BLASTOZOANS: CHARACTERISTICS, NUTRITION AND REPRODUCTION - BIOLOGY - 2025

The blastozoa (Blastozoa) are an extinct subphylum of the phylum Echinodermata. They are marine invertebrate animals that belong to the group of stars, urchins, stars, lilies, daisies and sea cucumbers.

Blastozoans are only known from the fossil record, dated more than 500 million years ago. They cover almost the entire Paleozoic era. They were animals of the epifauna, that is, they lived in the marine sediment.

Fossil blastozoans. Author:]

It is believed that they were anchored to the seabed by means of a peduncle of variable length according to the species. Possibly from surface waters to great sea depths.

Blastozoan fossils have been found in various parts of the planet, so their distribution in the Paleozoic must have been very extensive.

General characteristics

-Morphology

From the successful reconstruction of their fossils, blastozoans show a pentaradial symmetry structure, basically similar to current sea lilies (subphylum Crinozoa).

Its body is divided into three sections: peduncle or stem, calyx or main body (teak) and arms (brachioles).

Peduncle

It is a hollow cylindrical projection that is connected to the coelom or internal cavity of the teak. Contains coelomatic liquid. The length of the peduncle or stem is variable according to the genera, some of which may become almost sessile.

Chalice

The body or teak is shaped like a cup, calyx or cone, covered with small calcareous plates interlaced with each other (ossicles). The shape of teak varies according to gender, and can be wide and low or narrow and elongated. Internally it forms a cavity or coelom.

The upper or distal part of the teak or calyx is flat and the mouth is arranged there. Nearby is the anus. In this flat area, five ambulacres or feeding channels radiated from the center are arranged.

Perimeter to the flat area or along the ambulacres there is a series of brachioles or arms.

arms

The arms or brachioles are appendages for feeding. Generally arranged in two series, one long and one short, and they move freely.

They are supported by a series of crescent-shaped plates (ossicles) or by a discoidal columnella. They fulfilled the function of directing the food to the oral area located in the center of the ring of brachioles.

-Physiology

Blastozoans have specialized pores for respiration called epispires.

The epispirae are distributed along the sutures of the plates. They consist of semicircular pores with a raised border covered by a thin calcified membrane (epistereoma).

In the coelom are the organs of the animal. This is the main body cavity and contains coelomatic fluid. It is likely that, like current echinoderms, an ambulacral system developed.

This ambulacral system consists of a series of tubes through which the coelomatic fluid circulates. Sea water can also circulate. This system allows both feeding and internal circulation, including breathing.

In blastozoans the ambulacral or ambulacral appendages tend to be short and confined to the distal flat area of ​​the theca, near the mouth opening.

In this group, it is thought that it is highly unlikely that the internal fluid had direct contact with seawater.

Probably the subepidermal layer of the coelom fulfilled a respiratory function. The fluids contained in this subepidermis communicated with the epispires allowing the exchange of diluted oxygen in the water.

Respiration apparently occurred by diffusion of gases through the membranes of the epistereoma.

Taxonomy

The blastozoans evolved in the Paleozoic, from the Cambrian period to the Permian, with special diversification in the Ordovician. This time extends from more than 500 million years ago to about 250 million years ago.

The blastozoa were initially classified in the subphylum Crinozoa (modern "sea lilies") of the phylum Echinodermata. Today they make up the subphylum Blastozoa.

Depending on the author, the phylum Blastozoa is divided into five to nine classes, all grouping extinct organisms, that is, only known from the fossil record.

Eocrinoid class

They lived between the early Cambrian and the late Silurian. They constitute the basal lineage of the blastozoa. Some authors do not consider them a valid group, they qualify it as paraphyletic.

Its earliest forms had a short peduncle and irregular structural blades. Later forms already showed a longer peduncle and blades in regular rows.

Paracrinoid class

They lived in shallow seas during the early Ordovician through the early Silurian. It is unclear what types of respiratory structures these blastozoans likely had.

They are characterized by a stem, teak and arms with pinnulate structures. The mouth with two to five feeding arms arranged asymmetrically or somewhat bisymmetrically.

Class Parablastoidea

They existed from the Lower Middle Ordovician. Teak or bud-shaped body with well-developed pentameral symmetry. Calcium plaques include small or large radial basal plaques, and sometimes other small plaques in the inferior theca.

Rhombifera class

They lived from the Lower Ordovician to the Upper Devonian. They inhabited reefs, coastal areas, and sandbanks. The theca was globular and the respiratory structures rhomboidal with a set of folds or channels.

Diploporite class

They existed from the Lower Ordovician to the Lower Devonian. They are identified by having a globular theca and specialized respiratory structures called diplopores.

These consisted of a double pore system seated in a depression in a teak plate or body. Each plate could have numerous diplopores.

Class Blastoidea

It existed from Silurian to Permian. They were organisms of small diameter, about 15 to 16 mm. They had a short peduncle or were sessile. The teak or body has 18-21 plates arranged in four rows. They had numerous feeding brachioles.

They inhabited agitated and transparent ocean waters, sedimentary.

Class Felbabkacystidae

The fossils have been dated to the Cambrian. Inhabiting deep seas, below the storm line. It has a relatively long, cylindrical peduncle and a teak or cup-shaped body. Its epispires are elongated.

Class Lepidocystidae

They are located in the Cambrian. They show an oral disc made of numerous adjacent plates, with simple sutural pores along the sutures. Teak in the shape of an elongated cone on a cylindrical stem and composed of numerous interlocking plates. Epispirae are restricted to the oral surface.

Class Coronoidae

Known from Ordovician to Silurian. They have a relatively long peduncle. Plaques in the oral region are deltoid.

Nutrition

From what is known of their body structure, way of life and habitat, blastozoans must have been stationary feeding animals. Possibly they filtered the water and thus captured free organic matter and plankton in suspension.

Once inside the coelom, the absorption of nutrients had to be carried out by phagocytic cells of the peritoneum or tissue that lined the interior of the coelom.

The expulsion of the waste was carried out by a structure called anispiracle, formed by the fusion of the anus and the spiracles adjacent to it.

Reproduction

The fossil remains available do not allow for a better understanding of the reproduction of blastozoans.

By mere analogy, it is inferred that blastozoans were able to perform their reproduction similar to modern Echinodermata. Being able to be sexual, mediating a larval state of a planktonic nature (larva pluteus), or asexual.

References

1. Bockelie J (1984) The Diploporita of the Oslo region, Norway. Palaeontology 27: 1–68.
2. Brett CE, TJ Frest, J Sprinkle and CR Clement (1983) Coroniodea: A new class of Blastozoan Echinoderms based on taxonomic reevaluation of Stephanocrinus. Journal of Paleontology 57: 627-651.
3. Nardin E, B Lefebvre, O Fatka, M Nohejlová, L Kašička, M Sinágl, and M Szabad (2017) Evolutionary implications of a new transitional blastozoan echinoderm from the middle Cambrian of the Czech Republic. Journal of Paleontology 91: 672-684.
4. Parsley RL and YL Zhao (2006). Long stalked eocrinoids in the basal Middle Cambrian Kaili Biota, Taijiang County, Guizhou Province, China. Journal of Paleontology 80: 1058-1071.
5. Sprinkle J (1973) Morphology and evolution of blastozoan echinoderms. Harvard University Museum of Comparative Zoology, Special Publication pp 1-283.
6. Sprinkle J and CD Sumrall (2008) New parablastoids from the western United States. The University of Kansas Paleontological Contributions 16: 1-14.
7. Sumrall CD and J Waters (2012) Universal Elemental Homology in Glyptocystitoids, Hemicosmitoids, Coronoids and Blastoids: Steps Toward Echinoderm Phylogenetic Reconstruction in Derived Blastozoa. Journal of Paleontology 86: 956-972.
8. Zamora S, CD Sumrall, XJ Zhu and B Lefebvre. (2017). A new stemmed echinoderm from the Furongian of China and the origin of Glyptocystitida (Blastozoa, Echinodermata). Geological Magazine 154: 465-475.