Nassellaria is an order of

Rhizaria The Rhizaria are a diverse and species-rich clade of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus '' Paulinella'' in the phylum Cercozoa, they are all non-photosynthetic, but many Foraminifera ...

belonging to the class

Radiolaria The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ect ...

. The organisms of this order are characterized by a skeleton cross link with a cone or ring.

Introduction

Nassellaria is an order of

under the class

Polycystina The polycystines are a group of radiolarians. They include the vast majority of the fossil radiolaria, as their skeletons are abundant in marine sediments, making them one of the most common groups of microfossils. These skeletons are composed ...

. These organisms are unicellular eukaryotic heterotrophic plankton typically with a siliceous cone-shaped skeleton. The most common group of radiolarians are the polycystine radiolarians, which are divided into two subgroups: the spumellarians and the nassellarians. Both spumellarians and nassellarians are common

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

-forming

microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

and are important in stratigraphical dating, as the oldest radiolarians are Precambrian in age. The nassellarians appear in the fossil record much later than their other polycystine relatives, the spumellarians. spumellarians can be seen as far back as the Precambrian, whereas nassellarians do not begin to appear until the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

. Nassellarians are believed to have been increasing in species diversity since the beginning of the

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

. Nassellarians have been and continue to be some of the most remarkable and aesthetically interesting protists both alive and in the fossil record. They occupy an important role ecologically and have done so since the

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

. There is still much we do not know about these

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s, and discovering more will require the continued interdisciplinary effort of many scientists.

Morphology

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

of the Nassellarian order as described by Anderson and Boltovsky et al. is of an egg-shaped central capsule (the part of the cell containing one or more nuclei,

Golgi bodies The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

lysosome A lysosome () is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...

s, and other bodies important for cellular function) located within a porous conical skeleton made of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

. The central capsule is typically located toward the

apical Apical means "pertaining to an apex". It may refer to: *Apical ancestor, refers to the last common ancestor of an entire group, such as a species (biology) or a clan (anthropology) *Apical (anatomy), an anatomical term of location for features loc ...

end of the skeleton. The basal end is usually in the shape of a hollow circle, giving the skeleton a conical look. Surrounding the central capsule, but still inside of the skeleton, is the extracapsulum. This material is primarily alveoli, gas-filled bubble-like structures that regulate the buoyancy of the organism. The alveoli are connected by a web of rhizopodia throughout the extracapsulum. Fusules connect the intracapsulum (central capsule) to the extracapsulum. Axopodia and additional fusules extend out of the basal opening and through the pores in the skeleton. These will act as feeding apparatuses for the nassellarian. ; skeletons File:Anthocyrtium hispidum Haeckel - Radiolarian (34986365113).jpg, ''Anthocyrtium hispidum'' File:Anthocyrtis grossularia Ehrenberg - Radiolarian (34728144180).jpg, ''Anthocyrtis grossularia'' File:Radiolarie - Radiolaria (fossile) - 250x - Artostrobus sp. (?) (14290903161).jpg, '' Artostrobus'' sp. File:Calocycloma sp. - Radiolarian (32163186535).jpg, ''Calocycloma'' sp. File:Calocyclas sp. - Radiolarian (33510408555).jpg, '' Calocyclas'' sp. File:Radiolarian - Eucyrtidium sp. (33199074973).jpg, '' Eucyrtidium'' sp. File:Eusyringium sipho Ehrenberg - Radiolarian (30857068434).jpg, ''Eusyringium sipho'' File:Lychnocanoma sp. - Radiolarian (33183346526).jpg, ''Lychnocanoma'' sp. File:Lithochytris pyramidalis Ehrenberg - Radiolarian (30866628421).jpg, ''Lithochytris pyramidalis'' File:Radiolarian - Lychnocanium ventricosum Ehrenberg (28749275976).jpg, ''Lychnocanium ventricosum'' File:Radiolarian - Podocyrtis princeps Ehr. (27756021040).jpg, ''Podocyrtis princeps'' File:Radiolarian - Podocyrtis (Lampterium) mitra Ehrenberg - 160x.jpg, ''Podocyrtis mitra'' File:Podocyrtis papalis Ehrenberg - Radiolarian (30448963206).jpg, ''Podocyrtis papalis'' File:Podocyrtis papalis - Radiolarian (36721107223).jpg, ''Podocyrtis papalis'' File:Theocotylissa ficus Ehrenberg - Radiolarian (34638920262).jpg, ''Theocotylissa ficus'' sp. There exist three basic types of nassellarian skeletons. The first is formed from the connection of three elongate

spicule Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes ( ...

s at a central point. The three spicules are arranged in a single plane so that they are 120 degrees apart from each other. This is typically referred to as a “tripod” structure. The second skeleton type common in nassellarians is the aforementioned conical, porous skeleton. The third type is of a ring surrounding a latticed shell. These three skeleton types can be mixed and matched to create seven skeleton categories in nassellarians. Even the simplest classifications of nassellarian morphology is complicated because of the wide variety of morphologies in the nassellarian order even when compared with their sister group, the spumellarians. The innate nature of the nassellarian geometry lends itself to diversity, which may be one of the factors leading to the rapid diversification of the nassellarians in comparison to the spumellarians during the

. Many nassellarians are segmented, meaning there are circular skeletal divisions that exist equatorially along the skeletal cone. These divisions are called strictures. The strictures divide the organism into segments, typically giving it a more bulbous shape. Some nassellarians also exhibit spines from the apical end or along the lateral sides of the cone. Spines along the basal aperture of the organism are called feet.

Ecology

Many nassellarians house

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

symbionts Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term

The nassellarian provides

ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders. There is evidence from small subunit ribosomal

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

analysis of these dinoflagellate symbionts that dinoflagellate symbiosis with radiolarians evolved independently of other dinoflagellate symbioses (e.g.

foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

). Small subunit ribosomal DNA analysis also shows evidence that no coevolution of the dinoflagellates and radiolarians has occurred. Nassellarian feeding is mainly done through extension of

pseudopodia A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...

out of the basal opening. The axial projection is a very long

pseudopodium A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...

that extend out of the basal aperture and is surrounded by shorter terminal projections which form a terminal cone around the axial projection. The feeding in segmented Nassellarians is done in three phases: 1) extension, 2) capture, and 3) retraction. The geometry of nasselarians is quite beneficial for this feeding mechanism, as food passes through the basal aperture, which is much larger than the pores in the rest of the skeleton, which is how most spumellarians must transport food. A second variety of nassellarians feed without using an axial projection at all. These organisms instead form a wide terminal cone, which they cast out behind them from the basal aperture like a fishing net. The three phases of feeding used by segmented nassellarians are also evident in these nassellarians. These “fishing net” nassellarians are much easier to see feeding in the wild and present quite a spectacular sight. Nassellarians feed on other plankton such as small algae, bacteria, diatoms, and small zooplankton.

Fossil History

Early

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

radiolarian fossil history is dominated by Spumellaria until the

period, during which nassellarian fauna experienced a sharp increase in diversity. Nassellarian and spumellarian diversities have been relatively similar since the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

, with drops in diversity after

mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp fall in the diversity and abundance of multicellular organisms. It occ ...

events and a rise in both spumellarian and nassellarian diversity during the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

. Symbioses between algae and radiolarians is observed frequently in

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

species, but the evolution and timing of this symbiosis is currently unknown, as the symbiotic algae do not leave behind hard skeletons to fossilize. It may be possible to answer this question using isotopic analysis, as algal symbionts preferentially uptake

carbon-12 Carbon-12 (12C) is the most abundant of the two stable isotopes of carbon ( carbon-13 being the other), amounting to 98.93% of element carbon on Earth; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-1 ...

, so symbiont-bearing calcareous organisms such as

Foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

become enriched in

carbon-13 Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth. Detection by mass spectrometry A m ...

compared to non symbiont-bearing calcareous organisms. If fossil evidence can be gathered for isotopic ratio analysis, the time window for appearance of algal symbionts with nassellarians and/or spumellarians should be able to be restricted in geologic time.

References

{{Taxonbar, from=Q3870511 Radiolarian orders Taxa named by Christian Gottfried Ehrenberg