Figure 4: Chemical analyses of the putative soft tissues in the Confuciusornis hindlimb.

(a) Comparison of FTIR spectra (1–2) obtained from two spots on the supposed soft tissues with the typical proteoglycan aggrecan (3) and collagen (4)²⁶. (b) Mass spectra of detailed region in 274–276.5 AMU of the supposed soft tissues. (c) Sulfur K-edge XANES spectrum from the high-sulfur region showing that the dominant sulfur species is sulfate (edge position for sulfate standard at 2,482 eV indicated by vertical dashed line). (d) False colour synchrotron microfocus XRF map of sulfur in the rectangular area in Fig. 1g shows that the high-sulfur regions (yellow-green colour, inset:#1–6) are correlated with the distribution of tendons/ligaments (#1–6). Scale bar, 100 μm. (e–j) Comparison of the distribution of the inferred fibrocartilages (#2–3, 5–7) and possibly mineralized fibrocartilages (#1, 4) in BSE-SEM image of the rectangular area in Fig. 3b (e) and FTIR map of the strong absorbance region of 1,400–1,700 cm⁻¹ in the same area (f) and ion images of the spatial signal intensity distribution in the square area in Fig. 4e for Al⁺ (g), Fe⁺ (h), NH₄⁺ (i) and the peaks at 275.16 AMU (j) in negative ToF–SIMS spectra. The FTIR map was collected using a Thermo IN10MX infrared microscope with a cooled MCT detector.