Homoplasy and extinction: the phylogeny of cassidulid echinoids (Echinodermata)

Abstract

Inclusion of fossils can be crucial to address evolutionary questions, because their unique morphology, often drastically modified in recent species, can improve phylogenetic resolution. We performed a cladistic analysis of 45 cassidulids with 98 characters, which resulted in 24 most parsimonious trees. The strict consensus recovers three major cassiduloid clades, and the monophyly of the family Cassidulidae is not supported. Ancillary analyses to determine the sensitivity of the phylogeny to missing data do not result in significantly different topologies. The taxonomic implications of these results, including the description of a new cassiduloid family and the evolution of some morphological features, are discussed. Cassiduloids (as defined here) most probably originated in the Early Cretaceous, and their evolutionary history has been dominated by high levels of homoplasy and a dearth of unique, novel traits. Despite their high diversity during the Palaeogene, there are only seven extant cassiduloid species, and three of these are relicts of lineages dating back to the Eocene. Future studies of the biology of these poorly known species, some of which brood their young, will yield further insights into the evolutionary history of this group.

A Total-Evidence Dated Phylogeny of Echinoids and the Evolution of Body Size across Adaptive Landscape

“…The first contains faujasiids, clypeolampadids, echinolampadoids and cassiduloids, all of which were at some point included within Cassiduloida, but many of them were removed by Kroh and Smith (2010) in an attempt to find natural subdivisions of the “cassiduloids”. Our results suggest that all of these lineages are closely related to each other, as suggested by Souto et al (2019). The other subdivision contains a clade of Scutelloida + crown group Clypeasteroida which is subtended by plesiolampadids, conoclypids and oligopygids (Fig.…”

“…The second clade includes all extant “cassiduloids” as well as faujasiids, clypeolampadids, pliolampadids and plesiolampadids. This clade lacks the Aristotle’s lantern in adult forms (although it is present in juveniles of all extant lineages; Gladfelter 1978; Ziegler et al 2012), and agrees with the composition of Cassiduloida by Souto et al (2019), which is expanded by the addition of Apatopygus and taxa not included in their analysis. We suggest these two clades should bear the names Oligopygoida Kier, 1967 and Cassiduloida Agassiz & Desor, 1847, respectively (with the latter subsuming Echinolampadoida Kroh & Smith, 2010).…”

“…In an attempt to establish a more natural classification of this lineage, Kroh and Smith (2010) reclassified oligopygids (including Oligopygus and Haimea ), faujasiids (here represented by Faujasia and Stigmatopygus ) and plesiolampadids within Clypeasteroida ( sensu Agassiz 1873, now reclassified within Clypeasteroida sensu Mongiardino Koch et al 2018; Kroh 2020), and placed echinolampadids in their own order. Some features of this classification have since been contested (Souto et al 2019), and are not supported by our time-calibrated reanalysis of morphology (Fig. 2).…”

“…A clade of archiaciids and claviasterids, as well as other lineages variously classified as among the earliest fossil neognathostomates or atelostomates (Durham and Melville 1957; Kier 1962, 1966; Barras 2007; Kroh and Smith 2010) also appear to belong to this clade. Surprisingly, the extant Apatopygus recens —regarded as a close relative of the Mesozoic Nucleolites (Hawkins 1920; Mortensen 1948a; Kroh and Smith 2010; Souto et al 2019)—does not resolve within this clade. Although the relationship between apatopygids and nucleolitids has been questioned previously based on morphological differences and their long separation in time (Kier 1962, 1966; see also Smith 2001), this result requires testing with molecular data.…”

“…This shift is likely a consequence of the radical reorganization undergone by “cassiduloids”, one of the most problematic groups of echinoids from a phylogenetic perspective. The high levels of homoplasy and character exhaustion that characterize their morphological evolution has rendered the clade a taxonomic wastebasket (Suter 1994; Souto et al 2019). The exact delineation of the group has varied through time, but traditionally included the diverse paraphyletic assemblage of neognathosomates subtending the clade comprised of extant Clypeasteroida + Scutelloida.…”