Comparative studies on bone microstructure have been extensively used to infer growth rates, and in some instances, physiological aspects of extinct vertebrates. The bone histology of Mesozoic birds is known only for a lew taxa. kt Hesperornis, the bone tissue is like that of its extant counterpartsTr, with uninterrupted, fastdeposited fibrolamellar bone. This type of bone is consistent with rapid growth; modern birds grow rapidly" reaching adult size within the first year72. A different pattern, however, has been found in Enantiornithes and Patagopteryx. Microstructural studies of these birds have documented punctuations interrupting bone deposition (lines olarrested growth- o_r LAGs), suggesting cyclical pauses during postnatal growth. Such a pattern contrasts with the uninterrupted growth pattern ofextant birds. Furthermore, although Patagopteryx shares the modern condition of a highly vascularized, flbroJamellar bone, the enantiornithines exhibit a lamellar, slowly deposited bone tissue, with virtually no vascularization, a pattern remarkably different from that of modern birds and their nonavian theropod relatives.