†<i>Henningsmoenicaris scutula</i>, †<i>Sandtorpia vestrogothiensis</i> gen. et sp. nov. and heterochronic events in early crustacean evolution

Abstract

ABSTRACT A detailed account of the morphology and ontogeny of the late Middle Cambrian crustacean † Henningsmoenicaris scutula is presented. Ten successive ontogenetic stages could be recognised in the material collected from various localities in Sweden. Morphogenetic changes include the development of a pair of stalked lateral eyes and the increase in the number and size of appendages and their setal armature. Notably, early stages lack ‘proximal endites’ on all post-antennular appendages; such a spine-bearing endite has previously been thought to appear simultaneously on these limbs. In † H. scutula a single functional endite appears on the third limb in an advanced stage; an additional endite appears on the second limb and, subsequently, further endites appear on more posterior limbs. Furthermore, a single specimen of † Sandtorpia vestrogothiensis gen. et sp. nov. is described. Based on this new information and data of other ‘Orsten’ taxa, particularly those assigned already to the early evolutionary lineage of Crustacea, a small-scale computer-based phylogenetic analysis was performed. This resolved the basal branchings of Crustacea s. l. as follows: †Oelandocarididae (=† Oelandocaris oelandica +† H. scutula +† S. vestrogothiensis )+(†Cambropachycopidae (=† Goticaris longispinosa +† Cambropachycope clarksoni )+ († Martinssonia elongata +Labrophora (=†Phosphatocopina+Eucrustacea))). Plotting ontogenetic data on the phylogram and comparing the ground pattern at every node led to the detection of three peramorphic heterochronic events in the evolutionary lineage towards Eucrustacea.