Specimens of Archaeopteryx
Archaeopteryx fossils from the quarries of Solnhofen limestone represent the most famous and well-known fossils from this area. They are highly significant to paleontology and avian evolution in that they document the fossil record's oldest-known birds.
Over the years, fourteen body fossil specimens of Archaeopteryx and a feather that may belong to it have been found, although the Haarlem specimen was reassigned to another genus by two researchers in 2017. All of the fossils come from the upper Jurassic lithographic limestone deposits, quarried for centuries, near Solnhofen, Germany.
The feather
The initial discovery, a single feather, was unearthed in 1860 or 1861 and described in 1861 by Christian Erich Hermann von Meyer. The fossil consists of two counterslabs, designated BSP 1869 VIII 1 and MB.Av.100, which are currently located at the Bavarian State Collection of Paleontology and Geology of Munich University and the Natural History Museum in Berlin, respectively. Although it was the initial holotype, there were indications that it might not have been from the same animal as the body fossils, but from another, as yet undiscovered, avialan.History
Discovery and acquisition
The feather was first described in a series of correspondence letters between Hermann von Meyer and Heinrich Georg Bronn, the editor of the German Jahrbuch für Mineralogie journal. Examining the fossil on both counterpart split slabs, von Meyer immediately recognized it as an asymmetrical bird feather, most likely from a wing, with an "obtusely angled tip" and a "here and there gaping vane", and noted its blackish appearance. Six weeks after writing this first letter in August 1861, von Meyer wrote again to the editor stating that he had been informed of a nearly-complete skeleton of a feathered animal from the same lithographic shale deposits, which would later be known as the London specimen. Coincidentally, von Meyer proposed the name Archaeopteryx lithographica for the feather, but not for the skeleton. Therefore, the official name of the animal was originally linked to the single feather rather than any actual skeleton, and is formally considered the original holotype.Though 1860 is often the year named for the feather's discovery, there is no proof of this date, and some authors consider it more likely to have been found in 1861, as it seems reasonable that von Meyer's original letter was likely sent not longer after it came into his possession. The feather was discovered in the partition of the Solnhofen Community Quarry located southwest of the municipality of Solnhofen in a forested district known as Truhenleite, which was opened in 1738. 25 meters of the limestone profile of Upper Solnhofen strata are exposed here, but no information was given about which horizon the feather originated from, though the fossil's dark colour may indicate that it came from a deeper level where it was protected from weathering. Today, the quarry is abandoned and the location is built up.
Original description
Von Meyer published his official description of the feather in the journal Palaeontographica in April 1862, wherein he expressed some doubt of its exact age and affinity. In his original description von Meyer mentioned neither the original discoverer nor the collector by name, and also omitted any diagnostic information about its prior ownership. It wasn't until the year of von Meyer's death, 1869, that the main slab came into the Munich collections. The feather's counterslab was acquired by the Natural History Museum of Berlin in 1876, after having been part of the private collection of von Fischer, a Munich physician.There was also some initial uncertainty as to whether the fossil represented a real feather as in modern birds, though von Meyer pointed out that he could detect no morphological difference between the fossil imprint and modern feathers, and was able to recognize the central shaft, the barbs and the barbules. He described the lower end of the shaft as being less clearly imprinted than the rest, and concluded that the feather may have belonged to a juvenile individual for whom the shaft was still soft. Von Meyer also noted that due to having been compressed, the vane was split in several places. As part of his original description von Meyer compared the feather to that of a partridge, noting that the only discernible difference was in being slightly smaller and less rounded at the end.
Some doubt also existed at first as to whether the feather was a real fossil from the Solnhofen lithographic limestone. The preservation was regarded by von Meyer as unusual and therefore suspicious, as it appeared to be "transformed into a black substance" that reminded him of dendrites, which are inorganic pseudofossils. He thought it was possible that someone had skillfully painted the feather onto the stone. However, he could not conceive of an artificial way in which such a perfect feather – especially as it was reproduced in a perfect mirror image on both counterslabs – could be created by a human hand, and therefore concluded the feather must be genuine.
Though von Meyer was certain that the fossil feather was real, he was very reluctant to assign it with certainty to a bird, noting that the concurrent feathered skeleton could be "a feathered animal which differs from our birds essentially". His reluctance to link feathers to birds with certainty was prophetic, as it predated the discovery of very advanced, birdlike feathers on non-avian theropod dinosaurs by well over a century.
Specimen
After von Meyer's original 1862 description, no additional analyses of the feather were performed until 1996, when a detailed assessment of its morphology, function, taxonomy and taphonomy was produced by Griffiths, and the feather was later studied under ultraviolet light in 2004 by Tischlinger and Unwin. The feather was further studied using scanning electron microscopy technology and energy-dispersive X-ray analysis in 2012, and laser-stimulated fluorescence in 2019.Morphology
The feather has a total length of, and the vane is at the widest margin. The end of the feather has an obtuse angle of 110°, while the barbs branch off from the rachis at an angle of around 25°. The barbs branch noticeably into barbules, as in the feathers of modern birds. The base of the feather consists of plumulaceous barbs which are unconnected to one another and result in a down-like appearance. This tuft of down led Griffiths to conclude that Archaeopteryx might have been endothermic in that it implies the use of thermal insulation.The feather is clearly asymmetrical, which has been interpreted by researchers to be a clear indication of its aerodynamic function as a flight feather. As the feather bears little resemblance to the rectrices of the other full Archaeopteryx skeletons, it is generally thought to be a wing feather: Griffiths and others have concluded it was a remex, while Carney and colleagues in 2012 interpreted it to be a covert feather. The feather has a relatively low degree of asymmetry, which Speakman and Thompson in 1994 concluded to indicate it as a secondary remex. If this is the case, then it would have originated from an animal smaller than even the Eichstätt specimen, which is the smallest Archaeopteryx specimen known to date. This is consistent with von Meyer's much earlier interpretation of the feather as having belonged to a juvenile.
Although the calamus of the feather is no longer visible, Kaye and colleagues identified a corresponding geochemical halo using laser-stimulated fluorescence in 2019. On this basis, they cast doubt against all of these interpretations. First, they noted that the feather is more curved and less asymmetrical than the known primary remiges of Archaeopteryx, and too short relative to the secondary remiges of other specimens. Unlike the coverts of modern birds, they also noted that the center line of the calamus is straight and not S-shaped, although they did not rule out a covert or contour feather identity. Carney and colleagues provided a rebuttal to Kaye and colleagues in 2020; they noted that Kaye and colleagues underestimated the length of the calamus based on von Meyer's description, and noted that coverts without S-shaped center lines are found at the wing tips of birds. They considered the feather to be more consistent with an upper major primary covert than any other type of feather.
Colouration
The feather was studied under UV light by Tischlinger and Unwin in 2004, and they found that a pattern of dark and light spots were visible on each side of the shaft. This was interpreted as remnants of the original pigment design that would have been on the feather in life, similar to the spots and bars on the feathers of modern partridges or birds of prey. It isn't possible to be certain of this, however, so conclusions about the colouration and patterning of the plumage cannot be drawn with certainty from this study.In 2012, Carney and colleagues performed the first colour study on an Archaeopteryx specimen with the fossil of this single feather. Using scanning electron microscopy technology and energy-dispersive X-ray analysis, the team was able to detect the structure of melanosomes in the fossil. The resultant structure was then compared to that of 87 modern bird species and was determined with a high percentage of likelihood to be black in colour. While the study does not mean that Archaeopteryx was entirely black, it does suggest that it had some black colouration which included the coverts. Carney pointed out that this is consistent with what we know of modern flight characteristics, in that black melanosomes have structural properties that strengthen feathers for flight. In 2020, they re-performed the analysis using the specimen's melanosome imprints and an expanded dataset of birds, and predicted a matte black colouration with high probability. They reconstructed the entire feather as black, with a darker tip.