Archaeopteryx that in an ostrich (4.2 micrometres per day). Using this range of bone growth rates, they calculated how long it would take to "grow" each specimen of "Archaeopteryx" to the observed size; it may have taken at least 970 days (there were 375 days in a Late Jurassic year) to reach an adult size of . The study also found that the avialans "Jeholornis" and "Sapeornis" grew relatively slowly, as did the dromaeosaurid "Mahakala". The avialans "Confuciusornis" and "Ichthyornis" grew relatively quickly, following a growth trend similar to that of modern birds. One of the few modern birds that exhibit
Archaeopteryx be related particularly closely to "Anchiornis" and "Xiaotingia", which were recovered as basal troodontids instead. Agnolín and Novas (2013) found "Archaeopteryx" and (possibly synonymous) "Wellnhoferia" to be the basalmost avialans (Avialae being defined by the authors as including "Archaeopteryx lithographica" and "Passer", their most recent common ancestor and all of its descendants), with Microraptoria, Unenlagiinae, and the clade containing "Anchiornis" and "Xiaotingia" being successively closer outgroups to the Avialae. Another phylogenetic study by Godefroit, "et al.", using a more inclusive matrix than the one from the analysis in the description of "Eosinopteryx brevipenna", also found "Archaeopteryx" to be a member