03-11-2020, 08:37 PM
RL history
The earliest fossils of Trilobites that have been discovered so far date from the Cambrian Period (c. 540—485 million years ago), which is the earliest stage in history from which many fossils that can clearly be assigned to surviving phylums have been found. They did not appear right at the beginning of this time, which is known mostly for its “small shelly fauna”, but were present and diversifying well before its mid-point. However, the earliest known forms were already clearly recognisable as fully-complete Trilobites in body-plan, diversified, and geographically widespread, which obviously suggests an earlier evolutionary history for which direct evidence has not yet been found: Maybe the earlier stages simply took place within a limited geographical range from which no fossiliferous rocks have yet been found (Maybe, even, IC they originated in the IDU’s world and then spread to RL Earth?
); maybe those proto-Trilobites, like some other Arthropods, had only relatively “soft” exoskeletons rather than the mineral-reinforced ones of the later forms (and of some Crustaceans), so that remains surviving to be fossilised was much rarer; maybe, perhaps because of that softer casing, those proto-Trilobites were much smaller than the later “true” ones and so any fossils that did manage to form would be harder to spot…
During the remainder of the Cambrian and through most of the following Ordovician (c. 485—434 million years ago) the Trilobites remained a major component of the marine fauna and different forms developed for a wide range of lifestyles: Most walked along or swam close to the sea-bed, as predators, scavengers, ‘deposit feeders’ (which “mined” the seabed for smaller organisms & organic debris), or filter-feeders; some swam higher up, apparently filter-feeding and maybe actively hunting in the plankton; it has been suggested that one family had become parasitic on some types of ‘sessile’ (i.e. fixed-in-place) animals, although this is still disputed; and one family is even thought to have developed a symbiotic relationship with sulphur-using bacteria which supplied them with nutrition. However, the end of the Ordovician featured a ‘mass extinction’ event: There was an Ice Age, which lowered sea-levels (as well as global temperatures) and thus not only drained wide expanses of the continental shelves over which the Trilobites had lived but also — probably — brought oxygen-poor water from the deeps close to the surface. Trilobites were among the [numerous] groups of animals that suffered quite heavy losses during this stage, but survived it although with their phylogenetic and ecological diversities significantly reduced: form then on, it seems that most if not all of them were restricted to primarily-scavenging lifestyles. They declined further during the subsequent Silurian and Devonian Periods (c.434—419 MYA; and c. 419—386 MYA), presumably due to the evolution of more effective competitors and/or predators, and a series of extinction events during the second half of the latter (possibly linked to another Ice Age?) reduced them to just a single Order whose surviving lineages seem to have been restricted to moderately deep water. This remnant stock then managed to persist through the Carboniferous Period (or ’Mississippian’ & ‘Pennsylvanian’ Periods; c. 386—291 MYA, either way…) and Permian Period (c.291—252 MYA)… but the end of the Permian was marked by a ‘mass extinction event — this time apparently linked to a rise in global temperature, rather than to a serious drop — which was so severe that some palaeontologists call it “The Great Dying”! No fossil Trilobites have ever been found for which dates later than this are considered appropriate, and so they are presumed to have been among the numerous groups that it exterminated completely. The overall pattern of changes in faunas (and to some extent in marine ‘floras’, as well, although less so among the land-based plants…) was so extreme that this event is used to mark the transition from a Palaeozoic Era (i.e. “Era of Ancient Life”) to a Mesozoic Era (i.e. “Era of Middle Life”; also popularly called “The Age of Dinosaurs”).
The earliest fossils of Trilobites that have been discovered so far date from the Cambrian Period (c. 540—485 million years ago), which is the earliest stage in history from which many fossils that can clearly be assigned to surviving phylums have been found. They did not appear right at the beginning of this time, which is known mostly for its “small shelly fauna”, but were present and diversifying well before its mid-point. However, the earliest known forms were already clearly recognisable as fully-complete Trilobites in body-plan, diversified, and geographically widespread, which obviously suggests an earlier evolutionary history for which direct evidence has not yet been found: Maybe the earlier stages simply took place within a limited geographical range from which no fossiliferous rocks have yet been found (Maybe, even, IC they originated in the IDU’s world and then spread to RL Earth?
); maybe those proto-Trilobites, like some other Arthropods, had only relatively “soft” exoskeletons rather than the mineral-reinforced ones of the later forms (and of some Crustaceans), so that remains surviving to be fossilised was much rarer; maybe, perhaps because of that softer casing, those proto-Trilobites were much smaller than the later “true” ones and so any fossils that did manage to form would be harder to spot…During the remainder of the Cambrian and through most of the following Ordovician (c. 485—434 million years ago) the Trilobites remained a major component of the marine fauna and different forms developed for a wide range of lifestyles: Most walked along or swam close to the sea-bed, as predators, scavengers, ‘deposit feeders’ (which “mined” the seabed for smaller organisms & organic debris), or filter-feeders; some swam higher up, apparently filter-feeding and maybe actively hunting in the plankton; it has been suggested that one family had become parasitic on some types of ‘sessile’ (i.e. fixed-in-place) animals, although this is still disputed; and one family is even thought to have developed a symbiotic relationship with sulphur-using bacteria which supplied them with nutrition. However, the end of the Ordovician featured a ‘mass extinction’ event: There was an Ice Age, which lowered sea-levels (as well as global temperatures) and thus not only drained wide expanses of the continental shelves over which the Trilobites had lived but also — probably — brought oxygen-poor water from the deeps close to the surface. Trilobites were among the [numerous] groups of animals that suffered quite heavy losses during this stage, but survived it although with their phylogenetic and ecological diversities significantly reduced: form then on, it seems that most if not all of them were restricted to primarily-scavenging lifestyles. They declined further during the subsequent Silurian and Devonian Periods (c.434—419 MYA; and c. 419—386 MYA), presumably due to the evolution of more effective competitors and/or predators, and a series of extinction events during the second half of the latter (possibly linked to another Ice Age?) reduced them to just a single Order whose surviving lineages seem to have been restricted to moderately deep water. This remnant stock then managed to persist through the Carboniferous Period (or ’Mississippian’ & ‘Pennsylvanian’ Periods; c. 386—291 MYA, either way…) and Permian Period (c.291—252 MYA)… but the end of the Permian was marked by a ‘mass extinction event — this time apparently linked to a rise in global temperature, rather than to a serious drop — which was so severe that some palaeontologists call it “The Great Dying”! No fossil Trilobites have ever been found for which dates later than this are considered appropriate, and so they are presumed to have been among the numerous groups that it exterminated completely. The overall pattern of changes in faunas (and to some extent in marine ‘floras’, as well, although less so among the land-based plants…) was so extreme that this event is used to mark the transition from a Palaeozoic Era (i.e. “Era of Ancient Life”) to a Mesozoic Era (i.e. “Era of Middle Life”; also popularly called “The Age of Dinosaurs”).