Fig. 4 in New insights into the origin and relationships of blastoid echinoderms

Fig. 4. Origin of Lovén's Law in echinoderms illustrated by the oral plating in edrioasteroid Walcottidiscus Bassler, 1935 (USNM 376690), early Cambrian, locality unknown. A–E, five ambulacra arranged in a 2-1-2 pattern so that three radiate from the mouth, shared DE, A, and shared BC. A1, BC1, and DE1 represent the first ambulacral plates in the three primary ambulacra. A2, BC2, and DE2 represent the second ambulacral plates. When the shared ambulacra bifurcated to give five there was no other position in which the new ambulacral plates could be added than in the acute angle between the dividing primary rays. Thus, the new plates were B2, C2, D2, and E2. Once all five ambulacra reached the stage of having two floor plates, the positions of the first plates obey Lovén's Law as restated by Paul and Hotchkiss (2020), i.e., AR, BL, CR, DL, ER, that is the plates labelled A1, BC1, BC2, DE1, and DE2. Redrawn and modified from Paul and Smith 1984: 454, fig. 7A.