The oldest ‘onychophoran’ Xenusion: a link connecting phyla?

Abstract

The second specimen of Xenurion auerswaldae Pompeckj 1927, found in an erratic boulder of the basal Cambrian Kalmarsund sandstone in Hiddensee island, GDR, appears to represent the part of the body missing in the holotype. This enables reconstruction of the whole animal, which, despite its remarkable size of about 20 cm in length, was of rather simple anatomical organization, lacking any specialized appendages or head organs. It is suggested that Xenusion was an ancestral walking articulate which had originated from a priapulid-like crawling ancestor. Xenusion may be ancestral for Middle Cambrian Aysheaia, which exhibits distinct tardigradan traits. It is proposed to classify these two lobopod animals in a new class Xenusia, ancestral for both the Onychophora and the Tardigrada. Possible derivations of the arthropods and annelids from the Xenusia are discussed.