Anisograptus mojotorensis sp. nov. in the early Tremadocian of northern Argentina.

Abstract

Anisograptus mojotorensis sp. nov., a new species of triradiate anisograptid which has pseudo-quadriradiate symmetry in horizontal position is made known. The fossil material comes from the lower third of the Floresta Formation (Tremadocian), that copps out in the Mojotoro Range, southeastern edge of the Argentinian Eastern Cordillera. The Floresta Formation is a thick shale body, in which the Anisograptus, Adelograptus and Bryograptus kjerulfi Zones are developed. The new species was recorded in the Anisograptus Zone and defines the Anisograptus mojotorensis Subzone, which preceedes to the Anisograptus matanensis Subzone. Well preserved material of A. mojotorensis sp. nov. includes more than one hundred specimens corresponding to proximal ends and complete and incomplete colonies of juvenile and mature forms. This material allows to describe in detail the proximal development of the new species, whose peculiar isograptid type have interest to the classification of the triradiate anisograptids showing a quadriradiate aspect in horizontal preservation. The pseudo-quadriradiate symmetry of the rhabdosomes of A. mojotorensis sp. nov. is adquired by the bifurcation of the first order stipe 1 inmediately after its origin. In most of the cases, the A. mojotorensis sp. nov. colonies are presented superimposed and horizontally conserved; in laterally preserved rhabdosomes, the three declined first order stipes are clearly observed, evidencing the typical triradiate pattern of the rhabdosome. Similarities and differences of A. mojotorensis sp. nov. with other species of Anisograptus are discussed, and the possible evolutive transition from the Staurograptus dichotomous group to the Anisograptus matanensis group is also proposed. The A. mojotorensis Subzone can be correlated with the lower part of the A. matanensis Zone elsewhere, being a temporal equivalent to the upper part of the Iapetognatus conodont Zone that may reach the base of the Cordylodus angulatus conodont Zone.