Considerations on the taxonomy of Late Jurassic Avialae (Theropoda) from the Solnhofen Archipelago

The iconic Archaeopteryx still represents the oldest known volant avialan, but its taxonomic history is complex. Several avialan specimens from the Solnhofen Archipelago have been variously assigned to distinct genera, most recently, the Haarlem and Mühlheim specimens. A recent reinterpretation proposed that all these Urvogel specimens belong to Archaeopteryx , based on similarities in limb proportions, with observed morphological differences attributed to taphonomic or ontogenetic variation. Here, the taxonomy of the Solnhofen Urvögel is reassessed by comparing limb proportions, conducting specimen-based phylogenetic analyses, reconstructing a forelimb morphospace, and re-evaluating the diagnosis of Archaeopteryx and possible morphological differences from the Haarlem and Mühlheim specimens. The phylogenetic analyses recover a monophyletic Archaeopteryx that also includes the Maxberg, Ottmann and Steil (“chicken wing”), and Karlsruhe specimens, which have so far only been tentatively referred to this genus, as they do not preserve its diagnostic characters. The Mühlheim specimen is excluded from Archaeopteryx and placed more crownward, a result consistent with its distinct position in the forelimb morphospace. The Haarlem specimen is found in various positions at the base of Avialae, indicating that its assignment to Archaeopteryx remains unresolved even when key character differences are silenced. Integrating these results with detailed morphological comparisons, it is concluded that the Mühlheim specimen is distinct from Archaeopteryx , supporting the validity of Alcmonavis poeschli , whereas the Haarlem specimen lacks definitive diagnostic features and is best regarded as Avialae indet . Specimens of Archaeopteryx probably represent more than a single species, but the results from different analyses are currently contradictory and inconclusive.