BRS Javier Ortega-Hernandez "Along came a spider - weaving the Cambrian origin of chelicerates"

host: Heather Bruce

Title talk: Along came a spider - weaving the Cambrian origin of chelicerates

Abstract: Chelicerates, whose living representatives include sea spiders, horseshoe crabs and arachnids, represent one of the most successful groups of euarthropods, and most of them play a critical role in modern terrestrial ecosystems as obligate predators. Despite their substantial extant diversity and ecological significance, the deep evolutionary origin of chelicerates is highly debated. Several extinct groups have been traditionally regarded as early chelicerate ancestors based on their resemblance to extant horseshoe crabs, most famously the biomineralized trilobites. However, recent discoveries of soft-bodied euarthropods from sites of exceptional preservation in early and middle Cambrian marine deposits around the world suggest that modern chelicerates evolved from a diverse and morphologically disparate ancestral lineage comprising several soft-bodied representatives. Major groups of Cambrian euarthropods implicated in chelicerate origins include the megacheirans, which are typified by the presence of raptorial first appendages, as well as the sanctacaridids, habeliidans, and more recently the mollisoniids. However, none of these taxa convincingly show evidence for the critical shared derived characters (synapomorphies) observed in all modern chelicerates, such as the specialization of the first appendage pair as jacknife or pincer-like chelicerae, or the modification of the opisthosomal limbs into respiratory book gills.

New data on exceptionally preserved fossils from the early Cambrian of South China and mid-Cambrian of North America cast new light on early chelicerate evolution. Through a multi-pronged approach combining micro computed tomography, paleoneuroanatomy, and preparation of museum specimens, it is now possible to fundamentally redefine our understanding of chelicerate origins from Cambrian ancestors. These discoveries demonstrate that the key characters that define extant chelicerates evolved in a complex stepwise pattern, including the organization of the nervous system, the segmental organization of the head, and critically the transformation of the first (deuterocerebral) appendages as the chelicerae. These new insights illuminate the origin of the archetypical chelicerate body plan during the Cambrian Explosion prior to the main diversification of morphologically modern representatives during the Ordovician, and their subsequent major transition into terrestrial environments.

Short biography: I am an invertebrate paleobiologist interested in the evolution of major animal groups during the Paleozoic, with particular emphasis on the Cambrian Explosion and the Great Ordovician Biodiversification Event. Originally from Mexico City, I obtained my undergraduate degree in Biology from the Universidad Nacional Autónoma de México, and in 2008 completed the MSc in Palaeobiology at the University of Bristol in the United Kingdom. I then moved to the University of Cambridge from 2009 to 2018, first as a PhD student at the Department of Earth Sciences, and then as a Research Fellow of Emmanuel College and the Department of Zoology. I have been part of the faculty at the Department of Organismic and Evolutionary Biology and curator at the Museum of Comparative Zoology at Harvard University since January 2019.