Data from: The shoulder girdle of early chondrichthyans grew by skeletal remodelling

A distinct shoulder region, defined by endoskeletal and dermal girdles and
associated pectoral musculature, is a major evolutionary adaptation of
jawed vertebrates. In teleost model species the large (macromeric)
pectoral dermal bones can be derived from multiple embryonic tissues,
identifying the shoulder of osteichthyans as a developmentally complex
area at the head-trunk boundary. The absence of bone in living
chondrichthyans makes Palaeozoic stem groups capable of dermal
ossification key to understanding the underpinnings of skeletal growth in
the shoulder of crown gnathostomes (osteichthyans and chondrichthyans).
Here, using synchrotron X-ray tomography we demonstrate that individual
pectoral plates in the oldest unequivocal jawed vertebrate, the Silurian
(c. 439 Mya) chondrichthyan Fanjingshania renovata, develop from five
separate growth centres. These centres correspond to pectoral bony spines
that fuse neighbouring dermal scales into a pinnal plate and their
expansion is accompanied by cyclical resorption and remodelling of bone
and dentine. Our phylogenetic analyses support an interpretation of these
processes as crown and stem gnathostome characters that co-occur only in
the shoulder girdle of stem chondrichthyans. The systematic hard tissue
remodelling in Fanjingshania reveals an unexpected growth dynamic within
chondrichthyans that relates to the formation of a macromeric skeleton
through integration of modular elements.