The origin of the Palaeoscolecida and Some Other Cambrian Worms  

Cambrian lagerstätten preserve an enormous variety of palaeoscolecidans and other ecdysozoan worms.  Palaeoscolecidans were first discovered 130 years ago, and for over a century were affiliated with the annelids.  However, the group possesses many characters which are absent in annelids: a terminal mouth and anus; an inverting proboscis with pointed scalids; a thick integument built of diverse plates; sensory trunk tubuli, tumuli, and flocculi; and caudal hooks.  These features have been taken to indicate an affinity with the Cephalorhyncha or Cycloneuralia.  These two phyla unite some “primitive” worms (priapulids, kinorhynchs, loriciferans, and nematomorphs) with a retractable proboscis (at least in their larval stages), a circumpharyngeal brain, and some other features also found in leg-bearing ecdysozoans. 

The Palaeoscolecida, and many other Cambrian ecdysozoan worms (including Ancalagon, Fieldia, and Louisella), are commonly treated as representatives of the Priapulida, or as relatives of the Nematomorpha.  However, the elaborated cuticle of these Cambrian worms is comparable with the cuticle of larval priapulids and other cephalorhynchs.  In contrast, introverts of the Palaeoscolecida and most Cambrian ecdysozoan worms are much less complex than those of modern cephalorhynchs.  Thus, we propose that these Cambrian worms represent a stem-group of the Cephalorhyncha. 

The Palaeoscolecida share some of their morphological features with the Xenusia (leg-bearing Cambrian ecdysozoans). Xenusia commonly possess a terminal mouth; proboscis (although not retractable); midgut diverticulae; and a thick integument built of diverse three-layered plates.  They also possess telescopic walking appendages (lobopods) similar to those of the Onychophora and the Tardigrada.  The most primitive Cambrian ecdysozoan worms bear a non-retractable proboscis and midgut diverticulae, while both the Palaeoscolecida and the Xenusia share a complete (although highly variable) set of cuticular plates, with a common microstructure.  This suggests that stem-group cephalorhynchs are decedents of xenusian-like ecdysozoans, whose body form was modified in adaption to a burrowing lifestyle.  This caused the loss of walking appendages, and acquisition of a retractable proboscis and “worm-like” body during an adaptation to burrowing life-style.  The loss of limbs occurs independently in many animal clades (over 100 times among the tetrapods, for example), and is often accompanied by axial elongation; however, the reaquisition of limbs is rarely observed.  These changes garner more efficient locomotion, the ability to navigate crevices, and the ability to burrow.  On this basis, we consider it more likely that cephalorhynchs were derived from limb-bearing xenusian-like ecdysozoans, and not vice versa.  We propose Facivermis as a representative “intermediate form” – this semi-burrowing sedentary worm bore lobopods around a terminal mouth; an annulated, limbless, plate-bearing trunk; and a terminal anus.