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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
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 Berlin specimen, HMN 1880
Cumulatively, there are eight specimens attributable to the urvogel: seven skeletons, and one asymmetric remex.
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.