Fig. 1. Extant avian MB and homologous dinosaurian bone tissues. (A) Domestic laying hen, midshaft femur cross section showing the extension of spongy MB deep into the mar­ row cavity and surrounding preexisting trabeculae (T). (B) Laying emu, midshaft cross section, with a thin layer of MB on the endosteal bone surface, separated from overlying CB by ELB. (C) Ostrich MB arising from CB. Convoluted bony projections surround large cavities and form by continued deposition on hairlike spicules of calcified bone (S). (D) MB on endosteal surface of MOR 1125 femur fragment delineated from overlying CB by large vascular sinuses and change in color, texture, and density. (E) Emu and (F) ostrich bone taken at same aspect as (D), showing mor­ phological distinction between bone types. (G) Higher magnification of dinosaur femur fragment in oblique view shows dense CB lined with newly described bone tissue, also seen in oblique view of emu (H) and ostrich (I) tibia. Ostrich MB is apparently unique in forming longitudinal tubules.