Phylogeny and classification of echinoids

Taxonomic reassessment of Tetrapygus niger (Arbacioida, Echinoidea): molecular and morphological evidence support its placement in Arbacia

The echinoid genus Tetrapygus was initially described by L. Agassiz (1841) based on a single species, Tetrapygus niger Molina, 1782. Since the extensive work conducted by Mortensen (1935), Tetrapygus has received limited taxonomic attention over the past century. Recent discoveries of new fossil species of Arbacia Gray, 1835 from the upper Pliocene of northern Chile revealed striking morphological similarities between the two distinct Arbaciidae genera Arbacia and Tetrapygus. These findings compelled new investigations to evaluate the taxonomic status of these genera. Based on molecular mitochondrial (COI), nuclear (28S), and morphological evidence, Tetrapygus niger is here recovered as the sister species to Arbacia dufresnii, both species forming a clade within the phylogeny of South American species of Arbacia. Consequently, the diagnosis and description of Tetrapygus niger are here revised, and the species is reattributed to Arbacia, as previously proposed by A. Agassiz in Agassiz & Desor (1846) under the species name Arbacia nigra. An emended diagnosis of Arbacia is also proposed in light of these new findings.

Arbacia ballenensis sp. nov. (Echinoidea, Arbacioida): A new species reveals diversification of the genus in Central America

A new species of Arbacia (Echinoidea, Arbacioida), A. ballenensis sp. nov., from the Late PlioceneEarly Pleistocene of Baja California Norte, Mxico, is described from a series of 62 specimens ranging in size from 6 to 20 mm test diameter. It differs from all extant species in the genus by its high number of interambulacral plates, numerous small interambulacral tubercles, and the size of the interambulacral tags at the peristomial edge. This new species yields new evidence concerning the diversification of the genus along the Pacific coast of North America where previously, only one extinct species was described, Arbacia abiquaensis from the Oligocene of Oregon. A. ballenensis sp. nov. is closely related to the recently described Arbacia quyllur from the late Miocene of Chile. Study of the new species in the context of events such as the mid-Pleistocene Transition helps shed light on patterns of extinction and speciation within the genus.

Exploring adaptive landscapes across deep time: A case study using echinoid body size

Adaptive landscapes are a common way of conceptualizing the phenotypic evolution of lineages across deep time. Although multiple approaches exist to implement this concept into operational models of trait evolution, inferring adaptive landscapes from comparative datasets remains challenging. Here, I explore the macroevolutionary dynamics of echinoid body size using data from over 5000 specimens and a phylogenetic framework incorporating a dense fossil sampling and spanning approximately 270 million years. Furthermore, I implement a novel approach of exploring alternative parameterizations of adaptive landscapes that succeeds in finding simpler, yet better-fitting models. Echinoid body size has been constrained to evolve within a single adaptive optimum for much of the clade's history. However, most of the morphological disparity of echinoids was generated by multiple regime shifts that drove the repeated evolution of miniaturized and gigantic forms. Events of body size innovation occurred predominantly in the Late Cretaceous and were followed by a drastic slowdown following the Cretaceous-Paleogene mass extinction. The discovery of these patterns is contingent upon directly sampling fossil taxa. The macroevolution of echinoid body size is therefore characterized by a late increase in disparity (likely linked to an expansion of ecospace), generated by active processes driving lineages toward extreme morphologies.