a Acipenser transmontanus
b Acipenseroxyrinchus
Figure 2.Pectoral fin spine of Acipenseridae (Character 2): a-Illustrations of the pectoral fin spine ofAcipenser transmontanusin an
intact fin (left) and isolated (right). The fin spine supports the leading edge of the pectoral fin. At higher magnification, dermal bone
contacts the propterygium (ptg) at the base and encompasses one fin ray completely (Iptlis exposed distally) and shows bone encroaching
onto the second ray (arrows onto lpt2). This juvenile specimen (cleared and double stained) shows only partial development of the spine.
but in adults the first two fin rays are fully encompassed by dermal hone. b – Illustrations of the base of the fin spine in a largeA.
oxyrinchus(2.4 111 total Iength) in posterior (left) and lateral (right) views. The propterygium is braced within the dermal spine and fused
to the dermal bone via its perichondral ossification (arrows). The canal bearing blood vessels and nerves into the fin (pca) is a portion of
the propterygium and shows the extent of fusion. Two fin rays are fused into this spine, but (they cannot be distinguished from the dermal
spine at the base where (the segmented Iepidotrichia merge full) into the new bone (scc = scapulocoracoid, ra = pectoral radials and
metapterygium).