When life got hard

By Dr Duncan Murdock, Research Fellow

Whether you’re a great white shark with a deadly conveyor belt of teeth, a deep sea snail with a coat of armour or a coral building the Great Barrier Reef one polyp at a time, mineralized skeletons are a crucial part of many animals’ way of life. These hard skeletons – shells, teeth, spines, plates and bones – are all around us.

The fossil record is full of the remains of the skeletons of long-extinct critters, so much so that entire layers of rocks can be composed almost completely of them. But this has not always been the case…

A piece of 425 million year old sea floor containing the skeletons of trilobites, brachiopods, bryozons, corals and gastropods preserved as limestone

Travel back some 570 million years to a time known as the Ediacaran and the picture is very different. Although there were large-bodied creatures that were possibly animals, they were entirely soft-bodied. Then, right at the end of the Ediacaran Period, the first animals with hard skeletons evolved, creating strange tubes, stacked cones, and other bizarre forms such as Namacalathus, which resembles a baby’s rattle!

Some of the first animals with skeletons, Cloudina and Namacalathus alongside the soft-bodied Ediacaran fauna. Reconstruction based on rocks from Namibia, Southwest Africa, from 543 million years ago. Image: Mighty Fossils.

 

In the following few tens of millions of years, in the early part of the Cambrian Period, a whole host of animals burst onto the scene baring their ‘teeth’, hiding in their shells, and bristling their spines. In fact, we can trace the origin of almost every kind of animal skeleton to this relatively short window of the Earth’s past.

In my research, I have compiled the evidence for how and when these skeletons first appear. Three key observations have emerged. First, skeletons evolved independently many times in different animal groups. Second, there is both direct and indirect evidence, such as exceptionally preserved fossils and trace fossils, for entirely soft-bodied examples of animal groups that later evolved skeletons. And lastly, the first animal skeletons are less complex and more variable than later examples.

Added to what we know about how living animals build their skeletons, this all points to one explanation: Animal skeletons evolved independently in different groups by utilising a common ‘toolkit’ of genes, inherited from their common ancestor but used in different ways in different skeletons.

In other words, the soft-bodied ancestors of animals with hard parts had inherited all they needed to build simple skeletons that were then honed into the array of shells, teeth, spines, plates and bones we see today. For these skeletal pioneers, armed with their genetic ‘toolkit’, the environmental and ecological pressures of the early Cambrian prompted the evolution of similar, but independent, responses to their changing world – when life got hard.

Murdock, DJE. 2020. The ‘biomineralization toolkit’ and the origin of animal skeletons, Biological Reviews, is available for free here.

Top image: Tiny fragments of early skeletons, shells and spines, from around 510-515 million years ago.

 

Life’s big bang?

by Harriet Drage and Scott Billings

You may have heard of the Cambrian Explosion, an ‘event’, starting roughly 540 million years ago, when all the major animal groups suddenly appear in the fossil record, an apparent explosion of life and evolution.

But was there really an evolutionary explosion of all these animal groups, or is the lack of evidence from earlier periods due to some peculiarity of the fossilisation process? The debate has rumbled on for a number of years.

Now, a new study from our research team, the University of Oxford’s Department of Zoology, and the University of Lausanne, claims that the early Cambrian saw the origins and evolution of the largest and most important animal group on Earth – the euarthropods – in a paper which challenges two major pictures of animal evolution.

Euarthropoda contains the insects, crustaceans, spiders, trilobites, and a huge diversity of other forms alive and extinct. They comprise over 80 percent of all animal species on the planet and are key components of all of Earth’s ecosystems, making them the most important group since the dawn of animals over 500 million years ago.

Exceptionally preserved soft-bodied fossils of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis from the Burgess Shale, Canada. Top left: Frontal appendage showing segmentation similar to modern-day euarthropods. Bottom right: Full body specimen showing one pair of frontal appendages (white arrows) and mouthparts consisting of plates with teeth (black arrow) on the head. Images: A. Daley.

A team based at the museum, and now at Lausanne, conducted the most comprehensive fossil analysis ever undertaken on early euarthropods, to try and establish whether these animals really did emerge in the early Cambrian period, or whether fossilisation just didn’t occur in any earlier periods.

In an article published today in the Proceedings of the National Academy of Sciences they show that, taken together, the total fossil record does show a gradual radiation of euarthropods during the early Cambrian, 540-500 million years ago, challenging other ideas that suggest either a rapid explosion of forms, or a much slower evolution that has not been preserved in the fossil record.

Each of the major types of fossil evidence has its limitation and they are incomplete in different ways, but when taken together they are mutually illuminating
Professor Allison Daley

Reconstruction of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis, based on fossils from the Burgess Shale, Canada. Reconstruction by Natalia Patkiewicz.

By looking at a huge range of fossil material the researchers ruled out the possibility that Pre-Cambrian rocks older than around 541 million years would not have preserved early euarthropods. The only plausible explanation left is that the origins of this huge animal group didn’t evolve until about 540 million years ago, an estimate which also matches the most recent molecular dating.

The timing of the origin of Euarthropoda is very important as it affects how we view and interpret the evolution of the group and its effects on the planet. By working out which groups developed first we can trace the evolution of physical characteristics, such as limbs.

Exploring all the evidence like this allows us to make an informed estimate about the origins of key animal groups, leading to a better understanding of the evolution of early life on Earth.

Model of the Cambrian stem lineage euarthropod Peytoia, based on fossils from the Burgess Shale. Top left: Closeup of the mouth parts and frontal appendages. Bottom right: Overall view of the body. Model and image: E. Horn.