Lysocystites, Miller, 1889

Paul, Christopher R. C., 2021, New insights into the origin and relationships of blastoid echinoderms, Acta Palaeontologica Polonica 66 (1), pp. 41-62 : 55-56

and Lysocystites

As mentioned above, the oral area of the Late Ordovician glyptocystitoid genus Rhombifera ( Fig. 20A ) is remarkably similar to that of the Wenlock (Silurian) genus Lysocystites ( Fig. 20B ). Rhombifera is the only glyptocystitoid with ambulacral facets (for unknown erect feeding structures) on its radial plates. In Lysocystites similar facets for equally unknown erect feeding structures, lie at the adoral ends of elongate thecal plates that alternate with the five plates that form the mouth frame. Thus, it is reasonable to suggest that the facet-bearing plates in Rhombifera and Lysocystites are homologous, which provides a link between the glyptocystitoid rhombiferans on the one hand and blastoids, coronates and Lysocystites on the other. The oral frame of blastoids is composed of five deltoid plates, one in each interambulacrum ( Fig. 23 : D). The CD interambulacrum also contains the anus and at least one other deltoid. Coronates have an identical thecal plate arrangement to blastoids, including a mouth frame composed of five deltoids ( Fig. 24 : L1–L5). A second posterior deltoid lies aboral to deltoid L1 and shares the gonopore ( Fig. 24 : G). Both posterior deltoids are adoral to the anus.

The thecal plate arrangement in blastoids, coronates and Lysocystites is basically identical and consists of four plate circlets. All three taxa have three basals, two large and one small, with the smaller in the AB interradius. They all have five “radials”, five “deltoids” and blastoids have five lancet plates. In coronates the so-called “trunk-mounting plates” ( Brett et al. 1983: 629) and in Lysocystites the “ambulacral plates” ( Sprinkle 1973: 140, fig. 34) are homologues of the lancet plates in blastoids ( Donovan and Paul 1985: 532, fig. 5).

Blastoids, coronates and Lysocystites have very different respiratory pore structures. Blastoids possess hydrospires ( Fig. 23 ), which are similar in basic construction to the pectinirhombs and cryptorhombs of glyptocystitoids and hemicosmitoids, respectively. All three structures are composed of endothecal canals that are shared between two plates and through which seawater flowed in life. Blastoid hydrospires only cross radial:deltoid sutures, but all four plate circlets may bear rhombs in dichoporites. Coronates have coronal canals in their coronal processes through which body fluids flowed ( Brett et al. 1983; Donovan and Paul 1985: 537, fig. 8; McDermott and Paul 2015: 176, fig. 3). Like blastoid hydrospires, coronal canals only cross the radial:deltoid sutures. Lysocystites has triradiate “exospires” at the corners of the radial plates, each one shared between three plates, either two basals and a radial, or two radials and a basal or one oral and two radials ( Sprinkle 1973: 140, fig. 34). When complete, the triangular channels within the plates were covered externally by a thin calcified roof. Beneath the roof was a large, circular pore connecting to the interior at the junction of the three plates, plus two smaller openings in the paired plates, also connected to the interior. Exospires had body fluids flowing through them in life. Finally, blastoids had recumbent ambulacra composed of alternating outer side plates and side plates, each pair of which shared a brachiole facet and also alternated across the food groove. As with the recumbent ambulacra of callocystitid glyptocystitoids, the smaller, adoral outer side plate was the first to form during growth of blastoid ambulacra. Coronates had erect, pinnate ambulacra, giving rise to biserial brachioles alternately and the first pair of brachiolar plates were modified to form the axis of each ambulacrum ( Fig. 7 ). The ambulacra of Lysocystites are unknown.

Figures

Fig. 20. Oral plating of two blastozoans showing similarities in arrangement of facet-bearing plates.A. Rhombifera bohemica Barrande,1867, Late Ordovician, Czech Republic. B. Lysocystites sculptus Miller, 1889, early Silurian, Indiana, USA. In both the mouth (M) is encircled by only five plates (O1–O5 in A), which alternate with five radial plates (R1–R5) that bear ambulacral facets (F) and are connected to the mouth by short food grooves. In Lysocystites (B) the facet-bearing plates are the homologues of eublastoid lancet plates.A–E, ambulacra A–E; An, anus; d, additional anal deltoids; L1–L5, lateral plates. For plate homologies see Table 2. A after Kesling 1962: 283, fig. 2; B after Sprinkle 1973: 141, fig. 35.

Fig. 23. Oral surface of the blastoid Codaster acutus McCoy, 1849, early Carboniferous, England, UK, showing interpretation of oral plating. Five ambulacra(A–E) are recumbent on the lancet plates (La) with the floor plates omitted for clarity. Five deltoid plates (d) form the mouth (M) frame and share the adoral parts of the hydrospire slits (H), the aboral parts of which are in the radial plates (R).An, anus. For plate homologies see Table 2.

Fig. 24. Oral surface of the coronate Stephanocrinus angulatus Conrad, 1842, early Silurian (Wenlock), New York, USA, showing similarity to blastoid oral surface. Five erect ambulacra (A–E) arise from the trunk mounting plates, which are homologues of the lancets of blastoids. Food grooves are covered by paired ambulacral cover plates (cp). The mouth frame is composed of five homologues of blastoid deltoids and dichoporite lateral plates (L1–L5), and covered by primary oral cover plates (1–5). G, gonopore; PD, posterior deltoid. For plate homologies see Table 2. Redrawn and relabelled from Brett et al. 1983: 638, fig. 4A.

Fig. 7. Structure of the erect ambulacra of the coronate Stephanocrinus angulatus Conrad, 1842, Wenlock (Silurian), New York, USA. Biserial brachioles (1–7) are numbered in order of formation and the first two plates (1a, 1b, and so on) are modified to form the main ambulacral trunk. Note that because this is an aboral view, brachiole 1 is anatomically left but on the right of the figure. Redrawn from Brett et al. 1983: 640, fig. 8.