Growth and Molting

For a number of trilobite species, the various stages of growth (ontogeny) from the larval to the adult form are known with great detail from the fossil record. Three major periods of growth are recognized. The protaspidperiod extends from the hatching of the egg to the first appearance on the single-piece dorsal shield of a transverse suture, defining the cephalon and the so-called transitory pygidium. During this period, a larval ridge may be the precursor of the axial lobe. The size of the protaspis is very small, typically 0.3 to 1.0 millimeters, and the lower limit for any species is clearly the upper limit for the size of the eggs of that species. During this period, the protaspis may develop features, such as marginal spines, which will disappear at a later stage. Observation of the fossil record of growth in the protaspis period has led to speculations concerning the affinities of trilobites with other arthropods, based on the notion that larval development may be a replay of ancestral history or phylogeny. The meraspidperiod is characterized by gradual separation of the cephalon from the transitory pygidium, by means of the progressive appearance of thoracic segments. Meraspid degrees correspond to the number of segments which made their appearance. The new segments originate at the thoracic-pygidial boundary. The number of molts required to complete the thorax does not necessarily correspond with the number of segments to be added to the thorax. The overall size of the trilobite may increase to more than ten times that of the protaspis. The meraspid period terminates when the thorax has reached the number of segments which characterize the adult individual.

At this stage, the adult form, or the holaspidperiod, has been attained. Here the growth is continuous through many moltings, so that the size of the individual is correlated with its age. Several changes in the relative size of the various parts of the exoskeleton do occur throughout this period. The cephalon usually represents a larger fraction of the carapace in the early growth stages.

The ontogeny of trilobites is beautifully illustrated in an article by H. B. Whittington in the Treatise of Invertebrate Paleontology (Moore 1959). The illustration of this aspect of trilobite life will be limited here to the presentation of a pictorial summary in plate 2. This picture contains evidence of some kind of trilobite nursery, containing examples of all stages of growth. The trilobite depicted is Elrathia kingii (Meek) from the famous Wheeler Formation of the Middle Cambrian of Utah. Although detail may be lost in the smallest specimens, this view gives a feeling for the range of sizes involved in trilobite growth. Present are several holaspid carapaces of various length, with their characteristic number of thirteen thoracic segments; at least one meraspid carapace with eight segments; and at least one protaspid shield about one millimeter long, showing the larval ridge quite distinctly.

Molting is clearly an integral part of the growth process in trilobites. The most abundant fossil remains of trilobites are the disarticulated exuviae, which, on account of their relatively large surface and light weight, could be easily transported and concentrated by wave motions and currents. Plate 3 shows a slab of shale from the Collingswood Formation, Ontario, which is densely covered with a multilay-ered assembly of carapace fragments of Pseudogygites latimarginatus (Hall). Such accumulation is most likely due to selective concentration, as is often seen occurring with small bivalve shells on the seashore. In such occurrences, complete carapaces are very seldom found. Since trilobites molted many times during their growth, it follows that each individual left a multiple fossil record.

In situations where the molting process occurred in a relatively undisturbed environment, exuviae may be found in the approximate posture in which they have been abandoned by the trilobite. There are at least two characteristic modes in which the exoskeleton was shed by the growing trilobite. In one of them, the phacopid mode, the cephalon separated from the thorax between the occipital ring and the first thoracic segment. The animal could crawl out of the old exoskeleton through such an opening, and in so doing would force the old cephalic shield to flip upside down. Plate 4 shows one of many examples, collected by the author, of exuviae shed by Phacops rana milleri Stewart, as found in the Devonian Silica Shale at Sylvania, Ohio. The cephalon is intact and usually is found in the immediate vicinity of the thorax-pygidium assembly. The latter occurs most often in the enrolled condition, showing that the integument must have contracted like a spring, after the former inhabitant crawled out. As mentioned previously concerning Arthropoda, the newly molted animal is provided with a soft exoskeleton, which would later harden through sclerotization and mineralization. Plate 5 shows a "soft-shelled

Various stages of growth of the trilobite

Elrathia kingii (Meek)

from the Wheeler Formation, Middle Cambrian, Utah (x2.7). (RLS coll.; now at FMNH.) The larger trilobites represent the holaspid stage, a meraspid carapace with eight segments is the center lowermost trilobite, a protaspid shield is located just below the lowermost complete trilobite on the right-hand side.

Slab coated with disarticulated exuviae of the trilobite Pseudogygites latimarginatus (Hall) from the Collingswood Formation, Ordovician, Collingswood, Ontario (xl.l). (RLS coll.; now at FMNH.)

trilobite," also from the Silica shale. We are dealing here once again with Phacops rana milleri Stewart. This soft-shell condition is revealed by the fact that the carapace is considerably thinner than in the average specimen. This gives a translucent quality to the shield, particularly apparent in the pleural regions. The axial lobe seems to have thickened somewhat more and appears darker. By accident or not, this trilobite overlaps the exuviae of another trilobite (or its own?).

The above description seems to ignore that we are dealing with a fossil and not with a living animal. However, here the fossilized exoskeleton is composed of the primary calcite that was originally present in the trilobite cuticle. Thus, the color differentiation that affects selected areas of the carapace in plate 5 may reflect incomplete mineralization of the exoskeleton as it existed at the time of burial.

Another pictorial view of phacopid molting is contained in plate 6. Here we deal with exuviae of Dalmanites verrucosus (Hall) from the Silurian Waldron Shale formation of Waldron, Indiana. In this example all trilobite parts are seen from the ventral side and are still partly imbedded within the shale matrix. The hypostoma appears displaced from its axial position, indicating opening of the hypostomal suture during molting. In the exuviae shown in plate 5, the hypostoma is missing altogether.

Another frequent molting procedure is the olenid mode, in which the free cheeks separate from the cranidium at the facial sutures, enabling the molting trilobite to make its way out of the opening thus created. In this case, the facial sutures are functional, representing natural fracture lines.

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  • helvi
    How often did trilobites molt?
    4 months ago

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