On the identification of feather structures in stem-line representatives of birds: evidence from fossils and actuopalaeontology

Abstract

Dinosaurs with fossilized filamentous integument structures are usually preserved in a highly flattened state. Several different feather types have been described on this basis, but the two-dimensional preservation of specimens during fossilization makes the identification of single feather structures difficult due to overlapping feather structures in vivo. Morphological comparison with the diversity of recent feather types is therefore absolutely vital to avoid misinterpretation. To simulate the preservation process, a cadaver of recent Carduelis spinus (European siskin) was flattened in a printing press. Afterwards, the structure of the plumage was compared with the morphology of a single body feather from the same specimen. In comparison with the single feather, the body plumage of the flattened bird looked rather filamentous. It was almost impossible to identify single structures, and in their place, various artefacts were produced. The investigation of plumage in a specimen of the Mesozoic bird Confuciusornis sanctus reveals similar structures. This indicates that flattening of specimens during fossilization amplifies the effect of overlapping among feathers and also causes a loss of morphological detail which can lead to misinterpretations. The results are discussed in connection with some dubious feather morphologies in recently described theropods and basal birds. Based on recent feather morphology, the structure of so-called proximal ribbon-like pennaceous feathers (PRPFs) found in many basal birds is reinterpreted. Furthermore, the morphology of a very similarlooking feather type found in the forelimb and tail of an early juvenile oviraptorosaur is discussed and diagnosed as the first feather generation growing out of the feather sheath. Thus, the whole plumage of this theropod might represent neoptile plumage. Keywords Feather morphology Á Taphonomy Á Preservation Á TheropodaKurzfassung Dinosaurier mit fossilierten, filamentösen Integument-Strukturen sind in der Regel stark zerdrückt erhalten. Basierend auf diesen Funden wurden mehrere Federtypen beschrieben, allerdings ist die zweidimensionale Erhaltung während der Fossilisation die Identifikation von einzelnen Federstrukturen erschwert, da Federstrukturen in vivo einander überlappen. Des Weiteren ein morphologischer Vergleich mit der Fülle von rezenten Federtypen absolut wichtig ist, um Fehlinterpretationen zu vermeiden. Um diesen Prozess nachzuvollziehen wurde ein Kadaver von Carduelis spinus in einer Druckerpresse zerdrückt. Im Anschluss wurde die Struktur des Gefieders mit der Morphology einer einzelnen Körperfeder von Carduelis spinus verglichen. Im Vergleich zu der Einzelfeder ist das Gefieder des zerdrückten Vogels eher faserig. Einzelne Strukturen lassen sich schwer nachweisen, es können jedoch artifizielle Strukturen beobachtet werden. Die Untersuchung des Gefieders eines Exemplars des mesozoischen Vogels Confuciusornis sanctus führt zum ähnlichen Resultat. Das bedeutet, dass das Zerdrücken der Kadaver während der Fossilisation den Effekt der...

Homology and Potential Cellular and Molecular Mechanisms for the Development of Unique Feather Morphologies in Early Birds

“…While throughout this paper we have followed morphological interpretations by Xu et al . [4] of the feather structures in STM4-1, alternative interpretations of these feathers exist, primarily that the proximally sheet-like portion of the feather represents the keratin sheath that protects the feather as it grows (pin-feathers), consistent with the interpretation of the specimen as a juvenile [27,44]. This has been refuted based on the considerable length of the feathers in Similicaudipteryx STM4-1 (pin-feathers are short) [66].…”

“…The recent discovery of two specimens comprising two different developmental stages of a single taxon reveals that, just like modern birds, some non-avian dinosaurs apparently experienced considerable ontogenetic variation in their plumage [4]: adults were marked by different feather types than juveniles of the same species. However, limited preservation and overlap of fossil feathers prevents unequivocal interpretation of integumentary structures in most specimens; as a result, many recently described extinct feather morphotypes are very poorly understood and interpretations regarding their morphology and function vary [3,4,6,27]. …”

“…While some workers regarded these paired rectrices as a primitive stage in the evolution of feathers from elongated scales [40], others inferred them to be more advanced structures based on inferences that the rachis itself is a derived feather feature [9,42]. Most recent interpretations have concurred with the latter hypothesis: these paired rectrices are modified pennaceous feathers [27,35,42] not present in living birds—they represent an extinct morphotype.…”

“…This feather morphology was described as the same morphotype seen preserved in the tail of the basal scansoriopterygid Epidexipteryx hui [6], another non-avian maniraptoran, and some basal avians (e.g., some confuciusornithiforms and enantiornithines), suggesting that these rectrices are retained in birds from a much earlier stage in feather evolution [4]. Alternatively, the PRPFs in juvenile Similicaudipteryx STM4-1 have been interpreted as pin-feathers, with the protective keratinous sheath still attached [27,44]. …”

Exceptionally preserved juvenile megalosauroid theropod dinosaur with filamentous integument from the Late Jurassic of Germany

“…The protofeathers probably are monofilaments, because no branching patterns are visible in the well-preserved, long filaments above the tail; apparent branching patterns in a few places probably are the result of compaction of these structures (16). Because of the state of preservation, it cannot be established if these structures were hollow, like the filaments found in other dinosaurs (3,14).…”