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This transition is well documented in several lineages of mammals and non\u2010avian reptiles, but has received relatively little attention in birds. This study presents new observations on the long bone microstructure of penguins, based on histological sections from two extant taxa (<jats:italic>Spheniscus<\/jats:italic> and <jats:italic>Aptenodytes<\/jats:italic>) and eight fossil specimens belonging to stem lineages (\u2020<jats:italic>Palaeospheniscus<\/jats:italic> and several indeterminate Eocene taxa). High bone density in penguins results from compaction of the internal cortical tissues, and thus penguin bones are best considered osteosclerotic rather than pachyostotic. Although the oldest specimens sampled in this study represent stages of penguin evolution that occurred at least 25\u00a0million years after the loss of flight, major differences in humeral structure were observed between these Eocene stem taxa and extant taxa. This indicates that the modification of flipper bone microstructure continued long after the initial loss of flight in penguins. It is proposed that two key transitions occurred during the shift from the typical hollow avian humerus to the dense osteosclerotic humerus in penguins. First, a reduction of the medullary cavity occurred due to a decrease in the amount of perimedullary osteoclastic activity. Second, a more solid cortex was achieved by compaction. In extant penguins and \u2020<jats:italic>Palaeospheniscus<\/jats:italic>, most of the inner cortex is formed by rapid osteogenesis, resulting an initial latticework of woven\u2010fibered bone. Subsequently, open spaces are filled by slower, centripetal deposition of parallel\u2010fibered bone. Eocene stem penguins formed the initial latticework, but the subsequent round of compaction was less complete, and thus open spaces remained in the adult bone. In contrast to the humerus, hindlimb bones from Eocene stem penguins had smaller medullary cavities and thus higher compactness values compared with extant taxa. Although cortical lines of arrested growth have been observed in extant penguins, none was observed in any of the current sampled specimens. Therefore, it is likely that even these \u2018giant\u2019 penguin taxa completed their growth cycle without a major pause in bone deposition, implying that they did not undergo a prolonged fasting interval before reaching adult size.<\/jats:p>","DOI":"10.1111\/joa.12367","type":"journal-article","created":{"date-parts":[[2015,9,11]],"date-time":"2015-09-11T07:40:58Z","timestamp":1441957258000},"page":"611-630","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":46,"title":["Bone histology in extant and fossil penguins (Aves: Sphenisciformes)"],"prefix":"10.1111","volume":"227","author":[{"given":"Daniel T.","family":"Ksepka","sequence":"first","affiliation":[{"name":"Bruce Museum Greenwich CT USA"},{"name":"Smithsonian National Museum of Natural History Washington DC USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sarah","family":"Werning","sequence":"additional","affiliation":[{"name":"Department of Anatomical Sciences Stony Brook University Stony Brook NY USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michelle","family":"Sclafani","sequence":"additional","affiliation":[{"name":"North Carolina Department of Environment and Natural Resources Raleigh NC USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Zachary M.","family":"Boles","sequence":"additional","affiliation":[{"name":"Department of Biology Drexel University Philadelphia PA USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2015,9,11]]},"reference":[{"key":"e_1_2_6_2_1","first-page":"315","article-title":"La structure du tissue osseux envisag\u00e9e comme expression de diff\u00e9rences dans la vitesse de l'accroissement","volume":"58","author":"Amprino R","year":"1947","journal-title":"Arch Biol"},{"key":"e_1_2_6_3_1","first-page":"329","article-title":"La struttura della ossa nei vertebrate. 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