Article: Taxonomy and evolution of Isograptus Moberg in Australasia
Isograptus Moberg is one of the most diverse, abundant, and stratigraphically useful graptolite genera in Australasia. It first appears in the Chewtonian stage (mid Arenigian), dominates the faunas of the following Castlemainian and Yapeenian stages, and becomes extinct by the end of the Darriwilian (Late Llanvirn or Llandeilo).The genus Isograptus is diagnosed and is distinguished by its proximal symmetry, apparently unique among reclined didymograptids; the sicula and theca I1 form a symmetrical pair such that the axis of rhabdosome symmetry passes between them. The relationships of Isograptus to Oncograptus, Cardiograptus, and Meandrograptus are discussed. The inferred phylogeny of the isograptid stock is presented.The Australasian isograptids are described from the viewpoint of population systematics, particular attention being paid to the wide range of morphologic variation exhibited both within and between species. 8 species, comprised of 14 subspecies and 3 informal forms, are recognized, and grouped in 4 main groups.The first, the victoriae group, comprises the subspecies of Isograptus victoriae described by Harris (1933), namely lunatus, victoriae, maximus, maximodivergens, and divergens, which represent successive stages in a continuously evolving lineage. The second group, the caduceus group, comprises those forms related to I. caduceus imitatus Harris, and the third group, the manubriatus group, is distinguished by the presence of a manubrium. Phyletic increase in rhabdosome size is a general trend exhibited by each of the three groups and is discussed in some detail.The fourth group comprises the earliest species, I. [caduceus] primulus and is tentatively regarded as distinct from the other three groups; its closest relative is thought to be the European form I. gibberulus (sensu Moberg 1892). Brief comments are given on other described Australasian isograptids.Growth and allometry of the isograptid rhabdosome are described, and variation within and between species is quantitatively described and discussed. The principal phyletic trends in the group are outlined.The utility of Isograptus lineages for local and inter-regional correlation is discussed.