The early ontogeny of blastoids

Abstract

Blastoidea) pass through a stage during growth in which the oral surface of the minute theca is composed of five oral plates only. The evidence that these thecae, which lack hydrospires, deltoids and lancet plates, are juvenile blastoids is that they occur together with mature blastoids and vice versa; they have growth lines on the exterior of the plates; and the outlines of their thecal plates are congruent with the earliest growth lines of the thecal plates of blastoids. It is proposed that this growth stage should be referred to as the passalocrinid stage, because the juvenile blastoids at this stage have previously been referred to Passalocrinus, originally described as a monocyclic crinoid. Coronoids also pass through a stage comparable to the passalocrinid stage of blastoids.

The importance of lithographic limestones for revealing ontogenies in fossil crustaceans

“…This is especially important for the reconstruction of ground patterns and for the understanding of the early evolution of a group. Developmental data of fossils have been successfully used in various non-arthropod taxa (e.g., Sevastopulo 2005;Nützel et al 2006;Schoch and Fröbisch 2006;Bandel 2007;Sumrall and Wray 2007). For fossil arthropods, dorsal hard parts of trilobites or ostracod shields have mainly been used for ontogenetic studies (e.g., Gramm 1973;Schweitzer et al 1986;Smith 2000;Hughes et al 2006 and references therein).…”

Early post-metamorphic, Carboniferous blastoid reveals the evolution and development of the digestive system in echinoderms

“…The small size of the theca and the absence of the anal deltoids, the deltoid bodies and the hydrospires indicate that the specimen was immature; the presence of the deltoid lips, the most proximal elements of the ambulacral feeding system, and (possibly) the lancets strongly suggests that the specimen had progressed past the passalocrinid stage of blastoid development (as defined by Sevastopulo [8]). The passalocrinid stage was inferred to be equivalent to the nonfeeding cystidean stage of crinoid ontogeny [8]; the studied specimen represents a more advanced post-metamorphic developmental stage that was capable of feeding (as shown A -E, ambulacra; an, anus; bf, brachiole facets; bp, basal plate; dp, deltoid plates; fg, food groove; gl, growth lines; hd, hypodeltoid; mo, mouth; rp, radial plate; sp, stereom pores; ts, tubular structure. Colours: cyan, radial plates; green, brachiole facets; red, deltoid plates.…”

Soft‐tissue Preservation of the Hind Gut in a New Genus of Cladid Crinoid From the Mississippian (Visean, Asbian) at St Andrews, Scotland

“…Eggs in living crinoids range from 135 to 150 µ m in diameter, and would easily pass through these pores (Mortensen 1920; Nakano et al . 2003; Sevastopulo 2005). Modern crinoids have a small tegmen with an anal cone and lack a comparable anal sac like those of the Cladida, and the gonads are located on the arms in genital pinnules (Breimer 1978).…”