Mark 2

After 1 day in the tanks, three mussels per day for 30 days were checked for R. pseudosericeus larvae. The presence of larvae on the four gills (left or right, outer or inner) of U. d. sinuolatus mussels was checked using a mussel‐opening device that enabled mussels to be opened to approximately 10 mm. The adductor muscle of mussels was cut to examine the position, number, and developmental stage of bitterling eggs/embryos/larvae. Mussels without bitterling eggs/embryos/larvae were housed in different tanks.
To evaluate the changes in larval position in the mussels, the gills were divided into nine parts (Figure 1); from the gill demibranch to its point of contact with the suprabranchial cavity, the gill was divided into lower part (L), middle part (M), and upper part (U); it was also divided into three parts in the other direction, 3 being the farthest from the outlet, followed by 2 and 1. Moreover, the larvae’s position was accurately recorded and photographed (Canon, Mark II, Tokyo, Japan) by measuring the transverse length of the siphon of the mussel and the longitudinal length from the suprabranchial cavity to the gill demibranch. The developmental stages of R. pseudosericeus larvae were determined under a stereoscopic microscope (Nikon, SMZ‐10, Tokyo, Japan) using AxioVision LE program (version 4.5, Carl Zeiss, Germany), as described by Kim, Kang, and Kim (2006).

Fossil materials of Yunnanolimulus luopingensis were obtained by splitting micritic limestone in the field. Further preparation was carried out in the laboratory with sharp blades under a binocular microscope. Fossils were photographed with a Canon Mark II Camera with an EF 100mm f/2.8 L IS USM close-up lens under incident light. Enlargements of details at mm scale were photographed with a Leica DFC295 camera mounted on a Leica M125 photo-micrographic system under fibre-optic lights. The images were processed in Adobe Photoshop CS3. Based on the observation of specimens with preserved soft-tissues, artistic reconstruction of Y. luopingensis is made in both dorsal and ventral views (Supplementary Fig. S3).