An article published in the journal “BMC Evolutionary Biology” describes a research on Habelia optata, an arthropod that lived in the mid-Cambrian period, about 505 million years ago. It’s one of the many organisms that lived at that time whose fossils caused perplexities in paleontologists with difficulty in their classification. Now Cédric Aria of the University of Toronto and Bernard Caron of the Royal Ontario Museum believe it was related to the ancestors of the current species of the chelicerate subphylum.
In 1909 Charles Walcott discovered the first fossils in the Burgess Shale and in the following years discovered and described many species, among them the Habelia optata. It’s one of the species that emerged during the so-called Cambrian explosion, the period in which the greatest diversification of species in the history of life on Earth occured and the phyla existing today emerged. The strange characteristics of many animals made them difficult to classify, in the specific case it was clear from the beginning that it was an arthropod but the assignment to one of the subphyla was difficult.
Over the course of a century, various studies attempted to classify Habelia optata for good, sometimes together with Molaria spinifera, another arthropod with similar characteristics also discovered in the Burgess Shale. This new research is based on the examination of 41 specimens in total of which 27 acquired in fieldwork parties led by the Royal Museum of Ontario in that area that’s become among the most famous for the abundance of fossils.
The specimens of Habelia optata were examined with a stereoscopic microscope and photographed in various ways to enhance their details, a crucial factor because they were very small animals with a length of around 4 centimeters, of which about half are their tail. Having a dataset on the characteristics of Habelia optata was essential to be able to add it into a wider set on which to apply a Bayesian technique, a probabilistic method for the construction of phylogenetic trees.
According to the researchers, Habelia optata has characteristics of early chelicerates, with among other things appendages similar to the chelicerae, pointed appendages specialized in nutrition. Other structures and in particular the division of the body into three distinct regions are also similar to those of other arthropods belonging to that subphylum such as scorpions and sea scorpions.
However, Habelia optata shows some characteristics different from those typical of chelicerates. In particular, its head developed in some ways more similar to that of another group of arthropods, Mandibulata. According to Cédric Aria, this species is close to the point of divergence between Mandibulata and chelicerates.
Habelia optata was a very small animal and yet it was a predator that hunted other small Cambrian animals such as little trilobites. Its head evolved for that kind of predation and this suggests that in its ecosystem there was a parallel evolution with its prey. This is another clue in the reconstruction of the early history of arthropods.