Ontogeny of the Food-Gathering System in Ordovician Crinoids

Athenacrinusn. gen. and other early echinoderm taxa inform crinoid origin and arm evolution

“…‘Primary’ was used for lateral, and ‘secondary’ for medial cover plates in previous work on the Cambrian basal pentaradiate Stromatocystites (Paul and Smith, 1984). ‘Inner’ and ‘outer’ were used by Brower (2006) for two-tiered Late Ordovician crinoids. The term ‘lappets’ was applied to camerate crinoids with a single biserial tier (Kolata, 1982) and, in some cases, to extant crinoid cover plates (Clark and Clark, 1915).…”

“…Changes included: (1) extraxial lateral field size reduction or disappearance, (2) floor plates nestling into the adoral channel, but later not expressed as skeletal elements, and (3) medial cover plates consisting of a single biseries or entirely absent. (Brower, 2006, fig. 1), suggesting paedomophosis as a mechanism for the reduction of tier numbers in other taxa.…”

“…two-tiered Table 2. Characteristic arm types and cover plates among diverse Late Ordovician crinoids (primarily from Brower, 2006 andSprinkle, 1982a, b (Brower, 1966) disparid non-pinnulate, ramulose 1 Ectenocrinus simplex (Hall, 1847) disparid pinnulate 1 Peltacrinus sculptatus Warn, 1982 disparid non-pinnulate 2…”

“…sp., paratype PE 52753; ( 4 ) rhodocrinitid camerate Archaeocrinus snyderi Guensburg, 1984, unnumbered brachial on same slab with UI X-5738; ( 5, 6 ) disparid Doliocrinus pustulatus Warn, 1982, brachial outline from 1113TX51 and 1279TX177, showing transition from secondary groove to nearly fully enclosed foramen within brachial, cover plates added from Doliocrinus monilicaulis Guensburg, 1984, UI X-5716; ( 7 ) Columbicrinus crassus Ulrich, 1925, UI X-5711, brachial outline from Guensburg (1992); ( 8 ) hybocrinid cladid Hybocrinus bilateralis Guensburg, 1984, UI X-5867 and 5868, and ( 9 ) cladid Cupulocrinus crossmani Brower, 1992, SUI 62895A and 62895C, secondary groove based on unnumbered specimens of Cupulocrinus gracilis in Field Museum collections.

Figure 8.Color-coded tracings of Early (Tremadocian) ( 1–4 ) and Late (Sandbian/Katian) ( 5–10 ) Ordovician crinoid mid-distal arm segments showing plating from one side: ( 1, 2 ) earliest-known crinoids, neither with exposed brachials: (1) Apektocrinus ubaghsi Guensburg and Sprinkle, 2009, 1984TX1, deeply weathered, many elements fragmentary, outlines incomplete, prepared from Figure 1.8; ( 2 ) Titanocrinus sumralli Guensburg and Sprinkle, 2003, PE 52720, weathered, most element outlines preserved, floor plates unexposed except on left where laterals have fallen away, traced from Figure 10.6; ( 3, 4 ) slightly younger taxa: ( 3 ) cladid-like Aethocrinus moorei Ubaghs, 1969, YPM 517777, latex cast, plate outlines partially exposed; ( 4 ) disparid-like Athenacrinus broweri n. gen. n. sp., paratype PE 52751, medial cover plates showing primary, secondary, and ?tertiary elements, plate outlines well preserved except for medial cover plates, these slightly weathered, traced from Figure 3.2; ( 5–10 ) line drawings modified from Brower (2006, fig. 1): ( 5 ) disparid Cincinnaticrinus varibrachialis Warn and Strimple, 1977, MCZ 136884; ( 6 ) disparid Ectenocrinus simplex (Hall, 1847), MCZ 133457; (7) camerate Euptychocrinus skopaios Brower, 1994, SUI 80173; (8) cladid Cupulocrinus crossmani Brower, 1992, SUI 62895C; ( 9, 10 ) cladid Hybocrinus conicus Billings, 1857: ( 9 ) SUI 49484, juvenile with single biseries, and ( 10 ) SUI 80030, mature specimen with two-tiered pattern.

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Ordovician–Silurian Lilliput crinoids during the end-Ordovician biotic crisis

“…Although there is not a direct correlation of calyx or crown size to column height, it is reasonable to assume that a crinoid with a one or more order of magnitude smaller body would simply be a smaller crinoid with a shorter column, thus closer to the sea floor in lower tiers (Ausich 1980;Ausich and Bottjer 1982;Bottjer and Ausich 1987). Brower (2006) demonstrated that calyx size is positively correlated to the length of the ambulacral tracts, thus the area of the filtration fan would also be smaller. Thus, a smaller crinoid would presumably have lower fecundity and a diminished feeding capacity relative to its larger ancestor.…”