Paleontologists have unearthed the remains of a previously unknown slug-like creature that lived during the Early Ordovician epoch, 478 million years ago. The discovery, reported in the journal Nature, sheds new light on the earliest stages in the evolution of mollusks, a group of invertebrates that includes clams, snails and squids.
Calvapilosa kroegeri from the Lower Ordovician epoch (Tremadocian age) Fezouata Formation, near Zagora, Morocco: part, imaged and illuminated from north west (left), interpretative drawing (right). Image credit: Jakob Vinther et al, doi: 10.1038/nature21055.
“Mollusks consist of a multitude of distinct groups, which all originated about 520 million years ago in a very short period of time, probably less than 20 million years,” said Yale’s Professor Derek Briggs, a co-author on the study.
“Their evolutionary history is squeezed into a brief interval of evolutionary time called the Cambrian explosion, which makes the sequence of events difficult to piece together.”
“The mollusks are amongst the earliest animals identifiable in the fossil record, however determining what their ancestor looked like is difficult since many of the groups appear within a small window of time, making the sequence of evolutionary events difficult to piece together,” added lead author Dr. Jakob Vinther, from the University of Bristol, UK.
One of the defining characteristics of the mollusks is the possession of a radula, a kind of toothed-tongue which is used to rake up or rasp food.
The radula houses hundreds of teeth, the patterns of which can be used to determine diet and identify species. Whilst not all mollusks have a radula, a radula cannot be found in any other group of animals.
The newly-discovered species, Calvapilosa kroegeri, resembles a slug covered with short spines all over its upper body and with a large ‘fingernail-like shell’ over its head.
In the center of the head of this new species are two rows of teeth which the researchers demonstrate is a radula.
The discovery of this structure firmly identifies Calvapilosa kroegeri as a mollusk.
Additionally, it suggests that similar fossil forms from the Burgess Shale in Canada (Orthrozanclus) and Sirius Passet in Greenland (Halkieria) are also mollusks and possessed a radula.
Reconstruction of Calvapilosa kroegeri, juvenile. Color scheme is speculative, whereas soft tissues are inferred by phylogenetic bracketing. Image credit: Jakob Vinther et al, doi: 10.1038/nature21055.
Following an analysis to determine the family tree of mollusks, Calvapilosa kroegeri was revealed to be the most primitive member of the lineage leading to chitons.
Chitons can still be found today and are characterized by their possession of eight shell plates and spines around their margin, similar to what is seen covering the body of the new species.
“If we trace back the evolution of chitons, we can see that the number of their shells has increased with time,” Dr. Vinther said.
“It is therefore likely that the ancestor to all mollusks was single-shelled and covered in bristle-like spines, not dissimilar to Calvapilosa kroegeri.”
Calvapilosa kroegeri came from a fossil-rich deposit near Zagora in southeastern Morocco known as the Ordovician Fezouata Formation.
“Morocco has revealed itself as a fossil treasure trove for ancient life,” said co-author Dr. Peter Van Roy, of Ghent University in Belgium.
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Jakob Vinther et al. Ancestral morphology of crown-group molluscs revealed by a new Ordovician stem aculiferan. Nature, published online February 6, 2017; doi: 10.1038/nature21055
