In a new study, palaeontologists from the University of Toronto and the Royal Ontario Museum (ROM) are helping resolve the questions surrounding the evolution and ecology of Odaraia, a taco-shaped marine animal that lived during the Cambrian period. Fossils collected by ROM in the Burgess Shale reveal that Odaraia had mandibles, and researchers are finally able to place it as belonging to the mandibulates — ending its long enigmatic classification among the arthropods and revealing more about early evolution and diversification.

The results were published in the journal Proceedings B. Lead author Alejandro Izquierdo-López completed this work as a PhD student in the Faculty of Arts & Science’s Department of Ecology & Evolutionary Biology (EEB) at U of T while based at ROM, alongside co-author Jean-Bernard Caron, an associate professor in EEB and the Department of Earth Sciences at U of T, and the Richard M. Ivey Curator of Invertebrate Palaeontology at ROM.

The authors were able to identify a pair of large appendages with jagged edges near the mouth of Odaraia for feeding, clearly indicative of mandibles which are one of the key and distinctive features of the mandibulate group of animals. This suggests that Odaraia is one of the earliest known members of this group. In a detailed analysis of its more than 30 pairs of legs, researchers made another stunning discovery, finding an intricate system of small and large spines. According to the authors, these spines could intertwine, capturing smaller prey like a fishing net, suggesting how some of these first mandibulates were able to leave the sea floor and survive in the water column, and setting the seeds for their future ecological success.

“The head shield of Odaraia envelops practically half of its body including its legs, almost as if it were encased in a tube. Previous researchers had suggested this shape would have allowed Odaraia to gather its prey, but the capturing mechanism had eluded us, until now,” says Izquierdo-López. “Odaraia had been beautifully described in the 1980s, but given the limited number of fossils at that time and its bizarre shape, two important questions had remained unanswered: is it really a mandibulate? And what was it feeding on?”

At almost 20 centimetres, a substantial size for this time, early mandibulates like Odaraia were part of a community of large animals that could have been able to migrate from the marine bottom-dwelling ecosystems characteristic of the Cambrian period to the upper layers of the water column. These types of communities could have enriched the water column and facilitated a transition towards more complex ecosystems.

Cambrian fossils record the major divergence of animal groups originating over 500 million years ago. This period saw the first diversification of many animal groups, including the mandibulates, one of the major groups of arthropods (animals with jointed limbs) and the evolution of innumerable innovations, such as eyes, legs and shells.

Mandibulates are an example of evolutionary success, representing over half of all current species on Earth. Today, mandibulates are everywhere: from sea-dwelling crabs to centipedes lurking in the undergrowth or bees flying across meadows, but their beginnings were humbler. During the Cambrian period, the first mandibulates were marine animals, most bearing distinct head shields or carapaces.

Fossils of Odaraia were first collected in the Burgess Shale over 100 years ago. “The Burgess Shale has been a treasure trove of paleontological information,” says Caron. “Thanks to the work we have been doing at ROM on amazing fossil animals such as Tokummia and Waptia, we already know a substantial amount about the early evolution of mandibulates. However, some other species had remained quite enigmatic, like Odaraia.”

Burgess Shale fossils are exceptional, as they preserve structures, animals and ecosystems that under normal conditions would have decayed and completely disappeared from the fossil record. Mandibulates, though, are generally rare in the fossil record. Most fossils preserve only the hard parts of animals, such as skeletons or the mineralized cuticles of the well-known trilobites, structures that mandibulates lack.

ROM/A&S News
With files from the Royal Ontario Museum.