P.E. Raymond’s scientific contributions

... Further reports of Ordovician xiphosurids have confirmed that a thoracetron was ubiquitous among these taxa (Van Roy et al., 2010;Lamsdell et al., 2023). This narrative is challenged, however, by conflicting interpretations of a variety of Devonian taxa that resolve Journal of Paleontology:1-20 phylogenetically crownward of Lunataspis according to prosomal characteristics (Lamsdell, 2020(Lamsdell, , 2021a but have been interpreted as possessing an unfused opisthosoma (Pickett, 1993;Bicknell et al., 2019;Bicknell and Smith, 2021), indicating either: (1) that the thoracetron developed independently multiple times within Xiphosura (a scenario previously suggested by Raymond [1944] and Fisher [1981]), (2) that the Devonian taxa diverged from the main xiphosuran lineage before Lunataspis, or (3) that the interpretations of an unfused opisthosoma in these taxa are erroneous. Complicating matters further, xiphosurids appear to exhibit conflicting patterns of segment boundary effacement within the thoracetron, with tergite expression occurring variably within Belinurina, Paleolimulidae, and Austrolimulidae, potentially suggesting a high degree of plasticity within the thoracetron or further support for its convergent development. ...

Reference:

Segmentation in early Xiphosura and the evolution of the thoracetron