Abstract
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. 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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