The taxonomic status of Eurypterus remipes and E. lacustris is in dispute, based on conflicting morphology and geographic and temporal differences. Although phylogenetic analyses recover them as sister taxa, there is no consensus regarding their taxonomy. This problem was investigated here with the goals to (1) compare the growth series of each taxon and (2) combine the data sets. Forty-five trees were recovered for E. remipes (TL: 45, CI: 0.67, RC: 0.52); five trees were recovered for E. lacustris (TL: 17, CI: 0.88, RC: 0.44). The combined matrix resulted in 43,960 trees (TL: 73, CI: 0.45, RC: 0.55). The mature specimens of both species formed a polytomy. Some differences were seen: E. lacustris has a convex anterior prosomal margin, which is nearly straight in E. remipes, and E. lacustris has a rectangular prosoma, which is trapezoidal in E. remipes. The juvenile specimen of E. lacustris also has the rectangular prosoma, meaning that character is diagnostic of the species. Shared species growth were seen: a posteromedial shift in eye position, a medial shift in eye long axis. In terms of species differences, E. lacustris retains the juvenile prosoma shape, whereas its juvenile prosoma shape is trapezoidal. This is evidence for species differences driven by differences in developmental timing.