Evidence corroborates identity of isolated fossil feather as a wing covert of Archaeopteryx

Abstract

The historic fossil feather from the Jurassic Solnhofen has played a pivotal but controversial role in our evolutionary understanding of dinosaurs and birds. Recently, a study confirmed the diagnostic morphology of the feather's original calamus, but nonetheless challenged the proposed identity as an Archaeopteryx covert. However, there are errors in the results and interpretations presented. Here we show that the feather is most likely an upper major primary covert, based on its long calamus (23.3% total length) and eight other anatomical attributes. Critically, this hypothesis is independently supported by evidence of similar primary coverts in multiple specimens of Archaeopteryx-including from the same fossil site and horizon as the isolated feather. We also provide additional insights, such as an updated colour reconstruction of the entire feather as matte black, with 90% probability. Given the isolated nature of the fossil feather, we can never know the anatomical and taxonomic provenance with 100% certainty. However, based on all available evidence, the most empirical and parsimonious conclusion is that this feather represents a primary covert from the ancient wing of Archaeopteryx.

Figure 1

Centerlines of the isolated fossil feather and modern upper major primary coverts (UMPCs) from Kaye et al. 2019. (a) Modified from von Meyer 1862: Plate VIII, Fig. 3. (b) Laser-stimulated fluorescence image of MB.Av.100, modified from Kaye et al. 2019: Fig. 1. In (a) and (b), the centerline comprises the calamus (red) and rachis (blue), and the feather is oriented so that calamus endpoints are vertically aligned. Images are reversed to match (c). Scalebar: 1 cm. (c) Reproduced from Kaye et al. 2019: Fig. 3. The modern UMPCs presented here exhibit a strong anterior bend (leftward, towards the leading edge) near the calamus-rachis junction, creating an inflection point (S-curve) in the centerline. Note the yellow zone, representing the purported range of modern centerlines. (d) Modified figure from (c), including a more representative range of modern centerline morphologies (Common Crane X’ and XI’ from Supplementary Fig. S6, oriented so that calamus endpoints are vertically aligned). The correct Archaeopteryx centerline from (a,b) is overlaid as a solid blue line. This centerline was also overlaid onto the incorrect centerline to replicate the proximal end (red), revealing an alignment error. Note the substantial discrepancy between the correct and incorrect Archaeopteryx centerlines, the former of which now falls within the range of these modern primary coverts (yellow zone).

Figure 2

(a) Relative calamus lengths of modern feathers and the isolated fossil feather (red), in descending order. In both the plot (Gallus) and wing (Archaeopteryx), black represents UMPCs, dark grey represents primaries, and light grey represents secondaries and upper major secondary coverts (UMSCs), as well as rectrices (tail feathers) and alulars (not present in Archaeopteryx). UMPCs are statistically significantly longer than all other tracts. Inset: anatomical reference illustrating dorsal view of the left wing of Archaeopteryx (modified from Carney et al. 2012; based on Wellnhofer 2009, 2008). Note that there are 12 primaries reconstructed in Archaeopteryx and 10 primaries in Gallus. See Supplementary Fig. S12 for anatomical reference of Gallus wing and the relative calamus lengths grouped by tract. (b) Schematic of the enumerated anatomical attributes.

Figure 3

Map of the Solnhofen-Langenaltheim quarry district, illustrating locations of the isolated feather and the London (type), Maxberg, Munich, and Ottmann & Steil (9^th) specimens of Archaeopteryx. Note that these five fossils are coincident in both space and time (see text). Reproduced by permission from Wellnhofer 2009, 2008: Fig. 5.98.

Figure 4

Altmühl specimen of Archaeopteryx, showing the dorsal surface of the right wing. (a) Key anatomical features denoted include two slightly curved rachis impressions (white arrows), two leading vane widths (small double arrows), the leading edge of the best-preserved UMPC (arrowhead), and a representative barb angle, which measures 25.1° (yellow lines; corresponding barb in isolated feather measures 25.2°; see Supplementary Fig. S15). Also note the posterior orientation of the UMPCs with respect to the manus and primaries, suggesting an absence of S-shaped centerlines. Inset: overview of specimen, denoting enlarged region. (b) Reconstruction of the isolated feather is overlaid at scale. Note the match in both size and shape to the underlying distal UMPC in the Altmühl specimen. Inset: black feather denotes prior hypothesis of the isolated feather’s approximate location, as a distal member of the UMPC tract (shown as a right wing to match that of the Altmühl specimen). Scale bar: 1 cm.

Figure 5

Under primary coverts in the Berlin and London specimens of Archaeopteryx. White arrows indicate proximal orientation of centerlines. (a) Berlin specimen (MB.Av.101), main slab. (b) Right wing region from white box in (a), from the counterslab under oblique lighting. Reconstruction of the isolated feather is overlaid at scale. White arrowheads denote the non-interlocking “split” barbs of open pennaceous vanes. Red arrow and arrowhead denote centerline of a uMPC, which exhibits a ventral furrow on the proximal end. Yellow arrow denotes S-shaped centerline in the penultimate primary. Note the orientation of the coverts, and their posterior divergence from the primaries. (c) London specimen, main slab (NHMUK PV OR 37001); reproduced by permission from Wellnhofer 2009, 2008. (d) Reconstruction of the isolated feather. Yellow lines indicate barb angle measured in the London specimen in (e); angle of the corresponding barb in the isolated feather measures 25.8°. (e) Left wing region from white box in (c), from a cast of the counterslab (image is reversed). Centerline is preserved as a positive cast. Yellow lines indicate a representative barb angle, which measures 24.6°. Shown at same scale as (d). (f) Left wing region from white box in (c), from the original main slab under oblique lighting. Centerline is preserved as a negative impression. Negative impression of digit II phalanx 2 is denoted by “p”. Note the very close match in size, curvature, and barb angles between the isolated feather and primary coverts from both specimens. Scale bars: 1 cm.

Figure 6

Feather colour reconstruction, based on quadratic discriminant analysis of melanosome morphologies from modern feathers representing various classes: brown (brown), grey (grey), iridescent (purple), matte black (black), and penguin type (blue). Ellipses represent 95% confidence bounds. Results from the isolated feather (MB.Av.100) represent measurements of melanosome imprints only (red, most reliable), imprints and 3D preservation combined (pink), and 3D preservation only (white). Canonical axes 1 and 2 are most strongly associated with aspect ratio and length, respectively.