Attention has been paid of late to syllable structure in ancient Indo-European languages, e.g. Sanskrit (Kobayashi, 2004), Latin (Marotta, 1999), Greek (Zukoff, 2012), Anatolian (Kavitskaya, 2001), and general Indo-European (Byrd, 2010; Keydana, 2012). There is little agreement in the field about some of the more difficult cases, most of which involve both word-initial and medial clusters that violate the Sonority Sequencing Principle ( SSP), particularly sibilant-stop clusters. Because sibilants are more sonorous than stops, [STV-] σ onsets to roots such as * steh 2- require special consideration. I will argue that there are three types of evidence we can and should employ in attempting to diagnose syllable structure in ancient languages: metrical, phonological, and morphological. I will apply all three to Latin forms, showing that in Pre-Literary Latin, sibilant-stop clusters formed true onsets, as Byrd (2010) has argued for Proto-Indo-European, but that by the Classical period these SSP-violating clusters were no longer licensed as onsets. In such sequences, Classical Latin allowed only [t] in the onset, while the [s] formed a coda in medial position and was housed extraprosodically in word-initial position. The various treatments of ST-sequences in Latin and other Indo-European languages, especially PIE, Sanskrit, and Gothic, will be modeled in Optimality Theory using constraints on phonotactics and extraprosodicity.