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The analysis of morphological characters exhibited by the earliest ontogenetic stages of Middle Jurassic, Cretaceous and Holocene thecideoid species, together with the application of techniques that reveal the presence of early juvenile morphological characters buried in the shell fabric of adult representatives of the species concerned, reinforce the idea of a development pattern common to thecideoids from the early Middle Jurassic to the present day. Basic and long-standing differences in the architecture of the brachidial skeleton of thecidellinids and lacazellins may be correlated with the morphology of thecospiroids and indicate that the Thecidellinidae and Thecideidae may have emerged as sister groups as early as the Triassic. [ABSTRACT FROM AUTHOR]