A combination of pseudovirgulae and lateral branching in a species of dichograptid

Abstract

The development of Sigmagraptus praecursor Ruedemann is described in some detail from flattened and three dimensional material from Quebec. It is shown that early growth stages possess long pseudovirgulae developed from the dorso-lateral side of the apertures of th1¹ and th1², and along which the first pair of lateral branches grow in an upwards direction. Pseudovirgulae are lacking in later lateral branching. Sigmagraptus praecursor exhibits dichograptid development, th1² forming a single crossing canal and originating low down on th11 which probably has its origin on the prosicula. Pseudovirgulae and a combination of these with lateral branching has not been previously recorded in Ordovician graptolites: it is postulated that the pseudovirgulae may have assisted buoyancy, stability and orientation of the early growth stages.