Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites – a hyperdiverse group of tens of thousands of species -moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans mean that it is unclear whether these sutures were evolved deep within Artiopoda, or evolved multiple times within the group. Here we describe new well-preserved material of Acanthomeridion, a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features between Acanthomeridion and trilobites. We used two coding strategies treating structures as homologous or independently derived across multiple phylogenetic analysis techniques (parsimony or Bayesian inference), and showed that regardless of these variables, the sutures crucial for the success and growth of the hyperdiverse trilobites evolved multiple times within Artiopoda.
Abstract A crucial step in the evolution of Euarthropoda (chelicerates, myriapods, pancrustaceans) was the transition between fossil groups that possessed frontal appendages innervated by the first segment of the brain (protocerebrum), and living groups with a protocerebral labrum and paired appendages innervated by the second brain segment (deutocerebrum). Appendage homologies between the groups are controversial. Here we describe two specimens of opabiniid-like euarthropods, each bearing an anterior proboscis (a fused protocerebral appendage), from the Middle Ordovician Castle Bank Biota, Wales, UK. Phylogenetic analyses support a paraphyletic grade of stem-group euarthropods with fused protocerebral appendages and a posterior-facing mouth, as in the iconic Cambrian panarthropod Opabinia . These results suggest that the labrum may have reduced from an already-fused proboscis, rather than a pair of arthropodized appendages. If some shared features between the Castle Bank specimens and radiodonts are considered convergent rather than homologous, phylogenetic analyses retrieve them as opabiniids, substantially extending the geographic and temporal range of Opabiniidae.
Abstract The classification of Radiodonta is primarily based on the morphology of their frontal appendages, a main feeding structure of this iconic group of mostly Cambrian stem‐group euarthropods. However, recent progress in the description and revision of radiodont taxa, particularly drawing on their frontal appendages, has exposed morphological variation that challenges reliable identification of higher‐level groupings. Here we describe a new taxon of Radiodonta, Laminacaris chimera gen. et sp. nov., from the Cambrian Series 2, Stage 3, Chengjiang biota of China, based on its unique frontal appendage morphology. Laminacaris is distinctive for its combination of characters shared by hurdiids and other early Cambrian radiodont families. Elongated, possibly unpaired endites on two proximal podomeres that bear small distally‐directed auxiliary spines oriented perpendicular to the long axis of the endite, are comparable with the elongated endites and their auxiliary spines of all known Cambrian members of Hurdiidae. In contrast, endites on more distal podomeres are similar to some species of Anomalocaris , and the dorsal spines at the distal end resemble those of Amplectobelua . The mosaic characters in the frontal appendage of Laminacaris chimera may capture morphology close to the divergence between the major radiodont groups.
Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites – a hyperdiverse group of tens of thousands of species -moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans mean that it is unclear whether these sutures were evolved deep within Artiopoda, or evolved multiple times within the group. Here we describe new well-preserved material of Acanthomeridion, a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features between Acanthomeridion and trilobites. We used two coding strategies treating structures as homologous or independently derived across multiple phylogenetic analysis techniques (parsimony or Bayesian inference), and showed that regardless of these variables, the sutures crucial for the success and growth of the hyperdiverse trilobites evolved multiple times within Artiopoda.
Journal Article Piers Plowman Manuscript Trinity College: Dublin 212—The Annals Revisited Get access Stella Pates Stella Pates Tewkesbury, Gloucestershire stella.pates@btinternet.com Search for other works by this author on: Oxford Academic Google Scholar Notes and Queries, Volume 56, Issue 3, September 2009, Pages 336–340, https://doi.org/10.1093/notesj/gjp124 Published: 03 December 2009
Abstract Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites – a hyperdiverse group of tens of thousands of species - moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans mean that it is unclear whether these sutures evolved deep within Artiopoda, or convergently appeared multiple times within the group. Here we describe new well-preserved material of Acanthomeridion , a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features between the ventral plates of this taxon and trilobite free cheeks. We used three coding strategies treating ventral plates as homologous to trilobite free cheeks, to trilobite cephalic doublure, or independently derived. If ventral plates are considered homologous to free cheeks, Acanthomeridion is recovered sister to trilobites however dorsal ecdysial sutures are still recovered at many places within Artiopoda. If ventral plates are considered homologous to doublure or non-homologous, then Acanthomeridion is not recovered as sister to trilobites, and thus the ventral plates represent a distinct feature to trilobite doublure/free cheeks.
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