Eleutherozoa is a subphylum of echinoderms. They are mobile animals with the mouth directed towards the substrate. They usually have a madreporite, tube feet, and moveable spines of some sort. It includes all living echinoderms except for crinoids. The monophyly of Eleutherozoa has been proven sufficiently well to be considered "uncontroversial."[3]
From Ancient Greek ἐλεύθερος (eleútheros, “free”) + ζῷον (zôion, “animal”), meaning "animal that moves freely."[4][5]
Classification
History
F. A. Bather's diagram of Echinoderm phylogeny from A Treatise on Zoology, Part III: The Echinoderma" (1900)
Originally defined by F. J. Bell in a sense that excluded Holothuroidea, Eleutherozoa was expanded by F. A. Bather in his 1900 taxonomy to include all free-living echinoderms. Bather considered the taxa within Eleutherozoa to have descended from the other subphylum in his two-subphylum system, the Pelmatozoa, either from different subgroups or at "widely different periods."[6] In cladistic terms, this would make Eleutherozoa a polyphyletic group.
In 1966–7, the Treatise on Invertebrate Paleontology adopted a four-subphylum system to replace Bather's system, in part because of the belief that Eleutherozoa was polyphyletic, although it retained "pelmatozoic" and "eleutherozoic" as descriptions of attached and free-living modes of life, respectively. This classification introduced Asterozoa and Echinozoa as subphyla. However, this version of Echinozoa was much more expansive than its modern sense, and included all non-stalked classes that were assigned to neither Asterozoa nor Homalozoa.[4]
With the advent of cladistics, the taxonomy of echinoderms was re-evaluated, finding new support for both Pelmatozoa (in its original sense, per Leuckart, encompassing only stalked forms) and Eleutherozoa, with Echinozoa now having its modern contents of Echinoidea as the sister to a clade containing Holothuroidea and the extinct (and possibly paraphyletic) Ophiocistioidea. However, Edrioasteroidea was also included in this version of Eleutherozoa.[7]
An obsolete cladogram based on Smith (1984) showing the now-disproven Cryptosyringida hypothesis[9]
While this initially restored Eleutherozoa as a monophyletic clade and defined the modern scope of Echinozoa, it followed embryological evidence grouping Ophiuroidea with Echinozoa. This clade, as sister to the Asteroidea, was given the name Cryptosyringida, constructed from the Greek "kryptos" (hidden) and "syringos" (pipe or fistula), referring to the hiding of certain anatomical elements during development.[10]
More recent work has shown, through multiple lines of evidence, that Asterozoa, consisting of the classes Asteroidea and Ophiuroidea, is the sister group of Echinozoa within Eleutherozoa, disproving the Cryptosyringida hypothesis.[11][12][2] As of 2024, the emerging consensus regarding echinoderm phylogeny restricts Eleutherozoa to Asterozoa plus Echinozoa, but has not fully resolved the nature of Eleutherozoa's relationship to Edrioasteroidea.[13][2]
Phylogeny
The living clades of echinoderms are related to each other as follows:[11][12]
Nanglu, Karma; Cole, Selina R.; Wright, David F.; Souto, Camilla (2023). "Worms and gills, plates and spines: the evolutionary origins and incredible disparity of deuterostomes revealed by fossils, genes, and development". Biological Reviews. 98: 316–351. doi:10.1111/brv.12908.
An echinoderm is any animal of the phylum Echinodermata, which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry, and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian.
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea. Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the seabed in all the world's oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.
Stelleroidea is a junior synonym of Asterozoa.
Dr Thomas Wright FRS FRSE FGS was a Scottish surgeon and palaeontologist.
The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a variety of very different reconstructions of their anatomy, how they lived, and their relationships to other organisms.
Crinozoa is a subphylum of mostly sessile echinoderms, of which the crinoids, or sea lilies and feather stars, are the only extant members. Crinozoans have an extremely extensive fossil history.
The taxonomy of commonly fossilized invertebrates combines both traditional and modern paleozoological terminology. This article compiles various invertebrate taxa in the fossil record, ranging from protists to arthropods. This includes groups that are significant in paleontological contexts, abundant in the fossil record, or have a high proportion of extinct species. Special notations are explained below:
Ambulacraria, or Coelomopora, is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.
Ophiocanops fugiens is a living species in the brittle star family Ophiocanopidae. Though once considered to be the only one living species in this brittle star family, recent research has brought to light three specimens of Ophiocanops that differ substantially from O. fugiens. It has been regarded as the most primitive brittle star, close to Paleozoic forms, though other authors have disagreed with the view. Classification of O. fugiens is highly argued. Ophiocanops is usually placed in the order Oegophiurida or regarded as a genus incertae sedis or even given its own subclass Oegophiuridea. Some recent data suggest its relationship to the extant family Ophiomyxidae.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into four phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is thought to be a member of Deuterostomia.
The Asterozoa are a subphylum in the phylum Echinodermata, within the Eleutherozoa. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the class Asteroidea, the class Ophiuroidea, and the extinct taxa Somasteroidea and Stenuroidea.
Ophiocistioidea is a class of extinct echinoderms from the Palaeozoic and early Mesozoic. They most likely form a paraphyletic grade along sea cucumber stem lineage, although some sources still consider the question of ophiocistioid monophyly unresolved.
Pelmatozoa was once a clade of Phylum Echinodermata. It included stalked and sedentary echinoderms. The main class of Pelmatozoa were the Crinoidea which includes sea lily and feather star.
Chantal Conand is a French marine biologist and oceanographer.
This list of fossil echinoderms described in 2014 is a list of new taxa of echinoderms of every kind that have been described during the year 2014. The list only includes taxa at the level of genus or species.
Haptista is a proposed group of protists made up of centrohelids and haptophytes. Phylogenomic studies indicate that Haptista, together with Ancoracysta twista, forms a sister clade to the SAR+Telonemia supergroup, but it may also be sister to the Cryptista (+Archaeplastida). It is thus one of the earliest diverging Diaphoretickes.
Xenambulacraria is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Xenacoelomorpha and the Ambulacraria.
Benthodytes is a genus of sea cucumbers in the family Psychropotidae.
Villebrunaster is an extinct genus of starfish-like animal belonging to Asterozoa that lived around 480 million years ago during Early Ordovician Period in modern-day southern France and Morocco. As of 2022, it contains two species, namely V. thorali and V. fezouataensis. V. thorali was described in 1951 and V. fezouataensis was described in 2021. Villebrunaster represents one of the oldest members of asterozoans, and perhaps, according to a description in 2021, the earliest divergent stem-group of Asterozoa.
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