Should you wish to consult the literature referred to in my Aspidella poster at the ISECT 2017 conference, it is as follows:
Billings, E. 1872: On some fossils from the Primordial rocks of Newfoundland. Naturaliste Canadien, 6, p. 465-479.
Boyce & Reynolds (2008): http://bit.ly/2sV42NX
Gehling et al. (2000): http://bit.ly/2suzZ1X
Menon et al. (2013): http://bit.ly/2suORO4