Extended Data Fig. 7: Taphonomy of SSN8DOR11.

a, Dissected Phocoena phocoena flipper to expose the skeletal system of digits II and III. Arrow denotes anterior. White line indicates the sectioned phalanx depicted in f, g and j (n = 3 specimens). b, Schematic representation of the forelimb where white indicates ossified elements (phalanges) and light grey un-mineralised interphalangeal cartilage. Note extension of the cartilaginous support beyond the distalmost phalanges (arrowheads). cc, calcified cartilage; ic, interphalangeal cartilage. c, Removal of all non-biomineralized structures (except the skin) mimics the condition in SSN8DOR11. d, Diagrammatic representation of SSN8DOR11 with incipient ossifications (io) in the distal fin blade. e, Photomicrograph of a distal ossification. f, Stained light microscopy section through the distalmost phalanx of digit III (P. phocoena). The region in the box is enlarged in g (n = 54 histological sections). ct, connective tissue; otf, osseous trabecular framework. g, Magnification of calcified and interphalangeal cartilage. c, chondrocytes; cl, chondrocyte lacunae. h, Transversely sectioned SSN8DOR11 phalanx. The region demarcated by a box is shown in i (n = 2 ground sections). i, FEG-SEM micrograph of the calcified cartilage cap and neighbouring osseous trabeculae (ot). During diagenesis, the intertrabecular spaces (its) have been filled with calcium carbonate. j, Single-element EDX maps of the distalmost phalanx of digit III and associated tissues (P. phocoena). Note localized enrichment of calcium (Ca; turquoise) and phosphorous (P; green) in the calcified cartilage and osseous trabecular framework, whereas intensities from carbon (C; red) derive from surrounding tissues (n = 4 measurements). k, EDX maps of SSN8DOR11 differentiating increased levels of phosphorous in the calcified cartilage and osseous trabeculae versus calcium enrichment in the intertrabecular spaces (n = 4 measurements). Scale bars, 5 mm (a), 2 mm (h), 1 mm (e), 500 µm (f, i), 50 µm (g).