Unlike the other animals I'll be covering as part of Scleritome Week, the Palaeoscolecida were actually known as entire animals long before their dermal armation was described, but they still meet the Scleritome Week qualifications because said armation was described as isolated problematic fossils before a connection was made between the animal and its armour (Ivantsov & Wrona, 2004). The photo above (from here) shows one of the isolated sclerites, originally described under the name Hadimopanella. Palaeoscolecid sclerites are round and button-like, with a central array of nodules that vary in different species from low and rounded to higher and pointed. Opinions on the nature of these microfossils (to appreciate how small they are, the scale bar on the photo above represents 0.03 mm) varied from some sort of dermal armour to the remains of reproductive cysts (Repetski, 1981). The dermal armour theory, of course, won out when the connection was made between the isolated sclerites and ornamentation on the compressed body fossils almost simultaneously by different authors in 1989 (Ivantsov & Wrona, 2004).
Palaeoscolecidans were a successful group of burrowing worms in the early Palaeozoic, when they were probably even more significant than the annelids (the image above of the holotype of Tabelliscolex hexagonus comes from Han et al., 2007). Originally interpreted as annelids, the segmented appearance is apparently only superficial, and results from alternating bands of larger and smaller plates (Ivantsov & Wrona, 2004). Well-preserved specimens from the Chengjiang Fauna possess an anterior spiny proboscis like that of the modern priapulids, and palaeoscolecidans have most often been regarded as priapozoans*. Other authors have suggested relationships with the modern nematomorphs, or as stem-panarthropods (Han et al., 2007). At the very least, a position within the Ecdysozoa, the clade uniting these three groups, seems well-established.
Tomorrow, I'll move on to chancelloriids.
*Bring on the nomenclatorial quibble. Most authors supporting this affinity have simply referred to palaeoscolecidans as "priapulids". The modern priapulids are a small, well-defined group of worms, while the various Palaeozoic taxa regarded as stem-priapulids show a much higher diversity of body plans (many of them, for instance, were far more elongate than any living priapulid, while no living priapulid possesses a dermal armour like that of palaeoscolecidans). Personally, I'd prefer to only refer to the crown group as priapulids, and use the name Priapozoa to cover the larger group including the stem forms.