THE ONTOGENY OF TRILOBITES:
GENERALITIES
EXOSKELETON AND GROWTH
   Like all extant arthropods, trilobites had a discontinuous growth. This kind of growth characterizes organisms that possess an exoskeleton, i.e. a rigid structure enveloping their body and enabling, for example, insertions of muscles. The animal, to increase its body size, need to reject this rigid envelop while molting. Then, it increases its body volume before synthesizing a new exoskeleton. The adult size is attained after a more or less important number of molts.
   The dorsal side of trilobites was protected by a highly mineralized exoskeleton (or cuticle). By contrast, the cuticle covering the ventral structures was probably of organic nature only (chitine?). The mineralization of the dorsal exoskeleton, or calcification, occurred very early in the ontogeny, possibly as soon as hatching. Thus, nearly all the post-embryonic development of trilobites can be described.
THE DIFFERENT PERIODS OF DEVELOPMENT
   The development of trilobites is traditionally subdivided in three periods. The protaspid period (Beecher, 1895) includes the first post-embryonic calcified stages. Two kinds of protaspides are recognized in most trilobite ontogenies: the anaprotaspides and the metaprotaspides (Fig. 1). In anaprotaspides, cephalic region (or protocephalon) cannot be differentiated from the post-cephalic region (or protopygidium). When a transversal furrow separating these two regions appears, larval stages are then called metaprotaspides. During the protaspid period, new segments generally appear in the posterior part of the protopygidium. However, all the segments of the body remain fused together, thus forming a unique structure.
Figure 1. Differences between ana- and metprotaspides in Dimeropyge sp. (drawings from Chatterton, 1994).
   The meraspid period starts when an articulation appears between protocephalon and protopygidium (Fig. 2). This latter is then called transitory pygidium. While new segments keep on appearing in the posterior part of this transitory pygidium, creation of new articulations enables segments to be released in its anterior part. These free segments constitute the developing thorax. According to the number of thoracic segments released, different meraspid degrees are recognized. For example, a degree 1 meraspis is a meraspis with one thoracic segment released.
   
When the full complement of thoracic segments is attained, the trilobite enters the holaspid period. At the rear of the body, a more or less important number of fused segments constitute, the pygidium. During the holaspid period, trilobites keep on molting periodically to grow. It is even possible that some trilobites, if not all, have molted the rest of their life. Also, the notion of adulthood remains rather vague when dealing with trilobites, especially if we consider that the onset of sexual maturity is unkown.
Figure 2. The different periods of trilobite development. Example of Dimeropyge sp. (drawings from Chatterton, 1994).
MORPHOLOGY & ECOLOGY OF PROTASPIDES
   According to Speyer & Chatterton (1989), two types of protaspides can be recognized with regard to morphology (Fig. 3). The first type is characterized by its globulous shape, the presence of marginal spines directed in virtually all directions, an enrolled doublure, and generally a high degree of effacement of furrows. Protaspides of this type are called "nonadult-like" protaspides and have been described in the ontogenies of various groups (e.g., Asaphina, Proetoidea …). Following Speyer & Chatterton (1989), they are interpreted as planktonic larvae (i.e., larvae living in the water column).
Figure 3. The two morphological types of protaspides. The nonadult-like protaspides (e.g., Isotelus sp.) are interpreted as planktonic larvae, while adult-like protaspides were probably already benthic (e.g., Physemataspis sp.). Drawings from Speyer & Chatterton, 1989. When both types co-occur in the ontogeny of a species, nonadult-like larvae preceeded adult-like ones and a metamorphosis probably accompanied the transition from one type to another.
   The second morphological type is only moderately arched dorsally and displays an inturned doublure (Fig. 3). When marginal spines are present, they lay on a single horizontal plan. In general, furrows are more visible, especially the dorsal furrows that run from front to rear. This enables the longitudinal trilobation of the body, characteristic of trilobites, to be more easily observed. Because, to some extent, larvae of this type already resemble miniaturized adults, they are called "adult-like" protaspides. Though their mode of life is still discussed by some specialists (Adrain, pers. com., 2003), they are thought to have been benthic (i.e. living on the sea floor, see Fig. 3) like most adult trilobites.
THE DEVELOPMENTAL STRATEGIES
to be continued ...
(LARVAE & TRILOBITE ORDERS)
Last modified: Mar., 23rd 2007
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