PMC

Caudal skeleton, a and b: Holtbyrnia anomala, 55 mm SL, 118 mm SL, c: Maulisia argipalla, 115 mm SL, d: Denticeps clupeoides, 31 mm SL (epineurals and epipleurals were removed), e: Clupea harengus, 83 mm SL, f: Dawkinsia tambraparniei, 27 mm SL (epineurals and epipleurals were removed). Cartilages are blue and bones are red. The star marks the uroneural, which is fused with the compound center and thus is a pleurostyl (Character 7). The arrow in a-c marks the common cartilaginous base of the hypurals 1 and 2 (Character 8). Abbreviations: CC: compound centrum = PU1 + UI + UII + napu1 + nauI + un1 + hy2; ep, epural; hepu, haemal arch of preural centrum; hspu, haemal spine of preural centrum; hy, hypural; mc, medial cartilage; napu, neural arch of preural centrum; nau, neural arch of ural centrum; nspu, neural spine of preural centrum; nsu, neural spine of ural centrum; ph, parhypural; PU, preural centrum; U, ural centrum; un, uroneural; un*, uroneural fused with the preural centrum (e) or compound centrum (f). Scale bar =1 mm

A bone-like tissue in the shark vertebral centrum. (A–H) Transverse sections of swell shark centra in 5-cm (A,C,E,G) and 9-cm (B,D,F,H) embryos. Safranin O staining (A) and Alcian blue of the HBQ protocol (C) distinguished two tissue layers in the centrum of vertebrae in the 5-cm shark embryo: lighter, more diffuse staining of an inner layer (il) of tissue adjacent to the notochord (n), and deeper, more concentrated staining in an outer layer (ol) of tissue. In 9-cm embryos, the centrum was divided into three tissue layers: inner (il) and outer (ol) layers stained with Safranin O, while a middle layer (ml) remained unstained by Safranin O (B). Note hypertrophic cells in the inner layer of the centrum (inset B′, which is a high-magnification of cells in the boxed area in B). Similar to Safranin O staining of the 9-cm centrum, Alcian blue stained inner and outer layers, but the middle layer stained slightly with direct red of the HBQ protocol (D). No Aniline blue staining in the Trichrome protocol was apparent in the 5-cm centrum (E), while only the middle layer of the 9-cm centrum stained slightly with Aniline blue (F). No alkaline phosphatase activity was present in the 5-cm centrum (G), but evidence of alkaline phosphatase was detected in all three layers of the 9-cm centrum (H). Abbreviations: AP = alkaline phosphatase; il = inner layer; ml = middle layer; n = notochord; ol = outer layer; SafO = Safranin O; Tri = Trichrome. Scale bars: A,C,E,G = 100 µm; B,D,F,H = 200 µm.

Deformation changes of T12 centrum, T12-L1 intervertebral disc, L1 centrum, L1-L2 intervertebral disc, and L2 centrum under seven operating conditions. (a) Total model; (b) T12 centrum; (c) T12-L1 intervertebral disc; (d) L1 centrum; (e) L1-L2 intervertebral disc; and (f) L2 centrum.

Comparison of centrum morphology between bilobate tails. a–e) The ratio of centrum width to height (solid lines and dots) and the ratio of centrum length to height (dotted lines and un-filled dots) for 10% increments of the dorsal lobes of shark species studied here. Values along the y-axis for d) the sandtiger tail are greater than other tails shown. f) The ratio of centrum width to height (solid line) and the ratio of centrum length to height (dotted line) for a dolphin, from the beginning of the fluke until the end of the fin. g–j) The ratio of centrum width to height (solid line) and the ratio of centrum length to height (dotted line) for Jurassic ichthyosaur species, from the beginning of the fluke until the end of the fin (f–j) mod from .