Eocrinoidea

The Eocrinoidea were an extinct class of echinoderms that lived between the Early Cambrian and Late Silurian periods. They are the earliest known group of stalked, brachiole-bearing echinoderms, and were the most common echinoderms during the Cambrian.

Eocrinoidea Temporal range: Cambrian Stage 3–Silurian PreꞒ Ꞓ O S D C P T J K Pg N
Eocrinoid holdfasts (Middle Ordovician, Utah)
Colourful reconstruction of Gogia ojenai
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	†Blastozoa
Class:	†Eocrinoidea Jaekel, 1899
Groups included[1]
†Ascocystitida †Ascocystitidae †Cambrocystidae[2] †Rhopalocystidae †Gogiida †Eocrinidae †Lyracystidae †Trachelocrinida †Heckerocrinidae †Trachelocrinidae †Cryptocrinitidae †Felbabkacystidae[3] †Lepidocystoidae[3] †Lichenoididae †Lingulocystidae †Palaeocystitidae †Rhipidocystidae †Ridersiidae
Cladistically included but traditionally excluded taxa
†Rhombifera[4] †Diploporita[4] †Paracrinoidea[4] †Parablastoidea[citation needed] †Blastoidea[4] †Coronoidea[5] Crinoidea ?[6]

The earliest genera had a short holdfast and irregularly structured plates. Later forms had a fully developed stalk with regular rows of plates. They were benthic suspension feeders, with five ambulacra on the upper surface, surrounding the mouth and extending into a number of narrow arms.^[7][8]

Phylogeny

See also: List of echinoderm orders

Eocrinoids were a paraphyletic group that are seen as the basal stock from which all other blastozoan groups evolved.^[9]

Early evolution

The following cladogram, after Nardin et al. 2017,^[10] shows the progression of early eocrinoid families, with all other eocrinoid families (including representatives Trachelocrinus and Ridersia) grouped with "derived Blastozoans" as their relationships with each other and with other blastozoans are not addressed.

Lepidocystoidae	Kinzercystis Vyscystis Lepidocystis
	Felbabkacystidae Felbabkacystis Lyracystis "Eocrinidae" Akadocrinus Gogia Sinoeocrinus Ubaghsicystis Lichenoidae Lichenoides Trachelocrinus Ridersia (derived Blastozoa)

Note that some other sources use a more restricted definition of Eocrinidae,^[11] or use the spelling Lichenoididae in place of Lichenoidae.^[12]

Relationships to other groups

Relationships among the eocrinidae and other blastozoan clades are an area of ongoing study. Below are two of many cladograms showing some aspect of eocrinoid paraphyly or polyphyly.

Kinzercystis Titanocrinus Aethocrinus Lyracystis Gogia Sinoeocrinus Akadocrinus Tatonkacystis Ubaghsicystis Dibrachicystis Pleurocystites Velieuxicystis Macrocystella   Ridersia Lepidocystoids (Eocrinoids) Crinoids Eocrinoids Dibrachicystids (Rhombiferans) Glyptocystitoids (Rhombiferans) Eocrinoids Stalked Cambrian (and select Ordovician) echinoderms, after Zamora & Smith, 2011.[6] All genera except those bracketed as "Crinoids" are blastozoans.

Akadocrinus Cambrocrinus Ridersia Sanducystis Cystobsastus Lepadocystis Echinoencrinites Cheirocystella Macrocystella Rhombifera Lysocystites Macurdablastus    Codaster Stephanocrinus Thomacystis Caryocrinites Hemicosmites Eocrinoids Glyptocystitoids (Rhombiferans) Blastoids Eublastoids (Blastoids) Coronoids (Blastoids) Hemicosmitoids (Rhombiferans) This cladogram, after Paul et al. 2024, shows several eocrinoids in a polytomy with a clade containing several other blastozoan groups.[5]