Despite certain areas of uncertainty the study of trilobites shows no sign of abating, even after more than two centuries of collecting.
New species are continually being discovered and described, illuminating previously unknown features and in turn increasing our understanding of the evolutionary history of this fantastic fossil group.
The families Telephinidae and Cyclopygidae are typical of this life style, having developed a more streamlined body and a wider thoracic axis, presumably due to the need for stronger swimming muscles (Fig. They also developed greatly inflated eyes, which allowed for 360-degree vision.
These species tend to be found in rock layers that formed in a deep marine environment that was penetrated by only a small amount of sunlight, and so usually contain blind trilobite species.
Gould as evidence for the theory of punctuated equilibrium, whereby species evolve with little net morphological change (stasis) which is occasionally interrupted by short periods of sudden change associated with speciation events (cladogenesis). (c) Cambrian trilobites described by Joachim Barrande in 1852. In some countries, trilobites have even taken on importance as cultural icons. This continues until a stable number of segments is reached. Predatory trilobites have other characteristics that further differentiate them from deposit feeders, including much greater variability in overall form, a more arched exoskeleton and a generally greater size range. These species appeared with the first trilobites in the lower Cambrian, but became extinct in a mass extinction during the Late Devonian period, with only the deposit-feeding proetids surviving until the end of the Permian. (a) Balcoracania dailyi (~10mm), (b) Paradoxides gracilis (110mm), (c) Olenellus (~20mm); (d) Acadoparadoxides briaerus (300mm) (e) Isotelus rex (720mm), (f) Parabarrandia bohemica (120mm), (g) Pricyclopyge binodosa binodosa (20mm), (h) Cybantyx anaglyptos (50mm), (i) Aulacopleura konincki (20mm); (j) Leonaspis (10mm), (k) Erbenochile erbeni (50mm), (l) Kathwaia capitorosa (~30mm), (m) Ditomopyge (~30mm). During moulting, the exoskeleton broke across joints, or sutures, between the segments, such as the facial sutures, which join the free cheeks to the rest of the cephalon (Fig. After moulting, the individual took on water to swell its body size before growing a new exoskeleton.Sources: (a) Paterson and Edgecombe, 2006; (c,j) Levi-Setti, 1993; (e) Rudkin et al., 2003; (g) Horný & Bastl, 1970; (h) Whittington, 1997; (i) (k) Chatterton and Gibb, 2010; (l) Owen, 2003; (m) Moulting was an important part of the trilobite life cycle.4), allowing the trilobite to process prey such as worms and small crustaceans. This is called the conterminant condition By contrast, in deposit-feeding species the hypostome was less tightly fixed to the doublure (the natant condition), and would have been used mainly for sieving through mud on the sea floor for food particles. The earliest scientific report of a trilobite fossil was by Reverend Edward Lhwyd in 1698. In the early stages of development, this process was used to increase the number of thoracic segments, as well as the overall size of the animal. The world’s biggest trilobite—Isotelus rex new species from the Upper Ordovician of northern Manitoba, Canada. This specimen was collected near Llandeilo in South Wales, and was originally described by Lhwyd as “some kind of flat fish” (Fig. Later, following more and more fossil discoveries, trilobites were recognized as arthropods, more closely related to crabs, spiders and lobsters than to fish. Trilobite development can be broadly divided into three main stages: protaspid, meraspid and holaspid (Fig. The protaspid and meraspid phases involve moulting followed by an increase in segment number (anamorphic growth pattern). 1i-j); the development of these structures and the simultaneous increase in diversity could be the result of the trilobites being preyed on by other invertebrate groups, especially cephalopods such as orthocones, which are related to the modern nautilus. Although large trilobite fossils are found in regions that were around the equator when trilobites were alive, they seem to be more abundant in areas that were located close to the South Pole, especially during the Cambrian and Ordovician. Annual Review of Earth and Planetary Sciences 35, 401–434. An increase in diversity due to predation pressure might sound counterintuitive, but is in fact very common in the natural world. The trilobite species Acadoparadoxides briareus, which is found in rocks called the Jbel Warmast Formation in the Middle Cambrian of Morocco (Fig. In addition, the Valongo Formation in the Middle Ordovician of Portugal contains a highly diverse variety of giant trilobite species including Hungioides bohemicus and Ogyginus forteyi, the latter of which is estimated to have reached around 90 centimetres.