Hybocrinid oral surfaces

All hybocrinids have oral surfaces that are generally well preserved by comparison with other crinoid clades, and all oral surfaces known in hybocrinoids are classified as PBS-A3 (Kammer et al., Reference Kammer, Sumrall, Zamora, Ausich and Deline2013). The hybocrinid oral surface has oral plates visible; undifferentiated, moveable PPCPs; SCPs; and moveable ACPs. Examples include Hybocrinus conicus (Hybocrinidae, Ordovician, Fig. 2.1, 2.2); Hybocrinus nitidus (Hybocrinidae, Ordovician, Figs. 1.1, 1.2, 1.4, 1.5, 2.3, 2.4); and Hybocystites eldonensis (Hybocystitidae, Ordovician, Figs. 1.3, 2.5, 2.6). Hybocystites eldonensis is unique in having recumbent ambulacra in the B and E rays and free arms in the A, C, and D rays (Fig. 2.5, 2.6). Not only is this PBS-A3 oral surface considered homologous with oral surfaces of various blastozoans (Kammer et al., Reference Kammer, Sumrall, Zamora, Ausich and Deline2013), but also the recumbent ambulacra growing over the surface of calyx plates is similar to recumbent ambulacra crossing thecal plates present in glyptocystitid rhombiferans, such as Callocystites or Spaerocystites (Kesling, Reference Kesling1967). If these recumbent ambulacra are homologous, rather than analogous, in origin they may be an atavistic feature from blastozoans, providing additional evidence for a shared ancestry with blastozoans. However, there are not enough data at this time to make such a determination.

Cyathocrine cladid oral regions

The majority of oral regions known among cyathocrine cladids, including the oldest preserved (Sandbian), are oral surfaces. Carabocrinus treadwelli Sinclair, Reference Sinclair1945 (Fig. 3.2), Palaeocrinus hudsoni (Fig. 3.1), Porocrinus elegans Kesling and Paul, Reference Kesling and Paul1968 (Fig. 3.5, 3.6) have plesiomorphic PBS-A3 oral surfaces (Kammer et al., Reference Kammer, Sumrall, Zamora, Ausich and Deline2013). However, Illemocrinus amphiatus Eckert, Reference Eckert1987 (Katian, Fig. 3.3, 3.4) has a modified PBS-A3 oral surface with a doubling of the PPCPs. There are paired PPCPs at each of the five orals, rather than one. The PPCPs and ACPs are differentiated and moveable. In addition, Illemocrinus amphiatus has a high anal sac that is positioned marginally on the oral surface.

The majority of post-Ordovician cyathocrines with known oral regions have an oral surface. A few examples include the following: Euspirocrinus spiralis Angelin, Reference Angelin1878 (Euspirocrinidae, Silurian, Fig. 3.13, 3.14), which has PPCPs doubled; Gissocrinus incurvatus (Angelin, Reference Angelin1878) (Cyathocrinitidae, Silurian, Fig. 3.7, 3.8); Bactrocrinites fusiformis (Roemer, 1844) (Thalamocrinidae, Devonian, Fig. 3.11, 3.12); Nuxocrinus crassus (Whiteheaves, Reference Whiteaves1887) (Thalamocrinidae, Devonian, Fig. 3.9); Cyathocrinites harrodi (Cyathocrinidae, Mississippian, Figs. 1.6, 3.10), which also has PPCPs doubled; and Mississippian-Permian codiacrinaceans. These forms have either the PBS-A3 or PBS-A4 oral surface, depending on whether the peristome is elliptical or circular, respectively, and whether any SCPs are present or absent, respectively. The Oral 1 plate may be modified into a madreporite as in N. crassus (Fig. 3.9) and Cyathocrinites iowensis (Owen and Shumard, Reference Owen and Shumard1850) (Kammer and Ausich, Reference Kammer and Ausich1996, fig. 7.22). The Permian forms Monobrachiocrinus ficiformis granulatus Wanner, Reference Wanner1920 (Fig. 4.1, 4.2) and Embryocrinus hanieli Wanner, Reference Wanner1916 (Fig. 4.3, 4.4) are PBS-A4 and have oral surfaces reduced to five PPCPs. In Embryocrinus hanieli the zig-zag sutures between adjacent PPCPs suggest that these plates were locked rather than moveable, but the lack of any radial openings for arms indicates the PPCPs must have opened.

Figure 4

Permian cyathocrine PBS-A4 oral surfaces composed of only five primary peristomial cover plates. (1, 2) Monobrachiocrinus ficiformis granulatus; (1) oral view; (2) lateral view (Permian; THDKA 11962); (3, 4) Embryocrinus hanieli; (3) oral view; (4) lateral view; (Permian; UMMP 58254). All scale bars represent 5 mm.

Oral surfaces are the norm for cyathocrine cladids, but several cyathocrines evolved a tegmen. Two lineages of Silurian cyathocrines with tegmens are the Crotalocrinitidae and Cyathocrinitidae. Among the Crotalocrinitidae, Crotalocrinites rugosus Miller, Reference Miller1821 (Fig. 5.4) has five PPCPs, numerous ACPs, and interambulacral plates. The regular arrangement of these plates is considered homologous with those on cladid oral surfaces. In contrast, Enallocrinus scriptus (Hisinger, Reference Hisinger1828) (Fig. 5.12, 5.16) and Syndetocrinus dartae Kirk, Reference Kirk1933 (Fig. 5.1, 5.2) have tegmens composed of undifferentiated plating. Two Silurian Cyathocrinitidae with tegmens are Conicocyathocrinites Frest, Reference Frest1977 and Levicyathocrinites Frest, Reference Frest1977. Conicocyathocrinites ramosus (Angelin, Reference Angelin1878) has five large fixed PPCPs and smaller irregular plating elsewhere on the tegmen (Fig. 5.7, 5.8), and Levicyathocrinites monilifer (Angelin, Reference Angelin1878) has at least one enlarged PPCP with irregular plating forming the remainder of the tegmen (Fig. 5.9, 5.10).

Figure 5

Silurian and Devonian cyathocrine tegmens. (1, 2) Syndetocrinus dartae tegmen composed of multiple undifferentiated plates; (1) oral view; (2) lateral view (Silurian; YPM 13650); (3) Sphaerocrinus geometricus oral view of tegmen with differentiated plating that retains a PBS-A4 configuration (the largest plate is O1 with the five primary peristomial cover plates all in contact directly above it) (Devonian; from Lane and Moore, Reference Lane and Moore1978, fig. 378.1f); (4) Crotalocrinites rugosus oral view of tegmen with differentiated plating (Silurian; NRM-Ec 9700); (5, 6) Vasocrinus turbinatus tegmen with large undifferentiated plates and an anal sac; (5) oral view; (6) lateral view (Devonian; USNM 2334); (7, 8) Conicocyathocrinites ramosus tegmen composed of a few large plates, plated anal sac absent; (7) oral view; (8) lateral view (Silurian; NRM-Ec 9132); (9, 10) Levicyathocrinites monilifer tegmen composed of numerous plates; (9) oral view; (10) lateral view (Silurian; NRM-Ec 9173); (11) Vasocrinus sp. with the anal sac expanded over the entire oral region to form a tegmen; oral view (Devonian; BSM E12872); (12, 16) Enallocrinus scriptus; (12) oral view of tegmen with undifferentiated plating; (16) lateral view of calyx (Silurian; PMU 26506); (13, 14) Nanocrinus paradoxus with tegmen composed principally of five primary peristomial cover plates, note the posterior primary peristomial cover plate is enlarged significantly; (13) oral view; (14) lateral view (Devonian; from Lane and Moore, Reference Lane and Moore1978, fig. 376). All scale bars represent 5 mm.

After the Silurian, cyathocrine tegmens are only known from the Codiacrinidae, Euspirocrinidae, Gasterocomidae, and Sphaerocrinidae. Sphaerocrinus geometricus (Goldfuss, 1831) (Sphaerocrinidae, Devonian, Fig. 5.3) has a tegmen composed of differentiated PPCPs and ACPs with adaxial ACPs fixed and abaxial ACPs moveable. Gasterocomids are Devonian and include Nanocrinus paradoxus Müller, Reference Müller1856 (Fig. 5.13, 5.14) that has five plates that form a solid tegmen. In addition to having a tegmen, this crinoid is unusual by having tetragonal symmetry and the periproct on the side of the aboral cup. Two potential interpretations of this tegmen are (1) the five plates are homologous with the five PPCPs, or (2) the largest plate is homologous with Oral 1 and the other plates are PPCPs, with one PPCP eliminated along with one ray.

Vasocrinus Lyon, Reference Lyon1857 is a Devonian Euspirocrinidae, and several species had tegmens. Vasocrinus turbinatus Kirk, Reference Kirk1929 (Fig. 5.5, 5.6) has a tegmen with differentiated, fixed peristomial cover plates, with or without well-defined ACPs, interambulacral plates, and a marginal anal sac. In contrast, a specimen identified as Vasocrinus sp. has the base of the anal sac expanded to cover the entire adoral surface of the cup (Fig. 5.11). Thus, the anal sac base covers the mouth and forms a tegmen.

Cyathocrine cladid morphological pathways

The oral region ancestral state is presumed to have been a PBS-A3 oral surface as preserved in Palaeocrinus hudsoni (Figs. 3.1, 6.1), and the PBS-A3 configuration of oral surfaces persisted in some lineages (Fig. 6.2) with undifferentiated PPCPs and SCPs. In some lineages, as early as the Sandbian, some cyathocrines developed PBS-A4 oral surfaces (Illemocrinus, Fig. 6.4), in this case a doubling of the PPCPs occurred. Evolution of PBS-A4 oral surfaces (Fig. 6.3, 6.4) may have occurred multiple times among cyathocrines, but limitations imposed by generally poor preservation of oral surfaces prevents documentation of iterative changes. Illustrated here is Cyathocrinus harrodi (Fig. 6.3), which has six large PPCPs. Cyathocrines like Embryocrinus hanieli (Fig. 6.7) reduced the oral surface plating to only five, moveable PPCPs.

Figure 6

Inferred cyathocrine cladid constructional pathways reflecting an unknown combination of phylogenetic and iterative evolution (thumbnails not to scale; compare with Figs. 3–5). (1) Palaeocrinus hudsoni; (2) Nuxocrinus crassus; (3) Cyathocrinites harrodi; (4) Illemocrinus amphiatus; (5) Conicocyathocrinites ramosus; (6) Sphaerocrinus geometricus; (7) Embryocrinus hanieli; (8) Levicyathocrinites monilifer; (9) Crotalocrinites rugosus; (10) Syndetocrinus dartae; (11) Vasocrinus turbinatus; (12) Vasocrinus sp. ACPs=ambulacral cover plates; PPCPs=primary peristomial cover plates.

Sphaerocrinus geometricus (Fig. 6.6) has a tegmen with identifiable fixed PPCPs, ACPs, and moveable ACPs. These plates are basically in the same plane as homologous plates in an oral surface. The plate configuration in Sphaerocrinus geometricus suggests that plates of a crinoid with a PBS-A4 oral surface were sutured more-or-less in place, exapting moveable plates covering the mouth and ambulacra into fixed plates permanently covering these regions. From this tegmen configuration, (1) the tegmen could be expanded with additional plates (Fig. 6.9), (2) the addition of plates and loss of clear homology among plates could occur (Fig. 6.8), or (3) the reduction of plates that retain a homology signature could occur (Fig. 6.5). From a tegmen lacking plate differentiation, a larger tegmen with undifferentiated plates could develop (Fig. 6.10) or a tegmen with an anal sac could form (Fig. 6.11). From a cyathocrine with a distinct tegmen and anal sac, a form evolved with the base of the anal sac expanded across the entire oral portion of the aboral cup, thus forming a tegmen (Fig. 6.12).

Dendrocrine cladid oral regions and constructional pathways

In general, the oral regions of dendrocrine cladids are very poorly known, and we regard the oldest preserved oral regions on a dendrocrine to likely be a PBS-A4 oral surface because of the circular peristome. Examples include various Devonian cupressocrinitids, such as Halocrinites schlotheimii schlotheimii Steininger, Reference Steininger1831 with five orals (Fig. 7.1, 7.2), and the Pennsylanian crinoid Stellarocrinus sp. (Stellarocrinidae, Fig. 7.6). In addition, Stellarocrinus sp. evolved an erect plated anal sac (Fig. 7.6).

Figure 7

Dendrocrine oral surfaces and tegmens. (1, 2) Halocrinites schlotheimi schlotheimi oral surface with moveable plates missing; (1) oral view; (2) lateral view (Devonian; GIK-1939); (3) lateral view of Rhopalocrinus gracilis tegmen formed by the proximal anal sac (Devonian; from Moore and Lane, Reference Moore and Lane1978a, fig. 430.1b); (4, 5) Delocrinus? malaianus with flat tegmen composed of a few large plates, anal sac absent; (4) oral view; (5) lateral view (Permian, THDKA 12062); (6) oral view of Stellarocrinus sp. PBS-A4 oral surface with a plated anal sac (Pennsylvanian; from Strimple, Reference Strimple1973, fig. 20); (7) lateral view of Zeacrinites wortheni tegmen formed by proximal anal sac (Mississippian; from Springer, Reference Springer1926b, pl. 23, fig. 2a); (8) lateral view of Tholocrinus spinosus tegmen formed by proximal anal sac (Mississippian; from Springer, Reference Springer1926b, pl. 26, fig. 10); (9, 10) Botryocrinus ramosissimus tegmen with a plated anal sac; (9) oral view; (10) lateral view (Silurian; from Moore and Lane, Reference Moore and Lane1978b, figs. 398,1g, 398.1h). All scale bars represent 5 mm.

Tegmens are relatively rare in dendrocrines and were formed by either evolving a typical rigid plated structure or by expanding the base of the anal sac to cover all or most of the adoral surface. From an oral surface, dendrocrines evolved tegmens in one of three ways. (1) Delocrinus? malaianus Wanner, Reference Wanner1916 (Catacrinidae, Permian; Figs. 7.4, 7.5, 8.2) formed a flat-topped, rigidly-plated tegmen that presumably lacked an erect, plated anal sac. (2) Parallel to cyathocrines, a tegmen was formed covering the oral region and a plated sac was also present, such as in Botryocrinus ramosissimus Angelin, Reference Angelin1878 (Botryocrinidae, Silurian, Figs. 7.9, 7.10, 8.3). (3) Furthermore, tegmens composed exclusively of an expanded proximal anal sac (Figs. 7.8, 8.5) were formed. Examples include Rhopalocrinus gracilis (Schultze, Reference Schultze1866) (Cupressocrinitidae, Devonian, Fig. 7.3), Tholocrinus spinosus (Wood, Reference Wood1909) (Zeacrinitidae, Mississippian, Fig. 7.8), and Zeacrinites wortheni (Hall, Reference Hall1858) (Zeacrinitidae, Mississippian, Fig. 7.7). Due to poor preservation, few examples are known where it is clear that the anal sac was also a tegmen, but this tegmen type probably evolved many times and was most likely relatively common.

Figure 8

Inferred dendrocrine cladid and flexible crinoid constructional pathways reflecting an unknown combination of phylogenetic and iterative evolution (thumbnails not to scale; compare with Figs. 7, 9). (1) Taxocrinus intermedius; (2) Delocrinus? malaianus; (3) Botryocrinus ramosissimus; (4) Proapsidocrinus permicus; (5) Tholocrinus spinosus.

Additionally, all of the plating could be lost, resulting in a soft-tissue oral region, as suggested by the numerous dendrocrine genera with non-preservation of the oral region (Moore and Teichert, Reference Moore and Teichert1978) and by Tubulusocrinus doliolus (Wright, Reference Wright1936) (Scytalocrinidae, Mississippian, Kammer and Ausich, Reference Kammer and Ausich2007) with an uncalcified anal sac, but it is uncertain whether the oral surface was an uncalcified oral surface or a tegmen because the arms are tightly closed.

Flexible oral surfaces and morphological pathways

The oral regions of flexibles are also very poorly known, but those known formed by greatly expanding the oral surface (Fig. 9.1). Examples include Homalocrinus liljevalli Springer, Reference Springer1906, (Homalocrinidae, Silurian); Onychocrinus ulrichi Miller and Gurley, Reference Miller and Gurley1890 (Onychocrinidae, Synerocrinidae; and Taxocrinus intermedius Wachsmuth and Springer, Reference Wachsmuth and Springer1888 (Taxocrinidae, Mississippian), which all have a PBS-A4 oral surface. In T. intermedius (Figs. 8.1, 9.1), all five oral plates are exposed on the surface, PPCPs are absent, the ambulacra that branch on the oral surface have moveable ACPs, and the interambulacral regions were either uncalcified or had a spiculate integument. In O. ulrichi (Fig. 9.2) only Oral 1 is exposed, PPCPs are differentiated, the ambulacra did not branch on the oral surface, and interambulacral regions have numerous plates. Only the under surface of the oral region is known in Homalocrinus liljevalli (Fig. 9.3), with the five orals tightly sutured around the mouth. Based on calyx size, many flexibles must have also had smaller oral surfaces.

Figure 9

Flexible crinoid oral surfaces. (1) Oral surface of Taxocrinus intermedius oral surface (Mississippian; USNM S1821); (2) oral view of Onychocrinus ulrichi PBS-A4 oral surface (Mississippian; USNM S1877); (3) view of the underside of the oral surface of Homalocrinus lijevalli. (Silurian; NRM-Ec 12984); (4, 5) Proapsidocrinus permicus pseudotegmen; (4) oral view; (5) lateral view (Permian; from Moore, Reference Moore1978b, fig. 529.1a, 529.1c). All scale bars represent 5 mm.

No true tegmens are preserved among flexibles; however, one “pseudo-tegmen” is known. In Proapsidocrinus permicus Wanner, Reference Wanner1924 (Prophyllocrinidae, Permian, Figs. 8.4, 9.4, 9.5), the left shoulder of all five radial plates extend distally and adaxially to form a solid structure covering the mouth and adaxial regions of the ambulacra. Constructionally, this functions as a tegmen; however, it was formed exclusively by elements of the aboral cup rather than oral surface plates. Because it is formed in a fundamentally different manner than a tegmen, it is referred to here as a pseudo-tegmen.

Diplobathrid camerate tegmens

All known oral regions on diplobathrid camerates are tegmens, with the oldest preserved tegmens Ordovician (Sandbian). Most of these examples belong to the Rhodocrinitidae, and differing morphologies existed, even within a genus. Paradiabolocrinus stellatus Kolata, Reference Kolata1982 (Rhodocrinitidae, Ordovician, Fig. 10.1, 10.2) and Diabolocrinus arbucklensis Kolata, Reference Kolata1982 (Rhodocrinitidae, Ordovician, Fig. 10.3) are examples of Sandbian diplobathrids with tegmens. Both tegmens are composed of numerous undifferentiated plates. The anal tube is subcentral in P. stellatus. In D. arbucklensis, the anal tube is marginal and abaxial ACPs are well-defined and moveable.

Figure 10

Ordovician and Silurian diplobathrid camerate tegmens. (1, 2) Paradiobolocrinus stellatus tegmen composed of numerous undifferentiated plates; (1) oral view; (2) lateral view (Ordovician, OU 8860); (3) oral view of Diabolocrinus arbucklensis tegmen composed of undifferentiated plates in the center and fixed and moveable ambulacral cover plates (Ordovician; OU 8887); (4, 5) Gaurocrinus nealli tegmen with numerous very small plates, interpreted as being a flexible tegmen; (4) oral view; (5) oblique lateral view (Ordovician, NHMUK E14942); (6, 11) Lampterocrinus tennesseensis tegmen composed of more, smaller plates that the congeneric L. fatigatus; (6) lateral view; (11) oral view (Silurian; FMNH UC11942a); (7, 10) Lampterocrinus fatigatus tegmen composed of a few large plates; (7) lateral view; (10) oral view (Silurian; OU 4607); (8, 9) Dimerocrinites inornatus tegmen with differentiated plates including moveable abaxial ambulacral cover plates; (8) oral view; (9) lateral view (Silurian, FMNH UC 6395a); (12, 13, 14) oral views of Gazacrinus inornatus; (12) tegmen with primary peristomial cover plates, elongate fixed ambulacral cover plates (Silurian; USNM S137a); (13) oral view of oral region with tegmen removed; five orals forming a solid circle around the mouth; (14) lateral view of calyx (Silurian; USNM S137b). All scale bars represent 5 mm.

Gaurocrinus nealli (Hall, Reference Hall1866) (Reteocrinidae, Katian) has a tegmen with innumerable, small, undifferentiated plates and a marginal anal openning (Fig. 10.4, 10.5). The ambulcaral regions are raised above the interambulacral and intraambulacral regions; and it is probable that despite tessellate plating the tegmen was flexible.

Several Silurian diplobathrids have preserved tegmens. These include forms from the Rhodocrinitidae, Anthemocrinidae, Dimerocrinitidae, Gazacrinidae, and Lampterocrinidae. Again, the type of tegmens varies. Illustrated here is Dimerocrinites inornatus (Figs. 1.7, 10.8, 10.9), which has distinctive tegmen plates with five fixed PPCPs, fixed and probably moveable ACPs, and many interambulacral plates. Gazacrinus inornatus Miller, Reference Miller1892 (Gazacrinidae, Fig. 10.12–10.14) is a particularly instructive taxon, because it has five PPCPs differentiated on the tegmen and had fixed/moveable ACPs on the tegmen surface. However, in one specimen (Fig. 10.13) the tegmen is not preserved, and five large oral plates form a frame around the circular mouth in a position that would be directly beneath the PPCPs if the tegmen had been intact (Fig. 10.12), indicating it has the PBS-A4 arrangement. Note that in this example the PPCPs have an asymmetry reminiscent of 2-1-2 symmetry, but the underlying ambulacra have a PBS-A4 construction. Lampterocrinus Roemer, Reference Roemer1860 has relatively few tegmen plates compared to Ordovician and other Silurian diplobathrids. Lampterocrinus tennesseensis Roemer, Reference Roemer1860 (Lampterocrinidae, Fig. 10.6, 10.11) has a moderate number of smaller plates, but Lampterocrinus fatigus Strimple, Reference Strimple1963 (Fig. 10.7, 10.10) has even fewer plates in the tegmen. This tegmen is composed of only five much enlarged PPCPs; one or two interambulacral plates in each interray; and perhaps a single, radially positioned, fixed ACP in each ray.

Tegmens of Devonian and Mississippian diplobathrids are only known from the Rhodocrinitidae. Rhipidocrinus crenatus crenatus (Goldfuss, 1831) (Devonian) has a large, multiplated tegmen with undifferentiated plating (Fig. 11.8, 11.9). Gilbertsocrinus Phillips, Reference Phillips1836 has a highly modified tegmen with large tubular appendages. However, different species have differing degrees of tegmen plate specialization. The Devonian Gilbertsocrinus ohioensis Stewart, Reference Stewart1940 (Fig. 11.1, 11.2) has numerous undifferentiated tegmen plates, and Gilbertsocrinus tuberosus (Lyon and Casseday, Reference Lyon and Casseday1859) has a much larger tegmen, larger tubular appendages, and a more nuanced plating pattern (Mississippian, Fig. 11.7). Rhodocrinites cavanaughi (Laudon, Reference Laudon1933) (Mississippian; Fig. 11.5, 11.6) has poorly differentiated, fixed ACPs, and interambulacral plates, but the PPCPs are not clearly differentiated. In contrast, Rhodocrinites douglassi (Miller and Gurley, Reference Miller and Gurley1897) (Mississippian, Fig. 11.3, 11.4) has only a few, not clearly identifiable plates that comprise the tegmen.

Figure 11

Devonian to Mississippian diplobathrid camerate tegmens. (1, 2) Gilbertsocrinus ohioensis tegmen composed of numerous undifferentiated plates; (1) oral view; (2) lateral view (Devonian; SUI 56021); (3, 4) Rhodocrinites douglassi tegmen reduced to a few plates; (3) oral view; (4) lateral view (Mississippian; SUI 2057); (5, 6) Rhodocrinites cavanaughi tegmen with fixed ambulacral cover plates differentiated; (5) oral view; (6) lateral view (Mississippian; SUI 2059); (7) oral view of Gilbertsocrinus tuberosus tegmen with bifurcating, interradially positioned tegmen appendages. (Mississippian; USNM S1); (8, 9) Rhipidocrinus crenatus crenatus tegmen composed of numerous undifferentiated plates; (8) oral view; (9) lateral view (Devonian, GIK-1938). All scale bars represent 5 mm.

Diplobathrid camerate morphological pathways

Three basic tegmen types existed among Ordovician diplobathrid camerates, including those (1) with a flexible tegmen composed of numerous, very small, tessellate, and undifferentiated plates (Fig. 12.1); (2) with a rigid tegmen with numerous undifferentiated plates (Fig. 12.2); and (3) with a rigid tegmen with some differentiated plating including fixed ACPs axially and moveable ACPs abaxially (Fig. 12.3).

Figure 12

Inferred diplobathrid camerate tegmen constructional pathways reflecting an unknown combination of phylogenetic and iterative evolution (thumbnails not to scale; compare with Figs. 10 and 11). (1) Gaurocrinus nealli; (2) Paradiabolocrinus stellatus; (3) Diabolocrinus arbucklensis; (4) Gazacrinus inornatus; (5) Gilbertsocrinus ohioensis; (6) Dimerocrinites inornatus; (7) Rhodocrinites cavanaughi; (8) Gilbertsocrinus tuberosus; (9) Rhodocrinites douglassi. ACPs=ambulacral cover plates.

From forms with numerous undifferentiated plates, the plate number could be reduced (Fig. 12.4); and further, specialized plating could develop. Gilbertsocrinus and Rhodocrinites Miller, Reference Miller1821 are examples within a single family where both constructional trends existed. In Gilbertsocrinus, tegmens became more complex with interradially positioned tegmen extensions (Fig. 12.8); in contrast, plate reduction occurred in Rhodocrinites (Fig. 12.7. 12.9). Some forms evolved at least some differentiated plating, such as Fig. 12.3, and differentiation of plating could also become more pronounced (Fig. 12.6) in some diplobathrids.

Monobathrid camerate oral regions

The stereotypical monobathrid has a tegmen, although a few middle and late Paleozoic monobathrids have oral surfaces with moveable PPCPs. Similar to the diplobathrids, the oldest preserved oral region on a monobathrid is a tegmen, i.e., Glyptocrinus decadactylus Hall, Reference Hall1847 (Glyptocrinidae, Ordovician, Fig. 13.1) from Katian strata. In this crinoid, fixed ACPs are present with the remainder of the tegmen composed of innumerable, small, undifferentiated plates. The topography on the tegmen surface is variable, with interradial and intrabrachial portions of the tegmen depressed relative to the ambulacral positions. As in Gaurocrinus nealli, it is possible that this tegmen was flexible.

Figure 13

Ordovician to Silurian monobathrid camerate tegmens. (1) Oblique oral view of (three of the five rays) the presumably flexible tegmen of Glyptocrinus decadactylus with fixed ambulacral cover plates (Ordovician; from Ubaghs, Reference Ubaghs1978b); (2–4) Silurian Periechocrinus costatus tegmens. Note different shapes interpreted as the result of the tegmen being flexible; (2) oblique oral view with narrow anal tube, NHMUK E4071; (3) oral view with subconical anal tube. (NHMUK E54140); (4) lateral view of crown, NHMUK E1322 (crown); (5, 6) Methabocrinus laevigatus tegmen composed of a few large plates; (5) oral view; (6) oral view (Silurian; TUG 1395-40); (7, 8) Patelliocrinus punctuosus tegmen with large primary peristomial cover plates and a few other fixed plates; (7) oral view; (8) lateral view (Silurian; NRM-Ec 11579); (9, 10) Fibrocrinus phragmos with primary peristomial cover plates fixed and moveable ambulacral cover plates, and numerous interambulacral plates; (9) oral view; (10) lateral view (Silurian; GSC 126768); (11) Allocrinus irroratus tegmen with a large primary peristomial cover plates and a few other large plates centrally, and all ambulacral cover plates moveable (Silurian; OU 4655). All scale bars represent 5 mm.

Preserved examples of Silurian monobathrids with tegmens are known from the Carpocrinidae, Marsupiocrinidae, Patelliocrinidae, Periechocrinidae, and Stelidiocrinidae). Silurian monobathrid tegmens are also highly variable. Periechocrinus costatus (Austin and Austin, Reference Austin and Austin1843) (Periechocrinidae, Silurian, Fig. 13.2–13.4) has a tegmen composed of numerous, small, undifferentiated plates. Different specimens of P. costatus appear to have had somewhat different tegmen morphologies, which could also be the result of a flexible tegmen, despite tessellate plating.

Fibrocrinus phragmos Ausich and Copper, Reference Ausich and Copper2010 (Carpocrinidae, Silurian, Fig. 13.9, 13.10) has strikingly differentiated PPCPs and ACPs and interambulacral plates. Both fixed ACPs centrally and moveable abaxial ACPs are present. Similar to some diplobathrid counterparts, the tegmens of Methabocrinus laevigatus (Ausich et al., Reference Ausich, Wilson and Vinn2015b) (Carpocrinidae, Fig. 13.5, 13.6) and Patelliocrinus punctuosus Angelin, Reference Angelin1878 (Patelliocrinidae, Fig. 13.7, 13.8) are reduced in the number of plates. M. laevigatus has five, large PPCPs, five fixed ACPs, and a few interambulacral plates. The tegmen of Patelliocrinus punctuosus has fewer than ten total plates. Allocrinus irroratus Strimple, Reference Strimple1963 (Patelliocrinidae, Silurian, Fig. 13.11) has an unusual camerate tegmen. The tegmen has five, large, fixed PPCPs adjacent to three large plates supporting the anal tube, and elongate interambulacral plates supporting the arm trunks. Moveable ACPs extend from the PPCPs to the free arms.

Tegmens are commonly preserved on Devonian and Mississippian monobathrids, including the striking contrast between the very large tegmen with innumerable small plates of Strotocrinus glyptus (Hall, Reference Hall1860) (Actinocrinitidae, Mississippian) (Fig. 14.1, 14.2) and the small, few-plated tegmens of some Platycrinitidae (Devonian to Permian) with basically five PPCPs (Fig. 15.15, 15.16). Tegmens are best known on Devonian Dolatocrinidae, Hexacrinitidae, and Periechocrinidae and on Mississippian Actinocrinitidae, Batocrinidae, Dichocrinidae, and Platycrinitidae. Again, there is considerable variability among taxa. Some have larger tegmens and numerous differentiated tegmen plates (e.g., Gennaeocrinus carinatus [Wood, Reference Wood1901], Periechocrinidae, Devonian, Fig. 14.5, 14.6 and Amphoracrocrinus amphora [Wachsmuth and Springer, Reference Wachsmuth and Springer1897], Acrocrinidae, Mississippian, Fig. 15.11, 15.12). Although less common, large tegmens may also have differentiated PPCPs, ACPs, and interambulacral plates, including Agaricocrinus americanus Roemer, Reference Roemer1854 (Coelocrinidae, Mississippian, Fig. 14.4).

Figure 14

Silurian to Mississippian monobathrid camerate tegmens. (1, 2) Very large tegmen of Strotocrinus glyptus composed of innumerable small plates; (1) oral view; (2) lateral view (Mississippian; ROM 6225); (3) oral view of Marsupiocrinus (Amarsupiocrinus) stellatus stellatus tegmen composed of differentiated plates (Devonian; USNM S1296); (4) oral view of Agaricocrinus americanus tegmen composed of greatly enlarged primary peristomial cover plates and fixed ambulacral cover plates, but interambulacral plates are small (Mississippian; USNM S1064); (5, 6) Gennaeocrinus carinatus tegmen with undifferentiated plates, note the small plates covering anal opening; (5) oral view; (6) lateral view (Devonian, SUI 18152); (7, 8) Laticrinus oweni tegmen with differentiated primary peristomial cover plates, fixed ambulacral cover plates, and a few interambulacral plates; (7) oral view; (8) lateral view (Mississippian; USNM S1341); (9, 10) Collicrinus shumardi with very few plates, but differentiated primary peristomial cover plates; (9) oral view; (10) lateral view (Mississippian; USNM S1368). All scale bars represent 5 mm.

Figure 15

Mississippian to Permian monobathrid camerate tegmens. (1, 2) Platycrinites s.s. burlingtonensis tegmen reduced to fixed primary peristomial cover plates and five fixed ambulacral cover plates and a few interambulacral plates; (1) oral view; (2) lateral view (Mississippian; USNM S1337); (3, 4) Talarocrinus inflatus tegmen with very large posterior primary peristomial cover plate and other differentiated plates; (3) oral view; (4) lateral view (Mississippian; USNM S5898); (5, 6) Dichocrinus douglassi with undifferentiated plates forming a tegmen; (5) oral view; (6) lateral view (Mississippian; SUI 2073); (7, 8) Pleurocrinus goldfussi tegmen composed of primary peristomial cover plates and a few other plates; (7) oral view; (8) lateral view (Permian; THDKA 11841); (9, 10) Thinocrinus brevispina tegmen with primary peristomial cover plates, fixed ambulacral cover plates, and interambulacral plates; (9) oral view; (10) lateral view (Permian; THDKA 11815); (11, 12) tegmen of Amphoracrocrinus amphora with only fixed ambulacral plates differentiated; (11) oral view; (12) lateral view (Mississippian; from Ubaghs, Reference Ubaghs1978b, fig. 283.2a, 283.2b); (13, 14) Pleurocrinus depressus with fixed ambulacral cover plates, but homology of other plates unclear; (13) oral view; (14) lateral view (Permian; THDKA 11837); (15, 16) very reduced tegmen of Neoplatycrinus dilatatus with five fixed primary peristomial cover plates and five fixed ambulacral cover plates; (15) oral view; (16) lateral view (Permian; SUI 134856). All scale bars represent 5 mm.

Medium- to few-plated tegmens with differentiated plating occur among Devonian and Mississippian monobathrids, including Laticrinus oweni Ausich and Kammer, Reference Ausich and Kammer2009 (Platycrinitidae, Mississippian, Fig. 14.7, 14.8); Collicrinus shumardi shumardi Ausich and Kammer, Reference Ausich and Kammer2009 (Platycrinitidae, Mississippian; Fig. 14.9. 14.10); Platycrinites s.s. burlingtonensis (Owen and Shumard, Reference Owen and Shumard1850) (Platycrinitidae, Mississippian, Fig. 15.1, 15.2); and Talarocrinus inflatus (Ulrich, Reference Ulrich1917) (Dichocrinidae, Mississippian, Fig. 15.3, 15.4). In these crinoids homologies to oral surface plates are clear. In contrast, crinoids such as Dichocrinus douglassi (Miller and Gurley, Reference Miller and Gurley1896) (Dichocrinidae, Mississippian, Fig. 15.5, 15.6) have undifferentiated plating.

Pennsylvanian and Permian monobathrid camerate tegmens are rare relative to those of the Devonian and Mississippian. However, a reasonable disparity of tegmen morphologies persisted. The relatively large Permian crinoid Thinocrinus brevispina (Wanner, Reference Wanner1924) (Actinocrinitidae, Permian, Fig. 15.9, 15.10) has five differentitated PPCPs, fixed ACPs, and interambulacral plates. Most of the Permian monobathrid crinoids from Timor have a reduced number of plates in the tegmen, from Pleurocrinus depressus Wanner, Reference Wanner1916 (Platycrinitidae, Permian, Fig. 15.13, 15.14) with greatly enlarged fixed PPCPs; to Pleurocrinus goldfussi Wanner, Reference Wanner1916 (Platycrinitidae, Permian, Fig. 15.7, 15.8) with five PPCPs and one or a few fixed ACPs; and to Neoplatycrinites dilatatus Wanner, Reference Wanner1916 (Platycrinitidae, Permian, Fig. 15.15, 15.16) with a tegmen composed of five large PPCPs and one to a few fixed ACPs.

Despite the overwhelming occurrence of robust tegmens among monobathrids, a few small Devonian hapalocrinids and Mississippian dichocrinids and Pennsylvanian acrocrinids have oral surfaces with moveable PPCPs and ACPs. These include Amblacrinus rosaceus Roemer, 1844 (Hapalocrinidae, Devonian, Fig. 16.7); Camptocrinus alabamensis Strimple and Moore, Reference Strimple and Moore1973 (Dichocrinidae, Mississippian, Fig. 16.3, 16.4); Cyttarocrinus jewetti (Goldring, Reference Goldring1923) (Hapalocrinidae, Devonian, Fig. 16.1, 16.2); and Globacrocrinus glomus (Goldring, Reference Goldring1923) (Acrocrinidae, Pennsylvanian, Fig. 16.5, 16.6). Globacrocrinus presumably has five moveable PPCPs and moveable ACPs. Plating on Amblacrinus and Cyttarocrinus jewetti are more difficult to interpret, but the large rhomic-shaped plates that narrow to a point adorally may be exposed oral plates and PPCPs, and ACPs were moveable plates. It is possible that these plates were fixed (thus this is a tegmen), but because they are commonly not preserved in place, it is more likely that the PPCPs were moveable. Camptocrinus alabamensis has a reduced and highly modified oral region, with four or five moveable PPCPs, probably five ACPs that may be fixed, and a highly modified circular dichroism PPCP with the second circular dichroism interray plate forming distinctive anal tube (Fig. 16.4).

Figure 16

Devonian to Pennsylvanian monobathrid oral surfaces. (1, 2) Cyttarocrinus jewetti oral surface moveable ambulacral cover plates, presumably the large interradial plates are orals and the primary peristomial cover plates are missing; (1) oral view; (2) lateral view (Devonian; BSM E21032); (3, 4) Camptocrinus alabamensis oral surface with highly modified PPCPs; (3) oral view; (4) lateral view (Mississippian; from Strimple and Moore, Reference Strimple and Moore1973, fig. 23.4, 23.8); (5, 6) Globacrocrinus glomus oral surface presumably the large interradial plates are orals and the primary peristomial cover plates and ambulacral cover plates are missing; (5) oral view; (6) lateral view (Pennsylvanian; SUI 32936); (7) oral view of Amblacrinus rosaceus presumably the large interradial plates are orals and the primary peristomial cover plates and ambulacral cover plates are missing (Devonian; UMMP 56510). All scale bars represent 5 mm.

Monobathrid camerate morphological pathways

From the oldest preserved tegmen on a monobathrid camerate, Glyptocrinus decadactylus (Figs. 13.1, 17.1), presumably, three tegmen types arose: (1) a flexible tegmen with undifferentiated plating, as in Periechocrinus costatus (Figs. 13.2–13.4, 17.2); (2) a rigid tegmen with undifferentiated plating (Fig. 17.3) (e.g., Gennaeocrinus carinatus, Fig. 14.5, 14.6); and (3) a rigid tegmen with differentiated plating (e.g. Fibrocrinus phragmos, Figs. 13.9, 13.10, 17.5; Marsupiocrinus (Amarsupiocrinus) stellatus stellatus, Fig. 14.3). From the second type, tegmens could become larger, as in Strotocrinus glyptus (Figs. 14.1, 14.2, 17.4) or smaller, as in Dichocrinus douglassi (Figs. 15.5. 15.6, 17.6). From forms with differentiated plating (Fig. 17.5), plating could be reduced while still maintaining moveable ambulacra. An example is Allocrinus irroratus (Fig. 17.10) with a few fixed plates centrally (presumably PPCPs and a few interambulacral plates) and all of the ACPs were moveable. Tegmens also evolved that were composed of ten or fewer plates (Methabocrinus laevigatus, Figs. 13.5, 13.6, 17.8) and were presumably derived from a form like Figure 17.6 by the loss of visible ambulacra.

Figure 17

Inferred monobathrid camerate tegmen constructional pathways reflecting an unknown combination of phylogenetic and iterative evolution (thumbnails not to scale; compare with Figs. 13–15). (1) Glyptocrinus decadactylus; (2) Periechocrinus costatus; (3) Gaennaeocrinus carinatus; (4) Strotocrinus glyptus; (5) Fibrocrinus phragmos; (6) Dichocrinus douglassi; (7) Agaricocrinus americanus; (8) Methabocrinus laevigatus; (9) Laticrinus oweni; (10) Allocrinus irroratus; (11) Collicrinus shumardi; (12) Globacrocrinus glomus; (13) Neoplatycrinus dilatatus. ACPs = ambulacral cover plates, APCPs, ambulacral peristomial cover plates.

Differentiation of plating could become more extreme with very nodose PPCPs and ACPs, as in Agaricocrinus americanus (Figs. 14.4, 17.7). From this tegmen form, plating could be progressively reduced (Fig. 17.9, 17.11, 17.13) to produce a tegmen composed of ten plates (five large PPCPs and five small ACPs), such as in various Platycrinitidae.

This is only a very broad scenario for monobathrids. It is very likely that differentiated plating was lost and gained iteratively in numerous monobathrid lineages. Although relatively rare, tegmen plates also became highly modified. For example, tegmen plates may be modified to provide niches into which arms may be placed when closed. This includes crinoids in the ellipsoid calyx design of Ausich (Reference Ausich1988) and includes Eucalyptocrinites Goldfuss, 1831 (Ubaghs, Reference Ubaghs1978b, fig. 299) from the Silurian and Devonian (note this type of tegmen modification also occurred in disparids (Haplocrinites mespiliformis Goldfuss, 1831; Hapalocrinitidea, Devonian; Moore and Strimple, Reference Moore and Strimple1978, fig. 341.2a) and cyathocrines (Teganocrinus sulcatus Wanner, Reference Wanner1916 (Codiacrinidae, Permian, Lane and Moore, Reference Lane and Moore1978, fig. 385.10a)). Many camerate crinoids have tegmen and/or anal tube plates modified into spines. These vary from short nearly nodose plates, to the long, spinose ACPs of Dorycrinus gouldi Roemer, Reference Roemer1854 (Coelocrinidae, Mississippian, Ubaghs, Reference Ubaghs1978b, fig. 277.3c), to the spinose or blade-like “wing plates” of Pterotocrinus Lyon and Casseday, Reference Lyon and Casseday1859 that extend from tegmen plates (e.g., P. bifurcatus Wachsmuth and Springer, Reference Wachsmuth and Springer1897) (Dichocrinidae, Mississippian, Ubaghs, Reference Ubaghs1978b, fig. 281.7a–281.7c).

Tegmens dominated among monobathrids; however, some monobathrids had oral surfaces, including Amblacrinus rosaceus, Camptocrinus alabamensis, Cyttarocrinus jewetti, and Globacrocrinus glomus (Fig. 16). In all cases, these crinoids were relatively small, and the oral surfaces may have been the result of paedomorphosis within a lineage that reverted to an open mouth and ambulcacra. Alternatively, it is also possible that through the Paleozoic monobathrid lineages with an oral surface persisted, and we only have these few examples preserved (Fig. 17.12).

Disparid oral regions

The temporal pattern among disparids is similar to that of monobathrid camerates. Although relatively few unequivocal oral regions are known among disparids, of the four examples of oral regions known on Ordovician disparids three are tegmens, each with very different architecture. Doliocrinus pustulatus Warn, Reference Warn1982 (Cincinnaticrinidae, Fig. 18.13, 18.14) has a small, multiplated tegmen with undifferentiated plating. Anomalocrinus incurvus (Meek and Worthen, Reference Meek and Worthen1865) (Anomalocrinidae, Fig. 18.5, 18.6) has a large tegmen with larger, irregular undifferentiated plating that may have formed a flexible integument. Cincinnaticrinus varibrachialis Warn and Strimple, Reference Warn and Strimple1977 (Cincinnaticrinidae, Fig. 18.1, 18.2) presumably had a leathery, spiculate tegmen. Acolocrinus crinerensis Sprinkle, Reference Sprinkle1982 (Acolocrinidae, Fig. 18.3, 18.4) has an oral surface composed exclusively of five, large PPCPs with a spiraled asymmetry.

Figure 18

Disparid crinoid tegmens and oral surfaces. (1, 2) Cincinnaticrinus varibrachialus tegmen composed of a presumably flexible network of spicules; (1) oral view; (2) lateral view with proximal arms nearly enclosing tegmen. (Ordovician; YPM 517519; scale bars represent 2 mm); (3, 4) Acolocrinus crinerensis tegmen composed of five primary peristomial cover plates with a spiral twist; (3) oral view; (4) lateral view, (Ordovician; OU 9050; scale bars represent 5 mm); (5, 6) tegmen of Anomalocrinus incurvus with irregular large central plates and fixed ambulacral cover plates; (5) oral view; (6) lateral view (Ordovician; YPM 517517a; scale bar represents 5 mm); (7, 8) oral surface of Pisocrinus (Pisocrinus) gemmiformis composed of five moveable primary peristomial cover plates; (7) oral view; (8) lateral view (Silurian; USNM S1958b; scale bar represents 2 mm); (9, 10) Zophocrinus howardi oral surface with five moveable primary peristomial cover plates and additional small plates; (9) oral view; (10) lateral view (Silurian; USNM S5910; scale bars represent 2 mm); (11, 12) oral surface of Synbathocrinus dentatus composed of five moveable primary peristomial cover plates; (11) oral view; (12) lateral view with anal plate and proximal brachials in a few rays (Mississippian; USNM S2019; scale bar represents 5 mm); (13, 14) Doliocrinus pustulatus tegmen composed of irregular plating; (13) side view; (14) oral view (Ordovician; UT 1391TX3; scale bar represents 5 mm); (15) oral view of Haplocrinites stellaris (Devonian; USNM 185820; scale bar represents 5 mm).

In contrast, the relatively few post-Ordovician disparids with oral regions known are interpreted to have had moveable PPCPs, making them oral surfaces rather than tegmens. Specimens of these taxa may be quite abundant, but preserved oral regions are exceedingly rare. Hence, we conclude that these plates were moveable, making them oral surfaces. This includes two Silurian and three Devonian crinoids. The two Silurian disparids are Pisocrinus and Zophocrinus Miller, Reference Miller1891with much reduced oral surfaces. Pisocrinus (P.) gemmiformis Miller, Reference Miller1879 (Pisocrinidae, Fig. 18.7, 18.8) has an oral surface reduced to five, moveable PPCPs. Zophocrinus howardi Miller, Reference Miller1891 (Zophocrinidae, Fig. 18.9, 18.10) is highly unusual by lacking normal arms, and the plating of the oral surface has five PPCPs and a few other plates that are probably associated with rays. Most Devonian through Permian disparids with known oral regions are reduced to five or fewer plates and belong to the Haplocrinitidae, Allagecrinidae, Synbathocrinidae, and the Paradoxocrinidae (e.g., Haplocrinites stellaris (Roemer, 1844), Haplocrinitidae, Devonian, Fig. 18.15) and Synbathocrinus dentatus Owen and Shumard, Reference Owen and Shumard1852; Synbathocrinidae, Mississippian, Fig. 18.11, 18.12). The Pennsylvanian disparid, Belskayacrinus turaevoensis Arendt, Reference Arendt1997 has more plating on the oral surface, but a reduced number of PPCPs (disparid incertae sedis, Pennsylvanian). The moveable PPCPs are reduced to three, and it has moveable ACPs and numerous interambulacral plates.

Disparid morphological pathways

The two oldest (Sandbian) disparids with a preserved oral region are Acolocrinus crinerensis (Figs. 18.3, 18.4, 19.2) with an oral surface composed of five oral plates and Doliocrinus pustulosus (Figs. 18.13, 18.14, 19.3) that has a tegmen. Both of the other Ordovician disparids with preserved oral regions also have tegmens: Cincinnaticrinus varibrachialus (Figs. 18.1, 18.2, 19.1) and Anomalocrinus incurvatus (Figs. 18.5, 18.6, 19.5). Later disparids (e.g., Pisocrinus, Fig. 19.4) had an oral surface composed of five moveable PPCPs. It is possible that these PPCPs were modified from five fixed PPCPs depicted in (Fig. 19.2) or that the condition present in Pisocrinus was present but unknown among the earliest disparids. We suspect the latter. This five plated condition was also present among numerous microcrinoids. It is possible that the five-plated oral surface construction of Pisocrinus evolved into an expanded oral surfaces, such as Belanskycrinus turaevoensis (Arendt, Reference Arendt1997).

Figure 19

Inferred disparid tegmen constructional pathways reflecting an unknown combination of phylogenetic and iterative evolution (thumbnails not to scale; compare with Fig. 18). (1) Cincinnaticrinus varibrachialus; (2) Acolocrinus crinerensis; (3) Doliocrinus pustulatus; (4) Pisocrinus (Pisocrinus) gemmiformis; (5) Anomalocrinus incurvus. PPCPs=primary peristomial cover plates.

Articulate oral regions

In Hess and Messing (Reference Hess and Messing2011, p. 23), part of the definition for the post-Paleozoic Articulata is “Mouth exposed on the tegmen surface... .” Thus, the Articulata have an oral surface. Many oral surface morphologies are present in living crinoids with varying degrees of well-defined plating. A study of living crinoid tegmens is warranted to determine whether oral surface moprhologies are confined within clades or interatively evolved similar to tegmen and oral surface morphological grades among fossil crinoids. In addition, some oral surfaces on living crinoids are composed entirely of soft tissue.

Although extremely rare, oral surfaces are preserved on Uintacrinus Grinnell, Reference Grinnell1876 (Cretaceous). Uintacrinus is a relatively large articulate crinoid, and its oral surface was a soft-tissue structure with plates embedded in the soft tissue (Springer, 1901; Meyer and Milson, Reference Meyer and Milson2001, fig. 5D). It has a large anal sac that displaces the ambulacra from a strict pentameral symmetry.

Tegmens are known in two fossil articulates. Traumatocrinus hsui Mu, Reference Mu1949 (Traumatocrinidae, Triassic, Fig. 20.1–20.3) had a tegmen that is composed of innumerable undifferentiated plates and lacks ambulacra along the tegmen surface. Traumatocrinus hsui is a one of a few crinoid clades that assumed a pseudoplanktonic lifestyle attached to floating logs (Hagdorn, et al., Reference Hagdorn, Wang and Wang2007; Hagdorn and Wang, Reference Hagdorn and Wang2015). The life position of this crinoid was with the crown hanging downward.

Figure 20

Articulate crinoid and hemicosmitoid tegmens. (1–3) Traumatocrinus hsui tegmens, photographs courtesy of Hans Hagdorn: (1) entire view of oral surface, from Hess and Messing, Reference Hess and Messing2011, fig. 18b, MHI 1882/3; (2) irregular, large plating of a tegmenMHI 1881/2; (3) enlargement of MHI 1881/2; (4) lateral view of Psalidocrinus armatus pseudotegmen (Jurassic; from Hess and Messing, Reference Hess and Messing2011, fig. 90.2c); (5, 6) Apiocrinites roissyanus elevated tegmen to secundibrachial level composed of small plates; (5) side view of partial calyx; (6) enlargement (Jurassic; from Hess and Messing, Reference Hess and Messing2011, fig. 81.2f, 81.2g); (7, 8) tegmen of Caryocrinites missouriensis with large fixed plates; (7) oral view; (8) lateral view (Silurian; OSU 54440). All scale bars represent 5 mm.

Additionally, Apiocrinites roissyanus d’Orbigny, Reference Orbigny1841 (Apiocrinitidae, Jurassic, Fig. 20.5, 20.6) has a relatively high tegmen that incorporates primibrachials and, perhaps, a few secundibrachials. The mouth was subtegmenal. Plating on the Apiocrinites roissyanus tegmen is innumerable, undifferentiated small plates.

The articulate Psalidocrinus armatus von Zittel, Reference von Zittel1870 (Psalidocrinidae, Jurassic to Cretaceous, Fig. 20.4) is another example of a pseudo-tegmen. Unlike its Paleozoic counterpart, Psalidocrinus armatus forms a pseudo-tegmen with both shoulders of the radial plates. The shoulders project symmetrically both distally and adaxially. These distal projections are sutured together, cover the mouth, and form a niche into which the arms retract when closed.