03-11-2020, 08:35 PM
Relationships
Trilobites have a body-plan which places them clearly in the Phylum Arthropoda: A segmented body with jointed limbs, a chitinous exoskeleton, and compound eyes. Thus they are related to the Horseshoe Crabs, Crustaceans, Arachnids (Spiders, Scorpions, Mites & Ticks, etc.), Insects, Centipedes & Millipedes, and several other groups that you are less likely to have heard of. They might even be Crustaceans, although apparently the current consensus among palaeontologists says not. If they branched off from the Arthropods’ basal stocks before the ancestors of the various groups that survive in RL diverged from each other then some taxonomists (favouring a ‘crown group’ model of classification) would place them outside (but as a ‘sister-group’ to) the Arthropoda proper, in a wider assemblage labelled ‘Panarthropoda’: Here in the IDU, however, their own survival into modern times would simply extend the scope of the ‘crown group’ and thus of the Arthropoda proper.
The distinctive three-lobed body-plan that all Trilobites share is no more “unusual” than the body-plans of various Crustacean groups, but there is one feature in which they differ from all other animals! This is the fact that the lenses in their eyes are composed not of organic materials, as in all of those others, but of Calcite crystals — one per individual segment in each compound eye — instead.
Trilobites have a body-plan which places them clearly in the Phylum Arthropoda: A segmented body with jointed limbs, a chitinous exoskeleton, and compound eyes. Thus they are related to the Horseshoe Crabs, Crustaceans, Arachnids (Spiders, Scorpions, Mites & Ticks, etc.), Insects, Centipedes & Millipedes, and several other groups that you are less likely to have heard of. They might even be Crustaceans, although apparently the current consensus among palaeontologists says not. If they branched off from the Arthropods’ basal stocks before the ancestors of the various groups that survive in RL diverged from each other then some taxonomists (favouring a ‘crown group’ model of classification) would place them outside (but as a ‘sister-group’ to) the Arthropoda proper, in a wider assemblage labelled ‘Panarthropoda’: Here in the IDU, however, their own survival into modern times would simply extend the scope of the ‘crown group’ and thus of the Arthropoda proper.
The distinctive three-lobed body-plan that all Trilobites share is no more “unusual” than the body-plans of various Crustacean groups, but there is one feature in which they differ from all other animals! This is the fact that the lenses in their eyes are composed not of organic materials, as in all of those others, but of Calcite crystals — one per individual segment in each compound eye — instead.