Phylogeny, ancestral ranges and reclassification of sand dollars

Abstract

Classification of the Class Echinoidea is under significant revision in light of emerging molecular phylogenetic evidence. In particular, the sister-group relationships within the superorder Luminacea (Echinoidea: Irregularia) have been considerably updated. However, the placement of many families remains largely unresolved due to a series of incongruent evidence obtained from morphological, paleontological, and genetic data for the majority of extant representatives. In this study, we investigated the phylogenetic relationships of 25 taxa, belonging to eleven luminacean families. We proposed three new superfamilies: Astriclypeoidea, Mellitoidea, and Taiwanasteroidea (including Dendrasteridae, Taiwanasteridae, Scutellidae, and Echinarachniidae), instead of the currently recognized superfamily Scutelloidea Gray, 1825. In light of the new data obtained from ten additional species, the historical biogeography reconstructed shows that the tropical western Pacific and eastern Indian Oceans are the cradle for early sand dollar diversification. Hothouse conditions during the late Cretaceous and early Paleogene were coupled with diversification events of major clades of sand dollars. We also demonstrate that Taiwan fauna can play a key role in terms of understanding the major Cenozoic migration and dispersal events in the evolutionary history of Luminacea.

Selective concentration of iron, titanium, and zirconium substrate minerals within Gregory’s diverticulum, an organ unique to derived sand dollars (Echinoidea: Scutelliformes)

“…5 ). The Astriclypeidae, a scutelliform taxon hypothesized to be sister to all other extant scutelliforms ( Lee et al, 2023 ) is not found in these regions ( Ghiold & Hoffman, 1986 ) and lacks Gregory’s diverticulum ( Ziegler et al, 2016 ). The oldest fossil records of scutelliform echinoids (Eoscutellidae and Protoscutellidae) occur in North America, and it has long been proposed that this clade originated there ( Stefanini, 1924 ).…”

“…The distinctive group of echinoids known familiarly as sand dollars is globally distributed along tropical to temperate coasts, and the flattened morphology is adapted to sandy, shallow littoral waters exposed to wave and tidal current energy ( Smith, 1984 ). The sand dollars are divided into the Laganiformes and Scutelliformes, considered to be either sister ( Mongiardino Koch & Thompson, 2021 ; Mongiardino Koch et al, 2022 ) or polyphyletic ( Lee et al, 2023 ) clades. Scutelliformes are often considered to be “true” sand dollars ( Mongiardino Koch et al, 2018 ).…”

Reply to: Embracing the taxonomic and topological stability of phylogenomics

“…We would like to re-emphasize our contribution on re-classification of Scutelliformes based on our new findings and appreciate the valid criticism raised by Koch 1 . In his reply to our work 2 , Koch 1 criticized the use of a dataset based on four molecular markers (two mitochondrial loci: Cox1 and 16S , and two nuclear ones: 28S and H3 ) for a reassessment of the classification of sand dollars. Koch 1 pointed out the incongruence of certain deeper level splits in the tree published in our recent work 2 with those based on their genome-scale datasets 3 .…”

“…In his reply to our work 2 , Koch 1 criticized the use of a dataset based on four molecular markers (two mitochondrial loci: Cox1 and 16S , and two nuclear ones: 28S and H3 ) for a reassessment of the classification of sand dollars. Koch 1 pointed out the incongruence of certain deeper level splits in the tree published in our recent work 2 with those based on their genome-scale datasets 3 . One of the taxonomic disagreements is the position of Laganiformes.…”

“…One of the taxonomic disagreements is the position of Laganiformes. In the original paper where Mongiardino Koch et al 2 proposed the new clade Luminacea, Laganiformes are represented by only two taxa that form a sister group to Scutelliformes. The position of Laganiformes appears more closely related to Clypeasteroida in our study 2 .…”