Previous studies have shown that the effect of concurrent nontemporal processing on time estimation may vary depending on the level of difficulty ofthe nontemporal task. This is commonly interpreted within the context of so-called distraction/interruption models of temporal processing, which propose that as concurrent task difficulty or complexity is increased, temporal processing receives less attention. We hypothesize that the effect of nontemporal processing does not depend on the level of difficulty as such, but rather on the extent to which the concurrent nontemporal task specifically involves processing in short-term memory. Four experiments were run in which the short-term memory requirements of concurrent tasks were systematically varied, although all of the tasks were of comparable levels of difficulty. In the first experiment, the effect of memory search on simultaneous temporal productions was proportional to the number of items to search. As with reaction time, produced intervals were shown to increase linearly with the number of items in the memorized set. In Experiment 2, a visual search involving some load on short-term memory interfered in the same way with time production, although to a lesser extent. The last two experiments showed that performing attention-demanding visual search tasks that did not involve short-term memory did not lengthen simultaneously produced time intervals. This suggests that interference of nontemporal processing on time processing may not be a matter of nonspecific general purpose attentional resources, but rather of concurrent shortterm-memory processing demands.Prospective timing occurs when one is required to estimate the duration of a temporal interval that is to be presented. In such a situation, it is often reported that estimated duration decreases as a result of increasing concurrent nontemporal processing demands. This is commonly interpreted within the context of so-called distraction/interruption models of temporal processing, which propose that as concurrent task difficulty or complexity is increased, temporal processing receives less attention (Block, 1990;Brown, 1985;Fortin & Rousseau, 1987;Fraisse, 1984;Hicks, Miller, & Kinsbourne, 1976;McClain, 1983; Rousseau, Fortin, & Kirouac, in press;Thomas & Cantor, 1978;Vroon, 1970). The reduction of attention to temporal processing is assumed to result in less temporal information and, consequently, to shorter perceived duration. In spite of the accumulation of evidence, the distraction models still remain quite informal, which led Block to state that "Terms like attention to time and temporal information processing are unacceptedly vague" (p. 22).We believe that two basic questions should be addressed for some progress to be achieved. First, temporal-

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The sedimentary–diagenetic structure stromatactis is widespread in Palaeozoic spiculitic carbonate mud mounds, but occurs only sporadically in Mesozoic sponge carbonate mud mounds. Comparative analysis of Palaeozoic and Mesozoic stromatactis limestones suggests that this variation results from the degree of siliceous sponge skeletal rigidity and the amount of internal sediment accumulation in the original cavity network. Partial to entire filling by internal sediment resulted in a continuum, from a small amount of internal sediment and large amount of cement (stromatactis, common in the Palaeozoic), to only internal sediments (aborted stromatactis, common in the Mesozoic). These observations match independent lines of evidence concerning the siliceous sponge evolution and sediment recycling (e.g. bioerosion) across the Palaeozoic to Mesozoic biotic revolution.