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Abstract Great strides have been made in understanding the phylogeny of the five extant echinoderm classes, however, many Palaeozoic groups have yet to be examined in a rigorous, quantitative framework. The aberrant morphologies of Paracrinoidea, an unusual group of Palaeozoic echinoderms, have hindered their inclusion in large‐scale phylogenetic and morphologic studies. This study uses a combined approach of phylogenetic analysis and morphological disparity to elucidate species relationships within the clade. Findings from this study suggest that Paracrinoidea is a monophyletic group and that respiratory structures, oral plate arrangement, and ambulacral morphologies are important for defining subclades within Paracrinoidea. Examination of paracrinoids in a quantitative framework, facilitates their inclusion in larger projects examining Palaeozoic echinoderm evolution, ecology and biogeography. 

The Paleozoic Era was host to many significant biotic events such as the Great Ordovician Biodiversification Event, the Late Ordovician Mass Extinction, and the Late Devonian extinctions. These events were likely catalyzed by abiotic (e.g. climate) versus biotic drivers. Echinoderms are globally distributed, temporally expansive, and easily identifiable; these qualities make them an excellent model system to test hypotheses relating biodiversity with abiotic factors. Biodiversity patterns of echinoderms are currently not well understood because of a lack of focus on the dynamics of the entire clade. To remedy this, we have worked to expand current understandings of Paleozoic echinoderm diversity patterns by investigating the global distribution and temporal occurrences of taxa spanning the entire clade. Results suggest patterns of diversity unique to previously established trends that predominantly centered on a limited number of echinoderm groups. To examine the connection between climate change and Paleozoic echinoderm biodiversity (i.e., diversification, extinction, and origination rates), we collated stable oxygen isotope data from the primary literature spanning the Ordovician to the Devonian. We compiled these data to create a continuous curve of δO values during the described period to better evaluate in tandem with echinoderm diversity metrics. When the δO curve is compared to the echinoderm biodiversity patterns, we found that cooling periods coincide with increased extinction rates, corroborating prior hypotheses that major end-Ordovician cooling triggered changes in echinoderm biodiversity at a global level and further identifying a potential pattern in abiotic drivers in echinoderm biodiversity. The connection between Paleozoic echinoderm biodiversity and other abiotic factors will be further studied by comparing these recovered patterns with paleolatitudinal distributions. 

Abstract The Cambrian and Ordovician radiations marked the origins of all major echinoderm clades and established their Phanerozoic ecological blueprint. Recent claims of modest innovation of early echinoderms and other animals suggest constraints on novelty during the origins of phyla. Here, we document the life‐habit richness, body size, tiering, habitat usage, mobility, diet and foraging habits of 366 Cambrian–Ordovician echinoderm genera across a time‐scaled phylogeny to identify the timing and impact of novelty. Most early echinoderms were sedentary, filter‐feeding microbivores, and their body sizes, diets and modes of locomotion were largely unchanged through time, despite major volatility in taxonomic composition. Cambrian echinoderms lived close to the seafloor, with stylophorans first evolving semi‐infaunality. Many Ordovician echinoderms lived farther from the seafloor, with more complex filter‐feeding organs and tiering structure, often using other organisms as substrates. Mobile carnivores and mass‐feeders emerge then, too, across diverse asterozoans and echinozoans. Most of these novelties coincide with the origins of individual clades during the early and late Cambrian (Terreneuvian/Series 2 and Furongian), and 10–20 million years pass before most become ecologically important. Life‐habit evolution within most taxa involved new variations in these traits, with crinoids and asterozoans better able to shift strategies. The Ordovician radiation records more new variability in pre‐existing Cambrian life habits rather than radical re‐invention, with important novelties (like mobile carnivory and mass‐feeding, increased height off the seafloor, and biotic substrates) becoming ecologically more impactful. Taxa with these strategies were those best able to capitalize on the newly heterogeneous Ordovician world.