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Growth stages in trilobites

From the very beginnings of trilobite research, wary collectors used to come across clusters of fossilized remains which obviously did not consist of different taxa but actually seemed to represent distinct growth stages of the very same trilobite species. In his great 1852 work, Systême Silurien du centre de la Bohême, BARRANDE described these growth stages, using the trilobite Sao hirsuta BARRANDE, 1846 as an example.

Meraspis of Sao hirsuta There is hardly a doubt that trilobites have to be regarded as oviparous arthropods. Due to the limited knowledge we can gain from fossils alone, we do not know exactly how they cared for their eggs, i.e. whether they deposited them in a protected area or perhaps attached them to plants, corals, sponges or rocks. They may even have carried them along until they had reached a certain degree of development. The latter idea received some support through the excavation of some unusual fossils:

On rare occasions, a strange ‘bubble’ on the preglabellar field had been witnessed in some trilobite species. Professional workers like FORTEY attributed these uncommon morphological peculiarities to their functioning as “brood pouches” (FORTEY & HUGHES, 1998). FORTEY emphasized that the recent Limulus or horseshoe crab, generally regarded as the closest living relative of trilobites, conducts this kind of care for its offspring and he did not fail to tell of the culinary aspects of this strange sort of ‘caviar’ in his book Trilobite! – Eywitness to Evolution. <g>

Unfortunately, finds of trilobites showing this bubble are very scarce and much more material would have to be examined to verify or rebut this theory. Apart from the explanation as being a brood pouch, other functions may have to be considered and even pathological mutations cannot be entirely ruled out. Even if we accept the theory as a fact, we have to ask for how long the trilobite kept his eggs in its pouch and when it finally released them into the open water of the seas.

Ontogenesis 1

Protaspid (A) and Meraspids (B-D) of Shumardia pusilla (SARS) from the Lower Ordovician of England (Source: Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobitomorpha, University of Kansas Press, 1959). Protaspid shown in 50x magnification, already showing a distinct rhachis.

Much more is known on the development of the trilobite itself, once it hatched from its egg. Today we distinguish between three stages, described as the protaspid, meraspid and holaspid stage respectively. The protaspid stage (see first plate, fig. A) denotes the earliest stage of development. In this condition the trilobite larvae shows a size between 0,25 and 1 mm and has already developed an early rhachis. The larvae itself is usually of a more or less ball-like shape, sometimes with little protrusions. There are no thoracic segments evident, but the carapace must have been a mineralized one already, as otherwise there would have been no fossilized remains. Protaspid trilobites differ in their morphological diversity as much from species to species as the adult animals do. They were spiny or non-spiny and their shell smooth or blotched with little knots.

Ontogenesis 2
Meraspids (E-G) of Shumardia pusilla (S ARS ) from the Lower Ordovician of England (Source: Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobitomorpha, University of Kansas Press, 1959). It is evident that the animal developed new thoracic segments by separating them from the tail shield, rather than from the cephalon as could be expected.

The meraspid stage of a trilobite is characterized by the development of articulated thoracic segments. However, the animal has not yet reached the number regarded as typical for an adult animal of a particular species. Going through regular intervals of molting, the animal continued to add further segments by separating them from the tail shield, rather than from the direction of its head as could be expected. Growth therefore primarily took place in the hind area of the animal. Apart from the adding of new segments other body parts continued in their development.
Ontogenesis 3

Holaspids (E-G) of Shumardia pusilla (S ARS ) from the Lower Ordovician of England (Source: Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobitomorpha, University of Kansas Press, 1959). The gerontic specimen on the right has developed an additional pygidial segment, exceeding the typical number for this species.

brood Reaching the holaspid stage, the trilobite had developed as many thoracic segments as being regarded typical for a particular species. It was now an adult. This does not mean, however, that it necessarily stopped growing. The fossil record gives evidence that some animals developed more than the typical number of segments as can be seen in fig. "I", plate 3.

There are strong indications that trilobites continued to grow in size throughout their entire lifetime. So-called gerontic (old) specimens could reach enormous growth ( Paradoxides, Isotelus , etc.). Naturally, every trilobite sooner or later fell victim to environmental hazards, natural enemies or diseases. Nor must it be forgotten that trilobites being arthropods could not cross the final boundary that limited the size that invertebrates could reach.

Unlike vertebrates, in which the soft tissues and organs are attached to a rigid inner skeleton, allowing the animal to reach enormous sizes (just have a look at dinosaurs or whales) the sheer weight of an invertebrate’s soft tissue encapuslated in its carapace would sooner or later place an end to its growth. A simple analogy: An orange tree can carry a huge amount of fruit at its branches, all of it deliciously palatable. However, if you stacked all these individual oranges on top of each other within a confined space you sooner or later would end up with an undefinable lump as the weight of the fruit on top squashes the one at the bottom. But that is exactly how an arthropod body is constructed – a lot of rather soft biological mass inside a confined space, namely the carapace. The largest arthropods on record are the extinct eurypterids or sea scorpions (Ordovician – Permian) which could reach a length of about 2 m. The buck stops there!

It should also be noted that trilobites could go through distinct metamorphic changes throughtout their growth. Not all species grew in such a homogenous way like our exemplary Shumardia pusilla. Genal spines, macropleurae and other details were developed and lost in consecutive stages. Small - young - specimens of Paradoxides, for example, can show a macropleural third segment which is no longer present in fossilized adults.

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Last Update : 01/30/2010 5:06 PM