Ontogenetic adaptations in some Pennsylvanian crinoids

Abstract

Three species of Upper Pennsylvanian crinoids, two inadunates and one flexible, exhibit a sufficiently complete growth series to describe and quantify their ontogeny. Apographiocrinus typicalis (Moore and Plummer), Erisocrinus typus (Meek and Worthen), and Euonychocrinus simplex (Strimple and Moore) were collected from the middle shale of the Millersville Limestone Member of the Bond Formation at the Charleston Stone Quarry, Coles County, Illinois. Ten other crinoid species were collected in insufficient numbers for growth studies. Both bivariate and multivariate statistical procedures were used to analyze the growth of these crinoids.In the inadunate crinoids skeletal growth was highly coordinated in the dorsal cup and cup plates. Arm length increased by addition of secundibrachials. Food groove width increased almost three times from immature to adult stages, thus allowing a larger range of food particles to be consumed by adults. Growth of the cup is isometric in E. typus and slightly anisometric in A. typicalis. Primibrachials in both species develop anisometrically. Immature specimens possess long, slender primibrachials, with the A ray plate being longest. In maturity, the primibrachials become equidimensional and equalized in all rays. The A ray arm is followed by the C and D ray arms, and finally the B and E ray arms, in sharp contrast to arm development in disparid inadunates. Pinnules appear late in the immature stage of growth. Primary emphasis on the expansion of the arms in young inadunates demonstrates a rapid establishment of a filtration-fan mode of feeding. Subsequent development involves widening of the plates for strength and support of the arms and to accommodate wider food grooves that permit a larger array of food particles to be captured.The flexible crinoid, E. simplex, experienced a less correlated pattern of growth. Cup plates and arms grew isometrically and all arms were added simultaneously. Development suggests that flexibles fed differently than the associated inadunates and the cup may have played a less important role in functioning of the calyx.The upright, elevated growth pattern of stalked crinoids exposed them to a sequence of competitive interactions with other epifaunal animals that had important adaptational ramifications for their evolution, diversity, and extinction. The fact that most living crinoids, stalked and unstalked, have abandoned a permanently fixed place of growth may have been an advantageous factor in their survival.

Ontogeny ofHypselocrinus Hoveyi, Mississippian Cladid Crinoid From Indiana

“…Other aspects of the arms grew allometrically. As is typical with most crinoids studied (e.g., Brower, 2002b;Peters and Lane, 1990) and for both pinnulate and non-pinnulate crinoids, arm characters typically did not grow isometrically. For the filtration fan as a whole, this meant that food capture capacity kept pace with the volumetric growth of the calyx.…”

“…Similar to Quintuplexacrinus, Hypselocrinus grew with a mixture of allometry and isometry. Individual infrabasal and basal plates grew consistently allometrically; although note that in the two cladid species analyzed by Peters and Lane (1990) one had cup growth interpreted as isometric and the other was allometric. In Hypselocrinus, the secundibrachial width grew with positive allometry with respect to aboral cup height.…”

“…Because of its importance to our understanding of the evolution of the group, crinoid ontogeny is documented where possible. Brower et al (1978) summarized crinoid ontogeny, and examples of other previous studies include Brower (1973, 1992a, 1992b, 1996, 1997, 2002a, 2002b, 2005, 2006), Kesling (1961), Lane and Sevastopulo (1982), Laudon (1967), Macurda (1968), McIntosh (1983, Moore, (1940), Meyer (1965), Pabian and Strimple (1985), Peck (1935), Peters and Lane (1990), and Sevastopulo and Lane (1988).…”

“…Thus, relatively little is known about the ontogeny of middle and upper Paleozoic flexible and cladid crinoids; indeed little is known about the ontogeny of upper Paleozoic crinoids in general. Two notable exceptions were Pabian and Strimple (1985) and Peters and Lane (1990) who studied crinoid ontogeny in cladids and one flexible from the Pennsylvanian. The rarity of very small cladid crinoids makes the discovery of a Mississippian cladid with an aboral cup height of 4 mm a significant specimen to evaluate ontogeny within this important lineage of Mississippian crinoids.…”

Presumed postlarval pentacrinoids from the Lower Devonian Hunsrück Slate, Germany

“…Aside from Palaeozoic microcrinoids, which are thought to have been paedomorphic adults that fed by extending podia through open oral plates, or along very simple arms (Sevastopulo ), these are possibly the smallest known fossil juvenile crinoids (Brower et al . ; Peters & Lane ), and perhaps the only known occurrence of pentacrinoids in the fossil record. Crinoid larvae are known to settle gregariously (Mladenov & Chia ).…”