Soluta is an extinct class of

echinoderm 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 ...

s that lived from the Middle Cambrian to the Early Devonian. The class is also known by its junior synonym Homoiostelea. Soluta is one of the four "carpoid" classes, alongside

Ctenocystoidea Ctenocystoidea is an extinct clade of echinoderms, which lived during the Cambrian and Ordovician periods. Unlike other echinoderms, ctenocystoids had bilateral symmetry, or were only very slightly asymmetrical. They are believed to be one of the ...

Cincta Cincta is an extinct class of echinoderms that lived only in the Middle Cambrian epoch. Homostelea is a junior synonym. The classification of cinctans is controversial, but they are probably part of the echinoderm stem group. Cinctans were sessi ...

, and Stylophora, which made up the obsolete subphylum

Homalozoa Homalozoa is an obsolete extinct subphylum of Paleozoic era echinoderms, prehistoric marine invertebrates. They are also referred to as carpoids. Description The Homalozoa lacked the typical pentamer body form of other echinoderms, but all were ...

. Solutes (or solutans) were asymmetric animals with a stereom skeleton and two appendages, an arm extending anteriorly and a posterior appendage called a homoiostele.

Biology

Most solutes were free-living, but the basal solutan ''Coleicarpus'' used its homoiostele as a holdfast, as did juvenile ''Castericystis''.

Classification

The phylogenetic position of Soluta is contentious. Solutans are widely agreed to be echinoderms, though the outmoded

calcichordate hypothesis The calcichordate hypothesis, formulated by British Museum paleontologist Richard Jefferies, holds that each separate lineage of chordate ( Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoder ...

held that they were ancestral to both echinoderms and

chordates A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

. Within echinoderms, one hypothesis holds that solutes are

stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

echinoderms which branched off before echinoderms evolved

radial symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symme ...

. Another hypothesis holds that they are specialized descendants of radiate echinoderms which lost radial symmetry, likely belonging to

Blastozoa Blastozoa is a subphylum of extinct echinoderms characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. It ranged from the Cambrian to the Permian. Biserial, triradiate, and pentaradiate ambulacra ...

. Solutes are divided into two orders, Syringocrinida and Dendrocystitida.

Distribution

The earliest solutes, ''Coleicarpus'' and ''Castericystis'', lived during the

Drumian The Drumian is a stage of the Miaolingian Series of the Cambrian. It succeeds the Wuliuan and precedes the Guzhangian. The base is defined as the first appearance of the trilobite '' Ptychagnostus atavus'' around million years ago. The top is de ...

age of the Cambrian. Solutes were the last of the four carpoid classes to appear in the fossil record. Solutes appear to have evolved in

Laurentia Laurentia or the North American craton is a large continental craton that forms the Geology of North America, ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of ...

, but became more widespread during the Ordovician.

References

Homalozoa Paleozoic echinoderms Prehistoric animal classes {{paleo-echinoderm-stub