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Research Fellow: Javier Ortega-Hernández - Weird Wonders of the Cambrian

I came to Emmanuel as a Research Fellow in October 2013, after finishing a PhD at the Department of Earth Sciences here in Cambridge.

I was initially trained as a biologist during my undergraduate degree at Mexico City (UNAM), and my research focuses on the origins of complex animals by combining the study of their fossilised remains in the rock record with their developmental biology in extant representatives. I primarily look at the early evolution of a major group of invertebrates known as the Panarthropoda, which includes the arthropods (such as arachnids, millipedes, crustaceans and insects), onychophorans (velvet worms) and tardigrades (water bears).

Panarthropods, with a staggering diversity of an estimated over ten million species, are among the most successful animals on the planet. Furthermore, they have dominated most of the available ecosystems since their origins during the ‘Cambrian Explosion’ more than 500 million years ago: a critical event in which most of the major animal groups appear for the first time in the fossil record. Given their ecological importance and ubiquitous presence throughout the Phanerozoic (spanning the time from 541 million years ago to today), panarthropods are ideal subjects with which to study the conditions leading to the origin of complex animals, and the processes that have shaped the structure of the biosphere.

During my time at Emmanuel, I have looked at diverse aspects of the natural history of early panarthropods. A significant fraction of this work has centred on illuminating the fossil ancestry of onychophorans and tardigrades. These are the less familiar members of Panarthropoda, partly because their diversity – consisting of a little over 200 and 1000 described species respectively – pales in comparison with that of the arthropods, with an estimated millions
of species. Despite their comparatively modest success, reconstructing the evolution of onychophorans and tardigrades is crucial for understanding better the origins of animal biodiversity. However, whereas arthropods have a durable exoskeleton that is prone to preservation, onychophorans and tardigrades are almost entirely soft-bodied, and thus their fossil record is extremely sparse. Luckily there are some exceptional fossils with soft-tissue preservation, which give invaluable insights into the early evolution of these enigmatic organisms.

The world-famous Burgess Shale in British Columbia – dated at 508 million years ago – is one of the most important Cambrian sites. It reflects an ancient open-water marine community that includes dozens of extinct soft-bodied species preserved in superb detail, most of which correspond to distant ancestors of extant animal groups. Emmanuel has enabled me to examine museum collections of Burgess Shale material housed at the Smithsonian Institution in Washington and the Royal Ontario Museum in Toronto.

I was able to identify complex morphological traits shared between these fossils and extant onychophorans and tardigrades. A good example is Hallucigenia, a legged worm-like animal that is typified by a dorsal armature of paired spines. Hallucigenia has had a colourful, if somewhat infamous, research history. Notwithstanding having been originally interpreted upside down, Hallucigenia has baffled evolutionary biologists because its body plan is quite unlike that of any animal group living today; this peculiar fossil has even come to epitomise the concept of a ‘Cambrian weird wonder’, popularised by Stephen J Gould’s book Wonderful Life.

However, exceptionally well-preserved fossils often adhere quite well to the adage ‘the devil is in the detail’. Martin Smith, a Research Fellow at Clare College, and I demonstrated the importance of studying detail by looking at the claws in the limbs of Hallucigenia. By using backscatter electron microscopy, we showed that they have a unique construction resembling stacked ice-cream cones. More critically, however, this discovery allowed us to make direct comparisons with the way claws are organised in extant onychophorans, and led us to realise that Hallucigenia is an early member of this lineage as common ancestry means it shares a complex trait.

A parallel case occurs in another legged worm in the Burgess Shale known as Aysheaia. Despite its superficial resemblance to onychophorans in terms of size and appearance, new investigations suggest that Aysheaia is actually more closely related to the tardigrades as the two share various features, including a similar feeding apparatus and up to seven claws per leg. This leads to the intriguing implication that the Cambrian ancestors of tardigrades were macroscopic, whereas extant species typically measure less than 2 mm long, and clarifies a significant portion of the enigmatic evolutionary history of this mesmerising group.

I am continuing to explore the relationships between Cambrian organisms and their descendants, with the aim of understanding better the origins of extant biodiversity. New fossil discoveries around the world and the development of analytical techniques offer an ever-increasing range of exciting opportunities for answering the most challenging questions in evolutionary biology through deep time. Little by little, the weird wonders of the Cambrian cease to be puzzling enigmas, and turn into vital sources of information for understanding the rise of animal life on Earth.

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