Charles Messing's Crinoid Pages: Articulations

A synostosis (SY-no-STO-sis) is effectively rigid, with apposed ossicle faces flat or shallowly concave, and united by relatively few, short ligament fibers. It appears externally as a straight or gently curved suture with no separation between the ossicles and may occur between columnals, brachials or calyx ossicles.

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In a symplexy (sim-PLEX-ee), found only between columnals (and thus not found in feather stars), ridges (culminae) on one joint face interlock with corresponding grooves (crenellae) on the apposed face; the articulation appears externally as a wavy suture. Only ligaments unite the ossicles. A ridge and adjacent groove taken together is called a crenula. In adult Isocrinida, the grooves and ridges border five petal-like regions (petaloid zone) around the central lumen of the axial canal. It is the lozenge-shape of the petaloid zone on the facet of some fossil crinoids that suggested to Georgius Agricola (1546) a resemblance to a lily (rather than the flower-like appearance of the whole animal, which was unknown at the time). With five petaloids on each facet, he called it pentacrinos.

As the articulation on the distal side of a nodal (cirrus-bearing columnal) in the heteromorphic column of Isocrinida matures, fine stereom is deposited over the juvenile symplexy, producing a flat rigid articulation called a cryptosymplexy, which is functionally a rigid synostosis. This is a site of autotomy—where the crinoid can sever the more distal portion of the stalk, either with growth or in response to predation (Baumiller et al., 2008). In adult hyocrinids (e.g., Parahyocrinus, Feracrinus), radially arranged crenulae lie in a zone called the crenularium.

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A syzygy (SIZ-uh-jee) is a rigid ligamentary articulation in which alternating radiating ridges and depressions or grooves on the two joint faces appose each other rather than interlock as in a symplexy; the external suture resembles a perforated line. They are widespread between brachials in feather stars and in the stalked crinoids Guillecrinus and Vityazicrinus, which arose from within the feather star family tree by retaining the stalk into adulthood. Syzygies represent fracture points for arm autotomy and typically occur at regular, sometimes diagnostic intervals along arms. They also typically (but not uniformly) occur between the third and fourth brachials of four-ossicle division series, and between the third and fourth brachials of undivided arms. The proximal brachial of a pair of ossicles joined by syzygy never bears a pinnule.

The synostosis between brachials in stalked Isocrinida can develop short marginal radiating ridges (or rows of tubercles) and has been called a cryptosyzygy. Oji and Okamoto (1994) discuss how syzygies and the similar synostoses (or cryptosyzygies) of Isocrinida may be distributed as an anti-predator adaptation for reducing arm loss. In the distal stalk of some crinoids (e.g., hyocrinids, phrynocrinids), a syzygial articulation can develop from either a symplexy or synarthry (Roux, 2015; Messing 2016).

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A ligamentary synarthry (sin-AR-three) bears a central fulcral ridge that separates a pair of semicircular fossae (also called ligament pits) that each houses a large ligament bundle. This articulation links most columnals in juvenile stalks and adult xenomorphic stalks (e.g., Bathycrinus, Phrynocrinus) and permits limited motion between successive ossicles. In external view, the ends of ossicles joined by synarthry converge where the fulcral ridges reach the edge of each articular face. Fulcral ridges on the opposite ends of a columnal orient at different angles to each other, permitting the stalk to bend in multiple directions.

Between brachials, the fulcral ridge orients through the oral/aboral (=ambulacral/abambulacral) axis of the articular face. Such ligamentary synarthries are most common in the two-ossicle first division series, between the first two ossicles of four-ossicle division series, and between the first two brachials of undivided arms. In some genera, secondary skeletal material fills in much of the two fossae, so the facet becomes more like a synostosis and is called a cryptosynarthry. The proximal ossicle of pairs joined by these ligamentary articulations never bear a pinnule.

In a muscular synarthry (often treated separately as a muscular articulation), found only in the arms of crinoids, the fulcral ridge orients transversely (across the facet) in the articulation between a radial and the first primibrachial, and obliquely between most other brachials. The facet has a large semicircular aboral ligament fossa on one side of the ridge, and one pair each of interarticular ligaments and muscle bundles inserted on the ambulacral side of the fulcral ridge, each in its own fossa: interarticular ligament fossae and muscle fossae. A deep depression in the middle of the aboral ligament fossa adjacent to the fulcral ridge is called the ligament pit. Brachials joined by muscular synarthries typically each bear a pinnule.

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In any kind of articulation, a general depression of one ossicle facet that corresponds to a complementary relief on the other is called a symmorphy (sih-MOR-fee). In a few isocrinids (e.g., Hypalocrinus naresianus and Neocrinus blakei), a juvenile synarthry between proximal brachials transforms with growth into a symmorphial synarthry, in which a strong ridge on the distal articular facet lodges in a corresponding valley in the facet of the proximal ossicle. In external aboral view, the articulation appears V- or U-shaped. In a transverse symmorphy, the ridge and valley run from side to side across the arm; the suture line is convex distally in aboral view and forms a deep V on each side of the ossicle pair. Both ossicles may bear weak crenellations along their aboral margins. In large adult specimens, such an articulation becomes rigid.

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Each plate, or ossicle, of the skeleton in all echinoderms consists of a three-dimensional mineral meshwork called stereom, a skeletal structure unique to echinoderms. Each ossicle is actually a single calcium carbonate crystal that first forms within an individual cell (e.g., Okazaki, 1960; Märkel, 1986). So, even though an echinoderm’s skeleton may appear in many cases to be external (e.g., a sea urchin’s ‘shell’ may superficially resemble a lobster’s exoskeletal carapace), it is actually an endoskeleton covered with at least a thin epidermis. Different types of stereom are associated with different soft tissues. As examples, the articular fossae anchoring ligaments (e.g., symplexies; paired fossae of synarthries; aboral ligament fossa and interarticular ligament fossae of muscular synarthries) are characterized by galleried stereom, which exhibits narrow parallel “corridors” into which long ligament fibers anchor. By contrast, muscle fibers anchor in fine-meshed labyrinthic stereom (e.g., Macurda et al., 1978; Smith, 1980).