Data from: A large new leanchoiliid from the Burgess Shale and the influence of inapplicable states on stem arthropod phylogeny

Characterized by atypical frontalmost appendages, leanchoiliids are early
arthropods whose phylogenetic placement has been much debated.
Morphological interpretations have differed, some of which concern
critical characters such as the number of eyes and head appendages, but
methodological approaches also have diverged. Here, we describe a new
leanchoiliid, Yawunik kootenayi gen. et sp. nov., based on 42 specimens
from the newly discovered Marble Canyon locality of the Burgess Shale
(Kootenay National Park, British Columbia; middle Cambrian). This new
morphotype demonstrates the presence of a four-segmented head in
leanchoiliids, along with two small antero-median eyes in addition to
lateral eyes. Yawunik is characterized by a 12-segmented trunk and a
carinate, lanceolate telson adorned with minute spines. The ‘great
appendages’ of the animal bear teeth on their two distal rami, which would
have enhanced their ability to grasp prey. Attitudes of specimens,
resulting from burial at multiple aspects of bedding, suggest the ‘great
appendages’ were flexible and capable of antero-posterior rotation. We
also discuss the nature of intersegmental tissues and filaments present
within the ‘great appendages’. Our phylogenetic analyses extend the
monophyly of leanchoiliids to include Haikoucaris and Yohoia in a new
clade, the Cheiromorpha nom. nov. (within Heptopodomera nom. nov.). Other
nodes are poorly resolved unless implied weights are used, and in this
case, the topology is critically sensitive to the coding prerogative of
inapplicable states (NAs). Both the traditional ‘Arachnomorpha’ hypothesis
(NAs as additional states) and the more recently favoured ‘Artiopoda +
Crustacea’ (NAs as uncertainties) were obtained using the same data set
and outgroup. This result stresses, first, the historical importance of
polarization over data content in scenarios of early arthropod evolution,
and second, a pressing need to investigate the impacts of coding
inapplicables, especially given the inflating effect of implied weights.