A microevolutionary event involving the conodont Paroistodus lineage is documented in the Gualcamayo Formation (Middle Ordovician), Argentine Precordillera. A detailed sampling of limestones throughout the upper part of the San Juan Formation and the lower member of the Gualcamayo Formation yielded over 14,000 well-preserved conodont elements. Paroistodus originalis (Sergeeva, 1963) was recorded through the upper 230 m of the San Juan Formation and the lower member (10 m thick) of the Gualcamayo Formation. The derived species Paroistodus horridus (Barnes and Poplawski, 1973) was recorded throughout the middle member of the Gualcamayo Formation (65 m thick). The intermediate linking forms between both species are identified as two new taxa: Paroistodus horridus primus Albanesi, 1998b, and P. h. secundus Albanesi, 1998b. They were recorded in the uppermost 70 cm of the lower member. Apparently, the speciation event occurred under stressed environmental conditions with the drowning of the carbonate platform, i.e., the San Juan Formation, and the beginning of a deeper and restricted environment represented by the Gualcamayo black shales. The demise of the carbonate production was caused by a sea level rise and a significant influx of volcanic ashes. The punctuated speciation event occurred within an allopatric setting while the Precordillera occupied an isolated (Iapetus) oceanic position in its overall drift from Laurentia to Gondwana.
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