SOLNHOFEN LIMESTONE:
HOME OF ARCHAEOPTERYX

Elaine Kennedy
Geoscience Research Institute

Geoscience Reports 30:1-4 (Winter 2000).
    Related page — | EDITORIAL |

Introduction

    Paleontologists have attempted for many years to establish an evolutionary link between dinosaurs and birds. Numerous candidates have been considered, then disqualified upon further examination. Cretaceous birds have been identified, feathered dinosaurs have been proposed; but none of the finds contain the truly striking characteristics of Archaeopteryx. Part reptile, part dinosaur, part bird — the creature still fascinates us. If Archaeopteryx is not an evolutionary link, how do creationists explain such a unique organism?
    The first reported Archaeopteryx fossil discovery was a single feather in 1860. The following year a patient paid Dr. Carl Häberlein with a complete skeleton with feather impressions for services rendered. That specimen was sold to the British Museum for £700.1 By 1999, a total of 8 specimens had been recovered.2

Geologic Setting

    North of Munich, Germany, lies the Southern Franconian Alb, a plateau made of upper Jurassic marine limestones called the Weißer Jura (White Jurassic). Scientists believe that this deposition of marine limestones occurred in a small area of the European continent that was part of a large region of land (Pangaea) across which a variety of seas ebbed and flowed. Between Europe and Africa, Pangaea was separated by a sea with a distinct fauna (the Tethys Sea). The Tethys was much larger than the present Mediterranean, extending from Europe, east past Asia, and north into the Arctic Ocean. Because Tethyan faunas are found in the upper Jurassic limestones of Germany, there is believed to have been a connection between the body of water depositing the Weißer Jura and the Tethys Sea. (Geologists think there must have been a connection to the Boreal Ocean to the north of present-day Germany that apparently contributed limited numbers of fauna to the region as well.)3

The Solnhofen Limestone

    The Solnhofen limestone lies within the Weißer Jura deposits. The distribution of the Solnhofen is discontinuously exposed from the city of Donauwörth to Regensburg, a distance of approximately 100 km by about 25 km wide extending through that region. Some of these isolated deposits have a maximum thickness of more than 150 m.
    The discontinuous nature of the limestone is thought partly to be due to the paleotopography of the sea floor, i.e., the swells and troughs that had been formed by the accumulation of sponge/algal mounds controlling sedimentation during hurricanes or providing a back reef setting in which cyanobacteria generated carbonates.4 Some of these mounds are overlain by continuous deposition of the upper Solnhofen limestone but others are capped by coral reefs that created isolated basins where much of the limestone was deposited.5 The Solnhofen is described as back reef basins/lagoons.
    Throughout the region the Solnhofen Formation is divided into the upper and lower units by a widespread slump of partially cemented calcareous muds. The upper Solnhofen is capped by another submarine slump. All of the local quarries are located in the upper Solnhofen deposits.
    The character of this upper limestone unit varies throughout the formation with alternating sequences of hard and soft beds. The hard calcium carbonate beds (96-98% CaCO3) are separated by thin clays that contribute to the splitting of the limestone. The hard units are the primary rocks quarried for their technical commercial value. Fossils are more easily found in these beds.
    It is unfortunate that although fossils also are found in the softer carbonate units (70-90% CaCO3), they are typically discarded without examination for fossil content, because the local quarries are not mining for fossils.6

Archaeopteryx

    The significance of the Solnhofen limestone goes beyond its depositional history. Prized for centuries for its commercial uses, the limestone contains a diverse accumulation of marine and terrestrial fauna, the most famous being Archaeopteryx lithographica, a "dinosaur" bird.
    The mixture of dinosaur and reptilian traits with distinctive avian characters is well documented (Table 1). Was it a link between dinosaurs and birds? Was it a tree- or ground-dwelling feathered dinosaur, or could it fly like a real bird? These questions have been difficult to resolve.
    Some have argued that because Archaeopteryx had only 9% of its muscle mass in its "wings," it could not maintain flight or fly up from the ground. Another researcher thinks that given the reptilian skeletal features, one could assume that Archaeopteryx had reptilian muscle tissue providing twice the strength of bird muscles having the same percentage of wing muscle to body muscle mass.
    One paper evaluating the merits of both sides concludes that reptilian muscles lose much of their power due to the production of body heat. He suggests that comparable muscle mass has the same net power output. Because the avian wing muscles are twice as massive (based on percentage of body muscle mass) as Archaeopteryx, the author concluded that it could not maintain flight nor could it take flight from the ground.7 It should also be noted that Archaeopteryx does not have a keeled sternum for the attachment of the flight muscles (see Table 1).

TABLE 1.* Distribution of Features of Archaeopteryx in Other Groups

Features of Archaeopteryx shared with reptiles (diapsids plus turtles) in general but not with other birds
  • Long bony tail. Modern birds have fused caudal bones for the attachment of the feathers that form the tail.8
  • Digits have claws. Juvenile ostriches have clawed digits.8
  • Vertebrae are reptilian in structure and lack the spiny, stemmed appearance of birds.8
  • Stomach ribs (gastralia) are present, as in plesiosaurs, crocodilians and dinosaurs.9
  • Presence of interdental plates.10
  • Carpals in the wrist are unfused with the exception of the third carpal. Birds have fused carpals and metacarpals.8
  • Tarsals in the ankle are free with the exception of the fused third tarsal. Birds have fused tarsals.8
  • The bones are solid and lack pneumaticity, with the exception of the cervical and anterior vertebrae.11
  • The sternum was not bony or keeled, leaving no attachment point for flight muscles. Birds have a bony sternum, but some birds lack a keel.8
  • The coracoid is rounded; in birds it is much elongated.9

Features of Archaeopteryx shared with birds,12 but not with dinosaurs or other reptiles13

  • On each foot the first digit extends backward (a semi-reflexed hallux). This may have been reported in some dinosaurs.14
  • The quadrate bone articulates with the braincase. This may be a general amniote condition.15
  • Fused clavicles form the furcula. This has also been reported in some theropods.16
  • The "wings" have fully formed, fully modern flight feathers (the main shaft is off-center to the windward side), with barbs and barbules that maintain each feather's shape.
  • Palatine structure is similar to that of thecodonts and crocodiles; distinctively avian.10,12
  • Lack of coronoid bone.10
  • Digits 2,3,4 in the forelimb. Until rumored recent revisions, dinosaurs have digits 1,2,3.

Features unique to Archaeopteryx10

  • The teeth of Archaeopteryx are widely spaced, conical, without serrations.
  • The prearticular rostral blade of the mandible is unique.
  • The pterygoid bone is distinct in its shape and its articulation with the quadrate.
  • The quadrate bone is long in proportion to skull size.
  • Presence of a prearticular rostral blade.
*The interpretations of these features are consistent with each cited author's perspective on the proposed evolutionary relationships among dinosaurs, birds, and reptiles.

The Controversy

    Although several different kinds of animals have been postulated as ancestors for birds, the most popular are the therapod dinosaurs known as coelurosaurs.17 Whether or not these dinosaurs gave rise to modern birds is a subject for intense debate in the public arena. The real question here is "What do we do with Archaeopteryx?" How are we to understand an animal that is so obviously a mix of traits? Is it the long-sought link between dinosaurs/reptiles and birds? 
    Recent research18 challenges the idea of linkage between dinosaurs or reptiles and birds. The authors argue that coelurosaurs do not have turbinates in their nasal cavities and have a diaphragm like crocodiles. In addition, coelurosaurs have a true first digit, and there is evidence that birds do not develop the first digit as embryos. An examination of the pelvic structures shows that the more bird-like pelvis of Archaeopteryx lies stratigraphically below the proposed coelurosaurian ancestor in the fossil record.

Conclusions

    The uniqueness of the animal is undisputed and the combination of traits is intriguing.19 The number of total specimens is small, and so much more research is needed. 
    Some individuals have proposed that the British Museum specimen is a hoax and that without feathers Archaeopteryx is just another dinosaur.20 Even if that were true, it would neither eliminate the other specimens with feathers that have been found through the years, nor would it end the controversy.21
    Despite all the conflicting data with respect to the linkage between dinosaurs/reptiles and birds, it seems clear that although Archaeopteryx is the best candidate, it is not the link.

 

ENDNOTES

  1. Barthel KW, Swinburne NHM, Morris SC. 1990. Solnhofen: A Study in Mesozoic Palaeontology. NY: Cambridge University Press, 10-11.
  2. Frickhinger KA. 1999. The Fossils of Sohnhofen. Korb: Goldschneck-Verlag, p 150.
  3. Barthel, Swinburne and Morris, p 17-31 (see Note 1).
  4. Viohl G. 1985. Geology of the Solnhofen lithographic limestone and the habitat of Archaeopteryx. In: Hecht MK, Ostrom JH, Viohl G, Wellnhofer P, editors. 1984. The Beginnings of Birds. Proceedings of the International Archaeopteryx Conference. Eichstätt: Freunde des Jura-Museums, p 31-44.
  5. Frickhinger, p 159-169 (see Note 2). See also: Barthel, Swinburne and Morris, p 17-31 (see Note 1).
  6. Ibid.
  7. Speakman JR. 1993. Flight capabilities in Archaeopteryx. Evolution 47(1):336-340.
  8. Romer AS. 1966. Vertebrate Paleontology. Chicago: University of Chicago Press, p 166-167, 374. (Information was also retrieved from Vertebrate Paleontology class notes recorded at Loma Linda University in the mid-1980s.)
  9. Carroll RL. 1988. Vertebrate paleontology and evolution. NY: W.H. Freeman & Co.
  10. Elzanowski A, Wellnhofer P. 1996. Cranial morphology of Archaeopteryx: evidence from the seventh skeleton. Journal of Vertebrate Paleontology 16(1):81-94.
  11. Britt BB, Makovicky PJ, Gauthier J, Bonde N. 1998. Post cranial pneumatization in Archaeopteryx. Nature 395:374-376. Though Britt et al. found evidence of vertebral pneumaticity in cervical and anterior thoracic vertebrae, this cannot be used as a strong argument for an avian feature, since it also occurs in therapod dinosaurs.
  12. Only two characters identify Archaeopteryx as avian: the furcula and the feathers. See: Feduccia A. 1985. On why the dinosaur lacked feathers. In: Hecht et al., p 75-79 (see Note 4).
  13. Thecodontia is an order of reptiles found stratigraphically lower in the fossil record than dinosaurs and some crocodiles and therefore, considered a possible ancestor for the two groups.
  14. Digital development of birds may or may not be similar to that of Archaeopteryx, but is clearly different from that of theropod dinosaurs. See: (a) Hinchliffe JR. 1985. 'One, two, three' or 'two, three, four': an embryologist's view of the homologies of the digits and carpus of modern birds. In: Hecht et al., p 141-147 (See Note 4); (b) Hecht MK. 1985. The biological significance of Archaeopteryx. In: Hecht et al., p 149-160 (see Note 4).
  15. Wellnhofer P. 1990. Archeopteryx. Scientific American 262(5):70-77.
  16. Gauthier JA, Padian K. 1989. The origin of birds and the evolution of flight. In: Padian K, Chure DJ, editors. The Age of Dinosaurs: Short Courses in Paleontology, No. 2 (The Paleontology Society), p 121-133.
  17. Ostrom JH. 1991. The question of the origin of birds. In: Schultze H-P, Trueb L, editors. Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Ithaca, NY: Comstock Publishing Associates, Cornell University Press, p 467-484.
  18. (a) Padian K, Chiappe LM. 1998. The origin of birds and their flight. Scientific American (Feb):38-47; (b) Ruben JA, Jones TD, Geist NR, Hillenius WJ. 1997. Lung structure and ventilation in therapod dinosaurs and early birds. Science 278:1267-1270; (c) Morell V. 1996. A cold, hard look at dinosaurs. Discover (Dec), p 98-108.
  19. Romer, p 166-167 (see Note 8).
  20. Citations for such proposals are published by the Center for Scientific Creationism on the web: http://www.creationscience.com.
  21. Ostrom, p 469 (see Note 17).

Limesteone quarry near Solnhofen, Germany. (Photo courtesy Elaine Kennedy) — color version (51K), large color version (220K)

Archaeopteryx fossil slab from the Buergermeister Mueller Museum in Solnhofen, Germany. (Photo courtesy of Elaine Kennedy) — color version (86K), large color version (637K)

Lateral view of reconstructed Archaeopteryx skull (modified from Figure 252 in Romer 1966, p 166). (Drawing by Katherine Ching)

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