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A reply to Ashby Camp and TrueOrigins on Avian Phylogeny
A Reply to Ashby Camp and “TrueOrigins” on the Derivation and Phylogeny of Aves
Ashby Camp, one of the more renowned pseudo-scientists of the creationist movement, has lent his considerable talents for distortions and dissimulations to the well known creationist counterpart to TalkOrigins -- TrueOrigins, in a series of essays. While his collected works merit refutation, the authors herein concentrates on one of the more farcical, entitled: On the Alleged Dinosaurian Ancestry of Birds. The essay makes 11 salient points, scattered incoherently through a nauseating assemblage of quote-mining and logical fallacies:
b) Protarchaeopteryx robusta and Caudipteryx zhoui are neoflightless avialians
e) Protoavis is both a valid taxon and a disproof of theropod origins
f) "Bird tracks" corroborate the existence of Triassic and Late Jurassic avians
g) There is an intractable temporal paradox in the theropod origin hypothesis
i) The list presented by Feduccia (1996, 68) contains valid objections to the theropod origin hypothesis
j) Theropoda possessed a pelvovisceral pump
k) Terrestrial origin of flight is a biophysical impossibility and thus theropods cannot be bird ancestors
All of these assertions are seriously flawed and must be called into question.
Feathery integument is synapomorphic of Aves, the phylogenetic status of Caudipteryx and Protarchaeopteryx, and the derivation of remigial fans on the manus
Contrary to Camp’s impression, which is admittedly out of date, there are currently at least nine (possibly thirteen) non-avian theropod taxa, including dromaeosaurids sensu lato, which display integumentary derivates regarded as feather homologues and/or antecedents: Sinosauropteryx prima (Ji & Ji 1996, Chen et al. 1998, Currie & Chen 2001), Protarchaeopteryx robusta (Ji & Ji 1997, Ji et al. 1998), Caudipteryx zoui (Ji et al. 1998, Zhou & Wang 2000, Zhou et al. 2000) Beipiaosaurus inexpectus (Xu et al. 1999a, Xu et al. 2003a), Sinornithosaurus millenii (Xu et al. 1999b, Xu et al. 2001b, Xu & Wu 2001), NGMC 91 ("Dave", which may or may not be referable to Sinornithosaurus; see Ji et al. 2001), Scansoriopteryx heilmanni (Czerkas & Yuan 2002), Epidendrosaurus ningchengensis (which is possibly referable to Scansoriopteryx; see Zhang et al 2002), Yixianosaurus longimanus (Xu & Wang 2003), Microraptor zhaoianus (Xu et al. 2000, Hwang et al. 2002), Microraptor gui (Xu et al. 2003b), and Cryptovolans pauli (Czerkas et al. 2002). The alvarezsaur Shuvuuia deserti (Chiappe, Norell, & Clark 1998) displays simple tubular integumentary structures, which according to biochemical analysis are apparently composed of beta keratins, ruling out the possibility that they are dermal in origin (Schweitzer et al 1999). However, the exact phylogenetic status of Alvarezsauria remains uncertain, and they may in fact be basal Aves.
Camp repeats the arguments of Geist et al (1997) and Feduccia (1999) in stating that the integumentary derivatives observed in Sinosauropteryx are not feather homologues/antecedents. Camp presents no data to support this assertion, merely arguing from authority that it is in fact true. Nevertheless, the view that the integumentary structures of Sinosauropteryx are non-homologous to feathers, and represent something else (e.g., a medial collagenous frill), is simply untenable.
The integumentary structures of Sinosauropteryx prima are distributed along the midline of the body, extending from the crown of the skull caudally to the tail, and at scattered locations throughout the rest of the body including the abdomen, the knee, and adjacent to the humerus and ulna (Chen et al 1998). Thus, the conclusion that the structures were strictly limited in distribution to the body midline is falsified (contra Geist et al 1997).
The derivates are filamentous, and densely distributed over the body surface. Padian et al (in Tanke & Carpenter 2001) report length range for the structures as 8-40mm, although most of the filaments range from 8-10mm. Approximately ten filaments are preserved per millimeter, providing an indication of how dense the pelage actually is (contra Feduccia 1999). Compare Plate 1A and 1C in Padian et al (in Tanke & Carpenter 2001) to the figure of dermal collagenous fibers in Varanus (Feduccia 1999, 378). The filaments appear to lack a central rachis, and may or may not be hollow. The preservation of the structures is not sufficient to further clarify their structure, and biochemical testing is infeasible.
From the available data, the suggestion that these fibers are dermal collagenous structures, similar to those seen in aquatic or semi-aquatic reptiles such as some species of Varanus, must be regarded as wanting of substantiation (contra Geist et al 1997, Feduccia 1999, Ruben & Jones 2000). First and foremost, the distribution of the fibers and the sheer density with which they appear, are difficult to reconcile with the argument that they represent collagen fibers. A comparison between Plate 1A and 1C in Padian et al (in Tanke & Carpenter 2001) and the photograph from Geist et al (1997) presented in Feduccia (1999, 378) of a skinned varanid tail, shows striking differences. Combined with the lack of any evidence for an aquatic habitus for Sinosauropteryx or any other compsognathid theropod, the arguments presented by Geist and his colleagues, as well as Feduccia and Ruben, are without merit. No compelling data has yet been advanced to conclusively refute the feather-homologue status of the integumentary derivatives seen in NGMC 2123 and all other Sinosauropteryx material.
Camp next considers the phylogenetic status of Protarchaeopteryx robusta and Caudipteryx zoui (Ji et al 1998), the latter of which is now known from a decent number of well-preserved skeletons. As the integumentary derivatives of these taxa are undeniable feather homologues, Camp is left with no choice but to parrot the argument that these taxa are merely neoflightless birds, completely unrelated to Theropoda (Feduccia 1996, 1999, Feduccia in Fischman 1999, Martin and Czerkas 2000, Ruben & Jones 2000). While a possible (though not necessarily plausible due the poor preservation of the holotype) case for the neoflightless, avialian status of Protarchaeopteryx has been made (Feduccia 1996, 1999, Paul 2002) similar arguments advanced on the behalf of Caudipteryx zoui must be called into question.
Given that Caudipteryx is the better known of these enigmatic taxa, Camp naturally focuses on it. At every turn these arguments are shown untenable by the material available. Since the most recent attack on the dinosaurian affinity of Caudipteryx (Ruben & Jones 2000), new material attributable to the species has been recovered and described (Zhou & Wang 2000, Zhou et al 2000, Witmer in Chiappe & Witmer 2002), which has concomitantly led to a more thorough understanding of caudipterygian osteology.
Caudipteryx displays multiple characters synapomorphic of Oviraptorosauria. Contrary to Feduccia’s assertions, said traits are not limited to the articulation of the temporal region. In the analyses offered by Sereno (1999), Zhou & Wang (2000), Zhou et al (2000), and Barsbold et al (2000), only the contact of the quadrate and quadratojugal are stressed as plesiomorphic comparative to Aves, as regards cranial characters. Traits allying caudipterygians and oviraptorosaurs in a holophyletic clade (sensu Sereno 1999, Barsbold et al 2000) include the structure of the pubes, the procumbent orientation thereof, the prominent obturator process, the shallow pelvic canal, the contact between the fibula and calcaneum, and the dorsolateral projections of the astragalus, which overlap the calcaneum (Sereno 1999, Barsbold et al 2000). The structure of the mandibles seems to further ally these two groups (Paul 2002), but until such time as crania preserved with greater fidelity are recovered, definitive statements in this regard cannot be made.
More curious still is the fact that Caudipteryx presents a number of plesiomorphic attributes which are most difficult to explain as reversals, if this taxon in fact represents a neoflightless avialian more derived than the urvogel. The presence of a prominent obturator prong on the ischium, the lack of a pygostyle, the orientation of the scapular blade lateral to the thorax, the orientation of glenoid fossa, the lack of a cranially expanded preacetabular portion of the ilium, the lack of dorsal processes of the ischia, and lastly the retention of a dorsal ramus of the jugal and deep mandibular fenestra, are all characters entirely irreconcilable with the argument that Caudipteryx is a neoflightless avialian (Ji et al 1998, Witmer in Chiappe & Witmer 2002).
The argument posed by Feduccia (1999) that gastroliths, or "gizzard stones", in caudipterygians are irrefutable evidence for avialian status, is directly falsified by the herbivorous or at least omnivorous paleobiology of oviraptorosaurs, ornithomimosaurs and particularly therizinosaurs—all theropod clades. Indeed, gastroliths have even been associated with carnivorous allosaurids, despite Mr. Camp's insistance, in footnote 111, that "the only other theropod found with gastroliths was almost certainly an herbivore". For example, Mateus (1998, pg 119) noted "...32 gastroliths...[and] the negative imprint of 3 additional gastroliths," in "the rib cage below the eleventh dorsal vertebra" of Lourinhanosaurus antunesi. "The high number, concentration and relative size of the gastroliths suggest," he says, "that they belong to this specimen, and that they had not been swallowed when eating other dinosaur's stomach."
In light of the osteology unifying caudipterygians and oviraptorosaurs, arguments that Caudipteryx is merely a neoflightless avialian, are at best unsubstantiated, and at worst entirely specious.
It is thus interesting to compare, given these data, the list presented by Camp as synapomorphic characters of Caudipteryx and Aves, with which he imputes avian status to this species: shortened tail with distal fusion, reduced fibula, ventral occiput, long premaxilla, edentulous maxilla and dentary, quadrate head streptostylic, pubic foot absent, ball-like head of the femur.
The first character Camp presents apparently refers to the erroneous argument presented by Feduccia (1999) that Caudipteryx displays a “proto-pygostyle.” The presence of such a structure has not been corroborated by further review of the Caudipteryx material, and all distal caudals appear unfused (Witmer in Chiappe & Witmer 2002). Even if it were the case, a fusion of distal caudals is present in other oviraptorosaurs (Barsbold et al. 2000), and therizinosaurs (Xu et al. 2003). Camp next goes on to isolate the reduced fibula as a synapomorphic trait of Aves, ignoring the fact that all the Caudipteryx fibulae are incomplete. However, as Ji et al (1998) observe, "NGMC 97-9-A has a socket for the distal end of the fibula formed by the calcaneum, astragalus and tibia." This clearly indicates that the fibulae reached the proximal tarsals, and were not reduced (Chiappe & Dyke, 2002).
The elongation of the premaxilla cited by Camp is of no clear phylogenetic importance. If anything it merely underscores the affinity of Caudipteryx and Oviraptorosauria and the general trend of premaxillary expansion at the expense of the maxilla in derived maniraptorans. The edentulous character of the maxilla and dentary are in no way synapomorphic of Aves, as such a character state is observed in Ornithomimosauria, and of course, Oviraptorosauria. If this is evidence for avian status, then Camp must paradoxically argue (not unlike Paul 2002) that Ornithomimosauria are really Aves!
Camp’s assertion that the quadrate of Caudipteryx was streptostylic is baseless (see Sereno 1999, Witmer in Chiappe & Witmer 2002). The supposed absence of the pubic foot is most curious considering that the distal pubes were not preserved in the type, and additional specimens have shown it to be totally false (Ji et al 1998, Zhou & Wang 2000). Lastly, all proximal femoral heads in Theropoda are “ball-like,” but Camp appears to be arguing that the greater and lesser trochanters in the femora of Caudipteryx are confluent. This condition is not supported by any of the available fossil material.
Thus, under critical examination, Camp’s pilfered phylogenetic assessment of Caudipteryx and Protarchaeopteryx is found wanting of grounding in reality. Like other critics of theropod origins who would make them into flightless "Mesozoic kiwis", he has honed in on a couple dubious "key" characters while ignoring the bulk of osteology that unites them with non-avians. More curious yet is Camp’s assertion that if “Protarchaeopteryx and Caudipteryx are flightless birds, they would provide little, if any, new ammunition for theropod ancestry.” Such a statement is difficult to take seriously, as both of these taxa (even poorly preserved Protarchaeopteryx in which anatomical particulars are difficult to assess) present startling mosaics of avian and theropod features. This mosaic nature alone underscores the relevance of both taxa to the debate on avian phylogeny, and the fact that they should be so closely dinosaurian—indeed in the case of Caudipteryx so much like Maniraptora that this species can be accurately grouped with Oviraptorosauria—contradicts Camp’s argument.
In a confused segment of his reply, Camp goes on to assert that Maniraptora could never have evolved feathers as the maniraptoran manus could not have supported a fan of remiges without losing its ability to function as a grasping organ. This simply makes no sense. To quote Gishlick (2001, pg. 315), "because the feathers are attached at an angle roughly perpendicular to the claws, they are oriented tangentially to the prey's body, regardless of prey size," and are not in the way.
Camp finishes his incoherent argument against the presence of feathery integument in Maniraptoriformes by more or less dismissing a priori the integument reported for Beipiaosaurus inexpectus and Sinornithosaurus millenii. Camp, like the authorities he cites, has presented no data to refute the feather-homologue status of the integumentary derivatives of these clades.
Basal Deinonychosauria are paradoxically more birdlike than derived Deinonychosauria
In a curious passage in his review, Camp asserts that as the most basal Deinonychosauria, e.g., Sinornithosaurus millenii and Microraptor gui, are more conspicuously birdlike than derived Deinonychosauria, such as Velociraptorinae, it follows that theropod origin is an “ad hoc” hypothesis and of questionable validity. Where exactly Camp has derived this conclusion from is beyond the author’s comprehension. Indeed, the distribution of osteological characters observed over time in Deinonychosauria is precisely what is to be expected if Deinonychosauria and Aves are sister clades. As basal Deinonychosauria will be closer to the common ancestry of Eumaniraptora, and furthermore closer to the urvogel in phylogenetic terms, it is only to be expected that the most basal Deinonychosauria will most closely resemble basal avians. The subsequent alteration of the deinonychosaur bauplan is a result of their pursuing a different evolutionary trajectory than Aves, and as a result, developing different autapomorphies.
Curiously, if hypotheses such as those put forward by Greg Paul (1988, 2002) or George Olshevsky (1994) are correct, then the character distribution over time which Camp so objects to, would be just as logically accounted for. Considering these data, it is inexplicable why Camp raises this objection.
Protoavis is both a valid taxon and a disproof of theropod origins
See The Protoavis controversy for a detailed critique of this charge.
There is an intractable temporal paradox in the theropod origin hypothesis
This is perhaps the argument, which no matter how often it is shown to be fallacious, refuses to die. The alleged “temporal paradox” inherent in the hypothesis that Aves is nested with Theropoda is a principal foundation of the argument for the “thecodont” ancestry of birds, and Camp has done a fine job of repeating the same litany of tired errors that Feduccia (1996, 1999) has before him. The entire argument that par-avian dinosaurs appear too late to be considered as ancestral taxa for Aves, is based on the assumption that Maniraptoriformes and indeed Coelurosauria, post-date Archaeopteryx. And yet, while creationists and their intellectual confreres in the “thecodont” camp dogmatically adhere to this concept, does the fossil record substantiate it?
That Coelurosauria dates from the terminal Jurassic (Tithonian) and thus is at least coeval with Archaeopteryx has been readily apparent since the discovery of Compsognathus longipes, recovered from the very Solnhofen beds in which the urvogel was found entombed. Considering that Huxley reviewed the osteology and phylogenetic status of Compsognathus in 1868, we thus have known that Coelurosauria is at the very least no younger than Aves for nearly a century and half. Indeed, Gauthier (1986) placed Compsognathus inside Maniraptora, and although this topology has been infrequently recovered, newly discovered basal compsognathids are adding to its support (Hwang et al. 2004) Further terminal Jurassic coelurosaurian forms include Ornitholestes hermanni (Osborn 1903) and Coelurus fragilis (Marsh 1879), the latter two possibly allied with Maniraptoriformes (Dodson et al 1990, Clark et al in Chiappe & Witmer 2002). Further as yet undiagnosed coelurosaurian post-crania are known from Upper Jurassic rocks, including scattered vertebrae (Jensen & Padian 1989, Makovicky 1998). Teeth bearing resemblance to the dental morphology of troodonts date from, at the latest, the Kimmeridgian (Chure 1994, Clark et al in Chiappe & Witmer 2002), arguing that the derivation of Maniraptora predates the temporal record of Archaeopteryx. Because of their highly apomorphic nature, Camp's argument that these teeth may represent a non-coelurosaurian is simply specious.
Concordant with the expectation that the earliest known fossil of a clade is merely an estimate for the derivation date of that clade, are tantalizing finds from the medial and early Jurassic which suggest that the primary adaptive radiation of Coelurosauria was well underway at this time. Isolated dentition displaying deinonychosaur affinities is known from at least two medial Jurassic locales in Great Britain, and as in the case of similar teeth recovered from Kimmeridgian rocks, this material argues that Maniraptora had appeared and diversified well before Archaeopteryx makes it’s entry in the Tithonian (Evans & Milner 1994, Metcalf & Walker 1994).
A partial braincase displaying a caudal tympanic recess, diagnostic of Coelurosauria, is known from the Lower Jurassic rocks of the La Boca Formation in Mexico, and provides yet more compelling data to support a early-medial Jurassic derivation and initial adaptive radiation of the coelurosaurian dinosaurs (Clark et al 1994). In a similar vein, if Witmer’s assessment (Witmer in Chiappe & Witmer 2002) of the braincase of “Protoavis texensis” is correct, there is evidence to suggest that Coelurosauria originated in the Upper Triassic, some 75 million years prior to the derivation of Archaeopterygidae. Curiously, there is intriguing albeit limited evidence to suggest that Therizinosauria, without question the most enigmatic and bizarre of coelurosaur lineages, may have its origin in the Lower Jurassic (Xu et al 2000).
Theropoda are too autapomorphic to have given rise to birds
The origin of this claim is apparently an antiquated reference to the arguments of Benjamin Mudge, a contemporary of T. H. Huxley’s on the other side of the Atlantic who felt that as a whole, Dinosauria was too Byzantine a lineage to have been the progenitors of birds. What, if any, factual basis this argument has in reality in the 21st Century is not readily apparent. In fact, Theropoda, being incomparibly more birdlike than any other archosaur clade, is the most appropriate ancestral stock for Aves. Indeed, those theropods which occupy the closest phylogenetic position to the ancestry of Aves, are by and by so astonishingly avian in their osteology that they are scarcely differentiated from the urvogel itself (e.g., Sinornithosaurus). And although Camp is quite right about dromaeosaurids not being directly ancestral to birds, he is quite mistaken about some of the particulars.
For example, Camp repeats the argument that all non-avian maniraptorans were much too large. This is incorrect. The diminutive Microraptor probably weighed no more than half a pound, and was certainly no heavier than Archaeopteryx itself (Xu et al, 2002). Camp also claims that "[a hyperextendable pedal digit II] is not present in Archaeopteryx, the basal bird...," and that this "...is a specialization that moves dromaeosaurids away from the actual avian lineage." But its unclear why he believes this to be the case. If Archaeopteryx indeed lacked a hyperextendible second pedal digit, Camp must explain why the distal condyles of phalanx I show dorsal enlargement, and why the ligament fossae on the first and second phalanges are situated more dorsally (Paul, 2002); both are adaptations for hyperextendability. He must also explain the flange of MT IV, underlying MT III, that strengthens the pair for greater load-bearing. This would be unusual if most of the bird's weight was frequently centered on MT II; that is, if MT II were not hyperextendible and frequently held above the ground.
The list presented by Feduccia (1996, 68) contains valid objections to the theropod origin hypothesis
Camp continues by quoting Feduccia (1996) on a number of ostensible pseudo-homologies of the pelvic girdle, manus, carpus, teeth, pes, and hindlimb, accepting them as such without argument. As it were, each claim fails to stand up to careful scrutiny.
a) Three principal arguments have been advanced by proponents of the “thecodont” hypothesis concerning the pseudo-homology of the pelvic girdle: the presence of a hypopubic cup in Archaeopteryx (Feduccia, 1996, 1999; Martin, 1991), the supposed lack of an auxiliary dorsal process of the ischium in non-avian theropods (Martin, 1991; Tarsitano, 1991), and the degree of opisthopuby seen in Archaeopteryx and non-avian theropods (Feduccia 1996, 1999; Martin, 1983, 1991; Ruben et al, 1997)).
Tarsitano & Hecht (1980), Martin (1981), Feduccia (1996,1999) and Ruben et al (1997) have all argued that the caudal or caudodorsal surface of the pubic symphysis of Archaeopteryx formed a hypopubic cup absent in non-avian theropods, rendering the two non-homologous. The supposed structure itself is putativly observable in two specimens, London and Eichstatt, but neither stands up to serious scrutiny. The pubes of the Eichstatt specimen are preserved in lateral view, and there is no indiciation of any cup-like area. The irregular calcite mass of the London specimen is darker than the associated bony elements and has a very distinct rough composition, arguing that it is instead a artifact. In any event, a hypopubic cup has been reported in Unenlagia by Novas & Puerta (1997), and Norell & Makovicky (1999) have cautiously reported the structure in a Velociraptor specimen (IGM 100/986).
A number of authors (including Tarsitano, 1991 and Martin, 1991) have argued that non-avian theropods lacked the dorsal (posterior) process of the ischium observed in Archaeopteryx and other basal birds. While this was certianly a contentious issue when the respective arguments were initially published, new finds, particularly the basal dromaeosaurids Sinornithosaurus (Xu et al, 2001), Bambiraptor (Burnham et al, 2000) and Micoraptor (Xu et al, 2000) has effectivly shown this to be false. The process is present in all three, and is strikingly similar.
The extreme degree of pubic retroversion restored in Archaeopteryx by Martin (1983, 1991) and cited by others (for example, Ruben et al, 1997), in which the pubis is nearly parallel with the ischium is both unsupported by earlier specimens and demonstrably incorrect in light of newer ones. Because the pubes are disarticulated in the only two specimens (London and Berlin) showing such an extreme degree of opisthopuby, this caudal orientation could very well have been achieved postmortem, as forcefully argued by (among others) Wellenhofer (1985). This was later conclusivly demonstrated by the specimens in which the pelvis was well preserved. According to a recent survey:
- The well-preserved pelvis in the Munich specimen (Wellnhofer, 1993) confirmed that the pubis was directed nearly vertically downward as in Unenlagia (Novas and Puerta, 1997) and Rahonavis (Forster et al, 1998). The pubis was only slightly retroverted, forming a cranial angle of 110d with the long axis of the ilium (Wellnhofer, 1985). (Elzanowski, 2002, p. 144)
b) Following Alberch and Gale (1983, 1985), Hinchliffe and Hecht (1984), Hinchliffe (1985, 1997) Martin (1991), Tarsitano (1991), Hecht and Hecht (1994), Feduccia (1996, 1998, 1999, 2003), and Burke and Feduccia (1997), it is claimed that the digital identity of the avian manus is II, III and IV, as opposed to the I, II and III traditionally assumed for theropods. Originally proposed as an extension of "Morse's Law," which claimed the patterns of amniote digital reduction always occur in the lateral most digits (I and V; Morse 1872), recent embryological studies have argued essentially the same.
In response, critics have traditionally charged that it was not entirely clear that this amniote "ground plan", as Chatterjee (1998) called it, so essential to the embryological argument's identification of visable anlagen as II, III, IV and V was applicable to birds. As he noted, salamanders first develop digit II, and although Burke and Feduccia (1998) are certainly correct that salamanders are not amniotes, it was not entirely out of the question to assume avian/theropod development begins with digit III (as per Shubin, 1994; Garner and Thomas, 1998), and is similarly extraordinary.
Although these doubts as to the identity of visable anlagen are still expressed (for example, Padian 2001), the position is growing incresingly untenable. Studies by Larsson & Wagner (2002) and Kundrat et al (2002) on Gallus, and the preparation of embryos of Struthio camelus by Feduccia & Nowicki (2002) have all identified the elusive fifth (digit I) anlagen. Other more classical lines of evidence, from the position of the pisiform (Hinchcliffe 1985), to the structure of the stereotypical chondrogenic "Y" (Burke & Feduccia 1997, Feduccia & Nowicki 2002), consistently support the identification of condensations as II, III and IV.
As clear as the above evidence may be, some authors (e.g. Galis et al. 2003) have expressed doubt, not about the digital identity of the avian manus, but of the non-avian theropod manus. Herrerasaurids, recently classified as basal theropods, are so plesiomorphic (specifically with regards to the medial wall of the acetabulum, elongated cervicals three and four, lack of epipophyses, development of MT I and IV, the distally rod-like ischium, and the subnarial foramen) that their phylogenetic position would more properly be described as basal saurischian, if not basal Dinosauria or even some sort of dinosauromorph, making their manal digit reduction possibly independent of Theropoda proper, and their sacral count a case of parallelism (Sues in Dodson et al, 1990; Novas, 1994; Sereno, 1994; Fraser et al, 2002; Galis et al, 2003; contra Feduccia 2003; Larrsson and Wagner 2003). Similar difficulties arise with regards to Eoraptor (Galis et al, 2003). Because there are, as of yet, no clearly unambiguous basal theropods retaining two vestigial manal digits, its entirely possible that the vestige retained in ceratosaurs represents V, not IV as traditionally assumed, having already lost I. In this view, both non-avian and avian theropod digits are numbered II, III and IV.
A third possibility championed by Gauthier and Wagner (1999) is that avian digits indeed develop from condensations II, III and IV, theropod digits are indeed I, II and III, but at some point before the derivation of Coelurosauria a homeotic transformation occurred such that condensation CII developed digit DI, condensation CIII developed digit DII, and so forth. The possibility is intriguing given the disconnect between morphogenesis and identity seen in other body segments (Burke et al, 1995), and recent work by Dahn & Fallon (2000) which has shown that the conserved patterns of development do not necessarly dictate digital identity in particular. Very simply, it shows that the number condensation a digit develops from does not necessarly correspond to a digit homologous with that number. The only major shortcoming is that there is no obvious evolutionary impetus for such a transformation. In any case, there is just not enough evidence to level convincing charges of digital pseudo-homology.
c) As it is a classic Ostrom character, the semilunate carpal element of Maniraptora and Archaeopteryx has also come under intense scrutiny subsequent to Ostrom's emphasis on the structure. The debate concerns the homology of the element in Maniraptoriformes and Avialae, with Tarsitano (1991), Martin (1991, 1997) and Feduccia (1996) asserting that the semilunate element of Maniraptoriformes is a proximal carpal, as opposed to a distal carpal as in Avialae (which subsequently fuses to the carpometacarpus to form the trochlea carpalis). Ostrom initially misidentified the semilunate in Maniraptoriformes and Archaeopteryx as a proximal element, considering it representative of the radiale (Ostrom 1976), an error which Gauthier corrected in his 1986 review of saurischian phylogeny. However, the identity of the semilunate element has now been firmly established as a distal carpal, and the semilunate element is indeed clearly distal to the proximal carpals in both the type Scipionyx samniticus and new material attributable to Sinornithoides youngi (Russell & Dong 1993, Sasso & Signore 1998). Considering these data, the argument that the unique semilunate carpal is non-homologous in theropods and Aves is without basis in reality.
d) Citing Martin, it is claimed the dentition of the urvogel is dramatically different than that seen in theropods. Its now quite obvious that conical (or nearly conical) teeth are seen in spinosaurs (Holtz,1998), some abelisaurids (Sampson et al, 2001), and various coelurosaurs like Archaeornithoides, whose unserrated crowns without wide carinae make them quite similar to the teeth of basal birds (Elzanowski & Wellnhofer, 1993). Lack of serrations are also quite common, with various ornithomimids including Harpymimus (Barsbold & Perle, 1984) and Pelecanimimus (Pï¿½rez-Moreno et al., 1994), and some basal deinonychosaurians, including Byronosaurus (Norell et al, 2000) and Scansoriopteryx/Epidendrosaurus (Czerkas & Yuan 2002) lacking them on all teeth, and a host of others having only slight serrations on the premaxillary (for example, Microraptor and Protarchaeopteryx). Also interesting is that juvenile troodontids had both unserrated and conical teeth, making neoteny a plausible explanation for the conditions retention in birds (Paul, 2002).
e) The argument pertaining to the reversed hallux is unusual for a number of reasons. Not only is the hallux of Archaeopteryx not reversed a full 180 degrees as seen in more derived birds, but it has been suggested that a partial reversal (at least) was present in a number of non-avian theropods, including Compsognathus (Ostrom, 1992), Sinornithosaurus (Xu et al, 1999), and Microraptor (Xu et al, 2000), despite their poor preservation. More odd is the requirement of a reversed hallux in non-avian theropods at all. Surely we cannot expect ones ancestors to be exactly the same as their phylogenetic progeny!
To illustrate, consider an example of evolution we are sure even Mr. Camp can agree with: the derivation of domestic dogs. Are we to say that because C. lupus does not have the sagging jowls of a bulldog, the latter could not have been derived from an ancestral stock similar to the former?
The closing point offered in the discussion of the orientation of the hallux is the most valid, in that an anisodactyl pes currently remains a valid synapomorphy of Aves, contrary to the impression of some researchers. Indeed there is no definitive evidence for the retroversion of the hallux in non-avian theropods. Nevertheless, this in no way bars theropods from avian ancestry--if an anisodactyl pes is an avian synapomorphy, then it is only logical that we should see it absent in their ancestral stock.
f) Perhaps one of the most confused arguments made by advocates of "thecodont" origins and parroted by Camp concerns the placement of tarsal process of the tibia. Martin (1991), Tarsitano (1991) and Feduccia (1996) argue that the process in dinosaurs is a centrally placed ascending process of the astragalus, while birds possess a non-homologous "pretibal bone"; a separate ossification whose contact is mostly with the calcaneum. Such a view, however, is inconsistent with the evidence. It is now known that the process present in dinosaurs was a distinct ossification as well (Welles, 1984), and there appears to be no consistent difference in its placement among birds. For example, McGowan (1984, 1985) has shown that the paleognath tarsal process only contacts the astragalus, whereas the process in neognaths appears to be a pretibial ossification. Considering the basal position of the paleognathous lineage (e.g. Cracraft & Clarke 2001) and associated osteology, the homology of the ascending process in such forms and theropods is difficult to reconcile with the argument of Martin & Stewart (1985) that there is a complete lack of homology between Aves and Theropoda in the tibial process.
In passing it is interesting to note that Camp is ostensibly familiar with these and other critiques (endnote 95), but dismisses them out of hand, saying that "neither side is persuaded." This is simply a cop-out; a non-argument in lieu of his and other critic's inability to sufficiently deal with the data. Nor is it particularly true. Alan Feduccia's last review of the case against theropod origins (2002) contains precisely three arguments: the supposed pseudo-homology of the carpus, dentition, and digital identity. All the others appear to have been dropped. Oddly, Feduccia tentativly endorses MANIAC in the same paper, from which it would seem to follow that the arguments he does provide are invalid by admission (Prum 2003).
It should also be noted that the characters dealt with above are by no means exhaustive of those shared by dinosaurs and birds; quite the opposite, in fact. To quote another essay (Avian Phylogeny and Origins) by one of the present authors, "the total number of traits runs into the hundreds." An extensive list is available in appendix 1 of Paul (2002).
See also: Furcula homology
Theropoda possessed a pelvovisceral pump
In his attack on the theropod ancestry of birds, Camp parrots the arguments of John Ruben and his colleagues in asserting that Theropoda possessed a pelvovisceral piston in which diaphragmatic musculature anchored to the procumbent pubes operates upon the hepatic capsule, ventilating the lungs (Ruben et al, 1996, 1997, 1998, 1999).
Pelvovisceral systems are further characterized by an airtight post-pulmonary septum inferior to the liver, which preserves a pressure differential between the cranial and caudal aspects of the thorax (Gans & Clark 1976, Perry 1988, 1990). Osteological characters associated with such a system include (following Mook 1921, Gans & Clark 1976, Duncker 1978, Perry 1988, 1990, Ruben et al 1997, Paul 2002):
a) Thoracic ribs articulate with the dorsal vertebrae via hyper-elongated transverse processes and single proximal heads
b) Gastralia do not meet medially, and are instead set in a cartilaginous sheet
c) Pubes mobile, procumbent
d) Lumbar region formed by reduction of the caudal thoracic ribs
e) Sternal ribs doubled
f) Diverticula largely absent, pneumatic excavation of bones inconsistent or absent, most especially caudal to the post-pulmonary septum
Characters a, b, c, and d, are biomechanical necessities of a pelvovisceral pump. The modified rib articulation with the dorsal vertebrae and failure of the gastralia to meet medially permits the cranial aspect of the thorax (superior to the post-pulmonary septum) to present a smooth surface both dorsally and ventrally in which the viscera can readily move in concert with the action of the diaphragmatic musculature. Similarly, such musculature requires mobility and a procumbent orientation of the pubes (and most likely a high breadth ratio). The presence of a lumbar region is consistently observed in all taxa in which a pelvovisceral pump is found, and is therefore most logically considered to be an essential aspect of the system as a whole. The doubling of the sternal ribs assists in regulation of the pelvovisceral mechanism. The absence or inconsistent presence of pneumatic excavation of the bones caudal to the post-pulmonary septum is particularly indicative of such an aerobic system, as the septum maintains a pressure differential between the fore and aft thoracic compartments.
Ruben et al presented their conclusions on the aerobic system present in Theropoda following the 1996 description of the holotype Sinosauropteryx prima (NGMC 2124) (Ji & Ji 1996) from the Lower Cretaceous Yixian lagerstatten of China, in which a concentration of carbonized material is preserved in the caudal region of the thorax. Ruben and his colleagues identified the material as the liver, and argued that the cranial aspect of this region was convex, demonstrating the presence of a post-pulmonary septum, and thus by inference, a pelvovisceral pump. Ruben et al further asserted that muscle fibers preserved in the type Sinosauropteryx were indicative of the requisite diaphragmatic musculature, inserting on the distal pubes (Ruben et al 1997, Feduccia 1999). Ruben and his colleagues have further maintained that in all regards, the type Scipionyx samniticus (Sasso & Signore 1998) recovered from Cretaceous strata of Italy validates said claims (Ruben et al 1999).
Ruben et al have relied exclusively on the data available from preserved soft-anatomy, minimizing osteological factors in their analyses (Ruben 1995, Ruben & Jones 2000). The soft anatomy of the type Sinosauropteryx displays an overall poor state of preservation, and thus data derived from this material is largely ambiguous until such time as soft-tissues are found preserved with greater fidelity in similar theropods (Martill et al 2000, Paul 2002).
Due to haphazard and unsupervised preparation of NGMC 2124 by amateurs, the slab was shattered symmetrically into at least twelve pieces (Ji & Ji 1996, Paul 2002), and subsequent restorative measures have largely obscured the original extent and form of the carbonized region (Paul 2002). Breakage and infilling have destroyed the original cranial aspect of the carbonized region, such that its form and the degree to which it extended cranially cannot be ascertained with certainty (Paul 2002). The dorsal, central, and ventral margins identified by Ruben et al (1997) as displaying a convex arc, in fact delineate a pseudo-margin resulting from the inadequate preparation of the specimen. Thus, the initial conclusion that the carbonized material displays a convex arc in cranial aspect is not unequivocally substantiated at this time.
Moreover, the liver of both birds and crocodiles, the two extant nodes of Archosauria, display convex cranial arcs, and the presence of such an arc in and of itself is not indicative of a pelvovisceral system (Duncker 1979, Brackenbury 1987, Paul 1988, Paul 2002). Ruben et al infer a post-pulmonary septum, in concordance with their argument that the carbonized material displays a convex arc in cranial aspect, however, the reason why such a septum must be present given a convex cranial arc, is not clear. Furthermore, the height of the liver (assuming the material represents as much) is not a reliable correlate of a pelvovisceral system. Liver anatomy is highly variable throughout the ontogeny of any given taxon and individual animal, and more germane, there is no consistent difference in the size and position of the liver in crocodiles, and birds (Siwe 1937, Brackenbury 1987, Secor & Diamond 1995, Paul 2002). Curiously, a post-pulmonary septum is observed in none of the remaining Sinosauropteryx material (Ackerman 1998, Chen et al 1998, Paul 2002).
Ruben et al (1997) attempted to mitigate these ambiguities with the soft-anatomy of Sinosauropteryx by arguing that preserved muscle fibers in the type Scipionyx were oriented in a position to be expected of diaphragmatic musculature. However, these fibers—if they are indeed muscle fibers—have subsequently been identified as components of M. obliquus and M rectus, muscles of the abdomini, by virtue of their configuration (Paul 2002).
Further doubt is cast on the conclusions pertaining to the aerobic capacity in Sinosauropteryx made by Ruben and his colleagues, by the soft anatomy of Scipionyx samniticus. In the type Scipionyx, the intestinal tract is preferentially preserved at the expense of other viscera, and a structure immediately caudal to the sternum, distal ends of the humeri and proximal blade of the scapula, is most logically associated with the liver (Sasso & Signore 1998, Ruben et al 1999, Paul 2002). Ironically, the cranial orientation of the liver in Scipionyx is entirely incongruent with the position of the alleged liver in Sinosauropteryx. The position of the intestines in Scipionyx and the carbonized matter in Sinosauropteryx are identical, and given these data, the most logical conclusion is that the latter represents the fossilized post-mortem decay of the intestinal tract (Paul 2002).
Thus, even offering Ruben’s research the benefit of the doubt (i.e., assuming they have identified the carbonized material in Sinosauropteryx correctly), the data presented in Ruben et al (1997, 1999), is not the sine qua non verification of a pelvovisceral pump in Theropoda, in that the soft-anatomy is sufficiently ambiguous to render definitive conclusions based on such, wholly speculative at this time.
Given these limitations, the osteological data is currently the most reliable in determining the aerobic faculties of Theropoda. While Ruben (1995) and Ruben & Jones (2000) have argued that osteological factors are largely inconsequential to the operation of aerobic systems, there is no apparent reason why this should be so, and indeed the suite of characters which are biomechanical necessities for the respective systems of aerobic regulation, seem to directly refute this assertion.
Theropoda lack all the osteological modifications permitting the operation of a pelvovisceral pump, and in particular, lack the modification of the proximal heads of the ribs and the communication of the gastralia seen in crocodiles. In Theropoda, the thoracic ribs articulate with moderate or reduced transverse processes of the dorsal vertebrae via bifurcated proximal heads, and thus the dorsal surface of the rib cage in theropods is sharply corrugated (Ostrom 1969, Paul 1988, Britt 1997, Paul 2002). The gastralia meet medially in Theropoda, and were not imbedded within a cartilaginous sheet (Ostrom 1969, Paul 1988, Dodson et al 1990, Paul 2002). These characters would have precluded the formation of a sub-cylindrical, smooth-walled tube in which the viscera could be acted upon by diaphragmatic muscles (Paul 2002), and these data alone, demonstrate that the presence of a pelvovisceral system in Theropoda, was a biomechanical impossibility.
The absence of a lumbar region, inadequacy of the distal pubes to support diaphragmatic musculature and immobility of the pubes (especially in opisthopubic taxa, e.g. Eumaniraptora) further argue against the presence of such a hepatic piston, as is seen in crocodiles (contra Ruben et al).
To preclude the assertion of negative evidence, review of the osteological characters associated with avian or paravian aerobic capacities, which are, or are not present in Theropoda, is germane. Characters associated with the avian aerobic system include (after Zimmer 1935, Bellairs & Jenkin 1960, King 1966, Schmidt-Nielsen 1972, Olson 1973, Perry 1983, Beale 1985, Paul 1988, Perry 1989, McLelland 1989, Bramble & Jenkins 1998, Paul 2002):
a)Postcrania consistently pneumatic, infiltrated by pulmonary diverticula
b)Proximal thoracic rib heads bifurcated, forming a corrugated dorsal rib cage
c)Caudal ribs not reduced
d)Sternocostal hinge present
e)Sternal ribs are singular, not double, and usually ossified
f)Ossified uncinate processes
g)Gastralia meet medially
In Theropoda the postcrania are consistently pneumatized, displaying invasion of pulmonary diverticula. Pneumaticity of postcranial elements extends to the sacrum and caudal vertebrae in some Theropoda (Russell & Dong 1993, Britt 1997, Paul 2002), in contrast to the pattern expected had an airtight post-pulmonary septum existed in life. The proximal heads of the thoracic ribs, in even the most basal theropods, are bifurcated and articulate with the dorsal vertebrae via moderate transverse processes, as detailed above. The caudal ribs in Theropoda are not reduced, and consequently, a lumbar region is absent. The sterna and coracoids of Maniraptoriformes articulate via a hinge joint permitting sternal kinesis (Paul 1987, 1988, Norell & Makovicky 1999, Burnham et al 2000, Paul 2002). Contrary to Ostrom (1969), the sternal ribs of Maniraptoriformes are singular, and ossified (Barsbold 1983, Paul 1988, Ji et al 1998, Norell & Makovicky 1999, Xu, Wang & Wu 1999, Paul 2002). Ossified uncinate processes are present in derived maniraptoran taxa (Paul 1988, Xu, Wang & Wu, 1999, Norell & Makovicky 1999, Xu et al 2000, Burnham et al 2000, Paul 2002). Lastly, the gastralia of Theropoda meet medially, and were not set in a cartilaginous sheet, as in crocodiles.
It is thus concluded, that the osteology of Theropoda is inconsistent with the presence of a pelvovisceral pump, and at least in two regards, the operation of such a pump would have been a biomechanical impossibility. On the contrary, all the modifications of the thoracic and pectoral skeleton, as well as the vertebrae seen in Theropoda, are congruent with those observed in Aves, arguing the presence of a similar aerobic system in at least some theropods. Considering the ambiguity of the soft-anatomy at hand, current analyses must rely on the osteological data until such time as better-preserved soft-tissues become available.
Terrestrial origin of flight is a biophysical impossibility and thus theropods cannot be bird ancestors
At the end of Camp’s sorry diatribe, we at last find something, which has some factuality to it: the terrestrial origin of bird flight is indeed a biophysical impossibility. This does not, however, bar theropods from avian ancestry nor does it equate to a disproof of that fact that birds are derived from somewhere within Archosauria. As Paul (1988, 2002) so rightly points out, to demonstrate that at least some Theropoda were not arboreal requires definitive evidence to illustrate that arboreal habit would have been implausible or impossible in such forms. Yet no data to this end has thus far been produced, and the idea that some theropods were arboreal to varying degrees has instead been categorically dismissed, perhaps for no other reason than it’s being so unorthodox.
Curiously though, the idea of arboreal theropods is scarcely a new heresy. Marsh argued as much in 1881, when he considered the origin of avian flight. Indeed, even Alan Feduccia, who today represents the staunchest critic of the very notion of arboreal theropods, in his 1980 review considered these taxa to be quite versatile in their locomotor faculties. Indeed, there is growing evidence to suggest that some Theropoda were arboreal, or scansorial, and in particular, the derived coelurosaurs.
Basal Deinonychosauria display classic signs of increased arboreal locomotion in the elongation of the distal pedal phalanges at the expense of the proximal phalanges, the dorsoventral excursion of the glenoid fossa, elongation of the forelimbs, and perhaps even the classical, functionally didactyl pes of deinonychosaurs (Chatterjee 1997, Paul 2002). Indeed, the discovery of Sinornithosaurus millenii and Microraptor spp. has provided dramatic confirmation of the prediction that we should find maniraptorans in which traits associated with arboreal ecology are found. The recent discovery of Yixianosaurus longimanus, which shows extremely elongated penultimate phalanges, suggests arboreality could have been widerspread than previously thought (Xu & Wang, 2003).
Yet perhaps the most startling corroboration of this hypothesis, can be found in the enigmatic, and poorly known Elmisauridae, a curious taxon believed to represent a basal assemblage of oviraptorosaurs. Currie (in Dodson et al 1990) and Sues (1997) have executed the most recent reviews of the taxonomy and osteology of Elmisauridae, although for purposes of taxonomic clarity and convention, the author retains the taxonomy of Elmisauridae enumerated by Currie as opposed to the revisions offered in Sues (1997).
Elmisaurs are known from the Maastrichtian of Mongolia, Alberta, and only recently, Montana (Varricchio in Tanke & Carpenter 2001). Smallish, gracile theropods, they display a curious mosaic of traits associated with arboreal habitat. The postacetabular blade of the ilium is reduced with a concomitant expansion of the preacetabular blade, a positive correlate of arboreal lifestyle (Currie in Dodson et al 1990, Chatterjee 1997). In this case, expansion of the cranial aspect of the ilium would accommodate the enlarged femoral protractor musculature, the iliotibularis cranialis (Chatterjee 1997).
Perhaps the strongest indication of arboreal habitat in Elmisauridae, however, can be found in the pedal osteology of these dinosaurs. The penultimate phalanges of digits III and IV are elongated comparative to the proximal phalanges, and the articular faces of the distal phalanges in these digits are strongly ginglymoid (Varricchio in Tanke & Carpenter 2001). These adaptations are strongly correlated with grasping function in an arboreal context (Clark et al 1998, Hopson & Chiappe 1998). Most interestingly, a quantitative analysis of the articular surfaces of the distal phalanges presented in Varricchio (in Tanke & Carpenter 2001) demonstrates that the height-width ratio of these elements in Elmisauridae, at 0.88, compares favorably to those of arboreal birds with a strong grasping pes, such as Aquila spp. (where the same ratio is 0.91). Contrast the ratio calculated for elmisaurs with that observed in obligate cursors, such as Dromaius spp., where the same ratio is 0.67.
Objections that no theropod was small enough to be arboreal are baseless (e.g., Ostrom 1986, Feduccia 1996). Not only do multiple extant animals larger than allegedly arboreal theropods manage to be arboreal without issue, but theropods displaying exceptional arboreal adaptations are often exceptionally small. As previously mentioned, the basal dromaeosaurid Microraptor weighs in at a mere 200g (Paul 2002). Significantly, this is less than the urvogel, which likely weighed between 1-2kg.
In light of these data, the classical argument that all theropods were obligate terrestrial cursors is indefensible.
In his 101st endnote, Camp quotes Padian & Chiappe (1998) as saying that bird lungs "are extremely complex and are unlike the lungs of any living animal." This actually in response to those who claim non-avian theropods could have evolved the respiration of birds. The complete paragraph reads (with Camp's quote bolded):
- [The] assertion [that the complex lungs of birds could not have evolved from theropod lungs] cannot be supported or falsified at the moment, because no fossil lungs are preserved in the paleontological record. Also, the proponents of this argument offer no animal whose lungs could have given rise to those in birds, which are extremely complex and are unlike the lungs of any living animal.
Obviously, quoting that setence without the proper context could leave his readers with the wrong impression.
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