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- Archaeopteryx lithographica: The Marvelous Ancient Wing
- “From its anatomy, Archaeopteryx might be characterized as a reptile in the disguise of a bird.”
-Gerhard Heilmann The Origin of Birds, 1926
- “Were it not for these feathers, Archaeopteryx would not have been recognized as a bird, as is demonstrated by the fact that one nearly complete skeleton in which the feathers were not recognized was initially identified as a dinosaur. In fact, there are no features of the bony skeleton of Archaeopteryx that are uniquely avian. All have been described in genera that are classified among the dinosaurs.”
-Robert Carroll Vertebrate Paleontology & Evolution, 1988
- “The Archaeopteryx fossil is, in fact, the most superb example of a specimen perfectly intermediate between two higher groups of living organisms—what has come to be called a ‘missing link,’ a Rosetta stone of evolution.”
-Alan Feduccia The Origin and Evolution of Birds, 1996
A reconstructed model
Archaeopteryx bavarica Detail.jpg
Few are familiar with the vast array of exotic animals, which have at one time or another, populated the planet since its inception some four and a half billion years ago. Yet, there is one name, which is almost immediately recognizable: Archaeopteryx lithographica. The eloquent Greek binomial means “Lithographic Ancient Wing,” an apt moniker for this primordial bird, and the fine-grained calcium carbonate tomb from which it was recovered. Removed from us by an unfathomable amount of time—145 million years—Archaeopteryx is an echo of the past, granted immortality by way of geology. To the dominant species of the Quaternary, Archaeopteryx is the urvogel--a snapshot of evolution in action.
Of all the fossils known, none are so marvelously intermediary. So exquisite in form and detail, and so congruent with Darwinian theory is the urvogel that the validity of evolution has been assured since 1861. Then, as now, the implications of so unambiguous a specimen are earth shattering.
A century and a half of concerted research yielding insightful data on the osteology and paleobiology of the urvogel notwithstanding, Archaeopteryx remains in many ways a riddle wrapped inside an enigma. Yet by synthesizing the many data gathered over the years, I feel that an introduction to the nature of the urvogel and the world in which it lived can be presented, within the constraints of an economical primer. If the presentation of this primer strikes one as too intense, one can only plead infinite passion for its subject. In the biased opinion of the author, the story of the urvogel is the greatest of all vertebrate paleontology—indeed, it is the story of evolution itself.
The Archaeopteryx Material—Scarce as Hen’s Teeth
All of the urvogel fossils have been recovered from the Southern Franconian Alb, consisting of calcium carbonate deposits of variable quality and thickness, cut by the Altmuhl and Danube Rivers, north of Munich, in Bavaria (Viohl 1985, Buisonje 1985, Barthel, Swinburne, Morris 1990, Feduccia 1996, Kemp & Unwin 1997). These deposits date from the Tithonian, the terminal Jurassic.
While most limestone will preserve organic material with a high degree of fidelity, the preservation quality of those, which line the Solnhofen basin, is astonishing. And it is in no small part due to this exceptional quality, that the Solnhofen region has been quarried for commercial purposes for centuries. Indeed, it for this very reason, that the Archaeopteryx fossils came to light in the first place.
The holotype and first articulated skeletal material assigned to Archaeopteryx lithographica was recovered from Solnhofen in 1861, shortly after Meyer’s description of the species had first appeared in the Yearbook of Mineralogy (Griffiths 1996, Feduccia 1996). In the Victorian world, still reeling from the publication of a comprehensive theory of organic evolution a mere two years earlier, the impact of such a find was profound. Multiple institutions scrambled to secure the slab and counter-slab from their owner, one C. F. Haberlein, but it was ultimately the British Museum of Natural History, acting at the behest of the brilliant Sir Richard Owen, which, for a small fortune in Pounds Sterling, procured the newly unearthed fossil. The material arrived in London in November 1862, where it has resided since, to be immortalized as the London Archaeopteryx (BMNH 37001). It was using this specimen that Owen and his implacable rival Thomas Huxley, "Darwin's Bulldog”—would produce diametrically opposite reviews of archaeopterygiform anatomy and the implications thereof, which first appeared in 1863.
A little over a decade later, the Solnhofen quarries would produce yet another Archaeopteryx specimen, which was recovered in 1876 and described in 1877. No doubt humiliated by their loss of the original urvogel material to the British, the Germans wasted little time in purchasing the new specimen and spiriting it off to the Humboldt Museum, in Berlin, where it rests to this day. The Berlin specimen (HMN 1880) was, if possible, more spectacular than BMNH 37001. It was the first complete skeleton of Archaeopteryx, and further underscored the plesiomorphic characters of the urvogel comparative to all other birds.
Despite an excellent state of preservation, the skull of HMN 1880 exhibits considerable postmortem distortion, such that the detailed morphology of the suspensorium is largely obscured in this material. Thus was born a debate over cranial kinesis in the urvogel, which is, in part, unresolved to this day (Romer 1966, Buhler 1985, Carroll 1988, Haubitz et al 1988, Feduccia 1996, Paul 2002). The difficulties in restoring the cranial osteology of Archaeopteryx based on HMN 1880 are graphically illustrated by Heilmann’s 1926 reconstruction of the skull, as well as those of Wellnhofer (1974) and Ostrom (1976). Despite these shortcomings, the Berlin specimen has become the iconic Archaeopteryx—the image, which invariably comes to mind when one speaks the name.
It would be nearly a century before further Archaeopteryx material would be discovered, and the recovery of specimen S5, was not unlike that of BMNH 37001 in its debt to serendipity. A local student stumbled across the slab and counter-slab in a quarry shed in Solnhofen in 1956, and the Maxburg Museum of the Solenhofer Aktienverein acquired the specimens that same year. Heller (1959) and Ostrom (1970, 1976) have described S5 in detail. The Maxburg Archaeopteryx consists of unarticulated post-crania with scattered impressions of the feathery integument, and overall, has been of little use in clarifying the osteology of the urvogel (Heller 1959, Ostrom 1970, 1976, Feduccia 1996, Paul 2002). In 1982, a private collector secured the specimen from the Maxburg Museum, and it remained in his possession until the time of his death in 1991, following which the slab and counter-slab, tragically, were stolen (Wellnhofer 1992).
A more spectacular find was the rediscovery of an Archaeopteryx fossil during John Ostrom’s 1970 review of Pterosauromorpha, in which Ostrom announced that based on the pectoral girdle and integument, a specimen which had been attributed to Pterodactylus crassipes by Meyer in 1859, was in fact conspecific with Archaeopteryx. The material had been displayed since 1855 in the Teyler Museum in Haarlem, Netherlands under the wrong binomial! While the Haarlem specimen (TM 6428) is not particularly well preserved, Yalden (1985), Stephan (1987), Peters and Gorgner (1992) and most notably Feduccia (1993, 1996) have authored detailed review of the material. These studies, particularly those of Feduccia and Yalden, have placed great emphasis on the geometry of the claw arcs measured in this specimen (Yalden 1985 and Feduccia 1996).
In 1973 F. X. Mayr and the Jura Museum announced the most significant Archaeopteryx find since the recovery of HMN 1880 from its limestone grave 97 years prior. JM 2257, which had been in the museum’s possession since 1951 and referred to Compsognathus longipes, had upon subsequent reevaluation by Ostrom (1970, 1973, and 1974) proven attributable to the urvogel. Considering the exquisite preservation of the specimen, quarried out of the Franconian Alb at Eichstatt, its misidentification as a gracile coelurosaur is astonishing, when one considers the voracity of those who argue against a theropod origin for birds.
Wellnhofer (1974) has provided the most detailed review of this material, and the exceptional preservation of JM 2257 has afforded detailed reevaluation of archaeopterygiform osteology. Most importantly, JM 2257 possesses the least damaged skull of any of the Archaeopteryx specimens and has played a prominent role in settling old controversies about the morphology of the temporal region in the urvogel (Wellnhofer 1974, Ostrom 1976, Carroll 1988, Paul 2002).
Following hot on the heels of the Eichstatt find, 1987 saw yet another Archaeopteryx specimen surface, when Gunther Viohl of the Jura Museum stumbled across an urvogel fossil in the collection of the former mayor of Solnhofen, which had gone unnoticed for decades. This second Solnhofen specimen (S6), proportionately larger than any of the other urvogel material, has shed considerable light on the post-cranial anatomy of Archaeopteryx. Wellnhofer (1988, 1992) and Ostrom (1992) have extensively described S6, as has Feduccia (1996).
The most recent, and hopefully not the last Archaeopteryx fossil to be recovered, BSP 1999, held at the Bayerische Staatssammlung fur Palaontologie, is marvelously preserved, lacking only cranial elements. It was unearthed in 1992 and Wellnhofer reviewed the specimen in an elegant monograph published in 1993. BSP 1999 has been an extremely important specimen in the study of the urvogel in that it has largely led us to reconsider the aerial abilities of this species. The presence of an ossified sternum in this specimen, however lacking a carina or other evidence of hypertrophied development indicative of precocial volant capabilities, has nonetheless upheld the argument that Archaeopteryx was indeed capable of powered flight (Hecht et al. 1985, Wellnhofer 1993, Feduccia 1996, 1999).
Similarly, Wellnhofer (1993) and Feduccia (1996, 1999) have emphasized the preserved keratinaceous horn sheaths on the ungual claws, seen elsewhere amongst urvogel fossil only in TM 6428, as compelling evidence for strongly arboreal ecology in Archaeopteryx. Simultaneously, the cranial osteology of BSP 1999 has more or less put to rest the debate concerning the articulation of the suspensorium and the presence or lack of a streptostylic quadrate.
And that is that: these eight specimens are the only tangible links to a creature which, millennia before either you or I would inquire as to its nature, took to the airs over the lagoons of Solnhofen. They are powerful tools—indeed the only tools—for deciphering the phylogeny and paleobiology of the urvogel, but they are in the end only bones, and the tales they tell are limited.
See also: Archaeopteryx is fully bird
While a detailed treatment of urvogel osteology is beyond the scope of a primer, it is nonetheless at the heart of any work on Archaeopteryx. Unfortunately the subject is a daunting thicket of esoteric jargon and abstruse terminology, through which the prospective student must slog. It is much as Richard Feynman, the famed physicist and lecturer, commented—math is to the expression of the physical universe as anatomy is to the expression of evolutionary biology and the fossils, which are pertinent thereto. Considering this, it is necessary that some basic terms be outlined. In referring to the orientation of a character relative to the body as a whole, the author has substituted the standard terms “anterior” and “posterior”, and “superior” and “inferior” with cranial, and caudal, and dorsal and ventral (see comments on this nomenclature in Weishampel et al 1990), respectively. In the following discussion the author will consider the digits of the avian hand to be II, III and IV as indicated by the totality of embryological evidence.
The braincase of Archaeopteryx is markedly inflated, as can be readily observed in JM 2257 (the Eichstatt specimen) and the Munich specimen (Buhler 1985, Elzanowski 2002), beyond the level observed in any potential reptilian outgroup. Elzanowski (2002) commented on the homologies of the postorbital process and noted that the laterosphenoid, frontal, and parietal all contribute to this process in Archaeopteryx, as in neornithines. Elzanowski (2002) was highly critical of Walker’s 1985 analysis of the urvogel braincase based on Whetstone (1983), and argued that prootic recess identified by Walker is not a dorsal pneumatic recess, but rather represents a site of accommodation for the jaw adductor musculature (e.g., pseudotemporalis or adductor mandibulae externus). This suggestion notwithstanding, Elzanowski (2002) has offered no compelling evidence in its defense and Walker’s interpretation is still eminently logical. Elzanowski (2002) dismissed Walker’s reinterpretation of Whetstone’s (1983) “facies articularis pro quadrato,” the alleged quadrate cotyla on the face of the prootic as the threshold to the caudal tympanic recess. Elzanowski argued that the “threshold” is in fact a distorted cotyla and that its topographic relationships support this statement, contrary to Walker’s (1985) arguments. If the structure is a cotyla for the reception of the quadrate it would mean that the quadrate in Archaeopteryx did in fact communicate with the braincase, as has been suggested before (e.g., Haubitz et al. 1988, Feduccia 1996).
The otic region is strikingly avian in Archaeopteryx, most especially in the opening of the caudal (=posterior) tympanic recess within the columella. Such a character state has been claimed for the intriguing evolutionary mosaic Sinovenator changii (Xu et al. 2002). However there appears to be some ambiguity regarding the identification of the structure labelled as the caudal tympanic recess in this study and whether or not it opens with the columellar recess cannot from the available information be adequately determined. In light of these difficulties it seems best to provisionally regard until such time as new data comes forth this character state as distinctly avian. The structure of the crista interfenestralis and the threshold of the caudal tympanic recess are comparable to those observed in juvenile birds from a variety of taxa, although Walker (1985) identified Daption capensis as perhaps the best match. For instance, the prootic/opisthotic contact is notched dorsally, at the apex of the fenestra ovalis. The opisthotic likely contributed to a contiguous loop around the foramen perilymphaticum, which though plesiomorphic in birds is observed in some gallinaceous taxa (Walker 1985). The opisthotic and prootic diverge distally, near the lower end of the recessus scalae tympani, as in nearly all carinates. The otic capsule displays both a fenestra ovalis and fenestra pseudorotundum, marking a level of organization higher than most potential avian outgroups. In these traits, Archaeopteryx is most closely linked to other birds, as opposed to non-avian theropods.
The morphology of the quadrate and in particular the quadrate head is at best unclear. Elzanowski & Wellnhofer (1996) and Elzanowski (1999) have noted inconsistencies between the elements identified as the quadrate in BMNH 37001 (the London specimen), and the Munich Archaeopteryx. Elzanowski (2002) argued against the presence of a deeply bifurcated quadrate head with equivalent capitula, the condition observed in neornithines based on the morphology of the Munich specimen. Elzanowski & Wellnhofer (1995, 1996) noted a small medial capitulum in the Munich urvogel and Elzanowski (2002) based upon this character postulated that the quadrate head in Archaeopteryx might have corresponded to that seen in Oviraptorosauria. In Oviraptorosauria the otic quadrate process is formed by the lateral capitulum whereas the medial capitulum is attached ventral thereto, on the medial slope of the lateral capitulum (Maryanska & Osmolska 1997). If this is the case, then the quadrate head would communicate with the braincase via the contested tympanic concavity, albeit akinetic, unlike the condition in neornithine birds. Given these data, it seems increasingly likely that the quadrate morphology of Archaeopteryx was more complicated than generally thought heretofore.
The squamosal in Archaeopteryx is highly autapomorphic. Contact with the quadratojugal has been lost, and the quadrate cotyla of the squamosal opens rostroventrally. A rostral process, forked slightly at its apex, has been consistently interpreted as a postorbital process of the squamosal (e.g., Paul 2002), and has been used to triumphantly deduce the presence of a complete dorsal temporal bar in the urvogel. Reality, however, interferes with the euphoric celebration. The rostral process of the squamosal forms an acute angle to the quadratojugal; ergo it was directed caudoventrally, automatically ruling out its communication with the postorbital (if indeed the element was present) and the integrity of the dorsal temporal bar (Elzanowski 2002). The rostral process of the squamosal is comparable with the rostral most portion of the zygomatic process in some neornithines (e.g., Gruiformes) and most likely represents a site of attachment for the adductor mandibulae externus.
In a similar vein, the vigorous assertion that the presence of the postorbital in Archaeopteryx is confirmed by the amorphous structure in HMN 1880 (the Berlin specimen), an argument dating from Heilmann (1926), must itself be questioned vigorously. Similarly ambiguous crescentic elements in both the Eichstatt and Munich urvogel specimens have been identified as the postorbital. As reviewed, the morphology of the squamosal seems to preclude any communication with the postorbital. There is no compelling evidence to support the identification of any of these structures as the postorbital, to say the least of Chatterjee’s (1997) conclusion that the postorbital was robust. Thus, the temporal region is fully confluent with the orbit in Archaeopteryx, as in neornithines.
The palate of Archaeopteryx remains poorly understood. Nevertheless, what data is available indicates that there is a considerably more precocial level of organization in the palate of the urvogel than previously envisioned. The palatine is strikingly avian and completely incongruous with the tetraradiate element in potential theropod ancestors of birds (Elzanowski 2002). The palatine bears marked similarities to that in later birds (e.g., Hesperornis, Gobipteryx) particularly in that the medial process of the palatine encloses the choana, while the choanal process itself is slender and hook-shaped, in which it most resembles the corresponding element in Hesperornithiformes. The palatine in Archaeopteryx is sufficiently advanced to merit comparison with select neornithines, and Elzanowski (2002) has compared it favorably with that of the non-struthioniform paleognaths, particularly in that the maxillary process of the palatine is both short and offset, failing to contact the palatal process of the premaxilla.
The pterygoid is highly autapomorphic, contrary to previous interpretations (Elzanowski 2002). The pterygoid expands rostrally forming a trapezoidal blade whereas caudally the pterygoid forms a distinct and triangular quadrate process. Elzanowski (1991, 2002) compared the rostral portion of the quadrate to the hemipterygoid of Hesperornis but noted that the caudal pterygoid is completely unlike that of any other known bird, or potential reptilian ancestor of birds. The pterygoid in Archaeopteryx is most unusual in its division into distinct major and minor components about the longitudinal axis (Elzanowski 2002).
As noted by Elzanowski (2002), the homologies of the elements in the antorbital cavity in Archaeopteryx remain highly uncertain. Wellnhofer (1974), Witmer (1997) and Paul (2002) all argued that there is a fenestrate ascending ramus of the maxilla present in the urvogel, represented by a bony capsule ventral to the nasal in Archaeopteryx, considering it homologous with a similar element in theropods. There are difficulties with this interpretation however. Whetstone (1983) and Martin (1991) both identified this structure as the mesethmoid, and thus argued that the antorbital fenestra was walled dorsally by the nasals. Elzanowski (2002) argued that the element in question bears no particular resemblance to the theropod ascending ramus of the maxilla, particularly in JM 2257 (the Eichstatt specimen) and it is more likely a rostral ethmoid ossification, observed in both Hesperornithiformes and at least some neornithines. Elzanowski & Wellnhofer (1996) and Elzanowski (2002) noted that there is a similar structure, caudal to this ethmoid ossification which may be interpreted as a mesethmoid, as noted by earlier studies. Interestingly, Elzanowski (2002) notes the presence of badly preserved “ossicles” rostral to the lacrimal, suggesting that though they cannot be with certainty identified, the ventral most of these elements resembles the uncinatum of neornithines.
The mandible presents a most curious disparity with that of potential theropod ancestors of birds. The mandibular rami are highly rigid and akinetic structures lacking fenestration completely as well as any other evidence of intraramal mobility (Elzanowski 2002). The lack of an intraramal joint and its associated mediolateral mobility of the mandibular rami is a character which has not been properly stressed in analyses of archaic birds and the ancestry of the class Aves. The vast majority of primitive birds, including Confuciusornis and Enantiornithes lack intraramal joints and this character has a most erratic distribution throughout the class Aves. Both Hesperornithiformes and Ichthyornithiformes possess intraramal joints, as well as Pelecaniformes (Elzanowski 1999, Zusi & Warheit 1992). These joints are constructed of different components and thus must have arisen separately in each lineage, representing a marvelous example of convergent development dictated by shared biomechanical stimuli. Gingerich (1973) heavily emphasized the presence of intraramal joints in the odontognath birds and argued that it represented a synapomorphic character directly inherited from the theropod ancestors of birds. There are serious problems with this assertion. Feduccia (1996) argued that the intraramal joints in the odontognaths bore a greater resemblance to those seen in mosasaurs and varanid lizards than those in theropods, and thus questioned the utility of interpreting this character as a synapomorphy of theropods and birds. Elzanowski (1999) convincingly demonstrated that the two forms of intrarmal joint present in birds are structurally more similar to each other than either is to those of theropods, arguing thusly for their independent origin. Further considering the difficulty in explaining the character distribution of intraramal joints in Mesozoic birds if one wishes to advanced this trait as synapomorphic of birds and theropods, it becomes clear this character offers no corroboration for a theropod/bird nexus. Based on the mandibular morphology and kinetics of the earliest birds, Elzanowski (1999) concluded that avian evolution began with an already akinetic lower jaw lacking in intraramal mobility, a conclusion, which seems robustly supported by the available data.
Finally, there has been considerable controversy and markedly little consensus on cranial kinesis in Archaeopteryx. Buhler (1985) and Feduccia (1996) both argued that the skull of Archaeopteryx was fully prokinetic with a highly mobile quadrate in communication with the braincase, an interpretation also favored by Haubitz et al. (1988) in their computerized tomography work on JM 2257, the Eichstatt specimen. This stands in stark contrast to classical interpretations of cranial kinesis in Archaeopteryx (e.g., Heilmann 1926) in which it was argued that the urvogel skull was strictly mesokinetic. Paul (2002) who strenuously argued that the quadrate in Archaeopteryx was opisthostylic and prokinesis absent has favored this interpretation. Though there remains contention regarding the morphology of the quadrate, a major component of cranial kinesis, there is compelling evidence to suggest that the skull of archaeopterygids was kinetic with a zone of flexion rostral to the braincase, matching the condition observed in all other birds (Elzanowski 2002). The lacrimal and jugal articulate via a sliding joint first noted by Wellnhofer (1974), and the palatine as noted by Elzanowski & Wellnhofer (1996) and Elzanowski (2002) is slatlike and thus indicative of a pterygopalatine bar for the transfer of force between the quadrate itself and the upper jaw. Elzanowski & Wellhofer (1996) and Elzanowski (2002) have both argued for the presence of propulsion joint between the quadrate and pterygoid, which though possible has yet to be conclusively demonstrated. Buhler (1985) demonstrated that the brain of Archaeopteryx was inflated, completely filling the braincase and thus conclusively ruling out the presence of flexion zones within the braincase. Elzanowski (2002) cautions however that there is insufficient data to pinpoint the precise position of a kinetic flexion zone rostral to the braincase. Given the autapomorphic nature of the jaws in Archaeopteryx, it is entirely possible, as Elzanowski (2002) has noted that the cranial kinesis observed in the urvogel has no modern analogue amongst the class Aves.
The dentition has been extensively described by Martin et al. (1980) and Martin & Stewart (1999). Following Elzanowski (2002) there are a total of four premaxillary teeth, either eight or nine maxillary teeth, and either eleven or twelve dentary teeth. Elzanowski (2002) argued that the teeth do not display sharply constricted crown-bases and lack an expanded root but these statements are in conflict with observable facts. Wellnhofer (1988, 1993) clearly illustrates in a number of figures the teeth of Archaeopteryx and the characteristic pattern of constriction and expansion noted elsewhere are plainly visible. It is further noteworthy that such a morphology was in fact noted prior to Martin et al.'s 1980 paper, as far back as Evans (1865) and was first confirmed by Edmund (1960). The teeth resorb the roots of their replacements in widely oval resorption pits in lingual aspect indicating a vertical tooth replacement family. Elzanowski & Wellnhofer (1996) argued that interdental plates are present in Archaeopteryx but as Martin & Stewart (1999) note, this claim is not without contention. The morphology of the alveolar intersepta in Archaeopteryx is not consistent with what is known of the morphology of interdental plates. Thus there is no compelling reason to suppose the presence of such structures in Archaeopteryx.
The post-crania, particularly the vertebral column, pectoral girdle, and pelvic girdle, exhibit a generally basal level of organization. The cervicals articulate with the braincase caudally, not ventrally, as is the case in modern birds. Most of the post-crania are pneumatic, especially the dorsal vertebrae, ribs, and cervicals (Wellnhofer 1974, Britt 1995, 1997, Britt et al. 1998, Paul 2002). The caudal vertebrae are not fused into a pygostyle, and the caudal count is 23 or less. Lastly, gastralia are present in the urvogel, corresponding to the plesiomorphic archosaurian pattern.
The pectoral girdle displays a typically avian arrangement with the scapula and coracoid meeting at an acute angle, and articulating with a fairly well developed furcula to form a triosseal canal. The flight architecture is still quite generalized, as evidenced by multiple characters, including: the lack of robust bicipital crests on the humeri, the absence of a pneumatic fossa on the proximal humerus, the lack of a proximal groove on the humerus for insertion of the deflected tendon of M. Supracoracoideus and thus the incipient development of the acrocoracoid process. Considering these data, it is most likely that while volant, Archaeopteryx certainly could not take off from the ground. A broad, partly ossified though non-carinate sternum was present in the urvogel, no doubt helping to anchor the modest flight muscles. Uncinate processes and ossified sternal ribs, so characteristic of Aves, and representing modifications of the rib-cage and pectoral region to the demands of flight and high-grade respiration, are both absent in the urvogel, telling evidence of its plesiomorphic anatomy.
The ulna lacks remigial papillae, despite having supported a fan of remiges, and and lacks a distal tendinal pit. The structure and homologies of the carpus in Archaeopteryx are much debated. Ostrom (1976a) identified three carpals in the wrist, the prominent semilunate, radiale and ulnare. Martin (1991) subsequently argued that there were four carpal elements, with a small distal carpal III complimenting the remaining three elements described in previous studies. Analysis of the carpus in HMN 1880 and particularly JM 2257 corroborates this observation (Zhou & Martin 1999, Elzanowski 2002, Feduccia 2002). Zhou & Martin (1999) illustrated that in several characters the carpal and manal morphology of Archaeopteryx was more derived than previously thought, though several of their conclusions seem tenuous at best. The ulnare is v-shaped, permitting it to rotate against the articular surface of the carpometacarpus, as in modern birds. This unexpectdely precocial trait in the urvogel is further evidence for its ability to sustain powered flight. The semilunate carpal would have facilitated avian arm folding. A carpometacarpus is absent. An extensor process is incipient, and would have permitted both lateral flexion and hyperextension, the latter to a degree not seen in other birds. Metacarpal IV is not strongly bowed, but it is most closely appressed to metacarpal III distally, foreshadowing the more derived avian condition. It furthermore appears to slant ventrally in distal aspect as in modern birds (Zhou & Martin 1999). Metacarpal II is tightly appressed throughout its length to metacarpal III. In proximal aspect though not fused the metacarpals are tightly associated. Remigial papillae are absent from the metacarpals. The manus is still capable of grasping/raking, and the digits are not fused; most notably digit IV is not confluent with digit III. A posterolateral flange of the proximal phalanx of digit III, characteristic of most birds and the most par-avian Maniraptora, is also absent. Digit II is the most robust, heralding the more advanced avian condition of the manus and relfecing a shift in the role of the hand as a grasping, raking implement, to one primarily used for supporting the remigial fan and climbing (Zhou & Martin 1999).
The pelvic girdle is not especially apomorphic, contrary assertions aside, and the pubis is only mildly opisthopubic in orientation, with an estimated degree of retroversion of around 110 degrees comparative to the iliac blade, though perhaps this angle was greater. The ischium and ilium are not fused into the ilioschiadic structure so characteristic of Ornithurae. A hypopubic cup is absent, and the avian antitrochanter is incipient.
The femur lacks the confluence of the lesser and greater trochanters characteristic of derived avian taxa, as well as a proximal pneumatic fossa. The fibula is continuous to the astragalocalcaneal unit, and is not reduced to a vestigial splint as is seen in Ornithurae. The tibia lacks the double cnemial crests seen in Ornithurae. A tibiotarsus and tarsometatarsus are absent entirely. There is an ascending ossification fused laterally with the calcaneum, as opposed to the medial astragalus (Martin et al. 1980). The pes was anisodactyl, and markedly asymmetrical, with digit IV longer than digit II (Martin 1991).
Most notable in BMNH 37001 and HMN 1880, there are exquisite feathers preserved with the skeleton including long, asymmetrical remiges (Feduccia & Tordoff 1979, Feduccia 1980). The remiges did not insert via papillae and thus are preserved some distance from the associated bony elements of the Archaeopteryx material. Though the number of primaries remains disputed their general distribution is clear. There are either 11 or 12 primaries (after Rietschel 1985) while Stephan (1985) suggested that a "remicle" is present in addition to these feathers. Rietschel (1985) demonstrated the presence of a vental furrow on the rachis providing greater flexural stiffness dorsoventrally. This feature is notably precocial and identical to that in mdoern birds. The microstructure of the feathers is essentially modern (Martin 1991, Feduccia 1996).
The paleoecology of the urvogel is as much debated as is its osteology. While Ostrom (1974, 1985) argued that Archaeopteryx was a highly cursorial form, without substantial arboreal capacity, multiple lines of evidence cast doubt on this assertion. First and foremost, the claw geometry of both the manus and pes, suggest that Archaeotperyx was very much an arboreal form. Yalden (1985) noted that the manal claws of Archaeopteryx most closely approximated those of highly arboreal birds such as the woodcreeper Dendrocopus minor, and found no close resemblance to those of highly predaceous forms. Feduccia (1993a) demonstrated convincingly that the pes claws of Archaeopteryx are also similar in their geometry to those of arboreal, perching forms, with a mean curvature of 120 degrees, well outside the range of terrestrial cursors such as Centropus, where the same curvature measures around 85.7 degrees. The presence of a hallux, clearly an adaptation for perching and thus arboreality, is difficult to reconcile with a cursorial existence (Feduccia 1996). Further evidence comes from the expansion of the distal phalanges, a positive correlate of arboreality (Zhou 1999), that is observed in Archaeopteryx in contrast to the proximal phalangeal expansion witnessed in cursors, such as Dromaius. It has also been pointed out that gross morphology of the pedal and manal claws, characterized by needle-like points and extreme lateral compression, is incompatible with those of a predatory, highly cursorial form as envisioned by some authors (Feduccia 1999). Considering these data, the view of Archaeopteryx as a cursorial form, with little ability or inclination to dwell amongst trees, is simply indefensible.
While Archaeopteryx was at least faculatively cursorial, arguments initially stemming from Ostrom's research in the 1970s, to the effect that the urvogel was a specialized terrestrial cursor, are utterly lacking in substantiating data, and if anything are directly refuted by the structure of the pes itself. The asymmetry noted earlier persuasively argues against the hypothesis that Archaeopteryx was principally a cursor (Martin 1991, Elzanowski 2002), and is more congruent with an arboreal mode of life for the urvogel.
Archaeopteryx has had a very convoluted taxonomic history, with as many revisions thereto, as there are scientists studying it. The senior binomial of Archaeopteryx lithographica is actually Pterodactylus crassipes von Meyer, 1859. Following Ostrom's 1970 reidentification of TM 6428, and referral of the material to Archaeopteryx lithographica, a formal petition to the ICZN was made to conserve the junior synonym for purposes of stability in the literature, which quite obviously, was granted. The derisive name Griphosaurus was applied by J. Andreas Wagner of Munich University of 1861, but this is in turn a junior synonym to Archaeopteryx lithographica and has been all but forgotten.
Huxley (1867) in his comprehensive classification of Aves, assigned Archaeopteryx to Sauriurae. Furbinger (1888) followed this classification and it has since become convention to place Archaeopteryx as the sole member of Archaeornithes, within this sauriurine subclass. Martin (1985, 1987) and later Hou et al. (1996) revised the phylogeny of Sauriurae and argued for the retention of Archaeopteryx within this taxon. Given the difficulties in supporting the holophyly of Sauriurae, most analyses elevate Archaeopterygidae to an order, Archaeopterygiformes, and place it at the base of Aves.
Throughout the history of this genus, the main taxonomic question has always been twofold: how many species, and how many genera? In 1897 W. B. Dames reevaluated HMN 1880--the Berlin Archaeopteryx, which he described--and opined that it represented a distinct species from the London specimen, and renamed his material Archaeopteryx siemensii. This ushered in an era of taxonomic revision which heralded the zealous splitting of taxa we so often see today. In the 1920's, Bronislav Petronievics, in a series of reviews of both the London and Berlin material, went one step further and argued that Archaeopteryx siemensii was in fact not even congeneric with Archaeopteryx lithographica, and placed it in a new genus, Archaeornis. Heilmann (1926) concurred with this assessment, although his focus was primarily phylogeny and not unravelling taxonomic semantics. It was the general consensus of the modernist era in paleornithology, that the differences between BMNH 37001 and HMN 1880 had been greatly exaggerated and that the material was conspecific, although this is not without dissenters (e.g., Elzanowski 2002).
The discovery of new Archaeopteryx material, notably JM 2257, the Eichstatt specimen, reignited the old controversy about how many species and genera of urvogel, there actually are. Howgate (1985) argued with much voracity that JM 2257 cannot possibly be even congeneric with the other Archaeopteryx material, dismissed the possibility of polymorphism or the juvenile status of JM 2257, and went on to elevate the Eichstatt remains to an entirely new genus and species, Jurapteryx recurva. Wellnhofer (1985) effectively demolished this argument, and Jurapteryx has long since faded into taxonomic oblivion. The discovery of BSP 1999, with the preservation of a clearly ossified sternum and proportional and other morphometric differences, suggest to some that this specimen represented a new species of Archaeopteryx and in his 1993 monograph, Wellnhofer labelled it Archaeopteryx bavarica. The general consensus in the paleontological community has been that Archaeopteryx bavarica is a valid taxon, but allometric differences are more than adequate to explain the proportional disparity Wellnhofer was so impressed by (Senter & Robins 2003). Similarly, preservational vagaries, including slab breakage in the Haarlem specimen and portmortem transport in the London and Maxberg specimens, can be invoked to account for the appearance of a sternum in this specimen, and not others. It seems readily forgotten that there has been evidence for the presence of a sternum, ossified or cartilaginous, in Archaeopteryx since 1877, when the gap between the thoracic ribs and gastralia and the region in which a sternum might be expected, was noted (Ostrom 1976b).
Most recently, Andrzej Elzanowski (2002) has offered a review of the systematic paleontology of Archaeopteryx and upheld the validity of Dames' A. siemensii, Wellnhofer's A. bavarica, and placed the sixth or Solnhofen specimen of Archaeopteryx, in an entirely new genus and species, Wellnhoferia grandis. There is absolutely no data to suggest that HMN 1880 (A. siemensii) is distinct from other specimens of A. lithographica, and as mentioned, it seems equally unsubstantiated to extricate BSP 1999 from A. lithographica. Elzanowski's referral of the Solnhofen Archaeopteryx to an entirely new genus and species, is wholly unwarranted.
The author favors a conservative, or at least traditional approach and regards Archaeopteryx as monotypic. I attribute the differences so emphasized by some authors to polymorphism, and preservational defects, and can think of no reason why they should be attributed undue phylogenetic weight.
The Solnhofen World of Archaeopteryx
The process of paleoecological reconstruction is essentially the quest to bring lost worlds to life. It is a tantalizing exercise, offering a window into the past, and to the student of archosaur paleontology, there is no lost world more tantalizing than that in which the urvogel lived, and died, 145 million years ago.
What we know of the Solnhofen world is pieced together from stratigraphic data, and though these data speak volumes, picturing Archaeopteryx in the habitat in which it lived, requires the power of imagination. In the Tithonian, the Solnhofen basin was more southerly than it is now, residing at around 25-30 degrees north latitude, in the subtropics. Solnhofen was a lagoon dotted with small, sparsely vegetated islands, and ringed by larger landmasses forming the European Archipelago to the northwest and northeast. The ancient Tethys Ocean bordered Solnhofen on the south. A suitable modern analogue would be the Gulf of Cariaco (Feduccia 1996, Paul 2002).
The climate would have been arid and tropical, and the lagoon and its back reefs were most likely hyper saline, and subject to mass seasonal algal and planktonic blooms. Both factors would account for the astonishing richness of the fauna preserved in the Solnhofen limestone, and the often tortuously slow death throes observed in the various fossils of marine invertebrates collected from these deposits. One tends to picture the waters of the lagoon and reefs as brilliant turquoise, not unlike the water of the Caribbean and Bahamas. The sun-baked islands spread over the region were by and by small, dry affairs, ringed with sandy beaches similar to those of Africa’s Gold Coast—and equally inhospitable. The quintessential Caribbean scene, with ivory surf washing up on white-sanded beaches fringed by coconut palms, would have been alien here. These islands were far harsher. The paleobotanical evidence indicates that only hardy and generally small coniferous species could eke out a living on these islands, and deal with a seasonal cycle of extensive drought followed by monsoon-like rains. Inland, on the larger islands, denser and lusher forest may have taken hold, but the distinct paucity of fossiliferous trees or tree pollen, suggests otherwise. Rather than a tropical paradise, Archaeopteryx made its living in a distinctly barren locale.
We know from the scarcity of urvogel fossils, that Archaeopteryx was rare, especially compared to the pterosaurs so copiously preserved in the Solnhofen plattenkalk. The figures provided by Wellnhofer (1991) and Davis (1996), underscore this rarity: the urvogel population must have been 1/30 to 1/40 the size of that of the pterosaur population, at any given time. We can picture our primordial bird scampering along the beaches, and clawing its way through the low-lying growth and sporadic trees, fighting amongst others of its species, displaying threateningly or provocatively to rivals or mates, establishing territories and defending nests (envision the immortal paintings of Charles Knight). It is not hard to picture the urvogel scavenging organic detritus washed up on the beach with the tides, and taking wing to fish the lagoon waters, perhaps. It is plausible that there were discrete Archaeopteryx populations spread across the islands, mingling with each other and interbreeding, or perhaps insular, and avoiding contact (see Olson 1987). If given time, this insular habitat may have led Archaeopteryx to neoflightlessness (as per Chatterjee’s 1997 argument).
If this scenario seems too much like a reverie, and insufficiently underwritten by facts, allow one to counter that this is the best, which one can do with the scant clues left us by the geological vagaries of the Earth. In reality, we will never know with certainty the way in which the urvogel lived, nor shall we know all the details about the world as it was in the terminal Jurassic. But this scarcity of unambiguous data does not dissuade those for whom the study of the urvogel exceeds mere academic interest, and becomes intellectual passion. When one sees the skeleton of Archaeopteryx, it springs to life in the imagination, superimposed on the backdrop of its ancient island haunts.
Ultimately, the most contentious question regarding Archaeopteryx reduces to its cladistic affinities—what is the urvogel most closely related to. And in turn, this question invariably comes down to two alternatives: either Archaeopteryx is derived from "thecodonts"; (a paraphyletic hodgepodge of basal and derived archosaurs), or Archaeopteryx is the phylogenetic offspring of Maniraptora.
The subject of archaeopterygiform phylogeny is far too complex to adequately detail in such a brief primer. Below is a simplified graphic of the relationship of Archaeopteryx to theropods, and other birds. Though this hypothesis is not without dispute (e.g., Feduccia 1996), there have been no explicit alternative hypotheses capable of generating any form of a topology with which to compare that offered forthwith:
Neotetanurae (=Avetheropoda sensu Padian et al 1999)
Deinonychosauria (Dromaeosauridae + Troodontidae)
Why Archaeopteryx is so Important
To those who are not familar with the field of avian phylogenetics, or paleontology, or even in the grander sense, evolutionary biology as a whole, the great commotion and passion which this solitary fossil bird invokes, is difficult to understand. To be sure, the fossils of Archaeopteryx are beautiful and exquisite in their preservation. HMN 1880 looks alive enough to jump up from its limestone matrix, and take to the trees. But its obvious aesthetic appeal aside, there is a much deeper significance to Archaeopteryx. As the attendants of the 1984 Eichstatt Conference noted, few animals in the history of life have been accorded the honor of their own conference, assembling the brightest minds in disparate fields, with the singular goal of unravelling the many mysteries about how that animal lived, and to whom it was related.
The profound importance of Archaeopteryx is historical, both in the traditional sense of the term, and the phylogenetic. The urvogel was recovered, as noted, only two years after Darwin published his revolutionary treatise On the Origin of Species. The discovery, so soon, of such a dramatic confirmation of Darwin's hypotheses, was very much a bolt from the blue that jarred the scientific community and shook the Victorian world. So profound was the shock that some, e.g. Wagner at Munich, could not accept the transitional status of Archaeopteryx. The otherwise brilliant anatomist Sir Richard Owen, similarly, could not bring himself to see the obvious. Even today we witness much the same, with authors like Hoyle & Wickramasinghe making the scurrilous and baseless allegation that Archaeopteryx is a fraud. It is not a stretch of the truth by any measure to say that Archaeopteryx is what to many, clinched the validity of evolution as Darwin and Wallace had elucidated it.
And then we have the phylogenetic significance of Archaeopteryx. It is as I have attempted to convey, literally a snapshot of evolution in action, frozen in rock, and in it we witness the most dramatic and amazing transformation in the history of life--that of reptile, to bird. Archaeopteryx, regardless of where it slots within the avian cladogram, is the oldest unambiguously known bird, and as such, it displays the most basal known organizational level amongst birds. Whether or not extant birds arose orthogenetically via Archaeopteryx (which they probably did not), or whether Archaeopteryx is the sister taxon of all other birds (which it almost assuredly is), in Archaeopteryx we have the only tangible evidence of the earliest, and most primitive level in avian evolution. Archaeopteryx is the cornerstone from which any consideration of avian evolution must start, and it is perhaps this fact that makes these fossils so amazingly important--they are the key from which we can unravel the ancestry and early evolution of Aves.
It is perhaps inevitable that one should feel as if the urvogel has been short changed, by discussing it in such a taciturn way. Yet this is, after all, a primer, and as such one has endeavored merely to introduce the reader to the creature nearer my heart than any other. From detailing the nature and number of the specimens available to us, to enumerating the most crucial anatomical characters substantiating the transitional status of Archaeopteryx, to discussing its paleobiology and phylogeny, and a little of its meaning. I think that one can come away from this primer secure in the basics, and ready to delve deeper into the story of the urvogel, and in so doing, discover the nature of evolution itself.
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- All about Archaeopteryx
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Creationists look at the Urvogel
- Archaeopteryx (unlike Archaeoraptor) is NOT a hoax — it is a true bird, not a ‘missing link’ - From the EvoWiki's good friend Jonathan Sarfati of AIG, we learn what the title of the piece clearly suggests; Archaeopteryx is not a hoax, but its also a "true bird".
- Is Archaeopteryx a ‘missing link’? - From Creation Tips, a short, silly creationist look at urvogel.
- As a Transitional Form Archaeopteryx Won't Fly - A classic (and one of the "better"; that is, least poorly informed) creationist look at Archaeopteryx by Duane Gish.
- Bird evolution? from Aig
- Creationist Claim: Archaeopteryx is a fake