Document is current

Article History

Received: 15 April 2024

Revised: 3 May 2024

Accepted: 25 May 2024

First Online: 23 February 2025

ETHICS APPROVAL AND CONSENT TO PARTICIPATE

: This work does not contain any studies involving human and animal subjects.

: The author of this work declares that she has no conflicts of interest.

: Ambulacral system: a hydraulic system specific to echinoderms, consisting of canals penetrating the body and tube feet or tentacles extending from their branches, performing motor, respiratory, tactile and excretory functions. Ambulacrum (ambulacral field): the part of the echinoderm skeleton located along the radial canal of the ambulacral system. Arms: food-gathering appendages of crinozoan echinoderms, the internal cavities of which are a continuation of the body cavity of the animal. Unlike brachioles (see below), often uniserial; may branch and bear appendages—pinnules. Axial canal: a lumen inside the stem of an echinoderm, extending from the base of the cup to the holdfast; sometimes it reaches the distal surface of the latter or the ends of its branches, but it can also end blindly. It can have different diameters and cross-sectional shapes. Basal layer: a distinct calcite layer on the distal surface of the holdfast, directly adjacent to the substrate. Bioherm: a raised limestone formation on the bottom of a body of water formed by sessile organisms. Brachioles: food-gathering appendages of blastozoan echinoderms, which are relatively short and thin, usually unbranched, biserial processes into which the outgrowths of the animal’s body cavity do not enter. Cirri (singular “cirrus”): segmented appendages of the stem of an echinoderm, regularly extending from the columnals and serving for fixation to the substrate. Columnal: a structural unit of the echinoderm stem. It is a symmetrical calcite formation with two articular facets at either end, pierced by a lumen of the axial canal. Crown: part of the body of a stalked echinoderm that includes the calyx and the appendages of the food-gathering apparatus (arms or brachioles). Distal part: the part located further from the reference point, which here, in relation to the anatomy of stalked echinoderms, is taken to be the calyx. Used to denote the end of the stem farthest from the calyx; the holdfast surface farthest from the calyx (and from the stem attached to it). Facet: the surface on a skeletal element of an echinoderm at which that element articulates with another skeletal element. Examples: stem articular facets on the proximal portion of holdfasts, arm articular facets on calyx plates, articular facets on stem/arm/cirrus segments. Firm ground: surface of the seafloor, which is a layer of compacted but not cemented sediment. It can serve as a substrate for the settlement of burrowing and free-lying organisms. Five-chambered organ: a structure consisting of coelomic cavities separated from the general body cavity and surrounded by five processes of the central part of the apical nervous system, and an axial cord containing many blood vessels. Hardground: the surface of the seabed that has been lithified prior to burial and forming a hard calcareous crust that can serve as a substrate for the settlement of boring and attached organisms. Hecker-type mounds: carbonate-clayey silt mounds that formed on the territory of the East European Platform in the first half of the Ordovician. They rest on a hardground surface and generally consist of a carbonate-clayey core, an overlying bed of fine-grained limestone, and a hardground bed capping the section. Holdfast: attachment structure of an echinoderm. Interambulacrum (interambulacral field): the part of the echinoderm skeleton located between the ambulacral fields. Lobe: a flattened lateral outgrowth of the discoid holdfast, usually lacking internal structures. Lobolith: hollow, bulb-like structure at the distal end of the crinoid stem. There are two possible interpretations: as a “float” that filled with gas during the animal’s life and allowed it to lead a planktonic lifestyle, or as a “lying anchor” that filled with sediment and prevented it from being carried away by the current. Morphotype: a formal taxonomic category distinguished on the basis of a specific set of morphological characteristics within a taxon with a high degree of polymorphism. Pentamers: structural elements of stem columnals that are plate-sectors divided by vertical boundaries. Plate: a structural unit of the skeleton of an echinoderm, usually of a calyx or other part that does not belong to the stem, which segments termed “columnals”, cirri (“cirrals”) or arms (“brachials”). Proximal part: the part located closer to the reference point, which here, in relation to the anatomy of stalked echinoderms, is taken to be the calyx. It is used to designate, for example, the end of the stalk closest to the calyx and the side of the holdfast closest to the stalk, which bears the stalk facet. Septae: radially arranged plates dividing the inner cavity into sectors. Stalk/stem: a supporting structure extending from the base of the cup of a stalked echinoderm; usually metameric. A distinction is made between a stalk in the broad sense as any such structure, and a stem in the narrow sense as a well-differentiated structure consisting of segments, in contrast to a stele (see below). Stele: a supporting structure extending from the base of the calyx of an echinoderm; relatively short, broad, and poorly differentiated, composed of irregularly arranged plates—in contrast to the stem (see above). Stereom: a type of skeleton of echinoderms, consisting of porous single crystals of high-magnesium calcite, the microstructure of which can vary depending on the kind of soft tissue attached to them. Theca (=calyx): the main part of the body of an echinoderm, on which the mouth opening is located and inside which, as a rule, the bulk of the animal’s soft tissues is located.