Origin and significance of Lovén's Law in echinoderms

Abstract

Lovén's Law described the position of larger basicoronal ambulacral plates in echinoids. The smaller basicoronal plates form first in ontogeny. We restate Lovén's Law to describe the position of first ambulacral plates using Carpenter's ambulacra and left (L) or right (R) as: AR, BL, CR, DL, ER, with EA the pair of ambulacra both identical and adjacent. First ambulacral plates of the Cambrian edrioasteroid, Walcottidiscus, code identically. The transition from a tri-radiate 1-1-1 pattern to the 2-1-2 ambulacral pattern of Walcottidiscus and other pentaradiate Paleozoic echinoderms results in Lovén's Law. This provides an hypothesis for the origin of Lovén's Law and predicts its widespread occurrence among echinoderm classes. The ‘BD different’ pattern of primary brachioles in pentaradiate glyptocystitoid Rhombifera results from subterminal branching of the ambulacra. The ontogenetic sequence was triradiate, then lateral ambulacra bifurcated, and finally second brachioles developed. Positions of second brachioles of pentaradiate glyptocystitoids code as AR, BL, CR, DL, ER. Other examples of Lovén's Law occur in Diploporita, Ophiuroidea, Edrioasteroidea, Edrioblastoidea, Holothuroidea, Ophiocistioidea, and Bothriocidaris. Our working hypothesis is that Lovén's Law arose with pentaradiality. An objective test requires a reliable independent landmark across all classes, which does not exist currently. Our hypothesis is only parsimonious if Lovén's Law arose just once in echinoderm evolution and is widespread within the phylum. So far, both conditions appear to be met, but edrioasteroid first ambulacral cover plates commonly exhibit an alternative Lovén-like pattern of AL, BL, CR, DL, ER, with AB the identical adjacent ambulacra. UUID: http://zoobank.org/4c40d338-8bfa-4515-9037-934f3bbab153

Pentaradiate diploporites (Echinodermata) from the Spanish Middle Ordovician and their taxonomic significance

“…The five ambulacra (A−E) are arranged in a 2-1-2 pattern (central branches). Then the first branches to the ambulacral facets as viewed along each ambulacrum from the centre of the mouth at (1) are to the right in ambulacrum A, left in B, right in C, left in D and right in E. This pattern coincides exactly with Lovén's Law in echinoids, if stated as the position of the first formed ambulacral plate in each ambulacrum (seePaul & Hotchkiss, 2020); C, Internal mould of the same specimen showing five ambulacra, lack of a 'donjon' style peristome and sutures between circumoral plates; D, Same view of the latex cast of the external mould with the hydropore highlighted and outlines of the oral frame plates and ambulacral facets superimposed; E, Interpretation of the oral frame plates. The mouth is surrounded by five, radial, circumoral plates (CO1-CO5), with a sixth, inter-radial perioral plate (PO7).…”

“…2C). This is another example of Palaeozoic blastozoans in which ambulacra B and D differ from the other three ambulacra (see Paul, 2015Paul, , 2017Paul & Hotchkiss, 2020). Our material suggests that in Spain Oretanocalix is represented by two species, O. murchisoni (Verneuil & Barrande, 1855), the type species and O. julioi n. sp.…”

“…In addition, specimen DPM-A1-997-2 shows that the two plates on either side of CO1 and CO4 meet adorally so that these two circumoral plates do not reach the inner margin of the peristome. This is totally unexpected, but probably results from the evolutionary process by which a triradiate ancestor developed into a pentaradiate echinoderm (Paul & Hotchkiss, 2020). Unlike O. julioi, in O. murchisoni specimen MGM-8564-O (Fig.…”

Developmental atlas of the indirect-developing sea urchin Paracentrotus lividus: From fertilization to juvenile stages

“…In this nomenclature, the five rays correspond to the position of the ambulacra and they are respectively called ambulacrum A, B, C, D, and E, with ambulacrum A facing the madreporite, a specific skeletal piece of the central disk on the aboral surface ( Figure 1E , Supplementary Figure S1 ). In addition, these letters are positioned counterclockwise when the juvenile is looked at from the aboral side, and clockwise when it is looked at from the oral side ( Carpenter, 1884 ; Paul and Hotchkiss, 2020 ) ( Supplementary Figure S1 ). Accordingly, the interambulacra are named AB, BC, CD, DE, and EA, respectively, with the madreporite being located in interambulacrum CD ( Supplementary Figure S1 ).…”

The diploporite blastozoan Glyptosphaerites (Echinodermata: Blastozoa) and the origin of diplopores

“…Thus, their ambulacra are uniserial, but still form part of the thecal wall, i.e., they are mural ambulacra. The protocrinitid genus Estonocystis Jaekel, 1918 has more than one facet per ambulacral plate (Paul & Hotchkiss 2020, fig. 9, p. 1098, but is otherwise identical to Protocrinites Eichwald, 1840.…”