Research Proposal: Expanding the Frontiers of the Xiagou Formation—The Ecological and Geochronological Case for Continued Fieldwork in the Changma Basin

1. Project Rationale: The Strategic Imperative of the Changma Basin

The Changma Basin in China’s Gansu Province represents a premier Aptian-age Lagerstätte, providing an unparalleled window into the mid-Cretaceous (~124–120 Ma). While the lacustrine sediments of the Xiagou Formation have yielded over 100 three-dimensionally preserved avian partial skeletons, historical research has been constrained by an avian-centric bias, treating the locality as a passive collection point for isolated taxa. This narrow focus has obscured the broader evolutionary dynamics of the region. To advance our understanding of Mesozoic biogeography, we must transition from mere specimen salvage to a systematic, multi-trophic architectural analysis of the basin’s paleoecology.

The discovery and description of Jian changmaensis (holotype GSGM-D050) serves as the strategic catalyst for this shift. As the first non-avian theropod recovered from this avian-dominant locality, Jian necessitates a transition from passive bird collection to active, systematic predatory-link exploration. The presence of a specialized dromaeosaurid amidst a workforce of ornithuromorph birds challenges the "bird graveyard" narrative and demands a re-evaluation of the ecological pressures driving the rise of modern avian lineages. This taxonomic pivot provides the necessary framework to investigate the co-evolutionary arms race between early birds and their specialized microraptorine predators in Northwest China.

2. Taxonomic Foundations: Jian changmaensis as a Diagnostic Pivot

Establishing a complete phylogenetic landscape in avian-dominant localities requires the precise identification of non-avian theropods to bridge the morphological gap between dinosaurs and birds. Jian changmaensis provides this diagnostic bridge. Though known primarily from an articulated pectoral girdle and forelimb, the high-fidelity preservation of GSGM-D050 allows for rigorous comparisons with Jehol Group relatives.

The taxonomic distinctiveness of Jian is defined by three primary autapomorphies:

Coracoid/Humerus Ratio: The coracoid is proportionally longer relative to the humerus (~36%) than in any other known microraptorine, exceeding values seen in Sinornithosaurus (33%) and immature Microraptor specimens.
Humeral Distal Condyles: Unlike the craniodistally oriented condyles of most microraptorines, Jian possesses condyles developed primarily on the cranial surface—a specialized trait shared with therizinosauroids and Aves.
Radial Foramen: The ventral aspect of the proximal radius features a well-developed foramen, a diagnostic character unique within Dromaeosauridae.

Comparative Skeletal Dimensions and Ratios (in mm)

Taxon	Specimen	Scapula	Coracoid	Humerus	Radius	Ulna	C/H Ratio
Jian changmaensis	GSGM-D050	~69.0	~37.0	101.6	84.9	89.2	0.36
Wulong bohaiensis	DNHM D2933	44.8	23.9	76.0	61.4	64.0	0.31
Zhongjianosaurus yangi	IVPP V22775	33.0	—	43.0	42.0	44.0	—
Sinornithosaurus millenii	IVPP V12811	85.0	44.0	134.0	116.0	110.0	0.33
Institutional Abbreviations: GSGM, Gansu Geological Museum; DNHM, Dalian Museum of Natural History; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology.

The 2026 phylogenetic analysis (167 OTUs, 853 characters) provides critical resolution for the microraptorine lineage:

Resolution of Controversies: The inclusion of Jian stabilizes the position of previously debated members Tianyuraptor and Zhenyuanlong within Microraptorinae.
Monophyletic Support: Jian is recovered within a derived clade, sharing character state 438:1 (radius width at midshaft less than half that of the ulna) with its closest relatives.
Phylogenetic Mapping: In 50% majority-rule consensus trees, Jian occupies a polytomy with Microraptor, Zhongjianosaurus, and Hesperonychus.

Given its placement within Microraptorinae, the "four-winged" gliding morphology of Jian is an obligate inference. This functional adaptation implies an arboreal lifestyle where long feathers on both fore- and hind-limbs served as aerodynamic surfaces for navigating the Early Cretaceous canopy.

3. Ecological Synthesis: Re-evaluating the "Bird Graveyard" Narrative

The emergence of Jian forces a paradigm shift from a static "bird graveyard" to a multi-trophic ecosystem. In this Aptian landscape, the dominant ornithuromorph Gansus yumenensis was not merely a fossil curiosity but the primary resource for a specialized aerial predator.

While most microraptors were crow-sized, Jian was an outlier—a barn-owl-sized glider with a wingspan of approximately four feet. This significantly larger size class allowed Jian to occupy the role of an apex aerial predator. The discovery of pulverized bone pellets within the Xiagou Formation provides the functional evidence required to confirm this role; Jian was likely the biological source of these pellets, having predated upon the smaller birds that populate the basin.

The ecological parallels between the Changma Basin and the Sihedang locality of the Jehol Group are striking. Both are dominated by specific ornithuromorph taxa and punctuated by the presence of microraptorines. This suggests that both sites capture a specific, under-represented paleoenvironment—a seasonally warm, arid habitat with a documented mean annual temperature of 20.2°C. Jian thus provides the ecological context for the survival and evolution of the modern avian lineage within these high-temperature, low-moisture corridors.

4. Geochronological Synergy: Resolving the Xiagou-Jehol Temporal Discrepancy

Resolving the geochronological standing of the Xiagou Formation is essential for mapping the global transition from archaic enantiornithines to modern ornithuromorphs. The discovery of Jian addresses a long-standing conflict:

Biostratigraphic Discrepancy: The "advanced" appearance of the Changma avifauna previously suggested a younger age, fueling the hypothesis that these modern birds were only just rising to dominance.
Chronostratigraphic Reality: Radiometric and chemostratigraphic dating place the Xiagou at 124–120 Ma (Aptian), making it contemporaneous with the Jehol Group.

As an "indisputable microraptorine," Jian acts as a strategic biostratigraphic marker that supports the older radiometric dates. This disproves the "initial stage of dominance" hypothesis, proving instead that modern birds were already fully established alongside specialized dromaeosaurid predators by the Early Aptian. Furthermore, as mapped in Figure 1 of the primary research, Jian bridges a 2,000 km geographic gap between the microraptorines of Gansu and Liaoning. This confirms that Microraptoria represented a trans-continental radiation across the Early Cretaceous of Asia, rather than a localized Liaoning phenomenon.

5. Research Trajectory: Objectives for Expanded Exploration

The Xiagou Formation is now positioned as a high-value rival to the Jehol Group. Jian changmaensis is not merely a new species; it is the strategic catalyst for a new era of paleontological inquiry. To capitalize on this pivot, we propose the following objectives:

Objective A: Confirming Arboreal Adaptations: Targeted recovery of manus and pes elements is a priority to confirm specialized climbing traits. We aim to verify if Jian possessed the slender, recurved pedal claws and elongated penultimate phalanges diagnostic of arboreal relatives like Microraptor zhaoianus.
Objective B: High-Resolution Imaging of 3D Soft-Tissue: 3D preservation in microraptorines is statistically rare. Given the exceptional state of the GSGM-D050 holotype, we will employ high-resolution CT and computed laminography (CL) scanning to search for skin and feather impressions currently obscured by the matrix.
Objective C: Comparative Basin Analysis: Exploration of the Yuing Xi Basin is required to compare the "bird-dominant" Xiagou outcrops with their "dinosaur-dominant" equivalents. This will provide a holistic view of regional resource distribution and faunal migration.

Jian changmaensis has identified the predator that stalked the birds of Changma. We now move to reconstruct the entire world that predator inhabited. Northwestern China's potential is no longer an avian footnote; it is the front line of Mesozoic research.