ECOLOGICAL NOVELTY AT THE START OF THE CAMBRIAN AND ORDOVICIAN RADIATIONS OF ECHINODERMS

The Cambrian and Ordovician echinoderm radiations marked the origins of all major clades, and established their Phanerozoic ecological blueprint. Recent claims of modest ecological innovation of early echinoderms and other animals suggest constraints to novelty during the origins of phyla. We test this by documenting the life-habit richness, body size, tiering, habitat usage, mobility, diet, and foraging habits among 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 unchanged through time, despite major volatility in taxonomic composition. Cambrian echinoderms lived close to the seafloor, with stylophorans first evolving semi-infaunality. Ordovician echinoderms lived farther from the seafloor, with more complex filter-feeding organs and tiering structure. Echinoderm life-habit richness (the number of unique ecological strategies that exist during an interval) mirrors genus richness, displaying two pulses of diversification peaking in the Mid Cambrian and Mid–Late Ordovician. Ecological evolution in most taxa involved new variations in body size, tiering, filter-feeding organ complexity, mobility, and substrate, with crinoids and asterozoans better able to shift ecological strategies. Echinoderm novelties were ingrained rapidly during clade origins in the early and late Cambrian (Terreneuvian and Furongian), with only two major additions henceforth: the usage of other living organisms as substrates and the earliest mobile carnivores. After the inception of most novelties, 20 million years pass before echinoderms achieved peak genus and life-habit richness.

These patterns suggest that most of the echinoderm ecological strategies that typify the phylum were rapidly ingrained during their earliest origins, primarily associated with the origins of several taxonomic classes. In contrast, the Ordovician radiation primarily records the propagation of lineages (and life habits) that first existed in the Cambrian rather than a period of novel ecological invention, and these successful lineages were those best able to capitalize on the newly heterogeneous Ordovician world.