New observations on morphological variation of genus Vetulicola with quadrate carapace from the Cambrian Chengjiang and Guanshan biotas, South China

Vetulicolians are enigmatic fossils with a debated phylogenetic position; they first appeared during the Cambrian evolutionary explosion. The early Cambrian Chengjiang and Guanshan biotas from South China exhibit the highest taxonomic diversity of vetulicolians, with ten species described to date. Here we conduct a comparative study on the most common members of Genus Vetulicola that bear a quadrate carapace from these two biotas. Two morphotypes are recognized both within Vetulicola rectangulata from the Chengjiang Biota and within a new undetermined species, Vetulicola sp. from the Guanshan Biota. In both localities morphotypes differ from each other in the presence/absence of a posteroventral projection in the anterior body, and this is interpreted to possibly represent sexual dimorphism. These two Vetulicola species from the Chengjiang and Guanshan biotas also exhibit other morphological differences. Most notable is the position of the junction between the anterior and posterior parts of the body which is further towards the anterior in the specimens from the Guanshan Biota than those from the Chengjiang Biota. In addition, the expansion of the distal portion of the posterior body occurs from the third segment in the Chengjiang specimens but from the fourth segment in the Guanshan specimens. The most likely interpretation of these differences is that they reflect evolutionary adaption of the swimming ability of Vetulicola with a quadrate carapace from the Chengjiang to the Guanshan Biota.