Bed-by-bed collecting of the La Chilca Formation at Talacasto, San Juan, Argentina has revealed a sequence spanning the Ordovician–Silurian boundary, and the recognition of the following graptolite zones: Glyptograptus persculptus, probable Parakidograptus acuminatus and possible Atavograptus. &dcl001; The Talacasto section thus allows the closest definition of the Ordovician–Silurian boundary in South America. The shales of the La Chilca Formation are 3.65 m thick, rich in graptolites, and have allowed us to recognize for the first time in South America several stratigraphically important forms: Glyptograptus persculptus, Climacograptus angustus, C. normalis, C. acceptus, C. rectangularis and Paraclimacograptus innotatus. Also identified for the first time are Rhaphidograptus and Lagarograptus. The following are described: Talacastograptus leanzai gen. et sp. nov., Pseudoclimacograptus (?Metaclimacograptus) robustus sp. nov. and Lagarograptus praeacinaces sp. nov. The closest affinity of the new biserial genus is considered to be with Pseudoclimacograptus. The evolutionary relationships of the three new graptolites are briefly discussed

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