Geologic History and Structure of Paleozoic Crinoids | 1 |
Brief Geologic History
Identifiable crinoids first began to markedly proliferate and diversify during the Ordovician (Simms 1999). The dramatic expansion in the numbers of genera during this geologic time was followed by a marked decrease with the end-Ordovician mass extinction (fig. 1.1) (Broadhead and Waters 1980). Following a subsequent recovery, crinoid genera plummeted a second time during the late Devonian mass extinction. At least some of the marked declines during these times occurred in association with environmentally induced lowering of the levels of seawater (Macdougall 2011).
The onset of the Mississippian Period saw a very dramatic worldwide rebound in both the diversity of crinoid genera and in the total numbers of individual crinoids (fig. 1.1). During this geologic time, crinoids achieved their greatest total number of genera, and as a result, the Mississippian is commonly referred to as the âAge of Crinoidsâ (Kammer and Ausich 2006). In fact, the expansion in crinoid diversity outpaced that of all other classification groups of echinoderms (Moore and Teichert 1978). Although additional factors were likely involved, it is generally believed that the widespread presence of warm, shallow sea levels during that geologic time was a major contributing factor.
Crinoid diversity declined somewhat in the late Mississippian. Although the numbers saw both increases and decreases during the Pennsylvanian and the Permian, crinoids in general tended to thrive. However, their numbers never again approached those of the middle Mississippian.
The end-Permian mass extinction nearly saw the demise of all crinoids (fig. 1.1). Possibly only one group, the Articulata (Simms and Sevastopulo 1993), now included in the proposed clade Articuliformes (Wright 2017), survived this devastating collapse of almost all life forms. It is believed that this group within the Articuliformes gave rise to all modern crinoids.
Although crinoids still exist in considerable numbers in certain localities of modern oceans (Macurda and Meyer 1983), the number of modern species represents only a comparatively small percentage of those that lived during the Paleozoic. For example, Macurda and Meyer (1983) have estimated that there are approximately six hundred species of modern crinoids, while there are roughly six thousand identified species of Paleozoic crinoids. In common with Paleozoic crinoids, one extant group retains their stems as adults and are commonly referred to as sea lilies due to the perception on the part of some observers that they resemble modern flowering plants.
The other extant group, the feather stars, encompass a much larger number of species and do not have stems as adults (Macurda and Meyer 1983). Interestingly, however, juvenile feather stars do pass through a stemmed stage.
Background
Both modern and Paleozoic fossil crinoids are members of the Crinoidea, one of five extant classes included within the Echinodermata, which incorporates all of the spiny skinned invertebrates. The other extant classes include the Asteroidea (starfishes), the Echinoidea (sea urchins), the Holothurioidea (sea cucumbers), and the Ophiuroidea (brittle stars). Modern as well as extinct Paleozoic crinoids, like all other echinoderms, are marine and as adults have a five-sided, or pentaradial, symmetry. However, they spend a short posthatching stage as free-swimming, bilaterally symmetrical larvae. Further details of the classification of the Crinoidea will be discussed in chapter 2.
Morphology
Throughout the Paleozoic, crinoids were relatively simple organisms whose basic structure consisted of a calyx with arms articulated at the upper, or adoral, surface and a stem attached to the underside, or aboral, surface (fig. 1.2). The structure of crinoids comprises primarily inorganic calcium carbonate plates adjoined together at sutures.
There are also more general terms used to describe relative position or location in crinoids. For example, ventral refers to the adoral, or oral, area, particularly of the calyx, while dorsal refers to the aboral, or stem side, of the calyx. Although this terminology may initially seem confusing to those who think of ventral as the underside and dorsal as the upper side of an animal, consider instead that ventral always refers to the side on which the mouth is located and dorsal is the opposite.
Proximal refers to positions ever closer to the plane of intersection at the calyx/column articulation, thus more proximal is toward the base of the calyx and toward the top of the column. Distal, on the other hand, is either toward the top of the calyx or the bottom of the column. The terms abaxial and adaxial designate away from or toward the central axis, respectively.
Calyx
The calyx is the centrally located structure of the crinoid, and the digestive system is the major organ system cloistered there. The calyx and its associated arms are often referred to as the crown. The region from the attachment site of the stem to where the arms of the crinoid become free is also referred to as the aboral cup.
The tegmen is located at the apex, or adoral surface, of the calyx and forms the roof of that structure. Depending on the species, the tegmen may be flat, have a low or high dome shape, or may be shaped like an inverted cone. The exterior of the membranous surface of the tegmen is typically covered by small calcitic plates, which may in different species be smooth, rounded, or nodular. The tegmen may also be a solidly plate structure. Collectively, the calyx and the tegmen are sometimes referred to as the theca. On the other hand, some authors use the terms calyx, aboral cup, and theca interchangeably.
1.2.An example of Hypselocrinus hoveyi showing the structural components of a typical Paleozoic crinoid. Collected in the Edwardsville Formation, Montgomery County, Crawfordsville, Indiana. Authorâs collection. (Note in this crinoid the calyx equals the aboral cup.) The scale of alternating black and white 1-cm squares serves as a size reference.
The mouth of the crinoid opens onto the surface of the tegmen or in some species is shielded from the exterior by calcitic plates. The orifice of the anus may be located on the surface of the tegmen in close proximity to the mouth, or it may be elevated above the tegmen on a structure called the anal tube.
The plates of the aboral cup may be fused together to form a tight, rigid structure or may be only loosely attached to one another. There are eleven recognized external structural designs for the aboral cup (Ausich 1988). However, due to the considerable overlap and convergence in aboral cup design across species, this feature is but one of several needed to classify and differentiate species.
Depending on the species, there may be two or three separate circumferential rings, or circlets, of plates that collectively make up the major portion of the aboral cup wall. Each circlet comprises a single layer of plates joined at their lateral margins.
The most apical, or adoral, circlet is the radial circlet, which comprises radial plates (fig. 1.3). In keeping with the pentaradial design of adult crinoids, there are five radial plates, which are usually, though not always, roughly equal in size. The radial circlet may consist exclusively of radial plates; but depending on the species, the radial circlet may be interrupted by an anal plate.
Basal plates compose the second circlet of plates, or basal circlet, which is located immediately below or proximal to the radial circlet (fig. 1.3). Depending on the species, there may be five or fewer basal plates, and these may or may not be of equal size. The sutures between the basal plates may be either well defined or fused, such that their margins are either partially or totally obscured. In different species, the basal plates may or may not be visible when the calyx is viewed laterally.
Some taxonomic groups of crinoids have a third circlet of plates, the infrabasals, which are located aboral to the basal circlet (fig. 1.3). If infrabasal plates are present, they are the site of attachment of the stem, and the calyx is defined as dicyclic. Although the calyx may be dicyclic, the infrabasals may be hidden by the stem, such that they are not easily discerned. However, if the infrabasal plates are truly absent, the basals are the site of the stem attachment, and the calyx is classified as monocyclic.
Anal plates may also be found on the calyx. Their number vary with the species but are usually found in association with the underlying rectum and are believed to provide extra internal space for that organ within the calyx. The locations of the anal plates and their number on the calyx may be a critical criterion for differentiating closely related crinoid species.
1.3.A close-up view of the calyx of Hypselocrinus hoveyi showing the radial, basal, and infrabasal circlets.
Stem and Holdfast
The stem both anchors and elevates the crown above the ocean floor, thus placing it in a clearer, more sediment-free, better-aerated e...
