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Geology Department, Oslo University, P.O. Box 1047, Blindern, N-0316 OSLO 2, Norway
The variation of taxonomically important characters commonly used in the classification of lower Ordovician graptolites has been studied in fairly large populations of Extensograptus ‘slemmestadi’ and ‘Trochograptus’ diffusus from the lower Ordovician of the Oslo Region, Norway. In Extensograptus ‘slemmestadi’, which belongs to the patulus-hirundo lineage, there is a good correlation between ontogeny and supposed phylogeny, but some of the commonly used taxonomic characters are not as useful as previously supposed. An astogenetic series of ‘Trochograptus’ diffusus shows that it started as a Tetragraptus stage, then acquired secondary, lateral branching, (Schizograptus stage), and passed through a Trochograptus stage before the rare, large specimens reached a Holograptus stage. In both species, the preserved population is—as is usual in marine invertebrates—dominated by small, young specimens in which important taxonomic characters have not been developed.
