant, however, com- pared to the great diversity that the muscle shows in other groups. It is not possible to do outgroup comparisons with "reptiles" in ana- lyzing this character, as it is an avian special- ization associated with the patagium of the wing. Because this condition appears to be unique to the Passeriformes, it is tempting to suggest that, if the Passeriformes are shown to be monophyletic by other characters, then by correlation this character is also presumably derived at the level of the order. Then to use this as an evidence of passerine monophyly, however, verges on circular reasoning. Because the determination of its polarity is not inde- pendent of other characters, I would hesitate to argue passerine monophyly on this character alone, were that possible. (10) Iliofernoralis externus absent.--The ab- sence of this hindlimb muscle (under the name gluteus medius et minimus) was noted by Hudson (1937) on the basis of limited dissec- tions. It is also absent in various other groups, including the Piciformes (Swierczewski 1977, Hudson 1937) and Coraciiformes (Maurer and Raikow 1981). The muscle tends to reappear as a developmental anomaly in passerine birds and is even thought to have become reestab- lished in some groups (Raikow 1975; Raikow et al. 1979, 1980). The arguments for the con- cept that the muscle became reestablished in some passerine groups indicate that its ab~ sence is a derived state within Aves. Because it is also absent in groups generally considered close to the passerines, however, one cannot argue that its apparently primitive absence within the Passeriformes is a unique, derived condition within Aves; hence, this character cannot support the concept of passeriform monophyly. (11) Ambiens lacking.--This is the most su- perficial muscle on the medial surface of the thigh. According to Lance Jones (1979), it is homologous with the muscle of the same name in "reptiles"; therefore, its absence in birds is a derived state. It is absent in Passeriformes but also in Piciformes, Coraciiformes, Trogon- iformes, Apodiformes, and many other groups (George and Berger 1966: 421). Because there is no reason to suppose that it was lost in Pas- seriformes independently of its loss in any oth- er group, it cannot be used as an argument for passeriform monophyly. (12) Iliofemoralis lacking.--Also known as M. piriformis pars iliofemoralis, this muscle passes from the ilium to the femur. It is present in many groups of birds and absent in many others, including the Passeriformes (George and Berger 1966: 407). Because its homology with reptilian muscles is uncertain, its polarity cannot be determined by outgroup compari- sons. This muscle has been found as an occa- sional developmental anomaly of an apparently atavistic nature in passerine birds, including a Fox Sparrow (Passerella iliaca, Raikow 1975) and a White-breasted Wood-swallow (Artamus leucorhynchus, Raikow et al. 1979). This indi- cates that the absence of the muscle in passer- ines is derived. As there is no reason to believe that the muscle was lost in passerines sepa- rately from its loss in other birds, however, this character will not serve as an argument for the monophyly of the Passeriformes. (13) Spermatozoa bundled, with coiled head and large acrosome.--McFarlane (1963) explored the use of spermatozoan structure as a taxonomic tool, reviewing earlier work and reporting new observations. In nonpasserines, the sperm cells are relatively simple in structure, being straight and having a small acrosome. In con- trast, the spermatozoa of passerines are coiled in the head region and have a large acrosome. The nonpasserine type is similar to the reptil- ian condition and hence may be considered primitive within Aves, the passerine form thus being derived. This condition occurs both in oscines and suboscines. Henley et al. (1978) noted that oscine spermatozoa occur in bun- dles and are nonmotile under certain experi- mental conditions. Feduccia (1979) reported that in two suboscine species the spermatozoa are also bundled. The use of this character complex at present is limited because of the relatively small number of species yet exam- ined and because proper outgroup compari- sons will require the observation of a diversity of oscine, suboscine, nonpasserine, and non- avian spermatozoa under identical laboratory conditions. The distinctions are sufficiently great, however, and the range of taxa already examined sufficiently broad, that it is a rea- sonable conclusion that sperm morphology does corroborate the hypothesis of passerine monophyly. (14) Foot anisodactyl.--There are various ar- rangements of the toes among birds. In the Passeriformes the condition is anisodactyl, meaning that the first toe (hallux) is directed backward, while the second through fourth toes are directed forward. This condition oc- curs in the majority of birds, including forms with widely divergent locomotor habits, while other toe arrangements are clearly associated with one or another functional specialization. For this reason, it appears probable that an- isodactyly is primitive among birds. In addi- tion, this arrangement occurs in Archaeopteryx and in the theropod dinosaurs from which birds are believed by some to have evolved (Ostrom 1976b). Therefore, this character does not constitute an argument for passeriform monophyly. (15) Phalangeal formula 2-3-4-5.--This refers to the number of phalanges in digits I, II, III, and IV of the hind limb. The same consider- ations apply here as for anisodactyly; hence, this character does not corroborate the hypoth- esis that the order Passeriformes is monophy- letic. (16) Hallux incumbent.--This means that the hallux in Passeriformes is at the same level as the forward three toes, rather than being ele- vated. The polarity of this character is uncer- tain, but, because an incumbent hallux is a common characteristic among birds, no argu- ment for its being uniquely derived in passer- ines is possible; hence, it does not corroborate passerine monophyly. (17) Hallux and its claw large.--Among pas- serines, the hallux and its claw are relatively large in relation to the other toes, as compared to the condition in most other birds, including the Piciformes and Coraciiformes. The large size of the hallux and its claw may be func- tionally related to the degree of independent action permitted the hallux in passerines (ex- cept Eurylaimidae) by the absence of a con- nection between their major flexors. Ridgway (1901) considered the hallux in the Eurylaimi- dae to be relatively weak, but Olson (1971) dis- counted this. The enlarged hallux appears to be a derived component of the specialized pas- serine perching foot, which will be discussed in more detail below. Therefore, it apparently does corroborate the idea of passeriform monophyly. (18) Type VII deep plantar tendons.--Birds have two deep flexor muscles of the toes, the flexor digitorum longus (FDL) and flexor hal- lucis longus (FHL) (Fig. 2A). In most birds the FDL tendon trifurcates and supplies digits II, III, and IV, so that the muscle flexes these three digits simultaneously. The hallux is flexed by FHL. There are exceptions to this, however. In addition, most bird groups have an intercon- nection between the FHL and FDL tendons, either by a tendinous slip (vinculum) passing from the FHL tendon to the FDL tendon or by complete fusion of the two tendons. These variations in the deep plantar tendons have long been used as taxonomic characters, es- pecially following Garrod (1875) and Gadow (1893-1896). Most passerines have the Type VII arrangement of Gadow's system, with no con- nection between the FDL and FHL tendons. Garrod (1875) confirmed earlier observations by Sundevall on this, and later (Garrod 1876) included this character as part of his diagnosis of the Passeriformes. Subsequently, Garrod (1877) reported an exception to this condition, namely the presence of a vinculum in the broadbills, Eurylaimidae. He concluded that either the character must be abandoned or that the Eurylaimidae are not passerine and chose the first alternative. Subsequent workers have minimized the importance of this character and have shown that the vinculum is occasion- ally absent in broadbills (see Olson 1971 for a review). The assumption has been that the presence of a vinculum is a primitive state and its absence derived. It does appear probable that the lack of a connection between the deep plantar tendons is a derived state in birds. At least in some EDL ,' FD L FHL ': FPD2 " "'"" " ' '"A FPD4 Fig. 2. Diagrammatic representation of the ten- dons of insertion of the extrinsic digital muscles of passerine birds, the bellies of which, not shown here, lie in the shank. A. extensor digitorum longus (EDL); flexor digitorum longus (FDL); and flexor hal- lucis longus (FHL). A vinculum (v) connects the FDL and FHL tendons only in the Eurylaimidae (Broad- bills). B. flexor perforatus digiti II (FPD2); flexor per- forans et perforatus digiti II (FPPD2); flexor perforans et perforatus digiti III (FPPD3); flexor perforatus dig- iti III (FPD3); and flexor perforatus digiti IV (FPD4). "reptiles" there is one deep flexor muscle that supplies all five digits, and the FHL-FDL di- vision may have arisen in birds or their direct reptilian ancestors along with the movement of the hallux into its opposing position. Also, the character (except for the Eurylaimidae) oc- curs in correlation with other derived states shown by the Passeriformes. Finally, the sep- aration of the two tendons appears to be cor- related with the enlarged hallux as part of a functional specialization of the passerine foot (see below). The problem of the Eurylaimidae would dis- appear if the presence of the vinculum in that group could be considered a derived (second- arily primitive) condition. I do not know of any previous suggestion of this hypothesis, but there is good reason to consider it. The Eury- laimidae possess a derived stapes morphology (Feduccia 1975) that clusters them together with most other suboscines as a subclade of the Passeriformes. If the vinculum is primitive, then the derived stapes of the Eurylaimidae must have evolved independently of that in other suboscines. Compared to the stapes, the vinculum is a relatively simple structure. Vari- ations in the plantar tendons of birds are nu- merous and diverse; members of a single order often show considerable diversity. It appears more likely that the vinculum of broadbills evolved within that group than that the stapes is convergent. The feeble vinculum of the Eu- rylaimidae, in my opinion, is not strong enough to exclude the Type VII deep plantar tendon arrangement from being considered a probable synapomorphy of the Passeriformes. For each character analyzed there are three possible outcomes: it may corroborate the hy- pothesis of passeriform monophyly, it may fail to corroborate the hypothesis, or it may refute the hypothesis. In order to refute the hypoth- esis a character would have to be shown to be derived once in a group containing some pas- serines plus some nonpasserines, but primi- tive in other passerines. The alcediniform stapes was originally argued by Feduccia (1975, 1977) to be a character of this type. None of the 18 traditional characters analyzed above was found to refute the hypothesis. Five of these characters (numbers 1, 9, 13, 17, and 18) cor- roborate the hypothesis, while the remaining 13 simply fail to do so. On this basis two conclusions may be made. First, passeriform monophyly is corroborated by some traditional characters with sufficient force that it appears to be a reasonably viable hypothesis, certainly one worthy of further investigation. Second, the large number of traditional characters that fail to support the hypothesis underscores the importance of not assuming that long recog- nized taxa are monophyletic in the contem- pora W sense, merely because they share some similarities. THE HIND LIMB MUSCLES My preliminary study of some of the hind limb muscles of representative suboscines has yielded results that contribute to the analysis of passeriform monophyly. As already noted, previous to this study our knowledge of pas- serine limb muscles was mostly limited to the oscines, so that it was difficult to generalize about the order Passeriformes as a whole. The species of suboscines dissected, and references to previous studies pertinent to the following discussion, are listed above under Methods. M. pubo-ischio-femoralis.--This large muscle of the thigh passes from the postacetabular pel- vis to the femur. In most birds it has two sep- arate bellies, although sometimes these are partly or completely fused. In nonpasserines the bellies are arranged so that one lies more or less completely superficially to the other, the superficial belly being called Pars lateralis and the deep belly Pars medialis. In passerine birds, however, one belly lies mostly cranially to the other, with only slight overlap. Here, they are termed Pars cranialis and Pars caudalis (Baumel et al. 1979). For illustrations of this point compare Figs. IX 52, IX 53, and IX 54 in George and Berger (1966), where this muscle is labeled with its old name M. adductor lon- gus et brevis. It is probable, based on details of their attachments, that the passerine Pars cranialis is the homologue of the nonpasserine Pars lateralis, while Pars caudalis corresponds to Pars medialis. In any case, the difference between passerines and nonpasserines is dis- tinctive. The polarity of this difference must be de- termined by comparisons within Aves, be- cause I lack information to do outgroup com- parisons with "reptiles." If we accept the above discussion of the characters as evidence that the order Passeriformes is monophyletic, then this character must also be derived by cor- relation with its common occurrence with those characters. In all of the suboscines dissected, this muscle had the distinctive passerine conditions rather than the nonpasserine form. This confirms the description given by Hudson (1937) for Corvus and Tyrannus. Thus, the form of M. pubo-is- chio-femoralis appears to corroborate the hy- pothesis of passeriform monophyly, although I would give it greater weight if its polarity had been determined by a more independent method than correlation with other characters. Intrinsic foot muscles.--Birds in general have a set of intrinsic foot muscles that arise from the tarsometatarsus and insert on the base of one or another of the digits. A list of these muscles and the digits on which they insert is TABLE 1. Avian digital muscles. The distributions shown here are those found in the Passeriformes and various other groups. Intrinsic muscles Extrinsic muscles a Digit I flexor hallucis brevis extensor hallucis longus Digit II adductor digiti IIb abductor digiti IIb Digit III Digit IV extensor proprius digiti III b extensor brevis digiti III  extensor brevis digiti IV 1/2 adductor digiti IV  abductor digiti IV  flexor hallucis longus flexor perforatus digiti II flexor perforans et perforatus digiti II [flexor digitorum longus] [extensor digitorum longus] flexor perforatus digiti III flexor perforans et perforatus digiti III [flexor digitorum longus] [extensor digitorum longus] flexor perforatus digiti IV [flexor digitorum longus] [extensor digitorum longus] Bracketed muscles have multiple insertions on different digits in different groups of birds. Absent in passefine birds. Absent (occasionally vestigial) in passefine birds. given in Table 1, and the function of each is given by its name. The number of these mus- cles varies in different groups of birds, but nonpasserines generally have fairly complete sets. George and Berger (1966) and Hudson (1937) record the occurrence of these muscles and give illustrations of them. It was previ- ously known that oscines lack most of these muscles (summary in George and Berger 1966), and our dissections of oscines have confirmed this, but little was previously known about suboscines. My dissections of the representa- tive suboscines listed above confirm the simi- lar absence of these muscles. The intrinsic muscles of the hallux are gen- erally present in passerines. Except for the oc- casional presence of extensor brevis digiti IV in a very reduced (essentially vestigial) state, however, all of the intrinsic foot muscles of digits II, III, and IV are lacking in the Passer- iformes, including both oscines and subos- cines. This set of muscles as a group is ho- mologous to the intrinsic foot muscles of other tetrapods. Because the class Aves is monophy- letic and because these muscles occur both in nonpasserine birds and in nonavian tetrapods, outgroup comparison clearly indicates that this absence is a derived state and thereby corrob- orates the hypothesis of passeriform mono- phyly. It must be noted, however, that the loss of all of these muscles may not have occurred at one time or at the level of the passeriform clade. In the Coraciiformes and Piciformes, which are widely regarded as being closely re- lated to the Passeriformes, there are also trends toward the loss of intrinsic foot muscles. In each of these nonpasserine orders, however, some families retain at least one of the muscles for each toe, and no families lack the entire set. Furthermore, the three orders show the evo- lution of quite different foot adaptations for perching: zygodactyly in Piciformes, anisodac- tyly leading to syndactyly in Coraciiformes, and a specialized form of anisodactyly in Pas- seriformes (discussed below). The greatest losses of intrinsic foot muscles in the Corac- iiformes and Piciformes occur in the most de- rived groups (data from Maurer 1977 and Swierczewski 1977). I conclude, therefore, that most of the losses of intrinsic foot muscles in the Passeriformes occurred independently of their loss in the other two groups. ADAPTIVE SIGNIFICANCE OF THE PASSERINE FOOT In this section I will accept the conclusion that the order Passeriformes is monophyletic and will speculate briefly on the functional and adaptive significance of the passerine foot. The hypothesis of monophyly does not depend upon the validity of these speculations, but they may help to give some insight into the biological significance of certain passeriform characteristics. As a functional mechanism, the passerine foot is distinctively specialized and is probably a factor in the success of the order in radiating into a variety of niches. Basically the foot is adapted for perching but in such a way that the development of terrestrial habits has not been precluded. Perching adaptations have arisen in a number of ways among birds (see Bock and Miller 1959 for a general discussion). The primitive anisodactyl foot has been mod- ified by rearranging the digits into zygodactyl or heterodactyl configurations, or by joining some of the forward toes at their bases in syn- dactyly. In different ways these modifications enhance the use of the foot as a grasping mech- anism. Among passerines, one modification of the retained primitive anisodactyl foot is the enlargement of the hallux, which presumably provides a more evenly balanced distribution of strength between the numerically unequal opposing sets of toes and permits the hallux to encircle the perch effectively in opposition to the other toes. The most striking aspect of the evolution of the passerine foot is the loss of the intrinsic muscles of the forward toes as described above. The pattern of losses and their func- tional consequences may be understood by considering the general structure of the limb. Extrinsic muscles.--There are a number of muscles activating the toes, the bellies of which lie in the shank and that may therefore be termed extrinsic foot muscles. The three for- ward toes have a common extensor, M. exten- sor digitorum longus (EDL), the tendon of which passes across the intertarsal joint and down the dorsal surface of the tarsometatarsus (Fig. 2A). Near the distal end of that bone, the tendon trifurcates, the branches inserting on the dorsal surfaces of the three forward toes. Thus, the EDL provides for simultaneous ex- tension of these digits. There is also a common flexor, M. flexor digitorum longus (FDL), the tendon of which passes down the plantar sur- face of the tarsometatarsus, trifurcating dis- tally to insert on the plantar surfaces of the three forward toes (Fig. 2A). This provides si- multaneous flexion of these digits. Thus, these two muscles produce simultaneous extension and flexion of the forward three toes. The hal- lux is likewise provided with an extrinsic flexor muscle, the flexor hallucis longus (FHL), but it has no extrinsic extensor in birds (Fig. 2A). In most passerine birds the tendons of the FIlL and FDL have no interconnection as they pass down the plantar surface of the tarsus, as noted above. The absence of this connection (except in the Eurylaimidae, which have a feeble vin- culum) presumably allows independent flexion of the forward toes and of the hallux, which may aid in the versatility of movement in ad- justing the foot to perches of varying sizes and shapes. This functional separation of the flex- ion of the forward toes from that of the hallux is probably associated with the enlarged size of the hallux as part of the passerine perching specialization. These extrinsic muscles EDL, FDL, and FIlL are always present. Each forward toe also has one or two indi- vidual extrinsic flexor muscles that insert at one or more points along the plantar surface of the digit, flexing it around one or another in- terphalangeal joint. These muscles, together with the FDL, permit varying patterns of flex- ion, so that the toes can conform to perches of different sizes and shapes. These muscles are the flexor perforatus digiti II (FPD2), flexor per- forans et perforatus digiti II (FPPD2), flexor perforatus digiti III (FPD3), flexor perforans et perforatus digiti III (FPPD3), and flexor perfor- atus digiti IV (FPD4) (see Table 1 and Fig. 2B). Again, these muscles are always present in passerine birds. Thus, the extrinsic flexor system consists of one muscle that provides simultaneous flexion of the forward three toes, plus individual flex- ors of each forward toe, and an independent flexor of the hallux. The extrinsic extensor sys- tem is simpler, as there are no individual ex- tensors, only the common extensor of the for- ward three toes. This entire extrinsic muscle system is always present in passerines, appar- ently representing a necessary minimum of complexity. Intrinsic muscles.--As discussed earlier, non- passerine birds have intrinsic foot muscles that arise on the tarsometatarsus and insert at the bases of the forward three toes, while in pas- serines this entire set is lost except for one oc- casional vestige. The order Passeriformes is therefore characterized by a major simplifica- tion of the foot mechanism. The hallux opposes the forward toes and, as noted above, has a single extrinsic flexor, FIlL. It also has two intrinsic muscles (Fig. 3). The intrinsic extensor, M. extensor hallucis longus (EHL), is the only extensor of the hallux, as there is no extrinsic extensor in birds, and it is never lost. The intrinsic flexor, M. flexor hal- lucis brevis (FHB), is a synergist of the much larger FHL. The forward toes have the capacity for si- multaneous extension and for simultaneous or individual flexion, including variation in shaping the flexed digits to fit the perch. The loss of the intrinsic forward toe muscles, how- ever, eliminates a repertoire of individual ab- duction, adduction, and extension move- ments. Passerines have thus retained a variable mechanism for grasping but have simplified the range of movements not associated directly with the grip. This presumably limits the func- tional capabilities of the foot where such subtle movements might be useful, such as in walk- ing or climbing over irregular surfaces or grasping variously and irregularly shaped ob- jects. The passerine condition is in striking contrast, for example, to the complexity of the intrinsic foot musculature of mousebirds (Co- liiformes), which hold food with their feet and have an astonishing variety of movements and postures (Berman and Raikow 1981). Clearly, passerines do not depend upon these capabil- ities; the bill is the usual manipulative organ, and passerines are prone to fly even short dis- tances rather than clamber over difficult ter- rain. The loss or major reduction of foot muscles in passerine birds tends to occur in certain pat- terns not unlike those described by Stegmann (1978) in the avian forelimb: muscles that are reduced or lost tend to be those that (1) are initially small in size; (2) are complementary to others in their actions, so that their loss may reduce the subtlety or variety of some move- ments but will not eliminate an action entirely; and (3) are usually smaller in size and simpler in structure (e.g. crossing fewer joints) than their synergists that are retained. Adaptive potential.--Nevertheless, although passerines have a limb muscle system greatly simplified in many respects, it does retain the capability of some adaptive variation. There is much variety in the two intrinsic muscles of the hallux. The EHL is never lost, presumably because it has no synergist and its loss would eliminate the necessary ability to extend the hallux. In the Ocellated Tapaculo (Acropternis orthonyx: Rhinocryptidae), however, it is re- duced to a vestige, probably in association with the terrestrial habits of this species. The intrinsic flexor, FHB, is lost entirely in this species and is often reduced to a vestige in passerines. Presumably this muscle is dispens- able, because it is a small synergist of the much Fig. 3. Diagrammatic representation of the two intrinsic foot muscles generally found in passerine birds, M. extensor hallucis longus (EHL) and M. flex- or hallucis brevis (FHB). larger FHL. On the other hand, passerines with a strong grip may have considerable enlarge- ment of the intrinsic hallux muscles along with other modifications, such as were discussed previously in the shrikes (Laniidae) (Raikow et al. 1980). CONCLUSIONS Some readers of an earlier version of this paper suggested that this analysis is hardly necessary, because the order Passeriformes is almost universally recognized as being mono- phyletic. One found my arguments quite un- convincing but agreed with my conclusions. To this I have two responses. First, the concept of monophyly often employed is vague and imprecise. Second, previous studies have am- ply demonstrated that many traditionally rec- ognized taxa are not monophyletic in the strict and meaningful sense, and therefore I urge that such assumptions should not be made. I men- tion these points because they illustrate a com- mon tendency for biologists to assume that tra- ditional taxa are monophyletic, perhaps because that which is familiar tends to appear "natural" in some sense. The task of systematic biology is to produce hypotheses of phylogenetic relationships among organisms, and the key to doing this is the ability to recognize monophyletic groups. The chief virtue of the dadistic school in con- trast to others is not that it accomplishes this goal more satisfactorily, although it does, but that its methodology permits the full exposure of every facet of an analysis for maximum ease of criticism. As a result, a hypothesis, like the one of passerine monophyly discussed herein, may be seen to rest on individual arguments that vary in their strength, weakness, and de- gree of ambiguity. On the basis of the arguments presented in this paper I conclude that the order Passeri- formes, originally defined as a phenetic cluster, is a clade or monophyletic group in the specific contemporary sense. ACKNOWLEDGMENTS For the loan of specimens I wish to thank Mary H. Clench and Kenneth C. Parkes (Carnegie Museum of Natural History) and Charles G. Sibley (Peabody Museum of Natural History, Yale University). Mary H. Clench, Joel Cracraft, Storrs L. Olson, Kenneth C. 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